LZ50车轴钢低周疲劳性能研究

研究了LZ50车轴钢的循环相关特性,循环疲劳特性及循环应力-应变关系。试验结果表明:LZ50车轴钢的循环疲劳特性曲线及循环应力-应变曲线很好地符合双对数线性关系;其循环相关特性表现为先循环软化,稳定后又发生循环硬化,而且循环软化和循环硬化的程度随控制应变的不同而不同。     

薄壁圆管试样高温多轴循环载荷下的力学行为

对GH4169镍基高温合金制成的薄壁圆管试样的多轴循环相关特性进行了研究。试验在MTS809-250 kN/2000 Nm电液伺服拉-扭低周疲劳试验机上进行,采用恒幅总应变控制,对称加载,轴向应变与扭转应变的加载采用三种不同相位(0°、45°和90°),其加载频率分别为0.1 Hz和0.2 Hz,试验温度为650℃.通过对薄壁圆管高温多轴比例和非比例循环加载试验,研究了该材料在多轴循环载荷下的应力-应变响应和循环相关特性。试验结果表明,GH4169合金在试验温度下不通加载路径下均呈现为循环软化现象,其循环软化程度与等效应变和轴向与剪切方向之间的相位差密切相关。     

TCS不锈钢典型焊接接头的疲劳性能研究

选取两种不同规格的玻璃纤维土工格栅,首先进行大量的静载强度试验,在此基础上,进行了不同应力水平下的恒幅疲劳试验,研究玻璃纤维土工格栅的P-S-N曲线、滞回曲线以及应变与循环次数的关系。结果表明:玻璃纤维土工格栅的应力-寿命关系可以用幂函数来拟合,其疲劳性能具有明显的脆性材料的特性,疲劳寿命较低;玻璃纤维土工格栅的应变-循环比特性曲线呈现出三阶段变化规律,第一阶段为循环的20%之前,应变增加较快,第二阶段为循环的20%~80%之间,应变变化平缓,第三阶段为循环的80%之后,应变剧增直至破坏。     

316不锈钢低循环特性研究

研究了316L和316LN不锈钢在常温下的低循环特性并对其组织结构的变化进行了观察.结果显示,材料在循环变形过程中呈现不同程度的硬化、软化现象。组织结构的演变与循环应力特性曲线的变化规律相呼应。     

冷却剂参数对铣槽喷管低周疲劳寿命的影响

采用有限体积流固耦合计算方法、非线性有限元热结构耦合分析方法和局部应变法研究大面积比铣槽喷管三维再生冷却槽道在循环工作条件下的热结构变形与低周疲劳寿命,并对比分析了冷却剂质量流量与入口温度对铣槽喷管疲劳使用寿命的影响。计算结果表明,铣槽喷管热结构响应呈现复杂的三维效应,应变较大位置主要分布在与肋连接的内衬区域,喷管中部的残余应变量最大;冷却槽道低周疲劳寿命分布和热结构响应基本一致,最小寿命位于喷管中部与肋相连的内衬区域燃气侧;随冷却剂质量流量增加,铣槽喷管低周疲劳寿命不断提高;随冷却剂入口温度增加喷管尾部低周疲劳寿命值不断降低,而喷管中前部的低周疲劳寿命值却不断提高,当冷却剂入口温度为280K左右时,本文的铣槽喷管总体使用寿命达到最大。      

超弹性形状记忆合金丝力学性能试验研究

超弹性是形状记忆合金（SMA）的重要特性之一。通过试验,研究了应变幅值和加卸载循环次数对SMA丝应力诱发马氏体相变应力、残余应变、最大相变应变和耗能能力的影响。研究结果表明,超弹性SMA丝具有良好力学性能,应变幅值对它的耗能能力具有较大的影响,循环次数的增加可使其力学性能更加稳定。     

材料的变幅循环应变疲劳寿命估算

分析了金属材料的循环性能、疲劳性能,给出了变幅循环下材料应变疲劳寿命算例,验证了残余压应力对疲劳寿命有益的结论.     

应变疲劳对钢材延性断裂韧度JIC的影响

应用应变疲劳能量损耗的理论和弹塑性断裂力学的J积分理论,初步探讨了钢材的应变循环加载对延性断裂韧度JIC的影响。通过对20G钢和16Mn热轧钢板两种钢材的应变循环加载前、后两种状态的延性断裂韧度JIC测试数据的比较,考察了这种影响的存在。测试结果表明,±0.3%全应变控制的循环加载,经历500周次后可使两种钢材的JIC数值产生15%以上的改变量。对于20G钢,这种变化与Rice给出的循环加载变形功与J积分之间的函数关系相对应;而对于16Mn钢则有较大的差异,其原因有待于进一步的研究。     

一种基于瞬时蠕变效应的棘轮行为研究

针对316L不锈钢进行了大量的单轴棘轮和蠕变试验,建立了描述该材料单轴短时蠕变演化模型。根据已有的棘轮效应研究结论,该材料的蠕变变形的多项特征与棘轮变形规律一致。提出了棘轮效应和蠕变变形机理一致的假设,认为棘轮变形实质上是材料在应力循环中的瞬时蠕变变形累积所致,在此基础上进一步推导出一套棘轮变形的蠕变算法以计算材料的棘轮变形,取得了较好的预测结果,证实该假设在一定条件下成立。     

GH4169镍基高温合金高温多轴低周疲劳性能

利用薄壁管试样,在MTS809疲劳试验机上对GH4169镍基高温合金进行了650oC下的高温多轴拉扭低周疲劳试验,通过对应力应变迟滞迴线分析发现,拉扭分量应力应变迟滞迴线形状与试件的加载波形和加载路径相关,塑性应变不断增大。利用FEI-Quanta 200扫描电镜和EDAX Genesis 2000 X-射线能谱仪对试样疲劳断口微观分析,结果表明,GH4169镍基高温合金材料在高温循环加载下有氧化现象。综合不同等效应变下材料的断口形貌差异,推断在高温循环加载下有蠕变氧化作用。     

Experimental study on uniaxial time-dependent ratcheting of a polyetherimide polymer

The uniaxial ratcheting behavior of a polyetherimide （PEI） polymer ＇TECAPEI＇ was studied using stress-controlled cyclic loading at room temperature, including both cyclic tension-compression with non-zero tensile mean stress and tension- unloading tests. The experimental observations were focused on the time-dependent ratcheting of the PEI polymer revealed in cyclic tests at diverse stress rates and with different peak stress holding times. The results showed that the PEI polymer shows obvious ratcheting deformation; i.e., the ratcheting strain accumulates progressively in the tensile direction during stress- controlled cyclic tests with non-zero mean stress. The ratcheting is highly dependent on the applied mean stress and stress am-plitude, and is also characterized by a strong time-dependency during the cyclic stressing at diverse stress rates and with different peak stress holding times. The time-dependent ratcheting of the PEI polymer is caused mainly by its remarkable viscosity. A comparison of the ratcheting occurring before and beyond the ultimate stress point of the PEI polymer showed that the ratcheting beyond the ultimate stress point is more significant than that occurring before that point.     

非比例循环载荷下的随动强化律

对于４２ＣｒＭｏＡ钢的非比例循环实验，讨论了几个随动强化律的适用性，其中包括Ｐｒａｇｅｒ、Ｚｉｇｌｅｒ、Ｃｈａｂｏｃｈｅ、Ｍｒｏｚ和Ｔｓｅｎｇ－Ｌｅｅ强化律。分析表明与双曲面本构模型相关的Ｍｒｏｚ和Ｔｓｅｎｇ－Ｌｅｅ强化律能给出较好的预测。文中还给出了一个简化的双曲面本构模型     

Damage behavior of steel beam-to-column connections under inelastic cyclic loading

Brittle cracks were observed in the welded beam-to-column connections of steel frames during an earthquake. The crack propagation and accumulated damage to the connections can lead to fractures at much lower ductility ratios. Understanding the connections’ damage behavior during an earthquake is crucial for the design of steel moment frames in seismic areas. Nine full scale beam-to-column connections were tested under constant amplitude and variable amplitude cyclic loading. The effects of loading amplitude, loading history, and peak load on the connection damage were analyzed. The damage characters were studied and three damage evolution models were calibrated and validated based on test results. The damage mechanism was investigated and an effective plastic strain index was developed to evaluate connection damage based on a ductile fracture mechanism. A fatigue fracture mechanics-based model, for evaluating the damage process of beam-to-column connections under cyclic loading, was proposed.     

Constitutive model of saturated soft clay with cyclic loads under unconsolidated undrained condition

An equivalent visco-elastic model of saturated soft clay was studied under unconsolidated undrained (UU) condition, which can be used to evaluate the stability of ocean foundation. Cyclic triaxial compression and extension tests were conducted to study the parameters of the model. Results showed that the relationships of the damping ratio and the octahedral shear modulus with the octahedral cyclic shear strain were nearly unique, when the initial octahedral shear stress ratios of specimens were equal to 0.3, 0.5 and 0.7. The relationships of the damping ratio and the octahedral shear modulus with the octahedral cyclic shear strain determined from the cyclic triaxial compression tests were basically the same as those determined from the cyclic triaxial extension tests. Furthermore, the relationships were not related to the initial stress condition, the test stress state and the octahedral cyclic shear stress ratio. The relationships determined from the cyclic triaxial tests under no deviatoric stress were basically the same as those determined from the cyclic triaxial tests under deviatoric stress. The change of the octahedral cyclic accumulative strain with the number of cycles was unique under different tests stress states. An equivalent visco-elastic constitutive model of saturated soft clay under UU condition was initially established.     

Investigation of low-cycle fatigue behavior of austenitic stainless steel for cold-stretched pressure vessels

Cold-stretched pressure vessels from austenitic stainless steels(ASS) are widely used for storage and transportation of liquefied gases,and have such advantages as thin wall and light weight.Fatigue is an important concern in these pressure vessels,which are subjected to alternative loads.Even though several codes and standards have guidelines on these pressure vessels,there are no relevant design methods on fatigue failure.To understand the fatigue properties of ASS 1.4301(equivalents include UNS S30400 and AISI 304) in solution-annealed(SA) and cold-stretched conditions(9% strain level) and the response of fatigue properties to cold stretching(CS),low-cycle fatigue(LCF) tests were performed at room temperature,with total strain amplitudes ranging from -.4% to -.8%.Martensite transformations were measured during the tests.Comparisons on cyclic stress response,cyclic stress-strain behavior,and fatigue life were carried out between SA and CS materials.Results show that CS reduces the initial hardening stage,but prolongs the softening period in the cyclic stress response.Martensite transformation helps form a stable regime and subsequent secondary hardening.The stresses of monotonic and cyclic stress-strain curves are improved by CS,which leads to a lower plastic strain and a much higher elastic strain.The fatigue resistance of the CS material is better than that of the SA material,which is approximately 1?03 to 2?04 cycles.The S-N curve of the ASME standard for ASS is compared with the fatigue data and is justified to be suitable for the fatigue design of cold-stretched pressure vessels.However,considering the CS material has a better fatigue resistance,the S-N curve will be more conservative.The present study would be helpful in making full use of the advantages of CS to develop a new S-N curve for fatigue design of cold-stretched pressure vessels.     

Longitudinal resistance performance of granular ballast beds under cyclic symmetric displacement loading

The longitudinal resistance performance of a granular ballast bed under cyclic symmetric displacement loading was studied based on a full-scale test model of ballast track structures. The change law of the longitudinal resistance characteristics of the ballast bed under variable displacement amplitudes was analyzed. The results show that: the resistance-displacement curves of a granular ballast bed are a set of closed hysteretic curves, indicating obvious energy consumption; a granular ballast bed softens gradually during the cyclic process with constant displacement amplitude, and the residual deformation rate increases nonlinearly with increasing cycle number; the peak value of the longitudinal resistance of lines decreases with increasing cycle number; the cyclic softening of a granular ballast bed is dependent on the displacement amplitude–the higher the displacement, the more severe the cyclic softening will become; after cyclic displacement loading is applied several times, the longitudinal resistance of the bed will degenerate obviously, and the higher the displacement amplitude, the higher the longitudinal resistance attenuation rate of the ballast bed will become.