A model of debonding instability for solid propellant rocket motor: Part II — Unequal longitudinal and transverse stress rate

Considered in Part I was the debonding instability of a three-layered composite system made of steel, adhesive and rubber which models the situation in a solid-fuel rocket motor. Under the condition of a uniform stress rate of 0.75 ksi/s applied to the longitudinal and transverse direction, severe inhomogeneity of the material response were found in regions next to the adhesive/rubber interface. This led to the prediction of a macrocrack about 5 × 10⁻³ in. after an elapsed time of one second and sudden fracture over a length of 1.88 in. at eight seconds after loading.This is Part II of the work that examines the influence of load transients on interface failure. As unequal stress rate state is assumed where loading in the transverse direction is removed causing a more severe disturbance in the longitudinal direction even though the same stress rate of 0.75 ksi/s is maintained. A slightly bent crack of the order of 10⁻³ in. is predicted to initiate near the adhesive/rubber interface; it led to the onset of fracture over a length of 1.60 in. at four seconds. Onset of instability is predicted to occur at a much earlier state.     

A pseudo-elastic material damage model,part I: Strain energy density criterion

A failure criterion is presented which relates the strain energy density of the material to both yielding and fracture. Cumulative material damage throughout a structural component may be monitored and the relative influence of yielding and stable crack growth assessed. The criterion is demonstrated, using finite element analysis, for center cracked panel specimens differing by material toughness values. From crack growth increment predictions using the uniaxial stress-strain behavior of the material, the criterion predicts the critical value of the strain energy density factor S_c governing crack instability.     

Nondestructive detection of damage in aluminum: Electromagenetic discharge imaging

Electromagnetic Discharge Imaging (EDI) is applied for the nondestructive detection of imperfections and inhomogeneities in solids. These imperfections were introduced mechanically by drilling holes of different sizes and chemically by applying corrosive agents to aluminum specimens. The results are influenced by a host of variables associated with the waveform generator, the dielectric air gap and the specimen type. Correlation is thus established between the electromagnetic discharge image and the degree of material damage. Detection of material damage was also made possible even when the specimen surface was a layer of opaque insulation material.     

Bifunctional chiral synthons via biochemical methods. : VI. C5 isoprenoid units.

Two methods for the preparation of the isoprenoid chiron $\text{4}$ have been developed using a microbial kinetic resolution of $\text{5}$ and an enantiotopically selective hydrolysis of $\text{7}$ catalyzed by PLE.     

Regiospecific synthesis of (±)-deoxyanthracyclinones

A versatile regiospecific route to tetracyclic anthracyclinones is described; the synthetic strategy entails the attachment of a nucleophilic C-D ring synthon, $\text{2b}$, to a suitably functionalized A ring intermediate such as $\text{3}$.     

Prediction of thermally induced fracture from notch and crack front

The strain energy density criterion is applied to predict fracture trajectories emanating from existing notch and crack front in nonisothermal environments. When temperature gradients are raised sufficiently high across a notch or crack, the resulting fracture trajectories are non-self-similar and curved in shape. Influence of mechanical loading is also considered in addition to stresses induced by thermal changes. Increase in the applied mechanical load tends to shift or restore the fracture trajectories toward the plane of notch or crack symmetry. The notch sharpness can be varied by adjusting the ration of the minor to major axes of an elliptical cavity. Narrowing the notch primarily increases the local intensity of the strain energy density function dW/dV that is inversely proportional to the radial distance measured from the focal point of the ellipse. This singular character of dW/dV prevails, in general, for all materials and loadings. Numerical results are obtained and displayed graphically for several examples involving fracture trajectory shapes that are not intuitively obvious.     

Damage analysis of tetragonal perovskite structure ceramics implicated by asymptotic field solutions and boundary conditions

Cracking of ceramics with tetragonal perovskite grain structure is known to appear at different sites and scale level. The multiscale character of damage depends on the combined effects of electromechanical coupling, prevailing physical parameters and boundary conditions. These detail features are exhibited by application of the energy density criterion with judicious use of the mode I asymptotic and full field solution in the range of r/a=10⁻⁴ to 10⁻² where r and a are, respectively, the distance to the crack tip and half crack length. Very close to the stationary crack tip, bifurcation is predicted resembling the dislocation emission behavior invoked in the molecular dynamics model. At the macroscopic scale, crack growth is predicted to occur straight ahead with two yield zones to the sides. A multiscale feature of crack tip damage is provided for the first time. Numerical values of the relative distances and bifurcation angles are reported for the PZT-4 ceramic subjected to different electric field to applied stress ratio and boundary conditions that consist of the specification of electric field/mechanical stress, electric displacement/mechanical strain, and mixed conditions. To be emphasized is that the multiscale character of damage in piezoceramics does not appear in general. It occurs only for specific combinations of the external and internal field parameters, elastic/piezoelectric/dielectric constants and specified boundary conditions.