高频冲击表面处理技术

高频冲击表面处理技术是利用高密度能量推动冲击工具(冲击头)以每秒二万次以上的频率冲击金属表面,使表层产生较大塑性变形,从而起到改变材料表面形貌和改善力学性能的作用。高频冲击可用于金属零件的表面强化处理,特别是对提高焊接接头的疲劳性能效果显著,其强化机理是:消除焊接残余拉应力,引入残余压应力场;使焊趾处产生圆滑的几何过渡,减低余高和凹坑造成的应力集中;闭合焊趾表层微小裂纹和消除熔渣缺陷。     

焊趾TIG重熔对钛合金T型接头疲劳性能影响

研究了焊趾TIG重熔对钛合金T型接头疲劳性能的影响,结果表明,经过焊趾TIG重熔和喷淋激冷处理后,焊趾部位残余应力可降低32%;在2×10~5疲劳次数循环基数下,焊趾重熔+喷淋激冷处理后接头疲劳强度可提高4倍;在260 MPa应力值水平下,焊趾重熔+喷淋激冷处理后,接头疲劳寿命提高3.3倍;焊趾重熔+喷淋激冷处理并进行超声波冲击消应力后,接头疲劳寿命提高9.9倍,疲劳性能明显优于采用两种方法单独进行处理时的接头疲劳性能。     

随机轮载下钢桥面顶板与纵肋焊缝疲劳裂纹扩展特性研究

车辆轮迹横向分布是钢桥面板焊缝处产生随机应力谱的一项关键因素,加之焊接缺陷形态的随机性,诱发钢桥面板疲劳裂纹随机扩展行为。为了研究钢桥面顶板焊缝处疲劳裂纹的随机扩展特性,基于断裂力学理论与扩展有限元方法分析了轮迹横向分布对钢桥面板顶板焊根和焊趾等效应力强度因子的影响规律,揭示了轮迹横向分布离散度、初始裂纹深度和初始裂纹形态比对焊缝处疲劳裂纹随机扩展路径分布的影响规律。结果表明：轮载中心处于U肋正上方中心和焊缝中心位置分别为顶板焊趾和焊根的最不利横向加载工况;车辆轮迹横向分布对顶板焊根和顶板焊趾疲劳裂纹前缘应力强度因子影响差异显著,顶板焊根最大等效应力强度因子为85.99 MPa·mm^1/2,比顶板焊趾增加了6.72%;轮迹横向分布离散度和初始裂纹深度与钢桥面板焊缝处疲劳裂纹随机扩展路径分布离散程度成正相关,初始裂纹形态比与其成负相关;焊缝细节初始裂纹深度越大,车辆荷载对其横向影响范围越大;焊缝细节初始裂纹形态比越大,车辆荷载对其横向影响范围变化不明显。     

超声冲击改善高强钢焊接接头残余应力的数值模拟分析

通过将数值模拟技术应用于超声冲击工艺的研究,合理处理超声冲击加载方式和接触条件,建立了超声冲击工艺有限元分析模型,计算结果与试验结果吻合较好。研究结果表明,10Ni5CrMoV钢焊趾部位存在高达660MPa的残余拉应力,超声冲击技术能够改善其残余应力分布,在焊趾部位形成超过400MPa的残余压应力。     

超声冲击对高速列车转向架焊接十字接头超高周疲劳性能的影响

针对高速列车转向架在实际运用中出现的疲劳问题,利用HJ-Ⅲ型超声冲击处理装置,对高速列车转向架用SAM490BW钢十字接头焊趾进行超声冲击处理,并对处理后接头的超高周疲劳性能进行研究。结果表明,原始焊态试样和超声冲击态试样的S-N曲线均出现了传统意义上的水平段。超声冲击处理试样的疲劳强度相比于未处理试样提高了25%。经过超声冲击处理后,疲劳裂纹源的数量由多个变为单个,焊接接头焊趾处的应力集中系数下降了32.4%,降低了裂纹萌生概率;超声冲击降低了焊趾区域的残余拉应力,并诱发产生残余压应力,高达-255.5 MPa;焊趾表层晶粒得到细化,晶粒平均尺寸小于100nm;裂纹在塑性变形区扩展路径变长。     

超声捶击提高超细晶粒钢焊接接头的疲劳性能

焊接接头疲劳强度是其最重要的服役性能，由于焊接残余应力的作用和焊接接头处的几何不连续性，焊接接头的疲劳强度一般大大低于母材，采用超声捶击方法提高超细晶粒钢焊接接头的疲劳强度，通过对对接接头焊践处进行超声波冲击处理，对比超声波冲击处理后焊接接头的疲劳强度，实验结果表明：超声捶击使得S-N曲线右移，FAT（循环寿命为10^6时的疲劳强度）提高幅度达到66%，在应力范围为200MPa的疲劳寿命提高58倍，研究表明，经超声捶击处理，焊趾处的应力集中系数相应减小，焊接残余应力由拉应力转换为压应力，这是超声捶击提高焊接接头疲劳强度的主要机制。     

随焊冲击碾压对LY12CZ铝合金接头组织和性能的影响

随焊冲击碾压是一种控制薄壁结构焊接应力变形、防止热裂纹的新方法.这种方法同时也改善了LY12CZ铝合金接头组织,提高了其力学性能.金相和拉伸断口SEM观察表明,随焊冲击碾压焊缝区晶粒明显细化,组织更加致密,气孔、缩松等缺陷大大减少.与常规焊接接头拉伸断口脆性和塑性断裂特征并存相比,随焊冲击碾压焊缝断口形貌呈现典型的塑性断裂的特征.随焊冲击碾压之后,接头区的硬度有了明显提高,表明接头软化区重新得到强化.而拉伸、疲劳和三点弯曲实验结果表明,随焊冲击碾压之后接头的抗拉强度、塑性及疲劳寿命等力学性能也都得到不同程度的提高.由于薄弱的焊趾部位得到强化,裂纹萌生位置也由焊趾转移到焊缝.     

喷丸改性对7050铝合金FSW接头性能的影响

对5 mm厚的7050铝合金搅拌摩擦焊接头进行喷丸表面改性处理,并对喷丸前后的接头进行残余应力测量、低周疲劳裂纹扩展试验,研究喷丸改性对焊接接头残余应力分布和焊核区、热影响区疲劳裂纹扩展速率的影响。结果表明,喷丸处理提高了接头表面粗糙程度,但接头表层及近表层产生硬化层,二次喷丸处理后的表层硬度值达198HV。二次喷丸最高硬度值比焊接态高33.1%。喷丸引入的残余压应力随喷丸次数增加而增大,最大压应力值位置也移向板材更深处。焊核区、热影响区疲劳裂纹扩展速率相对焊接态均降低,表明喷丸改性提高了接头两微区抗疲劳断裂的能力。     

LF2铝合金燃油导管接头焊缝开裂原因及改进

焊接于LF2铝合金燃油系统导管上的传感器管接头,使用一段时间后在焊缝焊趾处出现裂纹。采用宏观观察、断口宏观、金相检验、焊接质量检查及焊接结构改进验证试验等方法对开裂原因进行了分析。结果表明:该LF2铝合金燃油导管接头的开裂性质为疲劳开裂。裂纹起源于导管外表面应力集中处,燃油管管接头焊缝焊趾处应力水平较高,且受到与导管轴向垂直方向的振动应力的叠加,最终导致焊缝开裂。     

拉压加载下纤维增强铝合金层板疲劳裂纹扩展的压载荷效应与预测模型

基于增量塑性损伤理论与纤维增强金属层板疲劳裂纹扩展唯象方法, 推导出在拉-压循环加载下, 纤维增强金属层板疲劳裂纹扩展速率预测模型。并通过玻璃纤维增强铝合金层板在应力比R=-1,-2的疲劳裂纹扩展实验对预测模型进行验证。结果表明, 纤维增强铝合金层板疲劳裂纹扩展的压载荷效应分为两种情况: 在有效循环应力比R_C>0时, 表现为压载荷对铝合金层所承受残余拉应力的抵消作用; 当R_C<0时, 表现为压载荷抵消残余拉应力后, 对纤维增强铝合金层板金属层的塑性损伤, 对疲劳裂纹扩展存在促进作用。纤维铝合金层板疲劳裂纹扩展的压载荷效应不可忽略, 本文中得出的在拉-压循环加载下疲劳裂纹扩展速率预测模型与实验结果符合较好。     

Fatigue of fillet welded A515 steel

A study has been made of the fatigue behavior of fillet welded ASTM A515 steel. As-welded and stress-relieved skip fillet weld specimens were tested under pulsed tension and altering cyclic load to determine stress-life and crack propagation behavior. Crack initiation and propagation features were determined from sectioned surfaces. All fatigue cracks were semi-elliptic and initiated from weld end toes. The length/depth ratio was approximately constant during propagation. There was no consistent effect of tensile residual stress on fatigue life under pulsed tension but there was a detrimental effect under alternating loads. An equivalent crack model has been proposed to quantify the stress concentration effect at the crack initiation site based on the application of the Paris equation. The test results show that the equivalent crack can give a reasonable prediction of the fatigue life of a welded structure and is a potentially convenient tool in fatigue design.     

Fatigue crack growth in a fillet welded joint

Existing theories for the growth of cracks at weld toes have proved difficult to verify because of a lack of experimental proof at short crack depths and slow growth rates. Arbitrary initial defect sizes have been employed in life calculations coupled with approximate two-dimensional stress analyses. In this study, the fatigue performance of a stress relieved fillet weld is determined by both theory and experiment. Crack growth results for shallow (less than 1 mm depth) elliptical cracks at weld toes are used to test an elastic expression for stress intensity using a correction factor from a three-dimensional stress analysis. No evidence of higher than expected growth rates, observed by others for very short cracks and cracks in notch plastic zones, is apparent. Integration of a growth law that includes the threshold stress intensity factor provides fatigue life predictions for various stress ratios and from experimentally measured defect depths. Needle peening the weld toe improves the fatigue life by retarding crack growth up to 1 mm below the weld toe.     

Crack aspect development curves and fatigue life prediction for surface cracks at weld toes in the presence of residual stress

An engineering procedure is proposed for estimating the crack growth behaviour and fatigue lives of semi-elliptical surface cracks at weld toes, based on a database of stress intensity factors. Some examples of crack aspect development curves (CADC) are given for some typical cracked welded joints subjected to service loading and residual stress conditions. The significance for predicting fatigue life according to the natural crack growth path, namely along the CADC, is emphasized through examples.     

DEFECTS AND CRACK SHAPE DEVELOPMENT IN FILLET WELDED JOINTS

Abstract— Accurate knowledge of defect sizes and crack shape development from initiating defects is essential for a realistic application of fracture mechanic? to the fatigue life prediction of welded joints. Defects at the toes of high quality longitudinal non-load-carrying fillet welded joints have been examined using two methods, and compared with those observed in plain unwelded plate. The defects observed at the weld toe were, on average, four times deeper than the largest observed in the unwelded plate but had similar root radii. For this particular geometry the defects could be considered continuous at the expected initiation site. A simple estimate of the stress concentrating effect of a single composite defect, coupled with experimental evidence, led to the conclusion that the proportion of total life spent in initiation is negligible for all stress levels which cause fatigue failure. Subsequent growth and crack shape development was observed to depend on the initial defect distribution and the complex stress field at the weld toe.     

载荷方向和焊缝余高对氩弧焊缝疲劳性能的影响

采用疲劳寿命测试和观测断口方法,研究焊接方向和焊缝余高对TC2钛合金氩弧焊缝疲劳性能的影响。结果表明:同种焊接方向,保留焊高试件的疲劳寿命低于去焊高试件;同种焊缝余高处理方式,斜焊试件的疲劳寿命高于直焊试件。去焊高试件于气孔缺陷处萌生裂纹,保留焊高试件疲劳裂纹起源于焊趾。裂纹扩展初期,裂纹均在焊缝内扩展,有明显的疲劳条带;扩展后期,斜焊试件裂纹穿过焊缝进入母材,存在典型的韧性疲劳条带。直焊试件疲劳瞬断区韧窝少而浅;斜焊试件在母材瞬断,韧窝多且密。     

FATIGUE CRACK INITIATION LIFE PREDICTION FOR WELDED JOINTS BY LOW CYCLE FATIGUE APPROACH

Abstract— Analytical procedures based on low cycle fatigue theory are used to estimate the fatigue crack initiation life (N_i) for a cruciform welded joint in mild steel under constant amplitude tensile cyclic loading; the fatigue crack initiating at the weld toe. Effects due to welding such as residual stresses, geometrical variability and changes in material properties are handled. It is shown that for high mean stresses the discrepancies observed between the N _i estimates provided by commonly used analytical procedures exceed an order of magnitude. For the base metal (BM) the discrepancies become negligible if cyclic relaxation of notch mean stress is taken into consideration. The differences betwen the N _i estimates for heat affected zone (HAZ) material (where fatigue cracks at the weld toe usually initiate) and for BM are quantified. The applicability of HAZ material properties, estimated from hardness, to N _i prediction is evaluated.     

Macro and microscopic observations of fatigue crack growth in friction stir welded aluminum joints

This paper presents the macro and microscopic fractography performed on fractures from fatigue cracks through friction stir welded joints. The welds were placed under different angles in the various specimens to study the influence of the yield strength and residual stress on fatigue crack growth. As a result, different behavior was observed at the macro level, depending on the type of alloy and orientation of the weld. The variations in rotation of the crack plane raised a number of questions regarding the mode of loading, i.e. mode I or mode II. The purpose of this study was to investigate the fracture surfaces at microscopic level to find explanations for the local macro behavior. Special focus was placed on the fracture surfaces on which features were observed indicating mode II fatigue crack growth.     

Effects of ultrasonic impact treatment on pre-fatigue loaded high-strength steel welded joints

Comparative fatigue tests were conducted on as-welded, weld toe peened specimens before and after fatigue loading. Fracture surface, residual stress, microstructure and hardness were determined. The test results showed that as the pre-fatigue loading period extended, deeper cracks may have initiated and propagated and the fatigue life improvement decreased. The processes of ultrasonic peening on welded joints with existing cracks were modeled by finite element analysis. The numerical results indicated that the mechanism of UIT improving fatigue performance included two factors: compressive residual stress and the change of crack orientation. Both effects reduced as the crack became deeper.     

High and low cycle fatigue behavior of linear friction welded Ti–6Al–4V

Linear friction welded Ti–6Al–4V was investigated in fatigue at various stress amplitudes ranging from the high cycle fatigue (HCF) to the low cycle fatigue (LCF) regime. The base material was composed of hot-rolled Ti–6Al–4V plate that presented a strong crystallographic texture. The welds were characterized in terms of microstructure using electron backscatter diffraction and hardness measurements. The microstructural gradients across the weld zone and thermomechanically affected zone of the linear friction welds are discussed in terms of the crystallographic texture, grain shape and hardness levels, relative to the parent material. The location of crack nucleation under fatigue loading was analyzed relative to the local microstructural features and hardness gradients. Though crack nucleation was not observed within the weld or thermomechanically affected zones, its occurrence within the base material in LCF appears to be affected by the welding process. In particular, by performing high resolution digital image correlation during LCF, the crack nucleation site was related to the local accumulation of plastic deformation in the vicinity of the linear friction weld.     

Measuring fatigue cracks in fillet welded joints

Accurate measurement of short (<1 mm depth) elliptical fatigue cracks that grow from the toes of fillet welds has proved to be an obstacle to the application of fracture mechanics principles to welding fatigue. This paper reports a DC potential drop technique which allows continuous measurement of the depth of such elliptical cracks. A delicate compromise between sensitivity and accuracy, combined with superior electrical stability displayed by the measurement apparatus, has allowed detection of: 1 — crack growth less than 0.01 mm and; 2 — crack growth rates less than 10^?7 mm/cycle for cracks less than 1 mm deep.Preliminary results have indicated the relative importance of stress ratio, defect size and material variation on the growth of these short elliptical cracks. When the weld toe is subject to high stress ratios the phenomenon may be considered propagation dominated whereas low stress levels increase the influence of threshold and initiation mechanisms.