纤维增强金属基复合材料层板热/机械疲劳性能试验研究

进行了B／Al层板250～350℃温度循环范围内的同相位、反相位的热／机械疲劳寿命试验以及250℃和350℃下的等温疲劳试验与宏微观分析研究。结果表明：同相位与反相位的热／机械疲劳S—N曲线出现相交，以交点做应力水平线FPF，在FPF以上，同相位的热／机械疲劳(TMF)比反相位的要短；而在FPF以下，同相位的TMF寿命比反相位的要长；无论是同相位，还是反相位的TMF寿命，均低于250℃和350℃下的等温疲劳寿命；疲劳裂纹起源于纤维与基体界面，并随着基体的横向开裂而扩展，但最终的疲劳损伤机理不仅取决于应力水平，还取决于试验环境条件；纤维与基体之间界面反应区在TMF的损伤扩展方面起了主要作用。     

循环温度范围对Ti-6-22-22合金热机械疲劳行为的影响

采用拉压对称的机械应变控制,研究了Ti-6-22-22合金在200～400℃和200～520℃两个温度范围的热机械疲劳(TMF)行为.结果表明,在200～400℃内,同相和反相热机械疲劳寿命均高于400℃等温疲劳寿命;在200～520℃范围,反相热机械疲劳寿命明显低于520℃等温疲劳寿命.在两个温度范围内,热机械疲劳的循环应力都与相应等温疲劳的循环应力响应有关.纵向剖面金相观察表明,520℃时等温疲劳表面的裂纹更长.循环温度范围扩大导致环境破坏作用增强是热机械疲劳具有明显破坏作用的原因.     

镍基高温合金GH4169的热机械疲劳行为

在不同温度区间、不同条件下进行GH4169合金的热机械疲劳实验测试其热机械疲劳数据,研究了这种合金的热机械疲劳行为。结果表明：GH4169合金在热机械条件下的迟滞回线具有明显的拉压不对称性;同相位时材料承受压应力,反相位时承受拉应力。拉应力,是影响疲劳寿命的主要因素。应变幅较高时GH4169合金出现平均应力松弛,在高温半周为先循环软化后循环稳定,在低温半周始终趋于循环稳定。     

保持时间对定向合金DZ125热/机械疲劳断裂行为的影响

对DZ125定向凝固铸造镍基高温合金进行了应变比为-1.0的同相位三角波和同相位梯形波,550℃()1000℃热/机械疲劳实验研究.实验结果表明:在相同应变幅下,同相位三角波载荷情况下的热/机械疲劳寿命比同相位梯形波载荷情况下的热/机械疲劳寿命长.研究了在两种载荷情况下材料的热/机械疲劳循环应力响应行为.试样断口的微观分析表明:在热/机械疲劳过程中,同时存在疲劳、蠕变和氧化损伤;在同相位三角波载荷下,穿晶 沿晶断裂为疲劳断裂的主要特征;在同相位梯形波载荷下,裂纹主要为沿晶萌生与扩展.这是导致在同相位梯形波载荷下疲劳寿命缩短的主要原因.     

热—机械疲劳影响因素的试验研究

本文对影响GH500、GH33A、DZ—22、DZ—4、GH37、GH132、GH36等高温合金和石墨铸铁的热-机械疲劳寿命的某些主要因素(温度、频率、保持时间、载荷波形、同相位和反相位、恒应力和恒应变)进行了试验研究。     

温度对GH34钢低周疲劳性能的影响

本文用应变控制的低周疲劳试验方法,研究了室温、350、450和550℃下,燃气轮机叶轮用钢GH34的低周疲劳性能。试验表明,各温度下的循环应力——应变特性均为循环软化。试验温度提高,低周疲劳性能下降。由于蓝脆性的发展,450℃的低周疲劳性能迅速下降,接近550℃的低周疲劳性能。COFFIN—MANSON方程中的指数α随试验温度提高而增大。高温低周疲劳裂纹的扩展,主要为Ⅱ阶段扩展,在所采用的试验条件下,没有发现蠕变引起的治晶疲劳断裂。     

热作模具钢在高温热机械应力循环下的疲劳断裂行为

研究了热作模具钢在应力控制下的等温疲劳和同相热机械疲劳寿命，发现在相同的应力幅下，同相热机械疲劳寿命低于上限温度的等温疲劳寿命。通过研究疲劳过程中的循环应变响应和疲劳断口特征时发现，等温疲劳条件下，滞后环朝压缩方向发展，疲劳裂纹主要为穿晶萌生与扩展；在热机械疲劳条件下，滞后环朝拉伸方向发展，疲劳裂纹主要沿晶萌生与扩展。这是导致同相热机械疲劳寿命低于等温疲劳的主要原因。     

高温合金材料时间相关热机械疲劳寿命预测技术

本文在变温非线性运动强化规律所描述的高温合金材料热机械疲劳应力－应变循环特性的基础上,重点讨论了应变控制的时间相关热机械疲劳寿命预测技术。对于温度循环的影响,采用由应变能密度表示的损伤参数,并且引入了温度损伤系数。对于循环时间的影响,引入了蠕变─疲劳相互作用的损伤机制,采用韧性耗散损伤模型。在确定模型的一些参数时,采用等温力学试验和疲劳试验的数据,把等温疲劳研究成果推广到变温疲劳分析领域。     

钛合金高温疲劳性能的试验研究

开展了钛合金材料高温疲劳试验研究。试验共设计3种工况,即:200℃,0.5kN-22.5kN-0.5kN力加载循环;300℃,0.5kN-22.5kN-0.5kN力加载循环;200℃,0.5kN-30kN-0.5kN力加载循环,每种工况下分配试样数为5个~7个。引入"肖维奈准则"对试验数据进行了筛选,通过寿命分布模型的检验,得到其有效数据点遵循正态分布模型的结论。根据试验数据分析了母体在三种工况下的中值疲劳寿命以及存活率为99.9%的疲劳安全寿命估计量,为相应工程提供基础数据。     

4Cr5MoSiV1，8407钢的热疲劳性能

采用自约束热疲劳试验方法,对比研究了相同热处理条件的4Cr5MoSiV1,8407钢的热疲劳特性,观察分析了疲劳裂纹形貌和深度,采用热疲劳损伤因子定量研究了二种钢的热疲劳过程。结果表明：两种钢的热疲劳裂纹萌生发生在100－200次之间。8407钢热疲劳裂纹的萌生较4Cr5MoSiV1钢均匀,细小;在1600次冷热循环前,二者的热疲劳损伤程度无明显差别,在1600次热循环后,前者的热疲劳损伤程度低于后者;在较低的回火温度条件下,8407钢的热疲劳抗力稍优于4Cr5MoSiV1;而在高温回火时,8407钢的热疲劳抗力属于4Cr5MoSiV1钢。分析了这二种钢的热疲劳机制,指出决定材料热疲劳裂纹抗力的是钢的热稳定性和钢的强度或硬度。     

Thermomechanical Fatigue Behaviour of a Nickel Base Superalloy

The isothermal low cycle fatigue (LCF)and thermomechanical fatigue (TMF) behaviourof a Ni-base superalloy was investigated. Theresults show that temperature plays an importantrole in both LCF and TMF. The alloy shows thelowest LCF fatigue resistance in the intermediatetemperature range (～760℃). For strain-controlledTMF, in-phase (IP) cycling is more damagingthan out-phase (OP) cycling. The high tempera-ture exposure in the TMF cycling influencesthe deformation behaviour at the low temperature.LCF lives at different temperatures, and IPand OP TMF lives are successfully correlatedby using the hysteresis parameter Δσ·Δε_p.     

Intergranular Fatigue Cracking Behaviour in Aluminium

In order to understand the basic mechanismof intergranular cracking in pure metals duringfatigue, stress-controlled push-pull fatiguetests were carried out with high purity aluminium.Tests were interrupted frequently so as to studythe grain boundary (GB) cracking behaviour bythe surface observation. The results show thatcrack initiation at GB was a process controlledby multi-factors, such as boundary structure,GB-slip interaction, GB sliding and so on. Ifthese factors are varied so that the incompati-bility at a GB increased, the possibility ofcyacking at the boundary will be raised. Someinteygranular cracking phenomena are not ableto be explained by the GB stepping mechanism.     

Fatigue Behaviour of Metal Matrix Composites

Based mainly on the work done at the authors'laboratory in recent years,this paper examines whatis currently known about the cyclic deformationand fatigue properties of metal matrix composites,with particular emphasis on discontinuous fiber(whisker or particulate)-reinforced Al composites.The following items are discussed:fatigue strengthand life,cyclic deformation and microstructuralevolution,microcrack initiation and growth,fatiguecrack propagation behaviour.     

圆管-方管混凝土T型节点疲劳性能试验研究

基于热点应力法的管节点疲劳性能研究思路中,热点应力集中系数和疲劳寿命是描述焊接管结构节点疲劳性能的两个主要方面。对8个支管为圆管主管为内填充C50等级混凝土的方管节点试件（CT1～CT8）进行热点应力试验以确定其热点应力集中系数（SCF）,在此基础上进行疲劳试验以确定其疲劳寿命（N3）,并将试验结果与具有相同几何尺寸和受力条件的圆管-方管节点试验结果以及相关规范进行比较分析。研究表明,支管受轴向荷载下,圆管-方管混凝土节点较对应空钢管节点而言,节点区刚度分布有明显改善,前者SCF最大值均小于后者SCF最大值,具有更好的疲劳性能;已有规范中关于空钢管节点疲劳性能的S-N曲线不适用于进行圆管-方管混凝土T型节点的疲劳性能分析。     

Influence of Temperature on the Fatigue Behaviour of Glass Fibre Reinforced Polypropylene

Abstract: The behaviour of a composite with a polymeric matrix subjected to fatigue was analysed. The study was conducted at ambient temperature, as well as at 50 and 100 °C. The material was observed to behave differently at each temperature. Furthermore, a statistical analysis was performed using Castillo’s model. Although certain similarities in behaviour were observed at ambient temperature and 50 °C, the material behaved differently at 100 °C. Structural modifications were found in the thermal analysis that justify these differences.     

Fatigue and Impact Behaviour of Carbon Fibre Composite Bicycle Forks

The use of carbon fibre‐based polymeric composites in bicycle components has dramatically increased in the last few decades. This paper presents the results of fatigue and impact testing on bicycle forks that are known to have quality‐related manufacturing defects. In the investigation performed, acoustic emission testing is used to monitor crack growth during fatigue loading and impact. The results show three distinct stages in composite fatigue failure related to initiation, propagation and final growth. Implications of the results on design and testing of composite bicycle components are addressed pointing to a greater need in understanding current design procedures.     

Thermal Fatigue Behaviour of a Chromium Electroplated 32NiCrMo145 Steel

Thermal fatigue behavior of hard chromium electroplated steel in three different crack intensities of high contraction(HC), medium contraction (MC) and low contraction (LC) was studied. Maximum and minimum temperaturesduring thermal cycle were 800 and 100℃, respectively. The topography and cross sections of the samples exposedto 50, 100 and 200 thermal cycles were studied. The thermal fatigue behavior was analyzed using the data obtainedfrom surface roughness, crack networks and stress induced during cycles. Although the as-coated sample with LCchromium contained no crack, it appeared to have a high crack density after only 50 cycles. The crack depth andwidth in cyclically oxidized LC coating were much less than those in MC and HC coatings. It was concluded that theLC coating protected the substrate from having cracks or subsurface oxidation during thermal fatigue. The cracksin the HC and MC coatings increased in density as well as in depth by thermal cycles. Moreover, the opening of thecracks to the s     

Influence of Stress Ratio and Loading Condition on Near-threshold Fatigue Crack Growth Behaviour

Near-threshold fatigue crack propagation behaviour of a cold-worked copper tested with distinct fa-tigue testing systems under different stress ratios has been investigated.The emphasis was placed onthe evaluation of crack closure effect as well as the effective fatigue threshold,It was found that thefatigue threshold for R=0 is higher than those for R=-1 and R=0.4,for which almost the samethreshold value was derived.Compared with the conventional closure evaluation method,the pro-posed new closure evaluation method can generally interpret the dependence of the fatigue thresh-old on stress ratio and loading condition,and leads to a higher and relatively constant effective fa-tigue threshold of about 3 MPa·m~(1/2) for the cold-worked copper at different stress ratios andloading conditions.     

16Mn钢在空气和3.5% NaCl溶液中的疲劳行为

采用哑铃状平板试样,分别研究了16Mn钢在空气中和3.5%NaCl溶液中的疲劳行为,获得了S-N曲线,并对疲劳试样的表面和断口形貌进行了观察。结果表明:3.5%NaCl溶液(与空气相比)使16Mn钢的疲劳强度有较大程度的降低,在空气中16Mn钢的疲劳极限为200 MPa,而在3.5%NaCl溶液中该钢则不存在疲劳极限;空气中的疲劳试样只有一个萌生于试样表面基体的裂纹源,而3.5%NaCl溶液中该钢的疲劳试样一般有多个裂纹源,除了极少数萌生于试样表面基体处,其余均萌生于表面的点蚀坑;空气中疲劳试样裂纹扩展区的断口形貌以疲劳辉纹为主,而3.5%NaCl溶液中的则以沿晶开裂等脆性特征为主。此外还对空气中16Mn钢的疲劳极限进行了预测,预测值与试验值基本吻合。