Now showing 1 - 10 of 21
  • 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.
      586Scopus© Citations 30
  • Publication
    Characterisation of the fracture energy and toughening mechanisms of a nano-toughened epoxy adhesive
    (Trans Tech Publications, 2011-09) ; ; ;
    In this study the adhesive joint fracture behaviour of a nano-toughened epoxy adhesive was investigated. Two experimental test methods were used; (i) the standard tapered double cantilever beam (TDCB) test to measure the mode I adhesive joint fracture energy, GIC, as a function of bond gap thickness and (ii) a circumferentially deep notched tensile test to determine the cohesive strength of the adhesive for a range of constraint levels. It was found that the fracture energy of the adhesive followed the well-known bond gap thickness dependency [1]. SEM analysis of the TDCB fracture surfaces revealed significant plastic void growth. Finally, numerical modelling of the experimental tests suggested that most of the fracture energy was dissipated via highly localised plasticity in the fracture process zone ahead of the crack tip.
    Scopus© Citations 3  498
  • 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.
      376Scopus© Citations 3
  • 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.
      244
  • Publication
    Effect of prepreg storage humidity on the mixed-mode fracture toughness of a co-cured composite joint
    The present work investigated the effect of the level of prepreg moisture content on the mixed-mode fracture toughness of a co-cured composite joint. It was found that moisture was stored in the prepreg as either free or bound water. It was also shown that the prepreg stores moisture from high humidity environments as free water, while the level of bound water remains unaffected. The excessive moisture was shown to plasticise the adhesive, lowering the glass transition temperature. The fracture toughness decreased under mode I and mode II loading as the humidity level was increased. The mixed-mode toughness also reduced with increasing storage humidity. However, the measured mixed-mode fracture toughness never reduced below that of the joints fabricated using the as-received material. This indicates that the moisture has a more pronounced effect on the bulk properties of the adhesive rather than on the interfacial adhesion between the composite and adhesive.
    Scopus© Citations 39  750
  • Publication
    The mechanical properties of polycrystalline diamond as a function of strain rate and temperature
    Polycrystalline diamond (PCD) materials are used in various applications, mainly as cutting tools for machining non-ferrous metals and non-metallic materials and for rock drilling operations. A better knowledge of their mechanical properties is of fundamental importance to PCD manufacturers and end users. In order to understand and predict the behaviour and structural integrity of the tools containing PCD, it is first necessary to study the behaviour of the material as a function of loading rate and temperature. In this paper, material behaviour is determined under testing conditions which correspond more closely to those in actual drilling, which is a significant improvement over investigations to date. Young’s modulus determined by four-point bending and a split-Hopkinson pressure bar apparatus was relatively constant with the rate, while a consistent decrease was observed with increase of temperature. The flexural strength was found to increase with the temperature, while decreasing with an increase in rate.
    Scopus© Citations 22  1769
  • Publication
    Micro-Mechanical Modelling of Void Growth, Damage and Fracture of Nano-Phase Structural Adhesives Using the Finite Volume Method
    (The European Structural Integrity Society Technical Committee, 2011) ; ; ;
    Significant toughening of structural adhesives is attainable with the addition of nano and/or micro particles1,2,3. A deep understanding of the effect of particle de-bonding and subsequent void growth to coalescence is key to evaluating the strengthening and failure mechanisms occurring in the damage and fracture of these adhesives. Tapered Double Cantilever Beam (TDCB) experiments, conducted at University College Dublin (UCD), have observed a significant dependence of the fracture toughness of these adhesives on bond gap thickness5. In conjunction with this change in fracture toughness, scanning electronmicroscopy (SEM) of the fracture surface has also revealed corresponding changes in void evolution as the bond gap is varied. Classical analysis suggests the change in toughness may be attributed to a physical constraint of the size to which the plastic zone around a crack tip may develop6. However, simulation of these TDCB tests using finite volume stress analysis has found that little plasticity develops in the bulk adhesive layer and is instead concentrated in the fracture process zone. The change in fracture toughness and void evolution present can be attributed to the change in triaxiality at different bond gap thicknesses and the results agree quite well with the void growth model of Rice & Tracey4. The variance of void growth with triaxiality is investigated here. The initial work considered here concerned 3D modelling of a void in an elastic perfectly plastic material with a view to verifying exponential dependence of void growth on the macroscopic stress triaxiality in the system in accordance with the Rice & Tracey model. The model examines void growth rate dependence on the stress triaxiality, for a given effective strain.
      415
  • Publication
    Transferability of Adhesive Fracture Toughness Measurements between Peel and TDCB Test Methods for a Nano-Toughened Epoxy
    Our previous work [1] on a nano rubber modified epoxy adhesive suggested that the observed bond thickness effect was due to the level of constraint (σhyd/σeq), a measure of the stress triaxiality, in the adhesive layer. In that study tapered double cantilever beam (TDCB) specimens were tested under quasi-static conditions for a range of bond gap thicknesses. The void diameters on the resulting fracture surfaces were measured from which the fracture strain was estimated in each case. The ratio of fracture strains corresponding to different constraint levels was found to agree with the predictions of the Rice and Tracey model. The current work attempts to further investigate the effects of constraint on adhesive joint fracture. Three experimental test methods are employed (i) the standardised LEFM tapered double cantilever beam (TDCB) test, in which the substrates experiences small elastic deformations, (ii) the fixed arm peel test where the substrate peel arm undergoes extensive plastic deformation and (iii) a recently developed circumferentially deep notched tensile (CDNT) test. Finite Volume simulations of the TDCB and CDNT tests were utilised to examine the role of constraint on the adhesive joint fracture.
      366