Key Engineering Materials Vols. 324-325

Abstract: The increase of magnetic field of ferromagnetic components under service load and geomagnetic field is induced by the residual magnetic induction and spontaneous magnetization. The stress concentration positions can be found by detecting the magnetic field and the fracture can be avoided. The variation of the residual magnetic intensity of bearing carriers under different damage conditions is studied in this paper; the statistical characteristic of the residual magnetic intensity of the specimens in different residual deformations is got, and a concept of residual magnetic intensity standard deviation is put forward with a lot of experiments and in-depth theoretic analysis. The results indicate that the curves of residual magnetic intensity standard deviation appear obvious turning points when the specimens reach the yield point in different original magnetic field conditions, and that the function between residual magnetic intensities standard deviation, tensile load and strain is established. Therefore, the plastic deformation can be judged by the variation of the magnetic field in their service process, and the damage level can be evaluated consequently, which gives a new reliable method to estimate the deformation of ferromagnetic material.

Abstract: Phase transformation temperatures of the SMA annealed at different annealing temperature were investigated through a series of DSC test. A new phase transformation model was proposed to predict the phase transformation and a numerical analysis was conducted to demonstrate the applicability of the new model. According to the result, phase transformation temperatures would be affected on annealing temperatures and the new phase transformation model would be applicable to predict the phase transformation under the stress-free state or the external stress state.

Abstract: In this paper, a new theoretical model is developed to characterize the damage of the adhesive joint. Elastic modulus of adhesive joints is an important parameter to represent damage characteristics. Based on the fact that the thickness of the adhesive layer is very small, it is reasonable to believe that damage will decrease the tension modulus of the adhesive joint while the compression modulus will keep unchanged. Modeling the adhesive joint as an interface with different modulus in tension and compression, and applying integral transform method, we solve the associated nonlinear boundary problem to obtain the nonlinear ultrasonic waves transmitting through the adhesive layer. With this nonlinear ultrasonic wave, variation of elastic modulus and damage variable of the adhesive layer are thereafter characterized nondestructively by a nonlinear coefficient.

Abstract: A new definition of vibration transmissibility, “the ratio of FRFs (Frequency Response Functions)”, is proposed to provide novel structural damage indices, i.e., vibration transmissibility and its real and complex curvatures. Using the FRF and its real and complex curvatures as references, the fundamental studies on performance of damage indices to reflect structural low-level damages are conducted using vibration-based nondestructive evaluation of a frame structure. The experimental results demonstrate that the sensitivity of the damage indices increases in the order of vibration transmissibility or FRF, and its real and complex curvatures. While the vibration transmissibility and its real and complex curvatures possess higher damage sensitivity than the FRF counterparts. The proposed novel damage indices are promising to develop viable and advanced structural nondestructive evaluation techniques.

Abstract: The damage characteristics of ferrite ductile cast-iron under uniaxial stress has been studied through measuring electric resistance. The results showed that the damage in ductile cast-iron happens when the applied stress is more than a certain extent, and the damage value increases with stress. The evolutional law of damage value as a function of stress was obtained. The damage threshold of ductile cast-iron exists and increases with nodularity, but this threshold is always below the yield strength, which has related significant reference to the design of machinery structure and the choice of materials. The critical damage value is not related to the nodularity, which is about 0.06~0.068 under the present test condition.

Abstract: Symmetric three-layer Al2O3/ZrO2 composites have been prepared by freeze casting and pressureless sintering at 1400-1600o C in air. The layered composite with Al2O3 containing 15 vol.% ZrO2 as an outer layer and containing 20 vol.% ZrO2 as an inner layer, sintered at 1600o C showed the maximum bend strength (300 MPa). Contact damage resistance was superior in layered composites compared with monolithic materials, possibly due to the development of compressive stress in the outer layer.

Abstract: Tetragonal zirconia polycrystal ceramics prepared by sintering compacts of 3 mol.% Y2O3- and 12 mol.% CeO2-doped ZrO2 powders, in air at 1400o and 1500o C have been characterized in terms of microstructure and mechanical properties. With increasing 12 mol.% CeO2-doped ZrO2 content, the average grain size of sintered materials increased in the range 0.27-1.93 μm. The bend strength and indentation fracture toughness were 374-962 MPa and 6-10 MPa.m1/2 respectively, dependent on batch composition.

Abstract: The experimental research on six steel plates strengthened with CFRP plates, loaded in tension, shows that their fatigue lives can be greatly increased compared with un-strengthened specimens. Linear elastic fracture mechanics (LEFM) is adopted to explain the mechanism of CFRP plates strengthening. The stress intensity factors of the steel plates are calculated with finite element method (FEM), and Paris law on crack propagation is used to predict the fatigue life of strengthened specimens. The comparison between experimental results and numerical predictions shows good agreements on the fatigue crack propagation.

Abstract: The influence of environmental corrosion on fatigue life of 2024-T3 aluminum alloy is investigated in this paper. Experiments with center-hole specimens and CC(T) specimens are conducted and reported. The test result shows that the corrosion influence is becoming serious while the environmental causticity is increasing, and the influence on crack initiation life is greater than that on crack propagation life. Analysis of the test data also indicates that the environmental causticity enlarges the dispersivity of fatigue life. Using the corrosion-modification-factor obtained by eigenvalue or mean value to prediction the fatigue life with high reliability will cause an unconservative conclusion.

Abstract: Surface and subsurface contact damage in dental ceramics and the influence of microstructure on damage tolerance were investigated in this study through indentation methods. The dependence of the damage tolerance on common materials properties was discussed. A simple expression to estimate quantitatively the damage tolerance via basic material parameters, based on the features of damage tolerance and fracture mechanics is confirmed. And the residual strength of damaged specimen was measured via three-point bending tests.