PSI - Issue 50

Kirill Guseinov et al. / Procedia Structural Integrity 50 (2023) 105–112 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

106

2

interlaminar shear stresses

τ 13

applied load

P

length of the gauge section

l t

specimen thickness

through-thickness strains in the material axes interlaminar shear strains in the material axes normal strains in the global coordinate system shear strains in the global coordinate system

ε 3

γ 13

ε x , ε y

γ xy

1. Introduction Carbon fibre-reinforced plastics (CFRP) generally have high in-plane and low out-of-plane properties. Mechanical behaviour of CFRPs during interlaminar shear significantly depends on the polymer matrix properties, and interlayer strength is determined by the adhesion forces on the contact surface «matrix - reinforcement» ( Tarnopol’skii et al.(1992)). The laminates behaviour during shear tests shows significant non-linearity due to progressive damage accumulation, which leads to the decrease in shear stiffness and, finally, to delamination. Accurate prediction of interlaminar shear properties is of great importance for assessing the static strength of aircraft structural elements. The influence of through-thickness stresses on the interlaminar shear strength and mechanical response of composites must be taken into account when developing thick-walled composite elements. The combination of through-thickness compression and interlaminar shear loads can increase the shear stress corresponding to the initiation of delamination (DeTeresa et al. (2004)). The increase in interlaminar shear strength needs to be considered, since a low shear strength estimate can lead to the decrease in the weight efficiency of the composite structure. It is necessary to use correct failure criteria that take into account the influence of the stress state type on the interlaminar shear strength (Guseinov et al. (2021)). The criterion of maximum stresses is also often used in the development of aircraft structural elements due to its simplicity (Anoshkin and Tashkinov (1997, 1998), Anoshkin et al. (2011, 2011)). However, the failure for each of the components of the stress state by this criterion is considered separately. In this regard, the strength assessment of thick-walled composite elements is very conservative under significant through-thickness compression stresses (Guseinov et al. (2021)). Several failure criteria consider changes in the material strength under combined compression-shear loading (Daniel (2007), Daniel et al. (2008), Christensen and DeTeresa (2004), Sun (2000), Tsai and Wu (1971)). Gan et al. (2013) showed that Daniel failure criterion is the most preferable for implementation of the hidden strength reserves of composites, since it is described by the material properties only, and it does not require the identification of additional parameters. The composites behaviour under shear testing is well studied, while studies of mechanical response under combined compression-shear are ongoing. In particular, Gan et al. (2013) modified the double notch symmetrical shear test to measure the interlaminar shear strength of laminated unidirectional CFRPs under moderate through thickness compression. These tests require complex testing machines equipped with four independent hydraulic grippers. In addition, only disproportionate loading is implemented by pre-compressing the specimen in the out-of plane direction during such tests. Thus, only the interlaminar shear strength is evaluated experimentally, without analysing the material behaviour under combined compression-shear. Gan et al. (2018) used a Modified Arcan Fixture (MAF) to study the mechanical response and strength of unidirectional fiberglass under biaxial in-plane loading. To study composites reinforced with carbon fibres, this fixture was modified by Hao et al. (2019) with guide assemblies to increase the rigidity of the setup. However, the authors did not study the interlaminar shear properties of laminates under through-thickness compression. Thus, there is no generally accepted approach between researchers for the reliable determination of the interlaminar shear properties of composites under biaxial loading. Test methods to assess interlaminar shear strength and to investigate mechanical response under combined compression-shear on simple test equipment are of particular interest. In this study, simplified experimental methods for determining the interlaminar shear properties of composites under through-thickness compression were compared. The combination of interlaminar shear and