H. H. Kausch

Round-robin interlaminar fracture testing of carbon-fibre-reinforced epoxy and PEEK composites

Abstract This paper summarizes results from a series of tests performed in eighteen laboratories with three specific aims: first, to establish the reproducibility of values from mode I and mode II tests carried out on two materials in different laboratories; secondly to investigate the differences between two data analyses; and finally to examine the influence of specimen thickness on mode I and mode II values. The materials tested, both unidirectional, were a relatively brittle carbon-fibre/epoxy laminate and a thermoplastic composite, carbon fibre/PEEK.

Measurement of GIc and GIIc in carbon:epoxy composites

Abstract The measurement of mode I and mode II delamination resistance of a unidirectional carbon/epoxy composite is examined. First, the results of a round-robin exercise organised by the Polymer and Composites Task Group of the European Group on Fracture (EGF), involving 10 laboratories, are presented. Values of GIc and GIIc at initiation and propagation have been measured using double cantilever beam (DCB), end notched flexure (ENF) and end loaded split (ELS) specimens. The influence of defect type and specimen geometry on the values of delamination resistance of this materials is then discussed.

A study of the delamination resistance of IM6/PEEK composites

Abstract This paper describes the delamination behaviour of unidirectional carbon fibre/PEEK (poly(ether ether ketone)) composites. Tests have been performed under mode I and mode II loading conditions, using double cantilever beam (DCB) and end-notched flexure (ENF) specimens. Mode I tests on specimens of different thicknesses indicated that propagation values of GIc were dependent on thickness, higher values being obtained or 5 mm than for 3 mm. This was due to increased fibre bridging and multiple crack formation in thicker specimens and may explain scatter in previously published values. Fibre distribution was inhomogeneous in IM6/PEEK specimens, with clusters of fibres forming. The delamination resistance of these specimens was independent of cooling rate over a range from 50 down to 0·3°C/min. The behaviour of IM6/PEEK composites is compared with that reported for AS4/PEEK materials.

Strain-rate dependence of the tensile fracture behaviour of woven-cloth reinforced polyamide composites

Abstract Tensile testing of plain-weave-cloth-reinforced polyamide composites has been carried out at various strain rates ranging from 1×10−2 to 4×101 s−1 in a servohydraulic testing apparatus. Four different polyamide composites studied were composed of two kinds of reinforcements, carbon fibre (CF) and glass fibre (GF), and two kinds of matrices, polyamide-6 (PA6) and modified polyamide-6 (mPA6). The experimental results showed that the tensile strength and failure strain of the composites tend to increase with increase in strain rate except in the case of GF/mPA6 whose fracture properties were stabilized at high rates (>1×100 s−1). Damaged regions of the fractured specimens were observed in order to study the micromechanisms of tensile failure by means of polarized optical and scanning electron microscopes. The microscope studies showed that in the composites with the mPA6 matrix, extensive microcracking occurred in the matrix and transverse threads region prior to the final failure. The effects of strain-rate on the tensile fracture behaviour of these types of composite systems are discussed on the basis of these experimental results.

Measurement of initiation values of GIC in IM6/PEEK composites

Abstract The double cantilever beam (DCB) specimen is widely used to determine delamination resistance under mode I loading. However, recent work has indicated the geometry-dependence of values of G IC measured during propagation of a delamination in unidirectional specimens of IM6/PEEK when fibre bridging and multiple cracking are observed. An alternative approach is to quote initiation values of G IC , but little information is available to allow a rational test procedure to be proposed. In this paper the influence of different types of starter defect on the initiation values of G IC in IM6/PEEK is examined. This has enabled a suitable starter defect type to be identified. Finally, results from static fatigue tests are presented which indicate a self-toughening mechanism in this material.

The effect of cooling rate on deformation and fracture in IM6/PEEK composites

Abstract A series of tensile tests have been undertaken on IM6/PEEK (poly ether ether ketone) in order to assess the effect of varying the rate of cooling on its short- and long-term fracture properties. Tests on (90°) laminates have shown that below the glass transition temperature of the PEEK matrix the tensile mechanical properties of the composite are not influenced by the microstructure of the matrix. Above the matrix T g , however, varying the percentage degree of crystallinity results in a change in both the short and long-term properties of the composite. Here, the slower-cooled laminate offered a superior ultimate stress and an improved creep resistance. Conversely, in the (+/−45°) composites significant cooling rate effects were observed only at lower temperatures. Optical micrographs taken from these specimens indicated that the fast-cooled laminate incurred considerably more internal cracking and delamination during the course of testing. It is believed that such volume damage results from the large thermal strains induced by the rapid cooling process. This information suggests that fast cooling, a procedure that is often encouraged in order to control the level of crystallinity, may be more detrimental to the mechanical properties than slow cooling.

A study of the effect of forming temperature on the mechanical behaviour of carbon-fibre/peek composites

Abstract In this paper, a study of the effect of forming temperature over the range 360–440°C on the mechanical behaviour of carbon-fibre/PEEK composites is presented. The properties examined include mode I and II delamination resistance of unidirectional specimens and the tensile strength of ( ±45° ) and (90°) specimens. A previous report suggested that GIC, Init can be doubled by forming specimens at a temperature 20K higher than the recommended forming temperature (380°C). Results in this paper show that other mechanical properties are also improved if higher forming temperatures are used, but the increase is most dramatic in mode I delamination behaviour. To explain this phenomenon, a microscopic analysis of fractured specimens and an evaluation of the preparation technique of delamination test specimens were carried out. Results of these analyses suggest that the improvement in mechanical properties with forming temperature is due to an increase in interfacial strength between fibre and matrix.

Examination of the processes of deformation and fracture in a silica-filled epoxy resin

A series of short-term fracture tests have been undertaken on a silica-filled epoxy resin in order to examine the processes of damage initiation, development and fracture in a particulatefilled polymer. Several different types of inelastic deformation and fracture mechanisms were observed within the volume of the material. These included localized shear yielding, particlematrix debonding and micro-cracking. The relative amount of each of these was found to depend upon the test rate and temperature. At low temperatures and high rates of loading, failure was associated with one single debonding event whereas at high temperatures and low rates, debonding and yielding were found to be extensive throughout the volume of the test specimens. A detailed examination of the fractured specimens identified several distinct regions on the fracture surface. Surrounding the defect the particles were often debonded from the matrix suggesting that the crack had propagated in a sub-critical manner. Beyond this zone was a smooth zone corresponding to the region over which the crack was accelerating unstably. The smooth zone then developed into a rough three-dimensional zone in which the crack was propagating at its maximum velocity. The size of each of these zones was found to vary considerably with test temperature and cross-head speed.

Viscoelastic creep crack growth: A review of fracture mechanical analyses

The study of time dependent crack growth in polymers using a fracture mechanics approach has been reviewed. The time dependence of crack growth in polymers is seen to be the result of the viscoelastic deformation in the process zone, which causes the supply of energy to drive the crack to occur over time rather than instantaneously, as it does in metals. Additional time dependence in the crack growth process can be due to process zone behavior, where both the flow stress and the critical crack tip opening displacement may be dependent on the crack growth rate. Instability leading to slip-stick crack growth has been seen to be the consequence of a decrease in the critical energy release rate with increasing crack growth rate due to adiabatic heating causing are duction in the process zone flow stress, a decrease in the crack tip opening displacement due to a ductile to brittle transition at higher crack growth rates, or an increase in the rate of fracture work due to more rapid viscoelastic deformation. Finally, various techniques to experimentally characterize the crack growth rate as a function of stress intensity have been critiqued.

A simple velocity gauge for measuring crack growth

ck growth. Only a few of the available methods [i] are able to measure continuously the crack length, a, with good accuracy over the whole range of speeds of interest. Moreover, they often make use of the specific properties of metallic specimens. Polymers, which are relevant here, and ceramics must, therefore, be tested by another method. A velocity gauge has been devised to fill this gap. A suspension of graphite particles is applied by spraying or by screen-printing over the surface where the crack is expected to propagate. A small quantity of binder facilitates handling without giving the gauge specific fracture properties. It is, indeed, intended that the particles follow locally exactly the behaviour of the specimen. The critical parameter is, therefore, the crack opening displacement (COD). Particles with diameters greater than the COD are liable to form bridges over the crack. Particle sizes in the order of I ~m were used satisfactorily with polymeric samples. The thickness of the layer should be as small as possible, a homogeneous layer of about 5 zm being obtainable in practice. Silver-ink electrodes permit the creation of a quasi-linear electric field. It is proposed that two such lines be drawn on the graphite, parallel to the crack using a drawing pen. Due to the high resistivity of graphite, as compared to metals, usual Ohmmeters are suitable for measuring the gauge resistance. Fig. 1 shows the final arrangement of the gauge. The initial resistance R measured between the electrodes is dependent on the restivity of [he graphite PC and the dimensions of the gauge, thickness ~, length l, and width b