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Nondestructiveevaluation (NDE)

Topic 7

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Nondestructive testing(NDT)

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Conventional NDE mthods Liquid penetrant inspection Ultrasonic inspection Acoustic emission Magnetic particle inspection Eddy current testing X-radiography

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Liquid penetrant inspection

For detection of surface defects Inexpensive and convenient Largely used on nonmagnetic

materials for which magneticparticle inspection is not possible.

Unable to inspect subsurfaceflaws. Loss of resolution on porous

materials.

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PenetrantHigh-visibility liquid

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Liquid penetrant inspectionmethod

1. Apply a penetrant to the surface.

2. Pull penetrant to the surfacecrack by capillary action.

3. Remove excessive penetrant.4. Extract some penetrant to thesurface.

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Ultrasonic inspection

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Audible frequency range20 20,000 Hz

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Ultrasonic inspection Ultrasonic wave has higher frequency

than audible sound. Typical frequency from 25 to 100,000

MHz. Method: Send an ultrasonic wave

(from a piezoelectric transducer)through the material (via a transmitting

medium) and measure the intensity ofthe reflected or transmitted wave, andthe time it takes for the wave to bedetected.

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Ultrasonic inspection

Ultrasonic wave has higherfrequency than audible sound. Typical frequency from 25 to

100,000 MHz. Method: Send an ultrasonic wave

through the material and measurethe intensity of the reflected ortransmitted wave, and the time ittakes for the wave to be detected.

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Through-transmission configuration(two transducers)

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14/52Pulse-echo mode (2 transducers )

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15/52Single transducer

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Single transducer connected to the materialb water an acoustic cou lin medium

Pulse-echomode

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Attenuation of ultrasonic wave upon traveling through thematerial. One cycle means traveling from the front surface tothe back surface and then to the front surface.

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Acoustic emission (AE)

testing Process of developing defectssuch as cracks causes the

emission of ultrasonic waves. AE measures the ultrasonicwaves produced by defects in a

material in response to anapplied stress. Transducer serves as receiver,

not ultrasonic emitter.

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A fiber composite experiencing delamination during loadingand friction between delaminated surfaces during unloading.

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AE applications Failure prevention

(warning of impendingfailure; rate of AE eventsrising sharply just prior to

failure) Locating defects

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Magnetic particle

inspection

Limited to magneticmaterials.

Inexpensive and convenient.

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Magnetic particle inspection method Magnetic flux lines in a ferromagnetic or

ferrimagnetic material (resulting from theapplication of a magnetic field) are distortedaround a defect.

Distortion causes magnetic flux lines to protrude

from the surface at the location of the surfacecrack. This is known as field leakage. Field leakage attracts magnetic particles (Fe or

Fe 3O 4) that are applied to the surface. Subsurface cracks near the surface can also be

detected. Applied magnetic field is preferably perpendicular

to the length of the defect.

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Distortion of the magnetic flux lines due toa surface crack in a magnetic material

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Distortion of the magnetic flux linesdue to a subsurface defect

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Little distortion of the magnetic flux lineswhen the length of the defect is

parallel to the applied magnetic field

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A time-varying magnetic fieldinduces a current in a copper ring.

Faradays Law

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Eddy currentAn eddy current is an electric currentinduced in an electrically conductivematerial due to an applied time-varying

magnetic field. Due to Faradays law, a voltage is generated in a conductor loopwhen the magnetic flux through the loop

is changed. The eddy current is in adirection such that the magnetic field itgenerates opposes the applied magnetic field.

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An axial magnetic fieldgenerated by

a circumferential electric current

A circumferential magnetic fieldgenerated by an axial electric

current

G i f dd b li d

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A flat sample

A cylindricalsample

Generation of an eddy current by an appliedmagnetic field

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Distortion of eddy current paths around a defect

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Eddy current testing method

The impedance of an inspection coil isaffected by the presence of an adjacent,electrically conductive test piece, in

which eddy current has been induced bythe coil.

By varying the frequency, the methodcan be used for both surface and

subsurface flaws.

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Eddy current testinglimitations

Limited to electrically

conductive materials. Qualitative

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X-radiography Good for detecting internal

defects. Method: Send x-rays through

the material and detect the

transmitted x-ray image using aphotographic film.

X b i

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X-ray absorption(not diffraction)

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Fractography Fracture surface

examination Different from

metallography

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Introduction

Broken cabin bolt

from an elevator

A il k h f d i b i l b

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IntroductionAn oil tanker that fractured in a brittle manner bycrack propagation around its girth

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Highly ductilefracture

Moderately ductilefracture

Brittle

fracture

Irreg lar & fibro s

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Ductile FractureIrregular & fibrousappearance

Ductile

fracture

Cup-and-cone fracture in aluminum

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Stages in cup-and-cone fracture

Initial necking Small cavityformation

Coalescence of cavitiesto form a crack

Crackpropagation Final shear

failure at a 45degree angle

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Ductile Fracturemoderately ductile: most common,moderate necking before fracture.Stages: i) microvoid formation, ii)

microvoid coalescence leading tomicrocrack formation normal tothe applied stress, iii) rapid crack

propagation at about 45 to tensileaxis ( max) ).cup and cone fracture.

B i l f i ild l

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Brittle FractureBrittle fracture in mild steel

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Brittle fracture

In hard & fine-grained metals,there will be no discernible pattern.In amorphous materials, thefracture surface is relatively shiny& smooth.

In most brittle crystalline materialscrack propagates along specificcrystallographic planes

cleavage.

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Brittle Fracture

Scanning electron micrograph ofductile cast ironshowing a transgranular fracture surface

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Brittle Fracture

Scanning electron micrograph showingan intergranular fracture surface

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Brittle fracture

The cleavage fracture is transgranular(transcrystalline)Macroscopically the fracture surfacehas a grainy or faceted texture due tochanges in orientration of cleavageplains from grain to grain.Intergranular fracture crackpropagates along grain boundaries.This shows evidence of a weakermaterial, embrittlement.

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Brittle fractureFracture with no obviousdeformation, fast crack growthnormal to the applied .

Relative flat fracture surface.

Some steels may show V-shapedchevron markings that pointback to crack initiation.

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Brittle Fracture

Photograph showing V- shaped chevron markings

characteristic of brittle fracture

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Brittle Fracture

Fractured pressure vessel shell showing chevronmarks. Note that marks point to the right.

Crack

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Initiation

Photograph of a brittle fracture surface