横向载荷对含表面裂纹对焊接头疲劳裂纹扩展速率影响的研究

研究了双向载荷作用下对焊接头表面裂纹的疲劳扩展规律。采用经精确标定应力强度因子幅值的双向十字形试样,在电液伺服双轴向疲劳试验机上对母材分别为16MnR 和 Q235-B 这两种钢的对焊试板进行了疲劳裂纹扩展试验。同时,将其结果分别与单向应力下对焊接头及母材的疲劳扩展速率进行了比较,为制定我国在役压力容器缺陷评定规范提供了参考数据。     

模拟压力容器接管的新型十字试板的角裂纹疲劳扩展性能

用新型十字试板进行了双轴载荷下的角裂纹疲劳扩展性能的研究,以模拟压力容器接管处角裂纹的疲劳扩展行为。试板角裂纹尖端的应力强度因子采用三维边界元法计算,裂纹疲劳扩展的形貌变化采用降载勾线法记录。试验结果表明,裂纹疲劳扩展规律与双向载荷比及裂纹初始形状有关,裂纹沿长度方向的扩展速率可以用 Paris 公式表示,但在深度方向则不然。     

16MnR钢表面裂纹的疲劳扩展

报告了在 P_x∶P_y=0,0.5和1三种双轴载荷作用下,对倾斜角等于0°、30°和45°的表面裂纹所作的疲劳扩展研究。讨论了表面裂纹扩展速率的计算方法,指出必须考虑不同的裂纹倾斜角和不同的双轴载荷比造成的裂纹扩展驱动力的变化对斜表面裂纹扩展速率的影响。因而采用裂纹投影法来处理斜表面裂纹的疲劳扩展问题并提出了修正的 Paris 方程。按照修正的 Paris 方程,根据不同的双轴载荷比和裂纹倾斜角分组整理了试验数据,并作了回归分析、假设检验,给出了含表面裂纹的16MnR 板材在三种不同的双轴载荷比情况下的裂纹扩展速率方程。     

研究生论文摘要

题目:16MnR钢在双轴向载荷下失效评定图(FAD)的研究作者:谈东明学校:南京化工学院教师:戴树和部门:化机系本文通过理论分析,有限元计算以及大量的双轴向应力状态下断裂试验,研究了16MnR钢十字形中心贯穿裂纹试样,在双向载荷下的断裂性能。同时,研究了双轴向载荷对失效评定图的影响。用弹塑性有限元计算了断裂参数 Jap,Je。并用局部直流电位法测出裂纹的启裂点。在此基础上,对英国中央电力局的CEGB双判据法和美国电力研究所 EPRI 的弹塑性工程方法进行     

双轴载荷作用下表面裂纹宏观疲劳断口形貌

报告了在不同的双轴载荷比和不同的裂纹倾斜角的情况下,表面裂纹疲劳试验的某些试验结果,并首次从裂纹扩展驱动力的角度,揭示了裂纹宏观疲劳断口的变化规律。     

CFRP加固损伤钢板的双轴拉伸性能研究

基于虚拟裂纹闭合法模拟受双轴向载荷的含裂纹塑性钢板断裂过程,利用减小裂纹裂尖处应力强度因子的方法确定了CFRP的加固方案,通过有限元仿真分析了双轴试样加固前后的对比效果,最后结合模型试验验证了加固方案和计算方法。研究表明,虚拟裂纹闭合法可以有效模拟受双轴向载荷裂纹的扩展过程,仿真结果与试验结果吻合较好。对于受双轴向载荷的含裂纹钢结构,CFRP加固后可以基本恢复原结构的承载能力,加固效果十分显著。该方法可以进一步用于复合型裂纹的加固方案确定。     

变幅载荷谱中加载顺序对疲劳裂纹扩展速率的影响

根据某高温高压换热器实际工况载荷波动所编制的载荷谱,用材料为2 1/4Cr-1Mo钢的CT试件,不同的载荷谱加载顺序,进行变幅载荷谱下的疲劳裂纹扩展速率试验研究。根据试验数据和超载迟滞效应理论,讨论了不同载荷谱加载顺序对疲劳裂纹扩展速率的影响。     

拉弯复合应力下焊接头表面裂纹疲劳扩展规律的研究

通过对16MnR钢对焊的弓形试极疲劳裂纹扩展试验，研究了在拉弯复合应力下对焊接头表面裂纹疲劳扩展规律。采用Newman－Raju公式分析了有限宽板在拉弯复合应力下表面裂纹的应力强度因子的计算。试验研究表明在拉弯复合应力下当a／t≤0．8时表面裂纹疲劳扩展规律仍可用Paris公式来描述，并且c向和a向的Paris系数之间仍存在着C_c＝0．9~nC_a的关系。     

变幅载荷疲劳裂纹扩展行为的试验研究

用某压力容器实际工况载荷波动所编制的载荷谱,对材料为16MnR的CCT试件进行了变幅载荷谱下的疲劳裂纹扩展试验。根据试验数据,分析了裁荷谱中大载荷级和小载荷级分别对裂纹扩展量的贡献,并对考虑和不考虑超载迟滞效应对小载荷裂纹扩展行为的影响进行了对比。作为这方面的初步探索,本研究获得了一些有意义的结论。     

2.25Cr-1Mo复杂应力状态下低周疲劳寿命预测

通过对无预裂纹圆柱形缺口试件的常温、高温低周疲劳总寿命试验以及对带有预裂纹圆柱形缺口试件的常温、高温裂纹扩展寿命试验,并利用ＮＨＲＤＳ有限元程序进行了缺口附近轴对称问题的循环应力和应变计算,研究了非均匀分布复杂应力状态下低周疲劳寿命。结果表明,２ ．２５Ｃｒ １Ｍｏ 材料复杂应力状态下低周疲劳总寿命和裂纹扩展寿命可采用当量形式的Ｍａｎｓｏｎ Ｃｏｆｆｉｎ 公式进行表征。     

Fatigue Crack Growth in Ferroelectric Ceramics Driven by Alternating Electric Fields

Electric-field-induced fatigue crack growth in ferroelectric ceramic PZT-5 with precracks was investigated. The experimental results showed that there were two distinct characteristics in the crack growth under electric loading. Under low electric loads, microcracks located ahead of the main crack emerged and grew and, as a result, impeded the growth of the main crack. On the other hand, under high electric loads, microcracks were absent, and the main crack was the only mode of fatigue cracking. The main crack grew macroscopically along the original path perpendicular to the electric field. Microscopically, the crack grew along the grain boundaries and grain breakaway was observed. The crack growth rate was nonlinearly related to the cyclic electric load. Similar to mechanical fatigue, there existed a crack growth threshold in the applied electric-field amplitude below which the crack ceased to grow. A steady crack growth occurred when the applied electric field exceeded this threshold. An empirical model for crack growth was obtained. Domain-switching effect and fracture-mechanics concepts were used to explain the observed crack closure and crack growth under electric loads.     

双轴载荷下失效评定图的研究

本文通过双轴载荷下的断裂试验和理论分析研完了16MnR钢所制成的含中心穿透裂纹的十字形板试样的断裂性能,并分别对英国中央电力局的CEGB R_6法和美国电力研究所EPRI工程估算方法的失效评定进行了双轴载荷效应的修正,作出了双轴载荷下估算含缺陷结构安全裕度的失效评定图(FAD)。试验结果表明,在不同的双轴载荷比下启裂的J积分J_i比较接近,而J控制裂纹扩展条件的J_R曲线受不同的双轴载荷比影响较显著。在几种评定方法的比较中,认为经双轴载荷修正的EPRI失效评定图与实验结果吻合得较好。     

Influence of High Pressure Drop Rates on Fatigue Crack Growth

Hydraulic components in mobile machines are subjected to varying conditions and loads, which limit their lifetime. In particular, high pressure drop rates are suspected to increase fatigue crack growth rates. Existing studies on the fluid-structure interaction inside fatigue cracks during fast pressure drops do not explain increased crack growth in hydraulic systems. A laminar flow model was developed to simulate the fluid flow inside a crack under periodic pressure loads. The viscosity restrained the oil inside the crack, and the mechanical resistance of the closing crack led to a high pressure increase. Knowledge of the pressure profiles inside fatigue cracks helps to estimate the effective fatigue damage during transient loads.     

Retardation of Fatigue Crack Growth in Ceramics by Glassy Ligaments: A Rationalization

In high-temperature fatigue crack growth (FCG) experiments on ceramic materials containing amorphous grain boundary phases, the crack growth rates under cyclic loads were observed to be lower than those predicted solely on the basis of crack growth velocities measured under static loads. In this paper, a rationalization was offered for such a behavior by means of a phenomenological glass-bridging model which takes the relaxation behavior of glass into account. In ceramics which exhibit subcritical crack growth through cavitation ahead of the crack tip, the maximum stress intensity factor of the fatigue cycle required to initiate FCG was observed to be always greater than or equal to the threshold stress intensity factor for crack growth under sustained far-field loads. This trend was also explained with the aid of the glass-bridging model and invoking the equivalence between bridging and damage zones. The elevated temperature FCG behavior of nitride-based ceramics which exhibit grain bridging in the wake during crack propagation was discussed and contrasted with oxide-based ceramics which show glass bridging.     

Fatigue crack growth in polyethylene: Material dependence. I: Tensile/compressive loading

Dynamic fatigue crack growth has been investigated in polyethylenes of various molecular weights and branch concentrations. Square load waveforms were applied to sharply notched samples and the load was adjusted so that for all tests the maximum stress intensity factor was constant and tensile, K_max, but different experiments featured different values of minimum stress intensity factor K_min, which was either zero or compressive. The crack growth rate increased as K_min became more compressive. Subsidiary experiments were made using static loads and these confirmed that, as previously reported, the slow crack growth rate decreased with increasing branch concentration and molecular weight. In all the materials studied, fatigue crack growth rates were significantly greater than those under steady loads. However, materials possessing higher branch concentrations showed a relatively greater increase in crack growth rates. For negative values of K_min, increasing branch concentration has a deleterious effect on the fatigue crack resistance. In the fatigue loading experiments studied here it was found that for K_min/K_max ≈ ?1, the ranking of materials in terms of crack growth resistance was reversed with respect to that seen under steady loads.     

Role of fatigue in damage development of refractories under thermal shock loads of different intensity

Refractories insulation of industrial furnaces often fail under repetitive thermal shock. Degradation of silica refractories under thermal shock loads of different intensity was studied. The load variation was achieved by utilisation of geometrically similar samples of different dimensions. Finite element method modelling predicted loads developing during the test. Resulting damage was determined by the ultrasound velocity and crack patterns. Tests involving up to 150 cycles demonstrated the role of fatigue in enabling sub-critical crack formation and countering the crack arrest. Repetitive cycles reduce crack wake friction and intensify loading due to crack debris re-location. Damage saturation, sigmoidal and near-exponential damage growth was typical for low, intermediate and high loads, respectively. Similar trends of damage accumulation were observed in mechanical displacement controlled cyclic fatigue tests performed in wedge splitting set-up. Strain and strain energy based criteria of thermal shock intensity seem to have complimentary value in predicting the crack formation and growth. Thermal shock damage after the first cycle seems to be an effective parameter to predict overall resistance to the degradation in the sample. Load reduction due to previous crack formation related to the fatigue potential for subsequent crack development can explain the crack size variation typically observed in refractories after multiple thermal shocks. For thermal shock tests, the variation of sample size, instead of the temperature interval, is a suitable alternative for refractories with strongly temperature dependant material properties.     

Subcritical Crack-Growth Behavior of Borosilicate Glass under Cyclic Loads: Evidence of a Mechanical Fatigue Effect

Amorphous glasses are generally considered immune to mechanical fatigue effects associated with cyclic loading. In this study surprising new evidence is presented for a mechanical fatigue effect in borosilicate glass, in both moist air and dry nitrogen environments. The fatigue effect occurs at near threshold subcritical crack-growth rates (da/dt 3× 10^-8 m/s) as the crack extension per cycle approaches the dimensions of the borosilicate glass network. While subcritical crack growth under cyclic loads at higher load levels is entirely consistent with environmentally assisted crack growth, lower growth rates actually exceed those measured under monotonic loads. This suggests a mechanical fatigue effect which accelerates subcritical crack-growth rates. Likely mechanisms for the mechanical fatigue effect are presented.     

Fatigue crack propagation behavior of polyether ether ketone under biaxial loading

Polyether ether ketone (PEEK) has become a promising material in total joint replacement. However, it still faces the risk of fatigue fracture during service. In this paper, the effects of biaxial stress ratio λ, cyclic stress ratio R, and load phase difference θ on fatigue crack propagation (FCG) behavior of PEEK are investigated. In the case of vertical cracks, results show that the FCG rate of PEEK increases with the R value, while decreases with the increase of λ value. Furthermore, the effective stress intensity factor range ΔK_eff can uniformly describe the biaxial FCG behavior at different cyclic stress ratios. In the case of 45° slant cracks, compared with mode-I intensity factor range ΔK_I, the energy release rate range ΔG is more accurate for describing the FCG behavior under various load phase differences. In addition, the investigation on the 45° crack propagation path shows that a bifurcated Y-shaped crack appears under 180° load phase difference, while no bifurcated crack appears under 90° load phase difference and uniaxial loading. Three different methods are used to predict the crack propagation path. The comparison results show that the maximum circumferential stress (MTS) criterion can well predict the crack propagation path under out-of-phase biaxial loading and uniaxial loading.