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.     

Beryllium metal matrix composites for aerospace and commercial applications

Abstract

There is a growing need in both aerospace and commercial markets for lighter weight, higher stiffness, higher thermal stability materials to solve the design engineers’ problems of reduced mass, higher access speeds, improved mechanical and thermal stability for today's advanced technology. To address those needs, Brush Wellman Inc. has developed, characterised, and put into high volume production a family of beryllium metal matrix composites. There are two classes of materials that have been developed to provide these engineering benefits to the designer in both the commercial and aerospace markets. The first family of materials is aluminium beryllium (AlBeMet, which is a registered tradename of Brush Wellman). This material is a metal matrix composite consisting of pure beryllium and aluminium, with 20–62 vol.-%Be and the remainder aluminium. The material is produced by both powder metallurgy and net shape technologies such as investment casting and semi-solid forming. The materials properties that make it attractive for the design engineer are a density that is 25% less than that of aluminium, a specific stiffness four times those of aluminium, titanium, steel, and magnesium, a higher dampening capacity than aluminium, and a coefficient of thermal expansion almost 50%lower than aluminium. The second family of materials is a beryllium–beryllium oxide metal matrix composite, which are called E materials. This material was developed to address the thermal management needs of the electronic packaging design engineer. The material properties that make this material attractive to the electronic packaging engineer are: a density 20–25% that of Kovar, Invar, and CuMoCu, and 30% less than Al–SiC; a thermal conductivity ranging from 210 to 240 W m^-1 K^-1and a tailorable coefficient of thermal expansion, ranging from 6×10^-6 to 8.7×10^-6 K^-1.     

Case studies on numerical simulations of the thermomechanical behaviour of Al SiC whisker composites

Simulations concerning the thermomechanical behaviour of a SiC whisker-reinforced aluminium alloy are carried out with the finite-element method. A representative three-dimensional unit cell with an overlapping whisker arrangement is derived by geometrical idealisations. Special importance is placed on the material model which should be as true to life as possible because the mechanical behaviour of the composite is decisively influenced by the inelastic behaviour of the matrix. Investigations showed pronounced inhomogeneous residual stresses in the matrix of the composite as a result of the cooling process during manufacture. These stresses have a considerable influence on mechanical behaviour, especially under transverse loading. The composite is anisotropic with higher stiffness and strength in the longitudinal direction than in the transverse direction. The elastic modulus of the aluminium alloy clearly increases due to the whisker reinforcement. However, the yield strength is limited by sharp stress concentrations which result from the cylindrical shape of the whiskers and especially the sharp edge between the shell and the upper surface. Furthermore, high hydrostatic stresses develop in the matrix in the regions of the whisker ends at tensile loads which can lead to damage and eventually to failure of the composite at low fracture strains. At cyclic loads, ratchetting can be observed and, as residual stresses decrease in the course of the cycles, the strength increases significantly.     

A compliant,high failure strain,fibre-reinforced glass-matrix composite

It is demonstrated that a unique form of composite material can be achieved by reinforcing glass matrices with discontinuous graphite fibres. The graphite fibres were utilized in the form of a paper, purchased in large sheets, and composites were formed by hot-pressing glass-powder-impregnated paper plys. The resultant composites exhibit high strength, high fracture toughness (compared to ceramics), low density and low thermal expansion coefficient. Of particular note is the unique tensile stress-strain curve achieved which exhibits both high strength and high failure strain. Its very non-linear shape differs markedly from that of either the unreinforced glass or a similarly reinforced epoxymatrix composite. In addition, the elastic modulus of the resultant composite, despite being reinforced with a high stiffness fibre, is lower than that of the parent matrix resulting in an unusually compliant ceramic material.     

Structure and properties of boron/aluminium composites

The influence of heat treatment on the structure and strength of boron/aluminium composites has been studied. Matrix material around a fibre is assumed to be hardened by dispersed particles of boron-containing compounds, presumably magnesium borides. Such a material is expected to have higher yield strength and greater brittleness compared to the original matrix material. A fibre and its surrounding zone is a prospective site of microcracking under loading. These zones grow as heat treatment proceeds. When neighbouring zones become linked to each other, the size of a possible brittle microcrack is doubled and a fall in the composite strength follows. A corresponding microstructural model of the composite failure is constructed.

Without having direct observation of magnesium borides, the model of composite behaviour is supported by (i) dependence of the composite strength on the size of the influence zone; (ii) dependence of the strength on the effective time, the latter being determined by the activation energy of magnesium diffusion in aluminium; (iii) observation of failure surfaces of composites subjected to various heat treatments; and (iv) a correlation of changes of matrix state detected by changes in X-ray diffraction patterns with a hypothetical picture of the development of the influence zones.     

Strengthening RC beams with composite fiber cement plate reinforced by prestressed FRP rods: Experimental and numerical analysis

This paper deals with the development of a new strengthening system for reinforced concrete beams with externally-bonded plate made of composite fiber cement reinforced by rebars made of fiber-reinforced plastic (FRP) [1]. The proposed strengthening material involves the preloading of FRP rod before mortar casting. The paper presents experimental and numerical analysis carried out on many large-scale beams strengthened by well-known reinforcement techniques, such as externally bonded Carbon Fiber-Reinforced Plastic (CFRP) plate and the Near Surface Mounted (NSM) technique, which are compared to the proposed new strengthening material through four-point bending tests. Results are analyzed with regard to the load-displacement curve, bending stiffness, cracking load, yield strength and failure load. The developed numerical model is in agreement with the experimental results. It clearly shows the effects of prestressed FRP rod on cracking mechanisms and internal strength distribution in the analyzed beams.     

Strength prediction of bolted joints in graphite/epoxy composite laminates

A parametric finite element analysis was conducted to investigate the effect of failure criteria and material property degradation rules on the tensile behaviour and strength of bolted joints in graphite/epoxy composite laminates. The analysis was based on a three-dimensional progressive damage model (PDM) developed earlier by the authors. The PDM comprises the components of stress analysis, failure analysis and material property degradation. The predicted load–displacement curves and failure loads of a single-lap single-bolt joint were compared with experimental data for different joint geometries and laminate stacking sequences. The stiffness of the joint was predicted with satisfactory accuracy for all configurations. The predicted failure load was significantly influenced by the combination of failure criteria and degradation rules used. A combination of failure criteria and material property degradation rules that leads to accurate strength prediction is proposed. For all the analyses performed, the macroscopic failure mechanism of the joint and the damage progression were also predicted.     

A finite element simulation and experimental validation of a composite bolted joint loaded in bending and torsion

The paper describes the FE modelling and experimental validation of a composite bolted joint loaded in bending and torsion. The selected material is a hybrid glass-carbon non-woven fabric in epoxy matrix; the manufacturing technology is hand lay-up with vacuum bagging. Following a material characterization run on standard specimens, six joints were subjected to monotonic or cyclic loading, followed by monotonic loading until failure. Results show that components exhibit lower reduction in stiffness and strength due to cycling compared to standard specimens. Also, the ratio between the maximum load for which no stiffness or strength degradation was observed in 10⁶ cycles and the static ultimate load were higher for components than for specimens. The modelling activity focuses on the simulation of the monotonic test. Numerical results show a good correlation with experiments in terms of material stiffness in the linear range and in predicting the regions of failure as highly stressed areas. However, being the analysis linear, a quantitative correlation between calculated stresses and ply strength properties was only found for the first ply failure.     

Fabrication of a carbon fibre/aluminium alloy composite under microgravity

Fabrication of a composite material with ultra low density and high stiffness under microgravity is the objective of the present investigation. The composite structure to be obtained is a random three-dimensional array of high modulus, short carbon fibres bounded at contact points by an aluminium alloy coated on the fibres. The material is highly porous and thus has a very low density. The motivation toward the investigation, simulation experiments, choice of the component materials and in-flight experiment during ballistic trajectory of a National Space Development Agency rocket are described herein. Supporting experimental evidence shows that the cohesion between the carbon fibre and the aluminium alloy is excellent, by which the achievement of desired properties of such composites seems probable.     

Acoustic Emission based on sentry function to monitor the initiation of delamination in composite materials

Delamination is the most common failure mode in composite materials, since it will result in the reduction of stiffness and can grow throughout other layers. Delamination is consisted of two main stages including initiation and propagation. Understanding the behavior of the material in these zones is very important, hence it has been thoroughly studied by different methods such as numerical methods, Acoustic Emission (AE), and modeling. Between these two regions initiation is a more vital stage in the delamination of the material. Once initiation occurs, which normally requires greater amount of force, cracks can easily propagate through the structure with little force and cause the failure of the structure. A better knowledge of initiation can lead to better design and production of stronger materials. Additionally, more knowledge about crack initiation and its internal microevents would help improve other parameters and result in higher strength against crack initiation. AE is a suitable method for in situ monitoring of damage in composite materials. In this study, AE was applied to test different glass/epoxy specimens which were loaded under mode I delamination. A function that combines AE and mechanical information is employed to investigate the initiation of delamination. Scanning electron microscope (SEM) was used to verify the results of this function. It is shown that this method is an appropriate technique to monitor the behavior of the initiation of delamination.     

碳化硅颗粒增强铝基复合材料颗粒表面改性技术研究现状

SiC颗粒增强铝基复合材料因具有高的比强度、比刚度、耐磨性及较好的高温稳定性而被广泛应用于航空航天、电子、医疗等领域,但由于SiC颗粒高熔点、高硬度的特点以及SiC颗粒与铝基体间存在界面反应,碳化硅铝基复合材料存在加工性差、界面结合力不足等问题,已无法满足航天等领域对材料性能更高的要求,因此开展如何改善基体与颗粒之间界面情况的研究对进一步提升复合材料综合性能具有重要的科学意义。结合国内外现有研究成果,总结了SiC颗粒与铝基体界面强化机制、界面反应特点、表面改性技术原理及数值建模的发展现状,结果表明,现有经单一表面改性方法处理后的增强颗粒对铝基复合材料性能的提升程度有限,因此如何采用新的手段使复合材料性能进一步提升将成为后续研究热点,且基于有限元数值模拟方法进行复合材料设计也是必然趋势。最后针对单一强化性能提升有限的问题,提出了基于表面改性的柔性颗粒多模式强化方法,同时针对现有的技术难点展望了后续的研究方向,以期为颗粒增强复合材料的制备提供理论参考。     

Modelling of fatigue damage progression and life of CFRP laminates

A progressive fatigue damage model has been developed for predicting damage accumulation and life of carbon fibre‐reinforced plastics (CFRP) laminates with arbitrary geometry and stacking sequence subjected to constant amplitude cyclic loading. The model comprises the components of stress analysis, fatigue failure analysis and fatigue material property degradation. Stress analysis of the composite laminate was performed by creating a three‐dimensional finite element model in the ANSYS FE code. Fatigue failure analysis was performed by using a set of Hashin‐type failure criteria and the Ye‐delamination criterion. Two types of material property degradations on the basis of element stiffness and strength were applied: a sudden degradation because of sudden failure detected by the fatigue failure criteria and a gradual degradation because of the nature of cyclic loading, which is driven by the increased number of cycles. The gradual degradation of the composite material was modelled by using functions relating the residual stiffness and residual strength of the laminate to the number of cycles. All model components have been programmed in the ANSYS FE code in order to create a user‐friendly macro‐routine. The model has been applied in two different quasi‐isotropic CFRP laminates subjected to tension–compression (T–C) fatigue and the predictions of fatigue life and damage accumulation as a function of the number of cycles were compared with experimental data available in the literature. A very good agreement was obtained.     

高强钢管超高强混凝土柱抗震性能试验研究

以轴压比、含钢率和钢材强度为参数,进行了8根高强钢管超高强混凝土柱和1根普通强度钢管超高强混凝土对比柱的拟静力试验,分析了各参数对破坏形态、荷载-位移滞回曲线、骨架曲线和各个抗震性能指标(如延性、耗能和强度与刚度退化等)的影响程度。结果表明:压弯破坏为主要破坏模式;弹性刚度受轴压比影响不大,但受含钢率和钢材强度影响较大;极限承载力受轴压比、含钢率和钢材强度影响较大,随前者增大而降低,随后两者增大而增大;延性受轴压比、钢材强度和含钢率影响较大,随前两者增大降低,随后者增大而增大;耗能能力随轴压比增大而减弱,随含钢率和钢材强度增大而增强;刚度和强度退化程度随轴压比增大而降低,随含钢率增大而增大,且前者随钢材强度增大而增大,后者则随钢材强度增大呈减小趋势。通过对比不同规程抗弯刚度计算方法,结果表明:受材料适用范围限制,各规程不适用于该类高强材料组合构件。     

Mechanical properties of an Al/Mg/Al trilaminated composite fabricated by hot rolling

An Al/Mg/Al composite with a trilaminate structure was fabricated by hot rolling and its mechanical properties at quasi-static rates of strain were investigated. The bonding strength of the trilaminated composite is about 40 MPa, mainly attributing to the mechanical bond at the interfaces. The first layer failure strength of the laminated composite increases from 305 to 372 MPa when the relative thickness of aluminium alloy layer increases from 0.235 to 0.265. The tensile and bending properties of the laminates were calculated based on the Classical Laminate Theory (CLT). The calculations of first layer failure strength based on CLT agree with the experimental data in the error of 2.9–18%. Thus, the first layer failure strength of the Al/Mg/Al trilaminated composite fabricated by hot rolling can be calculated by CLT with the maximum stress criteria. The calculations also show that the tensile modulus, the tensile rigidity, the specific tensile rigidity and the first layer failure strength of the laminated composite increase almost linearly with the relative thickness of the aluminium alloy component. The bending rigidity of the laminated composite increases with the relative thickness of aluminium alloy, and approximates to a fixed value after the relative thickness over 0.3. The specific bending rigidity increases with the relative thickness of aluminium alloy and reaches a maximum value when the relative thickness is 0.25.     

Microstructure,tensile and fatigue properties of AA6061/20 vol.%Al2O3p friction stir welded joints

Metal matrix composites reinforced with Al₂O₃ particles combine the matrix properties with those of the ceramic reinforcement, leading to higher stiffness and superior thermal stability with respect to the corresponding unreinforced alloys. However, their wide application as structural materials needs proper development of a suitable joining processes. The present work describes the results obtained from microstructural (optical and scanning electron microscopy) and mechanical evaluation (hardness, tensile and low-cycle fatigue tests) of an aluminium alloy (AA6061) matrix composite reinforced with 20 vol.% fraction of Al₂O₃ particles (W6A20A), welded using the friction stir welding process. The mechanical response of the FSW composite was compared with that of the base material and the results were discussed in the light of microstructural modifications induced by the FSW process on the aluminium alloy matrix and on the ceramic reinforcement. The FSW reduced the size of both particles reinforcement and aluminium grains and also led to overaging of the matrix alloys due to the frictional heating during welding. The FSW specimens, tested without any post-weld heat treatment or surface modification showed lower tensile strength and higher elongation to failure respect to the base material. The low-cycle fatigue life of the FSW composite was always lower than that of the base material, mainly at the lower strain-amplitude value. The cyclic stress response curves of the FSW composite showed evidence of progressive hardening to failure, at all cyclic strain-amplitudes, while the base material showed a progressive softening.     

A high performance fibre reinforced cement based plaster for retrofitting RC members

A high performance fibre-reinforced cementitious composite (HPFRCC) material is developed to be used for retrofitting reinforced concrete members. It can be applied to the face of a concrete member to the desired thickness as a wet mix or as an adhesively-bonded prefabricated slab or strip. The material is compatible with concrete and possesses favourable strength and ductility properties, desirable for seismic retrofit. It overcomes some of the problems associated with the current techniques based on externally bonded steel plates and fibre-reinforced polymer (FRP) laminates caused mainly by the mismatch of their tensile strength and stiffness with that of the concrete member being retrofitted. An extensive rheological analysis is undertaken to develop the appropriate mixes using different types and mix proportions of constituent materials including; fine steel fibres, fine quartz sand, silica fume, cement and superplasticizer. Much reduced amounts of steel fibres are used compared to the previous studies so that ordinary mixing procedures could be applied and a more cost-effective retrofitting material could be developed. Samples made of the optimum mixes are shown to posses very high compressive and tensile strengths and sufficient ductility for the composite plaster to be used externally for strengthening and seismic retrofitting of concrete members.     

Effects of interphase strength on the damage modes and mechanical behaviour of metal–matrix composites

A numerical version of the generalized self-consistent method previously developed by the authors is combined with the Gurson model to undertake a parametric investigation of the damage mechanisms and their relations with the macroscopic tensile properties of SiC reinforced aluminium, for three different interphase strengths. The results show that the interphase strength is a governing factor for damage propagation in the composite. Thus, transformation of the failure mechanism from reinforcement fracture to void nucleation and growth can be achieved by reducing the interphase bond strength, although the strengthening effects on the composite decrease unfavourably.     

A new damage model for composite laminates

Aircraft composite structures must have high stiffness and strength with low weight, which can guarantee the increase of the pay-load for airplanes without losing airworthiness. However, the mechanical behavior of composite laminates is very complex due the inherent anisotropy and heterogeneity. Many researchers have developed different failure progressive analyses and damage models in order to predict the complex failure mechanisms. This work presents a damage model and progressive failure analysis that requires simple experimental tests and that achieves good accuracy. Firstly, the paper explains damage initiation and propagation criteria and a procedure to identify the material parameters. In the second stage, the model was implemented as a UMAT (User Material Subroutine), which is linked to finite element software, ABAQUS™, in order to predict the composite structures behavior. Afterwards, some case studies, mainly off-axis coupons under tensile or compression loads, with different types of stacking sequence were analyzed using the proposed material model. Finally, the computational results were compared to the experimental results, verifying the capability of the damage model in order to predict the composite structure behavior.     

Joining of Aluminium Metal Matrix Composites

Aluminium metal matrix composites (AlMMCs) offer several advantages relative to monolithic aluminium alloys such as high stiffness, strength, wear resistance, low thermal expansion coefficient, etc. However, despite considerable improvements in developing AlMMCs, the lack of reliable joining methods restrict their greater application. Fusion welding of AlMMCs has not proved successful because high temperature nature of the process normally causes unfavorable reactions between the reinforcement and the matrix, leading to the formation of a variety of defects. On the other hand, solid-state welding and diffusion bonding may not be suitable due to the presence of chemically stable surface layer of aluminium oxide, which, being insoluble in aluminium, inhibits metal-to-metal contact during diffusion bonding. Furthermore, diffusion bonding requires a very smooth and clean contact surface, which is difficult to obtain in industrial applications. As an alternative, transient liquid phase (TLP) diffusion bonding, which operates at a lower temperature, can be used to circumvent the problems associated with the oxide layer. The formation of liquid phase (eutectic) can assist the disruption of the oxide layer and promote metallic contact. The composite material used in the present study consisted of 6061 alloy containing 15 volume % of SiC particulates of 23 μm diameter. TLP bonding was carried out at 560 ˚C in argon atmosphere using copper as an interlayer with different pressures and holding times. TLP-bonded AlMMCs were characterized by optical and scanning electron microscopy, microhardness survey, and shear tests. The results indicated that adequate bond strength could be achieved with suitable bonding parameters such as holding time and initial pressure.