连续退火与连续热镀锌590MPa级双相钢的疲劳性能

采用拉-拉疲劳实验研究了连续退火与连续热镀锌590MPa级冷轧双相钢的疲劳性能,绘制了它们的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,连续退火与连续热镀锌冷轧双相钢在应力比R=0.1,加载频率为15Hz的条件下的疲劳极限分别为217和216MPa,通过对疲劳试样表面的观察发现,表面铁素体/马氏体界面开裂是疲劳失效的主要原因。     

等静压铍材高周疲劳寿命研究

采用拉-拉加载方式对等静压铍材进行高周疲劳试验,根据试验数据绘制了S-N曲线,获得其疲劳极限,并运用扫描电镜观察疲劳断口形貌。结果表明,等静压铍材在指定疲劳寿命10⁷次下的疲劳极限为385 MPa;疲劳裂纹萌生于铍材试样边缘处,裂纹穿过晶界与相邻晶粒内的微裂纹连接合并长大;铍材试样发生瞬时解理断裂,形成主要由疲劳源和放射区构成的断口形貌;疲劳源尺寸随最大循环应力的减小而增大。     

CSP流程Ti微合金化高强钢的疲劳性能

采用升降法对CSP工艺生产的2mm厚Ti微合金化高强钢的疲劳性能进行研究.结果发现：高强钢的抗拉强度为830 MPa;疲劳强度为685 MPa,约为抗拉强度的0.83倍;伸长率为18.8%.绘制了高强钢的S-N曲线,并拟合出疲劳寿命与最大应力的关系.通过扫描电镜对疲劳断裂机理进行了分析.宏观疲劳断口可见明显的裂纹源区、扩展区和瞬断区形貌.疲劳裂纹起始于带钢表面微裂纹;疲劳扩展区存在微观疲劳辉纹、二次裂纹和宏观疲劳贝纹线;瞬断区出现撕裂棱,兼有韧窝存在.     

Q235B镀锌抱箍断裂失效分析

某工程腕臂底座镀锌Q235B抱箍在使用一段时间后一端加固件焊接附近发生断裂,对断裂件进行金相组织、化学成分和力学性能测试,以及断口宏观和微观形貌分析。结果表明:该抱箍为疲劳断裂,疲劳源区严重氧化,源区材料基体中也含有Zn元素,说明疲劳源区的裂纹产生于热镀锌工艺之前,腕臂底座抱箍在实际使用过程中承受交变载荷,基体材料原有微裂纹在该交变载荷下扩展,最终导致疲劳破坏和断裂失效。     

加热段露点对镀锌高强钢产品抑制层形貌的影响

为研究加热段露点温度对镀锌高强钢产品抑制层形貌的影响,通过600MPa级热镀锌双相钢产品在连续热镀锌线上于不同的加热段露点温度下退火,采用SEM、EDS对锌层形貌、抑制层成分进行分析。研究结果表明,在-30℃~15℃露点温度范围内加热时抑制层颗粒致密完整,当露点在0~15℃时,抑制层更薄更致密,颗粒尺寸约为0.1μm,锌层的附着性更好。     

1000MPa热镀锌双相钢组织和断裂机理

模拟热镀锌工艺,在实验室生产了1000MPa级热镀锌双相钢.利用原位拉伸实验,对其断裂行为进行了观察,进一步探讨了其断裂机理.结果表明:实验用钢经820℃退火后,可以获得抗拉强度为1022MPa、延伸率为9.5%的F+M双相钢;动态拉伸过程中,裂纹尖端的塑性区会萌生新的微裂纹,塑性区内的铁素体晶粒内部会产生\     

400 MPa级薄钢板的疲劳性能及断口分析

 利用液压伺服疲劳实验机研究了400 MPa级板带在对称循环应力下的高周疲劳性能,测量了S N曲线,确定了应力幅低于疲劳强度时疲劳寿命与应力幅之间的关系式;用扫描电镜观察该钢的疲劳断口,分析了疲劳断裂特点;在透射电镜下观察了断口附近的微观组织,并对其疲劳断裂机理进行了分析,结果表明：疲劳裂纹起源于表面驻留滑移带挤出脊、侵入沟或表面缺陷,裂纹源扩展最终导致断裂。     

铝硅镀层对淬火热成形钢疲劳性能的影响

本文以淬火后的镀铝硅热成形钢T1500HS+AS为研究对象,对淬火后板材的镀层、组织及拉伸性能进行了检验。随后采用磨抛去除镀层的方法获得热成形后的裸板,对镀层板和裸板的疲劳性能进行了检验,分析镀层板和裸板的性能差异,同时对两种样品的疲劳断口进行了扫描分析,以阐释二者之间产生差异的原因。试验结果表明：淬火T1500HS+AS镀铝硅热成形钢的镀层厚度约为36微米,淬火后镀层表面会形成大量微裂纹,大部分裂纹终止在过渡层,也有部分裂纹穿过过渡层接近基体组织。铝硅镀层的能谱分析表明铝在镀层中含量最多,分布于整个镀层,硅元素在镀层的次表层和扩散层中最广,由于元素的扩散作用,镀层中也含有一部分铁元素。镀层中存在明显的岛状析出相,其成分主要是硅和铁。淬火后板材强度达到了1 500 MPa级别,同时具有较高的屈服强度1 087 MPa,断后延伸率在5%左右。疲劳测试表明镀铝硅板的疲劳性能低于裸板,断口分析发现镀层板的裂纹源存在两处,一处为镀层表面裂纹形成的裂纹源,另一处为基体内部夹杂物形成的裂纹源,而裸板的裂纹源则仅为基体内部夹杂物形成的裂纹源。淬火镀层裂纹会向基体扩展,由于镀层裂纹的存在,增加了裂纹源...     

B_4C/6061铝基复合材料疲劳性能及断裂机制

基于轻质、高强和耐磨等诸多优势,铝基碳化硼复合材料已成为集结构/功能一体化的新型材料。本文采用粉末冶金及轧制方法,制备出厚度3.5 mm、碳化硼质量分数为33%的B4C/Al复合材料板材,并对其疲劳性能和断裂机制进行分析。在1×107循环次数下,铝基碳化硼复合材料板材的疲劳强度达到110 MPa。采用SEM对疲劳断口进行观察,结果表明B4C/Al复合材料疲劳断口可清楚的看到裂纹的萌生、扩展和失稳断裂的典型特征,但存在多种形式的疲劳启裂源。疲劳裂纹扩展路径取决于裂纹尖端塑性区的半径和B4C颗粒的间距大小,当增强颗粒的间距小于塑性区半径时,裂纹主要沿着颗粒的连接界面或断裂的碳化硼颗粒扩展,当增强颗粒的间距大于塑性区半径时,有利于裂纹尖端钝化,减缓裂纹的扩展和方向改变。     

TC4-DT钛合金高周疲劳行为研究

对TC4-DT钛合金的高周疲劳性能及断口形貌进行了研究。结果表明：TC4-DT钛合金的S-Ⅳ曲线（应力比R=-1）不出现呈水平线的疲劳极限,10^7次不被破坏的条件疲劳极限为550MPa,置信度为95％;疲劳裂纹源均出现在试样的表面,疲劳裂纹扩展区较大说明材料具有较高的断裂韧度;疲劳裂纹扩展区由许多解理小刻面组成,解理面上可见疲劳条带及二次裂纹,以解理断裂为主;断裂区断口表面由许多互相连接的凹坑所组成,主要表现为韧窝断裂。     

一种单晶高温合金不同温度的高周疲劳性能

研究了一种单晶高温合金700 ℃和800 ℃的高周疲劳性能，采用扫描电镜和透射电镜分析了断口和断裂机制.结果表明，随着温度升高，合金的疲劳强度系数降低，Basquin系数增加，高周疲劳极限降低.合金700 ℃与800 ℃具有相同的高周疲劳断口，都有几个{111}面平面组成，为类解理断裂机制.疲劳断口由裂纹源区、扩展区和瞬断区3部分组成.裂纹起源于试样的表面或亚表面，并沿{111}面扩展.扩展区可见河流状花样、滑移带、疲劳弧线和疲劳条带特征.瞬断区可见解理台阶和撕裂棱.断裂后γ′相仍保持立方形状，位错不均匀分布在γ基体通道中.      

Fatigue crack propagation near fatigue threshold in various structural steels

The investigations have been conducted by measuring fatigue crack propagation near fatigue threshold in various structural steels differing in chemical composition and strength level. The fatigue crack propagation measurements were carried out using the constant-load-amplitude test in Paris-region, R-constant and K_max-constant method in near fatigue threshold region. Scanning electron microscopy at fatigue crack front on fracture surface was applied to interpret the influence of crack closure effects on the measured fatigue threshold. Marked fretting oxide deposits distributed on the fracture surface at threshold level were observed in a low load ratio resulting from the combined action of plasticity- and oxide-induced crack closure under laboratory atmosphere. Fatigue threshold dependent on the load ratio appeared to be related to the extent of the crack closure effect. By considering the relationship of reversed plastic zone size and grain size the fatigue threshold in region of crack closure was calculated theoretically. The result has shown a good agreement with the experimentally measured values.     

Titanium alloy fatigue fracture facet investigation by selected area electron channeling

Flat regions (facets) found on fracture surfaces caused by initiation and propagation of fatigue cracks through the titanium alloys IMI-685 tested with and without a 5 min load dwell and Ti-11 have been examined using selected area electron channeling. The crystallographic planes of the facets have been identified as being near basal for the IMI-685 and more random for the Ti-11. None of the dwell specimens showed a pure basal facet orientation. The plastic zone size was also assessed and found to correlate approximately with stress intensity, allowing confirmation of crack initiation sites as determined by fractography. One case of beta to alpha-beta transformation crystallography is also examined. A partial channeling map for titanium is presented.     

15Cr14Co12Mo5Ni2齿轮钢的扭转疲劳特性及裂纹扩展行为

 通过扭转疲劳试验,研究了15Cr14Co12Mo5Ni2钢的扭转疲劳断裂的裂纹扩展行为和夹杂物尺寸与扭转疲劳寿命之间的关系。得到了钢的扭转疲劳极限强度和[τ-N]曲线,15Cr14Co12Mo5Ni2钢的扭转疲劳极限强度为350 MPa,扭转疲劳寿命分散度较大。通过断口观察,发现15Cr14Co12Mo5Ni2钢的疲劳破坏模式以表面破坏和近表面破坏为主,主要由氧化物夹杂引起。通过计算应力强度因子[ΔK]和裂纹扩展门槛值[ΔKth]分析15Cr14Co12Mo5Ni2钢的疲劳裂纹扩展的断裂力学条件,试验钢在断裂过程中受载荷情况为,II型载荷—I型载荷—II型载荷—I＋II型载荷,分别对应起裂源区、纤维区、疲劳裂纹扩展区和瞬断区;当有大裂纹产生时,则不会产生纤维区,受载荷情况则为：II型载荷—I＋II型载荷。通过公式推导和数据拟合得到夹杂物尺寸和15Cr14Co12Mo5Ni2钢扭转疲劳寿命的关系,发现随着夹杂物尺寸减小,钢的[τ-N]曲线向高寿命区移动。当引起裂纹萌生的夹杂物尺寸小于5 μm时,在350 MPa应力下,15Cr14Co12Mo5Ni2钢的扭转疲劳寿命超过107循环周次。     

Crack tip area in fatigued copper single crystals

The area ahead of the crack tip in single crystals of copper fatigued in pull-pull was studied using optical and electron microscopy techniques. Through microhardness measurements, it was found that a zone of higher hardness near the fatigue crack tip corresponds roughly to the size of the fatigue crack plastic zone calculated from fracture mechanics criteria. Electron microscopy observations revealed a distinct dislocation structure associated with this zone of higher hardness. Graduate Research Assistant, Materials Science Department, Northwestern University, Evanston, III. 60201 This paper is based upon a portion of a thesis submitted by A. H. Purcell in partial fulfillment of the requirements of the Ph.D. degree at Northwestern University.     

钛铝异质自冲铆接头疲劳性能及失效机理

对纯钛(TA1)和铝锂合金(AL1420)异质单搭自冲铆接头进行静力学实验和疲劳实验研究,通过三参数经验公式采用S-N曲线拟合法绘制接头的S-N曲线,分析接头的疲劳性能;采用扫描电子显微镜对接头疲劳断口进行观测,研究接头的微观疲劳失效机理.结果表明:接头的静力学性能与疲劳性能不具有一致性,TA1-AL1420(TA)接头静力学性能更优,但其疲劳性能比AL1420-TA1(AT)接头差;TA接头疲劳失效形式为下板断裂,疲劳裂纹萌生于靠近铆钉脚处的板材区域,随后沿着板宽方向扩展,最终导致下板完全断裂.AT接头在短寿命区因铆钉断裂失效,铆钉的断口属于脆性疲劳断裂;在中长寿命区出现铆钉断裂和下板断裂的混合失效形式,疲劳裂纹从下板一端萌生,沿着板宽向另一端方向扩展,导致下板断裂失效.     

22Si2MnCrNi2MoA钎杆断裂失效分析

运用金相观察、扫描电镜观察和能谱定量分析等实验手段,从组织状态、夹杂物、断口形貌等方面分析了22Si2MnCrNi2MoA钎杆内螺纹处断裂原因,同时对其疲劳裂纹起源和扩展进行了探讨.22Si2MnCrNi2MoA钎杆内螺纹处断裂破坏并不是由组织异常和夹杂物引起的,而是由于22Si2MnCrNi2MoA钎杆存在明显的壁厚不均,在高频应力、严重的内外耗同时存在的应力状态下持续工作,壁厚较薄处极易成为受力薄弱区,疲劳裂纹更倾向于在此处优先形成,从而致使壁厚较薄处优先断裂,最终导致钎杆断裂失效.该钎杆疲劳破坏起源于内表面,属于多源的疲劳断裂.起源区微观形貌为韧窝形貌,扩展区的微观形貌为韧窝和沿晶的混合形貌.     

A micromechanical model for cleavage-crack reinitiation

A crack-tip screening analysis of cleavage fracture of steel is developed. The analysis incorporates evidence that reinitiation of an arrested cleavage crack requires less stress intensity than cleavage initiation from a fatigue precrack. Fractographic evidence as well as metallographic sectioning of arrested cracks have previously shown that the mechanism of rapid crack propagation by cleavage is affected strongly by partial crack-plane deflection which leaves unbroken ligaments in its wake. The tearing of these ligaments by dimple rupture is the dominant energy-absorbing mechanism. Earlier etch-pit experiments using an Fe-Si alloy showed that the crack-tip stress intensity based on plastic zone size is extremely low. These observations are incorporated into a model in which cleavage crack reinitiation is analyzed using a sharp crack that is shielded by a distribution of pinching forces along its faces. During reloading of the arrested crack, the ligaments restrict crack-tip blunting, leading to higher local stresses. As a result, lower stress intensities are needed for reinitiation than for initiation from a fatigue precrack.     

A micromechanical model for cleavage-crack reinitiation

A crack-tip screening analysis of cleavage fracture of steel is developed. The analysis incorporates evidence that reinitiation of an arrested cleavage crack requires less stress intensity than cleavage initiation from a fatigue precrack. Fractographic evidence as well as metallographic sectioning of arrested cracks have previously shown that the mechanism of rapid crack propagation by cleavage is affected strongly by partial crack-plane deflection which leaves unbroken ligaments in its wake. The tearing of these ligaments by dimple rupture is the dominant energy-absorbing mechanism. Earlier etch-pit experiments using an Fe-Si alloy showed that the crack-tip stress intensity based on plastic zone size is extremely low. These observations are incorporated into a model in which cleavage crack reinitiation is analyzed using a sharp crack that is shielded by a distribution of pinching forces along its faces. During reloading of the arrested crack, the ligaments restrict crack-tip blunting, leading to higher local stresses. As a result, lower stress intensities are needed for reinitiation than for initiation from a fatigue precrack.     

Local fatigue damage accumulation around notch attending crack initiation

The subsequent recrystallization technique was used to study the process of local damage accumulation around a notch under conditions of low-cycle fatigue. A 0.8-in. compact tension specimen of 304 stainless steel with a notch radius of 1 mm was used. The accumulated plastic zone around notch increases with the number of cyclesN. The accumulated plastic strain within the zone also increases withN, producing the strain gradient (damage gradient). A fatigue crack initiates when the accumulated plastic strain at the notch root reaches a critical value equal to the fracture strain of the material; that is, when the accumulated plastic work at the crack initiation site becomes critical. The fatigue crack emanating from a notch root grows through the pre-existing damaged zone. It is shown that this local damage accumulation approach can explain the fast growth of a short crack from a notch.