Now showing 1 - 10 of 21
  • Publication
    Arbitrary crack propagation in multi-phase materials using the finite volume method
    An arbitrary crack propagation model using cell-centre nite volume based method is presented. Crack growth in an elastic solid, across an interface perpendicular to the initial crack path and into a second elastic solid is analysed. Crack initiation and the subsequent path of propagation are shown to arise naturally out of the selection of appropriate cohesive parameters. It is shown that the allowable crack propagation path is restricted by the underlying mesh. Results are presented for a number of values of interfacial strength and ratios of elastic properties between the two elastic solids. For higher values of interfacial strength, the crack is shown to propagate straight through the interface, while for lower values of interfacial strength, the crack is shown to change direction and propagate along the interface. It is shown that with careful selection of material and interface parameters it is possible to arrest a propagating crack at the interface. The method represents a useful step towards the prediction of crack propagation in complex structures.
      573Scopus© Citations 28
  • Publication
    A numerical investigation of spherical void growth in an elastic–plastic continuum
    Significant toughening of structural epoxy adhesives has been achieved with the addition of nano and micro-scale particles. However, the toughening mechanisms introduced by the addition of these particles is not very well understood. The ultimate aim of this research is to develop an understanding of the toughening mechanisms present and investigate the parameters which a ect the degree of toughening, i.e. particle size, particle volume fraction and particle distribution to guide future adhesives development. The current work examines the growth of a single void in an elastic-plastic material as a function of constraint and compares the results with the predictions of the classic Rice & Tracey model.
      321Scopus© Citations 3
  • Publication
    Evolution of dynamic fractures in PMMA : experimental and numerical investigations
    (WIT Press / Computational Mechanics, 2004-10-20) ; ; ; ;
    A combined experimental/numerical study has been conducted to investigate dynamic fractures in poly(methyl methacrylate) (PPMA). The results obtained from single-edge-notched-tensile (SENT) fracture tests support the idea that the evolution of fracture in PMMA is governed by nucleation, growth and coalescence of penny-shaped micro-cracks. The density of the microcracks and therefore the roughness of the fracture surface increase with the crack velocity. Both the surface roughness and the size of the process region increase with the crack length for a given specimen. Microscopy of the virgin material and fractured surfaces showed no consistent evidence of pre-existing flaws, dust particles or other impurities that would provide nucleation sites for the micro-cracks. Instead, it was observed that molecular weight significantly affects the fracture, and therefore must play an important role in the nucleation of micro-cracks. The crack velocity measurements show rapid initial crack acceleration followed by a nearly constant mean velocity, which was in some cases well above previously reported terminal crack speed. The mean velocity is found to increase with decreasing initial notch depth. Oscillations in the crack velocities were also observed and they were more pronounced at higher crack velocities. To a large extent, the degree of crack velocities oscillations is dependent on the filtering technique applied to process the raw experimental data. Therefore, no conclusive correlation between the fracture histories and fracture surfaces was obtained. Finite Volume (FV) method was developed for the numerical simulations of the experiments. Global material behaviour was approximated as linear elastic, while a Cohesive Zone Model (CZM) was used for defining the local separation process of the material. Numerical predictions show good agreement with experimentally observed variations of the process region and the crack velocity with initial crack length. Oscillations in the crack speed are also predicted.
      358
  • Publication
    The Influence of Plasma Surface Treatment on the Fracture Toughness Peel Ply Prepared Bonded Composite Joints
    The increasing use of composite materials in various industries, such as aerospace, automotive and renewable energy generation, has driven a need for a greater understanding of the fracture behaviour of bonded composite joints. An important prerequisite for the adhesive bonding of composites is the existence of a uniform surface free from contaminants and mould release agents. While there are several ways in which this may be achieved, the use of peel plies has emerged as the preferred choice for many industries due to the repeatable nature of the resulting surface, particularly in the highly regulated aerospace industry. The use of peel plies can present some problems. It is possible that contamination from the peel ply can be transferred to the composite substrate and adversely affect the adhesive joint [1]. Plasma treatments have been shown to improve the fracture toughness of adhesively bonded composite joints [2] and can be used to remove contaminants, such as mould release agents, from the surface [3]. The aim of this work is to evaluate the influence of various peel ply treatments on the mode I fracture toughness of different aerospace grade bonded composite joints and to assess the subsequent benefits of employing an atmospheric pressure plasma (APP) surface treatment prior to adhesive bonding in each case.
      183
  • Publication
    Mode I fracture toughness of co-cured and secondary bonded composite joints
    The mode I fracture toughness of a single co-cured and two secondary bonded joint systems were determined using the double cantilever beam test. The initiation values of fracture toughness from the PTFE film insert and a mode I crack-tip were considered as well as propagation values. It was found that the starting defect had a large influence on the initiation values for fracture toughness. It was also found that the two secondary bonded systems predominantly resulted in cohesive failure while the co-cured joiailed interfacially. Thermogravimetric analysis coupled with mass-spectrometry was used to show how moisture in the composite prepreg and adhesive affected the toughness of the joints. Microscopy methods were used to gain further insight into the damage mechanisms of the three joint systems.
      535Scopus© Citations 24
  • Publication
    Thermal shock resistance of polycrystalline cubic boron nitride
    The effect of thermal shock on the exural strength has been investigated experimentally. It was found that the variation in exural strength with quench temperature was influenced by the CBN grain size. Polycrystalline material containing small CBN grains showed a discontinuous drop in measured exural strength above a material dependent critical quench temperature difference, delta Tc. The sharp decrease in measured strength is accompanied by unstable crack propagation. Material containing a significantly larger CBN grain size, exhibited a gradual decrease in strength above the critical quench conditions. The experimental observations agreed with an established theory developed for thermal shock of alumina. The theoretically calculated critical temperatures agree well with the observed experimental data for each material when a aw size equal to the CBN grain size is employed.
      745Scopus© Citations 16
  • Publication
    The Effect of Prepeg Storage Humidity on Co-cured Composite Joints
    The increasing use of composite materials in the aerospace industry has driven a need for a greater understanding of bonded composite joints. There are generally two types of composite joint used in the aerospace industry; secondary bonded joints and cocured joints. Secondary bonded joints are produced by bonding two cured composite laminates together with an adhesive. However, when composites and adhesives are used to manufacture large parts in the aerospace industry, it is often convenient to co-cure the two materials at the same time. This helps to reduce the high costs associated with autoclave curing and also to reduce processing time. However, despite the apparent advantages, co-curing is not without its drawbacks. Any moisture stored in the composite material prior to co-curing is released during the cure cycle and has a negative effect on the joint. This can also result in interfacial failure. A way around this problem is to either dry the composite material prior to curing or to engineer the composite surface using a variety of surface treatments to promote adhesion, such as an atmospheric pressure plasma treatment [1]. The former option will be investigated in this work. The effects of moisture on the fracture performance of secondary bonded composite joints is well publicised. Moisture can be introduced into the composite laminate prior to [2] or after [3] secondary bonding. The moisture can plasticize the adhesive and reduce the glass transition temperature of the adhesive [4]. However, compared to secondary bonded joints, relatively little work has been carried out on co-cured joints. In the present work, the effect of the level of moisture in the composite prepreg prior to co-curing will be examined.
      198
  • Publication
    An Experimental and Numerical Investigation of the Mixed-mode Fracture Toughness and Lap Shear Strength of Aerospace Grade Composite Joints
    The increasing use of composite materials in various industries, such as aerospace, automotive and renewable energy generation, has driven a need for a greater understanding of the fracture behaviour of bonded composite joints. An important prerequisite for the adhesive bonding of composites is the existence of a uniform surface free from contaminants and mould release agents. While there are several ways in which this may be achieved, the use of peel plies has emerged as the preferred choice for many industries due to the repeatable nature of the resulting surface, particularly in the highly regulated aerospace industry. However, the use of peel plies can present some problems. It is possible that contamination from the peel ply can be transferred to the composite substrate and adversely affects the adhesive joint [1]. Composite joints are typically evaluated using lap shear type tests. While these tests are relatively simple to perform and post-process compared to their fracture mechanics based counterparts, the results can often be misleading and are greatly dependent on the overlap length, the thickness of the substrate and the type of fillet employed [2, 3]. The aim of this work is to show that composite joint systems can be modelled using material properties determined from fracture mechanics based tests. The fracture parameters will be used to develop numerical models of the fracture tests that accurately predict the wide-area lapshear test.
      564