Narrow Results

The cracking of GaN films and the associated cracking of substrates are described. The geometry, structure, and evolution of fracture demonstrate that GaN films crack under tensile stress during growth and are subsequently overgrown and partially healed. The film cracks channel along the (1010)_GaN planes and also extend a distance of ~5 μm into the sapphire substrate. These incipient cracks in the substrate form a set of initial flaws that leads to complete fracture through the sapphire during cooling for sufficiently thick films. Each stage of this cracking behavior is well described by a fracture mechanics model that delineates a series of critical thicknesses for the onset of cracking that are functions of the film and substrate stresses, thicknesses, and elastic properties. Similar cracking behavior is found to occur independently of the choice of substrate between sapphire and SiC and is traced to a tensile stress generation mechanism early in the growth process, such as that provided by island coalescence. Cracking is the dominant stress relief mechanism, as opposed to dislocation generation or diffusion, because of the island growth mode and because of optimized growth temperatures at or below the brittle-to-ductile transition. Lateral epitaxial overgrowth (LEO) of GaN is shown to minimize substrate fracture even though film cracking remains unaffected. This effect explained in terms of the limits placed on the initial extent of insipient substrate cracks due to the LEO geometry.

Following the empirical-computational methodology, the contemporary investigations deal with inelastic stability and dynamics of concrete beam-columns. Even under service loads, the concrete structures exhibit physical nonlinearity due to presence of axio-flexural cracks. The objective of the present paper is to analyze the static and dynamic stability of conservative physically nonlinear fully cracked flanged concrete beam–columns. In this paper, using proper reference frames, analytical expressions are developed for the lateral displacement and stiffness of a flanged concrete cantilever under axial compressive and lateral forces. Two critical values of both the axial and lateral loads are identified. For constant lateral force smaller than its first critical value, the concrete beam–columns exhibit brittle buckling mode. Higher lateral forces lesser than the second critical value introduce alternate stable and unstable domains with increase in axial force. The lateral stiffness is predicted to vanish when the axial loads reach the critical values and when the limiting displacement is reached for axial load exceeding its second critical value. The load-space is partitioned into stable and unstable regions. Accessibility of these equilibrium states in the load space has been investigated. Such distinguishing aspects of the predicted behavior of elastic concrete beam–columns are discussed. Their dynamic stability is investigated in second part of the paper.

In the first part of this paper, elastostatic stability of cracked conservative flanged concrete beam-columns has been analyzed. Using the derived expression for the lateral stiffness under constant axial force, their elastodynamic stability is investigated in this second part. As expected, the instantaneous values of the stiffness and the damping coefficients of the lumped-mass underdamped SDOF nonlinear structures are found to depend upon the vibration amplitude. The natural frequency has been found to vanish at the two critical axial loads defined in the first part. For axial load exceeding the second critical value, the concrete beam-columns in the second equilibrium state are shown to exhibit loss of dynamic stability by divergence. Depending upon the initial conditions, the phase plane has been partitioned into dynamically stable and unstable regions. Under harmonic excitations, the nonlinear dynamical systems exhibit subharmonic resonances and jump phenomena. Loss of dynamic stability has been predicted for some ranges of damping ratio as well as of peak sinusoidal force and forcing frequency. Sensitivity of dynamic stability to the initial conditions and the sense of the peak sinusoidal force have also been predicted. The theoretical significance and the methodology adopted in this paper are also discussed.

In order to know the impact behavior of turbine blade-grade monolithic silicon nitride ceramic, particle impact tests have been carried out at room and elevated temperatures with and without tensile load, which simulates the centrifugal force of blade rotation. In the experiments, a 1 mm diameter samarium-cobalt particle is impacted at velocities up to 900 ms^-1. The main results are : 1) Degradation of impact strength was clearly observed at elevated temperature and under tensile stress. 2) The critical stresses for the ring cracking were evaluated from both dynamic and static loading tests and were compared with each other. For a candidate material the reasonable stress value was supposed to be 14 GPa or less. 3) Moreover, X-ray inspection revealed that the radial cracks were prevailing in impacts at elevated temperatures.

Ceramic injection moulding is a well established processing technique, but is still limited to thin section components. This paper gives an overview of a variety of defects which appear preferentially in thick moulding sections. The generation of porosity and voidage during packing and solidification are discussed and related to the conditions prevailing during solidification. The use of an insulated sprue extended gate solidification and eliminated voids in thick sections and the use of a polyoxymethylene binder system enabled the progressive removal of binder from large 35 mm sections. Low hold pressure, applied by using a modified injection moulding machine reduced residual stress-induced cracking. Pronounced differential sintering was traced to particle alignment during mould filling and could be eliminated by using equiaxed powders.

Hot stamping of high strength steel (HSS) can significantly improve overall mechanical properties of part and thus meet the increasing demands for weight reduction and safety standards in vehicles. However, cracking prediction using traditional forming limit curve (FLC) in hot stamping is challenging. In this paper, to predict HSS cracking in hot stamping a temperature-dependent forming limit surface (FLS) is developed by simulations combined with experiments. Different from the FLC the newly developed FLS, where temperature and phase transformation are included, suits the hot stamping of HSS. A finite element (FE) thermo-mechanical coupled numerical model of the hot stamping process is developed and implemented under ABAQUS/Explicit platform. Finally the developed FLS is used to predict crack initiation in a hot stamping. Results show that effectiveness of the developed FLS is verified in cracking prediction for hot stamping of HSS.