Defects Welding

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Non Destructive Testing

NDT is concerned with finding defects. So, a knowledge of the types of

defects is essential, both to determine the best NDT technique to use and to

help to determine the type of defect and to assess how significant the defect

is.Defects are formed during:

Primary processing – Casting, Forging, Rolling, Welding etc.

Secondary processing – Heat treatment, Machining etc

Service induced – fatigue cracking, stress corrosion cracking.



Defects in Wrought Products

Wrought products are materials or components that have been produced by

mechanical working processes such as forging, rolling or extrusion. The

other basic method of producing shapes is casting, where molten metal is

poured into a mould of the required shape and it solidifies to take the shapeof the mould cavity.

Defects in wrought products include the following:

Laps

Seams

Pipe/laminationInclusions

Hydrogen flakes

Forging bursts



Extrusions

In direct extrusion a ram forces the preheated aluminum billet through the

die. This can be likened to squeezing toothpaste out of a tube. Using this

method it is possible to extrude up to six lengths from one die.Direct extrusion is usually used for the manufacture of profiled sections and

hollow bar products.

Indirect extrusion is the reverse of direct extrusion, the die being forced on

to the billet rather than the billet being forced through the die.



Extrusions When introduced into production, pre

cut billets (slugs) are heated in furnace

up to extrusion temperature. Billets, as

input material, are pressed in a

horizontal powerful hydraulic press.



The Press supplies the force

necessary to squeeze the billet

through the extrusion die. It

consists of:

The container where the billet

is put under pressure.The main cylinder with the ram

for pushing the billet into the

container and through the die.

The front platen giving counter

support to the die package.

The main columns fixing the

front platen and the cylinder

together.

The die is supported by a

series of back dies or backers

and bolsters for transferring the

main press load to the front

platen.



Forging The forging process involves deforming a hot work piece with dies attached to

a mechanical or hydraulic press. Forging is used to produce some of the

highly stressed parts in tools and aircraft because forged parts have high

resistance to shock and fatigue. Since forged parts are plastically deformed,

they are stronger and more ductile than parts produced with die-casting.



Rolling



Rolling



Casting

Tempered sand is packed onto wood or metal pattern halves, removed from

the pattern, and assembled with or without cores, and metal is poured into

resultant cavities. Various core materials can be used. Molds are broken to

remove castings. Specialized binders now in use can improve tolerancesand surface finish. Most metals are castable.

If the casting is to be hollow, as in the case of pipe fittings, additional

patterns, referred to as cores, are used to form these cavities. Cores are

forms, usually made of sand, which are placed into a mold cavity to form the

interior surfaces of castings. Thus the void space between the core and

mold-cavity surface is what eventually becomes the casting.



The pattern is a

physical model of the

casting used to make

the mold.

The mold is made by

packing some readilyformed aggregate

material, such as

molding sand, around

the pattern. When the

pattern is withdrawn,

its imprint provides

the mold cavity, which

is ultimately filled

with metal to become

the casting.



Laps

These are found in rolled or forged products. Laps in hot rolled bars are

longitudinally oriented folds on the surface of the product due to rolling over

of projections on the surface.



Laps

In cross-section laps tend to „hook‟ under the surface. They generally

contain oxide or scale and may be partially welded at the tip.Because of their method of formation, laps tend to be very long although

they are usually quite shallow, say less than 1 mm in depth.

The preferred NDT to detect laps in steel is magnetic particle testing. Eddy

current testing is the best method for non-ferrous metals. Penetrant testing

is generally not suitable as laps usually contain scale or oxide.





Pipes and laminations

Pipe and lamination defects are a by-product of ingot steel production.Modern steelmaking practice uses continuous casting technology where

these defects are much less common.

Both pipe and laminations defects are centrally located and, in the case of

lamination the defect is planar and parallel to the flat faces. The preferred

NDT method for pipe and lamination is ultrasonic testing. In smaller sections

pipe may also be detected by radiography



Pipe / lamination

These two defects are grouped together since they have the same origin.Piping is a cavity formed during solidification of an ingot due to the fact that

when molten metal solidifies there is a reduction in volume called shrinkage.

Piping may be open at the ingot top when it is called a primary pipe. It may

also be within the ingot when it is called a secondary pipe. Ingot pipe can

persist in material right through a rolling sequence from the ingot stage tofine wire or thin sheet to produce a pipe or lamination defect. In some cases

secondary pipe can weld up and so disappear during rolling operations.

The difference between pipe and lamination is that pipe occurs in sections

such as rounds, hexagons and squares and lamination occurs in flat

products such as plate or sheet.



Inclusions

These are non-metallic material such

as:

Products of steelmaking reactions, for

example, sulphides, silicates, slag.Refractory material dispersed through

the metal.

Inclusions are always present to some

degree in steel but are of concern in

gross form or at excessive levels.

Inclusions tend to be orientated in the

direction of metal working.

Effect of rolling on inclusions

The preferred NDT method for

detecting gross inclusions is ultrasonic

testing. For smaller sectionsradiography may be used.





Forging bursts

These are surface or internal ruptures

due to the inability of metal to

withstand internal tensile stresses

generated in forging. They arepromoted by such factors as

processing at too low a temperature,

excessive working in forging or forging

steels with higher sulphur contents

(hot shortness).

Bursts are often large and seldom

heal during subsequent working. They

may take the form of an open cavity or

a tight faced crack and may be

longitudinally or transversely

orientated. The best method ofdetection is ultrasonic, or radiography

in smaller sections.



Cracks hese are surface or

internal ruptures due to the

inability of metal to withstand

internal tensile stresses

generated in forging. They arepromoted by such factors as

processing at too low a

temperature, excessive working

in forging or forging steels with

higher sulphur contents (hot

shortness).

Bursts are often large and

seldom heal during subsequent

working. They may take the form

of an open cavity or a tight faced

crack and may be longitudinallyor transversely orientated. The

best method of detection is

ultrasonic, or radiography in

smaller sections.



Defects in Cast Products

include the following :

Porosity

Gas holes

Air locksShrinkage cavities

Hot tears

Cracks

Inclusions

Cold shuts



Defects in Cast Products

Defects in cast products include the following:

Porosity

Gas holes

Air locksShrinkage cavities

Hot tears

Cracks

Inclusions

Cold shuts



Porosity

Porosity is small smooth-faced cavities, generally smaller than 1.5 mm

diameter. Porosity is usually caused by the release of gas from the molten

metal as it cools. Gases such as hydrogen may be dissolved in the liquid

metal. As the metal cools, the dissolved gas separates out to form bubbles,

which are trapped in the solidifying metal.

Porosity

The preferred NDT method for detecting porosity is radiography. Ultrasonic

testing may also detect porosity.



Gas holes

The main distinction between gas holes and porosity is the size. Gas holes

are smooth-faced cavities greater than 1.5 mm diameter. Typical causes are:

Evolution of gas from molten metal during solidification.

Gas trapped as the molten metal enters the mould.

Reactions between the metal and the mould, also known as blowholes.

Again the best method to detect gas holes is radiography. Ultrasonic testing

can also be used. Blowholes are similar in origin to porosity in welds.

Dissolved gases precipitate from the liquid metal and leave rounded gas

filled cavities. Vacuum degassing of liquid metal before pouring has greatly

reduced the occurrence of blowholes.





Shrinkage cavities

Shrinkage cavities form during solidification

as a result of the reduction in volume when

metal changes from the liquid to the solid

state. Shrinkage cavities occur in situationswhere molten metal in not available to

compensate for the volume decrease

during solidification. Shrinkage flaws

typically occur where there is a localised

variation in section thickness but may occur

in parallel sections where penetration of the

liquid feed metal is difficult.

Shrinkage defects vary in form from open

cavities (piping) to branched interconnected

fine cavities. The defects tend to have a

rough surface profile.Formation of shrinkage cavities

Once again the best method to detect gas

holes is radiography. Ultrasonic testing

again can also be used.



Cracks

These are discontinuities due to the fracture of the metal during or after

solidification.

A particular type of cracking is „stress cracks‟ which are approximatelystraight and which form when the metal has become completely solid as

shown below. Stress cracks may be described in terms of the conditions

producing the cracks, for example, stress cracks due to contraction, residual

stress, shock or service.

Stress crack

The preferred NDT technique for ferromagnetic materials is magnetic

particle testing and for other metals liquid penetrant testing is used. Cracks

occur when the casting is of insufficient ductility, and consequently cracks

during solidification. These cracks are jagged type discontinuities resulting

from stresses imposed on the cast metal when it is just below the

solidification temperature and in a weak condition.



Hot tears

These are jagged crack type defects resulting from stresses imposed on the

cast metal when it is just below the solidification temperature and so is in aweak condition. The stresses usually arise when the casting is restrained

during contraction by the mould, or by an already solid thinner section. The

defect occurs mainly at or near a change of section and may or may not

extend to the surface.

Formation of hot tearsThe best NDT method for detecting hot tears, if they are at the surface, is

magnetic particle testing for ferromagnetic materials or liquid penetrants for

other metals. If the defects are sub-surface radiography or ultrasonic testing

should be used. Hot tears are similar to hot cracks in welding. As the liquid

metal solidifies, the remaining liquid surrounding the solid grains form a

crack propagation path under the contraction stresses of cooling.





Cold shuts

These are in effect a „lack of

fusion‟ defect caused by the

failure of a stream of molten

metal to form a continuousbond with a second stream, or

solid metal such as an internal

chill or splash. They are most

prevalent in thin-walled

castings.

Formation of cold sult in

casting

The preferred NDT method for

detecting cold shuts is

magnetic particle testing for

ferromagnetic metals and liquidpenetrant testing for other

metals.



Unfused chaplet/unfused chill

Chaplets and chills are metal inserts

placed in a mould for various casting

purposes. If the liquid metal fails to

fuse to these devices, a planardiscontinuity may result. The

presence of rust on the chaplet or

chill will generally give rise to

porosity around the chaplet or chill.



Welding Defects

Any of these defects are potentially disastrous as they can all give rise to

high stress intensities which may result in sudden unexpected failure below

the design load or in the case of cyclic loading, failure after fewer load

cycles than predicted. Welding defects include the following:

Porosity

Trapped slag

Lack of fusion

Lack of penetration or excess penetration

Undercut

Hot cracking

Hydrogen induced HAZ cracking

Lamellar tearing

Welding Defects

L ll t i



Lamellar tearing

This is mainly a problem with low quality steels. It occurs in plate that has a

low ductility in the through thickness direction, which is caused by non

metallic inclusions, such as suphides and oxides that have been elongated

during the rolling process. These inclusions mean that the plate can nottolerate the contraction stresses in the short transverse direction.

Lamellar tearing can occur in both fillet and butt welds, but the most

vulnerable joints are 'T' and corner joints, where the fusion boundary is

parallel to the rolling plane.

These problem can be overcome by using better quality steel, 'buttering' the

weld area with a ductile material and possibly by redesigning the joint.

F i



Fusion processes

The surfaces of two components to be joined are cleaned, placed close

together and heated while being protected from oxidation. A pool of molten

metal forms and connects the components, a filler rod may be used to add

metal to the joint.

A W ldi



Arc Welding

In this process an electrical machine

( which may be DC or AC ) supplies

current to an electrode holder which

carries an electrode which is coatedwith a mixture of chemicals or flux. An

earth cable connects the workpiece to

the welding machine to provide a

return path for the current. The weld is

initiated by tapping ( striking ) the tip

of the electrode against the workpiece

which initiates an electric arc. The

high temperature generated (about

6000oC) almost instantly produces a

molten pool and the end of the

electrode continuously melts into thispool and forms the joint. The operator

needs to control the gap between the

electrode tip and the workpiece while

moving the electrode along the joint.



In this process a filler metal is stored



In this process a filler metal is stored

on a spool and driven by rollers

(current is fed into the wire) through a

tube into a 'torch'. The large amount

of filler wire on the spool means that

the process can be considered to becontinuous and long, uninterrupted

welds can easily be made. An inert

gas is also fed along the tube and into

the torch and exits around the wire.

An arc is struck between the wire and

the workpiece and because of thehigh temperature of the arc a weld

pool forms almost instantly. In this

process they key issues are selecting

the correct gas mixture and flow rate

and the welding wire speed and

current. Once these have been set,

the skill level required is lower than

with the oxy acetylene process, and it

can readily be automated and MIG

welding is now commonly carried out

by robots. The MIG process is widelyused on steels and on aluminium.







Interpretation of weld radiographs



Interpretation of weld radiographsThe final stage in radiographic testing is the viewing, interpretation and

reporting the results of a radiographic inspection. After all, the real purpose of

a radiographic inspection is to provide information about the acceptability, or

otherwise, of the product being tested.

The viewer must include a uniformly illuminated diffusing screen



The viewer must include a uniformly illuminated diffusing screen

Procedures state that the examination of radiographs shall be carried out “by

diffused light in a darkened room”. Most illuminators also include a rheostat

that enables the brightness to be adjusted to accommodate radiographs of

varying densities. In addition, it must be possible to mask the viewer so that

bright, direct light is excluded from the eyes of the inspector.

A very important requirement is the brightness of the viewer Film viewers



A very important requirement is the brightness of the viewer Film viewers

should provide a source of defused, adjustable, and relativity cool light as

heat from viewers can cause distortion of the radiograph.

AS3998 requires the minimum intensity of light transmitted through a

radiograph being examined to be 30 candella per square meter (cd/m2). To

achieve this, the brightness of the viewing facility must be at least that shownin the following table: A film having a measured density of 2.0 will allow only

1.0 percent of the incident light to pass. A film containing a density of 4.0 will

allow only 0.01 percent of the incident light to pass. With such low levels of

light passing through the radiograph the delivery of a good light source is

important.

Minimum illuminator brightness required for radiograph density Density ofRadiograph Minimum Illuminator Brightness in (cd/m2)

1.5 1,000

2.0 3,000

2.5 10,000

3.0 30,000

3.5 100,000

It follows that the upper limit of film density is determined by the brightness of the



It follows that the upper limit of film density is determined by the brightness of the

available illuminator. The above values are the minimum brightness to view film,

based on 30 cd/m2 intensity of transmitted light. The standard suggests that 100

cd/m2 is a more reasonable value.

The brightness of an illuminator can be checked with a photographic light meter by

following these steps:Set the film speed indicator to 100 ASA or 200 ASA

Place the sensitive element of the meter close to the screen of the illuminator

Record the „exposure‟ in hundredths of a second against a camera aperture setting

of f10, f14.3 or f20

Use the table below to relate photographic exposure time to screen brightness

This illuminator must be used in a darkened room




There should be only sufficient background light to enable recording of details on the

viewing record. Too much background lighting may cause reflections off the film,

effectively reducing contrast and making interpretation more difficult.

Furthermore, the room used as a viewing room should be quiet and comfortable

to avoid unnecessary distractions.




There should be only sufficient background light to enable recording of details on the

viewing record. Too much background lighting may cause reflections off the film,







There should be only sufficient background light to enable recording of details on theviewing record. Too much background lighting may cause reflections off the film,







There should be only sufficient background light to enable recording of details on theviewing record. Too much background lighting may cause reflections off the film,




Radiographs are veiwed for short intervals



Radiographs are veiwed for short intervals

This practice is followed to prevent eye strain and maximise your concentration

level. Although each interpreter will differ, it is recommended that no more than

five minutes be spent viewing a radiograph.

Upon commencing a viewing session, the interpreter must allow sufficient time for

his or her eyes to become adjusted to the darkened conditions.

Radiographs should be dried before viewing



Radiographs should be dried before viewing

Wash water on a radiograph has a significant effect on sensitivity and increases the

difficulty of detecting fine discontinuities. Be sure to dry you radiographs before

viewing.

The radiographic process should be performed in accordance with a written



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procedure, code, or as required by contractual document. The required

documents should be available in the viewing area and referenced as necessary

when evaluating components. Radiographic film quality and acceptability, as

required by the procedure, should first be determined. It should be verified that

the radiograph was produced to the correct density on the required film type andthat it contains the correct identification information. It should also be verified that

the proper image quality indicator was used and that the required sensitivity level

was met. Next, the radiograph should be checked to make sure that it does not

contain artifacts that could mask discontinuities or other details of interest. The

technician should develop a standard process for evaluating the radiographs so

that details are not overlooked.

Single Wall Single Image



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With the single wall single image (SWSI) technique, radiation from the source

passes through the weld and is recorded on the film. This technique is invariably

applied for the radiography of plate butt welds and for the examination of pipe or

vessel butt welds where access to inner and outer surfaces is available.

Panoramic



The panoramic technique is a version of SWSI where the source of radiation is

positioned at the center of a cylindrical component such as a pipe or vessel with

the film wrapped around the outer surface of the weld. In this way the entire

length of weld can be examined with one exposure. A single piece of film or a

series of overlapping films may be used to cover the entire weld length.



DWDI



DWDI

Double Wall Single Image



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In the double wall single image (DWSI) technique radiation from the source

passes through both walls of the component, but only the image of the weld

region closest to the film is suitable for evaluation since the weld section nearest

to the source appears blurred and distorted in the image. On larger diameter

pipes, or if the source can be moved closer to the pipe surface the upper weldimage can be moved completely off the film leaving the area of interest clear for

evaluation, as seen in figure 3 frame 3. Imparting this complex information

without the use of animation would be both difficult and time consuming.

Alignment of Radiation



The detection of planar defects such as cracks is sensitive to the radiation beam

direction. This animation shows how alignment of the radiation beam changes

the appearance of the defect in the image. Figure 4 shows an inclined crack that

appears as a faint broad shadow in the radiograph. When the radiation is

directed parallel to the plane of the crack, its image becomes darker and moresharply defined. However, when the radiation is directed obliquely to the plane of

the crack the image becomes faint and eventually disappears as the angle of

incidence increases.

Requirements for Viewing There are several requirements which must be met when



carrying out the viewing and interpretation of weld radiographs. These are

described with the aid of photographs and audio commentary. A pre-requisite for

satisfactory interpretation is that the interpreter must have adequate eyesight,

whether corrected or uncorrected, and be able to recognise features in the image

caused by various conditions. The ability to recognise the features on aradiograph comes largely with experience. To assist in the interpretation of a

radiograph the interpreter should be aware of the radiographic technique used

and should have some knowledge of the weld configuration and welding

procedure used.

Viewing radiographs should be carried out using a film viewer in a darkened room.

Care must be taken to avoid marking or damaging the film.

Film Quality Section The interpretation process requires that film quality be of an



acceptable standard so that weld quality can properly be assessed. In order to

satisfy relevant codes and standards it is necessary that the stated requirements

for radiograph identification, density and image quality sensitivity be achieved.

This section discusses the monitoring of these parameters.



Film density



The density or blackness of a radiograph affects the contrast of the image

produced, contrast increasing with increasing density. For this reason minimum

density requirements are specified in codes and standards. The influence of

density on image quality is examined. The section includes an interactive task

where the student is asked to simulate the measurement of radiograph densityusing the mouse by pointing and clicking at selected points on the image. The

student is expected to evaluate the acceptability of the densities displayed

against prescribed criteria.

Radiographic sensitivity



This section examines how contrast and definition influence radiographic

sensitivity and how the quality of the image can be evaluated through the use of

image quality indicators. It highlights the importance of ensuring acceptable

image quality. Different types of image quality indicators are described and an

interactive presentation shows the effect of contrast and definition on thesensitivity of the radiographic image.



Film artefacts

Radiographs can sometimes be misinterpreted due to images appearing on the

radiograph that are not associated with the weld. These indications, referred to

as 'artefacts', can be due to handling damage or film processing faults. Thosedue to film damage may sometimes be identified by viewing under reflected light.

This section presents some of the more commonly encountered artefacts. Figure

6 shows a radiograph having an artefact caused by the presence of static

electricity.





Weld Surface Features

Common weld surface conditions that can appear in the radiograph are

described and shown as both photographic and radiographic images. When a

condition is selected from the weld surface features list, a detailed description ispresented together with a photograph or diagram and thumbnails of radiographic

examples (Figure 7). Clicking on a thumbnail image displays the full screen

radiograph including detailed information relating to the weld itself (Figure 8).



Weld Defects

This section shows a few of the many possible radiographic images produced by

internal weld defects. Examples are described using diagrams, photographs and

radiographic images. As in the weld surface features section, selection of an itemfrom the list displays a detailed description (Figure 9) and clicking on a thumbnail

(Figure 10) shows the full screen view of the radiograph (Figure 11)

Once a radiograph passes these initial checks it is ready for interpretation.

R di hi fil i t t ti i i d kill bi i i l it ith



Radiographic film interpretation is an acquired skill combining, visual acuity with

knowledge of materials, manufacturing processes, and their associated

discontinues. If the component is inspected while in service, an understanding of

applied loads and history of the component is helpful. A process for viewing

radiographs, left to right top to bottom etc., is helpful and will prevent thetechnician from overlooking any area on the radiograph. This process is often

developed over time and individualized to the technician. One part of the

interpretation process, sometimes overlooked, is rest. The mind as well as the

eyes need to rest when interpreting radiographs.

When viewing a particular region of interest, techniques such as using a small light

d i th di h th ll li ht h i th



source and moving the radiograph over the small light source, or changing the

intensity of the light source will help the radiographer identify relevant indications.

Magnifying tools should also be used when appropriate to help identify and

evaluate indications. Viewing the actual component being inspected is very often

helpful in developing an understanding of the details seen in a radiograph.Interpretation of radiographs is an acquired skill that is perfected over time. By using

the proper equipment and developing consistent evaluation processes, the

interpreter will increase his or her probability of defect detection.

Before beginning the evaluation of a radiograph, the viewing equipment and area

should be considered The area should be clean and free of distracting materials



should be considered. The area should be clean and free of distracting materials.

Magnifying aids, masking aids, and film markers should be close at hand. Thin

cotton gloves should be available and worn to prevent fingerprints on the

radiograph. Ambient light levels should be low. Ambient light levels of less than 2

fc are often recommended, but subdued lighting, rather than total darkness, ispreferable in the viewing room. The brightness of the surroundings should be

about the same as the area of interest in the radiograph. Room illumination must

be so arranged that there are no reflections from the surface of the film under

examination.

Check the quality of the radiograph

Before inspection proper can begin the radiograph is checked for processing and



Before inspection proper can begin, the radiograph is checked for processing and

handling artefacts and film density, and the IQI sensitivity is determined. The

person interpreting the radiograph must be sure that the quality of the radiograph

is adequate, and is in accordance with the requirements of the code or

specification, so that relevant discontinuities can be detected. The results ofthese preliminary checks and measurements should be recorded on the viewing

report.















Check the quality of the radiograph

Before inspection proper can begin, the radiograph is checked for processing and

handling artefacts and film density, and the IQI sensitivity is determined. The

person interpreting the radiograph must be sure that the quality of the radiograph

is adequate, and is in accordance with the requirements of the code or

specification, so that relevant discontinuities can be detected. The results of

these preliminary checks and measurements should be recorded on the viewing

report.

Weld discontinuities are designated by standard abbreviations

There is a standard set of abbreviations used to describe most weld discontinuities



There is a standard set of abbreviations used to describe most weld discontinuities.

These abbreviations are listed in AS4749-2001, “Non-Destructive Testing –

Terminology of and Abbreviations for Fusion Weld Imperfections as Revealed by

Radiography”. Description of each discontinuity are provided, plus prints taken

from an actual radiograph or a sketch to describe discontinuity. You are stronglyadvised to obtain a copy of this standard from Standards Australia if you are at all

involved with weld radiography.

Weld imperfections are either surface or internal

There are two classes of weld discontinuities:

surface imperfections

internal imperfections.

All radiographs should be interpreted to determine their compliance with a

code or standard



code or standard

A typical standard is Australian Standard AS4037 which includes acceptance levels

for various weld imperfections in pressure vessels. It states:

No planar imperfections (e.g. crack or lack fusion defects) are allowed.

In main butt welds (class 1 vessels), slag inclusions can have:a maximum length of 6 mm for thicknesses of up to 18 mm

a maximum length of T/3 for thicknesses between 18 mm and 60 mm

a maximum length of 20 mm for thicknesses greater than 60 mm.

Some standards include porosity charts which are typically illustrations to provide a

visual comparison to help determine the acceptablility of porosity discontinuities.

Porosity imperfections may be classified as:isolated pores (maximum diameter 0.3T but not greater than 6 mm)

uniform porosity

clustered porosity

linear porosity.



Burn through (BT)

A localised collapse of the weld pool leaving a



A localised collapse of the weld pool leaving a

hole in the bottom of the weld run. Appears as an

irregularly shaped globular dark area

Localised porosity (PG)

A group of gas pores confined to a small area of a




weld. Appears as a cluster of small round

indications. These discontinuities are sometimes

elongated, where they are referred to as “worm

holes”.





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weld. Appears as a cluster of small round

indications. These discontinuities are sometimes

elongated, where they are referred to as “worm

holes”.




A group of gas pores confined to a small area of a weld. Appears as a cluster of small

round indications. These discontinuities are sometimes elongated, where they are

referred to as “worm holes”.




A group of gas pores confined to a small area of a weld. Appears as a cluster of small

round indications. These discontinuities are sometimes elongated, where they are

referred to as “worm holes”.



Excess penetration

Weld metal protruding through the root of the



Weld metal protruding through the root of the

weld. Excess penetration arises from to high

a heat input and / or too slow transverse of

the welding torch (gas or electric). Excess

penetration - burning through - is more of a

problem with thin sheet as a higher level of

skill is needed to balance heat input and

torch traverse when welding thin metal.

Appears as a light continuous or more often

intermittent, irregularly shaped band withinthe image of the weld.

Excess penetration

Weld metal protruding through the



Weld metal protruding through the

root of the weld. Excess

penetration arises from to high a

heat input and / or too slow

transverse of the welding torch(gas or electric). Excess

penetration - burning through - is

more of a problem with thin sheet

as a higher level of skill is needed

to balance heat input and torchtraverse when welding thin metal.

Appears as a light continuous or

more often intermittent, irregularly

shaped band within the image of

the weld.

UnderfillA continuous or intermittent channel at the top



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surface of the weld and running along the

length of the weld. It may be at the centre of

the weld, where it is sometimes known as

external concavity or insufficient fill, or may beat the edges of the weld where it is known as

incompletely filled groove.

UnderfillA continuous or intermittent



channel at the top surface of the

weld and running along the length

of the weld. It may be at the centre

of the weld, where it is sometimesknown as external concavity or

insufficient fill, or may be at the

edges of the weld where it is

known as incompletely filled

groove.





weld and running along the length

of the weld. It may be at the centre

of the weld, where it is sometimesknown as external concavity or

insufficient fill, or may be at the

edges of the weld where it is known

as incompletely filled groove.





weld and running along the

length of the weld. It may be at

the centre of the weld, where it issometimes known as external

concavity or insufficient fill, or

may be at the edges of the weld

where it is known as incompletely

filled groove.

UndercutAn irregular groove at the top edge (toe) of a



weld caused by contraction of the weld

metal, or by burning away (gouging) of the

parent metal. Appears as a dark irregular

band along the top edge of the weld metal.Undercut can also occur at the root of the

weld, although this can easily be confused

with lack of root fusion.

In this case the thickness of one (or both)

of the sheets is reduced at the toe of the

weld. This is due to incorrect settings /

procedure. There is already a stress

concentration at the toe of the weld and

any undercut will reduce the strength of the

join.



Incomplete root penetration (LP)

Failure of the weld metal to extend

i t th t f j i t A



into the root area of a joint. Appears

as a dark continuous or intermittent

band with mostly straight edges. In

close square butt joints it may appearas a continuous or broken line. There

is often a line of fine porosity

associated with this defect.



Incomplete root penetration

(LP)

F il f th ld t l t



Failure of the weld metal to

extend into the root area of a joint.

Appears as a dark continuous or

intermittent band with mostlystraight edges. In close square

butt joints it may appear as a

continuous or broken line. There

is often a line of fine porosity

associated with this defect.



Root concavitySometimes called suck-back. A

h ll i th t f b tt



shallow groove in the root of a butt

weld. Appears as a dark area along the

centre of the weld.

Welding Defects



Lack of inter-run fusion (LI)

A lack of union between adjacent weld runs in a

multi run weld It appears as a faint dark line with



multi-run weld. It appears as a faint dark line with

sharply defined edges.

Slag Inclusions

Slag or other foreign matter trapped

b t ld b t th ld



between weld runs or between the weld

and the parent metal These can occur

when several runs are made along a V join

when joining thick plate using flux cored orflux coated rods and the slag covering a run

is not totally removed after every run before

the following run. Appears as mostly

irregular shapes.

Slag Inclusions

Slag or other foreign matter

t d b t ld



trapped between weld runs or

between the weld and the parent

metal These can occur when

several runs are made along a V join when joining thick plate

using flux cored or flux coated

rods and the slag covering a run

is not totally removed after every

run before the following run. Appears as mostly irregular

shapes.

Slag Inclusions

Slag or other foreign matter trapped

b t ld b t th ld



between weld runs or between the weld

and the parent metal These can occur

when several runs are made along a V join

when joining thick plate using flux cored orflux coated rods and the slag covering a run

is not totally removed after every run before

the following run. Appears as mostly

irregular shapes.

Inclusion (IN)


trapped between weld runds or



trapped between weld runds or


metal. Appears as mostly irregular

shapes.

Inclusion (IN)







metal. Appears as mostly irregular

shapes.



Slag Inclusion

pieces of slag on the surface of a finished weld. These slags may get

entrapped during welding.

Linear inclusion

Also known as a slag line. Caused by lines

of slag trapped generally between the weld



of slag trapped, generally between the weld

metal and parent metal, in a multi-run weld.

Appears as one or more dark bands, mostly

with irregular edges, running along a weld.

Lack of side wall fusion

A lack of union between the weld metal and

the parent metal at the side of a weld Its



the parent metal at the side of a weld. Its

image appears as a straight dark line or

band, depending on the orientation of the

beam of radiation. Its detection depends onits orientation relative to the beam

orientation, and sometimes requires an

additional exposure with the beam aligned

parallel to the weld preparation face.



Lack of fusion



Lack of fusion





Lack of fusion



Lack of fusion

Lack of root fusion

A lack of union of the weld metal with the

parent metal at the root of a weld Lack of



parent metal at the root of a weld. Lack of

fusion results from too little heat input and /

or too rapid traverse of the welding torch (gas

or electric). Appears as a straight line or bandat one or both edges of the weld root image.

Cracking

This can occur due just to thermal shrinkage or due to a combination of

strain accompanying phase change and thermal shrinkage



strain accompanying phase change and thermal shrinkage.

In the case of welded stiff frames, a combination of poor design and

inappropriate procedure may result in high residual stresses and cracking.

Where alloy steels or steels with a carbon content greater than about 0.2%are being welded, self cooling may be rapid enough to cause some (brittle)

martensite to form. This will easily develop cracks.

To prevent these problems a process of pre-heating in stages may be

needed and after welding a slow controlled post cooling in stages will be

required. This can greatly increase the cost of welded joins, but for highstrength steels, such as those used in petrochemical plant and piping, there

may well be no alternative.

Longitudinal Cracks Cracks appear a fine dark lines, mostly jagged

edges, sometimes discontinuous. Its detection



edges, sometimes discontinuous. Its detection

is dependent on its orientation relative to the

radiation beam.

Longitudinal Cracks Cracks appear a fine dark lines,

mostly jagged edges, sometimes



y j gg g ,

discontinuous. Its detection is

dependent on its orientation relative

to the radiation beam.





y j gg g ,


dependent on its orientation

relative to the radiation beam.





y j gg g ,


dependent on its orientation relative

to the radiation beam.





y j gg g ,








y j gg g





mostly jagged edges,



sometimes discontinuous. Its

detection is dependent on its

orientation relative to theradiation beam.

Longitudinal Cracks Cracks appear a fine dark

lines, mostly jagged edges,



sometimes discontinuous. Its

detection is dependent on its

orientation relative to theradiation beam.

Longitudinal root crack

This form of crack occurs mostly in the

parent metal adjacent to the root run of the



p j

weld. It appears as a fine dark line, mostly

jagged edges, sometimes discontinuous. Its

detection is dependent on its orientationrelative to the radiation beam.

Linear misalignment

may have a linear indication associated

with it caused by the protruding edge of



y p g g

one of the plates. This has the

appearance of a lack of penetration

indication.

Linear misalignment ( Hi – Low )

A planar misalignment of the two sides

being welded. May appear as light and



g y pp g

dark sides.

Linear misalignment ( Hi – Low)

A planar misalignment of the two



p g

sides being welded. May appear

as light and dark sides.





p g











Linear porosity A line of mostly small round images aligned

along a weld. Note that this can sometimes



indicate a lack of fusion defect which may

not be immediately obvious.

PorosityThis occurs when gases are trapped in the

solidifying weld metal. These may arise



from damp consumables or metal or, from

dirt, particularly oil or grease, on the metal

in the vicinity of the weld. This can beavoided by ensuring all consumables are

stored in dry conditions and work is

carefully cleaned and degreased prior to

welding. Porosities are mostly spherical gas

hole in the weld metal. Appears as one ormore circular dark images.

Transverse Crack A transverse crack runs across the weld

bead and sometimes into the parent metal.



It appears as a fine dark line, mostly jagged

edges, sometimes discontinuous. Its

detection is dependent on its orientationrelative to the radiation beam.

Transverse Crack A transverse crack runs across

the weld bead and sometimes



into the parent metal. It appears

as a fine dark line, mostly

jagged edges, sometimesdiscontinuous. Its detection is



Tungsten inclusion An inclusion of tungsten from a tungsten

electrode used in the gas tungsten arc



(GTAW) process. Appears as small white

sharp edged images in the weld metal due to

the fact that tungsten is much denser thansteel or aluminium.





(GTAW) process. Appears as small

white sharp edged images in the weld

metal due to the fact that tungsten ismuch denser than steel or aluminium.



(GTAW) A ll hi



(GTAW) process. Appears as small white

sharp edged images in the weld metal

due to the fact that tungsten is muchdenser than steel or aluminium.

An inclusion of aluminiumoxide in a arc welding

process. Appears as small

hit i l i i th



white irregular images in the

weld metal due to the fact

that oxide is much denserthan steel or aluminium.





Welding Defects







Welding spatters



Welding spatters



Film radiography



Real time radiography

vii





PorosityThis occurs as a series of fine cavities, generally spherical, but sometimes

tubular in form (worm holes). Porosity can occur in various patterns, for

example linear porosity scattered porosity and start porosity The defect is



example, linear porosity, scattered porosity and start porosity. The defect is

caused by such factors as:

Excessive gas content generated by chemical reactions in the weld.Gases or other hydrocarbon contamination.

Damp flux.

Porosity in weld

The preferred NDT techniques are radiography, ultrasonic testing and, if the

porosity is at the surface, liquid penetrants.

Trapped slag A number of welding processes deliberately form a flux or slag covering

over the molten weld pool as it solidifies. This isolates the weld metal from

the atmosphere and helps purify the weld metal Some of this slag can be



the atmosphere and helps purify the weld metal. Some of this slag can be

trapped in the weld metal due to insufficient slag removal between runs or

insufficient back gouging of the root. Depending on the circumstances offormation the slag is generally in an isolated or linear pattern. Slag can be

classed as a „volume‟ defect.

Slag entrapment in weld

Preferred NDT technique for detecting trapped slag is radiography or

ultrasonic testing.

Lack of fusionThis refers to incomplete fusion between the weld metal and the parent

metal or weld metal with previously deposited weld metal. Three distinct

types of fusion defect occur depending on the location of the defect within



types of fusion defect occur depending on the location of the defect within

the weld zone:

Lack of side wall fusion.Lack of inter-run fusion, that is, between weld runs.

Lack of root fusion.

Lack of fusion defects

Causes include such factors as:

Poor welding technique.Incorrect electrode size.

Inadequate weld preparation.

Lack of fusion defects are generally planar and crack-like in nature. The

best NDT method is ultrasonic testing. Radiography may be used for lack of

side wall and root fusion.

Lack of penetrationThis is where the weld metal has failed to penetrate into the root of a joint as

opposed to lack of root fusion where weld metal has penetrated into the root

area but has failed to fuse to one side



area but has failed to fuse to one side.

The causes of lack of penetration are the same as for lack of fusion defects.

Lack of penetrationThe preferred NDT technique for detecting lack of penetration is

radiography or ultrasonic testing.

Hot crackingThis is also called solidification cracking because it occurs when the weld

metal has just solidified and so is in a weakened condition. Most weld metal

cracks are of this type for example centreline cracking as shown below



cracks are of this type, for example, centreline cracking, as shown below,

and crater cracks.

Centreline crackingHot cracks result from the combined action of stress and lack of ductility of

the weld metal at high temperatures. Contributing factors are:

restraint

weld chemistry (for example, high sulphur content)

weld shape, (for example, concave fillet welds).Preferred NDT techniques for detecting hot cracking is magnetic particle

testing or liquid penetrant testing.

Heat affected zone (HAZ) cracksThese are also called underbead cracks or toe cracks.

The heat affect zone, HAZ, of a weld is that part of the parent metal

adjacent to the weld fusion line where the metal has been heated to a



adjacent to the weld fusion line where the metal has been heated to a

sufficiently high temperature by the weld to alter its grain structure.

Underbead cracks occur in the weld HAZ and lie parallel to the fusion linewhile toe cracks commence at the weld toe and angle across the HAZ as

shown below. HAZ cracks form at temperatures around room temperature

and may form shortly after welding or take hours or even days to form.

Forms of HAZ cracking in welds

The cracks occur under the combined action of:Hydrogen in the HAZ – hydrogen can originate, for example, from using

damp electrodes.

Weld restraint – that is, stress.

A hard HAZ – this relates to parent metal chemistry and cooling rate after

welding.

The tendency to cracking is influenced by:

The type of steel used (it is favoured by higher carbon and alloy steels).

Material thickness.

Type of joints.

Type of welding process.

The best NDT techni ue for underbead cracks is ultrasonic testin while



Other Types of DefectsSome other types of defects include:

Quench cracks

Grinding cracks



Fatigue cracks

Stress corrosion cracks

Quench cracksThese cracks develop in the later stages of the quenching operation of a „quench

and temper‟ heat treatment on steel. Quenching involves heating the steel to about

850°C and cooling rapidly in a water or oil bath. Quench cracks result where the

residual surface stresses produced in the quenching exceed the tensile strength of

the steel.

Quench cracks characteristically run from the surface in a straight line towards the

centre as shown below. They tend to occur at points of stress concentration such as

section changes, sharp corners, etc.

Contributory factors to formation include:

Too severe a quenching medium for the steel-section combination.

Quenching steel out cold.

Delay between quenching and tempering.

Quench cracks in steel bar

The preferred NDT technique for detecting quench cracks is magnetic particle

testing.

Other Types of DefectsSome other types of defects include:

Quench cracks

Grinding cracks



Fatigue cracks


Quench cracksThese cracks develop in the later stages of the quenching operation of a „quench

and temper‟ heat treatment on steel. Quenching involves heating the steel to about

850°C and cooling rapidly in a water or oil bath. Quench cracks result where the

residual surface stresses produced in the quenching exceed the tensile strength of

the steel.

Quench cracks characteristically run from the surface in a straight line towards the

centre as shown below. They tend to occur at points of stress concentration such as

section changes, sharp corners, etc.

Contributory factors to formation include:

Too severe a quenching medium for the steel-section combination.

Quenching steel out cold.

Delay between quenching and tempering.

Quench cracks in steel bar

The preferred NDT technique for detecting quench cracks is magnetic particle

testing.



Fatigue cracksFatigue cracks represent a major area of application in maintenance NDT.

Fatigue occurs under the repeated application of a stress which is insufficient to

cause failure when applied statically. It accounts for 80% to 90% of the fractures of

h i l t



mechanical components.

The fatigue process involves a slowly progressing crack over an extended time

period and fatigue cracks almost always start at the surface. This makes themideally suited to detection by NDT techniques. The great bulk of fatigue cracks start

at points of stress concentration such as sharp corners, thread roots, keyways, oil

holes and so on.

The preferred NDT techniques are magnetic particle testing for steel components

and liquid penetrant testing for non-ferrous metals. Ultrasonic testing may be used

for in situ inspection of assemblies.

Stress corrosion crackingLike fatigue cracking, stress corrosion cracking is a

service-generated defect.

Stress corrosion is the corrosion of a metal

l t d b t Th d t i ti d th



accelerated by stress. The deterioration under these

conditions is much more harmful than the separate

effects of stress and corrosion. For a given metal,stress corrosion only occurs in certain environments

peculiar to that metal.

Some classic combinations are:

Brass in mercury or ammonia compounds – „season

cracking‟.

Steel in sodium hydroxide – „caustic embrittlement‟.

Austenitic stainless steel in chlorides.

Stress corrosion cracking commonly takes the form of

a multitude of branched inter-granular cracks with little

or no corrosive attack to the surface as shown below. It

is not normally visually detectable.


The best NDT technique for detecting stress corrosion

cracks in ferromagnetic metals is magnetic particle

testing. For other metals liquid penetrant testing is

used.



Stress corrosion cracking





Corrosion pitting and wall thinning



Cracks



AdvantagesSimple and easy to conduct

Will detect surface and near surface flaws

Can detect flaws filled with contaminants e.g. oxide or non metallic inclusions

Sensitivity of testing can be specified and checked



Sensitivity of testing can be specified and checked

Disadvantages

Can only be applied to ferromagnetic materialsWill not detect deep internal flaws

High currents applied to component may cause damage

Components usually have to be demagnetised

Lack of PenetrationLack of penetration results

from the failure of the weld

metal to fully penetrate the root

section resulting in a surface



section, resulting in a surface

connected area of incompletely

welded material. Generallypresents as a strong corner

reflector detectable from both

sides of the weld.

Lack of Root FusionLack of root fusion results from the failure of the weld metal to fully fuse with the root

area of the weld. Lack of root fusion may occur in areas of full penetration. Generally

presents as a strong corner reflector detectable from only one side of the weld.

http://onlineshowcase.tafensw.edu.au/ndt/content/ultrasonic/glossary/glossary.htm






Root undercutSlight melting of the parent metal at the toe of a weld resulting in an irregular shallow

groove at the edge of the weld. Generally presents with variable amplitude as a

slightly ragged corner reflector at the edge of the weld.

Root undercut



Root undercut

Roll your mouse over the red numbered bullet to read the label.

Scanning the root area should be carried out with a line scan using a steep probeangle.

Orbital scanThe probe is positioned for maximum reflection from the discontinuity, and the

screen height adjusted to around 80% FSH. The probe is then moved in an orbital

movement around the discontinuity, trying to keep the discontinuity at the centre of

the orbit This type of scan may be impractical if the surface contour does not permit





the orbit. This type of scan may be impractical if the surface contour does not permit

the free movement of the probe.

Shrinkcastings



Sandinclu

castings





The end

thank you







Double sided (including tee butt) weld testing, where the welding and inspection canbe undertaken from both sides. This is generally the simpler case, and is the subject

of this section.

Single sided weld testing, where the welding and inspection access is from one side

only. This is more complex than the double sided weld testing, and will be discussed



y p g,

in the next task.

Double sided (including tee butt) weld testing, where the welding and inspection canbe undertaken from both sides. This is generally the simpler case, and is the subject

of this section.

Single sided weld testing, where the welding and inspection access is from one side

only. This is more complex than the double sided weld testing, and will be discussed



y p g,

in the next task.

CracksCracks are discontinuities caused by fracture under stress. They can occur at high

or low temperatures

Cold Cracks: occur when due to embrittlement the solid weld metal cannot withstand

the contraction stresses. Embrittlement is often due to retained hydrogen and



y g

excessive hardness caused by rapid cooling. Cold cracks may be delayed in their

formation.Underbead Cracks, Toe Cracks: are a variation of cold cracks, which occur in the

HAZ due to the presence of hydrogen and excessive hardness from rapid cooling of

higher carbon and alloy steels.

Hot Cracks: occur as the metal cools from liquid to solid and cannot withstand the

solidification stresses. Hot cracking is more likely to occur when the weld deposit is

deep and narrow, such as the weld root area. The most common type is thecentreline crack. It rarely occurs in flat capping passes

Lamellar Tears: are cracking in the HAZ of welds where nonmetallic inclusions are

oriented across the shrinkage stresses, just beneath the surface – especially in

heavy section tee butt welds. This type of cracking was common, but is now less

common due to improved steelmaking practice.

Chevron Cracks: are small transverse cracks that can occur in submerged arc

welding of heavy sections, 50 mm and thicker.

Entrapped gases (porosity, wormholes)Entrapped gases are a result of gases held in the liquid weld metal. The gases

separate (precipitate) when the metal cools and solidifies to form gas filled cavities.

Porosity can occur as clusters of porosity (more common with improperly used low

hydrogen electrodes), or as single pores. Porosity of this type is due to shortcomings









y g ) g p y yp g

in the shielding process.

Wormholes occur when the weld metal interacts with surface contaminant such asoils, or a cavity (such as a lamination or partial tack weld) containing gas, and the

resulting pocket of gas expands under the heat of welding, to be entrapped in the

weld metal. These wormholes generally have a tadpole-like shape with the tail of the

tadpole pointing to the likely source of the gas.





Inclusions (slag inclusions, wagon tracks)Inclusions are the result of incomplete cleaning of one pass before the following

pass is made.

It is sometimes difficult to distinguish between inclusions and lack of fusion.

Inclusions are generally regarded as „Volumetric‟ (three-dimensional) while lack of



fusion is regarded as „planar‟ (two-dimensional). The two discontinuity types may

occur together. Penetration discontinuities

Penetration discontinuities occur where the weld metal does not penetrate its

intended extent. Lack of penetration generally appears as a vertical planar

discontinuity at the weld root.

Shape and profile

The weld is required to have an acceptable shape both for its intended service andto make it testable. It is not generally the role of the ultrasonic technician to make

judgment on the shape of the weld, but if the weld has poor shape, it can obstruct

ultrasonic interpretation.

Undercut is a sharp groove that sometimes occurs at the weld toe (the edge of the

weld cap).

Shrinkage cavities (shrinkage grooves)

Shrinkage grooves occur when the weld deposit solidifies and there is not enough

molten metal to compensate for the volume reduction. Shrinkage grooves are rare in

double V welds, but may occur more frequently in single V welds.

Some basic guidelines for detection of discontinuities:Try and strike all discontinuities as close to square as possible.

Study the weld preparation for orientation of the fusion face

Try and select an angle of incidence within 10° of the fusion faces.







This may mean using a number of probe angles and scanning surfaces for

effective scanningIf this is impossible, some extra gain will assist. We will see how codes and

standards deal with this problem.

Use lower frequencies for initial detection.

Although higher frequencies will be better for measurement and evaluation, you

may miss discontinuities. You will never get the chance to evaluate if you donot find them first!

Strange as it may sound, for planar reflectors at unfavourable incidence, the

larger the reflector, the lower the probability of detection.

Use the largest practical transducer diameter, as this will give greater effective

coverage per scan.

The Ultrasonic Flaw Detector (UFD) provides an electrical signal to the probe.The probe converts the electrical signal to a pulse of mechanical vibration.

The couplant allows transmission of the mechanical vibration into the test piece.

The compression wave travels across the sample.

The backwall reflects the compression wave back to the probe.














The discontinuity (if present) reflects the compression wave back to the probe.

The amplitude of the discontinuity signal reaches its maximum when thediscontinuity area is equal to the beam width.

The probe reconverts the received compression wave back to an electrical signal.

The UFD display shows the transit time (X axis) versus signal amplitude (Y axis) in

real time.

The horizontal location of the reflected signal on the UFD screen is proportional to

the time for the ultrasound to travel from the probe to the discontinuity and back.The amplitude of the discontinuity signal is proportional to the area of the reflecting

surface - if the discontinuity is smaller than the beam, and reflection conditions

are ideal.

The UFD contains the essential elements to control and interpret the test that

include:

a timer to control the pulse ratea pulse generator to energise the probe

a sweep generator to drive the display

an amplifier for the weak signals returned by the probe

a screen (digital or analogue) to display the signals from the probe

electrical connections to the probe.





















Lamb waves are a mode of propagation produced in thin materialsLamb waves may be produced in thicknesses below three times the wavelength,

where shear waves cannot exist. There are a number of variants of Lamb waves,

and their application is limited to thin materials. Lamb waves are also known as

plate waves.







Corner ReflectorsIn optics, corners act as almost perfect reflectors of light.

Corner reflector

This property is very useful in manufacturing reflectors for the rear of cars and „cats

eyes‟ for road markers. If you have ever seen two mirrors set at right angles, you



will notice that no matter where you are, you receive a perfect reflection of your

own image as the incident beam is reflected back along its own path. Two otherthings are also evident when you look at your image in a corner mirror:

With one mirror, the image is laterally inverted, and your left hand looks like your

right hand, but with a corner mirror the image is right way round.

With a corner mirror, the reflection is often darker because it has undergone two

reflections, and lost more light.

Corner reflection relies on reflection at two complimentary anglesUnfortunately with ultrasonics, the reflection from a corner is not quite so simple,

because every corner reflection involves two reflections at complimentary angles.

(Complimentary angles are two angles that add up to 90°.)

If, for example we strike a corner at 60°, it will require a reflection at 60° and 30° to

produce the return reflection. You will remember from our discussion of oblique

incidence and the demonstration we looked at to calculate the angles and soundpressure, that the 60° reflection will be 100% with no mode conversion, but the

30° reflection will only be a 13% reflection of the shear mode. It will also produce

a significant compression mode at an angle of 67°. For these reasons, corners

can be very deceptive reflectors – very easy to manage in some situations and

very difficult at other times







































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Defects Welding