缺口应力集中系数对TC4 ELI合金低周疲劳性能的影响

研究了缺口应力集中系数不同的深海潜水器耐压壳用TC4 ELI(Extra-low-interstitial)合金板材在恒总应变幅控制下的低周疲劳行为。结果表明,在应变幅较低(0.7%以下)和应变幅较高(0.8%和0.9%)条件下的光滑试样在循环初期分别发生了循环硬化和循环软化,而缺口试样在0.2%～0.7%应变幅条件下的循环初期均发生了循环硬化。通过循环载荷作用下材料滞回能的变化描述了TC4 ELI合金试样低周疲劳的损伤程度,得到了缺口应力集中系数与低周疲劳性能参数之间的关系,建立了相对裂纹萌生寿命预测模型。利用该模型能较好地预测缺口应力集中系数较低的TC4 ELI合金在高应变幅条件下的相对疲劳裂纹萌生寿命。     

考虑循环塑性修正的薄片材料低周疲劳试验方法

对N18锆合金和SAPH440板材采用薄片漏斗试样进行了室温下的等幅低周疲劳试验,基于局部应力、应变相等的疲劳损伤等效原理,建立了用于估算N18和SAPH440材料疲劳寿命的Manson-Coffin模型和循环应力-应变方程。对于符合Masing律的材料,提出了一种获得循环本构关系的新方法,用转子材料B65A-S在300℃高温下的低周疲劳试验进行了验证。通过基于标定循环本构关系的有限元分析,建立了两种材料薄片漏斗试样平均应力到漏斗根部轴向应力、测试应变到漏斗根部轴向应变的转换模型。完成了SAPH440材料等直圆棒试样的低周疲劳试验,证明了采用薄片漏斗试样的低周疲劳测试新方法具有较高的精度。     

Ti-2Al-2.5Zr合金的高温低周疲劳行为

研究了Ti-2Al-2.5Zr合金的高温(673K)低周疲行为,结果表明:Ti-2Al-2.5Zr合金的循环变形表现出初始硬化、随后软化和最后二次硬化的特征。在高温和较高应变幅((Δε_t/2)1.0%)条件下,材料的疲劳寿命高于室温下的疲劳寿命。随着应变幅的提高,高温673 K材料疲劳变形后形成的位错组态由位错墙逐渐演变成成熟的位错胞。多系滑移开动和塑性变形均匀性增强是高温Ti-2Al-2.5Zr合金疲劳寿命提高的原因。     

应变速率对一种无铼单晶高温合金低周疲劳性能的影响

研究了一种无铼镍基单晶高温合金在1223 K、不同应变速率(5×10^-4s^-1、1×10^-3s^-1、5×10^-3s^-1、1×10^-2s^-1)条件下的低周疲劳行为。结果表明:在四种应变速率条件下,合金均表现出循环稳定。随着应变速率的增加,合金的疲劳寿命逐渐增加,且其半寿命稳定滞后回线环内面积逐渐减少,表明低应变速率合金更容易积累蠕变塑性变形。疲劳裂纹源均萌生于试样表面,随着应变速率的增加,疲劳过程中产生的塑性变形越来越少,疲劳裂纹扩展区的面积逐渐增大。低应变速率时,较大的塑性变形导致合金取向发生明显的偏转,诱发多滑移系开动进而形成位错网;反之,高应变速率时,合金没有产生明显的塑性变形,只有单一方向的位错塞积形成位错束。     

漏斗试样扭转低周疲劳寿命预测方法研究

针对传统圆棒试样扭转低周疲劳存在的问题,采用漏斗形试样对材料C250马氏体钢(Cr强化)进行4种温度(20,150,200,350℃)、两种应变幅比(-1、0.1)下的扭转低周疲劳试验,得到一系列S-N曲线。对比分析MansonCoffin模型、拉伸滞后能损伤模型以及三参数幂函数模型3种低周疲劳寿命预测模型,评述其基本假设、应用范围和特点。应用3种模型分别对上述8种工况的试验结果进行寿命预测对比分析,并对比分析温度对扭转低周疲劳寿命的影响。结果显示:三参数幂函数公式能更好地拟合试验结果,明显克服另外两种预测模型的缺陷。因此,对于扭转低周疲劳寿命的预测宜采用三参数幂函数模型,其预测结果更接近真实值。     

船用Ti-31钛合金的低周疲劳特性

研究了Ｔｉ ３１合金的疲劳饱和特性、过渡疲劳寿命及疲劳断裂性能。研究结果表明,Ｔｉ ３１合金是疲劳饱和型材料,其应变过渡疲劳寿命２Ｎｆ＝７５８次,其疲劳断裂呈典型的韧性特征。     

TA17合金薄片材料毫小试样疲劳性能研究

关键工程结构、小尺寸零部件和焊接区的疲劳寿命评估中往往无法采用传统大试样进行疲劳试验,因此本文提出了一种采用毫米级别薄片试样获取材料循环本构关系和低周疲劳寿命的新方法。在Care原位试验机上完成毫米级别薄片漏斗试样的加载工装和低周疲劳试验的基础上,通过变幅对称循环试验和等辐循环试验分别实现了材料循环本构关系和低周疲劳性能的获取。该文提出了一种对不同幂律材料和不同几何尺寸构型均具有良好普适性的材料循环本构关系预测模型,并通过有限元实现了模型准确性的正反向预测验证。将循环本构关系用于有限元计算中,给出了薄片漏斗试样漏斗两侧名义应力、名义应变和漏斗根部真实应力、真实应变的转换方程,进而预测材料的低周疲劳寿命。该文完成了TA17合金等直圆棒试样和1.2 mm厚度薄片漏斗试样的对称变幅循环试验和多级等辐循环试验。由模型预测获得的TA17合金循环本构关系与等直圆棒试样的试验结果比较表明：两种曲线的弹性段和0.009 mm/mm~0.011 mm/mm应变段吻合良好,在弹塑性过渡段（0.004 mm/mm~0.009 mm/mm）模型预测结果最大相对误差小于9%。根据两组应力和应变转换方程获得的漏斗试样材料代表性体积单元疲劳寿命和Manson-Coffin寿命预测模型与等直圆棒试样试验结果均吻合良好。     

TC21合金315℃高周疲劳光滑和缺口试样断口分析

开展了TC21研究合金光滑和缺口试样的315℃高周疲劳实验,并对疲劳断口进行详细观察,研究了缺口对TC21合金疲劳寿命的影响.结果表明,光滑和缺口试样的疲劳强度比值随循环寿命降低而降低;光滑试样的失稳疲劳裂纹长度随循环应力升高而降低;瞬断区所承受的断裂应力随循环应力升高而降低;这说明裂纹失稳决定试样的断裂,缺口试样断口有多个裂纹源,以缺口试样名义应力乘以应力集中因子与光滑试样的应力相等作为比较时,缺口试样主裂纹长度大于光滑试样的裂纹长度,缺口试样的裂纹扩展寿命更长.     

高周疲劳和低周疲劳统一的能量表征方法研究

对高周疲劳和低周疲劳寿命预测模型进行了研究,提出了一种能够将高周疲劳和低周疲劳统一表征的能量形式参量.用统一的能量形式表征参量对高温合金GH141的760℃高周疲劳和低周疲劳数据进行处理,得到理想的能量-寿命方程.用1Cr11Ni2W2MoV钢500℃和粉末盘材料FGH95的600℃高温低周疲劳和高周疲劳数据对统一表征方法进行验证,验证结果表明,用能量形式的表征参量能够得到理想的能量-寿命方程.     

基于材料低周疲劳的裂纹扩展预测模型

该文基于疲劳裂纹尖端循环应力-应变场, 定义了基于应变幅的平均单位循环损伤参量D, 并引入Miner累积损伤率, 从而从理论上建立起材料低周疲劳性能和疲劳裂纹扩展行为之间的联系。以裂尖扩展方向上的单调塑性区尺寸作为疲劳过程区大小, 并提出了基于弹塑性应变疲劳累积损伤的疲劳裂纹扩展预测模型。模型改进了前人提出的疲劳裂纹扩展预测模型, 考虑了单调塑性区内所有材料的弹塑性应变疲劳损伤贡献;模型中参数均有物理意义, 不需要人为调试。基于完成的Cr2Ni2MoV 材料的低周疲劳结果所建立的该文新模型对该材料裂纹扩展速率的预测结果与实验结果有良好一致性。并且, 借助手册数据, 在TC4钛合金材料上进一步得到了验证。     

A simple method to analyse the notch sensitivity of specimens in fatigue tests

A simple method to analyse the notch sensitivity of specimens in fatigue tests is presented. The parameter m, which can be used to measure the notch sensitivity, the nominal stress and the stress concentration factor (K_t) are used to establish the method. In order to verify the feasibility of the method, notch fatigue test results from our group and literatures were collected. The results reveal that an optimal value of parameter m does exist for each material. Life predictions indicated that the model is able to describe the life evolution for notched specimens under high cycle fatigue and low cycle fatigue tests. Because the geometry effect is accounted for K_t, the method is suitable for the conditions when the notch geometries and the absolute dimensions are similar to the tested specimens.     

Low cycle fatigue of thin-walled hollow cylindrical specimens of mild steel in uni-axial and torsional tests at constant strain amplitude

An attempt has been made to examine any differences in behavior between uni-axial fatigue and repeated torsion fatigue. In order to eliminate the stress gradient effect, thin-walled hollow cylindrical specimens were used. Low cycle fatigue tests of mild steel were conducted in uni-axial and torsional loading at constant strain amplitude.

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Zusammenfassung Die Unterschiede zwischen dem Dauerbruch infolge einachsiger Belastung und dem infolge wiederholter Torsionsbelastung wurden untersucht. Um den Einfluss der Spannungsgradienten auszuschalten wurden duennwandige Hohlzylinder als Versuchsgegenstand benutzt. Nieder-Frequenzversuche an weichem Stahl under einachsiger- and unter Torsion-Belastung mit konstanter Dehnungsamplitude wurden durchgefuehrt.

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Résumé Un essai a été fait pour examiner la différence dans le compottement entre la fatigue uni-axiale et la fatigue dùe à une torsion répétée. Dans le but Weliminer Veffet du gradient de contrainte, des models cylindriques creux à paroies minces ont été utilisés. Des tests de fatigue à basses frequences sur de Vacier doux ont été effectués dans le cas de chargement uni-axial et torsionel, en gardant constante Vamplitude de la tension.

     

The effect of notch geometry on critical distance high cycle fatigue predictions

A critical distance method for predicting the fatigue limit stresses of notched specimens was implemented for notched specimens with a wide range of notch dimensions. Circumferentially notched cylindrical specimens (k_t=1.97–4.07) taken from Ti–6Al–4V forged plate were cycled to failure (R=0.1 and 0.5) using a step loading method for estimating the 10⁶ cycle fatigue limit stresses. These experimental data were used in combination with finite element solutions for all specimen geometries to determine a ‘critical distance’, a quantity or parameter determined from the stress distribution surrounding the notch in combination with fatigue limit stress data from unnotched specimens. A unique parameter was not found for all of the specimen geometries. However, predictions for the fatigue limit stresses of the larger notch geometries may be made with some amount of accuracy using a single value of the critical distance parameter, while reasonable predictions for the specimens with the smallest notch dimensions may be made upon the recognition of an apparent size effect.     

Life fatigue estimation of welding seams based on notch strains

When based on structural stress acting on shell elements, the stress simulation for welding seams on thin sheets provides no sufficient correlation with damages occurring during real component tests under operational load.The calculation technique on hand is based on the analysis of the operational strength behavior of crucial positions such as toe notches and root notches. The notches are fictitiously rounded with radii of 0.05 mm. The notch root stress reduced by the support effect (especially the microstructural support effect) has proved to be an appropriate stress criterion.In order to verify the universality of the method for other welding seam geometries as well, the fatigue strength behavior of H-specimen was not only simulated for MAG-welding seams but also for laser welding seams and spot welding.The fatigue life estimation was carried out in accordance with the critical plane method and by using the P_SWT–stress-cycle curve. The Miner rule was used as hypothesis of damage accumulation.     

Low cycle fatigue behavior of constraint connections

The general influence of constraint effects on reversed low-cycle fatigue performance was investigated. Constraint effects are being considered to have contributed to the failure of welded Steel Moment Resisting Frames (SMRFs), were cracks initiated in constraint regions during recent earthquakes. However, this study revealed that high constraint enhanced the resistance to crack initiation during cyclic loading by reducing the local strains at the notch tip. Furthermore, changes in toughness due to various constraint severities had almost no influence on the low cyclic performance of steel under constraint. This applies to low and high stress levels as well as low and high constraint, covering the whole range of possible scenarios of low-cycle and even towards high-cycle fatigue. While high toughness is still important for enhancing the fracture strength to accommodate the maximum imposed stresses/strains in earthquake loading, it most likely does not play a key role in cyclic performance.     

低周疲劳试验寿命预测模型分析及其对断口硬度影响

为能够对材料的疲劳失效有更加科学的预测,采用Instron 8803液压伺服疲劳机对45#钢在±0.5%,±1.0%,±1.2%,±1.5%,±2.0%5个应变幅值下进行低周疲劳试验,对疲劳试验数据进行分析,并对寿命预测模型进行验证评测,发现三参数幂函数能量法具有更加广泛的适用性;使用Hitachi S-3400N扫描电镜观察比较不同应变幅下疲劳断口微观形貌,分析裂纹源形成原因及扩展趋势;用Matsuzawa显微硬度计测量疲劳断口截面径向及轴向硬度值变化,以评测循环加载对疲劳特征区强化程度的差异,发现断口径向硬度值沿近中心位置至边缘呈波动递增趋势,而轴向硬度值随与断口的距离增大变化不大,且经过疲劳强化后,断口径向截面平均硬度明显高于原始状态。     

The effect of friction on pin jointed single edge notch fracture toughness test specimens

A theoretical estimate is made of the effect of pin friction on values of the stress intensity factor for the ASTM type single edge notch fracture toughness test specimen. It is shown that for a coefficient of friction of 0.2 between pins and specimen the stress intensity factor is reduced by approximately 10 per cent. It is therefore concluded that precautions must be taken to keep friction to a minimum when measuring fracture toughness using this type of specimen.

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Zusammenfassung Eine theoretische Schätzung des Einflusses der Stiftreibung auf die Werte der Spannungsintensitätsfaktor für die Bruchduktilitätsprobe des Types ASTM mit einzelner Randkerbe ist gemacht geworden. Erwiesen ist, dass wenn die Reibungsfaktor zwischen Stiften and Probe 0,2 beträgt, so wind die Spannungsintensitätsfaktor um etwa 10 Prozent vermindert. Hieraus lässt sich folgern, class man Vorsichtsmassregeln treffen muss, um die Reibung bei einem Mindestwert.zu halten, wenn man die Bruchduktilität bei Anwendung dieser Probentyp misst.

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Résumé L'influence de la friction de la cheville sur les valeurs du facteur d'intensité de contrainte pour l'éprouvette du type ASTM de ductilité à la cassure a entaille unique de bord a été évaluée théorequement. Il est démontré que pour un coéfficient de friction de 0,2 entre chevilles et éprouvette, le facteur d'intensité de contrainte s'abaisse d'approximativement 10 pour cent. 11 est donc estimé qu'en mesurant la ductilité à la cassure au moyen de ce type d'éprouvette il faut prendre de precautions pour maintenir la friction à une valeur minimale.