Prediction of low velocity impact damage in carbon–epoxy laminates

It is well known that composite laminates are easily damaged by low velocity impact. This event causes internal delaminations that can drastically reduce the compressive strength of laminates. In this study, numerical and experimental analyses for predicting the damage in carbon–epoxy laminates, subjected to low velocity impact, were performed. Two different laminates (0₄,90₄)_s and (0₂,±45₂,90₂)_s were tested using a drop weight testing machine. Damage characterisation was carried out using X-rays radiography and the deply technique. The developed numerical model is based on a special shell finite element that guarantees interlaminar shear stresses continuity between different oriented layers, which was considered fundamental to predict delaminations. In order to predict the occurrence of matrix failure and the delaminated areas, a new failure criterion based on experimental observations and on other developed criteria, is included. A good agreement between experimental and numerical analysis for shape and orientation of delaminations was obtained. For delaminated areas, reasonable agreement was obtained.     

Fatigue damage of carbon–epoxy laminates with embedded optical fibres

Abstract

The present paper is concerned with the fatigue behaviour ofcarbon-epoxy laminates with embedded optical fibres subjected to bending loads. The main goal of this investigation was to evaluate quantitatively the effect of the presence of optical fibres within the host structure on its whole fatigue behaviour. Two optical fibre positions were investigated: in the mid-plane of the laminate and near the surface subjected to loading. Two distinct geometries of the ply stacking sequence were also considered, namely unidirectional and crossply. In order to evaluate the fatigue life and the fatigue damage, two different loading levels were used, both at 6 Hz frequency, room temperature and R = 0.1. Fatigue damage was monitored using dynamic stiffness decay and acoustic emission techniques. Failure mechanisms were analysed by means of optical and scanning microscopy. The results obtained lead to the conclusion that the embedding of optical fibres markedly prejudices the fatigue performance of the material only for certain configurations. It was also possible to speculate on the fatigue failure mechanisms, and to relate them with relevant experimental parameters, such as the lay-up geometry and optical fibre position.     

The fracture mechanics of some graphite fibre-reinforced epoxy laminates part 2: (± 45) ns laminates

The fracture behaviour of (± 45)_ns laminates of T300/934 graphite fibre-reinforced epoxy was studied using compact tension specimens of several widths and thicknesses, centre-notched tension, and three-point bend specimens. The process of damage initiation and propagation was studied and is discussed in detail. The critical stress intensity factor was evaluated and its variation with specimen size and type is shown. On the basis of these investigations, a suitable specimen for the evaluation of meaningful fracture toughness is suggested.     

“Mimetic” molecular composites of Kevlar® aramid/poly(p-phenyleneterephthalamide)*

Mimetic molecular composites can be viewed as hybrids of conventional and molecular composites, and are prepared from a matrix and reinforcing fiber consisting of a single polymer composition. The aim of the work was to obtain a good chemical, physical and thermal property match at the interface for an overall excellent balance of composite properties. Kevlar® aramid 49/poly(p-phenyleneterphthalamide, an all-PPD-T composite, was used as a model system in the work, and, in theory, should be ideal for testing the merit of the mimetic molecular composite concept. The key to the successful preparation of all-PPD-T infusible composites was the acid catalyzed thermal transformation of a fusible precursor, poly(N,N-di-sec-butyl-p-phenyleneterephthalamide), into PPD-T. The composites were prepared by embedding Kevlar® aramid 49 fibers in poly(N,N-di-sec-butyl-p-phenyl-eneterephthalamide) resin, which, on heating in the presence of benzene sulfonic acid catalyst, dealkylated to a PPD-T matrix. In this way, Kevlar® aramid 49/PPD-T(8/92 to 40/60 v/o) composites with densities in the range of 0.2 to 1.2 g cm–3 (versus 1.4 g cm–3 for a fully consolidated PPD-T composite) have been prepared and their thermal and mechanical properties characterized. Some of the foamed composites prepared in this work bear a remarkable resemblance to wood, a natural fiber reinforced foam composite, but with the advantages of flame and rot resistance.     

Dynamic buckling of elastic–plastic square tubes under axial impact—II: structural response

Dynamic elastic–plastic buckling of thin-walled square tubes is studied from the viewpoint of elastic–plastic stress wave propagation, which originates from an axial impact loading. The influence of the impact velocity and the striking mass on the development of the buckling shape is discussed when considering the transient deformation process. It is shown that the maximum load, which results from a high velocity impact load and occurs at t=0, is a function of the impact velocity and is related to the speed of the elastic–plastic stress waves propagating along the tube. The predictions for the initiation of buckling based on a numerical simulation of the axial impact of strain rate insensitive square tubes using the FE code ABAQUS show good agreement with the results from experiments on aluminium alloy tubes impacted at various initial velocities. A comparison between the buckling initiation in square tubes and geometrically equivalent circular tubes reveals differences in the response, which are attributed to the stress wave propagation phenomena and to the structural differences between the two structures.     

Shot peen forming of fiber metal laminates on the example of GLARE®

GLARE (GLAss-fiber REinforced aluminum) is a sandwich material that combines thin aluminum sheets with intermediate layers of glass fiber that are bonded using epoxy. Due to the resulting low specific weight and high strength as well as superior deterioration resistance the material has found its application in aircraft structures. GLARE parts are typically manufactured using the so-called self-forming technique, which is a very expensive and labor-intensive manufacturing process. If it was feasible to form GLARE from flat stock material using conventional forming processes, substantial savings could be achieved. Several attempts to form GLARE from flat stock reported in the literature are restricted by the limited formability of the glass fibers and/or delamination of the layers. This work analyses the possibilities to form GLARE using shot peen forming (SPF), which is an established forming process, e.g. for the production of fuselage parts. It is shown that GLARE shows a similar deformation behavior as monolithic sheets under quasi-static indentation with single steel balls. The process limits are analyzed using SPF tests and lock-in thermography, which is a non-destructive testing procedure for the detection of delamination. A process window for shot peen forming of GLARE is established, and it is shown that curvature radii of less than 2500 mm can be accomplished with no evidence of failure, which is a typical curvature radius of fuselage components for the Airbus A380.     

Simulation of the response of fibre–metal laminates to localised blast loading

The response of Fibre–Metal Laminates (FML) to localised blast loading is studied numerically in order to interpret the deformation mechanism due to highly localised pressure pulses causing permanent deformations and damage observed experimentally in FML panels comprising different numbers of aluminium alloy layers and different thickness blocks of GFPP material [Langdon GS, Lemanski SL, Nurick GN, Simmons MS, Cantwell WJ, Schleyer GK. Behaviour of fibre–metal laminates subjected to localised blast loading: part I – experimental observations and failure analysis. International Journal of Impact Engineering 2007;34:1202–22; Lemanski SL, Nurick GN, Langdon GS, Simmons MS, Cantwell WJ, Schleyer GK. Behaviour of fibre–metal laminates subjected to localised blast loading: part II – quantitative analysis. International Journal of Impact Engineering 2007;34:1223–45; Langdon GS, Nuric GN, Lemanski SL, Simmons MS, Cantwell WJ, Schleyer GK. Failure characterisation of blast-loaded fibre–metal laminate panels based on aluminium and glass-fibre reinforced polypropylene. Composite Science and Technology 2007;67:1385–405]. The influence of the loading and material parameters on the final deformation characteristics is examined. Particular attention is paid to the transient deformation process by using finite element and analytical models to analyse the panel behaviour. It is shown that the response of the FML panels is extremely sensitive to the spatial and temporal distribution variation of the pressure caused by the blast loading. The study reveals that the properties of GFPP in the through-thickness direction play an essential role in the velocity transfer, which influences considerably the failure and final deformed shape of the FML panel. Good agreement between the experimental and numerical results is observed. Comparisons between the responses of relatively thin FML panels, monolithic aluminium alloy plates of equivalent mass and a foam-core panel to localised blast are also presented and discussed.     

Fatigue behavior of carbon/epoxy composites under pretorsion and low‐energy impact effects

Abstract

This work presents a systematic study of the fatigue behavior and macroscopic analysis of carbon/epoxy [0/45/90/‐45]_2S quasi‐isotropic composite laminates. The failure mechanism and fatigue effects of the composites under pretorsional twist and low‐energy impact were investigated in this research. The coupling effects of the laminates under twist and low‐energy impact and the residual tensile strength and the S‐N curve under various stress levels were also studied.     

Fretting fatigue studies on carbon fibre/epoxy resin laminates: III—Microscopy of fretting fatigue failure mechanisms

With increasing application of carbon fibre reinforced plastic laminates (CFRP) the fretting fatigue properties become more and more important. In the case of fretting against the load-bearing 0°-fibres a reduction in fatigue life of about three orders of magnitude can be observed as a result of wear of the fretting pads on the carbon fibres. However, fretting against ±45°-plies protects the 0°-fibres and causes no reduction in fatigue life by comparison with plain fatigue.     

Impact response and damage tolerance characteristics of glass–carbon/epoxy hybrid composite plates

Impact behaviour and post impact compressive characteristics of glass–carbon/epoxy hybrid composites with alternate stacking sequences have been investigated. Plain weave E-glass and twill weave T-300 carbon have been used as reinforcing materials. For comparison, laminates containing only-carbon and only-glass reinforcements have also been studied. Experimental studies have been carried out on instrumented drop weight impact test apparatus. Post impact compressive strength has been obtained using NASA 1142 test fixture. It is observed that hybrid composites are less notch sensitive compared to only-carbon or only-glass composites. Further, carbon-outside/glass-inside clustered hybrid configuration gives lower notch sensitivity compared to the other hybrid configurations.     

Fretting fatigue studies on carbon fibre/epoxy resin laminates: I—design of a fretting fatigue test apparatus

A freeting fatigue test device was designed and built for special fretting fatigue studies with composite materials. The device is attached to the specimen grips of a servohydraulic testing machine and allows symmetrical fretting loading of the specimen surface by two freeting pads while the specimen is fatigued. Positioning of the fretting pads can be either on the edges of laminated samples or on their flat surfaces. Different fretting slip amplitudes can be specified at given fatigue conditions, and the fretting load can be exactly adjusted and controlled.     

Assessment of cure-residual strains through the thickness of carbon–epoxy laminates using FBGs Part II: Technological specimen

This paper is a continuation of our previous study [Mulle M, Collombet F, Olivier P, Grunevald Y-H. Assessment of cure residual strains through the thickness of carbon–epoxy laminates using FBGs, part I: elementary specimen. Compos Part A 2008. doi:10.1016/j.compositesa. 2008.10.008] pertaining to the assessment of autoclave cure-induced strains through the thickness of carbon–epoxy laminates. In this first part, postulates and measurement procedures were established for cure of elementary specimens. Based on these, this study undertakes investigation on what are called technological specimens. These specimens are of the beam type and contain geometrical specificities which represent typical structural issues. In-plane process-induced strains were studied through the thickness of a thick reinforced zone using several optical fibre Bragg gratings (FBGs) sensors embedded at different levels of the ply stack. A non-uniform distribution of residual strains was detected. Once cured, the technological specimen was subjected to a heating test whose cycle was comparable to the cure cycle. Thermally induced strains were measured with the embedded FBGs. The values recorded were compared with those of cure-induced residual strains and FEM simulation. Discrepancies were observed that strongly suggest the possible influence of environmental effects and the need for the calculation to take into account the through-the-thickness variability of thermal properties.     

Assessment of cure residual strains through the thickness of carbon–epoxy laminates using FBGs,Part I: Elementary specimen

Variability of thermo-mechanical properties within a composite part is a well known issue. It is problematic at the early stage of structural design. This variability being generated during the processing phases, the characterisation of the initial state of a structure becomes a crucial operation. A first step to this objective is presented in this paper. It deals with the assessment of autoclave cure induced strains in carbon epoxy laminates. Distribution and amplitude are considered through the thickness of carbon/epoxy elementary and technological specimens thanks to a series of FBGs embedded at different levels of the ply stack. Part I of this work concerns the analysis of unidirectional reinforced specimens. It begins with an overview of the various phenomena that cause the presence of residual stresses after cure and how they can be measured in experiments. Their development being closely related to the rheology of the matrix, an analysis of the M21 resin is given and assumptions are made to justify the following assessment procedures. A discussion leads to select the onset of gelation as the key point to start measuring residual cure stresses. The autoclave cure monitoring of the [0₈] specimen is then carried out and analyzed. Depending on their location through the laminate thickness, the FBG sensors give different information highlighting the effects of interaction between the laminate and the tool/plate.     

Mechanical response of barium-titanate/polymer 0–3 ferroelectric nano-composite film under uniaxial tension

Ferroelectric polymer based 0–3 composite films are attractive for applications such as capacitors and electric energy storage devices. In this paper, deformation and fracture behavior under uniaxial tension is characterized for BaTiO₃/poly(vinyledene fluoride-trifluoroethylene) (abbreviated as BT/P(VDF-TrFE)) ferroelectric composite film. Compared with the pure P(VDF-TrFE) copolymer film, the composite film with a small volume fraction of BT powders shows an enhanced ductility in accompany with reduced stiffness and fracture strength. Scanning electron microscope (SEM) observation and X-ray diffraction (XRD) analysis are carried out to examine the morphology and microstructure change during uniaxial tension. It is demonstrated that addition of a small amount of BaTiO₃ powders into the copolymer matrix inhibits the growth of the crystallite size, causes reduction in the crystalline content and a loosely packed molecular chain structure. Consequently, the fracture strain increases while the stiffness and fracture strength decreases for the composite films.     

On the influence of preloading in the nonlinear viscoelastic–viscoplastic response of carbon–epoxy composites

On an ongoing research for the nonlinear viscoelastic response of composites and polymers, a study of the influence of preloading applied to composite laminates subjected to creep–recovery loading is performed. In cases where high stress levels are applied, this response becomes highly nonlinear and has to be taken into account when designing composite parts. A major problem encountered in the experimental investigation of the nonlinear viscoelastic behaviour is the mode of the initial applied loading and its effect in the overall viscoelastic response of the test sample. The damage that occurs due to the instantaneous application of the load leads to an additional viscoelastic/viscoplastic strain component. In order to investigate this effect as well as to compare different preloading modes, as far as viscoelastic/viscoplastic response is concerned, a test program was initiated and the experimental data were investigated in the current study. A preloading mode is applied in each specimen prior to the creep–recovery testing at different applied stress levels. Useful results concerning the effect of preloading in the time dependent response of the material are concluded. Variation of the values of viscoplastic strain in respect to the preloading mode is also of great concern.     

Microstructural investigation and indentation response of pressureless-sintered α- and β-SiC

The microstructure and indentation response of pressureless-sintered - and -SiC were studied using a high-resolution electron microscope and analytical electron microscopy. The materials were manufactured with boron and carbon as sintering aids. It was found that the overall porosity of the materials was very low but a large number of carbon inclusions were present. X-ray diffraction revealed the fabricated -SiC material was of the same 3C polytype as the initial starting powder; however, electron microscope observations indicated that the material contained a high density of faulting of the -forms. High-resolution imaging of grain boundaries in these materials indicated that the boundaries were very clean, and when they contained an amorphous intergranular film it was at most 0.5 to 1 nm thick. The presence of boron was not detected. Deformation due to identation took several forms. Firstly, radial cracks extending from the corners of the indent suffered little hindrance from the matrix microstructure, such that transgranular fracture was the dominant mode. Secondly, the deformation zone beneath the indentations showed copious lattice microcracks with some preferred orientation during crack formation and propagation.     

Influence of embedded gap and overlap fiber placement defects on the microstructure and shear and compression properties of carbon–epoxy laminates

This paper presents results from an experimental study of the influence of embedded defects created during automated fiber tape placement, on the mechanical properties of carbon/epoxy composites. Two stacking sequences have been examined, [(−45°/+45°)₃/−45°] and [90°₄/0°₃/90°₄], in which gaps and overlaps have been introduced during fiber placement. These materials have been cured in an autoclave either with or without a caul plate, then analyzed by ultrasonic C-scan. The microstructures were characterized by scanning electron microscopy. In-plane shear tests were performed on the ±45° laminates and showed that the use of a caul plate does not affect mechanical behavior of plies in the embedded defect region. Compression tests were performed on 0°/90° laminates and in this case the presence of a caul plate is critical during polymerization as it prevents thickness variations and allows defects to heal.     

The ballistic impact characteristics of Kevlar® woven fabrics impregnated with a colloidal shear thickening fluid

This study reports the ballistic penetration performance of a composite material composed of woven Kevlar® fabric impregnated with a colloidal shear thickening fluid (silica particles (450 nm) dispersed in ethylene glycol). The impregnated Kevlar fabric yields a flexible, yet penetration resistant composite material. Fragment simulation projectile (FSP) ballistic penetration measurements at 244 m/s have been performed to demonstrate the efficacy of the novel composite material. The results demonstrate a significant enhancement in ballistic penetration resistance due to the addition of shear thickening fluid to the fabric, without any loss in material flexibility. Furthermore, under these ballistic test conditions, the impregnated fabric targets perform equivalently to neat fabric targets of equal areal density, while offering significantly less thickness and more material flexibility. The enhancement in ballistic performance is shown to be associated with the shear thickening response, and possible mechanisms of fabric-fluid interaction during ballistic impact are identified.