Key Engineering Materials Vols. 306-308

Abstract: According to laboratory accelerated test data, stress corrosion cracking (SCC) in structural metal materials occurs by initiation and coalescence of micro cracks, subcritical crack growth, multiple large crack formation and final failure under the combination of materials, stress and corrosive environment. In this paper, a computer simulation model for the process of SCC has been proposed. The procedure is as follows: The possible number of crack initiations is set for a given space and the initiation times for all cracks are assigned by random numbers based on exponential distribution. The sites and length of the cracks are assigned by uniform random numbers and normal random numbers, respectively. The coalescence of cracks and the subcritical crack growth are determined based on the fracture mechanics. The simulation is terminated when the maximum crack length reaches a critical value or all of the possible number of cracks is initiated. The results obtained in this paper indicate the applicability of the present model to predict the SCC behavior in real structures based on the laboratory accelerated test data.

Abstract: In the recent development of internal combustion engine, considerable increase in speed and power has been accomplished. This achievement, however, brought up various problems due to the excessive temperature of engine parts, which becomes a crucial factor in engine durability. In the present paper, temperature measurement of a marine engine was investigated experimentally. The adapter is made to pull out a thermocouple through the safety valve hole of the engine. The thermocouple is welded on the surface of the cylinder cover to measure the average temperatures of the engine cylinder cover. Ceramic adhesive was used for preventing the affect of high temperature combustion gas. The cylinder cover temperatures of the engine were measured by means of surface welding method. As a result, average temperatures of the engine cylinder cover were successfully in the range of 85~335oC. In the present study, the surface welding method was confirmed from the temperature measurement of cylinder cover in an experimental engine, and can be applied to large-sized marine engine without damage.

Abstract: Durability assessment for the suspension system of the vehicle is very important. In order to secure reliability on the results of durability assessment and fatigue design criterion, accurate stresses for the interesting component must be basically calculated. In this paper, numerical stress analyses for the automobile’s coil spring were performed. Residual stresses by shot peening were also analyzed. The effects of the stress analysis results on fatigue strength of the coil spring were discussed. From the results, fatigue durability of the shot peened coil spring could be considerably improved by the effects of compressive residual stress. However, being attacked by corrosive environments, it could decrease by corrosion fatigue degradation.

Abstract: A novel integral equation method is developed in this paper for the analysis of two-dimensional general piezoelectric cracked bodies. In contrast to the conventional boundary integral methods based on reciprocal work theorem, the present method is derived from Stroh’s formalism for anisotropic elasticity in conjunction with Cauchy’s integral formula. The proposed boundary integral equations contain generalized boundary displacement (displacements and electric potential) gradients and generalized tractions (tractions and electric displacement) on the non-crack boundary, and the generalized dislocations on the crack lines. The boundary integral equations can be solved using Gaussian-type integration formulas without dividing the boundary into discrete elements. The crack-tip singularity is explicitly incorporated and the generalized intensity factors can be computed directly. Numerical examples of generalized stress intensity factors are given to illustrate the effectiveness and accuracy of the present method.

Abstract: Parameter optimization of a static micro-mixer with a cantilever beam was accomplished for maximizing mixing efficiency using a sequential approximate optimization method. The objective function and design variables were chosen as mixing index, and the length and the angle measured from the horizontal of the cantilever beam, respectively. The Optimization problem of the mixer was considered as a series of sub-problems. Approximation to solve the sub-problems was performed by response surface methodology. To verify the reliability and the accuracy of the approximated objective function, ANOVA table and variable selection method were implemented, respectively. It was verified that the sequential approximate optimization method worked very well, and the mixing efficiency was significantly improved compared with the initial design.

Abstract: Theoretical and experimental methods dealing with the effect of a viscoelastic substrate on a cracked body under inplane load are presented in this article. A generalized trimaterial solution is solved as a convergent series in terms of complex potentials via the successive iterations of the alternating technique in order to satisfy the continuity condition along the interfaces between dissimilar media. This trimaterial solution is then applied to the problem of a finite thickness layer bonded to a half-plane substrate. Using a standard solid model to formulate the viscoelastic constitutive equation, the real time stress intensity factors can be directly obtained from the Laplace domain. In the experiment, an aluminum cracked plate bonded to a polymer substrate is tested in a tension machine, where the displacement of the specimen is recorded by a high precision digital camera. The time-dependent stress intensity factor is determined by an inverse calculation through the crack opening displacement. The comparisons between the theoretical and experimental results are discussed in the final.