A7085铝合金Ⅰ-Ⅱ复合型疲劳裂纹扩展及其数值模拟

为探究不同加载角度下A7085铝合金Ⅰ-Ⅱ复合型疲劳裂纹扩展机理,在MTS疲劳试验机上采用紧凑拉伸剪切试件（CTS）对A7085铝合金进行不同加载角度的疲劳实验;用有限元分析计算不同裂纹扩展长度的裂纹尖端应力强度因子,通过七点递增多项式法对数据进行处理,计算出A7085铝合金Paris公式中的参数C和m.结果表明不同加载角度的裂纹基本沿着与外载荷垂直的方向扩展,裂纹扩展路径近似为一条直线,裂纹扩展角测量结果基本符合最大环向拉应力理论;Ⅰ-Ⅱ复合型裂纹一旦发生扩展,Ⅱ型应力强度因子K_Ⅱ所占比例急剧减小,Ⅰ型应力强度因子K_Ⅰ不断增大,此后K_Ⅱ远远小于K_Ⅰ,有效应力强度因子（K_Ⅰ和K_Ⅱ的组合）基本等于K_Ⅰ,相当于裂纹扩展主要受Ⅰ型应力强度因子控制,研究结果有助于对Ⅰ-Ⅱ复合型疲劳裂纹扩展机理的理解.      

铸造奥氏体不锈钢的疲劳裂纹扩展

用紧凑拉伸试样研究了载荷比、单峰过载和两步高-低幅加载对Z3CN20-09M铸造奥氏体不锈钢疲劳裂纹扩展速率的影响.当应力强度因子范围相同时,疲劳裂纹扩展速率随载荷比的增大而增大.单峰过载使裂纹扩展速率先有短暂的增加后长距离的减速扩展,出现裂纹扩展迟滞现象.两步高-低幅加载时,若两步的最大载荷不同,第二步裂纹扩展也会出现迟滞现象.用两参数模型和Wheeler模型能够预测恒幅载荷和变幅载荷下的疲劳裂纹扩展行为.      

工业纯钛TA2压载荷下含表面裂纹板应力强度因子

以压载荷下工业纯钛TA2含表面裂纹板为研究对象。采用有限元方法,建立了压载荷下含复合型半椭圆表面裂纹板模型,计算了裂纹体应力强度因子,研究了裂纹倾角β、裂纹相对深度a/t、裂纹相对形状a/c、裂纹面摩擦系数μ和侧压系数λ对应力强度因子的影响。结果表明：裂纹倾角,裂纹相对形状对应力强度因子影响显著,裂纹相对深度对应力强度因子影响不明显。摩擦系数和测压系数的增大可明显减小应力强度因子,抑制剪切破坏。最后基于应力强度因子有限元解,回归得到了适用于受单轴压载下含半椭圆表面裂纹板的应力强度因子K_Ⅱ和K_Ⅲ解。研究结果对工业纯钛TA2压载荷下含半椭圆表面裂纹结构安全性评价具有参考价值。     

γ-TiAl金属间化合物的压缩断裂行为研究

通过室温压缩试验,研究全片层γ-TiAl基合金在不同加载速度和不同卸载载荷下的压缩断裂行为。结果表明:随着加载速度的增加,γ-TiAl基合金试样的屈服强度及抗压强度相应增大;试样的最终断裂是通过裂纹的形核、扩展以及相互贯通而形成的,断裂面主要由剪应力形成的撕裂区和压应力形成的解理断裂区域组成,并且在不同加载速度下,断口也呈现出规律性的变化。在不同载荷加载-卸载-再加载的过程中,小载荷(4.67、9.42、18.94 k N)下卸载和加载的名义应力-名义应变曲线完全重合,大载荷(26.60、37.24、53.20 k N)下卸载后产生的不可逆应变依次增大;裂纹面密度随着卸载载荷的增大而逐渐增大,材料的损伤程度不断增加。     

超细晶工业纯钛在3.5%NaCl溶液中的高周疲劳

采用Instron-8801型电液伺服疲劳实验机对复合细化工艺制备的超细晶工业纯钛进行高周腐蚀疲劳实验,实验条件为水浴恒温(17.9±0.5)℃,3.5%NaCl(pH=8.1±0.2),加载频率(f)=25 Hz、应力比(R)=-1。利用得到的实验数据拟合实验加载应力幅(σ_a)与疲劳断裂循环周次(N_f)间的关系,绘制应力-寿命曲线(S-N曲线)。研究材料的腐蚀疲劳性能,并对腐蚀疲劳宏观断口形貌进行观察分析。结果表明：在3.5%NaCl溶液中,超细晶工业纯钛的腐蚀疲劳极限值(σ_-1)为403.1 MPa(44.35%)。与常规疲劳相似,随着加载应力的增加,材料的疲劳断裂循环周次急剧减小,并与常规超细晶工业纯钛和粗晶工业纯钛相比较,发现细晶强化极大地提高了材料的疲劳极限。腐蚀疲劳裂纹源萌生于材料表面,可以直观地观察到材料腐蚀疲劳裂纹的萌生、扩展、断裂典型区域的特征。断口表面较为平整光滑,观察发现有大量的疲劳辉纹和少量的二次裂纹。材料具有较强的疲劳裂纹扩展抵抗能力,耐腐蚀疲劳性能优良。     

Ni—Cr／hBN自润滑复合材料的摩擦学性能研究

采用粉末冶金技术制备Ni—Cr／hBN自润滑复合材料,研究hBN含量及摩擦栽荷对Ni—Cr／hBN复合材料的性能及摩擦磨损行为的影响．结果表明：随hBN含量增大,Ni—Cr／hBN复合材料的密度、抗弯强度和摩擦系数均逐渐减小．当载荷为20N时,Ni—Cr／hBN复合材料的磨损速率随hBN含量的增大而减小：当栽荷为60N和100N时,磨损速率随hBN含量增大呈先减小后增大的趋势．当hBN含量不变时。磨损速率随载荷增大而逐渐增大．通过探讨Ni—C汕BN自润滑复合材料的润滑与磨损机理可知。材料的摩擦系数取决于hBN的含量,而磨损速率与材料的力学性能有关．当hBN含量为9％（质量分数）,摩擦载荷为60N时,Ni—Cr／hBN自润滑复合材料具有最佳的摩擦学综合性能．     

SLM-316L细丝脂润滑摩擦磨损性能

为研究金属橡胶用选择性激光熔融(SLM)技术制备的316L不锈钢细丝在脂润滑条件下的摩擦磨损性能,探讨了不同载荷、不同摩擦速度以及载荷(F)和摩擦速度(v)共同作用的Fv因子对SLM-316L细丝摩擦系数和磨损率的影响规律,利用扫描电镜观察细丝磨损表面形貌,利用能谱仪（EDS）检测磨损表面元素种类与原子分数,分析其磨损机制。结果表明:在脂润滑条件下,摩擦系数随着载荷的增大而减小,磨损率随载荷的增大呈先降后升的趋势。摩擦系数和磨损率均随摩擦速度的增大呈先升后降趋势。低载荷下SLM-316L细丝磨损机制主要为磨粒磨损和轻微的氧化磨损,较高载荷下氧化磨损加剧并伴随疲劳磨损。低摩擦速度下SLM-316L细丝磨损机制主要为疲劳磨损和氧化磨损,较高摩擦速度下氧化磨损减弱,以磨粒磨损为主。摩擦系数随Fv值的增大而减小,磨损率随Fv值的增大呈先升后降再升的变化趋势。因此用SLM-316L细丝制备的金属橡胶在脂润滑条件下最佳工作参数:Fv等于0.04 N?m?s?1,即载荷10 N、摩擦速度240 mm?min?1。      

不同加载条件对聚甲醛复合材料的摩擦学性能影响

用摩擦磨损试验对转矩流变仪-模压成型方法制备的Ekonol/POM和Ekonol/G/MoS2/POM复合材料在不同加载条件下的摩擦学性能进行了研究.结果表明:在标准加载条件下,POM及其复合材料的摩擦系数和磨损量都是随着Ekonol含量的增加而呈现先下降后上升的趋势;在特殊加载条件下,随着载荷的增加,POM及其复合材料的摩擦系数呈先增大后减小趋势,而材料的磨损量则随着载荷的增加而增大,经测定得出的ZOGM20的极限PV值比POM提高14.5%.     

MoSi2/SiC配对副在不同载荷下的高温磨损性能研究

在XP-5型高温摩擦磨损试验机上考察了MoSi2/SiC配对副在1000℃不同载荷下的摩擦磨损性能,并用SEM扫描电镜对MoSi2和SiC的磨损表面进行了观察,用X射线分析了MoSi2的磨损表面相组成。结果表明：MoSi2/SiC摩擦副在1000℃高温滑动时,随载荷增大摩擦系数逐渐减小;40 N时MoSi2的磨损率达到极大值;随载荷的增大,MoSi2的主要磨损机制依次表现为粘着、研磨、疲劳断裂和氧化磨损等形式。20-50 N时,SiC出现磨损增重现象,归因于氧化增重作用高于磨损失重作用。     

中碳车轮钢复合型裂纹疲劳扩展行为

为了准确评估含缺陷车轮轮辋的安全性,通过对I型裂纹、II型裂纹及不同加载条件下的I+II复合型裂纹的疲劳试验,对中碳车轮钢疲劳裂纹扩展方向及门槛值进行了研究,得到了车轮钢复合型裂纹疲劳扩展的门槛值,验证了适合中碳车轮钢材料的复合型裂纹疲劳扩展预测准则。结果表明,在I+II复合型裂纹疲劳扩展试验中,裂纹扩展方向与最大切向应力(MTS)裂纹扩展准则预测值基本吻合。不同加载状态下I+II复合型裂纹疲劳扩展等效门槛值ΔKth,equ(力值比R=0.5)为3.0～3.8 MPa·m1/2。II型裂纹疲劳扩展时,微裂纹主要在主裂纹尖端剪应力作用下形核,受到拉-剪应力的一侧裂纹持续扩展,而受到压-剪应力的一侧裂纹可以形成,但扩展几十微米后停止扩展。     

Crack deflection during fatigue and monotonic fracture of a SiC whisker reinforced 2009 aluminium alloy

Fatigue crack growth under Mode I loading, and static fracture in both symmetrical and asymmetrical notched four point bend specimens, have been examined in SiC whisker reinforced 2009 aluminium alloy. In the fatigue tests a range of orientations of the starter notch, with respect to the extrusion direction, L, was examined. Slanted crack propagation was observed in all specimens except that in the T-L configuration. For the monotonic tests the specimen orientation (L-T) remained constant whilst the ratio of Mode I to Mode II loading was varied. Again crack deflection was observed in all cases apart from the L-T specimen under pure Mode I loading. Whisker debonding was found to be the dominant factor controlling crack deflection, independent of the mixity of the loading mode. Under mixed-mode static loading, the deflection angle was controlled by the average orientation of the whiskers subject to the asymmetrically distributed maximum principal stress. In fatigue loading, however, the crack tended to follow the most frequently observed whisker orientation. These contrasting observations are interpreted in terms of the different sampling volumes at the crack tip in monotonic fracture and in fatigue crack growth.     

基于DIC的含3D打印起伏节理试样破裂特性及损伤本构

受地质构造的影响,岩体工程中经常赋存起伏结构面(如扭转皱褶),由于形态复杂,目前起伏节理岩体的破裂及损伤本构研究仍不充分. 采用3D打印技术制作不同倾角的起伏节理模型,通过单轴压缩试验和数字图像相关技术(DIC)对起伏节理试样的力学及破裂特性进行研究,并基于断裂力学原理,首次提出采用DIC位移场求解节理尖端应力强度因子(SIF:一型SIF,K_I;二型SIF,K_II)进而探究损伤本构特性的思路. 结果表明:通过分析最小强度确定了起伏节理对试样的损伤上限为46.6%,起伏节理试样单轴强度对倾角的敏感性大于平直节理试样;起裂发生在峰值应力附近,破裂过程可分为破裂路径上微裂隙的产生和同步贯通,破裂模式表现为多条裂隙张剪组合模式;峰前SIF随荷载增加而增加,峰后同一荷载下K_II>K_I,节理左右两端均匀以剪切裂隙形式扩展;起伏节理对试样的损伤与倾角呈正弦关系,节理和荷载对试样的总损伤与应变均大致呈“S”型曲线.      

N型组合节理类岩体单轴压缩破坏试验

针对实际工程中平行与交叉裂隙组合呈N字形裂隙岩体的稳定性问题，以N型组合节理为研究对象，开展了N型节理类岩体试件超声波检测试验和单轴静载试验，结合断裂力学理论，分析其强度特征、破坏特征和超声波波速衰减规律。结果表明：（1）N型组合节理类岩体的裂纹类型依次有翼型裂纹和次生倾斜裂纹，其扩展路径最终均趋向于主应力方向，不同于单节理下的裂纹发展规律；（2）当主节理倾角一定时，主次节理夹角和节理条数对试件的物理性能有一定的影响。各组合节理试件的波速衰减率范围在0.9%～9.6%之间，且15°和90°夹角节理试件的波速衰减最快，而60°夹角节理试件的衰减最慢；（3）组合节理类岩体的本构关系、峰值强度和破坏特征均表现出非线性特征。峰值强度分布规律基本服从M型分布，不同于单节理下的U型分布，其中15°、30°、45°、75°和90°夹角试件，以及完整试件呈准脆性破坏，其他夹角试件呈脆性破坏。     

Combined mode I-mode III fracture toughness of a spherodized 1090 steel

The aim of this investigation was to determine the fracture behavior of a spherodized 1090 steel under combined mode I-mode III loading conditions. Suitably defined formulations of the J integral denoted J_ic and J_iiic were used to characterize the elastic-plastic fracture of this steel. As the mode III component in the system is increased, the resolved mode I J integral at initiation decreases, its mode III counterpart increases and the total J value remains nearly a constant. This implies a constant energy requirement for fracture initiation under mixed mode loading. As the crack plane becomes less inclined to the load line, the slopes of the mode I and total J resistance curves increase from their pure mode I values until a crack inclination angle of about 65° is reached. Somewhere in the region of 65-55°, a maximum in these values is reached and they fall off rapidly for larger mode III components. This drop is accompanied by the breakup of the crack front into mode I and mode III steps, which is shown to be an energetically more favorable process for this steel.     

Faceted fatigue fracture and its relation to the crystallographic slip systems in Cu-16 at. Pct al single crystals

In order to explore the relationship between the characteristic linear marks on the fracture surfaces of fatigued planar slip materials and the activity of cross slip and secondary slip systems, fatigued single crystals of Cu-16 at. pct Al alloy with a common single slip orientation were examined by scanning electron microscopy (SEM). This paper documents and explains the fractographic details of stages I and II crack propagations as a function of crack depth, strain amplitude, and loading history in a planar slip metal. The linear marks on the fracture surfaces in stage I for the specimen cycled at low strain amplitude are found mostly parallel to the traces of the cross slip plane, which is the plane on which propagation occurs to link up closely parallel cracks on the primary slip plane. With increase of crack depth, the contribution of conjugate slip for the linking up of parallel primary cracks increases. By geometrical consideration of the secondary slip planes with respect to primary cracks, the cross slip plane was found to be the most favored for the linking up of parallel primary cracks. This linking-up phenomenon produces the dominant fracture surface morphology. The fracture surfaces in the final stages show the typical features of ductile fracture in striations or catastrophic rupture. This result indicates that the observation of crystallographic traces on the fracture surfaces in stages I and II need not be associated with a brittle cleavage mechanism of fatigue fracture. The present authors support a ductile, plastic mechanism of crack propagation in both stages I and II. Formerly Graduate Student, University of Pennsylvania,     

含齿形裂隙类岩石材料单轴压缩试验研究

为研究含齿形裂隙岩石在单轴压缩下的破坏特征及强度特性，制作了含不同裂隙倾角和起伏角的齿形裂隙类岩石材料试件，并采用岩石力学伺服试验机进行单轴压缩试验。试验结果表明：（1）试件主要产生拉伸、剪切和拉剪复合裂纹，且根据裂纹的扩展路径可划分为A型（拉伸破坏）、B型（剪切破坏）、C型（复合破坏）3种破坏模式，裂隙倾角对试件最终破坏模式影响显著；（2）当裂隙倾角较小时，试件应力—应变曲线为多峰曲线，随着裂隙倾角的增大，曲线呈单峰形式，表现为延性减弱，脆性增强，而裂隙倾角相同但起伏角不同的试件应力—应变曲线大致相同；（3）当裂隙起伏角相同时，试件当量峰值强度随裂隙倾角的增大呈先减小后增大的规律，且裂隙起伏角对试件当量峰值强度的影响小于裂隙倾角。     

Combined mode I-mode III fracture of a high strength low-alloy steel

Special formulations of theJ integral were used to quantify the combined mode I-mode III fracture behavior of ASTM A710 Grade A steel, a tough material which displays elastic-plastic behavior at room temperature. Experimental fracture initiation loci for two heat-treated conditions were linear inJ _i-J_iii space, agreeing with analytical predictions based on linear elasticity. As commonly observed in mode I fracture behavior, large tearing modulus values accompanied largeJ values for mode I and mode III components, individually as well as for totalJ values. There was a pronounced tendency toward antiplane shear flow in the modified compact tension specimens tested. This may result from the concentration of mode III shear strains in the plane of the propagating crack as predicted by slip line theory. Shear fractures with the crack propagating at inclined angles to both load line and plate free surfaces are favored energetically over pure mode I cracks even though the total surface area formed by the latter is much smaller. formerly Graduate Research Assistant, The Ohio State University     

Load ratio,notch, and scale effects for bridged cracks in fibrous composites

This paper considers the problem of Mode I matrix cracks growing under either monotonic or cyclic loading in the presence of unidirectional, bridging fibers coupled to the matrix by friction. Scaling relations facilitate the presentation of exhaustive solutions for specimens of various shapes. Particularly simple results are found numerically for the ratio of the net to the applied crack tip intensity factor in monotonic loading or the ratio of their ranges in cyclic loading. Material properties that control how finite specimen size and applied stress level determine the relative strength of bridging are identified for monotonic and cyclic loading. Bending is shown to be important in edge notch specimens, even under uniform remote loading. Conditions are found for fracture surface contact during cyclic loading.     

Effect of compression point load on the path and life of fatigue crack growth in mixed mode loading

In this study, fatigue crack growth of edge crack in plate made from 5083 aluminum alloy under mixed mode fracture was numerically investigated. Effect of various parameters include initial crack direction angle (30°, 60°, 90°), initial crack length (20 and 25 mm) and load ratio (R=0 and 0.25) were considered. We also consider the effect of point compression load on crack growth life with 20 mm initial crack length under 30°, 60°, 90° at various parts of crack growth path. Mentioned models were simulated using the finite element softwares, ABAQUS and FRANC2D and their results were compared together. For validation we compare the numerical results for a specimen with its experimental data. Numerical results show good agreement with the experimental data for fracture path. It is observed that the point compression load can affect the path and life of crack growth in fatigue loading. If this load was applied in the first cycles of crack growth, its effect is more than another cases. For instance in the crack growth with initial angle 30o applying point load in 5, 10 and 15 mm of crack growth length, increase the life 79.4%, 18.9% and 6.2% respectively.