Key Engineering Materials Vols. 577-578

Abstract: Friction stir channelling (FSC) is a relatively new solid-state manufacturing technology able to produce conformal channels in a monolithic plate in a single step. During the FSC process the metal workpiece material is softened by the heat energy generated from dissipation during: plastic deformation, internal material flow and frictional work between the tool and the metal workpiece. The mechanical performance of a friction stirred channel aluminium alloy is affected by microstructure surrounding the channel. A new methodology that simulates a realistic 2D microstructure from experimental metallographic characterization and tensile tests was developed using the commercial software ABAQUS to study the mechanical behaviour of the friction stirred channel 5083-H111 aluminium alloy. Fourpoint bending tests were simulated and compared with experimental results. The RambergOsgood model was also adopted in the finite element analysis. It is seen from this investigation that microstructure can significantly affect the bending strength of friction stirred channel plates.

Abstract: Thermo-mechanical coupled modeling of air-plasma-sprayed (APS) thermal barrier coatings (TBCs) on Ni-based alloy was investigated. In the computational models, the stress distribution in the depth direction of the TBCs and also the influence of mechanical properties in heating, dwelling and cooling thermal cycles, were investigated. Nonlinear relationship (e.g., convective heat transfer between surrounding environment and coatings, and thermal transfer between the different layers etc.) was considered in the modeling. The results showed that the stress significantly reduced in the dwelling stage because of stress relaxation. The maximum stress occurred in the peak at the BC/TGO interface and it was amplified at the cooling stage. Moreover, the internal stress in the BC and TGO layer had a slight increase when TGOs thickness increased whilst the stress in the TBC and Sub were essentially unchanged. In the present work, the cracks in BC coating and the BC/TGO interface cracks were simulated as well. The failure mechanism I/II of TBCs had been investigated and the results showed that there was no stress concentration in the vicinity of cracks near the peak at the top coating layer, however, due to crack propagation, factures happened near the peak at the BC/TGO interface.

Abstract: Rolling contact fatigue (RCF) will eventually become an issue for machine elementsthat are repeatedly over-rolled with high contact loads and small relative sliding motion. Thedamage consists of cracks and craters in the contact surfaces. Asperities on the contact surfacesact as local stress raisers and provide tensile surface stresses which can explain both initiationand propagation of surface initiated RCF damage. A parametric study was performed to inves-tigate the contribution of surface roughness, friction and a residual surface stress to the RCFdamage process. The effects on initiation, crack path and fatigue life at both early and devel-oped damage were examined for a gear application. Both a one-parameter-at-a-time approachand a 2-level full factorial design were carried out. Surface roughness and local friction prop-erties were found to control crack initiation, whereas the simulated crack path was primarilyaffected by the residual surface stress, especially for developed damage. Reduced surface rough-ness, improved lubrication and a compressive residual surface stress all contributed to increasethe simulated fatigue life. The asperity point load model could predict effects on RCF that areobserved with experiments. The results further support the asperity point load mechanism asthe source behind surface initiated RCF.

Abstract: This article presents a new material model developed with the aim of analyzing failure of blunt notched components made of nonlinear brittle materials. The model, which combines the cohesive crack model with Hencky's theory of total deformations, is used to simulate an experimental benchmark carried out previously by the authors. Such combination is achieved through the embedded crack approach concept. In spite of the unavailability of precise material data, the numerical predictions obtained show good agreement with the experimental results.

Abstract: Advanced ceramics are a class of materials used as cutting tools in some of the most demanding material removal operations. Their high hardness makes them extremely suited for use at these extreme conditions. However they have a relatively low fracture toughness when compared to other conventional tool materials. A combined experimental-numerical method was used to investigate the role of microstructure on the fracture of advanced ceramics. In particular, the effect of grain size and matrix content were examined. Representative finite volume (FV) microstructures were created using Voronoi tessellation. It is shown, by comparing with real micrographs, that the method captures the features of real microstructures in terms of grain size distribution and grain aspect ratio. It was found that the underlying microstructure significantly affects the failure of this class of materials. Furthermore, it was found that by altering the microstructural parameters in the numerical model, such as grain size and matrix content, it is possible to specify material improvements.

Abstract: Strength data of three advanced ceramics were fitted to the Weibull, normal and lognormal distributions. The three ceramics had similar grain size and varied in binder content. The role of microstructure in the failure mechanism of such ceramics was analysed in terms of the chosen strength distributions. The best-fit distributions were determined using the maximum log-likelihood criteria and a comparison between the best and worst fit was conducted using the Akaike Information Criteria (AIC). Both large and small samples were tested to investigate possible scaling effects for these ceramics. It was found that for two of the three ceramics tested that a lognormal distribution rather then the conventionally used Weibull distribution was preferable in characterising the strength data. A small drop in strength was noticed between large and small samples but this trend was not thought to be a result of scaling rather due to the decrease in binder content.

Abstract: In mixed-mode crack propagation the crack faces frequently touch each other. The ensuing friction is expected to decrease the crack propagation speed. This effect is usually not taken into account, however, a realistic prediction of this effect may increase the calculated life and consequently increase the length of the inspection intervals. In this paper, penalty contact conditions are introduced in between the crack faces of the automatically generated mesh in a cyclic crack propagation. Special attention is given to the contact formulation and the area in which contact is defined. It is shown that the resulting crack propagation rate is significantly reduced by the introduction of friction provided that positive Mode-I is not significantly involved.

Abstract: This work reports a study of the ensemble interactions in hierarchical structures where damage events present, a significant issue in the analysis of material failure. Our approach was data-enabled in which we constructed a multi-variate composing of the amplitude spectrum matrix of responses of the evolving hierarchical microstructure to the applied stress. The columns and rows of this matrix are vectors of the amplitude scale and the observation vectors of multiscale random damage events, respectively. The results showed that damage mechanisms occurred in ensemble clusters, and the multiscale random damage was strongly correlated in spacetime.

Abstract: The paper presents a study performed on steel specimens under static loading using thermal infrared techniques to determine whether thermal analysis can also provide information on the fatigue behavior. In particular, the traditional static elastic zone was investigated. The results highlight that, even in the macroscopically elastic phase, a variation from the theoretical behavior is also detectable using thermographic techniques. In some previous works the authors demonstrated the possibility to estimate the fatigue limit of the material by mean of static tensile tests. In this paper, the procedure is adopted to analyze the effect of the cross-head speed on the thermoelastic limit, confirming the possibility to define the fatigue limit as the end of the totally linear thermoelastic behavior.

Abstract: In this study, effect of Carbon Milled Fiber (CMF) addition on interlaminar fracture toughness of carbon fiber reinforced plastics (CFRP) was investigated. Plain woven carbon fiber was used as reinforcement. Epoxy resin was used as matrix. The addition amounts of CMF are 0.5wt%, 0.8wt%, 1.0wt% and 1.2wt% for the epoxy resin. Mode I and mode II interlaminar fracture toughness tests were conducted based on JIS K 7086. As a result, mode I and mode II interlaminar fracture toughness increased with an increase of addictive amount of CMF. But excess addition was not effective. Pull out of CMF in matrix was found after mode I and mode II interlaminar fracture toughness tests. The mode I and mode II interlaminar fracture toughness of CMF added CFRP can be improved by fiber bridging of CMF.