THE EFFECT OF PRESSURE OVERLOAD ON FATIGUE CRACK GROWTH IN PRESSURE VESSELS

Abstract— The pressure-overload method for proof-testing pressure-vessels was investigated for hydrofilters made of an Al-alloy. After a vessel had been overloaded using a hydrostatic fluid, the fatigue crack growth rate of the material was studied at three levels of hydrodynamic pressure. The period of fatigue crack growth N_p and the uniaxial equivalent stress σ_e were calculated on the basis of fractographic analyses for various combinations of the hydrostatic overload and the hydrodynamic pressure. Increasing the overload level increased N_p, and decreased σ_e. For a pressure overload of 3 times the normal pressure the initiation of a fatigue crack did not occur after 2 × 10⁶ cycles of loading at an hydrodynamic pressure of 20.6 MPa. Striation spacing could not be correlated against the value of the hydrodynamic pressure after an overload of 2.5 times the normal pressure. A unified relationship between the durability N_d and the ratio N_p/N_d (=N^d_p) at different hydrodynamic fluid pressures was demonstrated.     

轴向高过载下固体推进剂结构完整性数值模拟

利用动力学有限元软件对轴向高过载状态下固体推进剂小变形时结构完整性进行了数值模拟。结果表明,最大轴向应力及应变均发生在固体推进剂和发动机接触区,周向及径向应力较小,但周向应变却相当可观。     

Fatigue crack propagation in API 5L X‐70 pipeline steel longitudinal welded joints under constant and variable amplitudes

Fatigue crack propagation (FCP) under constant and variable amplitude loading in base metal (BM), weld metal (WM) and heat affected zone (HAZ) of longitudinal welded joints of an API X‐70 pipeline steel was investigated. Constant amplitude loading tests were performed at R = 0.1 and 0.5, whereas for variable amplitude testing single peak tensile overloads (OLs) alternating between 75 and 100% of maximum load were applied at 2.5 mm intervals in crack growth. Results of SE(B) specimens tested under constant and variable amplitude loading revealed that BM, WM and HAZ regions subjected to R = 0.5 and low ΔK‐values presented the highest crack growth rates. At higher ΔK values FCP rates in all the studied regions were similar and the R effect on FCP rate was no more observed. Crack growth retardation due to OLs was observed at the three studied regions, showing a decrease on the FCP delay with a decreasing on ΔK.     

Fatigue crack growth and crack closure in an AlMgSi alloy

This study reports an experimental investigation of fatigue crack propagation in AlMgSi1-T6 aluminium alloy using both constant and variable load amplitudes. Crack closure was monitored in all tests by the compliance technique using a pin microgauge. For the constant amplitude tests four different stress ratios were analysed. The crack closure parameter U was calculated and related with Δ K and the stress ratio, R . The threshold of the stress intensity factor range, Δ K _th , was also obtained. Fatigue crack propagation tests with single tensile peak overloads have been performed at constant load amplitude conditions. The observed transient post overload behaviour is discussed in terms of the overload ratio, Δ K baseline level and R . The crack closure parameter U trends are compared with the crack growth transients. Experimental support is given for the hypothesis that crack closure is the main factor determining the transient crack growth behaviour following overloads on AlMgSi1-T6 alloy for plane stress conditions.     

The effect of overloads on threshold and crack closure

The effect of tensile and compressive overloads on the threshold stress intensity level and crack closure behaviour of one aluminium alloy and three steels has been investigated. A few tensile overloads significantly decreased the crack propagation rate and increased the threshold stress intensity. An initially decreased and then increased opening stress was mostly responsible for the delayed retardation and crack arrest. Intermittant compressive overloads significantly accelerated the crack propagation and decreased the threshold stress intensity which was a function of the frequency of overloading. The opening stress was decreased to below zero after a large compressive peak load, and it took >10⁵ cycles for the opening stress to return to its stable level. During this period an initially high crack propagation rate also gradually decreased to the stable value.     

基于AHP的高速公路超载因素影响分析

从法制、税费、货运市场以及供应链的角度对高速公路超载的影响因素进行深入分析,将超载的成因层次化,采用“定性一定量一定性”的研究思路,利用AHP法对各层次的影响因素进行权重评价。通过计算分析,得到法制,税费和物流因素所占比重极大,根据权重计算结果以及我国的实际情况,从行政、经济以及技术方面提出一些具有建设性的意见。     

Overloads in ductile and brittle materials

The first part of the paper presents fatigue crack propagation experiments with single overloads at different overload ratios and specimen thickness in a very ductile austenitic steel. The results show that in the Paris regime in a ductile material, the overload effect can be explained solely in the framework of the change of the plasticity‐induced crack closure. Other effects such as strain hardening, blunting, additional damage, crack deflection and branching are not significant. Whether or not this behaviour can be observed in less ductile materials and also in the threshold regime is investigated in the second part. Periodic overload experiments were performed on a relatively ductile 2124, and a more brittle 359, particle‐reinforced aluminium alloy. In the Paris regime, the retardation in the 2124 reinforced alloy showed the expected behaviour for a ductile material, whereas in the 359 reinforced cast alloy, an acceleration of the mean growth rate was observed. Near the threshold the difference between the two alloys and the effect of the periodic overloads decreased.     

The effects of overloads in fatigue crack growth

Several theories have been proposed to explain the transient fatigue crack growth decelerations and accelerations which follow overloads. The mechanisms that have been proposed to explain retardation after a tensile overload, for example, include residual stress, crack deflection, crack closure, strain hardening, and plastic blunting/resharpening. These mechanisms are reviewed in the light of recent experimental results, and implications with regard to their applicability are examined. It is suggested that no single mechanism can be expected to represent observed effects over the entire range of da/dN versus ΔK; eg, behaviour ranging from the near threshold region to the Paris region.     

超载对桥梁构件受弯性能影响的试验研究

本文结合12根钢筋混凝土梁的超载试验,主要针对不同的预加荷载幅值和不同超载次数对不同配筋率的钢筋混凝土桥梁受弯承载力性能的影响进行研究,分析超载对钢筋混凝土梁桥受弯性能的影响规律。试验结果表明超载对桥梁构件承载力降低作用不显著,但是超载作用后的桥梁构件裂缝和挠度明显增大,从而导致正常使用性能降低,后期失效风险增大。