冲击和非冲击疲劳载荷下裂纹扩展的研究

对几种典型材料分别进行了四点弯曲冲击和非冲击疲劳裂纹扩展试验。结果表明,相同ΔK条件下,冲击和非冲击疲劳裂纹扩展速率之间存在三种类型的关系,这三种类型的关系与材料在两种载荷制度作用下的微观断裂机制紧密相联,统一了过去在冲击和非冲击疲劳裂纹扩展对比试验中的矛盾结果。     

激光冲击对金属板料裂纹的影响模型

研究了含有裂纹的金属板料在激光冲击波载荷作用下裂纹尖端应力强度因子和裂纹扩展速度的变化,利用断裂力学理论,对激光冲击加载下裂尖参数计算模型进行优化,采用应力强度因子叠加法,将外加载荷引起的应力强度因子和激光冲击后残留的残余压应力引起的应力强度因子叠加,推导出下裂纹尖端应力场强度因子表达式,由此可精确计算出金属板料的裂纹萌生寿命和裂纹扩展速度,实验验证了航空钛合金Ti6Al4V激光冲击后残余应力对裂纹扩展速度的影响,从而建立了激光冲击作用对板料裂纹扩展的影响的理论模型。     

超高压水射流破碎子午线轮胎机理

研究子午线轮胎回收处理的新方法。将超高压水射流技术应用于子午线轮胎橡胶的破碎回收,进行非淹没条件下超高压水射流破碎分离子午线轮胎橡胶试验,得到不同驱动压力破碎后的橡胶粉末和橡胶破碎断面,对轮胎橡胶、橡胶粉末进行粒度分析和低场核磁共振分析以评价破碎回收效果。采用流固耦合方法建立超高压水射流冲击轮胎橡胶的有限元分析模型,计算超高压水射流冲击作用下橡胶材料内部应力分布规律,应用材料失效准则,研究轮胎橡胶受水射流施加高速动载冲击下的破碎行为并与试验结果相验证。对轮胎橡胶断面和橡胶颗粒进行扫面电镜观察,分析轮胎橡胶和橡胶粉末的宏观形貌和微观形态,结合有限元模型计算结果,研究轮胎橡胶的微观破碎机理,认为在橡胶破碎过程中存在拉伸和剪切两种破坏机制,主要微观失效形式有沿晶破坏和穿晶断裂。     

刀具的疲劳断裂与高速钢的冲击疲劳强度

<正> 由于刀具的使用期限不很长,通常认为很少发生疲劳断裂。但是长期断续切削时刀具的疲劳断裂并不能排除,在重复冲击载荷作用下可引起疲劳。变形高速钢圆盘铣刀(φ160×7.3)失效的主要原因是切削齿剥落。其断口特征是多次冲击载荷作用下因疲劳过程才出现的断口,从中可看出疲劳裂纹扩展前沿的痕迹——平行带式的沟痕。断口的电子显微分析表明,铣刀使用过程中存在疲劳断裂,其表现是裂纹解理前沿呈明显的方向性,断裂处出现一系列平行平面并分布有碳化物斑。一次冲击断裂的断口没有发现这种情况。     

冲击和非冲击疲劳载荷下过载对随后裂纹扩展的影响

对12CrNi3A钢在冲击和非冲击疲劳载荷下的过载裂纹扩展延迟效应的研究表明:过载对随后裂纹扩展的影响不仅取决于过载力作用下的裂尖塑变,还取决于随后基载力作用下的裂尖塑变。过载对随后裂纹扩展的影响存在两个互为矛盾的方面:一是裂尖塑变损伤促进裂纹扩展:另一则是产生的裂尖残余压应力场和闭合效应延缓裂纹扩展,两方面共同作用结果才能最终决定对随后裂纹扩展的影响。相同条件下,冲击疲劳载荷下的延迟期M_d高于非冲击疲劳载荷下的延迟期。     

多处损伤的疲劳裂纹扩展分析方法研究

多处损伤(multiple site damage，MSD)是影响老龄飞机结构完整性的重要因素之一。文中以线弹性断裂力学为基础，将应力强度因子组合模型、裂纹连通准则、断裂准则以及MSD扩展中相关性处理综合在一起，建立一种等幅载荷谱下MSD裂纹扩展分析方法，并用所编程序对三种典型MSD模式的扩展与断裂过程进行计算分析。结果表明，寿命预测结果与试验结果吻合很好。     

Ⅰ—Ⅱ复合型裂纹断裂机理与断裂准则的研究

本文采用四点弯曲试件考察了常用韧性材料16MnR钢中Ⅰ-Ⅱ复合型裂纹的断裂机理。实验测定了裂纹扩展方向和裂纹尖端位移参量的启裂值；与此同时，采用有限元程序计算了试件中复合型裂纹尖端邻域的应力、应变场，分析了各种控制空洞成核的力学参量的几何分布曲线，结合实验研究和有限元分析表明，Ⅰ-Ⅱ复合型断裂存在两种不同的断裂机理，它们的发生取决于Ⅰ-Ⅱ复合型载荷比的大小，当Ⅱ型载荷分量小于一临界值时、断裂为张开型．裂纹扩展方向与本文的бm峰值线准则一致；而当Ⅱ型载荷分量大于此临界值时，断裂表现为不稳定的剪切型断裂。     

汽轮机中压调速汽门门杆断裂分析

某电厂汽轮机中压调速汽门门杆发生断裂,采用断口分析和性能分析等方法对门杆的断裂原因进行了分析。结果表明：门杆断裂位置渗氮时未做防护,渗氮后会使此处脆性增加,进一步加剧了应力集中效应,门杆中心的压力平衡孔严重偏离门杆中心,使门杆受力时形成偏载,门杆是在交变冲击载荷、偏载和应力集中效应的共同作用下形成裂纹源,裂纹扩展至临界裂纹尺寸时发生疲劳断裂。     

在役桥(门)式起重机箱形梁结构的疲劳安全评定

针对桥(门)式起重机箱形梁结构的疲劳评定问题,以断裂力学理论为基础,结合起重机结构的实测裂纹和随机载荷的统计情况,探索出一套适用于起重机箱形梁结构疲劳评定的流程和方法。以36t集装箱门式起重机为例,通过对箱形主梁裂纹的简化分析和随机载荷的等效处理,采用一阶段和两阶段的裂纹扩展规律,计算出起重机箱形梁在随机载荷作用下的临界裂纹尺寸和裂纹扩展寿命,并对不同位置、不同扩展准则条件下疲劳裂纹扩展结果进行了对比分析。评定流程和方法简单且易操作,计算结果具有较高可信度,对桥(门)式起重机疲劳评定检测和减少疲劳断裂事故发生具有重要的工程实用价值。     

循环压缩应力作用下的疲劳裂纹扩展机制研究

裂纹在循环压缩载荷作用下的扩展已经被实验证实，但是由于难以进行观测因而无法准确描述，另外裂纹面在压缩载荷的作用下会出现闭合现象也增加了问题的复杂性。文中通过有限元方法，建立三种几何条件的裂纹模型，考虑裂纹面接触问题，进而对循环压缩载荷作用下的裂纹萌生扩展进行分析。结果表明，裂尖区域在循环压缩载荷作用下的残余拉伸应力是导致裂纹扩展的重要因素。同时，还对不同裂尖几何在裂纹描述的合理性方面提出一些看法。研究表明，裂纹在循环压缩载荷作用下的扩展能力是有限的、稳定的。     

一种先进可靠的除大块装置

介绍浙江北仑发电厂从克虏伯公司引进的煤系统除大块装置，它由滚轴筛及液压驱动破碎轮2大部分组成，其液压系统由多台同轴串连的变量轴向柱塞泵驱动液压马达构成。     

循环流化床筛碎系统的探讨

发电厂中,越来越多地使用循环流化床锅炉。因其没有破碎系统,燃煤破碎须在进炉膛之前完成,这对输煤系统提出了新的要求。现以实际工程为例,结合以往国内外电厂的经验,对循环流化床火力发电厂中的筛碎系统的设计及设备选型进行了探讨。     

Repair of notched beam under dynamic load using piezoelectric patch

In this paper, the repair of a cracked beam under an external dynamic load employing the electro-mechanical characteristic of piezoelectric material to induce a local moment is presented. Conceptually, an external voltage is applied to actuate a piezoelectric patch bonded on the beam to effect closure of a crack so that the singularity at the crack tip under dynamic load may be decreased. Globally, this has the effect of altering the resonant frequency of the cracked beam towards that of the healthy beam, which is the criterion used for the repair. To demonstrate the repair methodology, a cantilever beam is used as an illustration, where the repair moment coefficient and the voltage required are mathematically derived. The relationship between repair moment coefficient, crack parameters and length of piezoelectric patch is investigated. The difference between the proposed repair criterion and an earlier published criterion for cracked beam under static load is also shown. A numerical example is used to study the effectiveness of the proposed repair methodology and its results are compared with those from 3-D finite element analyses using ABAQUS 6.4 as one means of verification.     

Load-Bearing Capacity in Quasi-Static Compression and Bearing Toughness of Silicon Nitride Balls

Results of ball-on-ball crushing tests on bearing-grade silicon nitride balls and a new analysis of the dependence of the peak load on the ball radius are presented. The variation of the peak load with the ball radius is consistent with a fracture-mechanics analysis, which suggests that the peak load corresponds to a transition from stable to unstable growth of cone cracks in the balls, and the critical cone crack size scales with the ball size. It is recommended that a ball-size independent bearing toughness be calculated and reported based on the crushing test results rather than a crushing strength, which decreases with increasing ball size.     

基于ANSYS/LS-DYNA轻薄型回收钢破碎过程仿真研究

利用非线性有限元法,以ANSYS/LS-DYNA为工具,在不同的锤击位置下对回收钢的破碎过程进行了计算机仿真,通过分析得出以下结论:(1)随着锤击点与铁砧间的距离不断增大,回收钢板被击穿的层数减少、被撕裂面积变小,破碎效果也变得越来越差,即锤击位置是影响破碎效果的重要因素.因此,在设计破碎机时,应充分考虑锤击点与铁砧间的距离;(2)在锤击点到铁砧的距离与破碎锤体积的关系不合理的情况下,锤头与夹持面之间的摩擦使得破碎冲击动能部分耗损,从而破碎效果变差,因此应考虑这种关系的合理性.     

Static and impact crushing of layered foam-plate systems

Kinetic energy absorbing systems comprising alternating layers of polyurethane foam and steel plate inserts are subjected to static and impact crushing by a falling mass. Four system geometries are investigated: uniform-width, tapered-width, hourglass and double tapered-width profiles. The pattern and degree of deformation and the crushing force are examined with respect to impact velocity, system geometry and system inertia. It is found that quasi-static force-deformation behaviour is governed by system geometry, while inertial resistance and material rate effects modify the response significantly for impact loading. Although the proposed analysis based on the load-deflection characteristics of constituent layers adequately describes the global quasi-static force-deformation response of the systems, it does not account for the markedly contrasting dynamic behaviour. This highlights the need for caution in evaluating impact dissipation mechanisms from purely static data and considerations.     

Axial crushing analysis of tubes deforming in the multi-lobe mode

Similar to the concertina mode, the wall of axially crushed tubes deforming in multi-lobe or diamond mode, will be laid down partly to the inside and partly to the outside of the tube generator with the latter part being smaller than the former. Such proportion of the folding length is known as the eccentricity factor, in both the concertina and multi-lobe modes. The present work examines the collapse of tubes in the multi-lobe in an attempt to evaluate the crushing load using this unique factor. The analysis produced a distinctive value for the eccentricity factor that simplifies the expression for the mean collapse load, which is a function of tube geometry and number of lobes. The analytical outcome for the mean crushing load, the total deflection is in a reasonable agreement with those obtained from experiment. Furthermore, the measured values of the eccentricity factor and the critical folding angles obtained for tubes of different materials and geometric ratios are also in good agreement with those produced by the analysis which postulates that the distinctive value of m is independent of the tube’s material and geometric ratios.     

Effects of defects on the in-plane dynamic crushing of metal honeycombs

The effects of defects and their distributions on the in-plane dynamic crushing of honeycomb panels were studied using explicit finite element modeling. The influence of defect locations and ratios is investigated on the deformation modes and the plateau stresses with respect to the impact velocity. Numerical results show that the dynamic performance of honeycomb displays a high sensitivity on the defect location, especially under low and moderate impact velocities. By introducing a defect correction factor β_m and using the one-dimensional shock wave theory, an empirical formula is given for the variation of honeycomb’s plateau stress with respect to the impact velocity and the defect ratio.     

基于机械物理法的热固性塑料粉碎及再生机理研究

提出了基于机械物理法的热固性塑料粉碎再生工艺,对在多种机械力复合作用下的粉碎过程进行了粉碎力学、断裂机理和粉碎效果的综合分析;通过对热固性酚醛塑料的试验研究,证明了在合适的转速、时间和温度条件下,由于机械物理作用、热固性塑料的物理特性及网状交联分子结构发生改变而在一定程度上恢复固化成形的能力。