The importance of surface layer on fatigue behavior of a Ti- 6AI- 4V alloy

The effects of precycling and surface removal on the fatigue life and fatigue limit of a Ti-6A1-4V alloy were investigated. It was shown that both the fatigue life and fatigue limit were strongly dependent on the severity of precycling. The fatigue limit lost its significance if the alloy was subjected to a precycling treatment with a high stress amplitude. Cycling with stress amplitude below the fatigue limit after precycling showed a dependence of the logarithmic number of cycles to failure on the fraction of prefatigue damage. The interdependence of fatigue life and fatigue limit to precycling history was attributed to microcrack formation, principally restricted to a surface layer of less than 100 μm. Depending on the severity of precycling and on the magnitude of the applied cyclic stress, the fatigue damage could be either partially or totally eliminated by surface removal. The α/β interphase region of the surface layer appeared to offer preferred sites for dislocation pile-ups and crack initiation.     

高温渗碳轴承钢旋弯疲劳裂纹萌生与扩展行为

 为了提高航空轴承的服役寿命,借助QBWP-10000X型旋转弯曲疲劳试验机,研究了高温渗碳轴承钢的旋转弯曲疲劳性能和裂纹萌生扩展行为。结果表明,钢的中值疲劳强度达到913.3 MPa。有效渗层中大量M₂₃C₆和少量M₆C碳化物显著提高了试验钢的表面硬度,渗层不同碳浓度导致马氏体先后发生相变而形成408 MPa表面压应力,进而提高了钢的疲劳性能。疲劳裂纹主要萌生在表面缺陷和次表面碳化物,分别占比71.4%和 28.6%。萌生裂纹缺陷特征尺寸及承载应力对应力强度因子和循环次数影响显著,深犁沟形状由于涉及应力集中而直接影响疲劳循环次数,承受相同加载应力碳化物特征尺寸越大,循环次数越低。裂纹萌生后沿渗碳层碳化物边界快速扩展同时向芯部缓慢扩展,最后在试样疲劳源对侧近边缘区域发生准解理和韧性混合断裂。     

Improvement of Ti alloy fatigue properties by Pt ion plating

The effect of Pt ion plating on the high cycle axial fatigue life of Ti-6Al-2Sn-4Zr-2Mo specimens was studied at room temperature and 455 °C. Unlike other coatings, the plated material tested in this work showed an increase in fatigue strength when compared to uncoated material. The fatigue strength improvement was greater at 455 °C than at room temperature. Coated specimens cycled close to the fatigue limit at 455 °C demonstrated the highest improvement, which was associated with subsurface fatigue crack initiation. The uncoated specimens, tested under similar conditions, failed by cracking at the surface. Sectioning analysis showed no defects in the subsurface initiation sites. The initiation was related to groups of similarly aligned coarse α platelets. It is suggested that the Pt ion plating caused some surface hardening as well as oxidation resistance at elevated temperatures leading to the suppression of surface crack initiation.     

Fatigue Life of Titanium Alloys Fabricated by Additive Layer Manufacturing Techniques for Dental Implants

Additive layer deposition techniques such as electron beam melting (EBM) and laser beam melting (LBM) have been utilized to fabricate rectangular plates of Ti-6Al-4V with extra low interstitial (ELI) contents. The layer-by-layer deposition techniques resulted in plates that have different surface finishes which can impact significantly on the fatigue life by providing potential sites for fatigue cracks to initiate. The fatigue life of Ti-6Al-4V ELI alloys fabricated by EBM and LBM deposition techniques was investigated by three-point testing of rectangular beams of as-fabricated and electro-discharge machined surfaces under stress-controlled conditions at 10 Hz until complete fracture. Fatigue life tests were also performed on rolled plates of Ti-6Al-4V ELI, regular Ti-6Al-4V, and CP Ti as controls. Fatigue surfaces were characterized by scanning electron microscopy to identify the crack initiation site in the various types of specimen surfaces. The fatigue life data were analyzed statistically using both analysis of variance techniques and the Kaplan-Meier survival analysis method with the Gehan-Breslow test. The results indicate that the LBM Ti-6Al-4V ELI material exhibits a longer fatigue life than the EBM counterpart and CP Ti, but a shorter fatigue life compared to rolled Ti-6Al-4V ELI. The difference in the fatigue life behavior may be largely attributed to the presence of rough surface features that act as fatigue crack initiation sites in the EBM material.     

激光冲击强化对Ti6Al4V合金表面完整性和四点弯曲疲劳性能的影响

采用扫描电镜、显微硬度仪、X射线应力测量仪及透射电镜等对激光冲击强化Ti6Al4V合金的表面完整性进行了分析,采用MTS疲劳试验机测试了疲劳性能,并采用扫描电镜分析了疲劳断口,探讨了激光冲击强化机制。结果表明:经功率密度为15.9 GW/cm^2的激光处理后,其四点弯曲中值疲劳寿命较未处理试样提高了4.2~23.5倍;激光功率密度越大,试样的中值疲劳寿命越长。激光冲击强化表现出比喷丸强化更优的疲劳寿命增益效果。经激光冲击强化后,Ti6Al4V合金表面形成了深度为600~1400μm的残余压应力场,表面硬度比未强化区域提高了约10%,且亚表层内部的位错密度也有显著提高。     

Effect of the Inclined Edge Notches on the Fatigue-Fracture Behavior of Soft Annealed and Quenched AISI O2 Tool Steel

In this research, the effects of edge notches with inclined angles on the fatigue-fracture behavior and fatigue life between soft annealed and hardened AISI O2 tool steel (DIN 1.2842) were investigated. Hardening was provided by the quenching and cooling by oil. A rotating bending fatigue machine was used to determine the fatigue strength. It was found that edge notches caused a reduction in the fatigue strength and fatigue life for soft annealed AISI O2 steel, but this reduction transformed to an increase for quenched AISI O2 steel. In addition to this, kink occurred on the all inclined edge notches for both materials. After fatigue tests, the fracture surfaces, specimen surface microstructures and specimen center microstructures were observed by scanning electron microscopy.     

The effect of heat treatment on the fatigue strength of microknurled Ti-6Al-4V

Microknurling, a high pressure surface indentation technique, was devised as an alternative to traditional heat-bonded porous coatings found on many orthopedic implants designed for fixation by tissue ingrowth. Heat-bonded porous coating can cause at the surface of an implant stress concentrations that reduce fatigue strength. However, microknurling may reduce stress intensification without eliminating it. Thus the purpose of this work was to explore surface thermal/mechanical processing of Ti-6Al-4V to improve the fatigue strength of microknurled specimens via the production of a Ti-6Al-4V dual microstructure. The latter consists of a surface layer of equiaxed grains known to be effective against crack initiation and a bulk microstructure of lamellar grains that possesses optimum fatigue crack propagation resistance. Rotating-bending fatigue tests showed that such a microstructure had some benefits, but this was offset by the reduction in compressive strains imparted to the surface by the heat treatments needed to obtain this microstructure.     

缺口对渗氮CrNiW钢超高周疲劳特性的影响

 为了研究缺口敏感性对渗氮CrNiW钢疲劳特性的影响,设计了一种光滑试样和两种缺口试样(应力集中系数K_t=1.20、1.55),开展了渗氮CrNiW钢的超高周疲劳试验,使用SEM观测疲劳断口的微观结构,分析了疲劳缺口系数K_f、疲劳敏感系数q和循环周次N_f之间的关系。结果表明,渗氮对钢材疲劳性能的提升不大,光滑试样和K_t=1.20的缺口试样存在表面和内部两种起裂模式,诱发内部起裂的缺陷均位于内部非渗氮区,K_t=1.55时发现一种新的渗氮层内不均匀基体组织诱发的失效模式。光滑试样未出现双重S-N特性,K_t=1.20时S-N曲线出现表面与内部起裂双重S-N特性,K_t=1.55时出现短寿命区和超高周寿命区表面起裂双重S-N特性。随着K_t的增大,多个内部缺陷同时萌生裂纹的现象被单裂纹源诱发失效的现象取代,并且诱发失效的内部缺陷位置呈现向试样表面靠近的趋势,鱼眼形状呈近似横向扁形→圆形→纵向椭圆形变化。缺口应力集中对表面起裂疲劳寿命影响较大,对内部起裂疲劳寿命影响很小。     

An increase in fatigue service life of eutectic silumin by electron-beam treatment

Silumin of the eutectic composition is treated by a high-intensity electron beam and a multiple increase in material fatigue service life is revealed. The structure of the modified layer and the failure surface of silumin subjected to fatigue multicycle destruction tests are investigated by scanning electron microscopy. Factors responsible for an increase in its fatigue service life are established and analyzed.     

通过等离子渗碳/CrN覆层改性处理提高Ti-6Al-4V合金的耐磨性和抗疲劳特性

In this study, a newly developed duplex coating method incorporating plasma carhurization and CrN coating was applied to Ti-6AI-4V and its effects on the wear resistance and fatigue life were investigated. The carburized layer with approximately150μm in depth and CrN coating film with 7. 5 μm in thickness were fomled after duplex coating. Hard carbide particles such as TiC And V4C3 were formed in the carburized layer. XRD diffraction pattern analysis revealed that CrN film had predominant [111] and [200] textures. The hardness (Hv) was significantly improved up to ahout 1960 after duplex coating while the hardness value of original Ti-6Al-4V was 402. The threshold load for the modification and/or failure of CrN coating was measured to be 32 N using the acoustic emission technique. The wear resistance and fatigue life of duplex coated Ti-6Al-4V improved significantly compared to those of un-treated specimen. The enhanced wear resistance can be attributed to the excellent adhesion and improved hardness of CrN coating film for the duplex coated Ti-6Al-4V. The initiation of fatigue cracks is likely to be retarded by the presence of hard and strong layers on the surface, resulting in the enhanced fatigue life.     

Tensile and fatigue properties of 17-4 PH stainless steel at high temperatures

The tensile and high-cycle fatigue properties for 17-4 PH* stainless steels in three different conditions were investigated at temperatures ranging from room temperature to 400 °C. Results indicated that the yield strength and fatigue strength for the three conditions at a given temperature took the following order: condition H900 > condition A> condition H1150. The yield strength of each condition decreased with increasing temperature except for condition A, which was tested at 400 °C with longer hold times, where a precipitation-hardening effect took place. The S-N curves showed that the fatigue strengths of each condition in the short-life regime were decreased with an increase in temperature. In the long-life regime, the fatigue strengths of condition A at 400 °C were greater than those at lower temperatures as a result of an in-situ precipitation-hardening effect. The fatigue strengths of condition H900 in the long life regime at 300 °C were superior to those at lower temperatures, due to the mechanisms of surface oxidation and thermal activation of dislocations. Fractography observations indicated that a shift of fatigue fracture from surface to internal crack initiation occurred at higher temperatures (300 °C and 400 °C) with long fatigue lives.     

Corrosion fatigue in nitrocarburized quenched and tempered steels

In order to investigate the fatigue strength and fracture mechanism of salt bath nitrocarburized steels, specimens of the steels SAE 4135 and SAE 4140, in a quenched and tempered state, and additionally in a salt bath nitrocarburized and oxidizing cooled state as well as in a polished (after the oxidizing cooling) and renewed oxidized state, were subjected to comparative rotating bending fatigue tests in inert oil and 5 pct NaCl solution. In addition, some of the quenched and tempered specimens of SAE 4135 material were provided with an approximately 50-μm-thick electroless Ni-P layer, in order to compare corrosion fatigue behavior between the Ni-P layer and the nitride layers. Long-life corrosion fatigue tests of SAE 4135 material were carried out under small stresses in the long-life range up to 10⁸ cycles with a test frequency of 100 Hz. Fatigue tests of SAE 4140 material were carried out in the range of finite life (low-cycle range) with a test frequency of 13 Hz. The results show that the 5 pct NaCl environment drastically reduced fatigue life, but nitrocarburizing plus oxidation treatment was found to improve the corrosion fatigue life over that of untreated and Ni-P coated specimens. The beneficial effect of nitrocarburizing followed by oxidation treatment on cor-rosion fatigue life results from the protection rendered by the compound layer by means of a well-sealed oxide layer, whereby the pores present in the compound layer fill up with oxides. The role of inclusions in initiating fatigue cracks was investigated. It was found that under corrosion fatigue conditions, the fatigue cracks started at cavities along the interfaces of MnS inclusions and matrix in the case of quenched and tempered specimens. The nitrocarburized specimens, however, showed a superposition of pitting corrosion and corrosion fatigue in which pores and nonmetallic inclusions in the compound layer play a predominant role concerning the formation of pits in the substrate.     

超声冲击对P355NL1钢焊接接头超高周疲劳性能影响

 通过超声疲劳试验探究超声冲击对P355NL1钢焊接接头超高周疲劳性能的影响。结果表明,由疲劳[S-N]曲线可知,在105～109寿命区间内,冲击态试样的疲劳性能要高于焊态试样,在1.0×108的疲劳寿命下,焊态试样的疲劳强度为139 MPa,冲击态的疲劳强度为217 MPa,冲击态疲劳强度相较于焊态提高了56%,这表明超声冲击可以明显提高焊接接头的疲劳强度。利用扫描电镜（SEM）观察断口形貌可以发现,裂纹源位于焊接接头焊根区表面。P355NL1钢焊接接头疲劳断裂为准解理断裂,超声冲击可以提高焊接接头的疲劳强度,但不会改变其疲劳失效机理。超声冲击可以降低焊根处应力集中,引入有益压应力和表面晶粒细化,从而提高焊接接头的疲劳强度。     

Fretting Fatigue Behavior of 304 Austenitic Stainless Steel Considering the Effect of Mean Stress and Tensile Overload

Effect of mean stress on fretting fatigue behavior of 304 austenitic stainless steel has been investigated by conducting fretting fatigue tests at a constant contact pressure of 100 MPa under three different mean stresses i.e., 0, 350 and 450 MPa. For comparisons, plain fatigue tests were also carried out. The influence of tensile overload on fretting fatigue life was also investigated. The results showed that with an increase in mean stress, the reduction in fatigue strength due to fretting increased drastically from 51% at 0 MPa mean stress to 71% at 450 MPa mean stress. The application of tensile overload during fretting fatigue had significant influence on the fretting fatigue lives when the tensile overload was above yield strength. The fretting variables, i.e., tangential stress and relative slip amplitude were measured during fretting fatigue tests. Fracture surfaces were examined using scanning electron microscope. The results have been discussed based on the tangential stress measurement, relative slip amplitude evaluation during fretting fatigue and fracture surface examinations.     

Rolling-contact-fatigue behavior in backup roll of 5 ％ Cr forged steel

With a focus on the backup roll,a rolling-contact-fatigue experiment was performed on samples of 5％ Cr forged steel.The P-S-N fatigue curves were determined and the fatigue strength was calculated.The emergence of cracks on the test-sample surfaces was observed at different fatigue cycles.A micro-hardness tester was used to measure the hardness of the subsurface fatigue layer.The microstructures were analyzed at various magnifications with an optical microscope,scanning electron microscope,and transmission electron microscope.Based on these tests,the rolling-contact-fatigue mechanism of the large forged steel backup roll was also considered.The results showed that the contact-fatigue strength of the tested backup roll steel was 1 249 MPa;the surface fatigue crack lengthened continuously as the number of cycles increased and followed an S-shaped curve;the subsurface fatigue hardness reached its highest value at about 90 (HV) increment from the matrix hardness of 540 (HV) in the backup roll;the subsurface martensite/bainite microstructure was crushed and the dislocation density was greatly increased.Under alternating contact stresses,the surface/subsurface material was damaged and exhibited many microdefects.At the least,the surface fatigue layer on backup rolls should be fully removed before the microcracks enter a period of rapid propagation.     

Influence of Surface Mechanical Attrition Treatment Duration on Fatigue Lives of Ti–6Al–4V

This work deals with the influence of surface mechanical attrition treatment (SMAT) duration on fatigue lives of Ti–6Al–4V. The SMAT process was carried out in vacuum with SAE 52100 steel balls of 5 mm diameter for 30 and 60 min at a vibrating frequency of 50 Hz. SMAT treated surface was characterized by electron microscopy. Surface roughness, nano-indentation hardness, residual stress, and tensile properties of the material in both SMAT treated and untreated conditions were determined. SMAT enabled surface nanocrystallization, increased surface roughness, surface hardness, compressive residual stress and tensile strength but reduced ductility. Samples treated for 30 min exhibited superior fatigue lives owing to positive influence of nanostructured surface layer, compressive residual stress and work hardened layer. However, fatigue lives of the samples treated for 60 min were inferior to those of untreated samples due to presence of microdamages or cracks induced by the impacting balls during the treatment.     

Correlating Scatter in Fatigue Life with Fracture Mechanisms in Forged Ti-6242Si Alloy

Unlike the quasi-static mechanical properties, such as strength and ductility, fatigue life can vary significantly (by an order of magnitude or more) for nominally identical material and test conditions in many materials, including Ti-alloys. This makes life prediction and management more challenging for components that are subjected to cyclic loading in service. The differences in fracture mechanisms can cause the scatter in fatigue life. In this study, the fatigue fracture mechanisms were investigated in a forged near-α titanium alloy, Ti-6Al-2Sn-4Zr-2Mo-0.1Si, which had been tested under a condition that resulted in life variations by more than an order of magnitude. The crack-initiation and small crack growth processes, including their contributions to fatigue life variability, were elucidated via quantitative characterization of fatigue fracture surfaces. Combining the results from quantitative tilt fractography and electron backscatter diffraction, crystallography of crack-initiating and neighboring facets on the fracture surface was determined. Cracks initiated on the surface for both the shortest and the longest life specimens. The facet plane in the crack-initiating grain was aligned with the basal plane of a primary α grain for both the specimens. The facet planes in grains neighboring the crack-initiating grain were also closely aligned with the basal plane for the shortest life specimen, whereas the facet planes in the neighboring grains were significantly misoriented from the basal plane for the longest life specimen. The difference in the extent of cracking along the basal plane can explain the difference in fatigue life of specimens at the opposite ends of scatter band.     

Dislocation distribution and prediction of fatigue damage

The dislocation density and distribution induced by tensile deformation in single crystals of silicon, aluminum and gold and by tension-compression cycling in aluminum single crystals and Al 2024-T3 alloys were studied by X-ray double-crystal diffractometry. The measurements of dislocation density were made at various depths from the surface by removing surface layers incrementally. In this way, a propensity for work hardening in the surface layers compared to the bulk material was demonstrated for both tensiledeformed and fatigue-cycled metals. Analysis of the cycled Al 2024 alloy as a function of the fraction of fatigue life showed that the dislocation density in the surface layer increased rapidly early in the fatigue life and maintained virtually a plateau value from 20 to 90 pct of the life. Beyond 90 pct the dislocation density increased rapidly again to a critical value at failure. Evaluation of the dislocation distribution in depth showed that the excess dislocation density in the bulk material increased more gradually during the life. Using deeply penetrating molybdenumK _α radiation, capable of analyzing grains representative of the bulk region, the accrued damage and the onset of fatigue failure could be predicted nondestructively for 2024 Al, cycled with constant stress as well as with variable stress amplitude. The dislocation structure produced in the bulk by prior cycling was unstable when the work-hardened surface layer was removed. It is proposed that the deformation response of the bulk material is controlled by the accumulation of dislocations and associated stresses in the surface layer. Formerly Research Associate, Rutgers College of Engineering.     

Mechanisms in fatigue strength improvement of thermomechanically manufactured automotive suspension springs

In a previous work, application of thermomechanical treatment (TMT) to suspension spring samples led to high increases in fatigue strength (up to 40 %). For the purpose of a better comprehension of these results, further investigations were carried out with major attention to the mechanical properties of quenched and tempered steel 50CrV4 and a Nb-microalloyed variant of this steel, respectively. First, the samples were ground prior to presetting, stress peening and subsequent dynamic testing. This was done to work out the influence of the surface state on the benefits of TMT. An extended program of materials testing was carried out with particular attention on the crack initiation and growth features of thermomechanically treated steel compared to conventional heat treatment (CHT). One of the results was that grinding of the spring surface causes further improvement of fatigue strength. On the other hand, the advantages of TMT compared to CHT are reduced as a result of surface grinding. The reason for this result is an overproportional increase in fatigue strength of CHT specimens due to grinding. An explanation imaginable for both the improvements caused by TMT and the reduction of these improvements after grinding could be the change in fracture mechanical properties. In particular TMT causes a significantly retarded crack initiation in the finite life range and an increased permissible stress intensity without crack formation.     

High cycle fatigue of weld repaired cast Ti-6AI-4V

In order to determine the effects of weld repair on fatigue life of titanium-6Al-4V castings, a series of specimens was exposed to variations in heat treatment, weld procedure, HIP cycle, cooling rate, and surface finish. The results indicate that weld repair is not detrimental to HCF properties as fatigue cracks were located primarily in the base metal. Fine surface finish and large colony size are the primary variables improving the fatigue life. The fusion zone resisted fatigue crack initiation due to a basketweave morphology and thin grain boundary alpha. Multipass welds were shown not to affect fatigue life when compared with single pass welds. A secondary HIP treatment was not detrimental to fatigue properties, but was found to be unnecessary.