直裂纹电磁热止裂与激光再制造的实验研究

利用ZL-2型超强脉冲放电装置对带有直裂纹的构件进行电磁热止裂,然后利用激光熔覆技术对止裂后的构件进行修复。对止裂前后裂尖处微观组织进行金相组织对比分析,对经过激光熔覆前后试件进行耐磨性能对比,发现熔覆后的试件耐磨性能得到显著提高,最后对修复前后的试件进行力学性能对比,结果表明:放电止裂与激光再制造的试件力学性能得到显著提高,实现了再制造的目的。     

表面裂纹疲劳止裂技术

将”人工楔块“止裂技术有于表面裂纹疲劳止裂。在对表面裂纹试件进行了低频拉伸和高频弯曲两种载荷下的楔,无楔试验后,发现”人工楔块“止裂技术对于表面裂纹同样有显著的止裂效果。     

动态爆生裂纹相互影响的试验研究

采用以有机玻璃(PMMA)为材料的模型试验方法,分析了试件的破坏形态和裂纹尖端的应力特征,研究了对向切槽炮孔的不同竖向间距对爆生裂纹扩展的影响。结果表明:爆生裂纹扩展过程中,裂纹速度震荡减小,裂纹尖端应力变化经历两个阶段,Ⅰ阶段(0~110μs)裂纹尖端动态应力强度因子迅速减小,试件破坏以拉伸破坏为主;Ⅱ阶段(110μs~止裂)裂纹尖端动态应力强度因子先增大后减小,试件破坏兼有拉伸破坏和剪切破坏。炮孔的竖向间距的不同对爆生裂纹Ⅰ阶段扩展影响不大,对Ⅱ阶段扩展影响显著。对向裂纹起到自由面作用并引导己方裂纹向其发生偏转,从对向裂纹面处反射的应力波加强了己方裂纹尖端的应力集中。     

金属构件中裂纹的电磁热效应局部跨越止裂

采用理论分析和实验研究的方法讨论了应用电磁热效应对金属构件中裂纹实施局部跨越止裂的技术。在带有裂纹的模具钢构件上取样,对裂纹处通过点电极进行局部跨越脉冲放电。实验研究和理论分析结果均表明:跨越止裂可以使导体中局部裂纹在裂纹尖端处很小的范围内熔化形成微小的焊口,实现了钝化止裂,遏制了裂纹的扩展。通过对止裂后裂尖的金相组织观察和力学性能测试发现:超细化的条状马氏体、极少量残余奥氏体和细颗粒碳化物的出现极大地提高了裂纹尖端的硬度、韧性和耐磨性。     

含圆形埋藏裂纹金属构件电磁热止裂时应变能密度分析

对带有圆形埋藏裂纹金属构件在脉冲放电瞬间的应变能密度进行了理论分析。在求解过程中,以张开型裂纹为例,应用了热传导、非定常热应力及汉克尔变换等理论知识,得出了含埋藏圆片裂纹在脉冲放电瞬间的温度场理论公式、热应力场公式和应变能密度公式。由热应力场公式和应变能密度公式可知,放电瞬间电磁热在裂纹尖端形成热压应力场,热压应力对金属构件做负功,减小了拉应力对构件的破坏程度,放电后构件的应变能密度降低了。以Cr12MoV模具钢中埋藏圆裂纹止裂为例,具体计算了脉冲放电前后不同拉应力作用下的应变能密度变化情况,为空间裂纹电磁热止裂技术的实际应用提供了理论基础。     

轴对称金属模具电磁热裂纹止裂温度场的分析

选择带有半埋藏环形裂纹的金属凹模为研究对象,通过金属凹模内外环面均匀通入强脉冲电流,应用电磁热效应实现了金属凹模中环形裂纹的止裂。采用复变函数的方法求解了脉冲电流放电瞬间裂纹尖端附近的温度场。由于脉冲电流放电瞬间,裂尖处电流绕流的热集中效应,在金属凹模内部,环形裂纹尖端金属的温升超过熔点,金属熔化,在金属凹模内部沿着环形裂纹尖端形成堆焊,致使环形裂纹尖端的曲率半径瞬间增大,阻止了干线裂纹源的开裂趋势。     

9Cr2凹模淬火裂纹的搭桥修复

9Cr2凹模淬火裂纹经脉冲放电止裂和电火花熔焊的搭桥修复后，裂纹尖端纯化，模具表面的淬火裂纹被焊合，模具的承载能力和表面光洁度可达到使用要求。     

拉伸情况下含裂纹16MnR钢的磁记忆检测实验

利用磁记忆方法检测压力容器用钢裂纹扩展和塑性变形的可行性.利用压力容器常用材料16MnR制作拉伸试件,在其上预制垂直于试件长度方向的不同几何尺寸的裂纹,在材料试验机上进行拉伸实验.利用磁记忆传感器检测试件表面裂纹尖端磁场强度法向分量信号,研究不同拉伸阶段裂纹尖端磁信号的变化规律,利用铁磁学基本理论研究和解释复杂实验现象.在对拉伸载荷变化和磁信号变化规律进行分析的基础上,提出了载荷梯度参数的概念,实验表明:利用该参数可以较好地表征压力容器用钢的弹塑性应变.     

Fe—3%Si 单晶体裂纹尖端位错行为及氢效应的 TEM 动态观察~*

在透射电镜中对 Fe-3%Si 单晶体薄膜进行拉伸试验,观察了在受力状态下裂纹尖端的位错行为及固溶氢的影响。结果表明,在受力时裂尖会自动发射位错。位错一旦从裂尖发射,便很快离开裂纹尖端以反塞积群形式存在于塑性区中。在裂纹尖端与塑性区之间存在一无位错区(DFZ)。裂纹扩展包括裂尖发射位错的塑性过程以及 DFZ 解理的脆性过程。固溶的氢改变了裂纹尖端的位错分布,促进了位错从裂尖发射,致使 DFZ 的长度减小甚至消失。     

含不同张开角度裂纹结构的动态断裂试验研究

不同结构的动态断裂行为受多个因素影响,其中裂纹缺陷对其影响最为显著。张开型裂纹是工程结构中十分常见的缺陷,而且张开角度往往并不固定。为研究结构的运动裂纹与不同预制角度裂纹相互作用的规律,以张开角度为单一变量,采用动态焦散线试验系统,在冲击荷载下对含不同张开角度裂纹的有机玻璃试件进行三点弯试验。研究发现：裂纹扩展经过不同张开角度的预制裂纹时,都会先扩展至预制裂纹夹角尖端,再从预制裂纹其中一端重新蓄能起裂,然后偏向落锤点方向扩展,最终贯穿整个试件;开口向上的预制裂纹会增加运动裂纹的扩展时长,而且预制裂纹的开口角度越大,对裂纹扩展产生的迟滞效应越大,裂纹贯穿试件的总时长越大;与预制裂纹相互作用阶段,动态裂纹都会先减速后增速至峰值,预制裂纹角度越大,裂纹扩展速度峰值越小,且当预制裂纹角度为180°时的峰值与无预制裂缝试件的峰值非常接近;运动裂纹再次从预制裂纹尖端处起裂时裂纹尖端应力强度因子迅速大幅增加后又迅速减小直至试件完全断裂。     

混凝土裂缝扩展过程中裂尖张开口位移(CTOD)与裂缝嘴张开口位移(CMOD)的变化关系分析

裂缝尖端张开口位移(CTOD)和裂缝嘴张开口位移(CMOD)是衡量裂缝张开程度的两个指标.该文进行了楔入式紧凑拉伸混凝土断裂试验,研究了在裂缝扩展的整个过程中裂缝尖端张开口位移和裂缝嘴张开口位移之间的关系.发现,裂缝尖端张开口位移和裂缝嘴张开口位移关系曲线可以用三线性模型来描述,两个转折点与裂缝的起裂和临界失稳扩展相对...     

THE INFLUENCE OF SPECIMEN GEOMETRY ON NEAR THRESHOLD FATIGUE CRACK GROWTH

Abstract— The near threshold fatigue behavior of a nickel base superalloy and a wrought 2024 aluminum alloy is examined as a function of specimen geometry. Experimental results revealed that for a given value of Δ K in Region I, crack growth rates were observed to increase as the specimen loading became more symmetric with respect to the load line. Compact tension type specimens exhibited lower crack growth rates than the more symmetrically loaded center cracked tension specimens. Consideration of the observed discrepancies is given in terms of the near field crack tip stress distribution and deformation behavior of the material affecting crack advance.     

The Effect of Micro Damage on Time-Dependent Crack Growth in a Composite Solid Propellant

The damage characteristics near the crack tip and crack growth behaviorin a centrally cracked sheet specimen of a solid propellant wereinvestigated. The specimen was subjected to a constant crosshead speed of0.508 mm/min at room temperature. The effects of local damage andfracture process near the crack tip on crack growth behavior wereinvestigated and results are discussed.     

CRACK GROWTH ARRESTING PROPERTY OF A HOLE AND BRINELL-TYPE DIMPLE

Abstract— Fatigue tests of sheet specimens having a central crack were carried out to study the effects of holes and dimples on the arrest of fatigue crack propagation. Two holes were drilled at some distance from, and at either side of, a crack tip, and the dimple of a certain diameter was introduced by pressing steel balls in the specimen at a crack tip. Results showed that the two holes produced an increase in crack propagation life (about 3 times) when the holes were drilled at an appropriate distance. On the other hand, the effect of a dimple on the fatigue strength was remarkably large, i.e. in the greatest case a 2.2 times increase in the fatigue endurance limit of cracked specimens and about a 50 times increase in the crack propagation life, at stresses above the fatigue limit. The main reason for the remarkable recovery of fatigue strength was the residual compressive stresses produced by the dimple. To evaluate the effect of residual compressive stresses on the da/dN vs. δK relation, a simple model is proposed. By using this model, the effect of residual stresses on crack propagation can be estimated quantitatively. Furthermore, the fatigue life of dimpled specimens was estimated based on the model.     

裂纹深度及尖端曲率对尖角裂纹周围应力场的影响

对于尖角裂纹,裂纹深度和裂纹尖端曲率对于裂纹附近应力场的影响有可能要大于裂纹张开角度的影响。根据弹性断裂理论和复变函数理论建立了一个计算模型,利用保角映射方法解决了尖角裂纹的边界条件问题,得到了具有不同深度和尖端曲率的裂纹周围应力分布,并对此应力分布规律进行了分析。     

DYNAMIC CRACK PROPAGATION AND CRACK ARREST INVESTIGATED WITH A NEW SPECIMEN GEOMETRY: PART I: EXPERIMENTAL AND NUMERICAL CALCULATIONS

A new test specimen geometry was advised to investigate unstable crack propagation and crack arrest. This geometry is a cracked ring which is subjected to a compressive load applied to its poles while the crack is located on the equatorial plane at the outer surface of the specimen. The main interest of this geometry lies in the variation of the stress intensity factor, K, with crack length which follows a bell-shaped curve numerically determined. The increasing part of the curve enables us to study unstable propagation and the decreasing one ensures crack arrest. This experiment has two major advantages in comparison with other specimen geometries; the boundary conditions are well controlled during the propagation, and the loading conditions of the crack are therefore precisely known. The round shape of the ring reduces wave reflection effects from free boundary surfaces. It is therefore shown that a static analysis can then be used to investigate crack arrest behaviour.