Coalescence of multiple flaws in a rock-model material in uniaxial compression

A number of specimens made of gypsum with three and 16 flaws have been prepared and tested in compression. Results from these experiments are compared with observations from similar specimens with two flaws. The comparisons indicate that the cracking pattern observed in specimens with multiple flaws is analogous to the pattern obtained in specimens with two flaws. Two types of cracks initiate from the tips of the flaws: wing cracks and secondary cracks. Wing cracks are tensile cracks that initiate at an angle with the flaw and propagate in a stable manner towards the direction of maximum compression. Secondary cracks are shear cracks that initially propagate along their own plane in a stable manner. Two types of secondary cracks are possible: coplanar or quasi-coplanar, and oblique. As the load is increased, wing cracks propagate in a stable manner and secondary cracks may propagate in an unstable manner and produce coalescence, which occurs when two flaws are linked together. Nine types of coalescence have been observed, and each type is characteristic of a particular flaw geometry. The stresses at which wing and secondary cracks initiate and coalescence occurs strongly depend on the geometry of the flaws and on the number of the flaws; as the flaw inclination angle increases, the spacing increases, or the number of flaws decreases, initiation and coalescence stresses increase.     

THE X-RAY MEASUREMENT OF AXIAL AND HOOP RESIDUAL STRESSES IN MICROCOMPOSITE SPECIMENS

Thermal residual stresses were measured in microcomposites (Le., the elementary cell of a composite) using the X-ray diffraction method In order to overcome the geometry problem caused by this type of specimen, three different theoretical methods were applied to analyze the X-ray results. The first method was used previously for larger microcomposites (φ = 140 μm) and cannot be applied successfully here due to the small size of our specimen (φ = 20 μm). The two other methods (three-dimensional methods) have been used successfully and lead to similar results. The effects of the interphase type and thickness were studied. However, the uncertainty on thermal residual stress values are still at about 20%.     

Structure,homogeneity ranges,magnetic, and electrical properties of the ordered Laves phases RENi4Mg with MgCu4Sn type structure

New ordered Laves phases RENi₄Mg (RE = Sc, Sm, Tb–Lu) were synthesized from the elements in sealed tantalum ampoules in an induction furnace. Six of the structures were refined on the basis of X-ray single crystal data. The diffraction experiments gave hint for small homogeneity ranges RE_1+xNi₄Mg_1?x. Magnetic susceptibility measurements show Curie–Weiss behavior for RE = Gd, Dy, Ho, Tm, Yb and the resulting effective magnetic moments suggest both stable trivalent states for all RE and a non-magnetic state for Ni. Gd_1+xNi₄Mg_1?x (x ≈ 0.12) orders antiferromagnetically at a Néel temperature of T_N = 4.6(5) K. Resistivity measurements reflect the metallic nature of these compounds.     

Crack coalescence in specimens with open and closed flaws: A comparison

A comparison is presented between experimental observations made from gypsum specimens loaded in uniaxial compression, with open and closed flaws. Three types of cracks are observed: wing cracks, coplanar and oblique secondary cracks. Wing cracks are tensile cracks that initiate at or near the tip of the flaws, are stable, and propagate towards the direction of maximum compression. Secondary cracks are shear cracks; they are initially stable and may become unstable near coalescence. Coalescence is produced by the linkage of two flaws by a combination of wing and secondary cracks. When coalescence occurs through wing cracks or a combination of wing and secondary cracks, the process is stable. When coalescence occurs through secondary cracks only, the phenomenon is unstable. Eight types of coalescence have been identified, which apply to specimens with open and closed flaws. The types of coalescence are classified based on the types of cracks that produce linkage, and are closely related to the orientation, spacing, and continuity of the flaws. Interestingly, coalescence produced through the linkage of shear cracks only occurs at higher stress than coalescence produced by a combination of shear and wing cracks. The smallest coalescence stress occurs when produced only by wing cracks. The main difference between experimental results from open and closed flaws is that initiation stresses and coalescence are higher for closed than for open flaws. This is explained by the friction along the closed flaws, which needs to be overcome before a crack can initiate, and also by the capability of closed flaws to transmit normal stresses. Thus the differences are not fundamental, and so a unified conceptual framework and a common terminology are proposed for fracturing phenomena in brittle materials with open or with frictional pre-existing discontinuities.     

Estimation of residual stresses in SiC/Ti-15-3 composites and their relaxation during a fatigue test

Residual stresses induced by cooling down from the elaboration process in SiC/Ti-15-3 composite were calculated by the finite element model using MARC and MENTAT software. An analytical model (derived from those of Eshelby and of Mikata and Taya) already used for ceramic matrix composites has also been used in this study for elementary composites. The influence of uncertainty on thermomechanical properties and also the influence of fibre packing and spacing were also studied. Then, evolution (relaxation) of the thermal residual stresses (TRS) during a fatigue test were investigated. Results show that the TRS decreased rapidly during the first cycles and then became almost constant at about one-third of their initial values. This revised version was published online in November 2006 with corrections to the Cover Date.     

A linear time-varying model of force generation in skeletal muscle

A model of isometric force production by skeletal muscle was developed in which the response to each stimulus in a train was described by a critically damped, linear second-order system. The parameters describing the system were constrained to be constant within an interstimulus interval, but were allowed to vary between interstimulus intervals. The ability of this model to match experimental data, and the time variation in the parameters (low-frequency gain and natural frequency) required to do so were examined in soleus and plantaris muscles of the cat stimulated by synchronous whole-nerve stimulation. The model produced good fits across firing rates from twitch to tetanus for slow and fast muscle, rested and fatigued muscle, and maximal and submaximal stimulation. Both gain and natural frequency generally varied smoothly and predictably under all conditions. Gain increased at intermediate stimulation rates and in potentiated muscle, and decreased with fatigue and submaximal stimulation. Natural frequency was higher in fast muscle, and decreased with stimulation rate and fatigue. This modeling approach may provide a useful alternative to current models of skeletal muscle force, as its implementation is simple and it can describe force under conditions (fatigue, potentiation) where the muscle dynamics change with time     

A Round Robin Test exercise on hydrogen absorption/desorption properties of a magnesium hydride based material

A Round Robin Test exercise on magnesium hydride (MgH₂) was performed by 14 laboratories with the aim to compare experimental isothermal data such PCI curves, kinetics curves and formation enthalpies together with a basic statistical evaluation of the results.     

Effect of pore water pressure on tunnel support during static and seismic loading

The support of underground structures must be designed to withstand static overburden loads as well as seismic loads. New analytical solutions for a deep tunnel in a saturated poroelastic ground have been obtained for static and seismic loading. The static solution accounts for drainage and no-drainage conditions at the ground–liner interface. Linear elasticity of the liner and ground, and plane strain conditions at any cross-section of the tunnel are assumed. For tunnels in which ground stresses and pore pressures are applied far from the tunnel center, the drainage conditions at the ground–liner interface do not affect the stresses in the liner. The analytical solution shows that the stresses in the liner are exactly the same whether there is drainage or not at the ground–liner interface. Hence, if the drainage conditions in the tunnel are changed from full drainage to no-drainage or vice versa the stresses in the liner are not affected. However, the stresses and displacements in the ground change significantly from drainage to no-drainage conditions. For seismic loading a new analytical formulation is presented which provides the complete solution for the ground and the liner system for both dry and saturated ground conditions. The formulation is based on quasi-static seismic loading and elastic ground response; for a saturated ground, undrained conditions are assumed which indicate that the excess pore pressures generated during the seismic event do not dissipate. The results show that the racking deformations of a liner in dry or saturated ground are highly dependent on the flexibility of the liner.