A new superhybrid composite material: Vinylon-reinforced aluminium laminate (VIRALL)

A new composite material, VIRALL, has been developed by alternately laminating Vinylon/epoxy prepreg layers and aluminium alloy sheets. Compared with the corresponding aluminium alloy, VIRALL exhibits about a 24% increase in tensile strength and a 36% decrease in tensile modulus. These results are discussed in terms of the rule of mixtures. The most fascinating feature is that VIRALL has a lower density and a lower price than the corresponding aluminium alloy. Therefore a future possible use of VIRALL could be as a partial substitute for civil aluminium alloys.     

维尼纶增强铝合金叠层板(VIRALL)——一种新型超混杂复合材料

本文采用胶接技术,制成维尼纶增强铝合金叠层板(VIRALL)复合材料。结果表明,同相应的 Al合金相比,VIRALL 层板密度小,强度有一定程度的提高,还具有良好的变形性,并且价格低于 Al 合金。VIRALL 层板可以作为民用 Al 合金的部分替代品。     

预拉伸形变对VIRALL复合板疲劳性能的影响

研究了预拉伸形变对ＶＩＲＡＬＬ复合板疲劳性能的影响，发现疲劳寿命随预形变量的增大而延长，对复合板片层中内应力的分析结果表明，预形变对ＶＩＲＡＬＬ复合板疲劳性能的 通过改变ＶＩＲＡＬＬ中片层内应力状态实现的。     

VINYLON REINFORCED ALUMINIUM LAMINATES(VIRALL)——A KIND OF NEWLY DEVELOPED SUPERHYBRID COMPOSITES

本文采用胶接技术,制成维尼纶增强铝合金叠层板(VIRALL)复合材料。结果表明,同相应的Al合金相比,VIRALL 层板密度小,强度有一定程度的提高,还具有良好的变形性,并且价格低于Al 合金。VIRALL 层板可以作为民用Al 合金的部分替代品。     

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.     

Fatigue behaviour of carbon fibre-reinforced aluminium laminates

A new hybrid composite (CARALL), consisting of thin layers of carbon fibre/ epoxy prepreg sandwiched between aluminium sheets, has been developed. It is shown that this class of materials offers higher modulus, higher tensile strength and lower density than 2024-T3 alloy in the longitudinal direction. Under tension-tension fatigue loading, the hybrid laminates showed superior fatigue crack propagation resistance in the longitudinal direction, which may be attributed to the bridging effect imposed by the intact fibres in the crack wake. It has also been shown that the effectiveness of fatigue crack growth reduction increases with the thickness of the carbon fibre/epoxy layer. The resistance to fatigue crack propagation can be further improved by introducing compressive residual stresses in the aluminium layer by postcure stretching the laminate in the plastic region of the aluminium alloy.     

Fatigue crack growth and delamination behaviours of advanced Al‐Li alloy laminate under single tensile overload

In this paper, fatigue crack growth and delamination behaviours of a new fibre metal laminate (FML) named as Al‐Li alloy laminate were tested under different single tensile overloads and compared with those of glass laminate aluminium reinforced epoxy. The results indicate that the crack growth rate of Al‐Li alloy laminate after overload applied can quickly get back to its original level when the crack grows outside of the overload plastic zone. The overload has no influence on the delamination shape and size of Al‐Li alloy laminate. These results are obviously different from those found in the present study for GLARE, in which the crack growth rate cannot recover after overload, even though the crack is far beyond the overload plastic zone. A kink nearby the location of overload applied was found in the obtained delamination shape. This study provides some new results for better understanding the damage tolerance mechanism of FMLs.     

碳纤维/铝/环氧复合板的初步研究

本文揭示了碳纤维/铝/环氧复合板的显微结构,比重、热胀性能与碳纤维含量的关系,探讨了增强组份的表面处理、碳纤维/铝的叠层结构设计、碳纤维含量、纤维混杂对复合板力学性能的影响。试验表明:碳纤维/铝/环氧复合板具有轻质、低热胀、高强度等特点,当碳纤维增强环氧的含量为55vol%时,它的拉伸,弯曲与剪切强度达到或超过芳纶纤维或玻璃纤维/铝/环氧复合板的相应值。     

Influences of artificial pre-stressing on ply stresses and tensile properties of Vinylon fibre-reinforced aluminium laminates (VIRALL)

A simple micromechanical model based primarily upon the rule-of-mixtures is developed which allows prediction of the effects of fibre preload on the ply stresses (i.e., the initial residual stresses of fibre, adhesive and matrix) induced within VIRALL laminates, and the tensile stress-strain curves and mechanical properties of VIRALL laminates. The analysis of the ply stresses of VIRALL laminates indicates that pre-stressing will dramatically influence the ply stresses. The predicted tensile stress-strain curves of VIRALL laminates are in good agreement with the experimental curves and the results show that the stress-strain curves of VIRALL laminates move upwards when the prestress increases. The predicted tensile mechanical properties of VIRALL laminates at room temperature show good agreement with those obtained experimentally; both show that prestress can improve the tensile properties (i.e., elastic limit strength, 0.2% yield strength and failure strength) of VIRALL laminates.     

Investigation of fatigue crack growth rate in CARALL,ARALL and GLARE

Fibre metal laminates (FMLs) are being used to manufacture many structural components in aerospace industry because of their very high strength to weight ratios, yet the exact model for estimating fatigue crack propagation in FMLs cannot be developed because of many variable parameters affecting it. In this research, tensile strength, fatigue life and fracture toughness values of 2/1 configuration carbon reinforced aluminium laminate (CARALL), aramid reinforced aluminium laminate and glass laminate aluminium reinforced epoxy specimens have been investigated. Mechanical, chemical and electrochemical surface treatments were applied to AA 1050 face sheets to improve the adhesive properties of the laminates. The specimens were prepared using vacuum assisted resin transfer moulding technique and were cut to desired shapes. Fatigue tests were conducted on centre notched specimens according to ASTM Standard E399. Real time material data and properties of adhesive were used in definition of numerical simulation model to obtain the values of stress intensity factor at different crack lengths. It was observed that CARALL shows very superior tensile and fatigue strength because of stress distribution during failure. Numerical simulation model developed in this research accurately predicts fracture toughness of aramid reinforced aluminium laminate, CARALL and glass laminate aluminium reinforced epoxy with less than 2% error. An empirical analytical model using experimental data obtained during research was developed which accurately predicts the trend of FMLs fatigue life.     

Grain size effects on the mechanical properties of some squeeze cast light alloys

Mechanical property-grain size relationships have been examined for squeeze cast Al-4.5% Cu alloy, for an aluminium alloy with a composition corresponding to wrought 7010, and for a magnesium alloy AZ91. The general trend of the results obtained showed that the tensile properties and the fatigue strength improved as grain size decreased and the reverse was found to be the case for the fatigue crack propagation resistance and fracture energy of these castings. However, the results also showed that no simple common relationship existed between grain size and the tensile properties of the different alloys. The results are discussed in respect of their microstructures.     

Experimental investigation on mechanical properties and wear characterization of Al-Si12CulMg1 aluminium alloy reinforced with titanium diboride and boron carbide particulate hybrid composites using stir casting process

LM13 aluminium alloy (Al−Si12CulMg1) with titanium diboride (TiB₂) and boron carbide (B₄C) particulate hybrid composites have been prepared using stir casting process. Wt% of titanium diboride is varied from 0–10 and constant 5 wt% boron carbide particles have been used to reinforce LM13 aluminium alloy. Microstructure of the composites has been investigated and mechanical properties viz., hardness, the tensile strength of composites have been analyzed. Wear behavior of samples has been tested using a pin on disc apparatus under varying load (20 N–50 N) for a sliding distance of 2000 m. Fracture and wear on the surface of samples have been investigated. Microstructures of composites show uniform dispersion of particles in LM13 aluminium alloy. Hardness and tensile strength of composites increased with increasing wt % of reinforcements. Dry sliding wear test results reveal that weight loss of composites increased with increasing load and sliding distance. Fracture on the surface of composites reveals that the initiation of crack is at the interface of the matrix and reinforcement whereas dimples are observed for LM13 aluminium alloy. Worn surface of composites shows fine grooves and delamination is observed for the matrix.     

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.     

碳纤维增强铝合金层压材料的研究

本文探索了碳纤维增强铝合金层压材料(CFALL)的制备工艺,并制得了具有优异力学性能、耐温、耐疲劳的层压材料.就纤维含量、纤维取向及温度等对层压材料力学性能的影响作了探讨,并对它的疲劳性能以及预应变对疲劳性能的影响进行了研究.试验表明,CFALL具有轻质、高强、耐温、耐疲劳等特点,它的拉伸、弯曲强度超过了ARALL和GLALL,它的疲劳性能与ARALL相近.     

Exceeding average rule of mixtures stiffness in composite materials with interconnected fibres as reinforcement

Abstract

In the present study, a galvanised iron wire mesh structure of two different types was used to reinforce aluminium alloy 1050. The composite was synthesised using conventional casting followed by hot extrusion. Microstructural characterisation of the composite specimens showed good interfacial integrity between the matrix and the reinforcement wire. Thermomechanical analysis demonstrated that the average coefficient of thermal expansion of the composites was decreased below the values predicted by theoretical models. Tensile tests conducted on the composite specimens revealed that the average elastic modulus of the composites exceeded the rule of mixtures predictions. The average values of 0.2 yield strength and ultimate tensile strength of the composites were found to be higher than those of the monolithic aluminium alloy irrespective of a low volume fraction 0.049 of reinforcement. This unusual variation in the properties clearly reflects the uniqueness of using interconnected wires as reinforcement in the metallic matrix. An attempt is made in the present study to rationalise particularly the tensile behaviour, especially the stiffness of the composite material, according to the type of reinforcement preform in the aluminium alloy matrix.     

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.     

A new structural health monitoring system for composite laminates

The increasing use of composite laminates in safety critical structures has prompted the development of a robust structural health monitoring system for laminates, which uses metastable ferrous alloy inserts embedded within the laminate during component construction to provide an indication of the peak tensile strain encountered by the laminate. The metastable ferrous alloy insert has an austenitic crystal structure at room temperature, but upon application of strain, this transforms to a thermodynamically stable martensite, resulting in a change in magnetic susceptibility, which can be correlated with the peak strain experienced by the material (strain memory effect). This paper presents the test results that show that it is possible to manufacture a smart laminate in this fashion, and that sufficient strain is experienced by the insert to provide a significant change in magnetic susceptibility, thereby warning of a high strain level in the laminate. Various insert geometries and laminate thicknesses are also tested for their effect on the susceptibility measurements.     

Study of fatigue behaviour of 7475 aluminium alloy

Fatigue properties of a thermomechanically treated 7475 aluminium alloy have been studied in the present investigation. The alloy exhibited superior fatigue life compared to conventional structural aluminium alloys and comparable stage II crack growth rate. It was also noticed that the fatigue crack initiated from a surface grain and the crack extension was dominated by ductile striations. Analysis also revealed that this alloy possessed fracture toughness and tensile properties superior to that noticed with other structural aluminium alloys. Therefore the use of this alloy can safely reduce the overall weight of the aircraft.     

Evaluation of the tensile and fatigue behaviour of ingot metallurgy beryllium/aluminium alloys

The tensile and fatigue behaviour of ingot metallurgy beryllium/aluminium alloys produced by Nuclear Metals, Inc., is determined as a function of temperature. The wrought alloy and the casting alloy are both shown to have a very high stiffness to density ratio compared with common structural materials. The wrought alloy was found to have superior fatigue strength, tensile strength and ductility relative to the casting alloy; it also maintained a greater fraction of its tensile strength as a function of temperature. The stiffness of the materials can be readily explained using standard composite theory, where the material is treated as a discontinuous beryllium-reinforced aluminium matrix composite. The strength of the casting alloy is controlled to a large extent by the strength of its aluminium alloy matrix. In contrast, strengthening increments from both dislocation-based mechanisms and load transfer appear to be operative for the wrought material. Fractographic analysis of tensile specimens showed that preferential failure of the aluminium regions or the beryllium/aluminium interfacial regions occurs under certain circumstances. Fracture analysis of fatigue samples revealed no obvious fracture initiation sites and no evidence of limited/controlled crack growth regions. This revised version was published online in November 2006 with corrections to the Cover Date.     

Analysis of fracture and delamination in laminates using 3D numerical modelling

The static failure behaviour of the fibre-metal laminate GLARE is examined using 3D finite element simulations. The configuration analysed is a centre-cracked tensile specimen composed of two aluminium layers sandwiching a cross-plied, fibre-epoxy layer. The crack and delamination growths are simulated by means of interface elements equipped with a mixed-mode damage model. The mode-mixity is derived from an energy criterion typically used in linear elastic fracture mechanics studies. The damage kinetic law is rate-dependent, in order to simulate rate effects during interfacial delamination and to avoid numerical convergence problems due to crack bifurcations. The numerical implementation of the interface damage model is based on a backward Euler approach. In the boundary value problem studied, the failure responses of GLARE specimens containing elastic aluminium layers and elasto-plastic aluminium layers are compared. The development of plastic deformations in the aluminium layers stabilizes the effective failure response, and increases the residual strength of the laminate. For a ‘quasi-brittle’ GLARE specimen with elastic aluminium layers, the residual strength is governed by the toughness for interfacial delamination, and is in close correspondence with the residual strength obtained from a closed-form expression derived from energy considerations. Conversely, for a ‘ductile’ GLARE specimen with elasto-plastic aluminium layers, the residual strength is also determined by the relation between the fracture strength and the yield strength of the aluminium. The amount of constraint by the horizontal displacements at the vertical specimen edges has a moderate to small influence on the residual strength. Furthermore, the ultimate laminate strength is lower for a larger initial crack length, and shows to be in good correspondence with experimental values.