Failure of AA 6061 and 2099 aluminum alloys under dynamic shock loading

Microstructural aspects of the deformation and failure of AA 6061 and AA 2099 aluminum alloys under dynamic impact loading are investigated and compared with their responses to quasi-static mechanical loading in compression. Cylindrical specimens of the alloys, heat-treated to T4, T6 and T8 tempers, were subjected to dynamic compressive loading at strain rates of between 2800 and 9200 s⁻¹ and quasi-static compressive loading at a strain rate of 0.0032 s⁻¹. Plastic deformation under the dynamic impact loading is dominated by thermal softening leading to formation of adiabatic shear bands. Both deformed and transformed shear bands were observed in the two alloys. The shear bands offer preferential crack initiation site and crack propagation path in the alloys during impact loading leading to ductile shear fracture. While cracks propagate along the central region of transformed bands in AA 6061 alloy, the AA 2099 alloy failed by cracks that propagate preferentially along the boundary region between the transformed shear bands and the bulk material. Whereas the AA 2099 alloy shows the greatest propensity for adiabatic shear banding and failure in the T8 temper condition, AA 6061 alloy is most susceptible to formation of adiabatic shear bands and failure in the T4 temper. Deformation under quasi-static loading is dominated by strain hardening in the two alloys. Rate of strain hardening is higher for naturally aged AA 6061 than the artificially aged alloy, while the strain hardening rate for the AA 2099 alloy is independent of the temper condition. The AA 2099 alloy shows a superior mechanical behaviour under quasi-static compressive loading whereas the AA 6061 shows a higher resistance to impact damage.     

Fracture mechanisms of aluminium alloy AA7075-T651 under various loading conditions

The fracture behaviour of the aluminium alloy AA7075-T651 is investigated for quasi-static and dynamic loading conditions and different stress states. The fracture surfaces obtained in tensile tests on smooth and notched axisymmetric specimens and compression tests on cylindrical specimens are compared to the fracture surfaces that occur when a projectile, having either a blunt or an ogival nose shape, strikes a 20 mm thick plate of the aluminium alloy. The stress state in the impact tests is much more complex and the strain rate significantly higher than in the tensile and compression tests. Optical and scanning electron microscopes are used in the investigation. The fracture surface obtained in tests with smooth axisymmetric specimens indicates that the crack growth is partly intergranular along the grain boundaries or precipitation free zones and partly transgranular by void formation around fine and coarse intermetallic particles. When the stress triaxiality is increased through the introduction of a notch in the tensile specimen, delamination along the grain boundaries in the rolling plane is observed perpendicular to the primary crack. In through-thickness compression tests, the crack propagates within an intense shear band that has orientation about 45° with respect to the load axis. The primary failure modes of the target plate during impact were adiabatic shear banding when struck by a blunt projectile and ductile hole-enlargement when struck by an ogival projectile. Delamination and fragmentation of the plates occurred for both loading cases, but was stronger for the ogival projectile. The delamination in the rolling plane was attributed to intergranular fracture caused by tensile stresses occurring during the penetration event.     

Multiaxial fatigue behaviour of AA6068 and AA2017A aluminium alloys under in‐phase bending with torsion loading condition

This paper presents an analysis of the multiaxial fatigue properties of two selected aluminium alloys. Several experimental results were used to perform the analysis e.g. the latest experimental results done in Opole University of Technology on PA6 (2017 A), PA4 (6068) under bending, torsion, and combined bending with torsion. Analyses of the results were done to find similarities of the multiaxial fatigue behaviour of selected aluminium alloys. Based on the (σ_a – τ_a) curves, prepared for a fixed number of cycles, it was possible to show some tendencies of the multiaxial fatigue behaviour of selected material group. This is an important indicator while selecting proper multiaxial fatigue failure criterion suitable to perform fatigue life assessment of aluminium alloys.     

Matrix strengthening in continuous fibre reinforced composites under monotonic and cyclic loading conditions

Unidirectional and cyclic tensile stress-strain testing has been carried out on continuous tungsten fibre reinforced copper composites, with fibre diameter from 11 to 48 m at a volume fraction of 0.37. In tensile tests the composites showed positive deviations from the rule of mixtures, the amount increasing with a decrease in fibre diameter and, therefore, interfibre spacing. This matrix strengthening continued to failure and was shown to be related in part to the matrix grain size.In the cyclic stress-strain tests the matrix strengthening was approximately the same for all the composites and was greater than for the tensile tests. The strengthening could be accounted for by the formation of a substructure during cycling of approximately 0.5 m cell size.     

Joining of alumina short-fibre reinforced AA6061 alloy to AA6061 alloy and to itself

The weldability of aluminium short-fibre reinforced AA6061 alloy (FRM) to AA6061 alloy and to itself using aluminium brazing materials has been investigated. AA4045 and BA03 were selected as brazing materials. When FRM was brazed to AA6061 alloy with AA4045 sheet, a disorder of fibre orientation near the interface was recognized at a brazing temperature above 863 K. Furthermore, the interface became very irregular and porous. The tensile strength achieved was about 100 MPa on brazing below 863 K. On the other hand, BA03 sheet, which has thin AA4045 layers on an AA3003 alloy layer, made the joint strong. The strength was about 200 MPa. BA03 induced little disorder of fibre arrangement and better contact at the interfaces. The BA03/AA6061 alloy interface was more porous than the FRM/BA03 interface and, hence, weaker. FRM/FRM joints with BA03 sheet had good strengths above 200 MPa.     

Substructure morphology in aluminium alloys AA 3003 and AA 3004

Abstract

The industrial alloys AA 3003 and AA 3004 have been experimentally rolled using a wide range of temperatures and reductions. The substructures obtained were extensively examined, and it is shown that when the rolling reductions are low the relationship between processing parameters and subgrain size (d) follows the well established form d^?m=a+bln Z, where Z is the Zener–Hollomon parameter. As the reduction is increased the orientations of the subgrains to the rolling direction decreases, and the subgrain size also decreases. It is concluded that the existing relationships require modification if they are to be useful in the industrial context.

MST/676     

Crack initiation and crack propagation under thermal cyclic loading

Theoretical and experimental investigations of crack initiation and crack propagation under thermal cyclic loading are presented. For the experimental investigation a special thermal fatigue test rig has been constructed in which a small circular cylindrical specimen is heated up to a homogeneous temperature and cyclically cooled down under well defined thermal and mechanical boundary conditions by a jet of cold water. At the end of the cooling phase the specimen is reheated to the initial temperature and the following cycle begins. The experiments are performed with uncracked and mechanically precracked specimens of the German austenitic stainless steel X6CrNi 1811.

In the crack initiation part of the investigation the number of load cycles to initiate cracks under thermal cyclic load is compared to the number of load cycles to initiate cracks under uniaxial mechanical fatigue loading at the same strain range as in the cyclic thermal experiment. The development of initiated cracks under thermal cyclic load is compared with the development of cracks under uniaxial mechanical cyclic load.

In the crack propagation part of the investigation crack growth rates of semi-elliptical surface cracks under thermal cyclic loading are determined and compared to suitable mechanical fatigue tests made on compact-tension and four-point bending specimens with semi-elliptical surface cracks. The effect of environment, frequency, load shape and temperature on the crack growth rate is determined for the material in mechanical fatigue tests.

The theoretical investigations are based on the temperature distribution in the specimen, which is calculated using finite element programs and compared to experimental results. From the temperature distribution, elastic and elastic-plastic stress distributions are determined taking into account the temperature dependence of the material properties. The prediction of crack propagation relies on linear-elastic fracture mechanics. Stress intensity factors are calculated with the weight function method and crack propagation is determined using the Paris relation.

To demonstrate the quality of the crack growth analysis the experimental results are compared to the prediction of crack propagation under thermal cyclic load.     

Control of earing quality in AA 5052 and AA 5454 aluminium alloys

Abstract

The effect of discrete operations during thermomechanical processing on the earing behaviour of two aluminium alloys (AA 5052 and AA 5454) has been investigated. In the homogenized condition AA 5454 contained a fine dispersoid distribution while AA 5052 was dispersoidfree. It was found that for the dispersoid-containing material the hot-rolling process was less dominant. Nevertheless, for both alloys the earing quality could be adjusted by suitable modifications to hot-rolling, cold-reduction, and annealing cycles. Pole-figure analysis indicated differences in hot-rolled texture caused by variations in processing, which became more significant as the total hot reduction was increased.

MST/41     

Developing Temperature–Time and Pressure–Time diagrams for diffusion bonding AZ80 magnesium and AA6061 aluminium alloys

The principal difficulty when joining magnesium (Mg) and aluminium (Al) lies in the existence of formation of oxide films and brittle intermetallic in the bond region. However diffusion bonding can be used to join these alloys without much difficulty. In this investigation, an attempt was made to develop Temperature–Time and Pressure–Time diagrams for diffusion bonding of AZ80 magnesium (Mg) and AA6061 aluminium (Al) dissimilar materials. The bonding quality of the joints was checked by microstructure analysis and lap shear tensile testing. Based on the results Temperature–Time and Pressure–Time diagrams were constructed. These diagrams will act as reference maps for selecting appropriate diffusion bonding process parameters to join AZ80 magnesium alloy and AA6061 aluminium alloy without trial experiments.     

Extrusion modelling of 6061 aluminium alloy and particle reinforced MMCs

Abstract

Extrusion modelling was performed for 6061 aluminium alloy and three particle reinforced MMCs (10%Al₂O₃/6061, 15%SiC/6061, 20%Al₂O₃/6061) using constitutive equations previously obtained from torsion test data. In applying the finite element software DEFORM, suitable heat transfer, friction, and velocity boundary conditions were chosen based on a direct extrusion press. Simulations were run for various extrusion conditions and the outputs for the four materials were compared. The simulation results were validated by comparison with real life extrusions and modelling of other researchers. The results showed that an increase in billet temperature, a reduction in ram speed, or a reduction in extrusion ratio had the effect of reducing the ram load. In consequence, extrusion conditions could be selected so that extrusion of the composite was carried out with the same peak ram load as the alloy.     

高周疲劳载荷下6061-T6铝合金的温度演化

为了研究金属材料在疲劳载荷下的温度变化,采用红外热像系统对高周疲劳载荷下6061-T6铝合金的温度演化进行分析,用热像图对疲劳裂纹尖端的塑性区进行测量.结果显示,疲劳加载作用下,循环次数达到107次时6061-T6铝合金试样表面温度的变化分为四个阶段:初始温升阶段、温度缓降阶段、温度二次缓慢上升阶段和温度快速上升阶段.结合热弹性理论、铝合金塑性变形的微观机制分析并预测疲劳载荷下温度的演化和宏观裂纹扩展时裂纹尖端塑性区域大小.宏观裂纹开始扩展时,裂纹尖端的塑性区域可达3.6 mm2,红外热像仪测得结果为3.46 mm2,测试结果与理论结果吻合.     

Inelastic response of confined aluminium oxide under dynamic loading conditions

Previous efforts to explore the compressive strength of ceramics as a function of confining pressure at high strain rates have been limited by the maximum hydrostatic pressure that could be achieved within the experimental apparatus. An alternate procedure, using an autofrettaged confinement ring, has been designed to achieve higher confining pressures. A 6.2 GPa stress pulse, of approximately 20 s duration, was used to load a 99.5% pure cylindrical aluminium oxide specimen under a hydrostatic load of approximately 650 MPa. The specimen remained intact and showed no evidence of fracture under scanning electron microscopy (SEM). Transmission electron microscopy (TEM), however, showed extensive evidence of plastic flow; the microcracks that were observed were associated with dislocation arrays. Static and dynamic yield strengths as a function of strain rate are compared.