SiC particulate reinforced glass matrix composites were fabricated by hot pressing. The volume fraction of SiC was changed from 0% to 30%. The elastic modulus of the fabricated composites was consistent with a law of mixture based on the micromechanics. In order to evaluate the mechanical properties of the materials we measured four-point bending strength and fracture toughness (KIC) by Single Edge Pre-cracked Beam (SEPB) method. Acoustic emission (AE) tests were performed during four-point bending and fracture toughness tests. We observed fracture surfaces after bending tests. The bending strength increased with increasing volume fraction of SiC particle. The bending strength of glass was 96MPa and that of the composite reinforced with 30vol% SiC particle was 144MPa. The fracture toughness KIC also increased with increasing volume fraction of SiC particle. The fracture toughness KIC of the composite reinforced with 30vol% SiC particle was 2.1MPam1⁄2. The number of AE events increased with increasing volume fraction of SiC particle. However AE behavior during bending tests is different from that during fracture toughness tests. The total number of AE during fracture toughness tests is more than that during bending tests.
The effect of reinforcement size and volume fraction on fracture toughness in powder metallurgy (P/M)-processed 6061 Al alloy matrix composites with different kinds of reinforcements (Si3N4 whiskers, and SiC particles) are analyzed. The acoustic emission (AE) activities has also been investigated during three point bending fracture toughness test.Comparing the fracture toughness results in MMCs containing different volume fractions (Vf) of whiskers and particles show higher fracture toughness in materials with lower Vf. On the other hand, the fracture behavior in 10 percent reinforced composites regardless of the type of reinforcement, particle or whisker, is ductile fracture, but it is semi-brittle in the case of 30 percent reinforced materials. Regarding the amplitude and the number of AE events during three point bending fracture toughness test, 30 percent reinforced materials with brittle fracture mode show very high amplitude signals with less number of events in the onset of plastic deformation. 10 percent reinforced composites with a ductile mode fracture show lower AE amplitude and higher number of events. Totally, a kind of interaction between the different parameters like volume fraction, reinforcement type and reinforcement size on the fracture behavior of 6061 Al alloy matrix composites are observed and discussed.
Due to the influence of welding thermal cycle toughness of structural steel degenerates. Recently, the intercritically reheated coarse grained zone (IC CG HAZ) was found display the worst toughness in welded joint, which was associated with its fracture mechanism. In this work, two IC CG HAZs of a structural steel were prepared by welding thermal-cycle simulation techniques. For the two IC CG HAZs the dynamic fracture toughness and the fracture mechanism were studied, and the correlation between fracture toughness and fracture mechanism was also discussed. Under both static and rapidly loading, cracks in the IC CG HAZ were found to initiate at the intersection of αB0 packets with different orientations, followed by propagating in cleavage. In some places of crack propagation, adjacent cleavage facets are connected by shear, producing dimple zones. Though the brittle fracture initiation mechanism remains unchanged, the cleavage facet size and the proportion of the dimple zones between facets vary with loading speed. These changes are shown being associated with the effect of strain rate on fracture toughness. In this work, the fracture behavior of IC CG HAZ was also compared with that of base metal and prestrained steel.
It is important to predict the stress driven hydrogen induced cracking at the weld joint on the basis of computational mechanics from the view point of engineering problem. One of authors has been proposed α multiplication method which magnifies the hydrogen driving term in the diffusion equation to realize correctly hydrogen concentration behaviors. In this study, on the basis of proposed numerical analysis, behaviors of hydrogen diffusion and concentration during cooling process of y-grooved weld joint were analyzed and the mechanism of hydrogen induced cracking was investigated. The behaviors of hydrogen diffusion and concentration for the model of y-grooved weld joint was analyzed by combining α multiplication method with the coupled analyses of heat transfer – thermal stress – hydrogen diffusion. As a result, hydrogen was found to diffuse from weld metal to base metal through HAZ (Heat Affected Zone), and concentrate at the position of blunt angle side of weld groove bottom. It was found that hydrogen concentrates at the position of the local maximum value of hydrostatic stress gradient. This analytical result was found to well predict the actual hydrogen induced cracking of the y-grooved weld joint. Therefore, it was considered that prediction of hydrogen induced cracking becomes possible using this method of analysis.
Non-linear ultrasonic and acoustic emission (AE) signals during ultrasonic fatigue testing were analyzed by using Laser Doppler Vibrometer (LDV) and continuous AE waveform analysis system (1 MHz/12bit). Notched specimens of a high strength low alloy steel were prepared for the ultrasonic fatigue test with exciting vibration frequency of 20 kHz. The detected surface velocity was longitudinal direction at the end of the specimen with frequency range from 100 Hz to 500 kHz. During the waveform monitoring of the fatigue test, a distorted exciting waveform was observed in the final stage of the test. Then the burst type noise mixed with the distorted exciting waveform was obtained just before the final failure in the case of the failure specimens. Contrary, the distorted exciting waveform and AE were not observed in the case of non-failure specimens. AE signal and upper harmonics of exciting frequency were analyzed by the FFT method. As the result, after the intensity of 2nd and 3rd harmonics increased rapidly, AE events were detected continuously in the case of the failure specimens. It can be concluded that non-linear ultrasonic and AE analysis were effective monitoring tool for fatigue damage progression.
In the present work, the ductile fracture of structural steels and the effect of plastic pre-strain on ductility have been investigated by tensile test. The fracture process consisting of void nucleation, growth and coalescence was observed by scanning electron microscopy (SEM). The nucleation strain of microvoid was measured experimentally. The critical interfacial strength of particle/matrix interface was calculated by dislocation model and continuum analysis, and the void growth was evaluated by Rice-Tracey model. It was found that longitudinal void coalescence does not affect fracture even though large cracks exist, but particle position affects void nucleation. Thomason's plastic limit-load model was used to predict void growth strain. Through the analysis of ductile fracture process, the influence of plastic pre-strain on ductility was discussed.
The purpose of this work is to compare the numerical calculation results of the hydrogen diffusion with the experimental results. We firstly conducted a y groove weld cracking test using 980 MPa grade steel and 780 MPa grade welding consumable to observe HAZ cracking. Next, we showed the present diffusion equation and the procedure to determine the physical properties for the calculations such as stress-strain curve, and using the apparent diffusion coefficient and the boundary condition obtained in the companion paper, we conducted the numerical calculations for the two cases, that is, the under-match case (980 MPa grade steel and 780 MPa grade welding consumable) and the even-match case (780 MPa grade steel and 780 MPa grade welding consumable). The calculation results of the under-match case show that hydrogen tends to accumulate in the stress concentration area of both the HAZ and the weld metal, which indicates crack may occur in both the HAZ and the weld metal. The results of the even-match case show not only the hydrogen accumulation around the stress concentration area but also high hydrogen concentration in the weld metal, which indicates crack may occur in the weld metal. Calculation results for both of the cases fairly agree with the experiments.
It is important to predict the stress driven hydrogen induced cracking at the weld joint on the basis of computational mechanics from the view point of engineering problem. In this study, On the basis of proposed numerical analysis, behaviors of hydrogen diffusion and concentration during cooling process of y-grooved weld joint were analyzed and the mechanism of hydrogen induced cracking was investigated. One of authors has been proposed a multiplication method which magnifies the hydrogen driving term in the diffusion equation to realize correctly hydrogen concentration behaviors. In this study, the behaviors of hydrogen diffusion and concentration for the model of y-grooved weld joint was analyzed by combining a multiplication method with the coupled analyses of heat transfer – thermal stress – hydrogen diffusion. As a result, hydrogen was found to diffuse from weld metal to base metal through HAZ (Heat Affected Zone), and concentrate at the position of blunt angle side of weld groove bottom. It was found that hydrogen concentrates at the position of the local maximum value of hydrostatic stress gradient. This analytical result was found to well predict the actual hydrogen induced cracking of the y-grooved weld joint. Using this method of analysis, prediction of hydrogen induced cracking becomes possible.
