Advanced Materials Research Vols. 41-42

Abstract: In this paper, the extensively-reported “size effect” phenomena in fracture mechanics tests are explained using the boundary effect concept. It is pointed out that the widely-observed size effect in fracture, including the dependence of the fracture energy on ligament, strength and fracture toughness on crack and/or ligament and the strength of geometrically similar specimens on characteristic size, is in fact, due to the boundary influence on the crack tip damage zone. Furthermore, the recently-developed asymptotic model is used to demonstrate that the dependence of strength on crack and ligament lengths as well as on the characteristic size of geometrically similar specimens is a result of the dominance of the distance of the crack tip to specimen boundaries on the specimen failure mode. To verify further the boundary effect concept, the asymptotic model is also applied to two sets of selected experimental data available in the literature, and the implications are discussed.

Abstract: Artificial neural network (ANN) is an intriguing data processing technique. Over the last decade, it was applied widely in the chemistry field, but there were few applications in the porous NiTi shape memory alloy (SMA). In this paper, 32 sets of samples from thermal explosion experiments were used to build a three-layer BP (back propagation) neural network model. According to the registered BP model, the effect of process parameters including heating rate ( ), green density ( ) and particle size of Ti ( d ) on compressive properties of reacted products including ultimate compressive strength ( v D σ ) and ultimate compressive strain (ε ) was analyzed. The predicted results agree with the actual data within reasonable experimental error, which shows that the BP model is a practically very useful tool in the properties analysis and process parameters design of the porous NiTi SMA prepared by thermal explosion method.

Abstract: Until recently, the solution heat treatment of conventional aluminum high pressure die cast (HPDC) parts has been considered impractical because the high temperatures involved cause surface blistering and dimensional instability. Now, a new heat treatment procedure has been developed by the CSIRO Light Metals Flagship in Australia which avoids these problems and, in many cases, allows tensile properties such as 0.2% proof stress to be doubled with little change to ductility. This development has the potential to reduce costs by allowing existing HPDC parts to be re-designed to use less metal and still achieve performance requirements. One issue, however, is the possibility that heat treating die castings to increase tensile properties may have an adverse effect on fracture toughness. This paper reports preliminary results of Kahntype tear tests conducted to assess the fracture resistance of as-cast and heat treated HPDCs. Studies of the alloys A360, A380 and C380 have shown that T4 and underaged (UA) T6 tempers produce an optimal combination of fracture resistance and tear strength. Furthermore, the fracture properties compare well with permanent mold and sand cast aluminium alloys that have similar tensile properties.

Abstract: The fatigue life data obtained under stress control conditions with and without mean stress are interpreted with reference to mean stress and ratcheting effects. Tests are conducted on nickel-base super-alloy Inconel 718 at 649°C and copper alloy Elbrodur-NIB at room temperature. The ratcheting strains at failure are obtained, and mean stress models such as the modified Goodman equation, the Smith-Watson-Topper parameter, and the Walker parameter are evaluated. The results are discussed in comparison for the two materials.

Abstract: A practical method to evaluate the thermal shock stress intensity factors (TS-SIF`s) associated with a flexibly restrained edge-cracked plate (FRECP) is outlined. It is assumed that the edge crack can be represented with a spring couple, and the deformations of the edge-cracked plate are consistent with Euler-Bernoulli deformation theory. It is then demonstrated how a compliance analysis of this simplified representation can be used with a finite element analysis of equivalent crack free plates to evaluate to TS-SIF`s associated with a FRECP. The accuracy of this method is shown to be excellent by comparing it against a direct fracture mechanic finite element analysis. This method is therefore advantageous to a fracture mechanic finite element analysis as it does not require an explicit analysis of a FRECP.

Abstract: A recoverable plate impact testing technology has been used for studying the growth mechanisms of mode II crack. The results show that interactions of microcracks ahead of a crack tip cause the crack growth unsteadily. Failure mode transitions of materials were observed. Based on the observations, a discontinuous crack growth model was established. Analysis shows that the shear crack grows unsteady as the growth speed is between the Rayleigh wave speed cR and the shear wave speed cs; however, when the growth speed approaches 2cs, the crack grows steadily. The transient microcrack growth makes the main crack speed to jump from subsonic to intersonic and the steady growth of all the sub-cracks leads the main crack to grow stably at an intersonic speed.

Abstract: Unexpected tube failure is the major factor causing unreliability in utility boilers. The first step in analyzing tube failures is to identify the active damage mechanisms. Three tube damage mechanisms related to overheating are presented and possible root causes are discussed to resolve these tube failures. Damage mechanisms can be recognized by metallurgical evaluation, comprising a combination of both visual and microexamination, complemented by chemical analyses of tube or fireside deposits, as appropriate. Characterizing the degree of microstructural degradation can also help to confirm and separate various potential high temperature tube damage modes, such as long or short term overheating, as well fuel ash attack. Carbon steel is the standard tube material for high pressure boilers (typically up to 625 psig steam) and has a normal design temperature limit of about 440 °C. However, microstructural changes occurring as a result of higher temperature exposures in service can include carbide spheroidization, graphitization and other transformations. Metallography is powerful tool for evaluating overheated failures and also for fire damage assessment. Evaluating overheated steel microstructures utilizes the principles of steel heat treatment and application of the iron-iron carbide equilibrium diagram.

Abstract: The cyclic fatigue threshold value (Kth) of PVC materials with (PVC-M) and without (PVC-U) impact modifier was determined and compared in air and water environments. The PVCM specimens contain 6 pphr of chlorinated polyethylene (CPE) impact modifier. The testing was undertaken at a stress ratio of R=0.1 and fatigue threshold was evaluated at 3 different frequencies: 1Hz, 7Hz and 20Hz. Frequency noticeably affected the fatigue threshold value; regardless of the testing environment; at low frequency the fatigue threshold of PVC-M was below PVC-U, however, this difference gradually decreased with increasing frequency as Kth of PVC-M increased but Kth of PVC-U remained constant. This trend was accelerated in water where a higher of fatigue threshold, Kth, was also observed. A lower fatigue threshold of PVC-M than PVC-U is associated with the presence of CPE particles. The absorption of water into the PVC matrix was evident with the formation of nodular structures observed on the fracture surface. The presence of the nodular structures (at regions close to the threshold) has retarded the fibrillation of crazes, which then blunted the crack propagation.

Abstract: Recently an effective block approach has been proposed to address the experimentally observed high growth rates of fatigue cracks at the critical locations on F/A-18 airframes. This approach treats a spectrum block as one equivalent constant amplitude cycle. Hence, the crack growth rate may be represented by a Paris-type equation, with the model parameters dependent on the load spectrum. This approach has been applied to the F/A-18 crack growth analysis, with good correlation to experimental data. In this paper, we summarise recent evaluation work of the approach using experimental data obtained under P-3C and F-111 spectra. An improved method for transferring the model parameters from a tested spectrum to an untested spectrum, suggested by the first author of the current paper, has been evaluated in comparison to the original procedure proposed. The soundness of relying on third-party tools based on constant amplitude model for the prediction of relative severity of spectra is also discussed. Finally, the advantages and limitations of the effective block approach are highlighted.

Abstract: Failures in hoist and drag ropes are studied and described based on site inspections on Marion and BE draglines. Causes of the failures are discussed. Some of the analyses of the wire rope failures centres on the wire rope accessories through which these ropes travel during machine operation. The influence of wire quality on wire rope life is also studied. Visual and metallographic examinations are both used in this study leading to a better understanding of deterioration mechanisms that lead to failure of hoist and drag ropes under service loading.