The effect of frequency on the giga‐cycle fatigue properties of a Ti–6Al–4V alloy

The effect of frequency on giga‐cycle fatigue properties was investigated in smooth and 0.3 mm‐hole‐notched specimens at three heats (Heats A, B, and C) for a 900 MPa‐class Ti‐6Al‐4V alloy. Fatigue tests were performed at frequencies of 120 Hz, 600 Hz, and 20 kHz using electromagnetic resonance, high‐speed servohydraulic, and ultrasonic fatigue testing machines, respectively. Heats A and B developed internal fractures, and in these cases, frequency effects were negligible. On the other hand, Heat C developed only surface fractures. In this case, high‐frequency tests showed a higher fatigue strength, indicating frequency effects were not negligible. The tests using the notched specimens showed almost no frequency effects regardless of the heat. The frequency effects observed in the cases of surface fracture were believed to be related to a delay in local plastic deformation in response to high‐frequency loading, since temperature increases in these specimens were successfully suppressed. The delay in the plastic deformation was observed to be reduced in the notched specimens because of stress concentration and limitation in the plastic deformation zone. In turn, the significant conclusion of this research is that high‐frequency tests can be applied not only to internal fractures but also to notch problems, but are not applicable to surface fractures of smooth specimens.     

镍基单晶合金中温疲劳性能应力集中敏感性

疲劳是涡轮叶片的一种主要失效模式.本文开展了DD11单晶合金在650℃中温条件下2种应力集中系数(K_t=1(光滑状态)、K_t=3(缺口状态))的旋转弯曲疲劳性能研究,对比了2种应力集中系数下的疲劳强度,并开展了相关断口分析.结果表明:应力集中系数由K_t=1增大到K_t=3时,疲劳极限由446 MPa降低为311 MPa,说明DD11单晶合金疲劳性能存在应力集中敏感性;疲劳寿命由10~5提高到10~7时,光滑状态由600 MPa降低为420 MPa,疲劳强度降低幅度为180 MPa,而缺口状态由370 MPa降低为290 MPa,降低幅度为80 MPa,说明应力集中条件下DD11单晶合金的疲劳寿命对于外载变化较敏感.断口分析表明,光滑试样断口(应力500 MPa/疲劳寿命9.7×10~5)由几个相交的光滑晶体学平面组成,疲劳源萌生在距表面100μm左右的铸造孔洞;缺口试样断口(应力340 MPa/疲劳寿命8.1×10~5试样)呈平面状,与应力轴垂直,为多源疲劳模式,疲劳源观察到小刻面,在加工刀痕不连续位置萌生.     

A MODIFIED FRACTURE MECHANICS APPROACH TO METAL FATIGUE

The fatigue lives, the fatigue limit stress ranges and fatigue notch factors for metallic specimens can be predicted using a modified fracture mechanics model for short cracks based on the combination of solutions for the non-uniform strains at the surface of a metal and the development of crack closure. The resulting local stress intensity factor exceeds that indicated by linear elastic fracture mechanics at short crack lengths. The model predicts a smooth and continuous variation of the fatigue notch factor with notch size between a lower bound of unity and an upper bound equal to the theoretical notch stress concentration factor. The model is verified using experimental data for a 2024-T351 aluminium alloy for smooth and notched specimens tested at various stress ratios.     

Größeneinfluß und Dauerfestigkeitseigenschaften unter Besonderer Berücksichtigung optimierter Oberflächenbehandlung

Size Effect and Fatigue Properties with Respect to Optimized Surface-Treatment. A hyperbolic function describes the geometrical size effect of notched specimens made from heat treated steel. An estimation of fatigue properties of components under one level fatigue tests is possible, if there are comparable materials and surface properties. The fatigue properties of specimens are well described by standardized stress-N graphs. The slope of the stress-N graphs in the range of load cycle depends on the concentration factor and not on the size effect. The fatigue properties of components are largely increased by thermal and mechanical surface strengthening. For the determination of the improvement of fatigue properties it is important to known the initiation of cracking. The improved fatigue properties of inductive surface hardened smooth specimens can be explained by the initiation of cracking below the surface. Mechanically strengthened notched specimens start cracking on the surface. The increase of fatigue properties for these specimens is explained by compressive residual stresses. The fatigue properties of notched specimens can be improved by the optimisation of mechanical strengthening, to higher values than for smooth surface strengthened specimens. This is due to compressive residual stresses. They decrease the tensile stresses which are responsible for crack propagation. If the tensile stress is below fatigue limit for initiation of cracking the crack arrests immediately.     

Notable size effects on very high cycle fatigue properties of high-strength steel

Very high cycle fatigue (VHCF) properties were compared between two types of specimens: enlarged specimens and our standard specimens. Fatigue tests were conducted by ultrasonic fatigue testing; the material used was commercial spring steel. All tests ended in internal fracture, with large-size effects observed, i.e., the enlarged specimens showed lower VHCF strength than the standard specimens. Most of the internal fracture origins were oxide-type inclusions that were larger in the enlarged specimens than in the standard specimens, indicating the size effect to be caused by the difference in oxide-type inclusion sizes at the origins of internal fractures. The large-size effect strongly urges the use of large specimens when conducting VHCF tests on high-strength steel. Moreover, the large-size effect implies that fatigue strength cannot in this case be determined using the conventional S-N curve approach, since the S-N curve depends on the specimen size. The evaluation of the VHCF strength thus needs two steps: an estimation of the maximal inclusion size, followed by an estimation of the VHCF strength based on the maximal inclusion size.     

残余应力对金属疲劳强度的影响

残余应力对光滑试样高周疲劳极限的影响可以用Goodman关系来描述，但必须要得到残余应力作用系数m、合理地提取残余应力的表征值和区分开其它因素的影响。残余应力对缺口疲劳极限的作用大于对光滑试样的作用，是由于残余应力也存在应力集中现象，而且不易衰减。残余应力的应力集中系数不仅与缺口几何因素有关，还与材料特性有关。试验研究还表明，表层残余压应力对于承受轴向载荷且疲劳残纹萌生于表面的零件也十分有益。     

FAILURE MECHANISMS IN IMPACT FATIGUE OF METALS

Abstract Impact fatigue tests were performed with smooth and notched specimens of low carbon steels under various impact loading conditions. The characteristic failure mechanisms in impact fatigue was discriminated by comparison with those in non-impact, ordinary fatigue. The fatigue life of smooth specimens was uniquely related to the range of plastic strain at the middle of the fatigue life in both impact and non-impact fatigue, although the characteristics of micro-structural deformation and cyclic stress-strain relationships were markedly different. The growth rate of a fatigue crack in impact fatigue of notched specimens was higher than that in non-impact fatigue when compared at the same stress intensity factor. Fractographic observations with scanning electron microscopy and the X-ray diffraction technique revealed more abundant cleavage facets and a smaller spread of the plastic zone beneath the fracture surface made by impact fatigue. Both nucleation and propagation lives in notched specimens were much shorter in impact fatigue than in non impact fatigue when compared at the same values of nominal stress and stress concentration factor.     

Low‐cycle fatigue/high‐cycle fatigue interactions in notched Ti‐6Al‐4V*

Combined low‐cycle fatigue/high‐cycle fatigue (LCF/HCF) loadings were investigated for smooth and circumferentially V‐notched cylindrical Ti–6Al–4V fatigue specimens. Smooth specimens were first cycled under LCF loading conditions for a fraction of the previously established fatigue life. The HCF 10⁷ cycle fatigue limit stress after LCF cycling was established using a step loading technique. Specimens with two notch sizes, both having elastic stress concentration factors of K_t = 2.7, were cycled under LCF loading conditions at a nominal stress ratio of R = 0.1. The subsequent 10⁶ cycle HCF fatigue limit stress at both R = 0.1 and 0.8 was determined. The combined loading LCF/HCF fatigue limit stresses for all specimens were compared to the baseline HCF fatigue limit stresses. After LCF cycling and prior to HCF cycling, the notched specimens were heat tinted, and final fracture surfaces examined for cracks formed during the initial LCF loading. Fatigue test results indicate that the LCF loading, applied for 75% of total LCF life for the smooth specimens and 25% for the notched specimens, resulted in only small reductions in the subsequent HCF fatigue limit stress. Under certain loading conditions, plasticity‐induced stress redistribution at the notch root during LCF cycling appears responsible for an observed increase in HCF fatigue limit stress, in terms of net section stress.     

Optimierung von Schwingfestigkeitseigenschaften beim Oberflächendrücken gekerbter Umlaufbiegeproben unter Berücksichtigung der Probengröße

Optimisation of Fatigue Properties by Surface-Rolling of Notched Specimens of Different Size The fatigue properties of notched specimens can be largely improved by surface-rolling. It is important to know the material strengthening in the rolling contact to improve the conditions of the surface-rolling process. A computer program is presented, which allows a determination of rolling pressures to any geometries of specimens. As a result of the fatigue investigations it is shown that surface-rolled notched specimens improve the fatigue limit above the fatigue strength of smooth specimens independed on the stress concentration. The improvement of endurance limit of notched specimens with non propagated cracks is based on the effect of constant compressive residual stresses.     

Stellite12钴基合金的疲劳性能及其断裂机理研究

采用三点弯曲疲劳法测得光滑试样和直缺口试样的S-N曲线以研究Stellite12钴基合金的疲劳性能,并通过断口形貌观察进一步探究该钴基合金的断裂过程。结果表明:光滑试样的疲劳极限为545 MPa,约为原始抗弯强度1552 MPa的25.4%;直缺口试样的疲劳极限约为101MPa,约为静态抗弯强度517.6MPa的19.1%。对于疲劳敏感性,光滑试样与直缺口试样的疲劳敏感性分别为397和31。此外发现疲劳裂纹多萌生于近表层聚集的碳化物处,同时表面缺陷也可诱发疲劳裂纹的萌生。疲劳裂纹的扩展主要表现为碳化物的穿晶断裂,钴基体在应力比R=0.1的疲劳加载条件下虽表现出一定的韧性且呈现出较多的撕裂脊,但也呈现出一定的脆性断裂模式,因此疲劳裂纹扩展模式为真疲劳与静态疲劳的混合模式。     

Ermüdungsverhalten ausgewählter Werkstoffe und Bauteile bei sehr hohen Schwingspielzahlen

Very High‐Cycle Fatigue of Selected Materials and Components Results of several fatigue tests using a servohydraulic testing machine (VHF 50 D) and a spring testing machine are presented. Investigations were carried out at room temperature with. – smooth and notched specimens made of aluminium wrought alloy EN AW 6082,. – screws M 8 made of EN AW 6056,. – smooth specimens made of die cast magnesium AZ91 hp (here also tests at 125 °C) and. – shot‐peened helical compression springs of four different high strength steels. Tests were done with a frequency of 20 Hz (spring testing machine) and between 250 Hz and 400 Hz (VHF50D) up to a maximum number of cycles of N = 1,5 x 10⁹. Crack initiation sites were investigated and could be found at high number of cycles below the surface for smooth magnesium and aluminium specimens and also for most of the shot‐peened helical compression springs, but not for screws and other notched specimens.     

Influence of sub-micrometer notches on the fracture of single crystal silicon thin films

The influence of notches on the fracture of single crystal silicon thin films was investigated. The tests were conducted on notched and smooth tensile specimens micromachined on a silicon wafer. The specimen geometry was 100 μm long, 50 μm wide and 5 μm thick. For the notched specimen, a V‐shaped sub‐micrometer notch was introduced on one edge of it by using a focused ion beam (FIB) process. The notch lengths ranged from 0.07 to 1.3 μm. Four types of specimens with different surfaces and tensile orientations were tested. The smooth specimens showed scattered fracture strengths and ‘collapsed’ fractures. For the restrictive‐shaped notches, the critical length was 0.5 μm. The short‐notched (<0.5 μm) specimens also showed ‘collapsed’ fractures, and the stress concentrations on notch tips decreased their fracture strengths. For the long‐notched (>0.5 μm) specimens, the notch was equivalent to a crack in the Griffith model and the crack mainly propagated on {111} cleaved planes.     

Investigation of high cycle and Very-High-Cycle Fatigue behaviors for a structural steel with smooth and notched specimens

The high cycle and Very-High-Cycle Fatigue (VHCF) properties of a structural steel with smooth and notched specimens were studied by employing a rotary bending machine with frequency of 52.5 Hz. For smooth specimens, VHCF failure did occur at fatigue cycles of 7.1 × 10⁸ with the related S–N curve of stepwise tendency. Scanning Electron Microscopy (SEM) was used for the observations of the fracture surfaces. It shows that for smooth specimens the crack origination is surface mode in the failure regime of less than 10⁷ cycles. While at VHCF regime, the material failed from the nonmetallic inclusion lies in the interior of material, leading to the formation of fisheye pattern. The dimensions of crack initiation region were measured and discussed with respect to the number of cycles to failure. The mechanism analysis by means of low temperature fracture technique shows that the nonmetallic inclusion in the interior of specimen tends to debond from surrounding matrix and form a crack. The crack propagates and results to the final failure. The stress intensity factor and fatigue strength were calculated to investigate the crack initiation properties. VHCF study on the notched specimens shows that the obtained S–N curve decreases continuously. SEM analysis reveals that multiple crack origins are dominant on specimen surface and that fatigue crack tends to initiate from the surface of the specimen. Based on the fatigue tests and observations, a model of crack initiation was used to describe the transition of fatigue initiation site from subsurface to surface for smooth and notched specimens. The model reveals the influences of load, grain size, inclusion size and surface notch on the crack initiation transition.     

Fatigue strength of Ti–6Al–4V at very long lives

The objective of this research was to investigate the fatigue strength of Ti–6Al–4V using an ultrasonic fatigue system. Fatigue testing up to 10⁹ cycles under fully reversed loading was performed to determine the ultra-high cycle fatigue behavior of Ti–6Al–4V. Endurance limit results were compared to similar data generated on conventional servohydraulic test systems and electromagnetic shaker systems to determine if there are any frequency effects. Fatigue specimens were tested with and without cooling air to determine the effects of increased specimen temperature caused by internal damping due to cycling at a very high frequency. An infrared camera was also used to record specimen temperatures at various load levels. Results indicate that the effects of frequency, including internal heating, on the very high cycle fatigue behavior of Ti–6Al–4V are negligible under fully reversed loading conditions.     

Fatigue of casting flaws at a notch root under an SAE service load history

Smooth and notched specimens of a 319 cast aluminium alloy were fatigue tested under a Society of Automotive Engineers service load history in the as-cast and hipped conditions. The hipping process, which includes subjecting the cast material to a high pressure at high temperature and then slowly cooling down to eliminate internal flaws, decreased the flaw size and improved the fatigue life of cast Al 319 smooth specimens. A 0.6-mm-diameter hole was drilled at the notch root of notched specimens to simulate a natural flaw at the notch root. Specimens with two different notch sizes were tested. Circular edge notches reduced the fatigue strength and a 0.6-mm-diameter drilled hole at the notch root resulted in a further reduction.
The fatigue lives of smooth specimens, notched specimens and notched specimens with a flaw at the notch root subjected to the service load history were predicted using the strain-life approach, an effective strain-life approach and a strain-based intensity factor crack growth model. In crack growth modelling of the fatigue life of smooth cast aluminium specimens the flaw was modelled as a circular edge notch having the same diameter as the flaw. However, in the case of a flaw at a notch root the flaw was modelled as a three-dimensional cavity subjected to the notch stress field and the crack length was predicted in the longitudinal and transverse directions of the specimen cross-section. The strain-life approach was unconservative for all specimen geometries studied. The effective strain-life approach gave good predictions for smooth and blunt notched specimens but gave very conservative predictions for the specimens with flaws in the notch roots. The crack growth calculations gave accurate predictions for all the specimen geometries.     

Fatigue properties of specimens similar to components deep rolled under optimized conditions

A method is presented for determining the rolling load that produced optimum fatigue strength improvement in deep-rolled specimens. Based on the surface stresses calculated using Hertzian theory and von Mises distortion energy failure criterion, the method yields an equivalent stress that is shown to be a suitable parameter for describing deep-rolling conditions. In addition, fatigue tests have been carried out on deep-rolled smooth and notched specimens of 37CrS4 steel. Specimens with optimized deep rolling are found to attain higher bending fatigue strengths than deep-rolled smooth samples, thus fully removing the notch effect. The greater fatigue strength improvement observed for notched samples is explained on the basis of fatigue behaviour. Adequate static strengths are demonstrated by fatigue-cracked specimens, particularly for ductile material conditions.     

Evaluating the fatigue limit of metals having surface compressive residual stress and exhibiting shakedown

The objective of the study described in this article is to evaluate the effect of shakedown of surface compressive residual stresses introduced by shot peening on fatigue limit of stainless steel. First, the tension‐compression fatigue tests were conducted on ASTM CA6NM specimens under controlled load and displacement conditions to acquire a fatigue limit diagram under various compressive mean stress. The results showed that shakedown of negative mean stress occurs under controlled displacement. We then carried out in‐plane–bending fatigue tests under controlled load conditions on welded ASTM 309 stainless steel specimens with surface compressive residual stress introduced by ultrasonic shot peening. The results provide a fatigue limit of 415 MPa, which agrees with the value of 404 MPa calculated based on a modified Goodman line considering shakedown. Therefore, it is suggested that the surface layer is restricted by the internal bulk that creates controlled displacement conditions and the shakedown of surface compressive residual stress occurs.     

Fracture mode identification of low alloy steels and cast irons by electron back-scattered diffraction misorientation analysis

The fracture modes of low alloy steels and cast irons under tensile and fatigue conditions were identified by electron back-scattered diffraction(EBSD) misorientation analysis in this research. The curves of grain reference orientation deviation(GROD) distribution perpendicular to the fracture surface were obtained by EBSD observation, and the characteristics of each fracture mode were identified. The GROD value of the specimen fractured in tension decreases to a constant related to the elongation of corresponding specimen in the far field(farther than 5 mm away from the fracture surface). The peak exhibits in GROD curves of two smooth specimens and a notched specimen near the fracture surface(within 5 mm away from the fracture surface), and the formation mechanisms were discussed in detail based on the influences of specimen geometries(smooth or notched) and material toughness. The GROD value of fatigue fractured specimen is close to that at undeformed condition in the whole field, except the small area near the crack path. The loading conditions(constant stress amplitude loading or constant stress intensity factor range K loading) and the EBSD striation formation during fatigue crack propagation were also studied by EBSD observation parallel to the crack path.     

表面超声滚压处理对高速列车车轴钢疲劳性能的影响

对EA4T型高速列车车轴钢棒状旋转弯曲疲劳试样实验段磨削加工后进行了表面超声滚压处理。观察了处理前后试样的表面形貌及表层微观组织,测量了处理前后试样的表面粗糙度、表层硬度及表层残余应力。利用旋转弯曲疲劳实验得到处理前后试样的疲劳极限。结果表明:表面超声滚压处理后,试样的疲劳极限由352MPa提高到401MPa。疲劳极限的提高主要由于表面超声滚压处理后试样表面粗糙度降低、表层强度及残余压应力增加。