Basic Aspects

7/31/2019 Basic Aspects

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BASIC ASPECTS

An initial step is to define what we call a laminate mesomodel. At the

mesoscale, characterized by the thickness of the ply, the laminate structure is

described as a stacking sequence of homogeneous layers through the

thickness and of interlaminar interfaces shown in the Figure-----1 below . The

main damage mechanisms are described as fiber breaking, matrix

microcracking, and debonding of adjacent layers as shown in Figure -----2).

The single-layer model includes both damage and inelasticity. The

interlaminar interface is defined as a two-dimensional mechanical model

which ensures traction and displacement transfer from one ply to the next. Its

mechanical behavior depends on the angle between the fibers of two adjacent

layers.

The damage mechanisms are taken into account by means of internaldamage variables. A mesomodel is then defined by adding another property: a

uniform damage state is prescribed throughout the thickness of the

elementary ply. This point plays a major role when trying to simulate a crack

with a damage model. As a complement, delayed damage models are

introduced.

One limitation of the proposed mesomodel is that material fracture is

described by means of only two types of macrocracks:

The layers - - in our sense m are assumed to be not too thick. Anotherlimitation is that very severe dynamic loadings cannot be studied; the

dynamic wavelength must be larger than the thickness of the plies.

Two models have to be identified: the single-layer model [16] and the

interface model [1,6]. The appropriate tests used consist of tension, bending,

and delamination. Each composite specimen, which contains several layers

and interfaces, is analyzed in order to derive the material quantities intrinsic

to the single layer or to the interlaminar interface.

Various comparisons with experimental results have been performed to

show the possibilities and limits of our proposed computational damage

mechanics approach for laminates [2, 8, 19, 20].The single-layer model is presented here. A similar model is used for the


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interface.

FIGURE------1

FIGURE-----2


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1.1 Background

In order to improve the quality of mass produced composites and in

order to undertake quality assessment of adhesive interfaces in these

materials it is first necessary to develop the theoretical basis describing both

qualitatively and quantitatively, the quality parameters of the composite, and

secondly to develop new non-destructive techniques for their testing and

evaluation.

Mechanical integrity of interfaces in composites plays a major role

in determining the serviceability of structures and their components. New

advanced materials are designed with specialty interfaces to increase fracture

resistance of composite materials and to accommodate residual stresses. Of

particular note is that the mechanical properties of the composites, usedmainly in (Structural engineering in Aircraft structures), may degrade

severely in the presence of damage, often with tragic consequences.

Therefore damage detection is a very important issue in the context of

structural health monitoring for Structural engineering infrastructure with

elements in glass fiber based composites.

Composites are complex materials exhibiting important anisotropic

properties. Commonly observed damage in these materials are: delamination

between fibers, debonding of fibers and adhesive layers, or Glass fibre

fracture.


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Delamination, which is a debonding of two adjoining layers in the laminated

Glass fiber composite, is probably the most frequently observed damage.

Delamination can occur at several scales of the cross section of laminateswith macroscopic delamination, while at a submicroscopic scale,

delamination can also be observed in laminates. Delamination may result

from manufacturing errors, by imperfect bonding, by separation of adjoining

piles of fibers, etc., or, during in service loading such as by accidentally

excessive loading produced for example by fatigue failures or

environmental conditions of temperature and humidity.


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DELAMINATION:

IS IT SERIOUS, DOCTOR?I have discussed fiberglass' problems several timeson Nautica with several articles on osmosis, achemical phenomenon which is sometimes appointedas the only fiberglass' disease. Osmosis is , in myopinion, the most important fiberglass' defect whichcan, on the other hand, be easily repaired withexpensive but effective cures. Other vices belong tofiberglass (although it is a very satisfactory boatbuilding material) which are sometimes more severethan osmosis itself and which can not always be fixed,or are very expensive to repair; in some extreme

cases the cost of repairing could be more than thevalue of the boat. Osmosis is surely the mostimportant fiberglass' defect, because it can be easilydetected (or at least many yachtsmen andprofessionals think so). This vice is, however, notalways easy to find, as some of you can rememberfrom the previous articles on this matter. Most of theboating fans ignore, on the other hand, otherfiberglass-related problems such as: cracks,delaminations, star cracking, structures failure, gelcoat failure (such as pinholing and wrinkling).

All this highlight a complex reality: just a gel-coat

survey is not enough to examine the laminate'scondition, and this is a rule often forgotten by boaters.One of the most severe failure of FRP materials isdelamination which occur when two layers of fiberseparate: it first effect is to improve the elasticity ofthe laminate (easy to feel by hand) in the effectedarea, which usually spread to the nearby surfaces sothat a hull or a deck become "soft". If, for instance,walking on deck we can detect a clear unusualbending, most likely it is due to delamination whichhas occurred due to a foam core damage or to theseparation of the skin from the core material.

There are four main faults which lead to delamination:

core material to skin separation in sandwich structures

interlaminar separation in single skin laminates

frames and longitudinals separation or deck fittings separation

teak to deck separation (on teak lined decks)

Lets now analyze what happen when delamination occur.
http://www.nauticalweb.com/info/maint/osmosi_e.htmhttp://www.nauticalweb.com/info/maint/osmosi_e.htmhttp://www.nauticalweb.com/info/maint/osmosi_e.htm


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SANDWICH STRUCTURES DELAMINATION

More and more boats benefit from the sandwichconstruction which consist of two layers of fiberglassseparated by a core material (balsa wood or closecell PVC foam). The outer layer is usually thicker.

The core material do not cover the entire hull surfaceand, in way of engine girder, deck fittings and chainplates is replaced by single skin laminate. This isdone to overcome the low compressive strength ofcore material, which can fail under concentratedloads, such as bolts. Sandwich delamination occurwhen one of the two fiberglass layers separate fromthe core material. This can be due to:

overload on the boat structure, like concentrated flexural, torsional or compressive stresses

collisions

high shear stresses or uncontrolled drilling

low resin to fiber ratio.

When sandwich delamination occur, the separated layer become"soft", even under a slight pressure. Delamination can involve thehull bottom as well; micro-cracks can be generated due to the lossof mechanical properties. The cracks may generate water leakageor, even worst, may let bilge water (usually greasy and oily)absorption in the laminate which spreads thanks to thefiberglass's high permeability. Delamination may occur in unusualways as well. Particular care must be exercised when the boat ishauled for the season: an asymmetrical load on the cradle or aninsufficient supporting area may cause serious, and sometimespermanent, damage to the laminate. Those damages anddelaminations produce, as mentioned before, a loss in the primarymechanical properties of the shell plate. Because delamination isa progressive phenomenon which spread with time and is responsible for the weakness of thestructure leading to cracks on the laminate, it must be repaired as promptly as possible. This is trueregardless delamination should occur on the hull or on the deck structure. The deck, in particular, isstressed by concentrated loads and is not always well supported by beams or longitudinals;because it is part of the boat structure and participate to the overall stiffness it is necessary to avoidany delamination problem. On delaminated decks, the most simple repairing method is to drill someholes on the interested surface and inject new resin to glue again the separated parts. If thismethod should not work, then it is necessary to rebuild the sandwich structure. In this case theinner skin have to be adequately supported, in order to replace the outer skin and the core material.

Afterward it will be possible to rebuild the inner skin.

Usually this expensive treatment is omitted and the

boat is sold... If we are facing hull delamination, thefirst thing to do is to dry out the interested area. Thisoperation is not simple and the result is notguaranteed. Then the separated parts will be gluedagain. On extreme cases, the sandwich structuremust be rebuild, following the above mentionedscheme. In this particular operation and for relativelysmall surfaces, the vacuum-bagged technique isrecommended to ensure a perfect bonding. Larger


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areas will require a vacuum-bagged technique with epoxy resin, which benefit from a longer curingtime and better performances. The vacuum-bagged technique is a lamination system which use theatmosphere pressure to scrimp the laminate. A particular film is placed on the surface and bondedwith a sealant. A pump is then connected to the film, eliminating the air between the film and thelaminate. In this way the difference of pressure between the outer face of the film and the laminate(approximately equal to the atmospheric pressure) will press on the fiberglass layers. Once theresin has cured, the bag is removed and the laminate is ready for finishing.

SINGLE SKIN DELAMINATIONS

Most of modern yacht hulls are built in single skinfiberglass, manufactured with glass mat and clothreinforcements: just like on sandwich construction, twolayers of fiberglass can separate thus causingdelamination. The main reason for single skindelamination are: - concentrated loads like frequent roadtransportation or inaccurate storage of the hull on thecradle - poor lamination shop condition, like uncontrolledenvironmental elements (humidity and dust percentage

must be controlled for a proper lamination) - low resin tofiber ratio or inappropriate resin type. A low resin to fiber ratio, in particular, can be easily obtainedwhen laminating heavy plies (say more than 1000 gr./m2) which are difficult to impregnate with theresin; if this is the case, the delaminated ply can be easily divided with a knife, showing dry glassfibers. All this may happen on Kevlar laminates as well. The separation of the first ply of mat fromthe rest of the laminate is a particular case of delamination, which may occur if the gelcoat is toothick, as shown on picture 8 and 9. Delamination can be easily seen because it highlight a whitesurface, as shown on picture 10, where the mat ply can be effortlessly separated, as shown onpicture 11. The single skin delamination is repaired following the procedures previously describedfor the sandwich construction. Single skin delamination is, unfortunately, rather frequent, but it hasto be repaired only if it involve a large surface; if the delamination process cover just few smallspots (say few square centimeters) then it is not something to worry about.

STRUCTURE AND FITTINGS SEPARATION

Deck fittings, like chain plates, generate concentrateloads which may cause delamination. Sail boats, dueto their rigging, are relatively more influenced by thisaspect. Fittings are, on same boats, glassed to thedeck and they require an accurate inspection becausea delamination can produce the chain plate shiftingwhich can be noticed, on the most severe cases, bythe continuous loosing of shrouds. This may nothappen on chain plates which are through-bolted ondeck as well, but delamination may still occur. On fastpower boats, engine girders delamination may occurdue to the engine's vibrations, especially in case theengine and the shaft are off-center one another. Thiscase is particularly difficult to repair, because oil, grease and fuel presence in the bilge make resincatalization almost impossible. Both of the delamination cases described in this paragraph areexpensive to repair, because they require a large disassembly job prior to the laminatereconstruction; in fact it will be necessary to remove internal fittings (like furniture) or , in the secondcircumstance, the engines and related equipment.


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TEAK TO DECK SEPARATION

On teak lined decks, delamination can be found bychecking the integrity of the deck structure with a smallhammer: as everybody knows, a change in the soundindicate a change in the underneath volume

distribution. This rather simple job require, on the otherhand, experience and should be carried out by aspecialized technician who will check the seriousnessof the delamination. First of all, all presumeddelamination spots must be highlighted as shown onpicture 13, trying to understand if a "delaminationscheme" exists (i.e. on some teak planks delaminationoccurs proportionally in the same area). Then thetechnician will determine the delamination type: it could be a simple teak to deck separation or aserious delamination in the fiberglass' layers. These two cases have really different repair costs. Asanyone could imagine, the cost of gluing some teak stripes is nothing if compared to the cost ofremoving entirely the teak deck to rebuild the laminate underneath.

In conclusion we should always remember that delamination is not always detectable at first sight,but it has to be found with an accurate survey were a professional opinion is essential becausedelamination reduce the safety of the boat and can be very expensive to repair.

Abstract


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Experimental results are presented from an investigation into angle cracks emanating

from delamination tips in crossply composite plates. The development and growth of the

delamination-intralayer crack system is examined under axial compressive cyclic(fatigue) loading. A study of the results indicate different behavior for the intralayer

crack formation depending on the composite material configuration and the position of

the delamination through the thickness of the specimen.

Introduction

Goals

We have been studying how cracks develop in the advanced composite materials used

in helicopter blades and in other machines. Such cracks may pose a threat to safety,

increase the need for inspections, and shorten the working lifetimes of the machines. Ourlaboratory observations help check theoretical models of the ways that composite

material deteriorate and break under service. Our studies lead to better design

methodologies, better damage detection and serviceability of helicopter components andthus increase the safety of helicopters.

Definitions

A material is characterized as a composite material if it has two or more constituents.

There are a lot of different kinds of composite materials. In this study we are interested

in polymer fiber reinforced composites. These type of composites have a matrix (mainbody) made of epoxy and we reinforce this epoxy with long fibers of graphite (here) in

order to increase the strength of the final product.

One way to manufacture polymer fiber reinforced composites is to obtain sheets (theyare called plies) of fiber reinforced epoxy and stack them together in the desiredorientation. This way you can manufacture [0]24 specimens which are composed of 24

unidirectional 0o plies, or [0/90]24 specimens which are composed of 12 unidirectional

0o and 12 unidirectional 90oplies. Once you stack the plies together you put them in anautoclave where high pressure and temperature are applied and you cure them.

A delamination or interfacial crack is a crack that initiates and grows between the

different plies of a composite material.

Aprimary delamination (PD), for the purposes of this work, is one created by embedding

a Teflon insert. A primary delamination is illustrated in Figure 1,below.

An intralayer crack(IC) is a crack that is initiated at the tips of an embedded

delamination and that grows through the neighboring 90o ply.

A secondary delamination (SD) is a delamination that follows the development of an

intralayer crack.
http://rutgersscholar.rutgers.edu/volume01/pelestra/pelestra.htm#fig1http://rutgersscholar.rutgers.edu/volume01/pelestra/pelestra.htm#fig1


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Figure 1. A primary delamination (PD) in a composite material

An interlayer delamination is a crack that grows in the interface of two plies withoutbreaking the plies. That means that it always has the same direction (Figure 2).

An intralayer delamination is a crack that while it grows in the interface occasionally

"jumps" to a neighboring interface. Then it breaks one, or more, of the plies and it alsochanges orientation (Figures 2 and 3).

Figure 2. Intralayer and Interlayer delaminations

Figure 3. Intralayer delamination in a graphite/epoxy specimen


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Figure 4. Results of axial compressive test for glass/epoxy sample S2/SP250, 4/29,

specimen #2

Experimental methods

Experimental setup

Figure 5. Evolution of intralayer delamination for Glass/Epoxy specimen 4/15 #3


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The numbers 4/29 and 4/15 in the figures above are referring to the specific stackingsequence of these specimens. In order to help the delaminations to grow in the composite

material, a predetermined delamination is fabricated into the specimen (Figure 6). As

such, in 4/29 the first number indicates where we put the delamination (between the

fourth and the fifth ply) while the second number indicates the total number of plies of

the whole specimen (29 plies).

Figure 6. Fabrication of a laminated composite with predetermined delamination

Conclusions

Experimental results

Our experimental results indicate that the damage in helicopter blades can be detectedvery accurately using non-destructive techniques, i.e., ultrasound. Various modes of

damage can exist, i.e. interlayer or intralayer delaminations.

The mechanical behavior and the life expectancy of a composite component depend in

part on interlayer and intralayer delaminations.

Intralayer delaminations are more likely to grow catastrophically and lead to

destruction of the component.


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If delaminations are of the intralayer kind, it is more probable that they will get arrest

and the component will be able to perform its function.

Future Goals

To advance the predictive methodology for composite materials in order to accurately

assess the remaining life of composite components used in aviation and automotiveindustries.

To improvise ways of controlling the damage process. For example our studies show

that if a delamination is of the intralayer kind eventually it will stop growing, i.e. it willbe arrested. To this extent, we may find ways to deviate cracks from their original path,

therefore turning them from interlayer to intralayer delaminations, in order to inhibit

their growth.

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Basic Aspects