Influence of artificial pre-stressing during the curing of VIRALL on its mechanical properties

A method for improving the mechanical properties of VIRALL has been proposed on the basis of the tensile behavior of Vinylon strands. The influence of artificial pre-stressing during the curing of VIRALL on its tensile properties has been experimentally investigated. It is revealed that fiber pre-stressing may lead to a dramatic increase in the elastic limit, yield strength and failure strength of VIRALL laminates. These results can be referred to the strengthening effect of Vinylon fibers and the modifications and recreations of the ply stress states in VIRALL laminates.     

The effect of resin failure strain on the tensile properties of glass fibre-reinforced polyester cross-ply laminates

An investigation has been made of the effect of resin properties on the transverse cracking behaviour of glass fibre-reinforced polyester resin three-ply laminates. The polyester resin properties were modified by the addition of a flexibilizing resin to produce five resin systems with failure stresses ranging from 1.75 to 11.1%. The mechanical properties of the resins which were determined, are observed to affect the stress level at which transverse cracking is initiated and the nature of the cracking behaviour. If fibre bunching is taken into account the Kies strain magnification theory can predict the general trend of the results. However, it is concluded that strain-rate effects associated with fibre bunching are worthy of further investigation.     

Influence of artificial pre-stressing during curing of CFRP laminates on interfibre transverse cracking

In this paper results are shown for a cross-ply laminate where the internal residual stresses in the 90° plies were influenced by mechanically pre-stressing the 0° plies during the curing process. After curing and cooling to room temperature the mechanical pre-stresses were released. The 90° plies were now under compressive stresses. This had the effect that under tensile loading the development of transverse cracks was delayed and shifted to higher strain and load values.     

Effect of fibre volume on tensile properties of real unidirectional fibre-reinforced composites

A simple theoretical model is proposed to account for the effect of high fibre loading on the tensile properties of real, unidirectionally reinforced fibre composites. Based on the reduction of interfacial surfaces due to fibre-fibre interaction, a modification to the rule of mixtures is proposed. The experimentally observed non-linear variation of tensile strength with fibre volume can be predicted by the modified rule. Experimental data from the literature are used to validate and verify the model and show good agreement with the predictions.     

Microstructure and mechanical properties of silicon carbide fibre-reinforced aluminium nitride composite

SiC continuous fibre (15 vol%)/AlN composite was fabricated using a sintering additive of 4Ca(OH)₂ · Al₂O₃ by hot-pressing at 1650 °C and 17.6 MPa in vacuum. Analytical transmission electron microscopy and scanning electron microscopy were used to investigate the microstructure of as-fabricated and crept SiC fibre/AlN composites. The room-temperature mechanical and high-temperature creep properties of the composite were investigated by four-point bending. The incorporation of SiC fibre into AlN matrix improved significantly the room-temperature mechanical properties. This improvement could result from the crack deflections around the SiC fibres. However, the incorporation degraded severely the high-temperature creep properties under oxidizing atmosphere. This could be attributed to the development of the pores and various oxides at the matrix grain boundary and matrix/fibre interface during creep test.     

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.     

Microstructures and tensile properties of spray-deposited high-strength aluminium alloys

Spray deposition is a new rapid solidification technique which produces bulk preforms directly from the melt metals. A spray deposition process was used to develop several high-strength aluminium alloys based on their commercial chemical compositions. These alloys include 2024 alloy, 7075 alloy and 7075 alloy modified with 1.0% Fe and 1.0% Ni.The deposits possessed rapid solidification microstructure with grain size of about 20 μm and a relative density of over 94%. The hardening phases of the materials in T4 or T6 conditions consisted of supersatured solid solution, stable and unstable ageing precipitates and disperse phases. The formation of the fine distributed disperse phases was due to the addition of iron and nickel to the 7075 alloy. The spray-deposited materials exhibited substantial improvement in tensile strengths and maintained acceptable ductility when compared to the corresponding ingot metallurgy processed materials. This revised version was published online in November 2006 with corrections to the Cover Date.     

Subzero tensile properties of vapour quenched aluminium alloys

Supersaturated and metastable aluminium alloy solid solutions containing a dispersed phase have been produced by a vapour quenching technique. Binary alloys contained 3.5% Fe and 5.5% Mn; ternary alloys contained 6 to 9% chromium and 0.5 to 1.2% iron. After rolling into sheet the tensile properties were determined in the temperature range 293 to 77 K. At 77 K tensile strengths of 1115 and 1036 MPa were obtained for two Al-Cr-Fe alloys, equivalent to E/82 and E/83, respectively. These are the highest strengths ever reported for an aluminium alloy. The deformation behaviour at subzero temperatures has indicated the potential for further strengthening of metastable rapidly solidified aluminium alloys by dislocations alone.     

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.     

Effect of silicon and aluminium in ferrite on tensile and impact properties

Two ferritic interstitial-free steels with approximately the same amount of solid solution strengthening by addition of 2?wt-% silicon and 4?wt-% aluminium are investigated using quasi-static tensile and dynamic impact tests. The addition of 2?wt-% silicon (2Si) results in brittle fracture in V-notched Charpy impact tests at ambient temperature, whilst the 4?wt-% aluminium-containing (4Al) steel has high absorbed energy of 320?±?12?J?cm^?2. In addition, the 4Al steel has a ductile-to-brittle-transition temperature (DBTT) ～60°C lower than the 2Si steel. It is proposed that the addition of silicon suppresses dislocation cross-slip at high strain rate and is responsible for the observed cleavage fracture and high DBTT in the 2Si steel. The ease of dislocation slip in the 4Al steel increases the impact toughness.     

Theoretical modelling of tensile properties of short sisal fibre-reinforced low-density polyethylene composites

The experimentally observed tensile properties (tensile strength and modulus) of short sisal fibre-reinforced LDPE with different fibre loading have been compared with the existing theories of reinforcement. The macroscopic behaviour of fibre-filled composites is affected by fibre loading, orientation and length of the fibres in the continuous medium. The interfacial adhesion between fibre and matrix also plays a major role in controlling the mechanical properties of the fibre-filled composites. In this study, a comparison is made between experimental data and different theoretical models. Composite models, such as parallel and series, Hirsch, Cox, Halpin–Tsai, modified Halpin–Tsai and modified Bowyer and Bader, have been tried to fit the experimental data. This revised version was published online in November 2006 with corrections to the Cover Date.     

Microstructure and strength of Si-Ti-C-O,fibre-reinforced aluminium and aluminium alloys

Microstructures of Si-Ti-C-O fibre-reinforced aluminium and aluminium alloys were investigated by scanning electron microscopy, and both conventional and analytical transmission electron microscopy. In the latter samples, some inclusions were observed between the matrix and the fibres. From the electron diffraction, high resolution microscopy and compositional analysis by energy-dispersive X-ray, the inclusions were identified as the α-Al-Si-Fe phase. Since the longitudinal three-point bending strength decreases with the increase of iron content, it was concluded that the α-phase inclusions on the surface of the fibre contribute to the lower strength of the composites based on this alloy.     

Effect of temperature and strain rate on tensile mechanical properties of ARALL-1 laminates

The combined effect of temperature and strain rate of the mechanical properties for unidirectional 3/2 ARALL^®-1 laminates was studied. In this paper, the effect of strain rates from 0.00083–0.833 min^–1 on tensile behaviour at temperatures up to 250°F (121 °C) has been conducted. It is demonstrated that tensile strength, tensile modulus, and fracture strain are found to depend on temperature and strain rate. However, the effect of strain rates at 75 °F (24 °C) and 180 °F (82 °C) was found to be insignificant except the lowest strain rate at 180 °F. It was also observed that the tensile yield strength decreased as the strain rate decreased. The tensile properties were moderately reduced at high temperatures and were higher at high strain rates than at low strain rates. The temperature effect is more significant than that due to the strain rate. Scanning electron photomicrographs of the fracture surfaces observed in the aramid/epoxy layer of ARALL-1 laminates at the lowest strain rate are shown to be significantly different only at 250 °F (121 °C). But this phenomenon is not obvious when the highest strain rate is employed.     

Influence of thermomechanical ageing on tensile properties of 2014 aluminium alloy

2014 aluminium alloy was subjected to various thermomechanical ageing (TMA) treatments which included partial peak ageing (25% and 50%), warm rolling (10% and 20%) and further ageing to peak hardness level at 160 ° C. The tensile tests reveal that TMA treatments cause a substantial improvement in tensile properties and thermal stability. The electron microscopic studies reveal that the TMA treatments affect substantially the ageing characteristics. The TMA Ib treatment yields the finest needles having longitudinal dimensions of 40nm. The TMA treatments also lead to precipitate-dislocation networks of different densities. It is observed that TMA IIb treatment results in the densest precipitate-dislocation tangles of all the TMA treatments. As a result, a significant improvement in the tensile properties of 2014 aluminium alloy has been observed.     

Influence of residual stresses and interfacial shear strength on matrix properties in fibre-reinforced ceramic matrix composites

Zircon matrix composites, uniaxially reinforced with a variety of SiC fibres were fabricated in order to create composites with different interfacial properties. Interfacial properties were varied by changing the nature of fibre coatings. The effect of changes in interfacial shear strength on important matrix properties, such as hardness and fracture toughness, was studied on a micro-scale using the microindentation technique. In addition, the relative orientation of the indented cracks with respect to the fibres was varied to investigate the existence of anisotropic behaviour of the matrix. The results indicated that the crack growth in the matrix was influenced by the presence of residual radial and axial stresses, such that relatively higher crack lengths were seen in certain directions in the matrix with respect to other directions. This asymmetric nature of the crack formation upon indentation was the reason for the observed anisotropic fracture toughness of the matrix. The residual stresses also led to anisotropic hardness and a critical load for crack initiation in the matrix.     

Microstructure,tensile properties and fracture behaviour of aluminium alloy 7150

A study has been made to understand the microstructure, tensile properties and fracture characteristics of aluminium alloy 7150. Detailed optical and transmission electron microscopical observations were used to analyse the intrinsic microstructural features of the alloy in the T77 condition. The alloy was deformed to failure over a range of strain rates in environments of 3.5% sodium chloride solution and laboratory air. The environment was found to have little influence on strength of the alloy. The strength only marginally increased with an increase in strain rate. However, for all strain rates, the ductility of the alloy degraded in the aggressive environment. The ratio of strain to failure in sodium chloride solution to that in laboratory air indicates that the alloy is only mildly susceptible to stress corrosion cracking. The fracture behaviour was different in the two environments. However, in a given environment the fracture behaviour was essentially the same. In the aggressive environment fracture was predominantly intergranular while fracture revealed a ductile transgranular failure in laboratory air. An attempt is made to discuss the kinetics of the fracture process in terms of competing mechanistic effects involving intrinsic microstructural features, matrix deformation characteristics, environment and strain rate.     

Effect of ply angle misalignment on out-of-plane deformation of symmetrical cross-ply CFRP laminates: Accuracy of the ply angle alignment

This paper discusses the accuracy of ply angle alignment and how it relates to out-of-plane deformation in carbon fiber reinforced plastics (CFRP) laminates. We investigated the deformation of symmetrical cross-ply laminates under hot and humid conditions. In spite of the symmetrically stacked laminates, unpredictable out-of-plane deformation occurred over time due to ply angle misalignment. The deformation was unstable and disproportionate to the absorbed moisture. A Monte Carlo simulation based on laminate theory was performed to quantify the deformation induced by the ply angle misalignment. Symmetrical cross-ply laminates were found to twist as they absorbed water when they underwent ply angle misalignments. By comparing the analytical results with experimental results, we concluded that a standard deviation of approximately 0.4° exists as ply angle misalignment in the laminates used in this study and that this slight ply angle misalignment can be a significant factor in out-of-plane deformation of cross-ply laminates.     

Retrogression and re-aging treatment on short transverse tensile properties of 7010 aluminium alloy extrusions

The influence of retrogression and re-aging treatment (RRA) on short transverse tensile properties of 7010 aluminium alloy extrusions was studied. The short transverse ductility of extrusions, which was much lower in the T6 condition, was improved to the optimum level after retrogression and re-aging treatment. It is found that short transverse ductility is influenced by the nature of precipitate particles located along the grain boundary. It is observed that coarsening of the grain boundary precipitates and its copper enrichment that occurs during RRA are found to be the factors responsible for improvement in stress corrosion cracking (SCC) resistance. The optimum retrogression and re-aging schedule is established that gives rise to the best combination of strength, ductility and SCC resistance.