The Importance of Viscoplastic Strain Rate in the Formation of Center Cracks during the Start-Up Phase of Direct-Chill Cast Aluminum Extrusion Ingots

A comparison of experimental observations and computer simulations shows that trends in the occurrence and severity of center cracks in direct-chill (DC) cast ingots due to different initial casting speed histories may best be explained by the changes in viscoplastic strain rate close to the center of the base of the ingot. The thermomechanical histories of five ingots were simulated and correlations between stresses, strains, strain rates, and liquid pressure drops due to feeding restrictions were considered

The role of rock joint frictional strength in the containment of fracture propagation

The fracturing phenomenon within the reservoir environment is a complex process that is controlled by several factors and may occur either naturally or by artificial drivers. Even when deliberately induced, the fracturing behaviour is greatly influenced by the subsurface architecture and existing features. The presence of discontinuities such as joints, artificial and naturally occurring faults and interfaces between rock layers and microfractures plays an important role in the fracturing process and has been known to significantly alter the course of fracture growth. In this paper, an important property (joint friction) that governs the shear behaviour of discontinuities is considered. The applied numerical procedure entails the implementation of the discrete element method to enable a more dynamic monitoring of the fracturing process, where the joint frictional property is considered in isolation. Whereas fracture propagation is constrained by joints of low frictional resistance, in non-frictional joints, the unrestricted sliding of the joint plane increases the tendency for reinitiation and proliferation of fractures at other locations. The ability of a frictional joint to suppress fracture growth decreases as the frictional resistance increases; however, this phenomenon exacerbates the influence of other factors including in situ stresses and overburden conditions. The effect of the joint frictional property is not limited to the strength of rock formations; it also impacts on fracturing processes, which could be particularly evident in jointed rock masses or formations with prominent faults and/or discontinuities

Neutron Diffraction Measurement of As-Cast Residual Stresses in Aa7050 Rolling Plate Ingots: Influence of A Wiper

During casting, thermally induced deformations give birth to ingot distortions and residual stresses. For some high strength alloys, ingot cracking can happen during casting per se or during cooling down. Ingot distortions such as rolling face pull-in, but curl and but swell are rather easy to quantify as opposed to internal stresses. As aluminium is rather transparent to neutrons, residual stress measurements using neutron diffraction appeared to be a good way to validate the thermomechanical models aimed at simulating the stress build-up during casting. This technique has been applied to DC cast AA7050 rolling plate ingots with special attention to the stress generation in the transient start-up phase, i.e. in the foot of the ingot. Additional results using the hole drilling method complement the measurements. The measured stress distributions are compared with the results of a numerical model of DC casting for ingots cast with and without a wiper

Informal care in Europe: Findings from the European Social Survey (2014) special module on the social determinants of health

Contains fulltext : 167516.pdf (publisher's version ) (Closed access)Background: Against the background of a rising demand for informal care in European societies, this study sets out to provide descriptive information by gender on (i) prevalence rates of (intensive) informal caregiving, (ii) characteristics of (intensive) informal caregivers and (iii) consequences of (intensive) informal caregiving in terms of mental well-being. Methods: Data from the European Social Survey, Round 7 were analysed with multilevel (logistic) regression techniques (n = 28 406 respondents in n = 20 countries). Results: On average, 34.3% of the population in 20 European countries were informal caregivers and 7.6% were intensive caregivers (providing care for minimum 11 h a week). Countries with high numbers of caregivers had low numbers of intensive caregivers. Caregiving was most prevalent among women, 50-59 year olds, non-employed - especially those doing housework - and religious persons. Determinants of providing care hardly differed by gender. Caregivers, especially female and intensive caregivers, reported lower mental well-being than non-caregivers. Conclusions: Our results suggest support for both crowding-in and crowding-out effects of the welfare state. Middle-aged women may become increasingly time squeezed as they are likely to be the first to respond to higher demands for informal care, while they are also the major target groups in employment policies aiming for increased labour market participation. Caregivers, and especially female and intensive caregivers, report lower levels of mental well-being. Supportive policies such as respite care or training and counselling may therefore be needed in order to sustain informal care as an important resource of our health care systems.6 p

Cold-Cracking Assessment in AA7050 Billets during Direct-Chill Casting by Thermomechanical Simulation of Residual Thermal Stresses and Application of Fracture Mechanics

Thermally induced strains and stresses developed during direct-chill (DC) semicontinuous casting of high strength aluminum alloys can result in formation of micro-cracks in different locations of the billet. Rapid propagation of such micro-cracks in tensile thermal stress fields can lead to catastrophic failure of ingots in the solid state called cold cracking. Numerical models can simulate the thermomechanical behavior of an ingot during casting and after solidification and reveal the critical cooling conditions that result in catastrophic failure, provided that the constitutive parameters of the material represent genuine as-cast properties. Application of fracture mechanics, on the other hand, can help to derive the critical crack length leading to failure. In the present research work, the state of residual thermal stresses was determined in an AA7050 billet during DC casting by means of ALSIM5. Simulation results showed that in the steady-state conditions, large compressive stresses form near the surface of the billet in the circumferential direction, whereas in the center, the stresses are tensile in all directions. Magnitudes of von Mises effective stresses, the largest component of principal stresses and the fracture mechanics concepts, were then applied to investigate the crack susceptibility of the billet.Materials Science and EngineeringMechanical, Maritime and Materials Engineerin

Nucleation and Arrest of Dynamic Rupture Induced by Reservoir Depletion

Seismic events induced by the depletion of hydrocarbon reservoirs can cause damage to housing and cause societal and economic unrest. However, the factors controlling the nucleation and size of production-induced seismic events are not well understood. Here we used geomechanical modeling of production-induced stresses and dynamic rupture modeling to assess the conditions controlling down-dip rupture size. A generic model of (offset) depleting reservoir compartments separated by a fault was modeled in 2-D using the Finite Element package DIANA FEA. Linear slip-weakening was used to control fault friction behavior. Fault reactivation was computed in a quasi-static analysis simulating stresses during reservoir depletion, followed by a fully dynamic analysis simulating seismic rupture. The sensitivity of reactivation and rupture size to in situ stress, dynamic friction, critical slip distance, and reservoir offset was evaluated. After reactivation, a critical fault length was required to slip before seismic instability could occur. In a subsequent fully dynamic analysis the propagation and arrest of dynamic rupture was simulated. Rupture remained mostly confined to the reservoir interval but could also propagate into the overburden and underburden or sometimes transition into a run-away rupture. Propagation outside the reservoir interval was promoted by a critical in situ stress, a large stress drop, a small fracture energy, and no or little reservoir offset. With increasing offset (up to the reservoir thickness), reactivation was promoted but dynamic rupture size decreased