Showing papers by "Michael R Wisnom published in 1996"
TL;DR: In this article, discrete voids of different sizes were simulated by embedding PTFE monofilaments, tubes and strips at the midplane of unidirectional glass fibre and carbon-fibre/epoxy plates.
149 citations
TL;DR: In this article, the average interlaminar tensile strength dropped from 109 MPa for the smallest specimens to 61 MPA for the largest, whilst the interlaminear shear strength was reduced from 97 MPa to 85 MPa.
Abstract: Curved beams tested in four point bending were used to measure interlaminar tensile strength, and straight beams loaded in three point bending to measure interlaminar shear strength. Results from specimens of 16, 32 and 64 plies with all dimensions scaled showed significant size effects. The average interlaminar tensile strength dropped from 109 MPa for the smallest specimens to 61 MPa for the largest, whilst the interlaminar shear strength was reduced from 97 MPa to 85 MPa.
58 citations
TL;DR: In this article, a finite element micromechanical modeling of SCS-6 SiC fiber reinforced with Ti-6A1-4V has been performed, where the effects of the low stiffness and highly anisotropic coating have been ignored in most other studies.
Abstract: Finite element micromechanical modelling of SCS-6 SiC fibre reinforced Ti-6A1-4V has been performed. The fibres have a relatively thick coating consisting of pyrolytic carbon and SiC. The mechanical and thermal properties are estimated by considering both components, among which the properties of pyrolytic carbon are highly anisotropic. The effects of the low stiffness and highly anisotropic coating have been ignored in most other studies on the same material. However, it is shown that the coating has a significant effect on the composite thermal residual stresses, elastic transverse tensile properties and interface debonding. Methods of estimating the interface tensile strength are proposed. The transverse loading-unloading-reloading behaviour of this material is modelled and compared with experimental results. Possible ways of improving the transverse properties of SCS-6/Ti-6-4 unidirectional composite are suggested.
35 citations
TL;DR: Good predictions of delamination load were obtained with an equation previously developed for symmetric specimens based on the strain energy release rate associated with the discontinuous plies, with the asymmetry of the specimens having only an indirect effect.
33 citations
TL;DR: In this paper, the effect of anticlastic curvature in pure bending of curved composite beams is investigated by finite element analysis, and a full 3D model of a slice through the beam is analysed, and also a 2D model assuming generalised plane strain.
Abstract: The effect of anticlastic curvature in pure bending of curved composite beams is investigated by finite element analysis. A full 3-D model of a slice through the beam is analysed, and also a 2-D model assuming generalised plane strain. It is shown that due to the curvature, there is restraint on anticlastic bending, giving rise to a significant variation of stresses across the width. The commonly assumed state of plane stress is therefore not strictly valid even for relatively narrow specimens. There can be significant discrepancies in fibre direction stresses and strains compared with those calculated on the basis of plane stress. As the width increases, the results at the centre of the beam approach those from the generalised plane strain analysis. This gives the same values of interlaminar tensile stresses as from the plane stress solution, but there are still differences in fibre direction strains. The results show the need for caution in interpreting strain gauge results on curved beams.
32 citations
TL;DR: In this paper, two panels of unidirectional T800/924 carbon fiber-epoxy were fabricated with a kink running across the width to produce fibre waviness.
Abstract: Two panels of unidirectional T800/924 carbon fibre-epoxy were fabricated with a kink running across the width to produce fibre waviness. One panel had maximum fibre misalignment varying through the thickness from about 1.7° to 3.9° with a wavelength of about 6.0 mm, and the other varied from 2.2 to 5.6° with a wavelength of about 6.6 mm. Specimens loaded in compression showed increasing bending followed by catastrophic failure at average stresses of 1085 and 840 MPa. This is well below the compressive strength of 1816 MPa reported elsewhere for nominally straight specimens, confirming the critical effect of fibre waviness. The stress-strain response of the specimens was predicted well from finite element analysis accounting for large fibre rotations and non-linear shear properties. Failure was predicted at stresses 5.6% and 8.0% below the measured values. The model showed that the failure was precipitated by shear instability rather than classical compressive instability. The good correlation obtained validates the modelling approach and shows that it can be applied to predicting the effects of fibre waviness in composite components.
27 citations
TL;DR: In this article, a two-dimensional finite element model is used with linear elastic continuum elements to represent the plies, and non-linear springs to model the interfaces, and the effect of cracks of varying length and at different positions is investigated.
Abstract: Short-beam shear tests of unidirectional carbon fibre/epoxy are analysed assuming shear deformations to be concentrated at the resin-rich interfacial layers between plies. A two-dimensional finite element model is used with linear elastic continuum elements to represent the plies, and non-linear springs to model the interfaces. This approach is shown to be satisfactory provided that there is a sufficiently large number of plies. The effect of cracks of varying length and at different positions is investigated. Linear elastic strain energy release rate analysis indicates that there is not sufficient energy for small cracks to propagate. In contrast, the non-linear interface modelling approach predicts that small cracks can have a significant effect on interlaminar shear strength. Excellent correlation is obtained with experimental data, and it shown that the presence of small defects is a plausible explanation for the scatter in results commonly observed in short-beam shear tests.
24 citations
TL;DR: In this article, a test specimen in the form of a hoop with two 90° curved sections is described, and it is shown that when loaded in three point bending, both interlaminar tensile and shear stresses arise in the curved sections.
Abstract: A test specimen in the form of a hoop with two 90° curved sections is described. When loaded in three point bending, both interlaminar tensile and shear stresses arise in the curved sections. Maximum tensile and shear stresses occur at similar locations, allowing the combined effect of these stress components to be investigated. Changing the geometry of the specimen alters the ratio between interlaminar shear and tension. Experimental results are presented for two sets of unidirectional glass fibre/epoxy specimens. It is shown that it is possible for specimens subjected to combined interlaminar tension and shear to withstand higher stresses than when subjected to only tension. This is believed to be due to the smaller volume subjected to the maximum tensile stress as a result of the stress gradient along the length of the specimen. An approach is presented to account for this effect based on Weibull statistical theory and a quadratic interaction equation between interlaminar tension and shear. Using stresses from finite element analysis, satisfactory predictions of the strength of the hoop specimens can be made.
20 citations