Effects of stitching on delamination of satin weave carbon-epoxy laminates under mode I, mode II and mixed-mode I/II loadings

The objective of the present study is to characterize the effect of modified chain stitching on the delamination growth under mixed-mode I/II loading conditions. Delamination toughness under mode I is experimentally determined, for unstitched and stitched laminates, by using untabbed and tabbed double cantilever beam (TDCB) tests. The effect of the reinforcing tabs on mode I toughness is investigated. Stitching improves the energy release rate (ERR) up to 4 times in mode I. Mode II delamination toughness is evaluated in end-notched flexure (ENF) tests. Different geometries of stitched specimens are tested. Crack propagation occurs without any failure of stitching yarns. The final crack length attains the mid-span or it stops before and the specimen breaks in bending. The ERR is initially low and gradually increases with crack length to very high values. The mixedmode delamination behaviour is investigated using a mixed-mode bending (MMB) test. For unstitched specimens, a simple mixed-mode criterion is identified. For stitched specimens, stitching yarns do not break during 25% of mode I ratio tests and the ERR increase is relatively small compared to unstitched values. For 70% and 50% of mode I ratios, failures of yarns are observed during crack propagation and tests are able to capture correctly the effect of the stitching: it clearly improves the ERR for these two mixed modes, as much as threefold

Blast resilience of composite sandwich panels with hybrid glass-fibre and carbon-fibre skins

The development of composite materials through hybridisation is receiving a lot of interest; due to the multiple benefits, this may bring to many industries. These benefits include decreased brittle behaviour, which is an inherent weakness for composite materials, and the enhancement of mechanical properties due to the hybrid effect, such as tensile and flexural strength. The effect of implementing hybrid composites as skins on composite sandwich panels is not well understood under high strain rate loading, including blast loading. This paper investigates the blast resilience of two types of hybrid composite sandwich panel against a full-scale explosive charge. Two hybrid composite sandwich panels were mounted at a 15 m stand-off distance from a 100 kg nitromethane charge. The samples were designed to reveal whether the fabric layup order of the skins influences blast response. Deflection of the sandwich panels was recorded using high-speed 3D digital image correlation (DIC) during the blast. It was concluded that the combination of glass-fibre reinforced polymer (GFRP) and carbon-fibre reinforced polymer (CFRP) layers in hybrid laminate skins of sandwich panels decreases the normalised deflection compared to both GFRP and CFRP panels by up to 41 and 23%, respectively. The position of the glass-fibre and carbon-fibre layers does not appear to affect the sandwich panel deflection and strain. A finite element model has successfully been developed to predict the elastic response of a hybrid panel under air blast loading. The difference between the maximum central displacement of the experimental data and numerical simulation was ca. 5% for the hybrid panel evaluated

Cohesive zone modeling for adhesives

Adhesives are very widely used in industry. In each application field, the adhesive that is used must fulfill specific requirements. Adhesive types can be classified for instance by their (thermo-) mechanical properties, their machining or their curing conditions. This paper describes, by way of example, the characterization criteria for structural and flexible adhesives with respect to differences in their mechanical properties under various test conditions such as loading rate or environmental temperature. For further increased industrial application of adhesives, for example to improve the crash performance of cars, the ability to predict the mechanical behavior by numerical simulation is required. Cohesive Zone Models (CZMs) are well suited for modeling adhesives. In this paper a tri-linear, strain-rate dependent CZM is presented. This model is compared to the bi-linear, strain-rate independent model implemented in ABAQUSTM. The parameters of these models are determined by direct testing of tensile bulk, tapered double cantilever beam, lap-shear and T-peel specimens. The model validation was carried out by comparing experiment results and simulations for a Ushaped specimen under different loading velocities. The application of these CZMs in offset crash test simulations is presented and compared to experimental data

A numerical approach to the disbonding mechanism of adhesive joints

The need for lightweight structures in aeronautics is leading to a strong interest in adhesively bonded joints. Incomplete knowledge of their fatigue behaviour is a major obstacle to their application. At present, the prediction of the disbonding growth is yet an open question. This work aims to develope a numerical model for the computation of the disbonding growth in an adhesive joint. The scope is calculating the energy release under quasi-static conditions in order to relate it to the fatigue disbond growth through the existing analytical models. A finite element model for the prediction of disbond growth under quasi-static loading has been implemented in Abaqus, by introducing a cohesive zone model which is able to capture the process zone around the crack tip and to enforce an energy-based failure criterion. The model, which had originally been developed for double cantilever beam specimens under mode I, was extended to mode II loading. Numerical simulations are validated by comparison with experimental results on double cantilever beam coupons in mode I and with literature results on end notched flexure coupons in mode II conditions. The results from tests and simulations are in accordance with each other. The presented model is a suitable option for the estimation of fracture mechanics parameters in cases in which complex geometry and loads prevent the application of analytical theories