TA1钛合金单搭自冲铆接头微动磨损机理

对TA1钛合金单搭自冲铆接头进行疲劳实验研究接头失效形式;用扫描电子显微镜和X射线能谱线扫描研究铆钉各部位微动磨损程度的差异和接头微动磨损机理;采用威布尔分布验证数据有效性。结果表明:接头疲劳失效形式主要为上板断裂,高周疲劳均为上板断裂,低周疲劳为上下板混合断裂;微动磨屑包含氧、钛、锌和锡元素,铆钉头部微动磨损程度高于铆钉腿部。微动磨损区出现严重脱层、微动磨屑堆积和微裂纹萌生等现象,随着微动磨损及剪切力共同作用导致接头断口部位出现大量微裂纹并逐步沿深度和宽度方向扩展为宏观裂纹,最终导致接头疲劳失效。     

TA1钛合金单搭自冲铆接头微动磨损机理

对TA1钛合金单搭自冲铆接头进行疲劳实验研究接头失效形式;用扫描电子显微镜和X射线能谱线扫描研究铆钉各部位微动磨损程度的差异和接头微动磨损机理;采用威布尔分布验证数据有效性.结果表明:接头疲劳失效形式主要为上板断裂,高周疲劳均为上板断裂,低周疲劳为上下板混合断裂;微动磨屑包含氧、钛、锌和锡元素,铆钉头部微动磨损程度高于铆钉腿部.微动磨损区出现严重脱层、微动磨屑堆积和微裂纹萌生等现象,随着微动磨损及剪切力共同作用导致接头断口部位出现大量微裂纹并逐步沿深度和宽度方向扩展为宏观裂纹,最终导致接头疲劳失效.     

轻合金自冲铆微动磨损及疲劳性能研究

选择4组轻合金自冲铆进行疲劳实验,用扫描电子显微镜和能谱仪对其断口进行微动磨损机理分析,并系统地研究了接头疲劳寿命和失效形式的影响因素。结果表明,下板与钉腿区的微动磨损是导致下板沿纽扣断裂和铆钉断裂的主要原因,两板间的微动磨损是导致上板靠钉头断裂的主要原因;微动磨屑主要成分为金属板材氧化物,并对微动磨损起缓冲作用。增加板厚可提高接头疲劳寿命,且疲劳载荷较大时寿命提高更为显著;增加板强可提高接头疲劳寿命,且寿命提高程度受疲劳载荷影响较小。增加板厚使失效形式从上板断裂变为下板断裂,增加板强使失效形式从板材断裂变为铆钉断裂。     

铝合金自冲铆接微动行为研究

选择两组不同厚度铝合金板制成SA15和SA25自冲铆接头并进行疲劳实验,用威布尔分布验证实验数据有效性,用疲劳三参数经验公式拟合接头S-N曲线并对比分析接头疲劳性能,用扫描电子显微镜研究接头微动行为以探索疲劳失效与微动磨损的关系。实验数据服从威布尔分布,表明数据可靠。三参数经验公式较好地拟合接头S-N曲线,SA25接头疲劳性能优于SA15接头,且在中低疲劳寿命区域SA25接头疲劳性能的优越性更为突出。SA15接头失效于上板靠铆钉头,而SA25接头失效于下板沿纽扣。SA15接头在铆接区两板间微动磨损较严重,SA25接头在下板与铆钉腿部微动磨损较严重。疲劳失效部位与微动磨损剧烈部位重合,表明微动磨损是导致接头疲劳失效的重要因素之一。     

TA1纯钛与1420铝锂合金异质薄板自冲铆接微动疲劳特性

基于自冲铆接技术研究TA1工业纯钛(TA1)与1420铝锂合金(AL1420)异质薄板组合的可铆性,并采用硬度≥44HRC和≥46HRC的铆钉分别制备TA1/AL1420 (TAF),AL1420/TA1(ATF)和TA1/AL1420(TAS) 3组接头。在拉伸-剪切实验的基础上进行了高周疲劳实验,拟合出各组接头的F-N曲线,进一步利用电子扫描显微镜和能谱仪分析了各组接头的微动磨损机理。结果表明:ATF接头的疲劳强度相对较优,TAS接头则在中高载荷水平下优于TAF接头;各组接头的疲劳失效均始于微动磨损区域,微动磨损导致疲劳裂纹的萌生;微动磨屑现象的剧烈程度是影响疲劳强度的重要因素。在不同疲劳载荷水平下,发生剧烈微动磨损的区域不同,使得疲劳裂纹的萌生区域存在差异,最终导致同种接头出现不同的失效模式。     

钛合金压印接头疲劳性能与微观分析

压印连接是近年来新兴的连接方式,因其具有简单高效、低耗环保等优点,在连接应用方面越来越受到重视,而疲劳破坏是机械零件失效的主要形式。对钛合金压印接头的疲劳性能进行了实验研究,与母材的力学性能进行了对比分析,并对疲劳失效断口进行了断口分析和能谱分析。实验结果显示钛合金压印接头的平均拉伸-剪切强度约为同等尺寸材料拉伸-剪切强度的36.7%;钛合金压印接头的疲劳极限约为材料疲劳极限的46%,平均载荷约为接头最大静强度的42%。微观特征显示断口呈脆性疲劳断裂特征,由于微动磨损和氧化作用产生了成分为氧化钛的微动磨屑,且其硬度较高,因此在微动过程中起到磨粒的作用,从而加速了磨损和裂纹扩展,最终导致疲劳失效。     

局部热处理对钢-钛自冲铆接头力学性能的影响

对钢-钛自冲铆接头进行局部热处理,并对已热处理和未热处理的接头进行拉剪实验,对比分析局部热处理对钢-钛自冲铆接头的静强度、能量吸收、失效模式和失效机理的影响。结果表明:淬火对钢-钛自冲铆接头的静强度有很大提高作用,对其能量吸收有减小作用;未淬火的钢-钛自冲铆接头失效模式为上板与铆钉分离且上板被撕裂,上板的断口形貌为典型韧性断裂特征;淬火后的钢-钛自冲铆接头失效模式为上板与铆钉分离且铆钉被拉断,铆钉的断口形貌为典型的沿晶断裂特征。     

TA1异质自冲铆接头力学性能及失效机理

通过自冲铆接对比试验获得接头最优铆接参数,并以此制备TA1钛合金板分别与Al5052铝合金板和H62铜合金板的异质自冲铆接头。通过静力学实验和疲劳实验研究异质接头的力学性能,并运用疲劳三参数经验公式拟合S-N曲线,最后利用扫描电镜和能谱仪进行断口分析和能谱分析进而研究接头的疲劳失效机理。结果表明,TA1-H62(STH)接头静失效载荷优于TA1-Al5052(STA)接头;且前者在低载荷下疲劳寿命优于后者,STA接头则在高载荷下优势明显。STA接头疲劳失效模式为下板断裂,STH接头则出现了两种失效模式;两板间及铆钉与上下板之间接触区域发生的剧烈微震磨损是导致疲劳裂纹萌生的主要原因。     

泡沫金属夹层板自冲铆接头的疲劳性能及失效机理

对铝合金自冲铆接头及泡沫金属夹层结构自冲铆接头进行疲劳实验,通过三参数经验公式采用S-N曲线拟合法绘制接头的F-N曲线,分析接头的疲劳寿命及泡沫金属夹层对自冲铆接头疲劳性能的影响;采用扫描电子显微镜对接头的疲劳失效断口进行观测,分析其微观失效机理。结果表明:泡沫金属夹层缩短了自冲铆接头的疲劳寿命,不同泡沫金属夹层对自冲铆接头疲劳性能的影响具有差异性,在高疲劳载荷下泡沫铜夹层接头疲劳性能更优。三组接头疲劳失效形式都为下板断裂,在高疲劳载荷下裂纹易在铆扣区域萌生,在中低疲劳载荷下裂纹萌生于下板一侧,沿铆扣区域下侧向板材另一侧扩展。     

18CrNiMo7-6合金钢的弯曲微动疲劳特性

对18CrNiMo7-6合金钢进行弯曲微动疲劳实验,建立弯曲微动疲劳S-N曲线,并对实验结果进行分析。结果表明:该合金钢的弯曲微动疲劳S-N曲线不同于中碳钢材料,也不同于常规弯曲疲劳,而是呈"ε"型曲线特征。随着弯曲疲劳应力的增加,微动运行区域由部分滑移区向混合区和滑移区转变,损伤区的磨损机制以剥层、磨粒磨损和氧化磨损为主。在混合区内,裂纹最易萌生和扩展,且裂纹均萌生于材料接触区次表面。受接触应力和弯曲疲劳应力影响,弯曲微动疲劳裂纹的萌生和扩展可分为三个阶段:初期,在接触应力控制下,裂纹萌生于次表面;随后,裂纹受接触应力和弯曲疲劳应力共同控制,转向更大角度方向扩展;最后,裂纹完全受弯曲疲劳应力控制而垂直于接触表面扩展,直至断裂失效。     

Experimental investigation of crack growth direction in multiple cracks

In this paper, the interaction between multiple cracks in crack growth direction is studied in an aluminium alloy under static and fatigue loading. Self similar as well as non‐self‐similar crack growth has been observed which depends on the relative crack positions defined by crack offset distance and crack tip distance. On the basis of experimental observations, the criterion for crack coalescence and crack growth direction are expressed in terms of the crack positions defined by crack offset and crack tip distances. The criterion presented in this study can be used to determine the limiting value of crack tip and crack offset distance and to determine the mode in which cracks coalesce during their growth process. Experimental results and crack interaction criterion presented under various crack positions and size conditions could be used to derive a new evaluation method of crack growth in multiple crack geometry.     

Experimental study of I + III mixed mode fatigue crack transformation propagation

In this paper, compact tension specimens with tilted cracks under monotonic fatigue loading were tested to investigate I + III mixed mode fatigue crack propagation in the material of No. 45 steel with the emphasis on the mode transformation process. It is found that with the crack growth, I + III mixed mode changes to Mode I. Crack mode transformation is governed by the Mode III component and the transformation rate is a function of the relative magnitude of the Mode III stress intensity factor. However, even in the process of the crack mode transformation the fatigue crack propagation is controlled by the Mode I deformation.     

A study of the propagation of an embedded crack in a composite laminate of finite thickness

In this paper, the mechanism for the propagation of a crack embedded in the central layer of a composite laminate is studied based on energy criteria. As the crack may propagate in the transverse direction or in the tunneling direction, the critical stresses for these two propagations are presented. The theoretical analysis is applied to a fibre-reinforced composite laminate. Critical stresses for the initiation of crack propagation are calculated taking into account the influence of the adjacent ply angle and the crack size. It is found that for the considered typical fibre-reinforced composite laminate, the critical stress for the transverse propagation is less than that for the tunneling propagation. Therefore, transverse cracking is a more possible fracture mode in composite laminates with initial crack-like defects. This finding supports the calculation of the in situ transverse strength based upon the transverse crack propagation.     

Path prediction of I + III mixed mode fatigue crack propagation

In this paper, compact tension specimens with tilted cracks under monotonic fatigue loading were tested to investigate I + III mixed mode fatigue crack propagation in the material of No. 45 steel with the emphasis on the propagation rate expression and the path prediction. It is found that during the mode transformation process, the crack propagation rate is still controlled by the mode I stress intensity factor; and Paris equation also holds for the relationship between and ΔK_I . Crack propagation path can be predicted only when both the crack mode transformation rate and propagation rate are available.     

A537钢疲劳裂纹扩展单次拉伸超载效应及裂尖形变,裂纹闭合行为

研究了恒定ΔＫ条件下，单次拉伸超载对Ａ５３７钢疲劳裂纹扩展速率的影响，并利用激光散斑技术原位研究超载前后的裂尖应变场，裂纹闭合效应。结果表明：超载后裂纹闭合效应呈增强趋势，裂尖应变呈下降趋势。伸超载有阻滞裂纹扩展的作用。     

Characteristics of fatigue crack propagation behaviour as identified by hysteresis loop at the crack tip

Fatigue crack propagation tests have been carried out under various load conditions. Hysteresis loops denoting the relationship between load and strain at the crack tip are obtained by using local compliance measurement. Crack growth acceleration, delayed retardation and non‐propagation phenomena are investigated by considering the variation of hysteresis loop expansion and hysteresis loop tail. Based on the physical meaning of hysteresis loops, two types of crack closure are ascertained and the effect of crack closure on fatigue crack propagation is studied. Results show that change of the effective amplitude of the stress intensity factor at the crack tip is the reason that crack propagation rates vary.     

Effect of crack closure on non-linear crack tip parameters

Crack closure concept has been widely used to explain different issues of fatigue crack propagation. However, some authors have questioned the relevance of crack closure and have proposed alternative concepts. The main objective here is to check the effectiveness of crack closure concept by linking the contact of crack flanks with non-linear crack tip parameters. Accordingly, 3D-FE numerical models with and without contact were developed for a wide range of loading scenarios and the crack tip parameters usually linked to fatigue crack growth, namely range of cyclic plastic strain, crack tip opening displacement, size of reversed plastic zone and total plastic dissipation per cycle were investigated. It was demonstrated that: (i) LEFM concepts are applicable to the problem under study; (ii) the crack closure phenomenon has a great influence on crack tip parameters decreasing their values; (iii) the ΔK_eff concept is able to explain the variations of crack tip parameters produced by the contact of crack flanks; and (iv) the analysis of remote compliance is the best numerical parameter to quantify the crack opening level. Therefore the crack closure concept seems to be valid. Additionally, the curves of crack tip parameters against stress intensity factor range obtained without contact may be seen as master curves.     

Examination of the fatigue crack growth equations

Most of the crack growth equations proposed so far correlate the crack growth rate (da/dN or da/dt) with crack tip parameters such as the stress intensity factor (SIF) or energy release rate (ERR). In our previous works, an experimental setup was designed to examine the applicability and the boundary of the functional relationship between da/dN and the crack tip parameters, particularly, ERR. In the present paper, the variation of the ERR along the experimentally observed curvilinear crack trajectories is obtained by means of the finite element method. The analysis shows that the Paris-Erdogan type of laws are applicable until the crack tip is located outside the strong crack-defect interaction region (SI region). A functional relationship between da/dN and ERR breaks down within this region. This suggests the existence of additional crack tip parameters that are not accounted for within conventional fracture mechanics. An approach to modeling the observed phenomenon is discussed following the concept of the Crack Layer theory.     

A numerical study on the effect of symmetric crack flank holes on fatigue life extension of a SENT specimen

The crack growth retardation is studied theoretically by drilling two symmetric holes at appropriate locations along the crack flanks. A fatigue crack growth code for two‐dimensional elastic problems is developed to approve the effectiveness of this method. The crack growth retardation was examined using a parametric study both on the arrangements of the flank holes and the diameters of holes. The numerical results reveal that for some cases, the presence of flank holes significantly decreases the stress concentration around the crack tip and its stress intensity factor. The best positions for crack flank holes to provide the highest crack growth retardation are shown to be where the line connecting the two centres of holes passes the crack tip. A larger hole diameter and a closer distance between the flank holes and the crack tip result in lower crack growth rate and higher fatigue life.