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Applied Composite Materials: An International Journal for the Science and Application of Composite Materials (v.5, #5)
Failure of Laminated Composites at Thickness Discontinuities – an Experimental and Analytical Study by Sangwook Lee; Wolfgang G. Knauss (pp. 273-287).
Failure initiation of laminated composites at a thickness discontinuity is studied experimentally with the aid of an optical microscope under combined loading of tension, transverse shear and bending, making use of three- and four-point bending arrangements. Because transverse shear produced relatively small effects in failure initiation, results are presented as tension-bending interactions. Two loading frames for three- and four-point bending were designed to apply moment and tension simultaneously to produce failure by generating a ply crack; this initiation was evaluated by finite element analysis using ABAQUS. For cross-plies bounding the interface at the base of the step it is found that a maximum strain criterion applied to the continuing (long) ply describes the failure initiation. Ultimate failure resulted at loads on the order of 25 to 35% than those at failure initiation.
Keywords: composite; failure; failure initiation; thickness discontinuity; combined loading
An Assessment of the Double-Notch Shear Test for Interlaminar Shear Characterization of a Unidirectional Graphite/Epoxy under Static and Fatigue Loading by Mahmood M. Shokrieh; Larry B. Lessard (pp. 289-304).
This research explores a modified testing technique for measuring interlaminar shear properties of orthotropic composite materials. An existing test method (double-notched test method) is examined here to characterize the interlaminar shear properties (strength and fatigue life) of a unidirectional ply under both static and fatigue loading conditions. No complicated fixture is required for the testing method which is beneficial for fatigue testing of the materials. The testing method is verified by a finite element technique where an optimized geometry for the specimen is found. AS4/3501-6 graphite/epoxy material is used in this study. The experimental results show that final failure occurs in the gage area for both static and fatigue loading conditions. Moreover, a reasonable amount of scatter for both the static strength and fatigue life is achieved.
Keywords: composites; interlaminar shear test method; static; fatigue; characterization; double-notched test
A Study of Structure Formation and Failure of Short Fibre Composites with Metal and Ceramic Matrices by N. S. Sarkissyan; V. M. Kiiko; A. A. Khvostunkov; S. T. Mileiko (pp. 305-317).
An experimental study of the fracture behaviour of metal- and ceramic-based composites containing randomly oriented short fibres, related to fabrication parameters of those composites produced by a powder metallurgy method, yields a suggestion to describe the failure of such composites with a multi-scale model. A most important feature of the failure behaviour is an occurrence of the maximum strength of a composite at rather low values of the fibre volume fraction.A multi-scale model is built of a non-homogeneous body which accounts for changes in the structure of the body during its formation and fracture. Despite the simplicity of the model it supplies an adequate description of the macroscopic behaviour of composites. In particular, the non-monotonic relationships between the composite strength and the fabrication parameters can be understood, and therefore the possibility to optimize a composite fabrication process becomes visible.The micro-level of the model is related to the fibre/matrix interactions in a composite. The meso-level of the model describes both formation of a structure on the many-fibres agglomerate scale when obtaining a composite body and its fracture on loading of the body.
Keywords: metal matrix composites; ceramic matrix composites; short fibres; fracture behaviour; powder metallurgy; modeling; multi-scale model
Effect of Temperature on the Tensile Strength and Failure Modes of Angle Ply CFRP Tubes under Hoop Loading by F. A. R. Al-Salehi; S. T. S. Al-Hassani; H. Haftchenari; M. J. Hinton (pp. 319-343).
A comprehensive study was undertaken to characterise carbon fibre reinforced plastic (CFRP) 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), +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 using a digital processing equipment.For a particular batch of tubes, tested at three different temperatures, a decrease in hoop strength and modulus of the 55° tubes with increasing temperature was recorded; the effect was less pronounced on the properties of 25° and 75° tubes. The use of a non-structural liner during the tests led to higher ultimate strength and strain of 55° tubes but had negligible effects on the behaviour of 75° tubes. The use of a liner in 25° tubes altered the mode of failure, resulting in a very large tube deformation with no noticeable increase in burst pressure. Micrographic analysis was also undertaken to study the failure mechanisms during pressurisation of lined and unlined tubes.
Keywords: CFRP tubes; modes of failure; tensile strength; static burst strength; temperature effects; lined tests; fractography