Fatigue Fracture Characteristics Research Articles

Fatigue Fracture Characteristics of Ti6Al4V Subjected to Ultrasonic Nanocrystal Surface Modification

The influence of ultrasonic nanocrystal surface modification (UNSM) on the fatigue fracture characteristics of Ti6Al4V was investigated. Two groups of specimens were separated due to different heat treatment conditions. Group one was stress-relief annealed at 650 °C, and group two was then treated with solid solution-aging. UNSM with the conditions of a static load of 25 N, vibration amplitude of 30 μm, and 36,000 strikes per unit produced about 40 μm surface severe plastic deformation (SPD) layers on both groups of specimens. UNSM improved the microhardness and the compressive residual stress. UNSM also helped achieve a neat surface, almost without changing the surface roughness. The fatigue strengths of these two groups were improved by 7% and 11.7%, respectively. After UNSM, fatigue cracks mainly initiated from the surface of the specimen before the fatigue life of 106 cycles, while they appeared at the internal compress deformed α-phase at the zone between the SPD layer and the core after the fatigue life of 106 cycles. The cracks usually extended along the deformation overflow bands and the process traces on the surface. Through the change of micro-dimples in the fatigue final rupture region, nanocrystals were achieved in the SPD layer. The crystal slip and the surface remodeling together influenced the energy field of crack evolution.

A comparison of axial fatigue strength of coarse and ultrafine grain commercially pure titanium produced by ECAP

Commercially pure titanium (CP-Ti) has been recently used as metallic biomaterials due to excellent biocompatibility and specific strength. CP-Ti has less static and dynamic strength as compared to other metallic biomaterials. Processing by the equal channel angular pressing (ECAP) as one of the most effective severe plastic deformation (SPD) method could lead to an increase in the mechanical strength of materials, significantly. In this study, Grade 2 CP-Ti billet is inserted into Al-7075 casing, and is then deformed by ECAP, with the channel angle of 135°, through 3 passes at route BC and room temperature. The purpose of using casing is to attain higher deformation homogeneity and more material ductility in the billet. The microstructural analysis shows that the coarse grain (CG) CP-Ti is developed to ultra-fine grain (UFG) structures after ECAP. In order to investigate the static and dynamic strength of CG and UFG CP-Ti, the tensile and axial fatigue tests are conducted. The results represent that UFG CP-Ti has much more tensile and fatigue strength than CG CP-Ti, and it could be utilized as biomaterials for production of implants. Surface features of fatigue fracture are also investigated. It should be noted that the investigation of fatigue strength of UFG CP-Ti produced by ECAP at RT utilizing casing, has not been conducted so far.

Experimental and numerical investigation of the breakage of a cutting tool with ultrasonic vibration

In this study, first, the breakage of a cutting tool with ultrasonic vibration was investigated using a scanning electron microscope. It was concluded that the breakage of a cutting tool with ultrasonic vibration has the features of fatigue fracture; i.e. it undergoes three stages: micro-crack generation, micro-crack spreading, and brittle fracture. Second, the reasons for the breakage of the cutting tool with ultrasonic vibration were analysed theoretically. The analytical results demonstrated two reasons: one is that the pulsating main cutting force and pulsating frictional force act periodically on the rake face of the cutting tool; the other is that the flank of the cutting tool is alternatively subjected to compressive stress and tensile stress. Furthermore, the influences of the main factors on the breakage of the cutting tool with ultrasonic vibration were studied experimentally. Finally, the breakage of the cutting tool was analysed using the finite element method (FEM). FEM simulation results demonstrate that the fatigue life and fatigue safety factor of the cutting tool decrease and the fatigue damage coefficient of the cutting tool increases with the increase in the amplitudes of the pulsating loads, reciprocating frictional force, and normal force.

High-temperature low-cycle fatigue behavior of a 9Cr-ODS steel: Part 1 - pure fatigue, microstructure evolution and damage characteristics

Low cycle fatigue (LCF) behavior of a tempered martensitic Fe-9%Cr-based oxide dispersion strengthened (ODS) steel at elevated temperatures is correlated to the microstructural evolution after cyclic straining. The fully reversed strain-controlled tests were conducted in air at 550°C and 650°C for different strain amplitude values ranging from ± 0.4% to ± 0.9%. Apart from the higher cyclic stress levels, the steel manifests complex cyclic softening which is significantly lower in comparison to that observed for similar non-ODS steels. This is due to the fact that highly stable nano-oxide particles act as persistent barriers for dislocation motion which as a consequence slows down the typical microstructural evolution realized in the conventional non-ODS steels. Upon cycling at 550°C, microstructure exhibits only minor changes. The main modifications are in respect to the dislocations rearrangement and/or annihilation which finally result in their reduced density. At 650°C, microstructural evolution hastens and becomes prominent mainly in nano-oxides/carbides deficient regions. Here, in addition to the reduced dislocation density, partially eliminated original sub-grain structures, grain growth, M23C6 carbides coarsening and Cr-W enriched Laves phase precipitation were evident. The microstructural modifications, concerning dislocation density and sub-grain structures, intensify even further with increase in applied strain amplitude. Nevertheless, annealing at 650°C for similar duration has no major influence on microstructure. Damage studies revealed expeditious as well as pronounced damage with increase in applied strain amplitude. Cracks initiation, eventuates at the early stage of the test, and their propagation were further assisted by oxidation. The stable crack growth region manifests secondary cracks, and at higher magnification the classical fine-scale transgranular ductile fatigue fracture features called striations. In addition, at 650°C, crack path also acquires an intergranular tendency under higher strain amplitude.

Quantitative Characterization of Microscale Fracture Features in Titanium Alloys

Quantification of the fracture mechanisms is important for design and sustainment of fatigue–critical components and for improved life prediction models. The traditional fractography in a scanning electron microscope (SEM) provides important, but qualitative, information on fracture mechanisms. In this study, the fatigue fracture features at the crack-initiation site in a near-α titanium alloy were characterized quantitatively in an SEM. The crack initiated in a surface-located primary α grain, with the resultant formation of a facet. A serrated feature was observed on the fatigue crack-initiating facet within an α grain for the first time in this study. The spatial orientations of the crack-initiating facet and the fracture lines forming the serration were determined in 3D with the quantitative tilt fractography technique, which consists of analyzing an SEM image pair acquired at different stage tilt angles. The electron backscatter diffraction (EBSD) data were collected nondestructively from the grain associated with faceted crack-initiation, without recourse to cross-sectional polishing. Combining the spatial orientation and the EBSD data established that the facet plane was basal and the lines forming the serrated feature were along 〈a〉 \( \left(\text{i.e.,}\; {\left\langle {2\bar{1}\bar{1}0} \right\rangle } \right) \)-type slip directions. The methodology presented in this study is applicable to quantification of crystallography for other planar and linear features in crystalline materials.

Grain boundary engineering: fatigue fracture

Grain boundary engineering has revealed significant enhancement of material properties by modifying the populations and connectivity of different types of grain boundaries within the polycrystals. The character and connectivity of grain boundaries in polycrystalline microstructures control the corrosion and mechanical behaviour of materials. A comprehensive review of the previous researches has been carried out to understand this philosophy. Present research thoroughly explores the effect of total strain amplitude on phase transformation, fatigue fracture features, grain size, annealing twinning, different grain connectivity and grain boundary network after strain controlled low cycle fatigue deformation of austenitic stainless steel under ambient temperature. Electron backscatter diffraction technique has been used extensively to investigate the grain boundary characteristics and morphologies. The nominal variation of strain amplitude through cyclic plastic deformation is quantitatively demonstrated completely in connection with the grain boundary microstructure and fractographic features to reveal the mechanism of fatigue fracture of polycrystalline austenite. The extent of boundary modifications has been found to be a function of the number of applied loading cycles and strain amplitudes. It is also investigated that cyclic plasticity induced martensitic transformation strongly influences grain boundary characteristics and modifications of the material’s microstructure/microtexture as a function of strain amplitudes. The experimental results presented here suggest a path to grain boundary engineering during fatigue fracture of austenite polycrystals.

Failure Analysis and Fatigue Investigation on Titanium Tubes in a Condenser

This paper presents an analysis of condenser titanium tube leakage in nuclear power plants. Chemical compositions, mechanical properties, metallographic structures, and microscopic morphologies were analyzed. The results show that the titanium tube leakage was mainly caused by fatigue failure on the basis of the fatigue fracture features. Fatigue tests had been carried out in both air and steam environments, and the fatigue resistance of titanium tubes decreased distinctly in a steam environment. Based on the investigations, proper recommendations have been proposed to enhance the prevention of fatigue fracture of titanium tubes in condensers.

Deformation and damage mechanisms of a bimodal 12Cr-ODS steel under high-temperature cyclic loading

The present study reports on the high-temperature low-cycle fatigue (LCF) behavior of a ferritic bimodal 12Cr oxide dispersion strengthened steel. The fully reversed strain-controlled LCF tests were conducted at a constant total strain rate with different axial strain amplitude levels. The measured cyclic stress response showed four distinct stages which include instant initial cyclic softening followed by gradual cyclic hardening thereafter continuous linear cyclic softening and finally crack initiation and growth stage. The rate and amount of these stages depend on both strain amplitude and testing temperature. The cyclic stress-strain and strain-life relationships were obtained through the test results, and related LCF parameters were calculated. Microstructural investigations, using electron microscopy, were carried out to shed light on the deformation mechanisms. At 550°C, no appreciable microstructural evolution occurred, which is consistent with the manifestation of Masing behavior. A propensity towards cross-slip and hence the formation of three-dimensional dislocation structures increased with increase in loading amplitude and testing temperature, which assisted in accumulating higher inelastic strain and led to slightly higher cyclic hardening at these conditions. Furthermore, W-enriched Laves phase particles precipitated at grain boundaries. The evolution of cell structures with strain amplitude at 650°C resulted in a small deviation from Masing behavior. The damage studies revealed multiple surface initiated transgranular cracks under higher strain amplitudes and a single surface initiated transgranular crack under lower strain amplitudes. The different stages of the crack path, whose durations depend on the testing conditions, revealed distinct fatigue fracture features.

Evaluation on Fatigue Performance and Fracture Mechanism of Laser Welded TWIP Steel Joint Based on Evolution of Microstructure and Micromechanical Properties

The fatigue performance and fracture mechanism of laser welded twinning induced plasticity (TWIP.) steel joint were investigated experimentally based on the evolution of microstructure and micromechanical properties. The optical microscopy was used to analyze the evolution of microstructure. The variation of composition and phase structure of fusion zone were detected by energy dispersive X-ray and X-ray diffraction spectrometers. The micromechanical behaviors of the various zones were characterized using nanoindentation. The static tensile test and high cycle fatigue test were performed to evaluate the mechanical properties of welded joint and base metal. The microstructures, tensile properties and fatigue strength of base metal as well as welded metal were analyzed. The fatigue fracture surfaces of base metal and welded joint were observed by means of scanning electron microscopy, in order to identify fatigue crack initiation sites and propagation mechanisms. Moreover, the fatigue fracture characteristics and mechanisms for the laser welded TWIP steel joints were analyzed.

Characteristics of cyclic fatigue fracture of nickel-titanium files in continuous rotary or reciprocating motion

Characteristics of cyclic fatigue fracture of nickel-titanium files in continuous rotary or reciprocating motion

Cyclic Bending Deformation and Fracture of Al and Al-1.0mass%Mg Alloy

The characteristics of cyclic bending deformation and fatigue fracture are studied on polycrystalline aluminum and Al-1.0mass%Mg alloy from the metallurgical point of view. It is found that the fatigue life mainly depends on grain size and the kind of materials. Cracks are preferentially formed at grain boundaries inclined 40 – 60°to the tension-compression direction, suggesting that shear stress affects the crack formation. EBSD measurements reveal the inhomogeneity of deformation. Intense development of sub-grain is seen in the grain interior close to a crack. It indicates that the work hardening close to grain boundaries triggers the crack formation.

Fatigue Properties and Fracture Characteristics of the Aluminum Alloys for High-Speed Train Carbody

The fatigue tests on two types of domestic aluminum alloys were conducted and the P-S-N curves of both aluminum alloys were obtained. The microscopic characteristics at the fatigue fracture surface were observed with scanning electron microscopy. The results show that the tensile strength of A7 is 376MPa, while the tensile strength of A6 is 311MPa. The fatigue strength of A7 is 75MPa at 107 circles, and it is just a little higher than that of A6, which is 71.57MPa. The fatigue cracks initiated from or near the surface of the specimen. The fatigue fracture can be divided into three parts, namely, the initiation zone, the propagation zone and the sudden fracture zone. Typical characteristics could be observed on fracture surface.A6 has better plasticity than A7.

On the Microstructural and Mechanical Characterization of Hybrid Laser-Welded Al-Zn-Mg-Cu Alloys

Butt-welded 2-mm-thick high-strength aluminum alloys have been welded using a hybrid fiber laser and pulsed arc heat source system with the ER5356 filler. The microstructure, size of precipitates, texture, grain size and shape, change of strengthening elements, mechanical properties, and surface-based fatigue fracture characteristics of hybrid-welded joints were investigated in detail. The results indicate that the hybrid welds and the unaffected base materials have the lowest and largest hardness values, respectively, compared with the heat-affected zone. It is resonably believed that the elemental loss, coarse grains, and changed precipitates synthetically produce the low hardness and tensile strengths of hybrid welds. Meanwhile, the weaker grain boundary inside welds appears to initiate a microcrack. Besides, there exists an interaction of fatigue cracks and gas pores and microstructures.

High Cycle Fatigue Behavior of a Wrought Nickel-Base Superalloy GH4698

Smooth and notched specimens of a wrought nickel-base superalloy GH4698 were subjected to high cycle fatigue (HCF) loading at room temperature and 650°C in ambient atmosphere. The experimental results show that the fatigue strength decreased with the increasing of temperature and cyclic stress, and fatigue limit of notched specimens was lower than that of smooth specimens. The fracture surface presents typical characteristics of fatigue fracture, where both stage I and stage II crack propagation can be found. For the smooth specimens, the single primary crack initiated on the surface or subsurface, while multiple primary cracks initiated on the surface for the notched specimens because of local stress concentration.

치과 임플란트에서 기성 지대주와 맞춤형 지대주의 응력분석 및 피로파절에 관한 연구

하악 제 1대구치에 티타늄과 지르코니아 소재의 기성 지대주와 맞춤형 지대주를 이용해 지르코니아 크라운으로 수복한 경우를 3차원 유한요소법을 통해 응력 분포를 분석해 보고, 피로 파절 강도 및 파절 형태를 고찰해 보았다. 정상교합자의 CT scan을 재구성한 하악골 모형상에 제 1대구치 임플란트 지지 지르코니아 크라운으로 수복한 6가지 유한요소 모형을 제작하고, 협측 교두 중앙에 수직과 30도 각도로 100 N의 하중을 가하는 조건으로 응력분포를 분석하였다. 현재 임상적으로 사용되고 있는 4가지 지대주를 이용하여, 군당 10개의 시편을 제작하고, ISO 14801에 따라 피로파절 실험을 실시하였다. 지대주나 고정체의 재질에 따른 응력분포의 차이는 없었고 형태에 따른 응력크기의 차이만 보였다. 맞춤형 지대주 군이 전반적으로 낮은 크라운 응력을 나타내었으며, 지대주 일체형 지르코니아 임플란트는 계면 골조직의 응력이 가장 낮게 나타났다. 피로수명의 평균값은 7군이 가장 높았고, 1군, 2군, 3군의 순서로 낮게 나타났으며(P<0.05), 기성 지대주 군은 피로수명의 편차가 적게 나타났다. 구치부의 임플란트 수복 시에는 맞춤형 지대주의 사용이 상부 보철물의 파절저항성 향상에 유리하며, 심미적 요구도가 높은 경우에는 고정체와의 접합부를 티타늄으로 제작한 지르코니아 맞춤형 지대주의 사용을 고려해 볼 만 하다. This study was to evaluate the stress distributions of prefabricated, customized abutments and fixtures according to their material and shape by three-dimensional finite element analysis. And to investigate the fatigue life and fracture characteristics. Mandibular models were fabricated by reconstruction of the CT scan of patients with normal occlusion. A total of six finite element models were designed, a load of 100 N was applied on the buccal cusps vertically, and 30 degree obliquely. 10 specimens each were fabricated for the more clinically widely used 4 type abutments and were loaded according to ISO 14801. Differences in stress distribution patterns were not found according to the materials of the abutments and fixtures. But a slight difference in the stress level was detected. Customized abutment groups showed lower crown stress levels. One-piece zirconia implant showed the lowest bone stress levels. In the fatigue test, highest values were measured in group 7. Prefabricated abutments showed less variation of fatigue life (P<0.05). Use of customized abutments can improve the fracture resistance of restorations. Especially, use of customized zirconia abutments reinforced by titanium screw connecting parts is recommended.

Fractal analysis of the fatigue fracture surface of metal of welded joints

The results of investigations of special features of fatigue failure of the metal of welded joints using the new methods of fractal analysis of the fracture surfaces in metals science of welding are presented. The fractal dimension of the fracture surface is determined and its relationship with the kinetic characteristic of the process – the fatigue crack growth rate – is investigated. The relationships governing the effect of the composition of electrode coatings on the fatigue fracture characteristics are determined.

S–N fatigue properties of a stable high-aluminum austenitic stainless steel for hydrogen applications

The fatigue properties of a novel high aluminum austenitic stainless steel with a high resistance against hydrogen embrittlement were investigated. S–N tests in 40 MPa H2 at −50 °C resulted in a reduction in fatigue life by a factor of about 2 compared to air. Striation analysis revealed no acceleration of crack growth rate but accelerated crack initiation or accelerated short crack growth in H2. No apparent difference in fatigue fracture characteristics and striation morphology between the air and H2 tested specimens could be identified.

Effect of equal channel angular pressing on microstructural and mechanical properties of as cast Al–20 wt-%Zn alloy

The microstructural and mechanical properties, mainly the hardness, tensile properties and impact toughness of two phase Al–20 wt-%Zn alloy subjected to severe plastic deformation via equal channel angular pressing (ECAP) up to two passes, were studied. High cycle fatigue behaviour of ECAP processed alloy was investigated for the first time. The ECAP almost completely eliminated the as cast dendritic microstructure including casting defects such as microporosities and formed a refined microstructure consisting of elongated microconstituents, aluminium rich α and zinc rich α+η eutectoid phases. As a result of this microstructural change, the tensile response, impact toughness and fatigue performance of the alloy improved significantly. The rates of increase in endurance limit and impact toughness after ECAP are 89 and 122% respectively. In addition, a relationship represented by an equation between the stress amplitude σa and number of cycles to failure N values was determined to express the fatigue behaviour of the alloy in all states. It was also observed that the nature of fatigue fracture characteristics of the as cast Al–20 wt-%Zn alloy changed after ECAP.

Failure of a helical gear in a power plant

Failure of high reliability components such as gears in power generation plants is a rare occurrence. This paper reports the results of failure analysis of a helical gear used in a power plant in Thailand. The gear failed after approximately nine years of service. Visual inspection as well as macro- and microscopic examinations of fracture surface were performed. Microstructure of the gear material was examined and microanalysis were carried out. Microhardness at the case of the gear at various distances from the surface were measured. The results showed that the fracture surface exhibited fatigue fracture characteristics. There were inclusions in the microstructure of gear material. Microanalysis results indicated that the inclusions are iron oxide surrounded by (FeMo)3C carbide. It is concluded that the gear failed by fatigue fracture. Fatigue cracks were initiated at the inclusions and propagated through the material until finally the gear tooth was overloaded and fractured. The inclusions were thought to be formed during the casting of the ingot that was used to make the gear blank, possibly due to inadequate degassing or deoxidation of liquid steel prior to ingot casting. It is recommended that cleaner steels be used to make such high reliability gears which could be achieved via improved steel making practice.

Mechanical Properties of Equal-Channel Angular Extrusion-Processed Al-20Zn Alloy

The mechanical properties mainly tensile properties, impact toughness and high-cycle fatigue properties, of two-phase Al-20Zn alloy subjected to severe plastic deformation (SPD) via equal-channel angular extrusion (ECAE) using route A up to 2 passes were studied. The ECAE almost completely eliminated as-cast dendritic microstructure including casting defects such as micro porosities. A refined microstructure consisting of elongated micro constituents, α and α+η eutectic phases, formed after ECAE via route A. As a result of this microstructural change, mechanical properties mainly the impact toughness and fatigue performance of the as-cast Al-20Zn alloy increased significantly through the ECAE. The rates of increase in fatigue endurance limit are approximately 74 % after one pass and 89 % after two passes while the increase in impact toughness is 122 %. Also the yield and tensile strengths of the alloy increase with ECAE. However, no considerable change occurred in hardness and percentage elongation of the alloy. It was also observed that the ECAE changed the nature of the fatigue fracture characteristics of the as-cast Al-20Zn alloy.