Simulation of cup-cone fracture using the cohesive model

The cohesive model is used for the prediction of the crack path during stable crack extension in ductile materials. The problem of crack-path deviation is investigated by means of simulation of crack propagation in a round tensile bar. The respective phenomenon is known as cup-cone fracture. It is shown that the model is able to predict the failure mechanism, which consists of normal fracture in the center and combined normal/shear fracture in the so-called “shear lips” at the specimen’s rim. The damage evolution and crack path predicted by the model are presented. The effect of the normal and shear failure parameters on the crack-deflection point and several aspects of the finite element mesh are discussed.     

Thermomechanical fatigue—damage mechanisms and mechanism-based life prediction methods

An existing extensive database on the isothermal and thermomechanical fatigue behaviour of high-temperature titanium alloy EVII 834 and dispersoid-strengthened aluminum alloy X8019 in SiC particle-reinforced as well as unreinv conditions was used to evaluate both the adaptability of fracture mechanics approaches to TMF and the resulting predictive capabilities of determining material life by crack propagation consideration. Selection of the correct microstructural concepts was emphasised and these concepts were, then adjusted by using data from independent experiments in order to avoid any sort of fitting. It is shown that the cyclic /-integral (δJ_eff concept) is suitable to predict the cyclic lifetime for conditions where the total crack propagation rate is approximately identical to pure fatigue crack growth velocity. In the case that crack propagation is strongly affected by creep, the creep-fatigue damage parameter δ_CF introduced by Riedel can be successfully applied. If environmental effects are very pronounced, the accelerating influence of corrosion on fatigue crack propagation can no longer implicitly be taken into account in the fatigue crack growth law. Instead, a linear combination of the crack growth rate contributions from plain fatigue (determined in vacuum) and from environmental attack is assumed and found to yield a satisfactory prediction, if the relevant corrosion process is taken into account.     

聚合物冷拉过程的流函数分析

用流函数法分析了聚合物拉伸成形时，颈缩的发生和扩展过程，文中论证了采用流函数法的优越性和适用性，并将流函数法计算结果和有限元法计算结果作了对比。     

On the mechanical behavior of aluminum alloys reinforced by long or short alumina fibers or SiC whiskers

This study was initiated to examine the dispersion of longitudinal and transverse waves in metal matrix composites in order to obtain the dynamic elastic modulus and to evaluate various models for predicting the composite's macroscopic elastic constants from the properties of its constituents. The materials chosen for this investigation were alumina continuous fibers on the one hand and alumina and SiC short fibers on the other hand, all embedded in an aluminum alloy matrix. In addition, some indications have been obtained experimentally for the acoustoelastic effect in the composite.     

分布系统中多米诺效应的分析与消除

分布系统中发生故障后,错误信息在进程间的传播,以及由此引起的系统恢复过程中的多米诺效应是分布系统容错恢复技术所面临的最重要与最困难的问题.目前各种研究工作偏重于具体的系统动态恢复技术.本文的研究重点在于首次揭示多米诺效应本身固有的数量特征及其与分布系统技术参数间的联系,给出多米诺效应动态消除技术,引入系统额外开销的下限,并针对动态技术的缺点,探讨多米诺效应的静态消除技术,为解决困难开辟新途径.     

An investigation of crack closure and the propagation of semi-elliptical fatigue cracks in Q1N (HY80) pressure vessel steel

The results of an experimental investigation of the effect of crack closure on the propagation of semi-elliptical fatigue cracks are presented. Load-shedding fatigue threshold tests were carried out at stress ratios of 0.2, 0.35, 0.5 and 0.7. Crack closure was measured at the surface and depth positions using backface strain gauges, near-tip gauges, and a clip gauge. Differences between the surface and depth growth behaviour are explained by considerations of the effects of the transition from plane stress conditions at the surface to plane strain conditions at the depth. The effects of stress ratios are attributed largely to differences in the crack opening displacement, which result in asperities coming into contact to induce roughness-induced crack closure.     

Thermodynamic state of reinforced interpenetrating polymer networks

The free energies of mixing of two networks in the interpenetrating polymer network based on crosslinked polyurethane and poly(ester acrylate) have been determined by the vapour sorption method. It was established that the constituent networks in the IPN are not miscible. The introduction of fillers of different chemical nature increases the compatibility. The thermodynamic affinity of the fillers to the individual networks and IPN was estimated. It was established that when the free energy of interaction of one or both components of the IPN with the filler is negative, reinforcement leads to the formation of a compatible and equilibrium system. For fillers having no affinity to the polymers, compatibilization is observed, which is connected with slowing down of phase separation in the system in the presence of filler.     

Effect of load ratio on the deformation behaviour of fatigue crack tips

To investigate the change in the manner of plastic deformation at a fatigue crack tip under different loading conditions, the fresh surface area which is produced in one fatigue cycle at the crack tip was measured by an electrochemical method under various load ratios. The fresh surface area produced in each fatigue cycle is in proportion to the crack propagation rate in the Paris regime. This indicates that the crack tip geometry at K_max maintains a similar profile through the Paris regime. Furthermore, the fresh surface area is independent of load ratio at a given da/dN in the Paris regime. This suggests that the shape of the fatigue crack tip at K_max should be independent of load ratio at a given da/dN, and that the shape at K_min is also independent of load ratio, whereas the size of the deformed region at the crack tip is in proportion to da/dN.     

Some experimental findings in compression-after-impact (CAI) tests of CF/PEEK (APC-2) and conventional CF/epoxy flat plates

Compression-after-impact (CAI) tests have been conducted for quasi-isotropic thick plates with 48 plies by using the NASA method and on plates with 32 plies by using the SACMA method. Specimens are made of CF/PEEK (APC-2) and conventional CF/epoxy for the NASA plates and CF/epoxy for the SACMA plates. In the NASA CAI tests, the sequence of delamination buckling and its propagation is clearly revealed through various experimental techniques. One major technique is moiré topography, and the other is thermo-mechanical stress analysis with a high-accuracy infrared sensor. The arrest of delamination propagation just before catastrophic failure due to high fracture toughness is clearly captured by the moiré camera. This behavior provides good CAI values of CF/PEEK. The initial buckling properties of the delaminated region by the impact are then extensively discussed. Numerical predictions of initial buckling stress have been obtained by modelled geometry of the delaminated region simplified from its precise structure clarified by ultrasonic C-scanning. They agree fairly well with the experimental results. The in-plane stress distribution in the delaminated region before initial buckling is measured by an infrared stress graphic system. This compared favorably with finite element predictions. Two types of symmetric buckling modes with respect to the central plate surface, twin and single peak ones, are experimentally captured.     

Fatigue crack growth analysis of welded aluminium RHS T-joints with manipulated residual stress level

The fatigue life of a welded aluminium T‐joint made from beams with rectangular hollow section (RHS) has been predicted using a crack propagation analysis and compared with experimental results from joints with different residual stress levels. To include the effect of the residual stresses, the stress ratio was calculated at the weld toe and, via Walker's equation, introduced into the analysis. How to obtain the Walker exponent has been discussed in detail. The introduction of a stress ratio at the weld toe provides good agreement between the experimentally and analytically found S–N curves. The effect of the residual stress was successfully included in the analysis.