Thermal residual strains in carbon fibre-reinforced aluminium laminates

The thermal residual strains in various carbon fibre-reinforced aluminium laminates (carall), which were generated during cooling from the curing temperature, have been evaluated by both experimental methods and theoretical analysis. The experimental methods used include the deflection of an asymmetric laminate and the yield point shift of the aluminium alloy in the carall laminate. The theoretical calculation performed was based on the classical lamination theory. Residual strains determined by each experimental method and by theoretical calculation show good agreement. In addition, the possible errors associated with each method were carefully assessed and shown to be acceptable. For carall laminates reinforced with unidirectional carbon fibres, the thermal residual stress in the aluminium layer was found to be roughly proportional to the volume fraction of the carbon/epoxy layer.     

Flexural fatigue of carbon fibre-reinforced PEEK laminates

Stress/number of cycles to failure relations were established for unidirectional, cross-plied and angle-plied laminates of APC-2 using three-point bend flexure tests. Results for constant stress ratio fatigue tests were modelled using both Weibull and fatigue modulus degradation analyses. Behaviour was compared with that of an established carbon/epoxy system.     

Compression strength of aligned carbon fibre-reinforced thermoplastic laminates

Compression strength measurements have been made on a number of unidirectional carbon fibre-reinforced thermoplastics fabricated by film stacking or by melt impregnation and comparisons made with a standard aerospace epoxide system. The results indicate that polyetheretherketone based laminates, despite having a lower matrix shear modulus, have similar compression strengths to the epoxide system.     

An investigation on the bearing test procedure for fibre-reinforced aluminium laminates

Excellent fatigue, static strength and damage tolerance characteristics together with low density make fibre-reinforced aluminium laminates a prime candidate sheet material for application in fatigue- and fracture-critical aircraft structures. Their use requires that mechanical property design allowables be established for incorporation in design handbooks (e.g. MIL-HDBK-5). An experimental programme based on statistical design was conducted to establish a meaningful test procedure for determination of fibre-metal laminate bearing strength design allowables. The test procedures investigated are the pin-type bearing test method (ASTM E-238) and the bolt-type bearing test method, a modified method based on the procedure for bearing strength determinations in plastics (ASTM D-953). Results are presented from an experimental programme which measured the bearing strengths of two grades of S-2 glass-based and one grade of aramid-based aluminium laminates. The influences of lateral constraint and ply orientation on bearing strength and failure mode are shown. The bolt-type bearing test method, which combines the attributes of the two aforementioned methods, is recommended. The study also showed that the bearing properties for edge distance ratio e/D = 2 can be predicted by correlation with the aluminium volume fraction in fibre-reinforced aluminium laminates. In addition, diagrams of joint structural efficiency, shown to be comparable to those of aluminium alloy sheets, have been established.     

Fatigue behaviour of metallic fibre-reinforced materials: a study of steel fibre-reinforced silver

Low-cycle fatigue tests were performed on silver matrix-steel-fibre-reinforced composites (FRC) with unidirectionally aligned fibres of constant volume-fraction,V _f=0.35. The influence of various material parameters, namely the fibre and matrix strengths, the interfacial bond strength and the mean fibre diameter, was examined. Low-cycle fatigue hardening and softening of FRCs deviate from the behaviour expected from the properties of the components. The differences are mainly based on a specific deformation or recrystallization structure of the composite matrix. The shapes of the stress—strain hysteresis loops are discussed qualitatively and compared to the experimental results. The comparison of fatigue life for different material conditions shows that the design of low-cycle fatigue-resistant metallic-fibre composites requires material criteria similar to those known to be responsible for high ultimate tensile strengths. The criteria may be varied if the composites are subjected to high-cycle fatigue loading.     

Fatigue behaviour of aluminium foam sandwiches

Aluminium foam sandwiches are subjected to four‐point bending fatigue test considering the effect of geometric parameters of panels, such as core and plate thickness, and loading mode, such as arm distance. Fatigue strength curves are expressed in terms of different stress amplitude parameters calculated using an analytical model based on laminated plate classical theory and a solid finite element method model. Despite, the notable fatigue data scatter, originated by foam intrinsic inhomogeneity, experimental fatigue curves are coherent and allow obtaining unified fatigue curves.     

On cross-ply cracking in glass and carbon fibre-reinforced epoxy laminates

The transverse tensile strength (or fracture strain) of unidirectional fibre-reinforced materials is an important mechanical property. The transverse fracture strain of a single ply in an angle-ply laminate, however, is not an independent mechanical property as it is influenced by its thickness and neighbouring plies. The present investigation describes the phenomenon of multiple fracture of glass and carbon fibre-reinforced epoxy 90° plies. Based on the model that a 90° ply consists of elements all of which can break, the applicability of Weibull statistics in describing the fracture strain is investigated.     

Fatigue delamination behaviour of unidirectional carbon fibre/epoxy laminates reinforced by Z-Fiber® pinning

Z-Pin reinforced carbon-fibre epoxy laminates were tested under Mode I and Mode II conditions, both quasi-statically and in fatigue. Test procedures were adapted from existing standard or pre-standard tests. Samples containing 2% and 4% areal densities of carbon-fibre Z-pins (0.28 mm diameter) were compared with unpinned laminates. Quasi-static tests under displacement control yielded a dramatic increase of the apparent delamination resistance. Specimens with 2% pin density failed in Mode I at loads 170 N, equivalent to an apparent G_IC of 2 kJ/m². Fatigue testing under load control showed that the presence of the through-thickness reinforcement slowed down fatigue delamination propagation.     

Fatigue behaviour of asphalt concrete beams reinforced by glass fibre-reinforced plastics

The aim of this study was to evaluate the enhancement effect of asphalt concrete beams reinforced by glass fibre-reinforced plastics (GFRP). First, the Cooper fatigue test machine was used to conduct the four-point bending fatigue tests. The test results showed that the mean fatigue life of hot mixture asphalt (HMA) beams had been extended to more than 8.81 times with 3-mm thick GFRP sheets pasted on the top. Second, the stress and strian behaviour of the four-point bending fatigue test specimen was simulated by the finite element method. The results showed that flexural stiffness of HMA beams had increased significantly with GFRP sheets pasted on the top. Finally, the fatigue failure process of the HMA beam with GFRP sheet pasted on the top was predicted by the theory of damage mechanics. The predicted results matched well with those obtained in the fatigue tests.Therefore, pasting a GFRP sheet of a certain thickness on a steel bridge deck could greatly improve the overall stiffness of the pavement layer and form a kind of durable steel bridge deck surfacing structure. The research results had important theoretical significance and value in engineering applications.     

Impact fatigue behaviour of carbon fibre-reinforced vinylester resin composites

Two types of unidirectional carbon fibre, one of high strength (DHMS) and another of medium strength (VLMS) reinforced vinylester resin composites have been examined for their impact fatigue behaviour over 10⁴ impact cycles for the first time. The study was conducted using a pendulum type repeated impact apparatus specially designed and constructed for the purpose. A well-defined impact fatigue behaviour (S-N type curve) curve has been demonstrated. It showed a plateau region of 10–10² cycles immediately below the single cycle impact strength, followed by progressive endurance with decreasing impact loads, culminating in an endurance limit at about 71% and 85% of the single impact strength for DHMS-48 and VLMS-48, respectively. Analysis of the fractured surfaces revealed primary debonding, fibre breakage and pull-out at the tensile zone of the samples and a shear mode of fracture with breakage of fibre bundles at the compressive zone of the samples. The occurrence of a few major macrocracks in the matrix with fibre breakage at the high load-low endurance region and development of multiple microcracks in the matrix, coalescing and fibre breakage at the low-load-high endurance region have been inferred to explain the fatigue behaviour of the composites examined.     

Temperature dependence of the tensile behaviour of aramid/aluminium laminates

Aramid/aluminium laminates (ARALL^® laminates) are a family of new hybrid composites made of alternating layers of thin aluminium alloy sheets with plies of epoxy adhesive prepreg containing unidirectional aramid fibres. The effect of elevated and cryogenic temperatures on these materials is critical to aerospace applications. ARALL 1, 2, 3, and 4 laminates have been tested in tension at temperatures ranging from –300F–400 °F (–184–204 °C) and at room temperature after exposure. This paper summarizes how tensile properties depend on temperature for these four ARALL laminates under the conditions described. At cryogenic temperatures, no degradation of ultimate tensile strengths, tensile yield strengths and moduli were found for either the longitudinal or transverse directions for ARALL 1–4 laminates. Furthermore, the mechanical properties remained the same or increased slightly as the temperature decreased. Longitudinal and transverse ultimate tensile strengths, tensile yield strengths, and moduli of ARALL 1–3 laminates at room temperature remain nearly constant after the laminates were exposed for 1, 10 and 100 h to temperatures up to 250 °F (121 °C), and up to 350 °F (177 °C) for ARALL 4 laminates. However, these properties determined at the elevated temperatures after 1, 10 and 100 h exposure showed a tendency to decrease with increasing temperature. The properties of ARALL laminates are much better in the longitudinal fibre direction than those of conventional monolithic aluminium alloys. Typical failure modes of the test specimens in the high-temperature range were examined using a scanning electron microscope. The discussions are also described in the paper.     

Characterization and analysis of carbon fibre-reinforced polymer composite laminates with embedded circular vasculature

A study of the influence of embedded circular hollow vascules on structural performance of a fibre-reinforced polymer (FRP) composite laminate is presented. Incorporating such vascules will lead to multi-functional composites by bestowing functions such as self-healing and active thermal management. However, the presence of off-axis vascules leads to localized disruption to the fibre architecture, i.e. resin-rich pockets, which are regarded as internal defects and may cause stress concentrations within the structure. Engineering approaches for creating these simple vascule geometries in conventional FRP laminates are proposed and demonstrated. This study includes development of a manufacturing method for forming vascules, microscopic characterization of their effect on the laminate, finite element (FE) analysis of crack initiation and failure under load, and validation of the FE results via mechanical testing observed using high-speed photography. The failure behaviour predicted by FE modelling is in good agreement with experimental results. The reduction in compressive strength owing to the embedding of circular vascules ranges from 13 to 70 per cent, which correlates with vascule dimension.     

Flight simulation behaviour of aramid reinforced aluminium laminates (ARALL)

Aramid reinforced aluminium laminates show excellent fatigue crack growth properties. Weight savings up to 30% combined with improved damage tolerance are possible as compared to monolithic high strength aluminium alloy structures. The improvement of the fatigue crack growth rate is merely due to the restraint on the crack opening by uncracked fibres in the wake of the crack. This effect becomes more active when favourable residual stresses are introduced by prestraining the whole laminate in the plastic region of the aluminium alloy.     

The fatigue of fibre-reinforced aluminium

In order to make use of the available strength of strong fibres, the metal matrix of a composite will have to undergo plastic deformation. The effect that this will have on the fatigue behaviour and damping capacity is discussed, with reference to aluminium reinforced with silica fibres and with stainless steel wires.     

Use of rule of mixtures and metal volume fraction for mechanical property predictions of fibre-reinforced aluminium laminates

This paper presents the results of a statistically designed programme conducted to validate feasibility of using the relationship of mechanical properties and metal volume fraction in fibre-metal laminates to make property predictions. Experimental and analytical practices employed to obtain these mechanical properties for tension, compression, in-plane shear, and bearing are described. Results from this pilot study show that use of the metal volume fraction may be useful for the prediction of strength mechanical properties in fibre-metal laminates. However, this needs further study to validate the concept. If the hypothesis is valid, the number of laminate configurations to be tested to qualify a fibre-metal laminate family can be minimized. The findings imply that the metal volume fraction approach using a rule of mixtures can be exploited to estimate design properties for a multitude of fibre-metal laminate variants, which is economically beneficial to the preliminary stages of aircraft design.     

Oxidation behaviour of carbon/aluminium and SiC/aluminium composites

Isothermal oxidation of 6061 Al, reinforced with chopped carbon fibres and SiC particles, was studied to investigate the applicability of these composites in the temperature range 300–500 °C, in terms of their degradation due to oxidation. Carbon/aluminium composite suffered a tremendous loss in weight at 350 °C. The extent of the damage due to oxidation for SiC/Al composites depended upon the concentration of nucleation sites, which formed the interface between the composite and the matrix. The number of such sites depended upon the volume fraction and size of the dispersed particles. Analysis of the oxide scale was carried out using SEM/EDAX and X-ray diffraction. The deterioration in strength of the composites, due to oxidation, was determined by tensile testing of exposed specimens.     

Fatigue crack growth in hybrid boron/glass/aluminium fibre metal laminates

The crack growth behaviour of hybrid boron/glass/aluminium fibre metal laminates (FMLs) under constant‐amplitude fatigue loading was investigated. The hybrid FMLs consist of Al 2024‐T3 alloy as the metal layers and a mixture of boron fibres and glass fibres as the fibre layers. Two types of boron/glass/aluminium laminates were fabricated and tested. In the first type, the glass fibre/prepreg and the boron fibre/prepreg were used separately in the fibre layers, and in the second type, the boron fibres and the glass fibres were uniformly mingled together to form a hybrid boron fibre/glass fibre prepreg. An analytical model was also proposed to predict the fatigue crack growth behaviour of hybrid boron/glass/aluminium FMLs. The effective stress intensity factor at a crack tip was formulated as a function of the remote stress intensity factor, crack opening stress intensity factor, and the bridging stress intensity factor. The bridging stress acting on the delamination boundary along the crack length was also calculated based on the crack opening relations. Then, the empirical Paris‐type fatigue crack growth law was used for predicting the crack growth rates. A good correlation between the predicted and experimental crack growth rates has been obtained.