Specimen Preparation

3.1 Introduction

There are a number of subsidiary, but vital, issues that are complementaryto the main activity of mechanical testing. These issues, taken together, con-stitute the preparatory work required to produce test specimens of ade-quate quality. If insufficient attention is given to any of these activities, theresults from a particular test could be invalidated. The following remarksrelate to the use of specimens of high performance composites fabricatedfrom continuous preimpregnated fibres, the subject of this text. The fourstages considered are: laminate production; quality checking; specimenmanufacture; application of strain gauges. The final three stages would, ofcourse, apply to any material.

3.2 Laminate production

Thin sheets, known as laminates, usually 1 or 2mm thick for coupon speci-mens, are manufactured from layers of fibres preimpregnated with partiallycured (if epoxy-based) resin prepreg. The matrix is usually an epoxy, butBMI (bismaleimide) and thermoplastic prepregs are also used. It should benoted that the following discourse relates mainly to epoxy prepregs (owingmainly to their popularity). It should, however, be pointed out that thepreparation of laminates with thermoplastic matrices is in many ways a similar but more straightforward process, because the plastic resin is not required to cure, but simply ‘melts’ at a suitably high temperature andresolidifies when cooled.

A single prepreg layer is usually 0.125 or 0.25mm thick and the fibres areeither continuous and parallel (unidirectional), or in the form of a wovenfabric. The prepreg is supplied as ‘tape’, normally 0.3m wide (but suppliershaving width preferences, woven materials being generally wider than uni-directional products), sandwiched between protective layers of paper orplastic and wound on a reel. If epoxy, the prepreg should be kept in a freezer

3Specimen preparation

F L MATTHEWS

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© 2000,Woodhead Publishing Ltd, except chapters 6, 8, 11 and 15, Crown copyright.

until it is required; if thermoplastic, low temperature is not a requirementbut it is advisable to store the material in a clean, light-free environment.Shelf-life (for epoxies) is normally around 18 months and will be clearlystated by the supplier; thermoplastics, on the other hand, generally degradevery slowly at ambient temperatures. If the prepreg has exceeded its life-time it can probably still be used for a further six months, at least. However,its suitability should be checked by moulding a test panel, or by checkingthe cure state of the matrix resin using differential scanning calorimetry(DSC).

Appropriate lengths are cut from the reel and placed on top of each otherwith the fibres in each layer oriented relative to one another in a prede-termined sequence. Hand tools, such as a ‘Stanley’ knife drawn against ahard edge, are usually satisfactory for cutting. Fabric prepreg can be cutusing shears or scissors. Where available, a rotary knife or water jet couldbe used. The protective layers are removed before each layer is placed onthat previously laid down, and the layer carefully smoothed out to preventair entrapment. It is essential that the layers are aligned with reference toa datum, since even a few degrees’ misalignment can cause a dramatic effecton mechanical properties. With properly prepared prepreg the edge of theprotective backing sheets can be used as a reference. Care must be takento ensure that twisted or knotted fibre bundles, or prepreg areas contain-ing gaps between bundles, are not included in the laminate.

Following completion of the layup, the stack of prepreg layers is preparedfor curing in the case of epoxies, or consolidation for thermoplastics. Theepoxy resin, which forms the matrix of the composite, is formulated forautoclave curing; the whole curing process lasts several hours and involvesa combination of vacuum, raised temperature (to 120 or 175°C for epoxies,often higher for thermoplastics) and raised pressure.The prepreg layers arecontained within a sealed ‘blanket’ as illustrated in Fig. 3.1.

To prevent the laminate sticking to the base and caul (pressure) plates,the latter can be coated with release agent, or layers of release fabric or apolymeric film are inserted between the plates and the prepreg. A dis-advantage of the second approach is that an impression of the fabric is lefton the surface of the laminate, thus making it difficult to detect the fibreorientation in the surface layers with the naked eye.

As an alternative to autoclave curing it is possible to use a heated press,in which case it is necessary to monitor separately the state of resin gela-tion, or a press-clave. The latter device, illustrated in Fig. 3.2, is placed in aheated press, in combination with a separate high pressure supply and avacuum source.A heated press, with facilities for rapid cooling of its platens,would be used for processing advanced thermoplastic prepregs. Clearly thesize of the laminate that can be produced will be determined by the size ofpress available.

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38 Mechanical testing of advanced fibre composites

Sealingtape

Melinex

Pressure plate

Vacuum bagging material Laminated composite plate Air breather cloth

Peel ply

Vacuum connection

3.1 Arrangement for producing laminates by autoclaving.

Baseplate

Pressure vessel Pressure connectionDiaphragm

Top plateLaminate, etc.FrameVacuum connection

Vacuum connection

Top plate

MelinexBleed clothPeel ply

LaminatePeel plyPerforated PTFE

Peel ply

Frame

Baseplate

3.2 Layout of a press-clave.

3.3 Quality checking

The manufacturing process, if not properly controlled, can introduce defectsinto the laminate. Typical defects are voids (small cavities in the resin),delaminations (unbonded areas between layers) or, unusually, longitudinalcracks (lack of bonding between fibre and matrix).

Voids can be caused if the prepreg is not allowed to warm to room tem-perature before laying-up, thus introducing moisture into the prepreg stack.Delaminations can be caused by entrapped air or the inclusion of pieces ofbacking sheet. Longitudinal splitting and delamination can occur in multi-directional laminates as a result of thermal stresses induced during cool-down from the curing temperature.

All the above defects will degrade mechanical properties, particularly in compression, shear and flexure. It is, therefore, important that their presence is detected so that faulty laminates can be discarded.

The standard method of detection is to use ultrasonic C-scan, which isgood at detecting inclusions, porosity and delaminations, or, possibly, X-raytechniques, which can detect through-thickness cracks.

3.4 Specimen manufacture

Specimens, as defined by the relevant standard, or test to be carried out,are cut from the laminates using a diamond-tipped saw. The normal bladehas 600 grit, but a cleaner cut, with less damage to the laminate, is obtainedwith 800 grit. In the latter case the blade can become clogged with debrisand frequent cleaning may be required. Laminates produced by autoclav-ing will have a feathered edge which must be removed.

It is clearly vital that edges produced after trimming, which effectivelyact as a datum for subsequent specimen cutting, are correctly aligned withthe fibres in the layers. A commonly used method for establishing the 0°direction of a cured laminate prior to cutting is to split off a narrow stripof material along this direction (in multidirectional laminates this can bedone if the 0° layer is made slightly wider), but it has been shown that thisapproach may not be sufficiently accurate, and a preferred method1 is tomark the outermost ply by scoring across in the 90° direction in a non-stressed region.

High temperatures are generated during dry cutting, which can causelocal degradation and damage at the machined edge. This can be largelyprevented by the use of a coolant (water), but subsequent drying-out stepsmust be taken to remove any absorbed liquid from the specimen. Gener-ally, specimen blanks are machined oversize, final dimensions beingachieved by grinding.

Drilling is readily achieved with tungsten carbide or diamond tipped bits,


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the laminate being supported by a (sacrificial) backing plate. It is advisableto use a drill bit tip angle of 55–60° for thin laminates and 90–100° for thicksections, rather than the usual 120°. Other operations are best achieved bygrinding. Clearly, appropriate support is needed for thin laminates ifthrough-thickness shaping is to be carried out. Kevlar fibre-reinforcedmaterials need special attention. Owing to the nature of the fibre it is difficult to avoid a ‘furry’ edge. Specially adapted impregnated wheels canbe obtained for cutting. Another alternative is to use a high pressure waterjet.

For many tests, for example tension and compression, it is often neces-sary to bond end-tabs to the specimen; this is done to diffuse the grippingloads and prevent failure at the specimen ends. According to the particu-lar requirements, the tabs may be of aluminium alloy, GFRP (glass-fibrereinforced plastic) or CFRP (carbon-fibre reinforced plastic). When thetabs are of composite, the preferred method is to stick strips to the trimmedlaminate before cutting into specimens. This approach is not only quicker,but it also ensures alignment of tabs and specimen.When the tabs are metalthis approach cannot be adopted and the tabs must be bonded to indi-vidual specimens, using a jig to give accurate positioning. Surfaces whereend-tabs are to be bonded should be abraded in order to remove surfacecontamination, whilst taking care not to damage the outermost fibres. Thisis done most easily, particularly if the laminate surface is rough, by grit blast-ing, the only objection to this method being that the surface may itselfbecome contaminated, either by grease carried by the grit or embedmentof the grit. Surfaces not needing to be abraded can be protected by maskingwith self-adhesive tape. The grade of grit used, typically 80–120 grade, doesnot appear to be critical if care is taken to avoid excessive abrasion and damage to the composite. The dust left behind on the material after grit blasting is most easily removed by flushing under running water.If the water lies on the surface in an unbroken film, a good standard of surface cleanliness is indicated. The amount of water absorbed by thelaminate will be small, particularly if the material is dried immediately afterwashing.

After drying, the surfaces are solvent wiped and bonded. Commercial‘two-tube’ epoxy resin has been found to be suitable for bonding end-tabsto be used for tests at room temperature and should be applied sparinglyto both bonding surfaces. The joint, when assembled, can be contained in a simple vacuum bag which will apply an adequate clamping force andremove entrapped air. The tab material can be located by using small pegsinserted in notches cut through the tab and test materials. An alternativeto low temperature curing adhesive is to use bonding film which is curedunder elevated temperature and pressure, although in some cases this cancause thermal stresses sufficient to split a 0° laminate. Whatever adhesive


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is used it should be tough, with a greater failure strain than that of the mate-rial under test. CRAG2 specifies that it should have a shear strength greaterthan 30MPa. Special adhesives would be preferred for fatigue testing orhigh temperature work. An incidental advantage in using GFRP end-tabsis that the material is translucent and any gaps in the glue film can be seenby visual examination. Even if the joint is strong enough to withstand speci-men failure loads, gaps in the glue line can result in uneven stresses in theunderlying composite and cause premature failure.1

The crucial issue when bonding is to ensure that both the specimen andthe tabs are properly prepared. Composites need to be degreased andabraded to remove all traces of release agent transferred during moulding.This procedure should be followed by wiping with a solvent. Similar pro-cedures should be followed when making bonded joints. In addition todegreasing, aluminium alloy tabs need to be etched in chromic acid or phosphoric acid.

3.5 Strain gauging

All mechanical tests will involve the measurement of displacements orstrains, as defined by the appropriate standard. When strain gauges arecalled for, it is important to follow the recommended procedures.The lengthof the gauge may be specified by the relevant standards, but should alwaysbe significantly shorter than the gauge length of the specimen. Compositescan cause particular difficulties not encountered with metals.3

The issues that must be addressed are as follows:

1 High gauge resistances are desirable because high voltages (2–4V) withlow current can then be used; this improves hysteresis effects and zeroload stability.

2 If possible, use gauges with lead wires attached, or solder wires to thegauge before installation; this should avoid soldering damage to thecomposite.

3 Ideally the pattern of the autoclave scrim cloth should be removedbefore gauge installation; this is particularly important if contact adhe-sives are used.

4 Corrections may be necessary to gauge transverse sensitivity effects;errors of over 100% between actual and measured strains can beobtained.

5 Gauges must be precisely aligned; errors of 15% can result from a 2°misalignment. There is no universally acceptable way of ensuring align-ment. The scrim cloth pattern can be misleading. Sometimes C-scan after installation can be useful, or checking with failure surfaces after fracture.


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6 Dummy gauges are the preferred method for temperature compensa-tion but, again, precise alignment is needed. It is necessary to mount thedummy gauges on an ‘identical’ piece of laminate, with the same orien-tation relative to the fibres as used for the active gauges.

3.6 Summary

In summary, it is essential that careful and consistent procedures are fol-lowed at every stage of specimen production. Failure to do so will throwdoubt on the validity of any data generated.

References

1. P W Manders and I M Kowalski, ‘The effect of small angular fiber misalignmentsand tabbing techniques on the tensile strength of carbon fiber composites’, 32ndInternational SAMPE Symposium, Anaheim, CA, USA, eds R Carson, M Burgand K J Kjoller, Society for the Advancement of Material and Process Engineering, Covina, CA, USA, 1987.

2. P T Curtis (Ed), CRAG Test Methods for the Measurement of the EngineeringProperties of Fibre Reinforced Plastics, Royal Aircraft Establishment, Farnbor-ough, UK, Technical Report 88012, 1988.

3. M E Tuttle and H F Brinson, Resistance Foil Gauge Technology as Applied toComposite Materials, Report No. VPI-E-83-19, Department of EngineeringScience and Mechanics, Virginia Polytechnic Institute and State University,Blacksburg, VA 24061, USA, June 1983.


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Specimen Preparation

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