Key Engineering Materials Vols. 462-463

Abstract: The resistance of metallic sandwich plates to blast loading is studied in this work. Two typical cores, i.e. trapezoidal plate core and metallic foam core are considered in the analysis. The normal and longitudinal compressive stresses of the trapezoidal plate core are analytically and numerically obtained. Large deflection and resistance of metallic sandwich plates to blast loading are numerically analyzed. The results are compared with that of monolithic solid plates made of the same material and having equivalent weight. It is shown that the well-designed sandwich plates possess higher resistance to blast loading than monolithic solid plates.

Abstract: In this paper, rotary bending fatigue tests for a structural steel were performed in laboratory air, fresh water and 3.5% NaCl aqueous solution, respectively, thus to investigate the influence of environmental media on the fatigue propensity of the steel, especially in high cycle and very-high-cycle fatigue regimes. The results show that the fatigue strength of the steel in water is remarkably degraded compared with the case tested in air, and that the fatigue strength in 3.5% NaCl solution is even lower than that tested in water. The fracture surfaces were examined to reveal fatigue crack initiation and propagation characteristics in air and aqueous environments.

Abstract: Using simplified benchmark models, representative of the behavior of real structures, a unified framework for quantification of elastic follow-up (EFU) in structures has been provided. Closed form analytical solutions for evaluation of elastic follow-up are presented for model structures. The impact of elastic follow-up on the relaxation i.e. the redistribution of residual stresses has been explored and hence its significance in the integrity assessment of structures in general and in classification of residual stresses in particular has been highlighted.

Abstract: Herringbone gears are extensively used in numerous engineering applications including gearboxes. Premature failures of such gears could lead to many serious consequences such as process downtime and late delivery which are critically important in this day and age of intense competition. This paper reports the results of failure investigation of a herringbone gear in a reduction gearbox used in a hot rolling steel re-bar mill in Thailand. The gear fractured after only 24 hours of service. The fractured gear was inspected visually and macroscopically, and all critical dimensions were measured. Chemical compositions of the materials were analysed. Fracture surfaces and microstructure of gear material were examined, and microhardness measured. Stress analysis was performed using finite element methodology. It was found that the premature failure of the gear, which appeared as a longitudinal crack, was due to excessive stress. The crack originated at the root of the keyway. Careful examination revealed that there was a slight misalignment between the key and the keyway which resulted in excessive stress. The lesson learned from this failure is that manufacturing defects, however slight, could lead to premature failures of key components such as gears, and could be very costly. It is recommended that apart from proper designs of components, good manufacturing practices are also necessary to minimize the chance of premature failures.

Abstract: Electrorheological (ER) fluids are a kind of smart material whose rheological properties can be controlled by an external electric field. In the present paper, the transient vibration of a rectangular three layer sandwich plate with electrorheological fluid core is analyzed based on the classical plate theory. The Bingham plastic model is used to consider the post-yield behavior of ER fluid. The structure is modeled using a finite element method. Hamilton’s principle is employed to derive the finite element equations of motion. The constant average acceleration scheme is used to integrate the equations of motion. The effects of change in electric field and core thickness on the structure settling time and its natural frequencies are studied for various boundary conditions. The results show that the thickness of the core layer and the electric field strength has significant effects on damping behavior of the sandwich plate. When the applied electric field increases a linear decay in transient response of the structure is observed. It is also found that the electric field changes have no influence on the system natural frequencies.

Abstract: This paper presents the similarity conditions between aeroelastic instability (flutter) and the free vibration frequency of cross-ply laminated plate. The method is based upon the direct use of governing equation of the system. Both complete and partial similarities are discussed. This approach, the natural frequency analysis of models and prototype is used to predict the flutter boundary of laminated plate. The results present herein indicate that there are some constrains such as geometry of the model, model material, and stacking sequence of laminates in designing a model with complete similarity. As working with complete similar scale model is usually difficult, some certain types of distortion (distortion in number of plies, and stacking sequence) are considered. According to the results, cross ply composite models can be found which have this ability to predict flutter boundary of the prototype accurately using free vibration data.

Abstract: Failure of thermal barrier coating (TBC) system is associated with the morphological imperfection of thermally grown oxide (TGO) layer. The objective of this work is to numerically investigate the crack nucleation and propagation in TBC system upon thermal cycling based on a cohesive zone model, in which TGO thickness imperfection effect is incorporated. The results show that TGO/BC (bond coat) interface is subjected to high tensile stress in the vicinity of TGO thickness imperfection during thermal cycling, thereby inducing crack nucleation. Owing to the plastic deformation of BC, fracture behavior of TGO/BC interface is related to BC yield strength for a typical thermal growth rate of TGO. Furthermore, the embedded oxide in BC could be also ruptured as a result of the increasing transverse stress, which will lead to the coalescence of adjacent cracks.

Abstract: The spinel growth induces undulation of the thermal growth oxide layer and decreases the service life of plasma-sprayed thermal barrier coatings. An analytical model is introduced to investigate the effect of spinel growth on the delamination of thermal barrier coating. The analytical results show that the number per unit area and the growth rate of spinel have significant influence on the delamination of thermal barrier coating. The stiffer and thicker thermal barrier coating is more easily to delaminate from the bond coat due to the existence of spinels. The effect of spinel on the delamination cannot be neglected. How to reduce the growth rate and the number of spinel is a key problem to prolong the service life of thermal barrier coatings.

Abstract: This paper provides a computational method for dislocation-precipitate interaction problems. The computational method is a combination of the parametric dislocation dynamics and the voxel-based finite element method, and has a potential to enable the simulation of interaction between dislocations and multiple precipitates. To reduce the computational time, a multi-level voxel element model is employed. The convergence behavior of numerical accuracy and the computational time of the proposed method are examined by solving a dislocation-precipitate interaction problem. The results show that the proposed method has a good convergence behavior, and the computational time can be drastically reduced by the use of the multi-level voxel element model. Finally, the interaction between dislocations and multiple precipitates is solved to demonstrate a potential of the proposed method with various average diameters and constant volume fraction of precipitate. As the result, the proposed method successfully captured the dependence of the critical resolved shear stress on the average precipitate diameter.

Abstract: Seven thousand sets of data consisting of mechanical properties, chemical compositions, and rolling parameters of industrial hot-rolled coils were analysed using multiple regression. This was to establish empirical formulas to predict mechanical properties of steel as a function of chemical compositions and rolling parameters. The empirical formulas predicting yield strength (YS), ultimate tensile strength (UTS) and percentage of elongation (EL) of low carbon steel strip were obtained, e.g. YS = 461+ 418 C + 61.6 Mn + 796 P ¬– 303 S + 159 Si + 146 Cu + 204 Ni + 49.7 Cr + 1127 V + 1072 Ti + 3674 Nb – 266 Mo – 6299 B – 76.3 Al – 557 Sn – 3.54 THK – 0.00758 WID – 0.114 FT – 0.223 CT. The rolling parameters in equation included finishing temperature (FT), coiling temperature (CT), thickness (THK) and width (WID) of strip. R-Square values for the formulas predicting YS, UTS, and EL were 82.3%, 90.1%, and 75.8% respectively. These equations were validated by using another 120 hot-rolled coils. The averages of absolute values of the difference between the predicted and actual values of YS, UTS, and EL were 9.6 MPa, 7.8 MPa, and 2.7 % respectively. Correlation of chemical compositions and rolling conditions with mechanical properties was discussed in the paper.