Polymerization shrinkage and contraction stress of dental resin composites.

Disorder and long-range interactions are two of the key components that make material failure an interesting playfield for the application of statistical mechanics. The cornerstone in this respect has been lattice models of the fracture in which a network of elastic beams, bonds, or electrical fuses with random failure thresholds are subject to an increasing external load. These models describe on a qualitative level the failure processes of real, brittle, or quasi-brittle materials. This has been particularly important in solving the classical engineering problems of material strength: the size dependence of maximum stress and its sample-to-sample statistical fluctuations. At the same time, lattice models pose many new fundamental questions in statistical physics, such as the relation between fracture and phase transitions. Experimental results point out to the existence of an intriguing crackling noise in the acoustic emission and of self-affine fractals in the crack surface morphology. Recent advances ...

Statistical models of fracture

Extremely low cycle fatigue tests on structural carbon steel and stainless steel

Correlation of filler content and elastic properties of resin-composites.

The effect of deposition conditions on characteristic mechanical properties – elastic modulus and hardness – of low-temperature PECVD silicon nitrides is investigated using nanoindentation. It is found that increase in substrate temperature, increase in plasma power and decrease in chamber gas pressure all result in increases in elastic modulus and hardness. Strong correlations between the mechanical properties and film density are demonstrated. The silicon nitride density in turn is shown to be related to the chemical composition of the films, particularly the silicon/nitrogen ratio.

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Effect of deposition conditions on mechanical properties of low-temperature PECVD silicon nitride films

Low-velocity impact tests were performed to investigate the impact behaviour of carbon fibre/epoxy composite laminates reinforced by short fibres and other interleaving materials. Characterisation techniques, such as cross-sectional fractography and scanning acoustic microscopy, were employed quantitatively to assess the internal damage of some composite laminates at the sub-surface under impact. Scanning electron microscopy was used to observe impact fractures and damage modes at the fracture surfaces of the laminate specimens. The results show that composite laminates experience various types of fracture; delamination, intra-ply cracking, matrix cracking, fibre breakage and damage depending on the interlayer materials. The trade-off between impact resistance and residual strength is minimised for composites reinforced by Zylon fibres, while that for composites interleaved by poly(ethylene-co-acrylic acid) (PEEA) film is substantial because of deteriorating residual strength, even though the damaged area is significantly reduced. Damages produced on the front and back surfaces of impact were also observed and compared for some laminates.

Impact damage characterisation of carbon fibre/epoxy composites with multi-layer reinforcement

This paper reports an investigation on techniques for determining elastic modulus and intrinsic stress gradient in plasma-enhanced chemical vapor deposition (PECVD) silicon nitride thin films. The elastic property of the silicon nitride thin films was determined using the nanoindentation method on silicon nitride/silicon bilayer systems. A simple empirical formula was developed to deconvolute the film elastic modulus. The intrinsic stress gradient in the films was determined by using micrometric cantilever beams, cross-membrane structures and mechanical simulation. The deflections of the silicon nitride thin film cantilever beams and cross-membranes caused by in-thickness stress gradients were measured using optical interference microscopy. Finite-element beam models were built to compute the deflection induced by the stress gradient. Matching the deflection computed under a given gradient with that measured experimentally on fabricated samples allows the stress gradient of the PECVD silicon nitride thin films introduced from the fabrication process to be evaluated.

https://iopscience.iop.org/article/10.1088/0960-1317/15/3/024/pdf

Determination of mechanical properties of PECVD silicon nitride thin films for tunable MEMS Fabry-Perot optical filters

The aim of this study was to investigate the effects of a single tensile overload on subsequent fatigue crack growth in a 316L stainless steel. Fatigue tests were conducted under the plane stress condition, and further supplemented with compliance measurements and field emission scanning electron microscopy (FESEM) observations. Effects of a tensile overload, e.g. initial acceleration and subsequent retardation of fatigue crack growth, were explained and quantified by FESEM and compliance measurements. The FESEM observations suggest that the initial crack growth acceleration stems from void and quasi-cleavage fracture within the fatigue damage zone in the vicinity of the crack tip. Systematic compliance measurements taken during fatigue crack growth suggest that the overall crack growth retardation is related to strain hardening and residual compressive stress produced by the plastic deformation associated with the tensile overload.

Effects of a single tensile overload on fatigue crack growth in a 316L steel

A recent asymptotic approach dealing with the size effect on the fracture properties of a large plate is further developed to consider the influence of both the front and back free surfaces of small sized specimens. The new extension is applied to experimental results found in the literature, and good agreements have been found between the predictions and the fracture resistance and energy measured using geometrically similar specimens and specimens with identical size but different initial crack or notch lengths. The physics behind the size effect are discussed based on the modified asymptotic approach. It is found that both the specimen geometry and crack length contribute to the size effect on fracture properties besides its physical size. In particular, the ratio of a fracture process zone, size over its distance to a free surface plays a very important role.

Xiao Hu

Papers

Effect of deposition conditions on mechanical properties of low-temperature PECVD silicon nitride films

Impact damage characterisation of carbon fibre/epoxy composites with multi-layer reinforcement

Determination of mechanical properties of PECVD silicon nitride thin films for tunable MEMS Fabry-Perot optical filters

Effects of a single tensile overload on fatigue crack growth in a 316L steel

Size effect on fracture resistance and fracture energy of concrete