Spalling of Flat Plates by Thermal Shock

A model is presented for the thermal shock damage in plates subjected to a sudden rise in temperature. Spalling is assumed to follow localized buckling caused by the thermally induced compressive stresses above subsurface flaws. The minimum size o flaw required to cause spalling is determined as a function of the material properties of the plate, the temperature rise, the position of the paw, and the time after the thermal shock.     

A hybrid ray-wave optics model to study the scattering behavior of silver metallic paint systems

The gonioapparent lightness of a metallic paint system depends strongly on the 3D microstructure of the platelet-containing basecoat layer, and on the platelet roughness. Current optical models which are used to simulate the paint’s appearance, however, ignore the multi-scale and multi-dimensional microstructural features, which limits their predictive power. Here, we describe a hybrid, ray-wave optics model for metallic paints. This model incorporates the ray-optics of the 3D platelet microstructure and the wave effects that result from the surface roughness of the platelets. This model is used to link the roughness to the reflection lobes of the aluminum platelets, and to the gonioapparent lightness of the paint system. Predicted lightness values from our model matched, at most viewing angles, measurements from physical paint samples. This model can be used to explore the effect of platelet surface roughness on the lightness of the complete paint system and predict the appearance of paints with different platelet microstructures.     

Iordanskii and Lifshitz-Pitaevskii Forces in the Two-Fluid Model

It has been known since the pioneering work of Onsager and Feynman that the statistical mechanics and dynamics of vortices play an essential role in the behavior of superfluids and superconductors. However, the theory of vortices in quantum fluids remains in a most unsatisfactory state, with many conflicting results in the literature. In this paper we review the theory of Thouless, Ao and Niu, which gives an expression for the total transverse force acting on a quantized vortex that is in apparent disagreement with the word of lordanskii and of Lifshitz and Pitaevskii. In particular, no transverse force proportional to the asymptotic normal fluid velocity was found. We use two-fluid hydrodynamics to study this discrepancy.     

Tensile Tests of Ceramic-Matrix Composites: Theory and Experiment

A model describing the salient features of tensile stress-strain curves of ceramic–fiber composites has been developed. The model incorporates statistics of fiber failure. Furthermore, the compliance of the testing machine is included so that the onset of instability can be predicted. An experiment conducted on a composite consisting of a glass-ceramic matrix reinforced with SiC fibers exhibits excellent agreement with the predicted behavior.     

Annoucement of a Round Robin on the Analysis of the Peel Test

There is some controversy over the analysis of the peel test when there is significant plastic deformation. To resolve this controversy a round robin on its analysis is announced contributions to which are invited. Details are given of test cases to be analysed.     

Tuneable elastomeric nanochannels for nanofluidic manipulation

Fluidic transport through nanochannels offers new opportunities to probe fundamental nanoscale transport phenomena and to develop tools for manipulating DNA, proteins, small molecules and nanoparticles. The small size of nanofabricated devices and the accompanying increase in the effect of surface forces, however, pose challenges in designing and fabricating flexible nanofluidic systems that can dynamically adjust their transport characteristics according to the handling needs of various molecules and nanoparticles. Here, we describe the use of nanoscale fracturing of oxidized poly(dimethylsiloxane) to conveniently fabricate nanofluidic systems with arrays of nanochannels that can actively manipulate nanofluidic transport through dynamic modulation of the channel cross-section. We present the design parameters for engineering material properties and channel geometry to achieve reversible nanochannel deformation using remarkably small forces. We demonstrate the versatility of the elastomeric nanochannels through tuneable sieving and trapping of nanoparticles, dynamic manipulation of the conformation of single DNA molecules and in situ photofabrication of movable polymeric nanostructures.     

Crack Deflection and Propagation in Layered Silicon Nitride/Boron Nitride Ceramics

Crack deflection and the subsequent growth of delamination cracks can be a potent source of energy dissipation during the fracture of layered ceramics. In this study, multilayered ceramics that consist of silicon nitride (Si₃N₄) layers separated by boron nitride/silicon nitride (BN/Si₃N₄) interphases have been manufactured and tested. Flexural tests reveal that the crack path is dependent on the composition of the interphase between the Si₃N₄ layers. Experimental measurements of interfacial fracture resistance and frictional sliding resistance show that both quantities increase as the Si₃N₄ content in the interphase increases. However, contrary to existing theories, high energy-absorption capacity has not been realized in materials that exhibit crack deflection but also have moderately high interfacial fracture resistance. Significant energy absorption has been measured only in materials with very low interfacial fracture resistance values. A method of predicting the critical value of the interfacial fracture resistance necessary to ensure a high energy-absorption capacity is presented.     

Analysis of the Symmetrical 90°-peel Test with Extensive Plastic Deformation

A numerical 2-D study of the symmetrical 90°-peel test (a similar geometry to the T-peel test) in which extensive plastic deformation occurs in the adherends is presented in this paper. A traction-separation relation is used to simulate failure of the interface, and the conditions for both crack initiation and steady-state crack growth are investigated. The numerical predictions for the steady-state peel force are compared with those based on elementary beam theory. It is shown that two competing effects dominate the mechanics of the peel test to such an extent that the results of beam-bending analyses cannot be used to predict the peel force. At one extreme range of parameters, delamination is driven by shear rather than by bending, resulting in a lower peel force than would be predicted by beam-bending analyses. At the other extreme, where delamination is bending-dominated, the constraint induced by the interfacial tractions cause an increase in the peel force. The numerical results are compared with the results of experiments in which adhesively-bonded specimens are tested in the symmetrical 90°-peel configuration. Excellent agreement between the numerical and experimental results validates the numerical approach.     

The effects of transverse shear on the delamination of edge-notch flexure and 3-point bend geometries

A general approach to develop complete and rigorous analytical expressions for the energy-release rates and phase angles for delamination of isotropic beam-like geometries is summarized. The analysis allows the effects of transverse shear to be correctly incorporated within the resulting expressions. The approach requires four effective crack-tip loads to be determined from the applied loads: a moment, an axial force, a double transverse shear force and a single transverse shear force. Each of these effective loads provides an energy-release rate and an associated phase angle; expressions for these quantities can be found in the literature. These fundamental expressions can be combined algebraically to generate analytical expressions for the total energy-release rate and phase angle for any geometry and loading configuration of interest. The approach is illustrated by general analyses of edge-notched flexure (ENF), end-loaded split (ELS) and 3-point bending specimens. In particular, it is shown that the equation for the energy-release rate for the ENF geometry reduces to a very simple form that has previously been proposed from numerical studies, when the geometry is perfectly anti-symmetrical.