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.     

Crosslinking of polybutadiene rubber without any vulcanization agent

Polybutadiene rubber with 10% vinyl unsaturation has been crosslinked without the addition of any vulcanization chemicals. The crosslinks have been created through treatment at high temperature (240–250°C) and elevated pressure (<293 MPa) in a plunger type mold. The treatment time needed to achieve a crosslinked material is of the order of a few minutes and the reaction rate is shown to be dependent upon the temperature as well as upon the pressure applied. As a result of the severe treatment, some degradation reactions may be expected but careful FTIR analysis shows no increased oxidation. There is some consumption of the vinyl unsaturation, to an amount comparable to the number of crosslinks formed. The presence of carbon black increases the measured crosslink density, as expected, but at higher pressures the amount of extra network material, because of the carbon black, is much larger than at lower pressures, suggesting that the carbon black plays a critical role in the network formation process. The stress relaxation properties in compression of the rubber material were investigated and found to be comparable to those of a peroxide vulcanizate.     

Classical and recent formulations for linear elasticity

Summary The paper presents a review of the classical formulations for linear elasticity, known as principles in solid mechanics. The analogy between the first variation of the energy functionals and the weak forms is pointed out. Several recent developments in first order system least squares, applied to elasticity are discussed. The paper concludes with a mathematical formulation of a new mixed least squares method and a discussion about its future development.     

Numerical Assessment of the Influence of End Conditions on Constitutive Behavior of Geomaterials

In this paper, we investigate the behavior of elastic–plastic specimens during testing in a triaxial apparatus. In particular, an investigation of the influence of a number of imperfections on the observed behavior of a specimen is performed. To this end, we present influences of end platen friction, end platen inclinations, and the shape of specimen on what we broadly understand by “constitutive behavior.” We investigate the issues related to the constitutive as opposed to boundary value behavior or elastic–plastic specimens. We present a number of examples to illustrate the differences between different types of response, which are usually, and wrongly, just called the constitutive behavior.     

Method for measuring jitter in VDUs

A method for measuring jitter in visual display units (VDUs) is proposed. The method is based on the fact that jitter is revealed by a variation in pulse amplitudes when the electron beam traverses in paths which are displaced in relation to each other. The obtained data demonstrate that frequency characteristics of jitter are of the white noise type. The pulse amplitude properties on the other hand appear to provide a promising basis for measuring jitter in VDUs.