The fracture of boron fibre-reinforced 6061 aluminium alloy

The fracture of 6061 aluminium alloy reinforced with unidirectional and cross-plied 0/90°, 0/90/±45° boron fibres has been investigated. The results have been described in terms of a critical stress intensity,K _Q. Critical stress intensity factors were obtained by substituting the failure stress and the initial crack length into the appropriate expression forK _Q. Values were obtained that depended on the dimensions of the specimens. It was therefore concluded that, for the size of specimen tested, the values of the critical stress intensity,K _Q, did not reflect any basic materials property.     

Thermal cycling studies of a cross-plied P100 graphite fibre-reinforced 6061 aluminium composite laminate

Response to thermal cycling of a 0/90 cross-plied P100 Gr-6061 aluminium composite laminate was studied between a minimum temperature (T _min) of 25 C and maximum temperatures (T _max) of 100 and 540 C. Strain hysteresis was observed between the heating and cooling half-cycles and was attributed to anelastic strains induced by matrix residual stresses. A residual plastic strain was also observed after the first cycle, and was seen to disappear after subsequent cycles. Alteration of the thermal residual stress state of the matrix via heat treatments was found to change significantly the magnitude of the plastic strain. These results were compared with those of studies on unidirectionally reinforced P100 Gr-6061 aluminium composites, and the differences were explained on the basis of the residual stresses resident in the matrix. Optical and electron microscopy were also utilized to observe thermal damage, which occurred predominantly along improperly bonded fibre-matrix interfaces.     

The theory of critical distances and fatigue from notches in aluminium 6061

Fatigue failure of metal components containing notches, cracks and other defects has been an active research topic for many years because of its important practical and theoretical implications. Recently, Taylor and his colleagues have re‐visited this topic and proposed the theory of critical distance (TCD), which summarizes the early work by Neuber, Peterson and others in a unifying theory and predicts fatigue fracture with the use of a critical distance, L_o. In this paper, an experimental and numerical study of the fatigue of notched and un‐notched 6061 aluminium alloys is used to verify the TCD and some of the limitations of the TCD are discussed on this basis.     

The fatigue properties of dynamically consolidated aluminium

Dynamic consolidation produces material made up of work-hardened particles bonded together by ultra rapidly solidified welds. Obviously such a structure will have unique mechanical properties. It has been found that the fatigue strength at 10⁷ cycles of dynamically consolidated aluminium is significantly above that of wrought ingot metallurgy material of similar composition. Annealing the compacts at 300° C only slightly reduces the fatigue strength. However, there is an appreciable reduction after annealing at 400° C and above. The fatigue strength appears equal to one-half the yield strength; a more complex relationship exists for the ultimate strength. The fatigue strength, like the yield strength, is probably related to the ratioR, which predicts the degree of interparticle melting. The value of this ratio may be calculated. The compacts undergo an insignificant amount of cyclic softening during low-cycle fatigue tests, although some softening does occur in specimens annealed at 300° C. Microscopy indicates that failure is of a ductile interparticular nature with some evidence of small areas of fatigue striations and “ridges”. The results, especially the reduction in properties on annealing, suggest that the improved fatigue properties reported for aluminium are a direct result of dynamic consolidation. Application to other alloys is, therefore, logical. The degree will depend upon the amount of work-hardening and of ultra rapidly solidified welding the alloy experiences.     

Mode III fatigue crack propagation rates in 6061-T6 aluminium

Fatigue crack growth rates were studied in type 6061-T6 aluminium alloy. Unlike the preponderance of previous studies, the present observations were carried out on cracks driven by a Mode III, or antiplane shear, type of loading. The observed crack growth rates were precisely correlated with the Mode III stress intensity factor range, ΔK_III. A simple power growth rate law, similar to that which predicts the growth rates of the more common Mode I driven crack, relates the incremental extension of the fatigue crack per cycle of loading to the stress intensity factor range. Fractographic examination of the fatigue crack surfaces indicated that the cracks propagated transgranularly, and did not seek out principal tensile stress planes, or Mode I growth habits.     

Material damping in 6061-T6511 aluminium to assess fatigue damage

Studies in fatigue can be summarized into two stages, fatigue crack initiation and crack propagation. Fatigue damage may increase the risk of failure under cyclic load. Energy dissipation, termed damping, occurs in engineering metals and is a function of the cyclic loading history. Damping behaviour of materials has been estimated using many different experimental techniques, and parameters i.e. the loss factor vs. strain amplitude, frequency range, etc. However, micro‐structural changes in the form of fatigue damage are also contributors to damping in engineering materials. In order to measure energy dissipation, a damping monitoring method has been used. Under a constant cyclic load up to the point of fatigue crack initiation, the effects of fatigue on damping factor were studied for 6061‐T6511 aluminium alloy. In the experiments, the stress levels were below yield point, 50% and 70% of ultimate strength. Experimental results showed that the damping factor changes with the number of fatigue cycles. Percentage increase in damping energy was calculated using experimental data.     

Tensile properties of thermally exposed aluminium borate whisker reinforced 6061 aluminium alloy composite

Abstract

The effect of thermal exposure on the tensile properties of aluminium borate whisker reinforced 6061 aluminium alloy composite was studied. The interfacial reaction was investigated by TEM and the mechanical properties were studied using tensile tests. The results indicated that the interfacial reaction had an influence on the mechanical properties of the composite, so that the maxima of Young’s modulus and ultimate tensile strength of the composite after exposure at 500°C for 10 h were obtained for the optimum degree of interfacial reaction. The yield strength, however, was not only affected by the interfacial state but also by many other factors.     

Repeatability of gel electrode measurements of fatigue deformation in 6061-T6 aluminium

In a previous report, using 5 V, 5 s pulses, the gel electrode method was shown to detect fatigue deformation in 6061-T6 aluminium, but each specimen was only measured once. In this study the technique is refined so that the measurements can be repeated many times to monitor the distribution and accumulation of deformation at intervals during a fatigue test. The current from the gel electrode produces both passivation (film formation) and corrosion at the sites of deformation on the metal surface. These two influences are balanced if the gel electrode measurement is performed with a 5 V, 350 ms pulse, and the charge flow during subsequent measurements is then repeatable to within ±15%. Although this pulse duration is an order of magnitude shorter than that employed in the previous study, excellent sensitivity is retained. Fatigue deformation is detected in 6061-T6 aluminium as early as 0.1% of the fatigue life. The charge flow measurements and gel electrode images show that the deformation accumulates at many sites during the early stages (5%) of fatigue life, but by about 50% of life clusters of sites of fatigue damage are well established, and provide a path for final fracture.     

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.     

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.     

A low cycle fatigue model of a short-fibre reinforced 6061 aluminium alloy metal matrix composite

A low cycle fatigue model has been developed to predict the fatigue life of both the unreinforced aluminium alloy and the short-fibre reinforced aluminium alloy metal-matrix composites based solely on crack propagation from microstructural features. In this approach a crack is assumed to initiate and grow from a microstructural feature on the first cycle. The model assumes that there is a fatigue-damaged zone ahead of the crack tip within which the actual degradation of the material takes place. The low-cycle fatigue crack growth and the condition for failure are controlled by the amount of cyclic plasticity generated within the fatigue-damaged zone ahead of the crack tip and by the ability of the short fibres to constrain this cyclic plasticity. The fatigue crack growth rate is directly correlated to the range of crack-tip opening displacement. The empirical Coffin–Manson and Basquin laws have been derived theoretically and applied to compare with total-strain controlled low-cycle fatigue life data obtained on the unreinforced 6061 aluminium alloy at 25 °C and on the aluminium alloy AA6061 matrix reinforced with Al₂O₃ Saffil short-fibres of a volume fraction of 20 vol.% and test temperatures from −100 to 150 °C. The proposed model can give predicted fatigue lives in good agreement with the experimental total-strain controlled fatigue data at both high strain low-cycle fatigue and low strain high-cycle fatigue regime. It is remarkable that the addition of high-strength Al₂O₃ fibres in the 6061 aluminium alloy matrix will not only strengthen the microstructure of the 6061 aluminium alloy, but also channel deformation at the tip of a crack into the matrix regions between the fibres and therefore constrain the plastic deformation in the matrix. The overall expected effect is therefore the reduction of the fatigue ductility.     

High-temperature mechanical properties of aluminium alloys reinforced with boron carbide particles

The mechanical properties of particulate-reinforced metal-matrix composites based on aluminium alloys (6061 and 7015) at high temperatures were studied. Boron carbide particles were used as reinforcement. All composites were produced by hot extrusion. The tensile properties and fracture analysis of these materials were investigated at room temperature and at high temperature to determine their ultimate strength and strain to failure. The fracture surface was analysed by scanning electron microscopy.     

The growth of fatigue cracks in aluminium

Experiments have been performed in order to assess quantitatively the rates of fatigue crack propagation in metals. Low stress fatigue tests have been performed on aluminium specimens under conditions of constant load and constant stress. The crack growth rates are found to be proportional to the fourth power of the appropriate stress intensity factors. Methods of processing fatigue test data are briefly discussed.

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Zusammenfassung Es wurden Versuche durchgeführt um die Fortpflanzungsgeschwindigkeit von Rissen in Metallen quantitativ zu erfassen. Aluminiumproben wurden in Dauerstandsversuchen unter konstanter Belastung und bee konstanter Spannung geprüft.Hierbei ergab, daß die Rißwachstumsgeschwindigkeit der vierten Potenz der entsprechenden Spannungsintensitäts-faktoren proportional ist. Die Verfahren zur Auswertung von Versuchsergebnissen wurden kurz besprochen.

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Résumé Des essais ont été exécutés en vue de déterminer, sur une base quantitative, les vitesses de propagation des fissures de fatigue dans les métaux. Des éprouvettes d'aluminium ont été soumises à essais de fatigue sons contraintes de faible amplitude, et l'on a examiné séparément les cas de la charge imposée et de la tension imposée.On trouve que les vitesses de propagation des fissures sont proportionnelles à la puissance quatrième du facteur d'intensité de contrainte que correspond aux deux cas envisagés. Suit une brève discussion sur la présentation des données expérimentales d'essais de propagation des fissures de fatigue.

     

A study on the low-cycle fatigue properties of SiCp/6061 Al composites

Metal matrix composites (MMCs) were produced using a powder metallurgy method. Fatigue and tensile specimens were extruded and rolled before being machined. The matrix of the composite was a 6061 Al alloy and the reinforcement was 180 mesh SiC particle (SiCp). Different weight fractions (10, 20 and 30 wt%) of 180 mesh SiCp were introduced to determine the influence of the SiCp content on the tensile and low-cycle properties. Reasonable Coffin-Manson plots have been obtained in low-cycle fatigue. More accountable data of fatigue ductility exponents and fatigue ductility coefficients have been obtained for the composites and monolithic Al alloy. Increasing the content of SiCp has been shown to encourage the development of particle cracks and resulted in the degradation of fatigue properties. This revised version was published online in November 2006 with corrections to the Cover Date.     

Effect of pre-existing substructures on the fatigue properties of pure aluminium

Pure aluminium was thermomechanically treated (cold rolled to 80% reduction of thickness and annealed at 200° C for various lengths of time) in order to obtain different kinds of substructures. These aluminium specimens were then fatigued with a stress amplitude of 9×10³ psi or 90% of their ultimate tensile strength. The substructures of the specimens before and after fatigue were compared with each other and were then correlated to their fatigue property. The size and shape of substructure introduced by fatigue depend on the pre-existing substructure formed by thermomechanical treatment. The sub-boundaries of the fatigued specimen are not as well-defined as the pre-existing subgrain. The substructure size of the fatigued specimen increases initially with annealing time and then reaches a saturation value (2.32 m) when the annealing time is longer than 2 h. A peak value of fatigue life against the annealing time was found if the stress amplitude of fatigue is 90% of the ultimate tensile strength. The peak value also occurs at an annealing time of 2 h. The reason why the specimen annealed for 2 h possesses the optimum fatigue and property is discussed in terms of the substructures before and after fatigue.     

Effects of side-groove and loading rate on the fracture properties of aluminium alloy AL-6061

The side-groove effect on the fracture behavior of aluminum alloy AL-6061 in single edge notch bending (SENB) specimens was studied. Aluminum alloys are broadly used to reduce the weight of vehicles as a structural component due to the excellent mechanical properties, lightweight characteristics, easy fabrication, and high specific strength, as aluminum alloys are exposed to high velocity and different loads during accidents. Therefore, acknowledging the impact behaviors of aluminum alloys are crucial. In the current research, 3-point bending test was performed at various loading rates of 10 mm/min, 30 mm/min and 50 mm/min on different specimen thicknesses of 10 mm, 15 mm, 20 mm and 25 mm and several side-groove depths. The side-groove depth is determined according to the American Society for Testing and Materials (ASTM E399) requirements standard. The result indicates that the shear lips ratio of fracture surface was decreased, which consequently showed a change of large scale yielding to small scale yielding with increasing the specimen thicknesses. Accordingly, J values were increased with increasing the specimen thicknesses. Fracture surface observation shows that the fracture manners are ductile and less ductile as the side-groove increased. Conclusively, the ductility of Al-6061 alloy slightly decreased with increasing side-groove depths and loading rates.     

Effects of heat treatments on the microstructure and mechanical properties of a 6061 aluminium alloy

This paper describes the mechanical behavior of the 6061-T6 aluminium alloy at room temperature for various previous thermal histories representative of an electron beam welding. A fast-heating device has been designed to control and apply thermal loadings on tensile specimens. Tensile tests show that the yield stress at ambient temperature decreases if the maximum temperature reached increases or if the heating rate decreases. This variation of the mechanical properties is the result of microstructural changes which have been observed by Transmission Electron Microscopy (TEM).