The detachment of an inclined micro-pillar adhered to a dissimilar substrate

We investigate the mechanics of the detachment of an inclined micro-pillar adhered to a dissimilar substrate when subjected to a combination of an axial load and end moment. When the micro-pillar has adhered to the substrate, singular stress fields exist at the bi-material corners. The order of singularity is estimated using asymptotic analysis. The first two terms in the asymptotic expansion lead to singular stress fields. The magnitude of the singularity is evaluated in terms of the elastic mismatch between the pillar and substrate and the micro-pillar inclination. The asymptotic stress due to the moment loading is more sensitive to the micro-pillar inclination when compared to that due to the axial loading. They are insensitive to the micro-pillar inclination when the micro-pillar is rigid when compared to the substrate. A short interfacial crack is further assumed to exist at the bi-material corner. This crack is embedded in the corner singularity region and is loaded by the singular fields due to axial and bending loads. A boundary layer analysis is performed on the singular zone to estimate the stress intensity factor when a short crack embedded in it is subjected to the singular fields. The stress intensity factors are also calculated for a long interfacial crack at the bi-material corner, which extends beyond the singular zone. Using the above results, we investigate the detachment of the inclined micro-pillar under the combination of an axial load and end moment.

Upper and Lower Bounds for the Parameters of One-Dimensional Theories for Sandwich Fracture Specimens

Upper and lower bounds for the parameters of one-dimensional theories used in the analysis of sandwich fracture specimens are derived by matching the energy release rate with two-dimensional elasticity solutions. The theory of a beam on an elastic foundation and modified beam theory are considered. Bounds are derived analytically for foundation modulus and crack length correction in single cantilever beam (SCB) sandwich specimens and verified using accurate finite element results and experimental data from the literature. Foundation modulus and crack length correction depend on the elastic mismatch between face sheets and core and are independent of the core thickness if this is above a limit value, which also depends on the elastic mismatch. The results in this paper clarify conflicting results in the literature, explain the approximate solutions, and highlight their limitations. The bounds of the model parameters can be applied directly to specimens satisfying specific geometrical/material ratios, which are given in the paper, or used to support and validate numerical calculations and define asymptotic limits.

Influence of Interface Proximity on Precipitation Thermodynamics

The formation of coherent precipitates is often accompanied by large elastic mismatch stresses, which suppress phase separation. We discuss the presence of interfaces as a mechanism for stress relaxation, which can lead to preferred zones of precipitation. In particular, we discuss the proximity of free surfaces and shear-coupled grain boundaries, for which we can obtain a substantial local energy reduction and predict the influence on the local precipitation thermodynamics. The latter case is accompanied by morphological changes of the grain boundary, which are less suitable for large-scale descriptions. For that purpose, we develop an effective description through an elastic softening inside the grain boundary and map the microscopic grain boundary relaxation to a mesoscopic elastic and phase field model, which also allows generalizing the description to multi-phase situations.

Development of elastically graded titanium alloys for biomedical applications

Recent works have shown that the elastic mismatch observed at the bone / implant interface could be responsible for stress shielding issues causing bone resorption phenomena and potentially implant failures. In the present study, new advanced thermomechanical approaches leading to titanium alloys with graded elastic properties are proposed. The underlying philosophy and the whole methodology is detailed here, from the selection of candidates with large elastic variability to the creation of gradients, involving the identification of microstructure-properties relationships and the use of appropriate thermo-mechanical treatments. Applied on Ti-Nb-Zr alloys, these original routes enabled to get the following graded properties: elastic modulus from 85 to 65GPa over 400μm for TNZ alloy by surface deformation, and from 130 to 75GPa over 100μm for Ti-13-13 by preferential dissolution. These promising results thus validated the previously designed material-strategy-process combinations.

Multi-Parameter Fracture Mechanics: Crack Approaching a Bi-Material Interface

Fracture behaviour of a crack approaching a bi-material interface is investigated. A three-point bending configuration of a cracked specimen is simulated numerically by means of the finite element method and the interaction between the crack and aggregate is studied. The crack deflection angle is estimated by means of the maximum tangential stress criterion in its classical as well as generalized (multi-parameter) form considering the Williams’ power series with various numbers of the higher-order terms for the tangential stress approximation. The influence of the elastic mismatch and of other parameters on the calculated initial crack propagation angle is discussed.

Experimental Investigation of Energy Dissipation in Presliding Spherical Contacts Under Varying Normal and Tangential Loads

Interfacial damping in assembled structures is difficult to predict and control since it depends on numerous system parameters such as elastic mismatch, roughness, contact geometry, and loading profiles. Most recently, phase difference between normal and tangential force oscillations has been shown to have a significant effect on interfacial damping. In this study, we conduct microscale (asperity-scale) experiments to investigate the influence of magnitude and phase difference of normal and tangential force oscillations on the energy dissipation in presliding spherical contacts. Our results show that energy dissipation increases with increasing normal preload fluctuations and phase difference. This increase is more prominent for higher tangential force fluctuations, thanks to larger frictional slip along the contact interface. We also show that the energy dissipation and tangential fluctuations are related through a power law. The power exponents we identify from the experiments reveal that contacts deliver a nonlinear damping for all normal preload fluctuation amplitudes and phase differences investigated. This is in line with the damping uncertainties and nonlinearities observed in structural dynamics community.