Key Engineering Materials Vols. 345-346

Abstract: The residual stresses in an epoxy film coated on Si wafer induced during polymerization at room temperature are investigated. The curvature measurement method and the boundary element method (BEM) are employed to investigate the residual stresses. An epoxy film is coated on a relatively thick Si wafer. The normal stress across thickness of the epoxy film is estimated from wafer curvature measurements to be 15- 20MPa . The boundary element method is employed to investigate the whole stresses in the film. The numerical result for the normal stress across thickness of the film, σ xx , shows good agreement with the experimental result obtained by using the curvature measurement method. The singular stress is observed near the interface corner. Such residual stresses are large enough to initiate interface delamination to relieve the residual stresses.

Abstract: Indentation method was used to determine the interfacial fracture toughness of epoxy coating on aluminum substrate. Tensile testing followed by finite element analysis was also performed to determine the interface fracture toughness. Fracture toughness values determined by two methods were consistent, giving reliability to indentation method for interfacial fracture toughness measurement.

Abstract: Young’s modulus and hardness data obtained from nanoindentation are commonly affected by phenomena like pile up or sink in, when elastic-plastic materials are tested. In this work, a finite element model was used to evaluate the pile up effect on the determination of mechanical properties from spherical indentation in a wide range of elastic-plastic materials. A new procedure, based on a combination of results obtained from tests performed at multiple maximum loads, is suggested.

Abstract: In this paper, an attempt is made to describe the structural-statistical aspects of the damage accumulation and fracture of structural steel by means of experimental and numerical micromacromodeling. The new approach of multiscale modeling of structure element fracture based on evolution of microstructure levels is proposed. The lifetime is considered as an ultimate state in a local volume of material, or a critical value of the local damage approaching.

Abstract: In this paper, a phenomenological, nonlinear constitutive relation of ferroelectric ceramics is established by considering the fact that domain switching happens gradually with the external loading. A simplified gradual switching model is suggested and implemented into the constitutive law of ferroelectric materials. The volume fraction of domain switching is used as the internal variable in the model, and its evolution equation with mechanical/electrical loading is given. Comparison with experimental results shows that this simple constitutive model can predict the nonlinear hysteresis responses of ferroelectric materials.

Abstract: The reliability and optimal design of Micro Electro Mechanical Systems (MEMS) can be achieved only with the determination of material properties at the micro-scale. The major challenges in performing fatigue tests at the micro-scale are related to the accurate measurement of tiny deformations, to the control of very low forces and to the preparation, handling and positioning of μm-sized samples. In order to investigate the fatigue behaviour of MEMS components a new experimental setup based on the Phase Lock Loop (PLL) technique and a continuum mechanical model were developed for the characterization of micro-sized test samples. The main advantage of PLL is the achievable resolution in the crack length measurement, which increases with the decreasing of specimen size. Therefore, micro-beams with notches and without notches were prepared by electroplating Nickel in a SU8 photoresist mold (UV-LIGA). Investigations on the initiation and near-threshold crack growth behavior were performed to improve the understanding of the micro-mechanisms involved in fatigue phenomena.

Abstract: In this paper, we investigated effects of aging at 473K on the relationship between microstructure in the vicinity of the grain boundaries and fatigue strength for Al-1.2%Si alloy. Results obtained show the following features. (1) As aging time, tA increase, the tensile strength (σB) and 0.2% proof stress (σ0.2) increase slowly, but gradually decrease after reaching a maximum at around 18 ks. On the other hand, fracture elongation shows an opposite trend, suggesting that at aging times above 18ks, over aging occurs. (2) The fatigue strength lowers with increasing aging time, however, when the aging time is more than 18 ks at 473K, the fatigue strength remains almost the same. (3) When the aging time is more than 6 ks, grain boundary precipitates with a size greater than several 10s of nm are observed. (4) When the aging time is 18 ks, an accumulation of dislocations are observed at the grain boundaries and in the vicinity of grain boundary precipitates, and dislocations increase with the number of stress cycles. (5) When the aging time is more than 6 ks, the fatigue fracture surface is mainly intergranular. These results suggest that reduction of fatigue strength results from propagation of micro-cracks which are initiated at the large precipitates on the grain boundaries.

Abstract: A micromechanical model representing two adjacent grains is developed. Rapid crack propagation from one grain into another driven by a constant global stress state is simulated. The normal of the crack face in the grain where the micro-crack initiates coincides with the principle loading direction. In the adjacent grain, the propagation direction changes and separation occurs in a mixed way, involving both normal and shear separation. The largest grain size that can arrest a rapidly propagating micro-crack is defined as the critical grain size. The effects of the global stress state and temperature on the critical grain size is examined. The influence of the mismatch in lattice orientation between two neighboring grains is qualitatively described. The influence of temperature is modeled by a temperature dependent viscoplastic response.

Abstract: An original instrumented microindenter capable of testing materials up to 1000°C in an inert atmosphere has been developed. The method of neural networks is used to solve the inverse problem, in order to determine the constitutive equation of the materials tested. To obtain a data basis for the training and validation of the neural network, finite element simulations were carried out for various sets of material parameters. To reduce the number of simulations a representative sampling of the loading-strain responses is performed using an unsupervised network, so-called self-organizing map.