Effect of inclusion size on the high cycle fatigue strength and failure mode of a high V alloyed powder metallurgy tool steel

The fatigue strength of a high V alloyed powder metallurgy tool steel with two different inclusion size levels, tempered at different temperatures, was investigated by a series of high cycle fatigue tests. It was shown that brittle inclusions with large sizes above 30μm prompted the occurrence of subsurface crack initiation and the reduction in fatigue strength. The fracture toughness and the stress amplitude both exerted a significant influence on the fish-eye size. A larger fish-eye area would form in the sample with a higher fracture toughness subjected to a lower stress amplitude. The stress intensity factor of the inclusion was found to lie above a typical value of the threshold stress intensity factor of 4 MPa.m^1/2. The fracture toughness of the sample with a hardness above HRC 56 could be estimated by the mean value of the stress intensity factor of the fish-eye. According to fractographic evaluation, the critical inclusion size can be calculated by linear fracture mechanics.

A fatigue damage model for asphalt mixtures under controlled-stress and controlled-strain modes

A fatigue damage model based on thermodynamics was deduced for asphalt mixtures under controlled-stress and controlled-strain modes. By employing modulus of resilience as the damage hardening variable, a damage variable related with dynamic modulus was extracted as the evaluation index. Then, the damage evolution law under two control modes was proposed, and it has a similar form to the Chaboche fatigue model with a nonnegative material parameter m related to its loading level. Experimental data of four loading levels were employed to calibrate the model and identify the parameter in both control modes. It is found that the parameter m shows an exponential relationship with its loading level. Besides, the difference of damage evolution under two control modes was explained by the law. The damage evolves from fast to slow under a controlled-strain mode. However, under a controlled-stress mode, the evolution rate is just the opposite. By using the damage equivalence principle to calculate the equivalent cycle numbers, the deduced model also interprets the difference of damage evolution under two control modes on the condition of multilevel loading. Under a controlled-strain mode, a loading sequence from a low level to a high level accelerates damage evolution. An inverse order under the controlled-stress mode can prolong fatigue life.

FATIGUE GROWTH MODELING OF MIXED-MODE CRACK IN PLANE ELASTIC MEDIA

This paper presents an extension of a displacement discontinuity method with cracktip elements （a boundary element method） proposed by the author for fatigue crack growth analysis in plane elastic media under mixed-mode conditions. The boundary element method consists of the non-singular displacement discontinuity elements presented by Crouch and Starfield and the crack-tip displacement discontinuity elements due to the author. In the boundary element implementation the left or right crack-tip element is placed locally at the corresponding left or right crack tip on top of the non-singular displacement discontinuity elements that cover the entire crack surface and the other boundaries. Crack growth is simulated with an incremental crack extension analysis based on the maximum circumferential stress criterion. In the numerical simulation, for each increment of crack extension, remeshing of existing boundaries is not required because of an intrinsic feature of the numerical approach. Crack growth is modeled by adding new boundary elements on the incremental crack extension to the previous crack boundaries. At the same time, the element characteristics of some related elements are adjusted according to the manner in which the boundary element method is implemented. As an example, the fatigue growth process of cracks emanating from a circular hole in a plane elastic plate is simulated using the numerical simulation approach.

Effect of Strain Ratio on Fatigue Model of Ultra-fine Grained Pure Titanium

The ultra-fine grained(UFG)pure titanium was prepared by equal channel angular pressing(ECAP)and rotary swaging(RS).The strain controlled low cycle fatigue(LCF)test was carried out at room temperature.The fatigue life prediction model and mean stress relaxation model under asymmetrical stress load were discussed.The results show that the strain ratio has a significant effect on the low cycle fatigue performance of the UFG pure titanium,and the traditional Manson-coffin model can not accurately predict the fatigue life under asymmetric stress load.Therefore,the SWT mean stress correction model and three-parameter power curve model are proposed,and the test results are verified.The final research shows that the threeparameter power surface model has better representation.By studying the mean stress relaxation phenomenon under the condition of R≠-1,it is revealed that the stress ratio and the strain amplitude are the factors that significantly afiect the mean stress relaxation rate,and the mean stress relaxation model with the two variables is calculated to describe the mean stress relaxation phenomenon of the UFG pure titanium under different strain ratios.The fracture morphology of the samples was observed by SEM,and it was concluded that the final fracture zone of the fatigue fracture of the UFG pure titanium was a mixture of ductile fracture and quasi cleavage fracture.The toughness of the material increases with the increase of strain ratio at the same strain amplitude.

Experimental study on mechanical properties of dowel bar embedded in concrete under fatigue loads

To investigate the mechanical properties of a dowel action under fatigue loads, nine reinforced concrete specimens were fabricated, and the monotonic and fatigue loadings were performed on these specimens, respectively. All of these specimens were divided into two series. Six specimens in SeriesⅠwith different bar diameters of 12, 20 and 25 mm were subjected to monotonic loads and were used to confirm the ultimate bearing capacity. The remaining three specimens in Series Ⅱ were subjected to fatigue loads and were designed to investigate the attenuation character of dowel action and the fatigue failure modes. The test results show that the accumulated fatigue damage due to fatigue loads can reduce the ultimate bearing capacity of specimens. With the increase in fatigue loads, the failure mode can transform to fatigue rupture of the dowel bar under the serviceability loading state,i. e. 55% of the monotonic capacity. The fatigue life is determined by the fatigue properties of steel and concrete.Based on the test data, the failure process of dowel action can be divided into two stages： the accumulation of fatigue damage and the fatigue rupture of dowel bar.

Residual Fatigue Life Prediction of Ball Bearings Based on Paris Law and RMS

Paris law can reflect the failure mechanism of materials and is usually used to be a method to predict fatigue life or residual fatigue life.But the variable which can represent the health of machine is hardly measured on line.To a degree,the difficulty of on-line application restricts the scope of application of Paris law.The relationship between characteristic values of vibration signals and the variable in the Paris equation which can describe the health of machine is investigated by taking ball bearings as investigative objects.Based on 6205 deep groove ball bearings as a living example,historical lives and vibration signals are analyzed.The feasibility of describing that variable in the Paris equation by the characteristic value of vibration signals is inspected.After that vibration signals decomposed by empirical mode decomposition（EMD）,root mean square（RMS） of intrinsic mode function（IMF） involving fault characteristic frequency has a consistent trend with the diameter of flaws.Based on the trend,two improved Paris models are proposed and the scope of application of them is inspected.These two Paris Models are validated by fatigue residual life data from tests of rolling element bearings and vibration signals monitored in the process of operation of rolling element bearings.It shows that the first improved Paris Model is simple and plain and it can be easily applied in actual conditions.The trend of the fatigue residual life predicted by the second improved Paris model is close to the actual conditions and the result of the prediction is slightly greater than the truth.In conclusion,after the appearance of detectable faults,these improved models based on RMS can predict residual fatigue life on line and a new approach to predict residual fatigue life of ball bearings on line without disturbing the machine running is provided.

Overall Evaluation of the Effect of Residual Stress Induced by Shot Peening in the Improvement of Fatigue Fracture Resistance for Metallic Materials

Before 1980s,the circular suspension spring in automobile subjected to torsion fatigue load,under the cyclic normal tensile stresses,the majority of fatigue fracture occurred was in normal tensile fracture mode（NTFM）and the fracture surface was under 45°diagonal.Because there exists the interaction between the residual stresses induced by shot peening and the applied cyclic normal tensile stresses in NTFM,which represents as＂stress strengthening mechanism＂,shot peening technology could be used for improving the fatigue fracture resistance（FFR）of springs.However,since 1990s up to date,in addition to regular NTFM,the fatigue fractures occurred of peened springs from time to time are in longitudinal shear fracture mode（LSFM）or transverse shear fracture mode（TSFM）with the increase of applied cyclic shear stresses,which leads to a remarkable decrease of FFR.However,LSFM/TSFM can be avoided effectively by means of shot peening treatment again on the peened springs.The phenomena have been rarely happened before.At present there are few literatures concerning this problem.Based upon the results of force analysis of a spring,there is no interaction between the residual stresses by shot peening and the applied cyclic shear stresses in shear fracture.This;means that the effect of＂stress strengthening mechanism＂for improving the FFR of LSFM/TSFM is disappeared basically.During shot peening,however,both of residual stress and cyclic plastic deformed microstructure are induced synchronously like＂twins＂in the surface layer of a spring.It has been found for the first time by means of force analysis and experimental results that the modified microstructure in the＂twins＂as a＂structure strengthening mechanism＂can improve the FFR of LSFM/TSFM.At the same time,it is;also shown that the optimum technology of shot peening strengthening must have both＂stress strengthening mechanism＂and＂structure strengthening mechanism＂simultaneously so that the FFR of both NTFM and LSFM/TSFM can be improved by shot peening.

Low cyclic fatigue performance of concrete-filled steel tube columns

Eight concrete-filled steel tubular(CFT) columns were tested subjected to cyclic loading under constant axial load. Experimental parameters included axial compression ratio, loading sequences, and strength of concrete and steel. The seismic performance of CFT columns and failure modes were analyzed. The test results show that different axial load ratios and loading sequences have effects on the load carrying capacity, ductility and energy dissipation capacity of CFT columns, as well as the failure modes of the CFT columns. The failure pattern can be categorized into two types: local buckling failure of steel tube in compression zone, and low cycle fatigue tearing rupture failure of steel tube. The seismic behavior was evaluated through the energy index obtained from each cycle.

EFFECT OF NON-PROPORTIONAL OVERLOADING ON FATIGUE CRACK GROWTH

The effect of proportional and non-proportional overloading on mode l fatigue crack growth have been studied,and the influences of crack tip plastic zone,crack tip blunting as well as crack closure were discussed.Proportional(model I)overloading may cause more serious crack growth retardation than non-proportional(mixed mode)overloading.Therefore,for estimating the fatigue life of engineering structures to simplify a real overload which may of- ten be non-proportional as a proportional one is not always safe.

A data-driven approach for predicting the fatigue life and failure mode of self-piercing rivet joints

In lightweight automotive vehicles,the application of self-piercing rivet(SPR)joints is becoming increasingly widespread.Considering the importance of automotive service performance,the fatigue performance of SPR joints has received considerable attention.Therefore,this study proposes a data-driven approach to predict the fatigue life and failure modes of SPR joints.The dataset comprises three specimen types:cross-tensile,cross-peel,and tensile-shear.To ensure data consistency,a finite element analysis was employed to convert the external loads of the different specimens.Feature selection was implemented using various machine-learning algorithms to determine the model input.The Gaussian process regression algorithm was used to predict fatigue life,and its performance was compared with different kernel functions commonly used in the field.The results revealed that the Matern kernel exhibited an exceptional predictive capability for fatigue life.Among the data points,95.9%fell within the 3-fold error band,and the remaining 4.1%exceeded the 3-fold error band owing to inherent dispersion in the fatigue data.To predict the failure location,various tree and artificial neural network(ANN)models were compared.The findings indicated that the ANN models slightly outperformed the tree models.The ANN model accurately predicts the failure of joints with varying dimensions and materials.However,minor deviations were observed for the joints with the same sheet.Overall,this data-driven approach provided a reliable predictive model for estimating the fatigue life and failure location of SPR joints.

EFFECTS OF SLIP MODE ON CYCLIC BEHAVIOR OF PRESTRAINED MATERIALS

The cyclic behaviours of two prestrained materials with different slip modes have been com- pared.From the analysis of hysterisis loops,it is suggested that the decrease in back stress dominates the cyclic process of Cu-Zn alloy with planar slip mode,while the decrease in frictional stress is the main factor affecting the behaviour of Cu with wavy slip mode.The re- lation between the dislocation configuration and internal stress has been also discussed.By means of the relation just mentioned,the variations of dislocation configuration before and after fatigue of the two materials,the differences in their softening mechanism and mechani- cal behaviour could be interpreted.

ROUGHNESS-INDUCED SHEAR RESISTANCE OF MODE Ⅱ CRACK GROWTH

It is obtained in this paper that the fatigue threshold value of mode H was 1.9 times of that of mode Ⅰ in dual-phase steel(DPS),and the normal stress intensity factor range oJ mode Ⅱ branch crack tip was 2.2 times of that of mode Ⅰ.Above results illustrate that the resistance of mode Ⅱ crack growth was higher than that of mode Ⅰ,the former resulting from roughness-induced shear resistance,the latter,crack closure. The mode Ⅱ component can play two important roles in near-threshold fatigue crack growth:(1)increasing crack tip plasticity which accelerates the crack growth and(2)intro- ducing crack surface contact and rubbing to reduce the crack propagation rate.By means of crack closure,the quantity of shear resistance was easily solved in this paper.The friction shear stress strength factor range of mode Ⅱ,K_,is still much higher than the closure stress strength factor range of mode Ⅰ,K_(Ⅰ,cl).This illustrated that the roughness enlarged the second role and played a role of shielding crack tip from mode Ⅱ crack.

SLEEP-MAD模式护理策略在结直肠癌病人术后护理中的应用

目的:探讨SLEEP-MAD模式护理策略在结直肠癌病人术后护理中的应用价值。方法:2022年5月—2023年5月选取医院收治的86例结直肠癌病人为研究对象,按照随机数字表法将病人分为观察组、对照组各43例,对照组行常规护理,观察组实施SLEEP-MAD模式护理策略,干预前后采用汉密尔顿焦虑量表(HAMA)、汉密尔顿抑郁量表(HAMD)、视觉模拟疼痛评分表(VAS)、匹兹堡睡眠质量指数量表(PSQI)、癌因性疲乏评分及欧洲癌症研究与治疗组织生命质量量表(EORTC QLQ-C30)评价病人的不良情绪、疼痛感、癌因性疲乏、睡眠质量及生活质量。结果:干预后观察组病人HAMA、HAMD、VAS、PSQI及癌因性疲乏评分低于对照组(P<0.05);干预后观察组病人EORTC QLQ-C30评分高于对照组(P<0.05)。结论:SLEEP-MAD模式护理策略可有效减轻结直肠癌病人焦虑及抑郁情绪,减轻病人术后疼痛感及癌因性疲乏,改善病人睡眠质量及生活质量。

基于有限元分析的连续油管寿命研究及预测模型

目前针对连续油管的寿命预测研究较少,但在实际的生产中连续油管的突然失效会造成极大安全隐患,急需解决方案。针对此问题,本文使用有限元仿真方法,针对三种规格的CT80连续油管在不同工况下进行有限元分析,得到了连续油管的受力、形变和疲劳寿命。用疲劳试验对有限元分析进行验证,证明了有限元分析的疲劳寿命的准确性。将连续油管的直径、壁厚、内压、弯曲半径参数转化为外壁应变和环向应力,通过数据拟合建立了关于外壁应变、环向应力和疲劳寿命之间的全经验公式,得到连续油管疲劳寿命预测模型。并用试验得出的结果与疲劳寿命预测模型进行比较,误差较小,可用于CT80连续油管的疲劳寿命预测,最后通过疲劳试验得到了连续油管在二级加载下的疲劳寿命模型,该模型可预测油管在第二级载荷下的剩余疲劳寿命。

冻融雁行裂隙花岗岩单轴压缩宏细观疲劳特性试验研究

对于季冻区的岩体工程,岩石经常受到冻融和周期荷载扰动引起的耦合疲劳损伤。为了研究冻融裂隙花岗岩的疲劳开裂扩展情况,量化裂纹的损伤程度和分类类型,对冻融40次后的双裂隙花岗岩进行了3种应力振幅下的周期荷载试验。采用实时声发射和高速摄影监测技术来揭示裂纹演化,并进一步对不同的裂纹开裂类型进行分类,以帮助理解动态压裂规律,利用核磁共振测试分析裂隙花岗岩细观孔隙特征及其宏观力学特性的关系。结果表明:随着周期荷载振幅应力比的增加,双裂隙花岗岩在周期荷载“沉寂期”内的声发射计数增加;周期荷载阶段双裂隙花岗岩产生的剪切裂纹占比大于单调加载阶段,高振幅作用下岩石容易形成剪切破坏裂纹;双裂隙花岗岩声发射的最大幅值变小即大尺度破裂减少,周期荷载下岩样预制裂隙外尖端产生的向端面扩展裂纹的角度越大;双裂隙花岗岩的疲劳寿命减小,内部微裂隙扩展发育减缓,能够储存的含H流体也越少,最终表现为核磁共振T2谱面积变化率减小。

正交异性钢-钢纤维混凝土组合桥面板疲劳极限状态研究

为了解正交异性钢-钢纤维混凝土(Steel Fiber Reinforced Concrete,SFRC)组合桥面板的疲劳性能及失效机理,以川南城际铁路临港长江大桥为背景,设计、制作正交异性钢-SFRC组合桥面板足尺模型进行疲劳试验,采用ANSYS软件建立试件有限元模型,研究关键细节的疲劳应力和开裂等情况。基于有限元模型,分析不同设计参数(钢顶板厚度、栓钉布置、SFRC抗拉强度及层厚)下组合桥面板疲劳极限状态的失效模式,并提出了主要控制参数取值建议。结果表明:200万次疲劳加载后足尺模型的实测最大裂缝宽度为0.136 mm,出现在SFRC层上缘,钢结构未开裂,组合桥面板疲劳性能良好;组合桥面板疲劳极限状态主要由SFRC层开裂控制,钢顶板厚度、栓钉布置对组合桥面板疲劳性能的影响较小,SFRC抗拉强度和层厚对组合桥面板疲劳性能影响较大,为主要控制参数;在常规正交异性钢桥面板上铺设薄层(厚度不超过50 mm)SFRC时,SFRC抗拉强度不应小于5 MPa,在常规正交异性钢桥面板上铺设普通SFRC(钢纤维体积含量不高于1%,抗拉强度不高于3 MPa)时,SFRC层厚不宜低于100 mm。

IKAP模式对乳腺癌化疗患者自我感受负担及癌因性疲乏的影响

目的探讨信息-知识-信念-行为(Information-Knowledge-Attitude-Practice,IKAP)模式对乳腺癌化疗患者自我感受负担及癌因性疲乏的影响。方法选取2021年1月—2022年12月枣庄市立医院收治的82例乳腺癌患者作为研究对象,按照随机数表法分为观察组和对照组,每组41例。对照组患者实施常规干预模式,观察组患者实施IKAP干预模式,比较两组患者的自我感受负担、癌因性疲乏和生存质量改善情况。结果干预前,两组患者自我感受负担和癌因性疲乏评分比较,差异无统计学意义(P>0.05);干预后,观察组患者自我感受负担和癌因性疲乏评分低于对照组,差异有统计学意义(P<0.05)。观察组患者生存质量5个评分均高于对照组,差异有统计学意义(P<0.05)。结论IKAP模式能缓解乳腺癌化疗患者负性情绪,降低患者自我感受负担和癌因性疲乏,提高患者生活质量。

膏摩顶法改善青少年近视视疲劳的临床疗效观察

目的观察应用摩顶明目膏进行膏摩顶法对青少年近视视疲劳的疗效。方法纳入2021年9月—2022年9月于湖南中医药大学第一附属医院就诊的青少年近视合并视疲劳患者94例(188只眼),随机分为治疗组和对照组,各47例(94只眼)。治疗组予摩顶明目膏为介质对头部五经进行推拿,对照组采用眼周穴位推拿。2组均观察28 d。分别于治疗前、治疗后测量患者双眼视疲劳症状评分、调节幅度积分、调节灵活度积分、调节滞后量积分、正、负相对调节积分的数据。结果2组治疗前视疲劳症状评分、调节幅度积分、调节灵活度积分、调节滞后量积分、正、负相对调节积分比较,差异均无统计学意义(P>0.05)。(1)双眼视疲劳症状评分:2组治疗后均较治疗前降低,差异均有统计学意义(t_(治疗组)=5.300,P=0.000;t_(对照组)=4.470,P=0.001)。2组间治疗后比较,治疗组视疲劳症状评分低于对照组,差异有统计学意义(t=2.950,P=0.004)。(2)双眼调节幅度积分:2组治疗后均较治疗前升高,差异有统计学意义(t_(治疗组)=7.560、t_(对照组)=3.800,均P=0.000)。2组间治疗后比较,治疗组高于对照组,差异有统计学意义(t=5.720,P=0.000)。(3)双眼调节灵活度积分:2组治疗后均较治疗前升高,差异均有统计学意义(t_(治疗组)=16.240、t_(对照组)=6.940,均P=0.000)。2组间治疗后比较,治疗组高于对照组,差异有统计学意义(t=9.980,P=0.000)。(4)双眼调节滞后量积分:2组治疗后均较治疗前降低,差异有统计学意义(t_(治疗组)=9.110、t_(对照组)=4.780,均P=0.000)。2组间治疗后比较,治疗组低于对照组,差异有统计学意义(t=2.973,P=0.003)。(5)双眼正相对调节积分:2组治疗后均较治疗前升高,差异有统计学意义(t_(治疗组)=5.920,P=0.000;t_(对照组)=2.530,P=0.012)。2组间治疗后比较,治疗组高于对照组,差异有统计学意义(t=11.300,P=0.000)。(6)双眼负相对调节积分:2组治疗后均较治疗前降低,差异有统计学意义(t_(治疗组)=10.930、t_(对照组)=8.810,均P=0.000)。2组间治疗后比较,治疗组低于对照组,差异有统计学意义(t=3.660,P=0.000)。结论应用膏摩顶法对头部五经进行推拿对青少年近视及视疲劳取得较好的治疗效果,具有较高的临床推广价值。

熔焊缺陷对镍基高温合金GH4065A疲劳行为的影响

采用SEM和EBSD显微分析手段研究镍基变形高温合金GH4065A熔焊焊点的缺欠组织,并对比研究无焊点、有密排焊点和疏散排布焊点3种GH4065A带中孔薄板试样分别在低周和低高周复合疲劳载荷下的寿命差异和断裂方式差异。结果表明:焊点组织中存在未熔合孔洞、凝固裂纹和液化裂纹,是导致含焊点试样低周和低高周复合疲劳寿命大幅下降的主要熔焊缺陷。这些熔焊缺陷的存在使得疲劳裂纹从无焊点试样的中孔内表面处转为在焊点处优先形成,导致700℃/700 MPa低周疲劳寿命的下降幅度可达44%~83%。在600℃和700℃低高周复合载荷(静应力700 MPa+动应力100 MPa)下,熔焊缺陷不仅使得裂纹源从中孔内表面处转为在焊点处优先形成,也改变了裂纹扩展方式,增大了沿晶扩展倾向。这使得低高周复合疲劳寿命在两种温度下均大幅下降超过85%。由于密排焊点因距离中孔结构更近,密排焊点试样低周疲劳寿命低于疏散排布焊点试样,但这种焊点情况差异对低高周复合疲劳寿命的影响不大。

司机操纵模式对岸桥起重机小车轨道疲劳寿命的影响研究

实践证明,岸桥起重机司机的操纵方法是影响岸桥起重机小车轨道寿命的关键因素,平稳操纵对防止小车轨道偶发冲击发挥着至关重要的作用,挖掘出司机的操纵模式并探究其对小车轨道寿命的影响具有重要意义.为研究司机的不同操纵对岸边集装箱桥式起重机的小车轨道疲劳寿命的影响,采集了两个月的应力数据和可编程逻辑控制器(PLC)数据,并对小车轨道的疲劳寿命进行分析.首先通过自适应共振理论神经网络(ART)聚类算法实现司机操纵模式数据的分类,然后将每类操纵对应的应力数据进行峰值压缩、无效幅值去除,再利用雨流计数法得出幅值统计直方图,最后利用Miner法则求出三类操纵下小车轨道的疲劳寿命.结果显示,第三类操纵下小车轨道的四个测点的疲劳寿命优于第一类和第二类.该文研究结果可为港口司机操作培训提供参考,同时也为厂家改进小车轨道设计提供依据.