This paper presents a study on delamination growth in Mode I, Mode II and mixed mode under fatigue loading in an HTA/6376C composite. The computed slopes of the modified Paris plots were high. Therefore, threshold values of the strain energy release rate for delamination growth were determined. Low fatigue threshold values revealed a significant effect of fatigue loading. The largest effect was found for the ENF test (Mode II) for which the fatigue threshold value was only 10% of the critical strain energy release rate in static tests. Threshold values for MMB (mixed mode) and DCB (Mode I) tests were 15% and 23% of the static values, respectively. Fractographic evaluation revealed identical initial failure mechanisms in fatigue and static loading conditions for the ENF specimen. The ENF specimen failed by formation and coalescence of microcracks. The low fatigue threshold for the ENF specimen was explained by microscopical observations on the specimen edge. It was also shown that the fracture surfaces generated in static and fatigue DCB and MMB tests were similar

The evaluation procedure for the mixed mode bend (MMB) delamination test is assessed with focus on analytically equivalent evaluation models, expressed in load-displacement or load-only parameters. In particular, the assessment concerns the sensitivity of the interlaminar toughness to the test rig forces as well as material and geometrical properties of the specimen. For a typical example, neglect of test rig forces causes a 10% relative error in the calculated mixed mode ratio when using methods based on load only. When all additional forces were considered, both evaluation methods produced almost identical results. However, evaluation based on load only is sensitive to variations in specimen flexural modulus and dimensions and had a larger scatter. The application of crack length corrections for calculating the Mode I component is discussed in detail. The findings of the study are summarized in recommendations for the MMB test procedure and its subsequent evaluation.

Four methods to determine composite interlaminar shear strength (ILSS) are evaluated. In particular, the recently devised inclined double-notch shear test (IDNS) is compared with three existing and more established methods: the Iosipescu test, the short three-point bending test (S3PB) and the double-notch compression tesl (DNC). The uniformity of strain field in the test region in a real test situation - which is the crucial test method quality parameter - is investigated by strain mapping using digital speckle photography. The measured strain fields are compared with FE-calculated strains representing ideal conditions and both known advantages and drawbacks of the different methods are confirmed. The IDNS test produces the most uniform strain fields and also consistently high ILSS values. A fractographic analysis indicates shear separation over a major part of the fracture surfaces of all specimen types; typical shear cusps were found over about 80% of the IDNS fracture surface and in about 50% to 70% in the other specimens. For the Iosipescu tests, failure initiation could be ascribed to initiation in tension at defects. Experimentally determined stress-strain responses in shear exhibit a distinct variation among the different methods. For the best methods, a notable material softening was observed prior to failure. Observed formation of shear cusps is believed to be the primary cause for this softening of the composite material studied here.

This paper reviewed fatigue damage mechanisms in composite materials, giving an account of the historical developments. The present understanding has resulted from a large number of investigations, some of which have required developing new techniques and modifying earlier techniques for observing the complex damage process. The development of damage involves a sequence of mechanisms that may not be separable into distinct damage modes but rather into dominant damage modes

Different models based on variational analysis are compared with respect to the thermo-elastic properties of cross-ply laminates with transverse cracks in the 90°-layer. This approach, pioneered by Hashin, applies the principle of minimum complementary energy to describe the stress state in a cross-ply laminate with transverse cracks. Apart from material data, only laminate geometry data and stress distribution assumptions are required. The model developed in the present study contains stress distribution assumptions that are closer to reality than previous models. Non-uniform stress distributions are included through-the-thickness of both the 0°- and the 90°-layers. An important result is the improved agreement with stiffness reduction data. As expected from the nature of the model, predictions generally provide lower bounds to the stiffness of the cracked laminate. However, at high crack densities, this is not the case. This additional stiffness reduction at high crack densities indicates the presence of new damage types, such as branched cracks and local delamination.