GIGACYCLE FATIGUE BEHAVIOR OF CAST ALUMINUM IN TENSION AND TORSION LOADING

An improved understanding of fatigue behavior of a cast aluminum alloy（2-AS5U3G-Y35）in very high cycle regime is developed through the ultrasonic fatigue test in axial and torsion loading.The new developed torsion fatigue system is presented.The effects of loading condition and frequency on the very high cycle fatigue（VHCF）are investigated.The cyclic loading in axial and torsion at 35 Hz and 20 kHz with stress ratio R=-1 is used respectively to demonstrate the effect of loading condition.S-N curves show that the fatigue failure occurs in the range of 105—1010 cycles in axial or torsion loading and the asymptote of S-N curve is inclined,but no fatigue limit exists under the torsion and axial loading condition.The fatigue fracture surface shows that the fatigue crack initiates from the specimen surface subjected to the cyclic torsion loading.It is different from the fatigue fracture characteristic in axial loading in which fatigue crack initiates from subsurface defect in very high cycle regime.The fatigue initiation is on the maximum shear plane,the overall crack orientation is on a typical spiral 45° to the fracture plane and it is the maximum principle stress plane.The clear shear strip in the torsion fatigue fracture surface shows that the torsion fracture is the shear fracture.

Flexibility of the Lumbar Intervertebral Disc in Bending and Torsion： Experimental and Finite Element Study

Intervertebral disc flexibility is influenced by lifestyle, loading history, trauma, preexisting conditions, age and degeneration. With regard to degeneration, intervertebral discs become less flexible and stiffer. In this study, a testing protocol using bending and torsion loading was developed to gain the flexibility curves and stiffness often cadaveric lumbar discs. Measurements of rotation in the sagittal plane （flexion-extension）, coronal plane （right-lefl lateral bending） and transverse plane （torsion） due to a 5 N-m load are reported. Results show that overall normal discs are more flexible and behave in a nonlinear fashion. The testing results were used in a develop t＇mite element model of an intervertebral disc to investigate the stresses and strains in the disc components： annulus fibrosus and nucleus pulposus with regard to degeneration. Simulation of bending and torsion loadings show large strains in the annulus and nucleus from a normal disc, in contrast higher stresses develop in the annulus from a degenerated disc. The proposed methodology is novel, versatile, functional and economic with implications in bioengineering, medical sciences and the clinical field.

Relationship between the Initial Fracture Stress and Fatigue Limit—Simple Prediction Method of Tensile Fatigue Limit of Composite

This article presents an experimental study that clarifies the relationship between the initial fracture stress and fatigue limit of glass fiber reinforced unsaturated polyester resin specimens with a laminated structure taken from a pultruded square pipe. Quasi-static bending and tension tests are performed with acoustic emission (AE) measurements to identifying the occurrence of initial fracture during testing. AE and observation results have clarified the occurrence of initial fracture was detected by maximum acoustic energy values and corresponding fiber breakage in the unidirectional (UD) bundles. Moreover, the ratio of initial fracture stress to ultimate strength is 32% in bending and 26% in tension, when comparing stress and strains on the tension side of the UD layer. These values are in good agreement with each other and with the measured tensile fatigue limit when the cyclic stress is at 25% of the tensile strength. Initial fracture stress obtained by static tests is close values to the fatigue limit which will greatly contribute to the prediction of the fatigue limit.

Fatigue Performance of Bridge Deck Pavement Materials

Three beam samples of bridge deck pavement were prepared, with gradation types of AC-13, and AC-16 and combined AC-13＋AC-16. Four-point bending test was adopted to investigate the fatigue performance of these beam samples. The experimental results indicate that the initial bending stiffness is related to the type of beam sample samples decreases as the increase of the controlled strain fatigue resistance and bigger limiting bending strain at pared with single beam sample, the fatigue performance and testing temperature. Fatigue life of these level. The AC-13 beam sample exhibits better the given strain level and temperature. Corn- of combining beam samnle is relatively poor.

A review on evaluation of crack resistance of asphalt mixture by semi-circular bending test

Although there are many kinds of fracture tests to choose from in evaluating the crack resistance of asphalt mixture,the semi-circular bending(SCB)test has attracted a lot of attention in the academic road engineering community because of its simplicity,stability,and flexibility in testing and evaluation.The SCB test has become a common method to study the cracking resistance of asphalt mixture in recent years.This paper mainly summarizes the overview of the SCB test,summarizes some research results and common characterization parameters of the SCB test method in monotone test and fatigue test in recent years,and predicts and suggests the research direction of the SCB test in the future.It is found that the research on the monotonic SCB test is more comprehensive,and the research on the SCB fatigue test needs to be further improved in the aspects of loading mode,characterization parameter selection,and so on.Researchers can flexibly adjust the geometric dimensions and the test parameters of semi-cylindrical specimens,and conduct comprehensive analysis combined with the results of numerical simulation.The crack resistance of asphalt mixture can be comprehensively evaluated by fracture energy,fracture toughness,stiffness,flexibility index and other fracture indicators,combined with the crack propagation of the specimen.The analysis of numerical simulation can confirm the test results.In order to standardize the setting of fatigue parameters for future application,it is necessary to standardize the setting of bending performance.

A NEW EXPRESSION OF FATIGUE SIZE FACTOR

An expression describing the size effect on fatigue strength is presented, which is deduced based on the fatigue design approach of stress field intensity. The fatigue size factors under rotating-bending or reversed torsion for smooth specimen and under rotating-bending for notched specimen are estimated, and the main effects on the fatigue size factor are discussed. Some experimental data are analysed and it is shown that the description of fatigue size factor developed in this paper is satisfactory.

Fatigue crack growth behavior of a 170 mm diameter stainless steel straight pipe subjected to combined torsion and bending load

In a nuclear powerplant,the rotary equipment,such as a pump directly fitted with hanger in the piping system,experiences torsional and bending loads.Higher crack growth rate occurs because of this torsional load in addition to the bending load.Hence,it is necessary to study the fatigue behavior of piping components under the influence of combined torsional and bending load.In this study,experimental fatigue life evaluation was conducted on a notched stainless steel SA312 Type 304LN straight pipe having an outer diameter of 170 mm.The experimental crack depth was measured using alternating current potential drop technique.The fatigue life of the stainless steel straight pipe was predicted using experiments,Delale and Erdogan method,and area-averaged root mean square-stress intensity factor approach at the deepest and surface points of the notch.Afterward,the fatigue crack growth and crack pattern were discussed.As a result,fatigue crack growth predicted using analytical methods are in good agreement with experimental results.

SHORT FATIGUE CRACK PARAMETER BASED ON THE TOTAL CRACK AREA

The progressive fatigue damage of a material is closely related to the whole population of cracks on its surface. In order to understand which parameter is a more useful indicator of fatigue damage, rotatory bending fatigue tests were carried out using smooth specimens of medium-carbon steel. The behavior of short crack propagation during fatigue was examined and a new parameter total crack area was suggested. The aim of this paper is to extend the research on fatigue damage in the already studied steel and to study how these damage parameters are correlated with the process of fatigue damage in order to evaluate the effectiveness of damage detection methods.

沥青混合料弯曲疲劳损伤强度的多尺度研究

目的为了验证多尺度数值试验模拟方法在预测沥青混合料半圆弯曲试验疲劳损伤过程中弯拉强度参数变化规律的有效性问题。方法基于多尺度算法思想和半圆弯曲试验,通过对沥青混合料试件进行细观结构分析,得到内部集料颗粒的几何特征分布规律;在此基础上,对沥青混合料划分5个微观区域尺度,并构建相应尺度下半圆弯曲试件的有限元(FEM)数值模型,结合多尺度迭代运算来预测各级沥青混合料计算在0%Nf,20%Nf,50%Nf,65%Nf,80%Nf(Nf为疲劳寿命)不同半圆弯拉疲劳损伤程度下的弯拉强度。将室内半圆弯曲试验获得不同粒径的沥青混合料的弯拉强度参数作为宏观验证试验,与通过多尺度算法数值模拟试验所得的弯拉强度进行对比分析研究。结果结果表明:在沥青混合料的疲劳损伤过程中,前期材料的抗弯拉强度衰减速率较为缓慢,当在中后期时,抗弯拉强度进入了急剧衰减阶段,多尺度数值模拟试验与室内试验得到的衰减规律相吻合,且弯拉强度相对误差的绝对值控制在8%以内。结论多尺度数值模拟用于预测疲劳过程中沥青混合料的抗弯拉强度参数是有效的,该算法可以为类似的实际工程提供一定的参考意义。

压弯扭作用下新型承插装配式桥墩的抗震性能

为明确并提升承插式拼装桥墩抵抗压弯扭等复合荷载的能力,提出了一种结合灌浆套筒和承插口组合连接的新型承插装配式墩,通过复合荷载作用下的拟静力试验对比了现浇(reinforced concrete,RC)、灌浆套筒(grouting and sleeve,GS)、承插口(socket with ultra-high performance concrete,SU)和结合套筒连接钢筋的新型承插(grouting sleeve and socket with ultra-high performance concrete,GSU)连接拼装桥墩的损伤机理和滞回性能,结合有限元模型重点讨论了承插口深度对滞回性能的影响。结果表明:4个构件的破坏模式都是以受弯破坏为主的弯扭破坏,其中SU构件出现了轻微拔起的现象,而对应的GSU构件并未出现该现象,与RC构件接近;各构件的剪力-墩顶位移骨架发展趋势比较一致,由于GSU构件纵向钢筋连续,具有更好的整体性能,其抗弯承载力与RC构件接近,且明显大于SU和GS构件,4个构件弯曲滞回耗能较为接近;承插口深度为1.0倍截面宽度的GSU构件抗扭承载力略高于RC构件,且明显大于其余装配式墩,GSU构件的扭转刚度、延性系数和耗能能力均大于其他3个墩;当承插口深度采用0.5倍构件截面宽度时,新型承插GSU构件的抗弯和抗扭承载力均略高于整体现浇构件,具有良好的抵抗压弯扭荷载的能力,可以实现浅承插口连接。研究结果可为压弯扭复合作用下装配式墩的应用提供试验依据。

沥青混合料半圆弯曲试验疲劳剩余强度分析与预测

为揭示沥青混合料在弯拉作用下的疲劳剩余强度规律,基于半圆弯曲试验,对AC-13C型沥青混合料及含不同粒径集料的AC-9.5、AC-4.75、AC-2.36、AC-1.18型沥青混合料开展了疲劳试验、疲劳剩余强度试验等研究,结合多尺度算法建立的数值模型,预测了不同粒径沥青混合料在不同损伤程度下的抗弯拉剩余强度和剩余强度衰减规律。预测结果与沥青混合料半圆弯曲试验结果对比分析表明,利用多尺度算法建立的数值模型,能够较精确地预测沥青混合料的抗弯拉剩余强度,预测结果与半圆弯曲试验结果吻合良好。

考虑骨料类型的沥青混合料疲劳预估模型

为了更准确预测不同骨料类型沥青混合料的疲劳性能,本研究基于JTG D50—2017《公路沥青路面设计规范》中的疲劳预估模型(JTG模型)进行了深入探讨。采用均匀试验设计法,对AC-16C型沥青混合料进行了四点弯曲小梁疲劳试验。通过67组玄武岩、石灰岩和花岗岩沥青混合料的疲劳数据,对JTG模型参数进行了修正。研究发现:(1)简化的JTG模型适用于沥青混合料疲劳寿命预估,不同骨料类型对模型参数a的取值有影响;(2)玄武岩沥青混合料疲劳寿命最长,石灰岩最短,花岗岩居中;(3)修正后的JTG模型与实际疲劳试验数据高度吻合。本研究为沥青混合料的疲劳性能提供了更为精确的预估模型。

寒区防渗层SBS改性沥青混凝土抗弯性能及三轴静动力特性研究

为提升寒区抽水蓄能电站面板防渗层服役能力,以低冻断温度为调控指标,制备了一种SBS改性沥青混凝土,考察了-1℃条件下该沥青混凝土弯拉应变、弯曲蠕变、弯曲疲劳、静三轴和动三轴性能,并与常规水工90号沥青混凝土对比。结果表明,SBS改性沥青混凝土冻断温度-46.3℃,比水工90号沥青混凝土降低10.1℃;弯曲应变、拉伸应变、弯曲蠕变、弯曲疲劳性能均不同程度提高;-1℃水工90号沥青混凝土静三轴和动三轴试验参数,如黏聚力、模量数、动应力、动弹性模量等显著增加,SBS改性沥青混凝土弹性模量、模量数、黏聚力等刚度指标相对较小,与常温条件水工90号沥青混凝土相当,低温柔性明显较好,能够弥补寒区环境对防渗层普通沥青混凝土造成的不利影响。

四辊轧机工作辊辊颈端部铸接修复部位寿命评估

轧辊作为带钢生产轧机的核心部件,其消耗对生产成本有重要影响。随着钢铁企业竞争加剧导致的降本增效需求逐步提高,轧辊再制造循环利用得到逐步推广。大型轧机大尺寸轧辊工作层消耗后改造用于小轧机为其中主要方式之一;对于局部尺寸尤其是辊肩尺寸不足的轧辊通常采用铸接方式加长以满足进一步使用的需求;铸接再制造后轧辊的使用寿命成为保障安全生产的关键。针对上述问题,在轧辊铸接部位取样开展了疲劳性能测试,同时选用扭转疲劳模型在证明计算精度的基础上,结合基于现场生产数据建立的载荷谱,对工作辊修复后的辊颈进行了寿命预测,预期服役寿命满足现场对修复后轧辊的性能需求,可为现场轧辊安全使用维护制度建立提供理论支撑。

蜂窝夹层结构弯曲疲劳寿命预测研究

复合材料夹层板兼具轻质高承载特性,但也面临多样的失效模式,深入研究其性能与失效机理,对确保结构安全与效能至关重要。针对一种玻璃纤维/氰酸酯织物层压面板和Nomex蜂窝复合的夹层板,采用特殊设计的试验夹具,开展三点弯曲静力和疲劳试验,分析该夹层板的弯—剪承载特性,观察其疲劳损伤和失效现象;同时,根据夹层板承力特点,对受面内力的面板和受面外剪切的芯材分别建立疲劳损伤模型,借助夹层板等效有限元模型,预测夹层板弯曲疲劳寿命。结果表明:所推荐的预测方法能够合理地反映夹层板弯曲疲劳损伤过程,疲劳寿命预测结果略为保守,与实测寿命误差属于可接受范围。

钢丝帘线/橡胶复合材料弯曲疲劳失效机理的研究

自主设计并搭建基于隧道磁阻传感器的无损检测系统,对钢丝帘线/橡胶复合材料试样(简称试样)弯曲疲劳失效机理进行研究。结果表明:无损检测方法可对疲劳试验后试样内部的损伤状态进行评估,无损检测系统可实现试样缺陷的在线检测;疲劳缺陷的产生是损伤累积,钢丝帘线单丝断裂主要发生试样疲劳寿命的最后阶段;试样内部钢丝疲劳断裂为穿晶解理断裂,裂纹扩展过程中存在一定程度的微观不连续性,包括多次角度偏折与裂纹分支;试样内部钢丝疲劳断口具有典型的低周疲劳特点,存在多个疲劳源与撕裂台阶,疲劳源间偶见撕裂棱,无明显贝纹样,疲劳源附近表面光滑,断口形成了较平滑的微撕裂形貌与粗糙的撕裂台阶形貌。

钢悬链线立管全尺寸共振弯曲疲劳试验系统与模态分析

钢悬链线立管(steel catenary riser,SCR)作为支撑海上平台和其他关键设备的重要组成部分,在极端的海洋环境中长时间服役运行时,其结构完整性和安全性至关重要。为确保SCR立管在恶劣环境下的安全运行,对立管进行共振弯曲疲劳试验研究成为必要的评估标准,同时也为后续立管疲劳寿命预测和失效分析提供基础。针对两种特定型号的SCR立管,推导并求解不同约束条件下的共振系统的固有频率与振型函数,分析管道长度、管道壁厚、偏心块和配重块质量对共振系统固有频率的影响;在此基础上开展不同特定工况下的全尺寸共振弯曲疲劳试验,同时对SCR立管在全尺寸共振弯曲疲劳试验系统中不同跨距及内压的振动响应特性进行了模拟。结合理论分析、有限元计算和共振弯曲疲劳试验结果,发现管道长度和两端的配重约束是影响共振系统1阶固有频率的重要参数,特别是在管道长度较短和壁厚较薄的情况下,影响更为显著。此外,注水加压和跨距增加会降低共振系统的固有频率,支撑跨距越短,系统的固有频率越大。分析所得结果为SCR立管的共振弯曲疲劳试验系统结构设计和试验方案优化提供了重要的参考依据。

乳化沥青冷再生混合料疲劳性能及机理分析

乳化沥青冷再生混合料的抗疲劳开裂性能会影响沥青路面结构层整体服役寿命。为了评价不同增强方式下乳化沥青冷再生沥青混合料的疲劳性能,以普通乳化沥青、外掺玄武岩纤维和SBR改性乳化沥青这3种冷再生混合料为研究对象,开展了乳化沥青冷再生混合料中梁试件的四点弯曲疲劳试验,建立了3种乳化沥青冷再生沥青混合料的疲劳方程,揭示了疲劳过程中动态弯拉模量和累积耗散能的演化规律。采用扫描电镜试验获取了乳化沥青冷再生混合料疲劳破坏界面的微观形貌。试验结果表明:动态弯拉模量衰减至初始模量的70%~80%即可能发生断裂。动态弯曲劲度模量比κ越小、累积耗散能越大,疲劳性能越好。SBR改性乳化沥青和外掺玄武岩纤维均可以改善乳化沥青冷再生混合料的抗疲劳性能。SEM试验揭示了乳化沥青冷再生混合料的抗疲劳性能增强机理。研究成果可为乳化沥青冷再生混合料的抗疲劳性能提升及其耐久性设计提供参考。

高抗折矿渣硫铝酸盐水泥混凝土抗疲劳性能研究

传统水泥混凝土公路路面面临的主要问题是路面易开裂劣化和抗疲劳性能较差,这些问题大大缩短了路面的使用寿命,严重制约了水泥混凝土路面的发展和推广应用。矿渣硫铝酸盐水泥(Slag-Calcium Sulfoaluminate Cement,S-CSA)是一种具有高抗折强度的新型胶凝材料,利用该水泥制备的混凝土路面具有解决现有路面抗疲劳性能差的潜力。通过三点弯曲疲劳试验和声发射试验,对比分析了高抗折S-CSA混凝土和普通硅酸盐水泥(Ordinary Portland Cement,OPC)混凝土的抗疲劳性能。研究结果表明:相同荷载作用下,高抗折S-CSA混凝土粗骨料断裂数量更多;不同强度等级下,高抗折S-CSA混凝土的疲劳寿命均高于OPC混凝土,且提升幅度随着强度等级的提高而显著增加,最高可达到约7.2倍;钙矾石的生成有效改善了界面过渡区,从而提升了高抗折S-CSA混凝土的抗疲劳性能。

长挑臂闭口钢箱组合梁桥设计及其关键技术

为满足黄河桥梁跨度需求和减少临时施工设施对黄河泄洪的干扰,该文以临猗黄河大桥为依托,其主桥采用(112+28×128+120) m连续长挑臂闭口钢箱组合梁,共分两联,最大联长1 912 m。钢箱组合梁采用无临时墩顶推法施工,闭口钢箱梁顶推到位后再安装桥面板,桥面板安装顺序为先跨中正弯矩区后支点负弯矩区。重点对组合梁负弯矩区力学性能优化措施、斜撑抗疲劳设计和2.5 m高声屏障抗风优化设计3个关键技术问题开展研究,结果表明:负弯矩区采用双结合构造可降低钢箱梁底板压应力和桥面板裂缝宽度,张拉体外预应力钢束的优化效果随着混凝土收缩徐变作用逐渐降低;相同截面积的方钢管斜撑抗疲劳性能优于圆钢管;折线形声屏障可有效抑制主梁涡振响应。