Fatigue Crack Propagation Behavior of as-Extruded AZ31B Mg Alloy Welded Joint

对AZ31B 镁合金焊接接头和热影响区的疲劳裂纹扩展行为进行研究,分析了焊接接头[L-T(W)]和热影响区的紧凑拉伸试验[C(T)],其中热影响区的C(T)试验包括焊缝平行于挤压方向[T-L(H)] 和垂直于挤压方向 [L-T(H)]两种。结果表明:对于L-T(W) 试样,裂纹沿挤压方向扩展,裂纹扩展经历先快后慢的扩展过程;T-L(H) 试样裂纹平行于缺口方向扩展,L-T(H)试样裂纹为平行于缺口方向和与缺口成一定角度两种扩展方向,裂纹扩展经历先慢后快的扩展过程。裂纹尖端扩展为穿晶和沿晶的混合模式,疲劳断口为准解理特征的脆性断口。     

AZ31B镁合金焊接接头的疲劳裂纹扩展行为(英文)

对AZ31B 镁合金焊接接头和热影响区的疲劳裂纹扩展行为进行研究,分析了焊接接头[L-T(W)]和热影响区的紧凑拉伸试验[C(T)],其中热影响区的C(T)试验包括焊缝平行于挤压方向[T-L(H)] 和垂直于挤压方向 [L-T(H)]两种。结果表明:对于L-T(W) 试样,裂纹沿挤压方向扩展,裂纹扩展经历先快后慢的扩展过程;T-L(H) 试样裂纹平行于缺口方向扩展,L-T(H)试样裂纹为平行于缺口方向和与缺口成一定角度两种扩展方向,裂纹扩展经历先慢后快的扩展过程。裂纹尖端扩展为穿晶和沿晶的混合模式,疲劳断口为准解理特征的脆性断口。     

复合材料单面修补铝合金裂纹板的疲劳破坏特性

利用热压成型工艺、采用预固化的单向碳纤维/环氧复合材料补片对铝合金裂纹板进行了修补,测试了裂纹板胶接修补前后的破坏强度、疲劳寿命及裂纹扩展情况,观察了破坏后的断口形貌,分析了复合材料补片的止裂机理.结果表明,经过单向碳纤维/环氧复合材料补片胶接修补后,其破坏强度和疲劳寿命均有显著的提高,破坏强度提高了34.28%,恢复到完好板的85.83%,疲劳寿命提高2.06倍;裂纹板的临界裂纹长度从17.86 mm增加到28.64 mm,从而延长了裂纹缓慢扩展阶段,延缓了裂纹快速扩展;其断口形貌方式发生生了明显的变化.     

复合材料单面修补铝合金裂纹板的疲劳破坏特性

利用热压成型工艺、采用预固化的单向碳纤维/环氧复合材料补片对铝合金裂纹板进行了修补，测试了裂纹板胶接修补前后的破坏强度、疲劳寿命及裂纹扩展情况，观察了破坏后的断口形貌，分析了复合材料补片的止裂机理。结果表明，经过单向碳纤维/环氧复合材料补片胶接修补后，其破坏强度和疲劳寿命均有显著的提高，破坏强度提高了34.28%，恢复到完好板的85.83%，疲劳寿命提高2.06倍；裂纹板的临界裂纹长度从17.86 mm增加到28.64 mm，从而延长了裂纹缓慢扩展阶段，延缓了裂纹快速扩展；其断口形貌方式发生生了明显的变化。     

H68黄铜断裂过程的透射电镜原位观察

采用透射电镜动态拉伸、原位观察研究了低层错能合金H68黄铜断裂的微观过程。结果发现：黄铜薄膜试样拉伸时，裂尖首先发射位错，平衡时形成无位错区和反塞积位错群；裂尖前方较厚区域可以发生孪生变形，形成形变孪晶，微裂纹在孪晶中形核、扩展，导致裂纹呈Z字形扩展；裂尖无位错区也可能形成微孪晶，微裂纹在微孪晶中形核，使裂纹呈不连续扩展；微裂纹也可以从主裂纹顶端连续形核、扩展。     

紧凑拉伸铍试样裂纹尖端应力分布研究

为了评估应力作用下铍试样裂纹尖端的微观断裂特性，设计了平面应变状态下铍的紧凑拉伸试样，采用X3000应力分析仪测试了不同载荷下铍试样裂纹尖端的应力分布，并对加载过程中铍试样内的应力应变进行了有限元计算。结果表明，离铍试样裂尖较远区域，有限元计算和实测值吻合较好，靠近裂尖区域，有限元计算高于实测值。根据铍试样拉伸时裂纹扩展的临界载荷，计算获得了铍试样裂纹尖端的最大应力、塑性区半径以及断裂强度因子。     

钴基高温合金裂尖塑性区位错结构的原位观察

走向凝固的钴基高温合金在原位拉伸研究中发现了两种裂尖塑性区位错结构,在拉伸轴为[511]时,从裂尖发射的是不全位错,在整个塑性区中位错始终以扩展位错的形式存在;在拉伸怕为[121]时,从裂尖发射的是全位错,位错在离开裂尖向试样内部运动过程中发生扩展分析了裂纹尖端应力状态对裂尖塑性区位错结构的影响。     

钴基高温合金尖塑性区位错结构的原位观察

定向凝固的钴基高温合金在原位拉伸研究中发现了两种裂尖塑性区位错结构,在拉伸辊为［５１１］时,从裂尖发射的是不全位错,在整个塑性区中全错始终以扩展位错的形式存在;［１２１］时,从裂尖发射的是全位错,位错在离开裂尖向试样内部运动过程中发生扩展,分析了裂纹尖端应力状态对裂尖塑性区位错结构的影响。     

蠕变性能失配对应力腐蚀裂尖力学场的影响

核电一回路中应力腐蚀开裂(SCC)是奥氏体不锈钢和镍基合金的主要失效形式。金属在高温水中会发生蠕变,从而影响了裂纹的扩展速率。以紧凑拉伸试样为研究对象,采用ABAQUS软件,分析了蠕变失配对SCC裂尖应力、应变以及蠕变速率和裂纹扩展速率的影响。结果表明,蠕变会使裂尖处的高应力在短时间内快速减少,但对应变的影响不明显。随着失配比的增加,C_t积分增大,同时裂纹扩展速率也越大,裂纹在蠕变主导下更易扩展。     

单轴加载下含共面双节理双层复合类岩试样的力学行为与破坏特征（英文）

为研究节理与层理面对裂隙层状岩体裂纹演化的共同作用,对不同层理面倾角下,含多角度共面双节理的双层复合类岩试样进行单轴压缩试验和数字图像相关试验。由试验结果可知,层理面和节理对应力-应变曲线特征和强度参数有显著影响。试件的破坏模式分为4种不同类型。层理面的存在抑制了岩桥区域裂纹的贯通,层理面倾角的变化对裂纹扩展路径和破坏模式的转变有较大影响。对于层理面倾角为30°和45°的试件,节理倾角越大,层理面对试件破坏特征的影响越小。此外,观测到一种从层理面上萌生并且更容易在试件上层扩展的拉伸裂纹。     

Mechanical properties and rockburst proneness of phyllite under uniaxial compression

To investigate the influence of the bedding angle, β,on the mechanical properties and rockburst proneness, uniaxial compression tests were conducted using cylindrical phyllite specimens with different bedding angles. According to the results, the peak stress, peak strain, cumulative acoustic emission counts, and potential energy of the elastic strain exhibited a U-shaped change trend. With an increase in β from 0° to 90°, the failure mode transformed from tensile splitting failure along the bedding plane to shear slip failure along the weak bedding plane. Finally, the failure mode evolved into a tensile splitting failure across the bedding plane. When β=15°, 30°, and 45°, the phyllite specimens exhibited strong, slight, and moderate rockburst proneness, with strong, slight, and moderate shear slip rockbursts, respectively. When β=0°, 60°, 75°, and 90°, the phyllite specimens had extremely strong rockburst proneness, and an extremely strong strain rockburst occurred.     

Application of a novel type Barkhausen noise sensor to continuous fatigue monitoring

A novel Barkhausen noise (BN) sensor with no need for external magnetization was tested and applied to continuous fatigue monitoring of mild steel and high strength steel specimens. This new type of sensor indicated an increase in the BN rms value under maximum tensile stress during one cycle in cyclic bending tests with increasing stress amplitude. The BN rms value under maximum compressive stress stayed, however, approximately constant. The reason for this behaviour was the stress-induced anisotropy of the BN. Bending fatigue experiments with constant stress amplitude and R=−1 were conducted at different stress levels. In addition to the BN also the acoustic emission of the specimen was measured. In the mild steel specimens the BN amplitude stayed constant after the initial saturation period, but just prior to the failure of the specimen the amplitude increased meaningfully. This increase occurred at the same time as the increase in the acoustic emission signal indicating the beginning of crack initiation and growth. In the high strength steel specimens the BN amplitude decreased after the initial saturation period. The increase of the BN signal started well before the failure of the specimen and even before the increase in the acoustic emission signal.     

用声发射监测16MnR带预制中心穿透裂纹试板的拉伸试验

用声发射监测16MnR带预制中心穿透裂纹试板的拉伸试验,以声发射信号分析预制裂纹的运动。与裂纹失稳扩展相应的声发射事件信号参数值差异很大,反映了其显微断裂机制的不同。     

Effect of electromagnetic bulging on fatigue behavior of 5052 aluminum alloy

The effect of electromagnetic bulging on the fatigue behavior of the 5052 aluminum alloy was investigated through tensile–tensile fatigue testing. The intriguing finding is that the bulged specimens exhibited enhanced fatigue strength as depicted by maximum stress vs the number of cycles until failure (S–N) curves, by comparison with these original aluminum alloys. Although the fatigue process of the original and budged alloys follows the same mechanism with three distinct steps, namely, crack initiation at a corner of the tested samples, stable crack propagation with typical fatigue striations and finally catastrophic fracture with dimple fractographic features. The typical crack propagation rate vs stress intensity factor range (da/dN–ΔK) curves derived from the spacing of striations reveal a lower crack propagation rate in the bulged specimens. The enhancement of fatigue strength in electromagnetically bulged aluminum alloy is further rationalized in-depth on the basis of strain hardening and dislocation shielding effect.     

不同孔隙率规则多孔钛的变形失效特征

设计和制备了孔隙率不同的3组规则多孔钛试样,采用分离式霍普金森压杆实验装置,开展了应变率范围在600~2100 s^-1的单轴压缩实验,研究了加载过程中整个试样和局部孔洞的变形失效特征。结果表明,试样展示出2种典型的变形模式：压缩变形模式和连通断裂模式。压缩变形模式主要发生在低孔隙率的试样,试样的外表面胞壁出现局部塌陷,而连通断裂模式易发生在高孔隙率的试样中,常在试样的一个或多个孔层出现连通的断裂。另外,2种变形模式下的局部孔洞也呈现出不同的形状变化特征,且在孔洞最小曲率的位置都形成明显的应力集中,相应的应力集中因子随试样孔隙率的增大而增大。微结构分析显示：剪切带是试样失效的主要机制,试样的断口容易产生韧窝和韧性条纹。     

MECHANISM ANALYSIS OF THICKNESS EFFECT ON MIXED MODE Ⅰ/Ⅱ FRACTURE OF LC4-CS ALUMINUM ALLOY

Mixed mode Ⅰ/Ⅱ fracture experiments of LC4-CS aluminum alloy were conducted by using tension-shear specimens with thicknesses of 2, 4, 8 and 14mm. Fracture mechanisms of thickness effect on mixed mode Ⅰ/Ⅱ fracture were first examined from fracture surface morphology to correlate with the macroscopic fracture behavior and stress state. It is found that specimen thickness has a strong influence on mixed mode fracture. As thickness varies from thin to thick the macroscopic fracture surfaces appear the characteristics of plane stress state (2mm, 4mm-thick specimen), threedimensional stress state (8mm-thick specimens), and plane strain state (14mm-thick specimens), respectively. The specimens of all kinds of thicknesses are typical of tensile type failure under mode I loading condition and shear type failure under mode Ⅱloading condition. Two distinct features coexist on the fracture surfaces under mixed mode loading conditions, and the corresponding proportion varies with loading mixity. Void-growth processes are the failure mechanism in both predominately tensileand shear-type fractures. The size and depth of dimples on the fracture surface vary greatly with thickness. Therefore, it is extraordinary necessary to take into account the thickness effect when a mixed mode fracture criterion is being established.     

制孔分层损伤对CF/PEEK复合材料拉伸性能和表面应变分布的影响

目的 研究钻削制孔表面分层损伤与拉伸载荷下开孔碳纤维增强聚醚醚酮（CF/PEEK）复合材料表面应变分布的相关性。方法 通过对CF/PEEK复合材料层合板进行钻削制孔实验,分析不同进给速度对钻削温度、钻削轴向力、制孔出口表面分层和孔壁表面损伤的影响。采用数字图像相关技术（DIC）和力学实验相结合的方法,研究分层损伤程度对开孔CF/PEEK复合材料层合板拉伸性能和表面应变分布的影响。使用扫描电镜观测开孔试件的断裂形貌,分析开孔试件受拉伸载荷时的破坏模式。结果 随着进给速度的增加,钻削温度降低,钻削轴向力提高,出口表面分层和孔壁损伤程度加剧。随着分层损伤程度的增加,层合板的拉伸强度呈现出降低的趋势,试件的拉伸强度从558.4 MPa降低到525.63 MPa,降低了5.87%。在中应力和高应力状态下,试件x方向的最大负应变随着分层损伤程度的增加而增加。在高应力状态下,试件y方向的最大正应变随着分层损伤程度的增加而增加。试件的断裂方式主要是基体开裂、分层和纤维撕裂,断口有纤维脱落和纤维拔出,垂直于载荷方向的纤维破坏模式为剥离破坏,与载荷方向一致的纤维破坏模式为拉伸破坏。结论 钻削制孔表面分层损...     

SCC evaluation of type 304 and 316 austenitic stainless steels in acidic chloride solutions using the slow strain rate technique

The stress corrosion cracking (SCC) of the commercial austenitic stainless steels type 304 and 316 was investigated as functions of strain rate, test temperature and pH in acidic chloride solutions using a slow strain rate technique. From the comparison between the parameters such as time to failure (t_f), maximum stress (σ_m) and strain at t_f (ε_f) obtained in inert and corrosive environments, the relationships between t_f^Cl(corrosive)/t_f^W(inert), σ_m^Cl/σ_m^W and ε_f^Cl/ε_f^W versus strain rate were divided into three regions for type 304 and 316 specimens: the strain rate-dominated, SCC-dominated and corrosion-dominated regions, respectively. Furthermore, it was found that the relationship between t_f^Cl/t_f^W and σ_m^Cl/σ_m^W in the SCC-dominated region became a good straight line for the type 304 and 316 specimens irrespective of strain rate, test temperature and pH. This suggests that σ_m^Cl/σ_m^W became the parameter for predicting t_f. The results obtained were compared with those obtained from the constant load method and are discussed in terms of the formation of slip steps, dissolution current and film formation rate.     

MECHANISM ANALYSIS OF THICKNESS EFFECT ON MIXED MODE I／II FRACTURE OF LC4-CS ALUMINUM ALLOY

Mixed mode I//II fracture experiments of LC4-CS aluminum alloy were conducted by using tension-shear specimens with thicknesses of 2, 4, 8 and 14mm. Fracture mechanisms of thickness effect on mixed mode I//II fracture were first examined from fracture surface morphology to correlate with the macroscopic fracture behavior and stress state. It is found that specimen thickness has a strong influence on mixed mode fracture. As thickness varies from thin to thick the macroscopic fracture surfaces appear the characteristics of plane stress state (2mm, 4mm-thick specimen), three-dimensional stress state (8ram-thick specimens), and plane strain state (14mm-thick specimens), respectively. The specimens of all kinds of thicknesses are typical of tensile type failure under mode I loading condition and shear type failure under mode II loading condition. Two distinct features coexist on the fracture surfaces under mixed mode loading conditions, and the corresponding proportion varies with loading mixity. Void-growth processes are the failure mechanism in both predominately tensile and shear-type fractures. The size and depth of dimples on the fracture surface vary greatly with thickness. Therefore, it is extraordinary necessary to take into account the thickness effect when a mixed mode fracture criterion is being established.     

Fatigue-life prediction of SiC aluminum composite using a Weibull model

The feasibility of the acoustic emission technique in predicting the residual fatigue life of 6061-T6 aluminum matrix composite reinforced with 15 vol.% SiC particulates (SiC_p) is presented. Fatigue damages corresponding to 40, 60 and 80% of total fatigue life were induced at a cyclic stress amplitude. The specimens with and without fatigue damage were subjected to tensile tests. The acoustic emission activities were monitored during tensile tests. The number of cumulative AE events increased exponentially with the increase in strain during tensile tests. This exponential increase occurred when the material was in the plastic regime and was attributed mainly to SiC particulate/matrix interface decohesion. Cumulative events during post fatigue tensile tests reduced with a decrease in the residual fatigue life. Based on the high cycle fatigue damage accumulation model, a Weibull probability distribution model is developed to explain the post fatigue AE activity of specimens during tensile tests. Using the model, the residual fatigue life can be predicted by testing the specimen in tension and monitoring the AE events. In high cycle fatigue, it was observed that the residual tensile strengths of the material did not change significantly with prior cyclic loading damages since the high cycle fatigue life was dominated by the crack initiation phase.