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Murphy, Neal
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Murphy, Neal
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Murphy, Neal
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- PublicationInfluence of an Atmospheric Pressure Plasma Surface Treatment on the Interfacial Fracture Toughness on Bonded Composite JointsThe 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.
193 - PublicationMicro-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.378 - PublicationTransferability of Adhesive Fracture Toughness Measurements between Peel and TDCB Test Methods for a Nano-Toughened EpoxyOur 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.
309 - PublicationThe Effect of Prepeg Storage Humidity on Co-cured Composite JointsThe 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.
204 - Publication
241 - PublicationThe Influence of Plasma Surface Treatment on the Fracture Toughness Peel Ply Prepared Bonded Composite JointsThe 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.
205 - PublicationEvolution 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.375 - PublicationAn Experimental and Numerical Investigation of the Mixed-mode Fracture Toughness and Lap Shear Strength of Aerospace Grade Composite JointsThe 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.
569 - PublicationImpact fatigue fracture of polycrystalline diamond compact (PDC) cutters and the effect of microstructureThe fatigue behaviour and failure of polycrystalline diamond compact (PDC) cutting tools under cyclic impact loading is investigated. These tools are composed of a polycrystalline diamond layer in-situ bonded onto a tungsten carbide substrate via a high temperature and high pressure sintering route. Their main application is in oil and gas drilling and non-ferrous machining. The tools were subjected to repeated impact loading until catastrophic failure occurred or up to 5000 impacts. Results show typical fatigue fractures, with cracks initiated and intermittently grown with each successive impact. Impact force or stress (S) was varied and the number of impacts (N) to crack initiation, growth and catastrophic failure recorded in order to generate S–N fatigue curves. PDC cutters with a coarser grain microstructure exhibited up to 70% better impact fracture resistance than their fine grain counterparts. Their fatigue endurance limit was also about 10–15% higher. The frequency at which impact loads occurred did tseem to affect the fatigue behaviour.
1868Scopus© Citations 33