S. T. S. Al-Hassani

Electrical resistance measurement technique for detecting failure in CFRP materials at high strain rates

Abstract A new electrical resistance technique is considered for detecting failure in balanced (± θs) angle-ply carbon fibre reinforced plastic materials (CFRP) under high strain-rate tensile testing. The electrical conductivities of balanced angle-ply laminates are measured for a number of angles and the results are compared with the conductivities of an off-axis lamina. The variation of electrical resistance of angle-ply CFRP tubes during expansion, when subjected to internal explosive loading pulses, is monitored. The change of the resistance of the tubes is compared with the deformations as measured by conventional strain-gauges. The method shows promise in detecting the instant of composite failure.

A method for investigating the mechanical properties of intracoronary stents using finite element numerical simulation

The proliferation of stent designs poses difficult problems to clinicians, who have to learn the relative merits of all stents to ensure optimal selection for each lesion, and also to regulatory authorities who have the dilemma of preventing the inappropriate marketing of substandard stents while not denying patients the benefits of advanced technology. Of the major factors influencing long-term results, those of patency and restenosis are being actively studied whereas the mechanical characteristics of devices influencing the technical results of stenting remain under-investigated. Each different stent design has its own particular features. A robust method for the independent objective comparison of the mechanical performance of each design is required. To do this by experimental measurement alone may be prohibitively expensive. A less costly option is to combine computer analysis, employing the standard numerical technique of the finite element method (FEM), with targeted experimental measurements of the specific mechanical behaviour of stents. In this paper the FEM technique is used to investigate the structural behaviour of two different stent geometries: Freedom stent geometry and Palmaz-Schatz (P-S) stent geometry. The effects of altering the stent geometry, the stent wire diameter and contact with (and material properties of) a hard eccentric intravascular lesion (simulating a calcified plaque) on stent mechanical performance were investigated. Increasing the wire diameter and the arterial elastic modulus by 150% results in the need to increase the balloon pressure to expand the stent by 10-fold. Increasing the number of circumferential convolutions increases the pressure required to initiate radial expansion of mounted stents. An incompressible plaque impinging on the mid portion of a stent causes a gross distortion of the Freedom stent and an hour-glass deformity in the P-S stent. These findings are of relevance for future comparative studies of the mechanical performance of stents, in designing newer stents and also in clinical practice.

Use of FEM in performance assessment of perforated plates subject to general loading conditions

Abstract Perforated plates, also called tube plates, are widely used in power generating plants and chemical processing refineries as components in heat exchangers and steam generators (particularly in nuclear power stations). The integrity of these plates under general (from hydrostatic pressure differentials) and other loading conditions (such as those generated by tubes flexing or the influence of constraints) is vital to the safety of plant. In this paper a general methodology is developed which is applicable to any regular perforation (or penetration) pattern. The solution procedure isolates a unit module from which, by successive reflections and translations, the whole plate can be constructed. The unit perforation module, composed of isotropic material, is replaced by a unit solid brick-shaped module of an equivalent anisotropic plate with identical overall dimensions. Then, by comparing the responses of the isotropic perforated and the anisotropic solid models, the equivalent “effective” elasticity constants E∗, v ∗ and G ∗ are calculated for the equivalent plate, which enable a nominal stress field, for any loading state, to be evaluated. A series of curves, referred to as stress multiplier factors, are also compiled which, together with the nominal stresses, allow the behaviour of the plate on the periphery of the perforation, the most severely stressed zone, to be estimated. The finite element method (FEM) involves the calculation of the elasticity matrix [K] (from {σ ∗ } = [K]{e ∗ } ) and then the evaluation of the anisotropic engineering constants-by the inversion of [K] into [φ]. Two particular penetration patterns (square and triangular) are examined but the method employed is general and can be readily extended to deal with other perforation patterns of different ligament efficiencies.

Residual Stress Assessment in Thin Angle Ply Tubes

This preliminary study aims to investigate the residual stresses developed in hot cured thin-walled angle-ply filament wound tubes made of E-glass/epoxy, Kevlar/epoxy and carbon/epoxy materials. The residual stresses were estimated from change in geometry of these tubes when axially slitted at ambient temperature. Three basic deformation modes; namely opening up, closing-in and twisting, were observed and these depended on the winding angle, material and wall thickness. The residual stresses were also determined from hoop and axial strain gauges mounted on both the inner and outer surfaces at various locations around the tube. The stresses were compared with theoretical prediction based upon a linear thermo-elastic analysis. Both the predicted and measured values were found to increase with increasing hoop stiffness but there was a large discrepancy between the predicted and measured data, reaching a factor of 5 for the thinnest case. When compared with predicted failure stresses, the experimentally determined stresses were some 15% of the computed compressive strength.

Effect of Temperature on the Tensile Strength and Failure Modes of Angle Ply Aramid Fibre (KRP) Tubes Under Hoop Loading

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.

Temperature and Rate Effects on GRP Tubes Under Tensile Hoop Loading

A comprehensive study was undertaken to characterise glass fibre reinforced plastic (GRP) tubes at different temperatures and strain rates. The tests were performed on tubes of 25°, 55° and 75° winding angle. The tubes were burst under internal radial pressure with minimum end constraints. Two separate rigs were used, one for the static and the other for the dynamic tests. The tests were carried out 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 on suitable digital processing equipment. For a particular batch of tubes tested at three different temperatures, there is in general a decrease in hoop strength with increasing temperature during quasi-static tests. The use of a non-structural liner during such tests led to an increase in ultimate hoop strain of 55° tubes, especially at high temperature. The corresponding increase in ultimate hoop strain was markedly less in the case of 75° and almost negligible in the case of 25° tubes. Testing the tubes at high strain rates resulted in substantial increases in burst strength and ultimate hoop strain as compared with the quasi-static and low strain rate values. The mode of failure of 75° tube is a catastrophic fibre breakage under all test conditions. The mode of failure of 55° tube is a combination of weeping and fibre failure. The 25° tubes are characterised by matrix failure, which is very severe at high strain rates.

Burst behaviour of ±75 ° filament-wound Kevlar/epoxy and carbon/epoxy tubes at high loading rates

Abstract Burst tests have been carried out on thin filament-wound angle-ply ±75 ° Kevlar-49/epoxy (KRP) and ±75 ° XAS carbon/epoxy (CFRP) tubes at strain rates up to 200/s by means of an internal explosive pressurisation technique. To overcome the limitations of premature failure of foil strain gauges, a wrap-wire gauge was developed which enabled instantaneous measurement of hoop expansion up to the final burst. Full hoop-stress/strain curves were obtained up to the final failure. For tests carried out at strain rates above 10/s, the dynamic response of both materials was characterised by surface cracking or ‘initial failure’, prior to final burst. Surface cracking of the specimens causes degradation of tube stiffness as well as failure of conventional type foil strain gauges. Effects of strain rate on the mechanical properties at initial and final failures are presented.

Effect of Temperature on the Tensile Strength and Failure Modes of Angle Ply CFRP Tubes under Hoop Loading

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.

FE investigation of a spirally slotted tube under axially compressive static and dynamic impact loading

In this study, helically gouged Steel tubes of relatively small thickness are discussed as a new design configuration for energy absorption, using the results of numerical simulations. The tubes are loaded in two different modes: (1) Static compressive axial loading applied via a displacement and the kinematic coupling option of the FE code, which constrains the nodes at each end of the tube to the respective reference nodes. (2) Dynamic axial impact loading via a mass element attached to the reference node of the rigid base platen which is energised by an initial velocity while appropriately maintaining its load sustaining resistance. One of the salient features of the deformation mechanism is that in both cases the axial shortening of the tube is accompanied by twisting. The boundary constraints of the reference nodes at both the loaded and supported ends are of significant importance and care has been exercised to specify the appropriate degrees of freedom for movement in both rotation and translation. The contact that occurs between various parts of the tube has been correctly identified and targeted. In the present study attention has been focused on the energy storage capacity of the model under both loading modes and in providing an explanation for the major differences in response that exist between the two cases. The material properties of steel are specified as linear elastic followed by non-linear work hardening in plastic range with moderate and high degree of sensitivity to strain rate effect. The solution reveals several important features, which are discussed in this paper. The proposed device could be used in clusters to limit damage in the event of cable failure in hoist & lift compartments. Although the procedure is applied to axial loading, the method is equally applicable to lateral static and impact loading as cited in the introduction.

Simultaneous determination of in-plane shear and transverse moduli of unidirectional composite laminae at different strain rates and temperatures

Abstract A semi-empirical method is proposed for the extraction, simultaneously, of the transverse tensile and in-plane shear moduli of unidirectional laminae, at various strain rates and temperatures, from tests on symmetric and balanced ±65 ° angle-ply composite laminates. The extraction method is applied to data obtained from tests on Kevlar-49/epoxy and carbon/ epoxy filament-wound tubes which were subjected to internal pressure loading at three key temperatures of −45, 20 and 70 °C at different strain rates of up to 80/s. The combined effect of strain rate and temperature on these extracted properties is studied by applying strain rate temperature equivalence principles. It is found that the variation of the mechanical properties of the two materials with strain rate and temperature can be adequately described by semi-empirical equations similar to the Arrhenius and Williams-Landel-Ferry relationships, usually used for homogeneous solids.