Now showing 1 - 8 of 8
  • 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
    Enhanced Carbon/Epoxy Composite Fracture Toughness Achieved Using Atmospheric Pressure Plasma Treatments
    Composite materials are used in a wide range of industry sectors including automobiles, aeronautics and sports equipment. Two types of composite joints, co-cured and secondary bonded joints are used in the industries. Co-curing of composite joints is an efficient and cost-effective method of joining composites. The objective of this research is to enhance the bond strength between the composite material and adhesive, specifically in this study the bond between carbon-epoxy prepregs and an epoxy adhesive. The research investigated how the use of atmospheric plasma treatments of the uncured composite prepreg influenced the fracture toughness of the co-cured composite joints. The use of atmospheric pressure plasma treatment in the surface activation of prepregs was examined using contact angle measurements and X-ray photoelectron microscopy (XPS). Failure mechanism of the co-cured composite joints was studied by double cantilever beam (DCB) tests. Composite interface morphology was examined by scanning electron microscopy (SEM/FIB).
      201
  • 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
    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.
      656
  • Publication
    Effect of an Atmospheric Pressure Plasma Treatment on the Mode I Fracture Toughness of a Co-cured Composite Joint
    (Informa UK (Taylor & Francis), 2014-04-21) ; ; ;
    In this study, the surface of a composite prepreg was treated using an atmospheric pressure plasma in an attempt to improve the fracture toughness of a co-cured joint system. Three gas mixtures were investigated; Helium, Helium/Nitrogen and Helium/Oxygen. The processing parameters of the system were varied to obtain the maximum increase in surface energy of the prepreg. A He/O2 plasma was found to be the most efficient treatment, giving the largest increase in surface energy in the shortest time. Co-cured joints were then fabricated using prepreg that had been treated with various plasmas. A modest 15–18% increase in the mode I fracture toughness was achieved. However, the locus of failure remained interfacial. It was also observed that a He/O2 plasma treatment could be detrimental to joint toughness for long treatment times.
    Scopus© Citations 19  855
  • 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.
      246
  • Publication
    Influence of an Atmospheric Pressure Plasma Surface Treatment on the Interfacial Fracture Toughness on Bonded Composite Joints
    The aim of this work is to investigate the influence of a variety of plasma treatments on the surface properties of an epoxy-based composite material and to establish a relationship between these properties and the subsequent mechanical behaviour of adhesively bonded joints. To this end, specimens were subjected to three different types of plasma treatment: two short treatments (2min) of Helium and Helium plus Oxygen, and one long treatment (15min) of Helium plus Oxygen. The variation in surface energy of the composite specimens was examined in each case over a period of up to 3 days using contact angle measurements. Initial results show that the surface energy was increased from an untreated value of approximately 40 mJ/m2 to a value of 65 mJ/m2 immediately after treatment. The surface energy then fell by approximately 10 mJ/m2 over the course of three days for each treatment. The composite substrates were then bonded together using an epoxy film adhesive and the Mode I fracture toughness of the joint was determined from a series of symmetric and asymmetric double cantilever beam (DCB) tests. It was found that for both test geometries the adhesive failed cohesively. As a result, the values calculated for the mean propagation strain energy release rate, GIC, were those of the cohesive fracture toughness of the adhesive as opposed to the interfacial fracture toughness between the composite surface and adhesive.
      242