焊接接头疲劳行为研究

为了研究焊接接头的疲劳行为和焊接疲劳裂纹在焊接接头不同区域的扩展规律,设计了焊接接头的轴向疲劳试验和疲劳裂纹扩展试验。通过轴向疲劳试验,得出了在不同条件下的S-N曲线和S-N方程;通过疲劳裂纹扩展试验,得出疲劳裂纹扩展的门槛值,以及从疲劳裂纹产生到失效的疲劳寿命。     

30 mm厚TA1电子束焊接头疲劳断裂原因分析

为探究30 mm板厚TA1电子束焊接头发生疲劳断裂的原因,针对接头开展疲劳试验,获得了接头S-N曲线及条件疲劳极限。针对TA1接头开展宏观、微观组织及力学性能检验,结合对典型低寿命接头的疲劳试样断口分析,获得了接头断裂特征。结果表明:电子束焊接过程使TA1焊缝及热影响区得到强化,整个接头无明显软化区。TA1电子束焊接头S-N曲线数据符合线性拟合规律,其拟合的疲劳极限值163 MPa,为接头抗拉强度的48.7%,接头疲劳强度符合金属材料结构件疲劳设计的一般规律。焊缝内部"微米级"焊接气孔的存在是导致接头发生疲劳断裂的主要原因,部分接头则是因焊接夹渣导致疲劳断裂。     

WC-Co类硬质合金的低周冲击疲劳性能研究

为研究WC-Co类硬质合金的冲击疲劳断裂机制和微观组织参数对其低周冲击疲劳性能的影响,对6种牌号不同WC晶粒尺寸及Co含量的硬质合金进行了冲击疲劳试验,并用扫描电子显微镜(SEM)观察冲击疲劳断口。结果表明:当硬质合金承受冲击加载时,裂纹从粉末冶金缺陷处萌生,萌生的裂纹与孔洞相互连接形成主裂纹,主裂纹快速扩展导致材料疲劳失效。Co含量越高的硬质合金其疲劳敏感性越强,冲击疲劳寿命越短;WC晶粒尺寸越大的硬质合金其冲击疲劳寿命越长。     

择优取向层片组织TiAl合金的室温疲劳行为

采用旋转弯曲加载方式,评价了择优取向层片组织Ti-47.5Al-2.5V-1.0Cr-0.2Zr(原子数分数,%)合金的室温高周疲劳性能,并采用扫描电镜对疲劳断口进行了观察和分析。结果显示,实验合金的应力-寿命(S-N)曲线呈现平直形态,符合Basquin方程;其条件疲劳极限为477MPa,相当于其抗拉强度的83%。断口观察发现,疲劳试样以穿层片解理方式发生断裂。疲劳裂纹主要沿位于试样表面层、与外加应力成30°~90°的层片界面萌生,之后以穿层片方式发生扩展。在同一应力水平下,疲劳寿命随疲劳源尺寸增加而减少,疲劳源尺寸波动是导致疲劳寿命大幅分散的主要原因。     

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。     

超声冲击对P355NL1钢焊接接头超高周疲劳性能影响

 通过超声疲劳试验探究超声冲击对P355NL1钢焊接接头超高周疲劳性能的影响。结果表明,由疲劳[S-N]曲线可知,在105～109寿命区间内,冲击态试样的疲劳性能要高于焊态试样,在1.0×108的疲劳寿命下,焊态试样的疲劳强度为139 MPa,冲击态的疲劳强度为217 MPa,冲击态疲劳强度相较于焊态提高了56%,这表明超声冲击可以明显提高焊接接头的疲劳强度。利用扫描电镜（SEM）观察断口形貌可以发现,裂纹源位于焊接接头焊根区表面。P355NL1钢焊接接头疲劳断裂为准解理断裂,超声冲击可以提高焊接接头的疲劳强度,但不会改变其疲劳失效机理。超声冲击可以降低焊根处应力集中,引入有益压应力和表面晶粒细化,从而提高焊接接头的疲劳强度。     

不同烧结条件下Fe-Cu-Ni-Mo-C合金的弯曲疲劳性能及断口分析

以部分扩散预合金Fe-2Cu-2Ni-1Mo-1C粉末为原料,利用模壁润滑温压技术与3种烧结工艺制备合金材料,研究不同烧结工艺下合金的疲劳性能。结果表明:3组不同烧结工艺制备的试样在104~109循环周次下的应力幅值-循环周次(S-N)曲线均为一条连续下降的曲线,不存在传统疲劳概念上的疲劳极限,只存在条件疲劳极限;在107循环周次下,3组试样的条件疲劳极限分别为280、264和239 MPa。断口分析发现,3组试样的疲劳裂纹均萌生在试样表面的棱角处,且均表现为多源萌生;疲劳裂纹扩展以穿晶断裂为主,不同的组织对裂纹的扩展有明显的影响;扩展区存在典型的解理和疲劳辉纹形貌;断裂区有塑性韧窝出现。     

82A和82B高强钢疲劳性能的试验研究

对某钢厂生产的82A和82B高强钢进行了高周疲劳性能试验研究,得到了这两种钢的疲劳应力-寿命曲线(S-N曲线)及疲劳强度。用扫描电镜对试样的疲劳断口进行了观察。结果显示,对夹杂物进行变性处理后的82A钢的疲劳裂纹大多从表面缺陷处萌生,而夹杂物没有变性处理的82B钢的疲劳裂纹则从非金属夹杂物及表面缺陷处萌生,且从内部非金属夹杂物处萌生的疲劳断口呈"鱼眼"特征。讨论了非金属夹杂物对高强钢疲劳性能的影响。     

淬火温度对86CrMoV7钢抗接触疲劳性能的影响

冷轧辊接触疲劳剥落是在接触负荷下表层疲劳裂纹萌生扩展的断裂过程,与组织结构和内应力状态有关,是主要失效方式之一。本文研究淬火温度对86CrMoV7钢抗接触疲劳性能的影响。选择840、870、910和950℃四个淬火温度。测定了试样的接触疲劳寿命、硬度、残余应力和残余奥氏体量。并对金相组织、裂纹形貌及剥落断口进行了观察。研究发现,840℃淬火有最高的接触疲劳寿命;硬度和疲劳寿命有一定的对应关系,硬度为62～63HRC时接触疲劳寿命最高,更高的硬度反而使之下降。残余应力与疲劳寿命的关系较为复杂。金相组织对钢的接触疲劳寿命有重要作用:板条马氏体组织比片状马氏体组织抗接触疲劳性能好,适量的残余奥氏体可改善钢的抗接触疲劳性能。不同的裂纹及断口形貌对应不同的接触疲劳寿命。     

激光沉积TA15钛合金高周疲劳性能研究

为了研究激光沉积TA15钛合金的高周疲劳性能,在720,760和800 MPa应力水平下进行了室温高周疲劳(HCF)测试并分析了疲劳断口,结果显示激光沉积TA15钛合金具有较高的高周疲劳寿命,720 MPa下疲劳源区断口形貌表现为很高的组织敏感性,敏感尺寸达单个α片层尺寸单元,800 MPa下敏感性较弱,只达到α片层集束尺寸单元。部分试样有气孔存在,气孔范围在20～40μm,气孔的存在使试样的高周疲劳寿命呈现不同于锻件的明显分散性,表现为同一应力水平下寿命量级的分散,且气孔的大小与位置对疲劳寿命有不同程度的影响。直径较大和距离表面越近的气孔对疲劳寿命损伤越大。采用数值模拟方法研究发现尺寸大、距离表面近的气孔应力集中系数大,导致裂纹萌生寿命降低。虽然有气孔的存在,但激光沉积TA15钛合金仍具有优异的疲劳性能,这与取向随机、尺寸细小的片层组织有关,细小的显微组织增加裂纹的萌生阻力,提高高周疲劳强度。     

Mechanical and fatigue properties of self-piercing riveted j oints in high-strength steel and aluminium alloy

Static tensile and fatigue tests were performed on shear and tensile self-piercing riveted aluminium-steel structures to evaluate their mechanical and fatigue properties.The influences of the thickness and the strength of the high-strength steel on mechanical and fatigue performances were investigated based on the tensile and F-N curves of the joints.The results show that mechanical and fatigue properties of the shear self-piercing riveted joints are much better than those of the tensile self-piercing riveted joints.Mechanical and fatigue performances of the two joints were significantly influenced by the thickness and strength of the steel sheet, and were markedly improved when the thickness of steel sheet increased.The steel strength showed significantly different effects on shear and tensile riveted structures, i.e., when the steel strength increased, the strength of the shear structure greatly increased while the tensile structure just had a slight increase in the strength.Fatigue failure generally occurred in the sheet materials and the fa-tigue crack location changed with increasing the sheet thickness and the sheet strength.     

应力比对TC4钛合金超高周疲劳失效机理的影响

采用频率为100 Hz的电磁谐振疲劳试验机进行疲劳拉伸试验, 研究了两种应力比(R=0. 1和-1) 对TC4钛合金的超高周疲劳失效机理的影响.结果表明, 两种应力比下的S-N曲线都呈现"双线"型, 但各自表示的意义及失效机理不同.当R=0. 1时, TC4钛合金的疲劳失效形式有两种, 即由加工缺陷诱发的表面失效和内部鱼眼失效, 这两种失效形式都伴随着颗粒平面(Facet) 出现; 而当R=-1时, 仅存在表面失效, 且无Facet的出现.基于断裂力学的讨论可知, 在正应力比及真空环境下, 对应小裂纹扩展的门槛值更低, 更有利于裂纹扩展及Facet的形成. TC4钛合金的整个内部疲劳失效过程及机理可解释为: (1) 滑移线或滑移带在部分α晶粒上的出现; (2) 微裂纹的萌生和接合; (3) 颗粒亮区(GBF) 的形成; (4) 鱼眼的形成; (5) 鱼眼外的失稳裂纹扩展; (6) 最终的瞬时断裂.      

Evaluation on Fatigue Performance and Fracture Mechanism of Laser Welded TWIP Steel Joint Based on Evolution of Microstructure and Micromechanical Properties

The fatigue performance and fracture mechanism of laser welded twinning induced plasticity(TWIP)steel joint were investigated experimentally based on the evolution of microstructure and micromechanical properties.The optical microscopy was used to analyze the evolution of microstructure.The variation of composition and phase structure of fusion zone were detected by energy dispersive X-ray and X-ray diffraction spectrometers.The micromechanical behaviors of the various zones were characterized using nanoindentation.The static tensile test and high cycle fatigue test were performed to evaluate the mechanical properties of welded joint and base metal.The microstructures,tensile properties and fatigue strength of base metal as well as welded metal were analyzed.The fatigue fracture surfaces of base metal and welded joint were observed by means of scanning electron microscopy,in order to identify fatigue crack initiation sites and propagation mechanisms.Moreover,the fatigue fracture characteristics and mechanisms for the laser welded TWIP steel joints were analyzed.     

340MPa级热镀锌高强低合金钢的疲劳性能

 采用拉-拉疲劳试验研究了厚1.75mm的340MPa级热镀锌高强低合金钢的疲劳断裂行为,绘制了其S-N曲线,并采用扫描电镜对其疲劳断裂特征进行了分析。结果表明,340MPa级热镀锌高强低合金钢在应力比为0.1,加载频率为15Hz的条件下的疲劳极限为190MPa,其疲劳裂纹源位于表面处,疲劳断口靠近裂纹源处锌层破坏不明显,瞬断处锌层破坏明显。     

Interface Behavior and Lap Shear Properties of the 7075-T6 Al FSLW Joint Using a Tip-Threaded Pin

For the purpose of reducing the hook and increasing the effective sheet thickness of the friction stir lap welding joint, a tip-threaded pin was used to weld 3 mm-thick 7075-T6 aluminum alloy in this work. Material flow behavior, joint formation mechanism and lap shear properties of the lap joints were mainly discussed. Results showed that when using the tip-threaded pin, lap joint with nearly aequilate stir zone (SZ) along thickness and rather flat hooks were obtained. When using high rotating speeds, the width of the lower SZ exceeded that of the middle SZ. A void, which resulted from the material loss near the lap interface, was observed at the same time. With increasing the rotating speed, the lap shear failure load of the joint initially increased and then decreased, while the fracture mode changed from tensile fracture to shear fracture mode. The maximum failure load of 17661 N was attained for the joint without void at 850 rpm.     

Experimental Investigation of Bonded Fiber Reinforced Polymer-Concrete Joints under Cyclic Loading

The strengthening of reinforced concrete structures by means of externally bonded fiber reinforced polymers (FRPs) is becoming an attractive technique for upgrading existing structures. Although previous laboratory investigations have shown that the bending capacities of beams can be increased considerably with this strengthening technique, premature failure by debonding of the FRP reinforcement can often limit its effectiveness. To gain insight into debonding phenomena, various experimental and analytical investigations of the behavior of bonded FRP-to-concrete joints have been carried out. However, such studies have generally been limited to monotonic (“static”) loading conditions. In this paper, we present results from an experimental investigation of bonded FRP-to-concrete joints under cyclic loading. First, we describe the experimental setup and test parameters. Next experimental results for the effects of cyclic loading on slip at the FRP–concrete interface, crack opening, and strain profiles along the bonded FRP joint are presented and discussed. A power-law expression for the so-called “S–N” curves (cyclic stress ranges versus numbers of cycles to failure) is proposed, and the parameters in this expression are determined from the experimental data. The influence of various parameters such as bond length, bond width, and cyclic bond stress levels on fatigue behavior are discussed.     

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。     

Fatigue of cold-work tool steels: Effect of heat treatment and carbide morphology on fatigue crack formation,life, and fracture surface observations

The fatigue properties of two types of cold-work tool steels tempered at various temperatures were evaluated. The microstructure and fracture surface morphology were correlated to the fatigue behavior. Cold-work tool steels using this study were a conventional tool steel (JIS SKD11; 1.4C-11Cr-0.8Mo-0.2V) and its modified steel (M-SKD11; 0.8C-8Cr-2Mo-0.5V). The fatigue strength of the M-SKD11 steel increased 20 pct over that of the SKD11 steel for any number of cycles. This is attributed to the refinement of primary M₇C₃ carbides. These M₇C₃ carbides fractured during fatigue and were found at the sites of fatigue crack initiation. Change in crack initiation behavior was confirmed by acoustic emission testing. The S-N curves of the steels are similar to those of most structural steels. However, the subsurface fatigue crack initiation was dominant at lower alternating stresses. This study points to a general approach of carbide refinement that can be used for the enhancement of fatigue properties.     

Fatigue Evaluation of Rib-to-Deck Welded Joints of Orthotropic Steel Bridge Deck

This study reported fatigue test results of 300-mm-wide specimens with three details: 80% partial joint penetration (80%PJP), weld melt-through (WMT), and both. The specimens were cut out from full-scale orthotropic deck specimens of 16-mm-thick deck plate. In the fatigue test, the deck plate was subjected to cyclic bending loading and the rib was free from loading. The fatigue fracture surfaces showed that the presence of WMT may affect the initiation of fatigue cracks. A propensity to root cracking rather than toe cracking was observed. Plotting fatigue test results in an S-N diagram showed that the specimens with WMT seemed to have slightly lower fatigue strengths than the 80%PJP specimens, but the difference is more likely to be within a usual scatter of test data, which means that both details have comparable fatigue strength. The present test results satisfied the S-N curves of JSSC-E (80?MPa at 2×106 cycles) or AASHTO-C (89?MPa at 2×106 cycles).