含低速冲击损伤复合材料层合板剩余压缩强度及疲劳性能试验研究

对T300/QY8911复合材料层板进行了低速冲击、 冲击后压缩以及冲击后疲劳试验研究。通过对冲击后的层板进行目视检测和超声C扫描获得了层板受低速冲击后的若干损伤特征; 在压-压疲劳试验中, 测量了损伤的扩展情况。讨论了冲击能量与损伤面积以及冲击后剩余压缩强度的关系, 分析了含冲击损伤层合板在压缩载荷及压-压疲劳载荷下的主要破坏机制。结果表明, 低速冲击损伤对该类层板的强度和疲劳性能影响很大, 在3.75 J/mm的冲击能量下, 层板剩余压缩强度下降了65%; 在压-压疲劳载荷作用下, 其损伤扩展大致可分为两个阶段, 占整个疲劳寿命约60%的前一阶段损伤扩展较为缓慢; 而疲劳寿命的后半阶段损伤则开始加速扩展, 并导致材料破坏。     

高强度钢动态冲击疲劳性能的研究与应用

对高强度钢ＨＱ－６０和ＨＱ－８０的动态冲击疲劳性能作了全面的试验研究，测定了两种材料的冲击疲劳性能曲线，即能量－寿命曲和和冲击疲劳下的裂纹扩展速度ｄａ／ｄＮ，以及动态断裂韧性ＫⅠｄ等。结合１５４ｔ电动轮自卸车的车架强度和寿命校核，提出了一种新的应用模型，为受冲击疲劳型载荷的零部件找到了一种实用的校核计算方法，具有普遍的推广应用价值。     

复合材料层板低速冲击后疲劳性能实验研究

通过对T300/5405复合材料层板进行低速冲击后的压-压疲劳实验,研究含不同冲击损伤层板的压缩性能与其在多级应力水平下的疲劳寿命与损伤扩展,并讨论冲击能量、应力水平、损伤扩展对层板疲劳寿命的影响。结果表明:冲击损伤明显降低层板的剩余强度;在低应水平下,冲击能量越大,含冲击损伤层板的疲劳寿命越小;疲劳实验中损伤经历平稳扩展和快速扩展两个阶段,其中平稳扩展阶段约占总体寿命的80%,快速扩展阶段约占总体寿命的20%,损伤扩展速率随着应力水平降低而减小。     

复杂面内应力状态下平面编织高铝纤维增强氧化铝基复合材料强度及疲劳寿命预测方法

针对平面编织氧化铝基复合材料提出了一种复杂面内应力状态下的强度准则和疲劳寿命预测方法。通过拉伸、压缩及纯剪切试验,分别获得了材料的静强度指标。考虑材料拉、压性能的差异和面内拉-剪联合作用对材料强度的影响机制,提出了修正的Hoffman强度理论。采用该强度理论预测得到的偏轴拉伸强度与试验结果基本一致,偏差不超过10%。开展了偏轴角θ=0°、15°、30°、45°,应力比R=0.1,频率f=10 Hz的拉伸疲劳试验,试验结果表明随着偏轴角的增加,相同轴向拉伸载荷下的疲劳寿命逐渐降低。由于面内剪切应力分量的作用,疲劳失效由纤维主导逐渐过渡到纤维和基体共同主导的模式。基于单轴疲劳寿命曲线,采用Broutman-Sahu剩余强度模型表征剩余强度随疲劳循环次数的变化规律,结合剩余强度演化模型和修正的Hoffman强度理论,提出了一种面内复杂载荷条件下的疲劳寿命预测模型,并引入疲劳剪切损伤影响因子表征拉-剪应力联合作用对材料疲劳行为的影响。采用本文提出的疲劳寿命预测模型,预测不同偏轴角拉伸疲劳寿命,预测结果与试验结果基本一致,偏差在1倍寿命范围内。比较结果表明在给定应力比、温度和疲劳载荷频率条件下,该疲劳寿命预测模型可以用来预测平面编织氧化铝基复合材料拉-剪复杂面内载荷条件下疲劳寿命。      

碳纤维复合材料假脚冲击后的疲劳试验

针对一种新型结构的碳纤维复合材料假脚,开展了自由落体冲击试验及冲击后疲劳寿命试验,分析了不同铺层参数、不同冲击能量等因素对其冲击损伤及疲劳寿命的影响规律。结果表明,不同铺层参数对碳纤维复合材料假脚U形结构件的冲击损伤具有显著影响,且随着0°铺层含量的降低,试件的冲击损伤面积增加,外观损伤越来越严重;碳纤维复合材料结构件的疲劳性能对冲击能量比较敏感,随着冲击能量的增加,碳纤维结构件的冲击损伤面积明显增大,其疲劳寿命逐渐降低。在冲击能量从4J增加到10J的情况下,碳纤维复合材料假脚的疲劳寿命大幅度降低了66.8%;尽管随着冲击能量的增加,试件的疲劳寿命逐渐降低,但二者之间并不符合线性关系,即冲击能量存在一门槛值,当冲击能量超过该门槛值后,其对碳纤维结构件疲劳寿命的影响将减弱;仅当碳纤维复合材料假脚的后龙骨厚度为2.7mm,且冲击能量≤4J的情况下,其疲劳寿命才可较好地满足相关安全标准的要求。     

疲劳过程中45钢力学性能变化规律的研究

针对疲劳强度设计的理论和方法所应用的均是材料在未受载下的力学性能,对45钢材料在三级应力水平下进行了旋转弯曲疲劳试验,并对疲劳试验后的试样进行了拉伸试验,研究了屈服极限σs 、强度极限σb和截面收缩率ψ的变化规律.研究结果表明,45钢的力学性能在疲劳过程中的变化规律很明显.在疲劳过程中,材料的屈服极限和塑性变形能力均呈逐渐下降的趋势,强度呈逐渐上升的趋势.在不同的载荷作用下,应力水平越高,材料的屈服极限和截面收缩率值越大,强度极限值越小.     

多损伤复合材料加筋壁板高周疲劳特性及剩余压缩强度

为研究常用于飞机垂尾的复合材料加筋壁板的冲击疲劳特性,设计了该型加筋壁板多点冲击试验、高周疲劳试验及剩余压缩强度试验。讨论了不同冲击能量对筋条边缘冲击损伤的影响,及施加低应力水平的疲劳载荷后各冲击损伤区域的扩展情况,对比分析了疲劳对冲击后剩余压缩强度的影响。结果表明:40J能量冲击后的损伤面积和凹坑深度较大,C扫描损伤形貌很不规则。100万次低应力疲劳后主损伤区附近衍生出新损伤,导致压缩破坏时产生向上、下夹具扩展的裂痕。该型加筋壁板疲劳后破坏载荷保持率为95.6%,有较好的抗冲击疲劳能力,为加筋壁板耐久性及后屈曲设计提供了思路。     

蜂窝纸板低周疲劳及蠕变复合作用下的损伤研究

对蜂窝纸板在低周疲劳和蠕变作用下的损伤进行了研究,并采用梁柔量作为损伤参量.研究结果表明:在80%应力水平下,梁柔量在疲劳和蠕变作用下演化规律可分为:快速增加、稳定增加和梁加速断裂3个阶段.随着蠕变保载时间的增加,蜂窝纸板的蠕变寿命分数和疲劳寿命分数均下降很快.蜂窝纸板蠕变寿命分数与蠕变保载时间几乎成线性关系下降,且蠕变寿命分数远远大于疲劳寿命分数.随着蠕变作用时间的增加,疲劳/蠕变损伤的负交互作用加剧.     

单个冲击对不锈钢管道焊接头低周疲劳寿命的影响

完成了单个冲击对１Ｃｒ１８Ｎｉ９１不锈钢管道焊接头试样低周疲劳寿命影响的试验研究。单个试验最大瞬时峰值应变率达４８０ｓ＾－１。试样未经消除焊接残余应力。采用成组法试验（每组７个试样）,对称加载模式,总应变幅为０．００２２８。结果表明,冲击影响受到焊接残余应力和冲击塑性导入机制的耦合作用。焊接残余应力与冲击应力叠加将增加材料损伤,而冲击塑性导入将减缓疲劳损伤和降低疲劳寿命分散性。前者扼制后者。考虑疲     

高温对核电工程材料低循环行为的影响研究

对核电工程材料（钛合金T42NG与T225NG和主螺栓材料18Cr2NiWA)开展了室温与350度高温下的低周疲劳性能试验研究,获得了各材料单调R－O本构模型和M－C寿命估算模型,基于这些模型,研究了材料的循环强化与软化规律,了温度系数λσ,λΔσ,和λNf对钛合金静强度,循环强度和低周疲劳规律的影响效应;根据温度对寿命的影响系数λNf与应变幅Δε/2呈线性规律的重要发现,提出了考虑温度效应的用于高温低周疲劳寿命估算的λ－M－C模型,进而总结出一种现行方法更简便的高温疲劳试验方法。     

HD钢在气门热精锻模具的生产应用初步试验

气门热精锻模具失效的主要形式为热磨损和压塌，提高模具使用寿命的关键是使材料具有更高的室温和高温的硬度，磨损抗力，屈服强度和热稳定性。ＨＤ（４Ｃｒ３Ｍｏ２ＮｉｖＮｂ）钢比传统应用的３Ｃｒ２Ｗ８Ｖ钢具有更高的上述性能及足够的韧塑性，因而其模具在理论上具有更长的工作寿命。生产应用试验表明，ＨＤ钢制气门热精锻模具的使用寿命是３Ｃｒ２Ｗ８Ｖ钢的２．２倍。     

Brittle Fracture of Hot-Forging Cast Dies

The site applications of cast hot-forging dies made from a new type cast steel were performed and several of the dies experienced brittle fracture. Analysis of the operating modes and evaluation of mechanical properties were used to establish relations among failure patterns and lives of dies. The results show that the dies experience elevated-temperature wear, plastic deformation and thermal fatigue and that brittle fracture was the predominant failure pattern. Brittle fracture of cast dies has two patterns: early brittle fracture and impact fatigue fracture. The early brittle fractures seldom occur but, when this failure mode occurs, the failure severely shortens the life of the cast dies. The early brittle fracture is attributed to abnormal heat-treatment and operation. Impact fatigue fracture is the dominant failure pattern in the cast dies. This failure mode usually initiates from sand inclusions, micro-cracks and/or thermal fatigue cracks. The specifications of the die material should include a requirement for impact toughness to assure that the dies are resistant to multiple impact loads. This means that strength-dominant toughness must be controlled and optimization of the heat-treatment processes must focus on this requirement.     

Improvement of wear resistance of hot working tool steel by hardfacing Part 2 – Case study

Abstract

The performance and service life of the die components are limited because of different reasons such as thermal and mechanical fatigue cracking, wear, plastic deformation, etc. To minimise these damages, the dies are normally subjected to a variety of surface engineering processes. In the present study, the dimensional loss and lifetime of hardfaced and unmodified H11 steel hot forging dies were monitored during service and compared. For this purpose, various experimental tools such as XRD, SEM, optical metallography and microhardness test were employed. Microhardness profiles from surface to the depth of the dies after service were determined to study the variations in mechanical properties. It was observed that the hardfacing with Stellite 21 and Inconel 625 alloys leads to an increase in wear resistance and the working life of the dies in industrial scale. In the case of hardfaced dies, the work hardening of surface produces a self-protecting layer which has a strong metallurgical bonding to the substrate and thus enhances more resistance to further wear, while hardness reduction during service in unmodified H11 steel die causes a reduction in wear resistance and plastic deformation of surface layer.     

Fatigue Failure of Extrusion Dies: Effect of Process Parameters and Design Features on Die Life

Analysis of die failure plays an important role in the prediction and prevention of die failure, and subsequently in improving economics of any metal-forming process. Industrial experience has shown that fracture is the most common mode of failure in the case of hot aluminum extrusion dies. The purpose of the present work is to implement fatigue damage models in a Finite Element code for identification of critical process parameters and die design features in the case of fatigue being the dominating failure mode. For the maximum number of billets extruded by the die before fatigue crack initiation (fatigue life cycles in extrusion), Morrow’s stress- and strain-life damage models are implemented for axisymmetric flat extrusion die. With the help of finite element software ABAQUS, extrusion process is simulated and dynamic stress and strain values were obtained by first identifying the potential fatigue location in the die. The evaluation of applicability of the damage models is done for specific hot extrusion die made of H13 steels with Al-6063 as billet material. By considering temperature and strain rate as process parameters and bearing length and fillet radius of the die as geometric features, different simulation runs are performed to investigate the effect of process and design features on the useful die life. Morrow’s stress life model shows a good correspondence between computed and actual failure of dies. By establishing correlations of die life with process and design parameters under different conditions, it was shown that the present investigation is a useful guideline at die design and extrusion process stages.     

精铸热锻模具的主要失效形式和分析

对新型精铸模具钢制热锻模具的失效进行了分析，并对不同形式下模具的失效原因作了探讨。结果表明，精铸热锻模的主要失效形式为热磨损、塑性变形、热疲劳裂纹、冲击疲劳破断和脆性断裂。     

Failure mechanisms of H13 die on relation to the forging process – A case study of brass gas valves

In the hot forging industry, die life is an important process factor because of the cost involved in lost production, replacement of die blocks and operative handling of the dies. There is still no consensus, however, on the type of wear affecting dies or the dominant mechanisms for die failure, which varies from one situation to another. This metallographic study of a failed industrial hot forging die used to forge gas cylinder valves has indicated various failure modes. Although plastic deformation and thermal fatigue are usually quoted as the main causes of damage, oxidative and abrasive wear, fatigue cracking and chipping appeared to bet he most important in this study. Feedback coupling of fatigue and wear effects are detected. Detailed scanning electron microscopy observations and energy-dispersive X-ray and optical profilometry analysis suggest that these failures might very depending on their localisation on the die surfaces and show a complex mechanisms related to the variation of process parameters.     

钢结硬质合金在热挤压模具上的应用

分析了气门顶杆热挤压工艺及模具失效形式 ,尝试用钢结硬质合金取代原模具材料 ,现场试验表明 ,钢结硬质合金模具的使用寿命为原模具的 1 0倍左右。电镜观察显示热疲劳是硬质合金热挤压模具失效的主要原因。     

WC-8Co与Al2O3-TiC两种拉拔模具磨损行为研究

采用冷、热压烧结技术分别制备了WC-8Co和Al2O3-TiC两种拉拔模具.研究了该两种不同拉拔模具用于实际拉拔加工45#钢线时的摩擦磨损行为,并对其磨损机理进行了分析.结果表明:两种拉拔模具均具有较好的强度和硬度,能较好地进行拉拔加工,其内孔磨损在工作区及定径区最为严重,磨损面有金属脱落元素、模具颗粒脱落元素及润滑脂残留物.WC-8Co和Al2O3-TiC两种拉拔模具的磨损机理不尽相同,WC-8Co模具主要为粘着磨损和磨粒磨损,而Al2O3-TiC模具则以磨粒磨损和表面疲劳磨损为主.Al2O3-TiC拉拔模具更适合于45#钢线材的拉拨.