BRAKE TEST OF SiCp/A356 BRAKE DISK AND INTERPRETATION OF EXPERIMENTAL RESULTS

Material properties are obvious different between aluminum matrix composites and iron and steel materials. After the brake disk braked at the same speed, the average temperature of the aluminum brake disk is 1.5 times as high as one of iron and steel brake disk, the thermal expansion value of the aluminum brake disk is 2 times as big as one of iron and steel brake disk. Mechanical property of the material decreases with the temperature increasing generally during braking, on the other hand, the big thermal stress in the brake disk happens because the material expansion is constrained. Firstly, the reasons of the thermal stress generation and the fracture failure of brake disks during braking are analyzed qualitatively by virtue of three-bar stress frame and sandwich deformation principles in physic, and then the five constraints which cause the thermal stress are summarized. On the base of the experimental results on the 1：1 emergency brake test, the thermal stress and temperature fields are simulated; The behavior of the fracture failure is interpreted semi-quantitatively by finite element analysis, There is the coincident forecast for the fraction position in term of the two methods. In the end, in the light of the analysis and calculation results, it is the general principles observed by the structure design and assembly of the brake disk that are summarized.

DETERMINATION OF CREEP PROPERTIES OF THERMAL BARRIER COATING(TBC)SYSTEMS FROM THE INDENTATION CREEP TESTING WITH ROUND FLAT INDENTERS

Indentation creep behavior with cylindrical flat indenters on the thermal barrier coating (TBC) was studied by finite element method (FEM). On ike constant applied indentation creep stress, there is a steady creep rate for each case studied for different creep properties of the TBC system. The steady creep depth rate depends on the applied indentation creep stress and size of the indenters as well as the creep properties of the bond coat of the TBC and the substrate. The possibilities to determine the creep properties of a thermal barrier system from indention creep testing were discussed. As an example, with two different size indenters, the creep properties of bond coat of the TBC system can be derived by an inverse FEM method. This study not only provides a numerical method to obtain the creep properties of the TBC system, but also extends the application of indentation creep method with cylindrical flat indenters.

Effects of humidity on shear behavior of bamboo

Bamboo is a naturally occurring biological composite, however its microstructure and hence its properties are very complex compared to the manmade composites. Due to optimization, it can be assumed that the variation in properties along the thickness of the culm be a smooth transition for better bonding strength between layers and to prevent non uniformity in stress concentration. As a consequence, biological structures are complicated and functionally graded. Hence, a realistic model that can capture the mechanical performance of bamboo is valuable in future design of robust multifunctional composites. This paper presents the results of experimental and numerical studies on the torsional （shear） properties of bamboo. The hierarchical and multi-scale structure of bamboo and the distribution of micro-scale fibers are revealed via laser scanning and atomic force microscopy. This information was incorporated into a finite element model to analyze the mechanical behavior of bamboo under torsion and to estimate the shear modulus of bamboo along the fibers. Moreover, the effects of humidity and therefore water content on the mechanical properties of bamboo were evaluated by performing torsion tests on samples maintained in environments with different humidities. Increasing the humidity does not cause a drop in the shear modulus, however, a jump in the shear modulus did occur at around 60% humidity. Results of this study indicate that the highest strength values in samples occurred in environments with humidity levels between 60% and 80% and undergo a significant drop after that. In higher humidities, the samples behave more ductile.

A new approach on necking constitutive relationships of ductile materials at elevated temperatures

A new method is presented to determine the full-range, uniaxial constitutive relationship of materials by tensile tests on funnel specimens with small curvature radius and finite element analysis(FEA). An iteration method using FEA APDL(ANSYS parametric design language) programming has been developed to approach the necking constitutive relationship of materials. Test results from SAE 304 stainless steel at room temperature show that simulated load vs displacement curve,diameter at funnel root vs displacement curve, and funnel deformation contours are close to modeled results. Due to this new method, full-range constitutive relationships and true stress and effective true strain at failure are found for 316 L stainless steel, TA17 titanium alloy and A508-III stainless steel at room temperature, and 316 L stainless steel at various elevated temperatures.

Effects of Vertical Angle of Conical Punch on Stretch Flangeability of High Strength Steel

To clarify the effects of the vertical angle of a conical punch on stretch flangeability, hole expansion forming tests were conducted. Test results showed that the hole expansion ratio becomes larger as the vertical angle decreases.Results also showed that the fracture strain at the fracture location on the hole edge was constant and independent of the vertical angle. This is because the hole expansion ratio was controlled not only by the fracture strain, which is independent of the vertical angle, but also by deformation uniformity along the hole edge. From the result of numerical analyses, it was determined that deformation uniformity depends on the gradient of circumferential stress along the radius direction. When the vertical angle is sharp, the circumferential stress showed a steep decline and the deformation localization was suppressed. Consequently, the hole edge deformed more uniformly and the hole expansion ratio became larger. It is concluded that in order to improve stretch flangeability of high strength steel, it is important to uniformly deform the hole edge by applying a conical punch with a sharp vertical angle.