O. Ishai

Interlaminar fracture toughness and toughening of laminated composite materials: a review

Abstract A review of the state of the art in the subject of interlaminar fracture toughness ( ift ), its relation to structural performance and the damage tolerance of polymeric composite materials is presented. The sources of low ift (high brittleness) of existing materials and methods to im4prove it by introducing tough interlayers or by using thermoplastic matrices are discussed. The IFT test methods, their analytical basis and utilization are described. Comprehensive ift data for GIc and GIIc for different composite systems and test methods, which was extracted from numerous publications, are presented.

The effect of adhesive thickness on interlaminar fracture toughness of interleaved cfrp specimens

Abstract The investigation concentrated on the adhesive size effect on interlaminar fracture toughness ( ift ) of interleaved cfrp specimens. In tests an artificial interlaminar edge crack was forced to propagate under two modes: separation (Mode I) and shear (Mode II). Two types of adhesive were used and adhesive thickness ranged from 0.04 mm–1.1 mm. Results indicated that ift increased with adhesive thickness up to 10-fold in the case of Mode I and up to 7-fold in the case of Mode II compared with the ift of non-interleaved counterparts. The toughening effect was more pronounced at the low range of thickness and tended to decrease in the case of thick layers. In Mode II ( enf test), ift was found to decrease above an adhesive thickness of 0.7 mm. It was concluded that optimum adhesive thickness for improved ift is in the range of 0.1 - 0.7 mm.

The Elastic Properties of Three-Phase Composites

A three-phase particulate composite, consisting of filler and voids within an epoxy matrix, was tested in tension and com pression, covering the volumetric filler- and void-content ranges up to 39% and 52% respectively.

Effect of selective adhesive interleaving on interlaminar fracture toughness of graphite/epoxy composite laminates

Abstract Interlaminar fracture toughness (IFT) characteristics of graphite/epoxy laminates were evaluated in tests on specimens with different tough adhesive interlayers. An artificial interlaminar edge crack was forced to propagate under two modes: separation, and shear. Results indicated that the interlayer, located in front of an existing flaw, may increase ift more than sixfold compared with an uninterleaved brittle-matrix composite reference. The effect was stronger in the separation mode than in the shear mode, and seems to increase with thickness of the interlayer. In all cases, delamination propagated through the adhesive layer in a cohesive failure process. The hygrothermal aspect was also studied in this context. It was concluded that selective interleaving at critical spots may produce a significant improvement in the ift of the composite laminate.

Effect of Fillers and Voids on Compressive Yield of Epoxy Composites

Two series of epoxy composites, filled and porous, were loaded in compression up to yield under several constant strain rates. Results indicate a linear relationship between yield stress and log strain rate, characterized by a constant slope for the different com positions, and almost linear dependence of yield stress on filler content. A simplified empirical formula was derived for the yield stress of filled composites as function of filler content and strain rate. Similar dependence was found for porous composites. The strain-rate dependence indicates non-Newtonian visco plastic behaviour obeying the corresponding Eyring equation. The filler content was found to have almost no effect on the activation volume terms, and its main effect is apparently with regard to the activation energy characteristics of the yielding process.

Effect of impact loading on damage and Residual Compressive Strength of CFRP laminated beams

Abstract The effect of flexural impact loading on damage characteristics and on Residual Compressive Strength (RCS) of CFRP composite beam specimens was investigated. Detection and follow-up of interlaminar damage initiation and propagation processes were conducted via video inspection of specimens subjected to quasi-static loading. It was found that the shear delamination mechanism is the most effective in the reduction of RCS. The edge delamination process, on the other hand, seems to have only a minor effect on residual failure characteristics. Compressive failure was mainly controlled by sub-laminate buckling, which seems to be dependent on delamination length, damage location and layer sequence. The presence of tough adhesive interlayers was found to delay and even prevent interlaminar damage formation and growth under flexure and compressive loading. The dependence of RCS on initial flexural energy seems to be similar under both quasi-static and impact loading modes. It was concluded that the inter-relationship of flexural energy, damage characteristics and RCS, as derived via beam specimen testing, may provide a sensitive tool for comparative damage tolerance evaluation of different composite material configurations.

Effect of delamination on stability of laminated composite strip

Abstract A general one-dimensional model of a composite delaminated elongated strip (wide beam) under arbitrary axial and transverse loading and boundary conditions is used for predicting the classical buckling load, with the bending-stretching coupling effect as well as the prebuckling geometrical nonlinearity taken into account. The differential equations are solved by Newtons method, using a finite-difference scheme. The contribution of the coupling effect is examined by means of parametric analysis.

Characterization of damage initiation and propagation in graphite/epoxy laminates by acoustic emission

Abstract A method of characterizing two major failure processes in graphite/epoxy compositestransverse cracking and mode I delamination, has been studied. Representative laminates were tested in uniaxial tension and flexure. The failure processes were monitored and characterized by acoustic emission (AE). The effects of moisture on AE were also investigated. Each damage process was found to have a distinctive AE output and to follow a definite trend which was significantly affected by moisture conditions. It was concluded that AE can serve as a useful tool for detecting and identifying these failure modes in composite structures in laboratory and in service environments.

Hygrothermal effects on the mechanical behaviour of graphite fibre-reinforced epoxy laminates beyond initial failure

Abstract The critical load levels and associated cracking beyond which a multidirectional laminate can be considered as structurally failed has been determined by loading graphite fibre-reinforced epoxy laminates to different strain levels up to ultimate failure. Transverse matrix cracking was monitored by acoustic and optical methods. The residual stiffness and strength parallel and perpendicular to the cracks were determined and related to the environmental/loading history. Within the range of experimental conditions studied, it is concluded that the transverse cracking process does not have a crucial effect on the structural performance of multidirectional composite laminates.

Aligned short glass fibre/epoxy composites

Abstract An orientation system based on the principles of the ERDE glycerine process was built and used to prepare short glass fibre mats of known orientation distributions. The mats were subsequently impregnated with epoxy, B staged to yield partially cured prepregs and finally compression moulded. Mechanical characterization and comparison with theoretical formulations were performed on longitudinal, transverse and off-axis specimens. The mechanical characteristics of specimens with different orientation patterns were compared with analytical predicitions based on ‘Laminate analogy’ methods. Fracture surfaces were studied by optical and scanning electron microscopy techniques.