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Applied Composite Materials: An International Journal for the Science and Application of Composite Materials (v.9, #2)
Characterisation of Transverse Cracking in a Quasi-Isotropic GFRP Laminate under Flexural Loading by Keiji Ogi; P. A. Smith (pp. 63-79).
Transverse cracking behaviour in a quasi-isotropic glass/epoxy (GFRP) laminate loaded in flexure is studied experimentally and theoretically. A theory developed for cross-ply laminates is applied to a [0°/90°/−45°/45°] S quasi-isotropic laminate. An equivalent laminate is introduced to derive the Young's modulus of a cracked transverse ply on the basis of a shear lag analysis. The model predicts the flexural stiffness, the neutral axis position and the residual curvature as a function of the transverse crack density and the in-situ ply stress at first ply failure. Experimental results are obtained with the use of the applied moment – strain data in four-point flexural tests and compared with predictions. Time-dependent behaviour of the residual curvature is also investigated.The theoretical predictions are in reasonably good agreement with the experimental results. It is found that the decrease in the residual curvature after unloading is mainly ascribed to viscoelasticity of the material.
Keywords: transverse cracking; flexure; shear lag; quasi-isotropic laminate; stiffness; residual curvature
Mechanical Properties of Stitched Multiaxial Fabric Reinforced Composites From Mannual Layup Process by Youjiang Wang (pp. 81-97).
This paper summarizes the consolidation behavior of E-glass stitched multiaxial non-crimp fabrics (NCFs) and the mechanical properties of the resulting composites from the manual layup process. NCFs offer many advantages over conventional woven and nonwoven fabrics in terms of processibility and properties. The reinforcement structures studied include six NCFs with different fiber orientation combination, number of sub-ply layers, and unit weight. The compressive and recovery behavior of the dry preforms is evaluated and related to their processibility in the hand lay-up process. Mechanical properties of specimens from the hand layup process are evaluated in tensile, compressive, and flexural tests. The test results and failure mechanisms are reported. The effect of consolidation quality on the mechanical properties is discussed.
Keywords: failure; manual layup; mechanical properties; multiaxial warp knitted fabric; NCF; non-crimp fabric; stitched multiaxial fabric; testing; textile preform
Effect of Temperature on the Tensile Strength and Failure Modes of Angle Ply Aramid Fibre (KRP) Tubes Under Hoop Loading by H. Haftchenari; F. A. R. Al-Salehi; S. T. S. Al-Hassani; M. J. Hinton (pp. 99-115).
A comprehensive study was undertaken to characterise Kevlar reinforced plastic (KRP) angle ply filament wound tubes at different temperatures. Quasi-static burst tests were performed on tubes of 25°, 55° and 75° winding angle. The tubes were burst under internal radial pressure with minimum end constraints. An experimental rig and two conditioning tanks were designed and built to test the specimens at three temperatures; −46°C (low temperature) and +20°C (room temperature) and +70°C (high temperature). For each test the internal pressure and the strains in both circumferential and longitudinal directions were recorded on suitable digital processing equipment.For a particular batch of tubes tested at three different temperatures, an increase in ultimate hoop strain and a decrease in hoop modulus of the 55° tubes with increasing temperatures was recorded; the temperature effect was less pronounced on the corresponding properties of 25° and 75° tubes. The use of a non-structural thin liner during the tests led to a higher ultimate strength of 55° tubes but had negligible effect on the behaviour of 25° and 75° tubes. The 75° tubes failed in a catastrophic fibre fracture under all test conditions. The mode of failure of 55° changed from weeping at 70°C to fibre fracture at −46°C. The 25° tubes failed by weeping with matrix cracking. The matrix cracking was particularly severe when a liner was used.
Keywords: KRP tubes; modes of failure; tensile strength; static burst strength; temperature effects; lined tests
Mode II Interlaminar Fracture of Filament Wound Angle-ply Specimens by A. B. de Morais; J. F. Silva; A. T. Marques; P. T. de Castro (pp. 117-129).
This paper presents a study of the interlaminar fracture of filament wound composites. Mode II end notched flexure (ENF) tests were performed on flat glass/polyester specimens. The tested specimens had asymmetric [±Θ]4 angle-ply stacking-sequences, with Θ values from 0.8° (hoop winding) to 30°. Due to the low stiffness and probable high toughness, it was not possible to propagate the crack in [±60°]4 specimens before they suffered considerable permanent deformations. A short support span had to be employed for [±30°]4 specimens in order to obtain crack propagation. Nevertheless, the results for those specimens should be viewed with caution, as some nonlinear behaviour and small permanent deformations were detected. No other unusual features, such as crack jumping to other interface, were observed. The scatter in the critical strain energy release rate values (G IIc ) was higher for [±10°]4 and [±30°]4 specimens than for the quasi-unidirectional ones. G IIc values from the insert were generally lower than those from mode II pre-cracks, except for quasi-unidirectional specimens. A plot of average G IIc values against Θ showed a minimum at Θ=5°.
Keywords: interlaminar fracture; filament wound composites; glass/polyester composites; angle-plies; mode II; end notched flexure (ENF) tests; permanent deformations