No abstract

The eVect of specimen gauge section ( lengthÖ width) was investigated on the compressive behaviour of a T300/924C [45/ 45/0/90] 3 s carbon bre-epoxy laminate. A modi ed Imperial College compression test xture was used together with an antibuckling device to test 3 mm thick specimens with a 30Ö 30, 50Ö 50, 70Ö 70, and 90Ö 90 mm gauge length by width section. In all cases failure was sudden and occurred mainly within the gauge length. Post-failure examination suggests that 0° bre microbuckling is the critical damage mechanism that causes nal failure. This is a matrix dominated failure mode and its triggering depends very much on initial bre waviness. It is suggested that manufacturing plays a signi cant role in determining the compressive strength and may be more important as the section thickness of the composite increases. Additionally, compressive tests on specimens with an open hole are performed. The local stress concentration arising from the hole dominates the strength of the laminate rather than the stresses in the bulk of the material. It is observed that the remote failure stress decreases with increasing hole size and specimen width but is generally well above the value one might predict from the elastic stress concentration factor. This suggests that the material is not ideally brittle and some stress relief occurs around the hole. X-ray radiography reveals that damage in the form of bre microbuckling and delamination initiates at the edge of the hole at~80% of the failure load and extends stably under increasing load before becoming unstable at a critical length of 2-3 mm (depending on specimen geometry). This damage growth and failure are analysed by a linear cohesive zone model. Using the independently measured laminate parameters of compressive unnotched strength and in plane fracture toughness the model predicts successfully the notched strength as a function of hole size and width.PRC/1855

In this paper, the failure mode of laser welds in lap‐shear specimens of non‐galvanized SAE J2340 300Y high strength low alloy steel sheets under quasi‐static loading conditions is examined based on experimental observations and finite element analyses. Laser welded lap‐shear specimens with reduced cross sections were made. Optical micrographs of the cross sections of the welds in the specimens before and after tests are examined to understand the microstructure and failure mode of the welds. Micro‐hardness tests were also conducted to provide an assessment of the mechanical properties in the base metal, heat‐affected and fusion zones. The micrographs indicate that the weld failure appears to be initiated from the base metal near the boundary of the base metal and the heat‐affected zone at a distance away from the pre‐existing crack tip, and the specimens fail due to the necking/shear of the lower left load carrying sheets. Finite element analyses based on non‐homogenous multi‐zone material models were conducted to model the ductile necking/shear failure and to obtain the J integral solutions for the pre‐existing cracks. The results of the finite element analyses are used to explain the ductile failure initiation sites and the necking/shear of the lower left load carrying sheets. The J integral solutions obtained from the finite element analyses based on the 3‐zone finite element model indicate that the J integral for the pre‐existing cracks at the failure loads are low compared to the fracture toughness and the specimens should fail in a plastic collapse or necking/shear mode. The effects of the sheet thickness on the failure mode were then investigated for laser welds with a fixed ratio of the weld width to the thickness. For the given non‐homogenous material model, the J integral solutions appear to be scaled by the sheet thickness. With consideration of the plastic collapse failure mode and fracture initiation failure mode, a critical thickness can be obtained for the transition of the plastic collapse or necking/shear failure mode to the fracture initiation failure mode. Finally, the failure load is expressed as a function of the sheet thickness according to the governing equations based on the two failure modes. The results demonstrate that the failure mode of welds of thin sheets depends on the sheet thickness, ductility of the base metal and fracture toughness of the heat‐affected zone. Therefore, failure criteria based on either the plastic collapse failure mode or the fracture initiation failure mode should be used cautiously for welds of thin sheets.