Papers by Author: Ian A. Ashcroft

Abstract: Advanced polymeric materials and polymer based nanocomposites are finding an increasing range of industrial and defence applications. These materials have the potential to improve combat survivability, whilst reducing cost and weight. This study deals with nanocomposites manufactured from blends of low density polyethylene (LDPE) with various nanofillers. The high strain rate behaviour of these materials was investigated using the split Hopkinson pressure bar (SHPB) test. The experimental results for non-reinforced materials were used as a reference to analyse the effect of the nanofillers on the properties and performance of the nano­composites. These results, together with those obtained from other mechanical tests, will be used as input into finite-element analyses to simulate the performance of these materials in lightweight armour applications. In the first step, the finite element model was validated by simulating the SHPB test and comparing the predicted results with those from the experiments. Explicit finite element analysis was used for the simulation. The fully developed model was able to demonstrate the behaviour of the test bar and specimen interaction correctly and reasonably good agreement between predicted and experimental results was observed.

Abstract: The use of structural adhesive joints to join carbon fibre reinforced polymer (CFRP) adherends is now well established in the aerospace industry. These joints are subjected to varied load spectra, of which one of the most damaging forms of loading is fatigue with intermittent low energy impacts, which is termed combined standard and impact fatigue (CISF) in this paper. It is seen that the rate of crack growth in impact fatigue is greater than that in standard fatigue for a given value of the strain energy release rate, moreover, it is seen that the fatigue crack growth rate (FCGR) in standard fatigue (SF) increases after a block of impact fatigue. In this paper a model is proposed to predict crack growth in bonded joints subjected to CISF. The model is based on numerical crack growth integration (NCGI) with a method of accounting for the accelerated crack growth in SF following IF. The model was seen to provide a good prediction of the fatigue crack growth in CISF.

Abstract: Fibre reinforced polymer composites (FRPCs) are being increasingly used in structural applications where high specific strength and stiffness are required. The performance of FRPCs is affected by multi-mechanism damage evolution under loading which in turn is affected by microstructural stochasticity in the material. This means that the fracture of a FRPC is a stochastic process. However, to date most analyses of these materials have treated them in a deterministic way. In this paper the effect of stochasticity in FRPCs is investigated through the application of cohesive zone elements in which random properties are introduced. These may be termed ‘stochastic cohesive zone elements’ and are used in this paper to investigate the effect of microstructural randomness on the fracture behaviour of cross-ply laminate specimens loaded in tension. It is seen from this investigation that microstructure can significantly affect the macroscopic response of FRPC’s, emphasizing the need to account for microstructural randomness in order to make accurate prediction of the performance of laminated composite structures.

Abstract: Steel catenary risers (SCR) are used in deepwater oil and gas developments to transfer produced fluids from the seabed to surface facilities. SCRs can be subject to fatigue loading from a variety of sources including wave and tidal motion, vortex induced vibration (VIV) and operating loads. When the produced fluids are sour (ie contain water and H2S) higher fatigue crack growth rates (FCGR) are expected, and this can have a significant effect on defect tolerance. The aim of this paper is to provide guidance on the current best practice methods for performing engineering critical assessments (ECA) on internal surface breaking defects in SCRs operating in a sour environment and subject to VIV fatigue loads. Example ECA calculations are presented for circumferential girth weld flaws, based on the failure assessment diagram (FAD) approach within the framework of BS 7910 [1]. The influence of certain key input variables is demonstrated, including the FCGR, determined from recent sour test data generated as part of this research.

Abstract: Various aspects of the effect of microstructural randomness exhibited by carbon fibre-reinforced cross-ply laminates on the delamination damage mechanism is investigated in this paper. In the first part, the matrix cracks with different spacings measured in experiments are simulated using finite elements in order to obtain the levels of degradation and effective properties for a composite beam loaded in bending. The results show significant levels of degradation of obtained effective properties depicting the importance of accounting for the inherent stochasticity in these laminates. In the second part of the paper, initiation of delamination at an interface between 0° and 90° layers due to stress concentrations at tips matrix cracks is simulated for a beam under tension. Stochastic cohesive zone elements with fracture parameters presented as random fields are used to model this interface in a composite. Different values of the axial stress are obtained for initiation of damage for a number of realisations based on this approach. The results emphasize the need to take into consideration the microstructural randomness in fibre-reinforced laminates for adequate predictions of damage and load carrying capacities.

Abstract: The paper presents results of studies into the effect of repetitive low-energy impacting (known as impact fatigue) on reliability and crack growth in adhesively bonded joints. This type of loading is compared to the standard tensile fatigue in order to assess severity of such loading regime. Another loading type studied is a combination of a small portion of repetitive impacts with tensile fatigue. Crack propagation in a joint exposed to these types of loading is studied experimentally and numerically (with finite elements). This analysis is accompanied by microstructural studies of various damage processes, active at different stages of the crack growth process.

Abstract: Triaxiality function (Rv) has been known as one of the important factors that responsible for damage initiation in adhesive bonding. Damage evolution law for low cycle fatigue (LCF) is function of Rv, von Mises equivalent stress (Seqv) and number of cycles (N). From previous research, it was found that the Rv values of two cases: bulk adhesives and single lap joint (SLJ), were close to unity. Those values are uncontrollable. Meanwhile, the damage equation for general solution contains Rv as an independent variable. There is need to choose another joint type that can characterise Rv as an independent variable. This paper presents the choice of scarf joint as specimen that can simulate variation of Rv. Several types of adhesive joints have been modelled and analysed using ANSYS as finite element analysis (FEA) tool. In ANSYS, Rv values were calculated directly from direct output results: von Misses equivalent stress and Hydrostatic stress. From FEA, it was shown that Rv changed as a function of adhesive bondline angle of the scarf joint. The values of Rv are constant along adhesive line except at the free edges. This choice is better than Cleavage joint where the values of Rv are not constant along adhesive line due to the presence of bending moment.

Abstract: Adhesive in joints will have complex stress state rather than bulk adhesives. This will lead to the assumption behind bulk adhesive that triaxiality function (Rv) is equal to one (uni-axial stress state) is not valid anymore. In this paper, new procedure to find damage parameters α and β for single-lap joints has been developed based on global damage of adhesive joints. With this procedure, damage parameters α and β have been found. Validating the procedure by calculating the number of cycles to failure (Nf) has been performed successfully. The accuracy of the damage evolution equation is less than 2 %.

Abstract: In recent decades the use of structural adhesive joints in the aerospace industry has increased considerably thanks to their high strength-to-weight ratio, low stress concentration and capacity to join different adherends. There is increasing interest in damage due to low-velocity impacts produced in adhesively bonded components and structures by vibrating loads. This type of loading is known as impact fatigue. The main aim of this paper is to investigate damage evolution in adhesive joints subjected to impact-fatigue and to compare this with damage evolution in standard fatigue (i.e. non-impacting, constant amplitude, sinusoidal fatigue). In this work, adhesively bonded lap joints were subjected to multiple tensile impacts tensile and it was seen that this type of loading was extremely damaging compared to standard fatigue. A number of methods of studying damage evolution in bonded joints subjected to fatigue and impact fatigue loading have been investigated and various parameters have been used to characterise these processes. Two modifications of the accumulated time-stress model [1-4] are proposed and it is shown that both models provide a suitable characterization of impact-fatigue in bonded joints.

Abstract: This paper describes a dynamic test carried out on intact and damaged FRP composite beams with fixed-fixed boundary condition. Hammer excitation is used to excite the beam at fixed locations. The modal parameters are extracted from the time response using a time domain analysis, i.e. the stochastic subspace identification technique. In order to introduce damage, two sections of the beam are bonded together using an epoxy adhesive, and then a static test is carried out. For the static test, a 3-point bending -configuration is used, i.e. the beam is fixed at both ends and a static load is gradually applied in the middle of the beam using a screw jack. Different static load steps, and in turn different damage stages, are considered. After each load step, dynamic measurements are carried out. The results obtained from both tests are presented and analysed.