Fatigue Crack Growth Rate of Ti-6Al-4V Considering the Effects of Fracture Toughness and Crack Closure

Fatigue fracture is one of the main failure modes of Ti-6A1-4V alloy,fracture toughness and crack closure have strong effects on the fatigue crack growth（FCG）rate of Ti-6A1-4V alloy.The FCG rate of Ti-6A1-4V is investigated by using experimental and analytical methods.The effects of stress ratio,crack closure and fracture toughness on the FCG rate are studied and discussed.A modified prediction model of the FCG rate is proposed,and the relationship between the fracture toughness and the stress intensity factor（SIF）range is redefined by introducing a correcting coefficient.Notched plate fatigue tests（including the fracture toughness test and the FCG rate test）are conducted to investigate the influence of affecting factors on the FCG rate.Comparisons between the predicted results of the proposed model,the Paris model,the Walker model,the Sadananda model,and the experimental data show that the proposed model gives the best agreement with the test data particularly in the near-threshold region and the Paris region,and the corresponding calculated fatigue life is also accurate in the same regions.By considering the effects of fracture toughness and crack closure,the novel FCG rate prediction model not only improves the estimating accuracy,but also extends the adaptability of the FCG rate prediction model in engineering.

Influence of Ultrasonic Surface Rolling Process and Shot Peening on Fretting Fatigue Performance of Ti-6Al-4V

At present,there are many studies on the residual stress field and plastic strain field introduced by surface strengthening,which can well hinder the initiation of early fatigue cracks and delay the propagation of fatigue cracks.However,there are few studies on the effects of these key factors on fretting wear.In the paper,shot-peening(SP)and ultrasonic surface rolling process(USRP)were performed on Ti-6Al-4V plate specimens.The surface hardness and residual stresses of the material were tested by vickers indenter and X-ray diffraction residual stress analyzer.Microhardness were measured by HXD-1000MC/CD micro Vickers hardness tester.The effects of different surface strengthening on its fretting fatigue properties were verified by fretting fatigue experiments.The fretting fatigue fracture surface and wear morphology of the specimens were studied and analyzed by means of microscopic observation,and the mechanism of improving fretting fatigue life by surface strengthening process was further explained.After USRP treatment,the surface roughness of Ti-6Al-4V is significantly improved.In addition,the microhardness of the specimen after SP reaches the maximum at 80μm from the surface,which is about 123%higher than that of the AsR specimen.After USRP,it reaches the maximum at 150μm from the surface,which is about 128%higher than that of AsR specimen.It is also found that the residual compressive stress of the specimens treated by USRP and SP increases first and then decreases with the depth direction,and the residual stress reaches the maximum on the sub surface.The USRP specimen reaches the maximum value at 0.18 mm,about−550 MPa,while the SP specimen reaches the maximum value at 0.1 mm,about−380 MPa.The fretting fatigue life of Ti-6Al-4V effectively improved after USRP and SP.The surface integrity of specimens after USRP is the best,which has deeper residual compressive stress layer and more refined grain.In this paper,a fretting wear device is designed to carry out fretting fatigue experiments on specimens with different surface strengthening.

Fatigue Property of Additively Manufactured Ti-6Al-4V under Nonproportional Multiaxial Loading

The low cycle fatigue strength properties of the additively manufactured Ti-6Al-4V alloy are experimentally investi-gated under proportional and nonproportional multiaxial loading.The fatigue tests were conducted using hollow cylinder specimens with and without heat treatments,at room temperature in air.Two fatigue tests were conducted:one for proportional loading and one for nonproportional loading.The proportional loading was represented by a push-pull strain path(PP)and the nonproportional loading by a circle strain path(Cl).The failure lives of the additively manufactured specimens were clearly reduced drastically by internal voids and defects.However,the sizes of the defects were measured,and the defects were found not to cause a reduction in fatigue strength above a critical size.The fracture surface was observed using scanning electron microscopy to investigate the fracture mechanisms of the additively manufactured specimens under the two types of strain paths.Different fracture patterns were recognized for each strain paths;however,both showed retention of the crack propagation,despite the presence of numerous defects,probably because of the interaction of the defects.The crack propagation properties of the materials with numerous defects under nonproportional multiaxial loading were clarified to increase the reliability of the additively manufactured components.

Coordinately Improving the Wear and Fatigue Properties of Ti6Al4V Alloy by Designed CrZr-alloyed Layer

A CrZr-alloyed layer was prepared through a pre-zirconizing and subsequent chromizing treatment on a Ti6Al4V substrate.After the removal of the top Cr deposit and Ti4Cr layers,a（Cr,Zr）-Ti solidsolution layer was obtained.The microstructure,composition,microhardness and toughness of the（Cr,Zr）-Ti solid-solution layer were evaluated.The results showed that the pre-addition of Zr played an important role in inhibiting the precipitation of the soft Ti4Cr phase,which in turn allowed us to obtain a material characterized by a remarkable hardness.Wear and fatigue tests showed that the（Cr,Zr）-Ti solid-solution layer could coordinately improve the properties of the Ti6Al4V alloy.This was mainly due to the good match of hardness and toughness of the（Cr,Zr）-Ti solid-solution layer.In addition,the gradual change in composition and mechanical properties was conducive to the coordinated deformation between the（Cr,Zr）-Ti solid-solution layer and the Ti6Al4V substrate during fatigue tests.This reduced the stress concentration in correspondence of the interface between the two materials.

S含量对18CrNiMo7-6齿轮钢中夹杂物和疲劳性能的影响

以w[S]为0.002%(低S含量)和0.022%(高S含量)的两种18CrNiMo7-6齿轮钢为研究对象,参照齿轮热处理工艺获得伪渗碳试样,并对其力学性能、显微组织、疲劳性能和夹杂物分布进行了分析表征。结果表明,两种齿轮钢的强度基本相当,而高S含量试验钢具有更好的塑性和低温冲击韧性,并且其疲劳极限、疲劳寿命均优于低S含量试验钢,疲强比由0.445提高到0.479。S含量显著影响钢中的夹杂物分布情况:低S含量试验钢中夹杂物以MnS-Oxide为主,数量较少,同时尺寸更大;而高S含量试验钢夹杂物以CaS-MnS-Oxide复合型夹杂物为主,数量提高1倍以上,但尺寸更为细小。高S含量试验钢采用Ca处理工艺对夹杂物进行改性,夹杂物尺寸明显细化,有利于钢的塑韧性提升,并改善疲劳性能。

渗碳工艺对18CrNiMo7-6合金钢缺口件疲劳性能的影响

为分析渗碳工艺对18CrNiMo7-6缺口构件疲劳性能的影响,对应力集中系数Kt=1.86的缺口试样进行渗碳热处理并测定其旋转弯曲疲劳性能,同时对试样表层组织、硬度以及应力场进行了表征。结果表明,渗碳热处理可以明显提升试样表面硬度并引入残余压应力,并且可以显著提升缺口试样的疲劳性能,相较于未渗碳试样疲劳极限提升超过100%;随着有效硬化层深度的增加,缺口试样的疲劳极限均呈现先增后减的趋势。未渗碳试样和渗碳试样的疲劳源均在缺口根部表面处,且均为多源断裂。疲劳过程中渗碳试样表面残余奥氏体在循环应力作用下诱导马氏体相变,其转变量存在一个临界值。

Using Cavitation Peening to Improve the Fatigue Life of Titanium Alloy Ti-6Al-4V Manufactured by Electron Beam Melting

Although Electron Beam Melting (EBM) is an innovative technology, the fatigue properties of materials manufactured by EBM may be lower than those of casted and wrought materials due to defects and surface roughness. In order to enhance the fatigue life of components or structures manufactured by EBM, a mechanical surface treatment technology, e.g., peening, would be effective because peening introduces high compressive residual stress at the surface which can extend the fatigue life considerably. In the present study, specimens were manufactured by EBM using titanium alloy Ti-6Al-4V powder. Two types of specimens were prepared: as-built and as-machined specimens. Specimens of each type were treated by cavitation peening or shot peening. The fatigue lives of the specimens were evaluated by a plate bending fatigue tester. The residual stress and surface roughness were also evaluated. The results obtained showed that the fatigue strength of as-built specimens can be improved by 21% by cavitation peening or shot peening, and the fatigue life under particular applied stresses can also be extended by 178% by cavitation peening.

Improving the high cycle fatigue property of Ti6Al4V ELI alloy by optimizing the surface integrity through electric pulse combined with ultrasonic surface rolling process

To improve the surface integrity and high cycle fatigue property of Ti6Al4V ELI alloy,the electric pulse has been introduced into the ultrasonic surface rolling process(USRP),which is called electric pulse-assisted ultrasonic surface rolling process(EUSRP).With the help of“electroplasticity”of the electric pulse,the thickness of the surface gradient deformation layer was about three times of the USRP specimens by adjusting the pulse current level.However,the surface hardness decreases due to the continuous effect of the pulse current and the“skin effect”during treatment.It is worth noting that the higher the applied pulse current,the more severe the softening.This paradox causes the fatigue performance of EUSRP specimens lower than that of USRP specimens.To break this paradox,the EUSRP treatment is followed by a USRP treatment.The EUSRP-2(with a pulse current of 200 A)+USRP specimens exhibit excellent surface hardness,a gradient deformation layer thickness of about 400μm,low surface roughness and high compressive residual compressive stress.Besides,the hardening mechanisms of the different surface strengthening specimens have been quantitatively analyzed in combination with microstructure analysis.The fatigue life of Ti6Al4V ELI alloy can be improved by about 25 times at 780 MPa using the EUSRP-2+USRP treatment,the main reason for the highest fatigue life is the deepest surface gradient layer and the deepest crack initiation site.The fatigue limit of the EUSRP-2+USRP specimens is not the highest because too much surface hardening causes compressive residual stress relaxation during cycling and the beneficial effect of compressive residual stress is eliminated.

Study on the Overload and Dwell-Fatigue Property of Titanium Alloy in Manned Deep Submersible

With the rapid development of ocean technology, the deep-sea manned submersible is regarded as a high-tech equipment for the exploration and exploitation of ocean resources. The safety of manned cabin has a decisive effect on the whole system. Ti-6 Al-4 V with the superior strength-to-weight ratio and corrosion resistance has been used for the manned cabin. The manned cabin experiences loading spectrum with different maximum stresses and different dwell time during their service life. The load sequence effects on dwell fatigue crack growth behavior of Ti-6 Al-4 V under different dwell time are investigated experimentally in this paper. The experimental results show that the crack tip plastic zone is enlarged by the dwell time and the overload retardation zone increases with dwell time under the same overload rate. A dwell fatigue crack growth model is proposed by modifying the crack tip plastic zone under the loading history with combinations of the single overload and dwell time factors are included in the modified model. Based on the experimental data, the overload retardation zone and the crack growth rates of Ti-6 Al-4 V are predicted by the modified model. A reasonable model for the load sequence effect on the dwell fatigue crack growth rates of Ti-6 Al-4 V is verified.

18CrNiMo7-6高铁齿轮钢的疲劳极限评定

在不同最大循环应力(600~880 MPa)和应力比0.1下对18CrNiMo7-6高铁齿轮钢进行棘轮试验和疲劳试验,先通过对稳定阶段的棘轮应变差值和温升与最大循环应力进行拟合来预测疲劳极限,然后再基于由棘轮应变差值和温升计算的断裂疲劳熵来预测疲劳极限,并将不同方法的预测结果与试验结果进行对比。结果表明:由稳定阶段的棘轮应变差值和温升与最大循环应力的线性拟合得到的疲劳极限分别为664.9,681.4 MPa,与由疲劳试验得到的疲劳极限(689.0 MPa)的相对误差分别为1.11%,3.50%,说明用这2种方法预测疲劳极限的精度较高;当最大循环应力为673.2 MPa时,断裂疲劳熵值由0.1 MJ·m^(-3)·K^(-1)以下突变增至0.46 MJ·m^(-3)·K^(-1),由此预测得到的疲劳极限为673.2 MPa,与疲劳试验结果的相对误差为2.3%,预测精度较高。

Effects of laser shock processing,solid solution and aging,and cryogenic treatments on microstructure and thermal fatigue performance of ZCuAl_(10)Fe_(3)Mn_(2)alloy

The materials used in variable temperature conditions are required to have excellent thermal fatigue performance.The effects of laser shock processing(LSP),solid solution and aging treatment(T6),and cryogenic treatment(CT)on both microstructure and thermal fatigue performance of ZCuAl_(10)Fe_(3)Mn_(2) alloys were studied.Microstructure and crack morphology were then examined by scanning electron microscopy(SEM)and energy-dispersive X-ray spectroscopy(EDS).The result showed that,after being subjected to the combination treatment of T6+CT+LSP,the optimal mechanical properties and thermal fatigue performance were obtained for the ZCuAl_(10)Fe_(3)Mn_(2) alloy with the tensile strength,hardness,and elongation of 720 MPa,300.16 HB,and 16%,respectively,and the thermal fatigue life could reach 7,100 cycles when the crack length was 0.1 mm.Moreover,the ZCuAl_(10)Fe_(3)Mn_(2) after combination treatment shows high resistance to oxidation,good adhesion between the matrix and grain boundaries,and dramatically reduced growth rate of crack.During thermal fatigue testing,under the combined action of thermal and alternating stresses,the microstructure around the sample notch oxidized and became loose and porous,which then converted to micro-cracks.Fatigue crack expanded along the grain boundary in the early stage.In the later stage,under the cyclic stress accumulation,the oxidized microstructure separated from the matrix,and the fatigue crack expanded in both intergranular and transgranular ways.The main crack was thick,and the path was meandering.

解郁除乏汤联合针灸对乳腺癌化疗后癌性疲劳患者SAS、SDS和FACT-F评分及血清CRP、IL-6和sTNF-R1表达水平的影响研究

目的:探讨解郁除乏汤联合针灸对乳腺癌化疗后癌性疲劳患者焦虑自评量表(SAS)、抑郁自评量表(SDS)和中文版癌症治疗功能评估疲乏量表(FACT-F)评分,血清C反应蛋白(CRP)、白细胞介素6(IL-6)和可溶性肿瘤坏死因子受体-1(sTNF-R1)表达水平的影响。方法:选取2019年7月至2022年7月该院收治的乳腺癌化疗后癌性疲劳患者117例,按照随机数字表法分为对照组(化疗后给予常规对症治疗)58例与观察组(化疗后给予解郁除乏汤联合针灸治疗)59例,治疗周期为3周。观察两组患者的临床疗效、中医症状积分(健忘、精神疲惫、失眠多梦和精力不足)和血清学指标(CRP、IL-6和sTNF-R1),以及SAS、SDS和FACT-F评分,并记录不良反应。结果:观察组患者治疗后临床总有效率为94.92%(56/59),高于对照组的65.52%(38/58),差异有统计学意义(P<0.05)。治疗后,观察组患者健忘、精神疲惫、失眠多梦和精力不足等中医症状积分低于对照组,差异均有统计学意义(P<0.05)。治疗后,观察组患者CRP、IL-6和sTNF-R1表达水平低于对照组,SAS、SDS和FACT-F评分低于对照组,差异均有统计学意义(P<0.05)。对照组、观察组患者的不良反应发生率分别为5.17%(3/58)、8.47%(5/59),差异无统计学意义(P>0.05)。结论:解郁除乏汤联合针灸可改善乳腺癌化疗后癌性疲劳患者的SAS、SDS和FACT-F评分,降低血清CRP、IL-6和sTNF-R1表达水平。

枸杞多糖和1,6-二磷酸果糖协同抗运动疲劳作用及其机制

目的:研究枸杞多糖与1,6-二磷酸果糖的协同抗疲劳作用,为改善和促进运动员体力恢复、提高训练和比赛成绩提供科学依据。方法:将140只小鼠随机分为空白对照组(生理盐水)、枸杞多糖组(20mg.kg-1)及1,6-二磷酸果糖组(300mg.kg-1),并将两者按2×2表分组,以负重游泳小鼠作为研究抗运动疲劳实验的小鼠模型。检测各组小鼠力竭游泳时间、肝糖原、肌糖原、乳酸脱氢酶(LDH)、超氧化物歧化酶(SOD)、血乳酸(LAC)和血尿素氮(BUN),分析两者协同抗疲劳的效果和机制。结果:与空白对照组比较,枸杞多糖和1,6-二磷酸果糖组小鼠负重游泳时间延长,两者剂量分别为20mg.kg-1和300mg.kg-1时最佳(P<0.01),两者之间存在交互作用。与空白对照组比较,枸杞多糖组小鼠LDH、SOD活力提高(P<0.01),LAC和血BUN水平降低,1,6-二磷酸果糖使小鼠肝糖原和肌糖原的储备量显著提高(P<0.01)。结论:枸杞多糖与1,6-二磷酸果糖配伍能起到协同抗疲劳作用,其原因可能是1,6-二磷酸果糖增加了小鼠的肝糖原和肌糖原的贮备,能够提供更多能量;枸杞多糖提高LDH、SOD活力,加快代谢产物的分解,使小鼠在耐力运动中坚持时间更长。

三维表面粗糙度对18CrNiMo7-6钢旋转弯曲疲劳寿命的影响

目的探究固定载荷下三维表面粗糙度Sa对18CrNiMo7-6钢旋转弯曲疲劳寿命的影响。方法通过砂纸研磨制备不同表面粗糙度及纹理方向的18CrNiMo7-6钢旋转弯曲疲劳试样,测量所有试样的表面粗糙度参数Sa及三维表面形貌参数Sq、Sz、Ssk、Sku。对试样进行旋转弯曲疲劳试验,分析疲劳寿命。结果在相同或相近粗糙度的情况下,轴向纹理疲劳试样疲劳寿命大于周向纹理疲劳试样疲劳寿命。相同纹理方向的情况下,表面三维粗糙度Sa越低,试样疲劳寿命越高。试样疲劳寿命次数与表面粗糙度参数Sa及三维表面形貌参数Sq、Sz、Ssk、Sku均有明显的相关性。结论对于18CrNiMo7-6钢旋转弯曲疲劳试样,拥有平行于疲劳应力的机械加工纹理比垂直于疲劳应力的机械加工纹理具有更小的危害性。降低18CrNiMo7-6试样表面粗糙度,能够有效提高试样旋转弯曲疲劳寿命。纹理方向平行于疲劳应力方向的试样,表面偏斜度Ssk对零件疲劳寿命影响不明显。

腹腔镜胆囊切除术对老年胆囊结石患者术后IL-6、C反应蛋白及疲劳综合征的影响

目的研究腹腔镜胆囊切除术对老年胆囊结石患者术后Il-6、C反应蛋白及疲劳综合征的影响。方法将54例老年胆囊结石患者采用单双号法分为对照组和观察组各27例,其中对照组行开腹胆囊切除术,观察组行腹腔镜胆囊切除术,观察两组患者手术前后Il-6、C反应蛋白水平,并记录疲劳指数变化情况。结果两组患者术前第1天IL-6、C反应蛋白值比较,差异无统计学意义(P>0.05),术后第2天观察组患者的IL-6、C反应蛋白分别为(34.51±5.29)ρ/ng·L-1、(12.67±2.55)ρ/ng·L-1,均较对照组低,差异有统计学意义(P<0.05)。两组患者术后疲劳综合指数均呈明显下降趋势,观察组下降幅度较对照组明显,差异具有统计学意义(P<0.05)。结论采用腹腔镜胆囊切除术治疗老年胆囊结石,能够降低机体应激反应,提高手术治疗质量,值得推广应用。

人参皂甙Rg1与1,6-二磷酸果糖配伍抗疲劳作用的研究

观察人参皂甙Rg1与1,6-二磷酸果糖配伍的抗疲劳效果,寻找最佳配伍剂量。根据雄性清洁级昆明种小鼠按体重随机分为安静对照组、运动对照组、人参皂甙Rg1对照组及4个配伍组。通过析因实验设计分析人参皂甙Rg1与1,6-二磷酸果糖提高运动耐力的效果及两者的交互作用。结果不同剂量人参皂甙Rg1对小鼠力竭游泳时间影响有显著差异(P<0.05)。各配伍组力竭游泳时间均有显著延长,C组延长最显著;与人参皂甙Rg1对照组相比,各配伍组力竭游泳时间均显著降低(P<0.01)。MDA含量A组显著低于运动对照组和D组,与安静对照组相比,各组均升高。SOD/MDA比值A、C组显著高于运动对照组,C组显著高于人参皂甙Rg1对照组,与安静对照组相比,各组均有下降趋势,D组下降最显著(P<0.01)。乳酸脱氢酶C、D组显著低于运动对照组和人参皂甙Rg1对照组(P<0.05)。与运动对照组及人参皂甙Rg1对照组相比,配伍在一定程度上减少心肌、骨骼肌细胞线粒体和其他细胞超微结构损伤。结论:与FDP配伍未能延长小鼠力竭游泳时间,配伍可减轻耐力运动对小鼠心肌和骨骼肌的细胞损害,一定程度上保护线粒体的呼吸功能,缓解细胞缺氧损伤。

频率及应力比对18CrNiMo7-6合金钢疲劳性能的影响

通过旋转弯曲疲劳试验和轴向疲劳试验方法得到了不同试验条件下的S-N曲线,研究了不加载频率及应力比对18CrNiMo7-6合金钢疲劳性能的影响。使用扫描电镜观察了试样的断裂表面。结果表明:常规加载频率下存在频率效应,并且随加载频率的增加,材料的疲劳强度呈增加趋势,并且屈服强度随应变速率的增大逐渐增大,发现不同加载频率下的断裂机理并无明显差别。在最大应力相同时,随应力比的增大,18CrNiMo7-6合金钢疲劳寿命逐渐增大。利用有限元方法使用BrownMiller模型计算了不同应力比条件下18CrNiMo7-6合金钢的疲劳寿命,计算结果与试验结果匹配较好。

18CrNiMo7-6钢旋转弯曲疲劳试样磨削工艺优化

渗碳淬火可以提高材料的疲劳性能,但在对热处理后旋转弯曲疲劳试样的磨削加工中存在废品率高、效率低等问题,为了高效高质量磨削出标准疲劳试样,提出一种径向圆跳动变形量检测方法以衡量热处理后变形程度,基于国标GB/T4337-2015中尺寸、形位公差以及表面质量要求,提出两种磨削加工方案,并进行同轴度误差检测与比较,对渗碳淬火后18CrNiMo7-6钢旋转弯曲疲劳试样毛坯在外圆磨床上的磨削加工工艺进行优化,确定最终磨削方案。磨削后试样同轴度误差为9.3μm,表面残余应力为-233.84 MPa,表面算术平均偏差Sa为0.208μm,符合国标要求,为疲劳试验中标准旋转弯曲疲劳试样磨削加工提供依据。

喷丸强化对18CrNiMo7-6渗碳齿轮表面性能的影响

采用气动式喷丸机对渗碳后的18CrNiMo7-6齿轮进行不同参数的喷丸强化,利用三维光学显微镜、扫描电镜、显微硬度仪和X射线残余应力分析仪对齿轮根部三维表面形貌、表层微观组织、显微硬度和残余应力进行分析。使用高频疲劳试验机对不同喷丸参数强化的齿轮进行疲劳试验。结果表明:喷丸强化可提高齿根的表层硬度,降低材料表面的残余应力,细化表层组织。通过优化喷丸工艺参数,试样的弯曲疲劳寿命提高了8.39倍。

渤中19-6构造下部钻具组合动力学特性

渤中19-6构造深井钻探时,Φ406.4mm井眼利用PDC和PowerV钻具组合钻进到2841m处,出现了扭矩增加、悬重降低的复杂情况,起钻后发现马达上活塞套筒与直壳体连接处发生断裂,严重耗损钻井工期与费用。为揭示下部钻具组合局部应力分布,基于Abaqus有限元分析软件建立了下部钻具组合-钻头-地层三维有限元模型,利用真实井下工况参数、采用显式时间积分法进行模拟求解,分析了下部钻具组合受力情况。结果表明,马达上存在应力集中点,应力随时间呈现周期循环往复状态,符合疲劳破坏的特征,且集中点基本与断裂位置重合,验证了本模型的准确性。该模型对于渤中构造深井的下部钻具组合动力学分析具有一定的指导意义。