Fatigue in SiC-particulate-reinforced aluminium alloy composites

The fatigue behaviour in SiC-particulate-reinforced aluminium alloy composites has been briefly reviewed. The improved fatigue life reported in stress-controlled test results from the higher stiffness of the composites; therefore it is generally inferior to monolithic alloys at a constant strain level. The role of SiC particulate reinforcement has been examined for fatigue crack initiation, short-crack growth and long-crack growth. Crack initiation is observed to occur at matrix-SiC interface in cast composites and either at or near the matrix-SiC interface or at cracked SiC particles in powder metallurgy processed composites depending on particle size and morphology. The da/dN vs ΔK relationship in the composites is characterized by crack growth rates existing within a narrow range of ΔK and this is because of the lower fracture toughness and relatively high threshold values in composites compared with those in monolithic alloys. An enhanced Paris region slope attributed to the monotonic fracture contribution are reported and the extent of this contribution is found to depend on particle size. The effects of the aging condition on crack growth rates and particle size dependence of threshold values are also treated in this paper. © 1991.

Influence of macroscopic residual stress fields on fatigue crack growth measurement in SiC particulate reinforced 8090 aluminium alloy

The effect of residual stresses, induced by cold water quenching, on the morphology of fatigue crack fronts has been investigated in a powder metallurgy 8090 aluminium alloy, with and without reinforcement in the form of 20 wt-%SiC particles. Residual stress measurements reveal that the surface compressive stresses developed in these materials are significantly greater than in conventional metallurgy ingot 8090, because surface yielding occurs on quenching. The yield stresses of the powder route materials are greater than those of ingot produced 8090 and hence greater surface stresses can be maintained. In fatigue, severe crack front bowing is observed in the powder formed materials as a result of the reduction of the R ratio (minimum load/maximum load) by the compressive residual stresses at the sides of the specimen, causing premature crack closure and hence reducing the local driving force for fatigue crack growth ΔKeff. This distortion of the crack fronts introduces large errors into measurements of crack growth rate and threshold values of ΔK.

The influence of ageing on fatigue crack growth in SiC-particulate reinforced 8090

A study of the influence of SiC-particulate reinforcement on ageing and subsequent fatigue crack growth resistance in a powder metallurgy 8090 aluminium alloy-SiC composite has been made. Macroscopic hardness measurements revealed that ageing at 170°C in the composite is accelerated with respect to the unreinforced alloy, though TEM studies indicate that this is not due to the enhanced precipitation of S′. Fatigue crack growth rates in the naturally aged condition of the composite and unreinforced matrix are similar at low to medium values of ΔK, but diverge above ≈ 8 MPa√m owing to the lower fracture toughness of the composite. As a result of the presence of the reinforcement, planar slip in the composite is suppressed and facetted crack growth is not observed. Ageing at or above 170°C has a deleterious effect on fatigue crack growth. Increased ageing time decreases the roughness of the fracture path at higher growth rates. These effect are though to be due to microstructural changes occurring at or near to the SiC/matrix interfaces, providing sites for static mode failure mechanisms to operate. This suggestion is supported by the observation that as ΔK increases, crack growth rates become Kmax dependent, implying the crack growth rate is strongly influenced by static modes.

Nanoindentation measurement of surface residual stresses in particle-reinforced metal matrix composites

The surface residual stresses in SiC particle-reinforced Al matrix composites are measured using a recently developed nanoindentation technique. The tensile biaxial residual stress in Al is found to increase with the particle concentration. The stress magnitudes are in reasonable agreement with those from numerical modeling.

Short and long fatigue crack growth in a SiC reinforced aluminium alloy

Fatigue crack growth behaviour in a 15 wt% SiC particulate reinforced 6061 aluminium alloy has been examined using pre-cracked specimens. Crack initiation and early growth of fatigue cracks in smooth specimens has also been investigated using the technique of periodic replication. The composite contained a bimodal distribution of SiC particle sizes, and detailed attention was paid to interactions between the SiC particles and the growing fatigue-crack tip. At low stress intensity levels, the proportion of coarse SiC particles on the fatigue surfaces was much smaller than that on the metallographic sections, indicating that the fatigue crack tends to run through the matrix avoiding SiC particles. As the stress intensity level increases, the SiC particles ahead of the growing fatigue crack tip are fractured and the fatigue crack then links the fractured particles. The contribution of this monotonic fracture mode resulted in a higher growth rate for the composite than for the unreinforced alloy. An increase in the proportion of cracked, coarse SiC particles on the fatigue surface was observed for specimens tested at a higher stress ratio.

Fatigue in metallic alloys containing non-metallic particles

This paper examines the effects of non-metallic particles on fatigue performance and, in particular, their influence on fatigue crack propagation at high ΔK (Kmax) levels. The nature and properties of a number of common non-metallic particles found in Fe- and Al- based alloys are described, and consideration is given to the consequences of mismatch of physical and chemical properties between particle and matrix. Effects of particles on fatigue in conventional alloys are illustrated and compared with the behaviour of Al/SiCp MMC. The problems associated with developing particulate reinforced MMC with adequate fatigue crack growth resistance and toughness for structural applications are discussed. © 1991.

Role of reinforcement/matrix interfacial strength in fatigue crack propagation in particulate SiC reinforced aluminum alloy 8090

A study has been made of the influence of the reinforcement/matrix interfacial strength on fatigue crack propagation in a powder metallurgy aluminum alloy 8090-SiC particulate composite. The interfacial region has been altered by two separate routes, the first involving aging of the 8090 matrix, with the subsequent formation of precipitate free zones at the boundaries, and the second consisting of oxidizing the surface of the SiC particles before their incorporation into the composite. In the naturally aged condition, oxidation of the SiC leads to a reduction in fatigue crack growth resistance at higher values of stress intensity range ΔK. This is due to a proportion of the crack growth occurring through voids formed in association with many of the weak SiC interfaces which have retained a layer of thick surface oxide after processing. On overaging no difference in crack growth rate is discernible between the oxidized and unoxidized SiC composites. It is proposed that this is due to similar levels of interfacial weakening having occurred in both composites, indicating that this is an important factor in the reduction of the high ΔK crack growth resistance of the unoxidized SiC composite on aging.

The structural properties of glassfibre reinforced plastics

The research work described in this thesis is concerned with the development of glassfibre reinforced plastics for structural uses in Civil Engineering construction. The first stage was primarily concerned with the design of GRP lamintes with structura1 properties and method of manufacture suitable for use with relatively large structural components. A cold setting, pressure moulding technique was developed which proved to be efficient in reducing the void content in the composite and minimising the exothermic effect due to curing. The effect of fibre content and fibre arrangement on strength and stiffness of the cornposite was studied and the maximum amount of' fibre content that could be reached by the adopted type of moulding technique was determined. The second stage of the project was concerned with the introduction of steel-wire "sheets" into the GRP cornposites, to take advantage of the high modulus of steel wire to improve the GRP stiffness and to reduce deformation. The experimental observations agreed reasonably well with theoretical predictions in both first and second stages of the work. The third stage was concerned with studying the stability of GRP flat rectangular plates subjected to uniaxial compression or pure shear, to simulate compression flanges or shear webs respectively. The investigation was concentrated on the effect of fibre arrangement in the plate on buckling load. The effect of the introduction of steel-wire sheets on the plate stability in compression was also investigated. The boundary conditions were chosen to be close to those usually assumed in built-up box-sections for both compression flanges and webs. The orthotropic plate and the mid-plane symmetric were used successfully in predicting the buckling load theoretically. In determining the buckling load experimentally, two methods were used. The Southwell plot method and electrical strain gauge method. The latter proved to be more reliable in predicting the buckling load than the former, especially for plates under uniaxial compression. Sample design charts for GRP plates that yield and buckle simultaneously under compression are also presented in the thesis. The final stage of the work dealt with the design and test of GRP beams. The investigation began by finding the optimum cross-section for a GRP beam. The cross-section which was developed was a thin walled corrugated section which showed higher stiffness than other cross-sections for the same cross-sectional area (i.e. box, I, and rectangular sections). A cold setting, hand layings technique was used in manufacturing these beams wbich were of nine types depending on the type of glass reinforcement employed and the arrangement of layers in the beam. The simple bending theory was used in the beam design and proved to be satisfactory in predicting the stresses and deflections. A factor of safety of 4 was chosen for design purposes and considered to be suitable for long term use under static load. Because of its relatively low modulus, GRP beams allowable deflection was limited to 1/120th of the span which was found to be adequate for design purposes. A general discussion of the behaviour of GRP composites and their place relative to the more conventional structural material was also presented in the thesis.

Interfacial effects on fatigue and fracture in discontinuously reinforced metal matrix composites

Interfaces in conventional monolithic alloys exert an important influence on fatigue and fracture behavior. In discontinuously reinforced metal matrix composites (MMCs), the role of interface is even more dominant. The interfacial is higher in MMCs and the interfaces are generally of high energy and chemically unstable. This paper reviews the factors which can affect interfacial strength in discontinuously reinforced MMCs, and the ways in which interfacial strength can be controlled. The effects of interfacial strength on fatigue crack propagation and fracture behavior are then illustrated.

Mechanisms of secondary dispersions separation in particulate beds

The mechanisms by which drops of secondary liquid dispersion ie. <100μ m, are collected, coalesced and transferred have been studied in particulate beds of different sizes and heights of glass ballotini. The apparatus facilitated different coalescer cell arrangements. The liquid-liquid system was toluene/de-ionised water. The inlet drop size distribution was measured by microscopy and using the Malvern Particle Size analyser; the outlet dispersion was sized by photography. The effect of packed height and packing size upon critical velocity, pressure drop and coalescence efficiency have been investigated. Single and two phase flow pressure drops across the packing were correlated by modified Blake-Kozeny equations. Two phase pressure drop was correlated by two equations, one for large ballotini sizes (267μm - 367μm), the other for small ballotini sizes (93μm- 147.5μm). The packings were efficient coalescers up to critical velocities of 3 x 10-2 m/s to 5 x 10-2 m/s. The saturation was measured across the bed using relative permeability and a mathematical model developed which related this profile to measured pressure drops. Filter coefficients for the range of packing studied were found to be accurately predicted from a modified queueing drop model. 

The durability of glass fibre reinforced cements made with new cementitious matrices

Widespread use of glass fibre reinforced cement (GRC) has been impeded by concerns over its durability. Three degradation mechanisms are proposed - fibre corrosion, Ca(OHh precipitation and matrix densification - although their relative importance is debated. Matrices with reduced alkalinities and Ca(OH)2 contents are being developed; the aim of this study was to investigate their hydration and interaction with alkali-resistant fibres to determine the factors controlling their long-term durability, and assess the relevancy of accelerated ageing. The matrices studied were: OPC/calcium-sulphoaluminate cement plus metakaolin (C); OPC plus metakaolin (M); blast-furnace slag cement plus a micro-silica based additive (D); and OPC (O). Accelerated ageing included hot water and cyclic regimes prior to tensile testing. Investigations included pore solution expression, XRD, DTA/TG, SEM and optical petrography. Bond strength was determined from crack spacings using microstructural parameters obtained from a unique image analysis technique. It was found that, for the new matrices - pore solution alkalinities were lower; Ca(OH)2 was absent or quickly consumed; different hydrates were formed at higher immersion temperatures; degradation under 65°C immersion was an order of magnitude slower, and no interfilamental Ca(OH)2 was observed .It was concluded that: fibre weakening caused by flaw growth was the primary degradation mechanism and was successfully modelled on stress corrosion/static fatigue principles. OPC inferiority was attributed partly to its higher alkalinity but chiefly to the growth of Ca(OH)2 aggravating the degradation; and hot water ageing although useful in model formulation and contrasting the matrices, changed the intrinsic nature of the composites rather than simply accelerating the degradation mechanisms.

Reinforced thermoplastics for engineering applications

The effect of mechano-chemically bound polypropylene modifiers on the mechanical performance and thermal-oxidative stability of polypropylene composites has been studied. The mechanical performance of unmodified polypropylene containing silane coupled glass and Rockwool (mineral) fibre was poor by comparison with a similar commercially produced glass reinforced composite; this was attributed to poor fibre-matrix adhesion. Mechano-chemical binding with unsaturated additives was obtained in the presence of a free radical initiator (di-cumyl peroxide). This process was inhibited by stabilisers present in commercial grades of polypropylene composites by chemical bond formation between the chemically bound modifier and the silane coupling agent on the fibre surface, resulting in a dramatic improvement in the mechanical properties, dimensional stability and retention of mechanical performance after immersion in fluids typically found in under-bonnet environments.A feature unique to some of these modifiers was their ability not only to enhance the mechanical properties of polypropylene composites to levels substantially in excess of currently available commercial materials, but their ability to act as effective thermal-oxidative polypropylene stabilisers. The mode of action was shown to be a chain-breaking mechanism and as a result of the high binding levels achieved during melt processing, these modifiers were able to efficiently stabilise polypropylene in the most severe volatilising and solvent-extracting environments, thus giving much better protection to the polymer than currently available commercially stabilised grades of polypropylene.

The mechanical and wear behaviour of B(SiC) fibre-reinforced composite materials

The mechanical properties and wear behaviour of B(SiC) fibre-reinforced metal matrix composites (MMCs) and aluminium alloy (2014) produced by metal infiltration technique were determined. Tensile tests were peliormed at different conditions on both the alloy matrix and its composite, and the tensile fracture surfaces were also examined by Scanning Electron Microscopy (SEM). Dry wear of the composite materials sliding on hardened steel was studied using a pin-on-disc type machine. The effect of fibre orientation on wear rate was studied to provide wear resistance engineering data on the MMCs. Tests were carried out with the wear surface sliding direction set normal, parallel and anti-parallel to the fibre axis. Experiments were perfonned for sliding speeds of 0.6, 1.0 and 1.6 m/s for a load range from 12 N to 60 N. A number of sensitive techniques were used to examine worn surface and debris, i.e: Scanning Electron Microscopy (SEM), Backscattered Electron Microscopy (BSEM) and X-ray Photoelectron Spectroscopy (XPS). Finally, the effect of fibre orientation on the wear rate of the Borsic-reinforced plastic matrix composites (PMCs) produced by hot pressing technique was also investigated under identical test conditions. It was found that the composite had a markedly increased tensile strength compared with the matrix. The wear results also showed that the composite exhibited extremely low wear rates compared to the matrix material and the wear rate increased with increasing sliding speed and normal load. The effect of fibre orientation was marked, the lowest wear rates were obtained by arranging the fibre perpendicular to the sliding surface, while the highest wear was obtained for the parallel orientation. The coefficient of friction was found to be lowest in the parallel orientation than the others. Wear of PMCs were influenced to the greatest extent by these test parameters although similar findings were obtained for both composites. Based on the results of analyses using SEM, BSED and XPS, possible wear mechanisms are suggested to explain the wear of these materials.

An X-ray micro-fluorescence study to investigate the distribution of Al, Si, P and Ca ions in the surrounding soft tissue after implantation of a calcium phosphate-mullite ceramic composite in a rabbit animal model

Synthetic calcium phosphates, despite their bioactivity, are brittle. Calcium phosphate-mullite composites have been suggested as potential dental and bone replacement materials which exhibit increased toughness. Aluminium, present in mullite, has however been linked to bone demineralisation and neurotoxicity: it is therefore important to characterise the materials fully in order to understand their in vivo behaviour. The present work reports the compositional mapping of the interfacial region of a calcium phosphate-20 wt% mullite biocomposite/soft tissue interface, obtained from the samples implanted into the long bones of healthy rabbits according to standard protocols (ISO-10993) for up to 12 weeks. X-ray micro-fluorescence was used to map simultaneously the distribution of Al, P, Si and Ca across the ceramic-soft tissue interface. A well defined and sharp interface region was present between the ceramic and the surrounding soft tissue for each time period examined. The concentration of Al in the surrounding tissue was found to fall by two orders of magnitude, to the background level, within similar to 35 mu m of the implanted ceramic.

The transference of axial load from reinforced concrete columns to bases

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