Finite element modelling of damage development during longitudinal tensile loading of coated fibre composites

In this paper, results of finite element calculations of the stress state near a broken coated fibre in unidirectional carbon fibre-epoxy matrix composites are presented. For this aim, a simplified two-dimensional axisymmetric model has been developed. A part of the model contains an equivalent composite medium and, to this end, elastic constants of a coated fibre composite were separately calculated and used as input data in this analysis. The influences of coating thickness, coating stiffness and assumed crack pattern on the stresses in the broken fibre and in adjacent, unbroken fibres are evaluated, and their possible impact on composite longitudinal tensile strength is discussed. (C) 1998 Elsevier Science Ltd. All rights reserved.status: publishe

Variational approach to the stress-transfer problem through partially debonded interfaces in a three-phase composite

In our previous study (Wu W, Verpoest I, Varna J. Compos Sci Technol 1998;58(12):1 863-77) of the stress-transfer problem around a single fibre, we presented a variational approach based on the principle of minimum complementary energy, not only in the perfectly bonded zone but also in the zone with a discontinuous interface of a two-phase composite. This approach is adapted in this paper to derive an accurate axisymmetric analytical model for the description of the stress state around the fibre breaks and partially debonded interfaces of a three-phase composite with a fibre, coating and matrix. The debonded fibre/coating interface is treated as a special external boundary on which a presumed interfacial shear stress with some free parameters is specified. Once the parameters are given, minimisation of complementary energy can be applied for both debonded- and bonded-interface zones together, to extract the most accurate closed-form solution. The shear stress at the debonded interface (the right values of free parameters) is finally found by substituting the calculated radial stresses in the Coulomb friction law and minimising the discrepancy by a simple numerical iteration. As the minimisation procedure is applied for the both debonded and bonded zones simultaneously, the strong interaction of the two zones is correctly described. This model also includes the matrix axial stress nonuniformity in the radial direction. The stress profiles along both axial and radial directions are presented and closely compared with the results from a finite element model and both agree quite well. A number of applications of this model are also discussed. (C) 1999 Elsevier Science Ltd. All rights reserved.status: publishe

High strain monitoring during fatigue loading of thermoplastic composites using imbedded draw tower fibre bragg grating sensors

This paper presents the experimental study of fibre Bragg grating sensors for measuring strain inside composite laminates during fatigue loading. The optical fibres are imbedded inside thermoplastic CFRP test-coupons which have an ultimate strain of about 1.1%. Tension - tension fatigue cycling at a rate of 5Hz is been carried out at 314MPa with a maximum strain of 0.51%. At such extreme strain levels the use of high strength sensors becomes inevitable. Neither the sensor nor the composite test-coupons showed any significant degradation even after more than 500000 cycles. Fibre optic Bragg grating sensors are known to be very accurate strain sensors but one should be very careful interpreting their response once they are imbedded inside composite materials. In this study high strength fibre Bragg grating sensors with coating are imbedded in composite test coupons and a pretty good correlation was found between the strain measurements of an electrical extensometer and the imbedded sensor during the complete cycling. The high strength sensor show to be very feasible for extreme and long term strain measurements

Experimental evaluation of the interphase region in carbon fibre composites with plasma polymerised coatings

A simultaneous improvement of both the fracture toughness and strength of composite materials is very hard to realise. In this study, plasma polymerised HMDSO/O-2 coatings have been deposited onto carbon fibres to provide a distinct interphase and a means of achieving this aim. Utilising a microwave energy source, coatings of various thickness were examined. By carefully controlling the how rate of oxygen into the plasma, the resultant coating modulus could be selected within the range 0.7-2.0 of the resin modulus. Thus, the technique can be used to achieve interfacial interlayers with a range of stiffness. The micromechanics of interfacial failure were evaluated by the fragmentation test. Various fibre pretreatments (with O-2, N-2, Ar) and coating post-treatments (with O-2 and acrylic acid) were also employed. Oxygen plasma pretreatment was found to significantly improve the adhesion of the coating to the fibre. (C) 1998 Elsevier Science Limited. All rights reserved.status: publishe