Micro-mechanical finite element analysis of Z-pins under mixed-mode loading

© 2015 Elsevier Ltd. All rights reserved.This paper presents a three-dimensional micro-mechanical finite element (FE) modelling strategy for predicting the mixed-mode response of single Z-pins inserted in a composite laminate. The modelling approach is based upon a versatile ply-level mesh, which takes into account the significant micro-mechanical features of Z-pinned laminates. The effect of post-cure cool down is also considered in the approach. The Z-pin/laminate interface is modelled by cohesive elements and frictional contact. The progressive failure of the Z-pin is simulated considering shear-driven internal splitting, accounted for using cohesive elements, and tensile fibre failure, modelled using the Weibulls criterion. The simulation strategy is calibrated and validated via experimental tests performed on single carbon/BMI Z-pins inserted in quasi-isotropic laminate. The effects of the bonding and friction at the Z-pin/laminate interface and the internal Z-pin splitting are discussed. The primary aim is to develop a robust numerical tool and guidelines for designing Z-pins with optimal bridging behaviour

Crack propagation in honeycomb cellular materials: a computational approach

Computational models based on the finite element method and linear or nonlinear fracture mechanics are herein proposed to study the mechanical response of functionally designed cellular components. It is demonstrated that, via a suitable tailoring of the properties of interfaces present in the meso- and micro-structures, the tensile strength can be substantially increased as compared to that of a standard polycrystalline material. Moreover, numerical examples regarding the structural response of these components when subjected to loading conditions typical of cutting operations are provided. As a general trend, the occurrence of tortuous crack paths is highly favorable: stable crack propagation can be achieved in case of critical crack growth, whereas an increased fatigue life can be obtained for a sub-critical crack propagation

Fracture parameters formulation for single edge notched AS4 stitched warp-knit fabric composite plate

The three-dimensional problem of the fracture for the single edge notched tension plate (SENT) of orthotropic material is considered in this paper. The finite element solution is used to evaluate the singular and non-singular terms of the William series, i.e. Stress intensity factor (SIF) and T-stresses namely T11, T13 and T33. Based on the obtained numerical results, a fitting procedure is performed in order to propose analytical formulations giving the fracture parameters near the crack tip. The obtained results are in good agreement with the finite elements calculation and other literature results

Fracture parameters formulation for single edge notched AS4 stitched warp-knit fabric composite plate

The three-dimensional problem of the fracture for the single edge notched tension plate (SENT) of orthotropic material is considered in this paper. The finite element solution is used to evaluate the singular and non-singular terms of the William series, i.e. Stress intensity factor (SIF) and T-stresses namely T11, T13 and T33. Based on the obtained numerical results, a fitting procedure is performed in order to propose analytical formulations giving the fracture parameters near the crack tip. The obtained results are in good agreement with the finite elements calculation and other literature results

Fracture parameters formulation for single edge notched AS4 stitched warp-knit fabric composite plate

The three-dimensional problem of the fracture for the single edge notched tension plate (SENT) of orthotropic material is considered in this paper. The finite element solution is used to evaluate the singular and non-singular terms of the William series, i.e. Stress intensity factor (SIF) and T-stresses namely T11, T13 and T33. Based on the obtained numerical results, a fitting procedure is performed in order to propose analytical formulations giving the fracture parameters near the crack tip. The obtained results are in good agreement with the finite elements calculation and other literature results

Influence of corrosion and creep on intergranular fatigue crack path in 2XXX aluminium alloys

In this paper, two examples of the influence of time-dependent processes on crack path in two 2XXX aluminium alloys are presented. The first example is concerned with corrosion–fatigue crack growth resistance of a 2024 T351 alloy cracked in the S–L direction in 3.5% NaCl solution at free corrosion potential. The second example deals with the elevated temperature crack growth resistance of a 2650 T6 alloy that might be used in future supersonic aircraft fuselage panels. The common idea is to correlate quantitative measurements of relevant fractographic features of crack path to the effects of time-dependent processes on crack growth rates

Investigating the micromechanisms of mode II delamination in composite laminates

The use of composite materials in primary aerospace structures is continually increasing due to their high stiffness and high strength to weight ratios. However, composite laminates are susceptible to delamination which can lead to the loss of global stiffness and potentially to catastrophic failure. Delamination is therefore a critical damage mechanism to account for when determining the durability and damage tolerance of composite materials. A large amount of research has already been carried out to investigate delamination and a number of failure criteria have been developed to predict delamination growth. However, the majority of these criteria are not based on the physical mechanisms that cause interlaminar fracture. Thus to be able to confidently predict delamination growth in real world applications a physically based failure criteria would be more appropriate. The key to developing physically based criteria is through fractographic observations to determine the dominant failure mechanisms of delamination. Thus the purpose of this research was to characterise the failure process of mode II shear fracture, and in particular to determine the formation of cusp features prevalent to mode II dominated fracture. A polyvinylchloride foam was used to macro-simulate mode II interlaminar failure of composite laminates. The in-situ fracture process was characterised successfully and the energy absorbed during cusp formation was determined. Results were compared with cusp formation in two carbon-fibre/epoxy laminates and the post fracture morphology of all the materials was similar. In addition the influence of parameters such as, material type, inter-fibre spacing, fibre diameter and ply orientation on the frequency, morphology and size of cusp formation were also investigated. Results indicated that cusp formation was via the initiation, saturation and propagation of tensile microcracks and that neither the cusp formation process nor the energy associated with crack formation was found to be influenced by the parameters studied