Fatigue Fracture Surfaces Research Articles

Research on Fatigue Properties of Micron Scale Copper Bonding Wires

Copper bonding wires are frequently used to connect to MEMS devices. Mechanical properties of copper wire are crucial to the reliability of MEMS system. The paper reported a symmetrical bending fatigue test on micron scale copper bonding wires. The test is based on the phenomenon that a micro-cantilever can be set into self-excited vibration between two electrodes under DC voltage. The results demonstrate that the yield strength, ultimate tensile strength and Young's modulus of copper wires with diameter of 20μm are higher than those with a diameter of 30μm and 40μm, which significantly performs size effect. In fatigue test, the number of cycles to failure is 104~107. Under the same stress condition, fatigue strength (N=106) of copper wires (d=20μm, 30μm, 40μm) is 140MPa, 97MPa, 70MPa respectively. The tensile fracture surface is the chisel-shaped peak, and the surface of the fracture appears many spaced strip drawing traces. The fatigue fracture surface is flat. Two cracks almost simultaneously originate from the surface, and the final rupture region is just like a narrow sheet.

Ensemble of secondary cracks near the fatigue fracture surface of a titanium alloy VT23M specimen

The secondary fragment cracks in a specimen made of a high-strength VT23M titanium alloy in the plane parallel to the specimen plane are studied with a scanning laser microscope. Two types of the secondary cracks are detected. Relations between the characteristics of an ensemble of secondary cracks and the cyclic loading parameters are obtained.

Intergranular crack during fatigue in Al-Mg-Si aluminum alloy thin extrusions

The fatigue properties and possible mechanisms of intergranular fatigue fracture were investigated in the Al-Mg-Si aluminum alloy. The fatigue fracture surface of the 6N01-T5 aluminum alloy exhibited thorough intergranular cracking and a “rock candy” morphology, where the stress amplitude had no effect on the fracture morphology. The three typical fracture regions were obtained after solution heat treatment, whereupon the crack paths changed from intergranular cracking to traditional brittle cracking. A chainlike continuous distribution of Mg-Si and Fe-Al-Si phases was observed by electron backscatter diffraction phase graph analysis. After solution heat treatment, the number of Mg-Si phases decreased sharply and the chainlike continuous distribution of the intermetallic phases transformed into an interrupted island distribution, which demonstrates that Mg-Si phases are the main cause of intergranular cracking.

Fatigue of Low Carbon Alloy Steel (JIS S45C) and a New Method of Fracture Surface Analysis

In this study, we observed cracks and fatigue surface of low carbon alloy steel (JIS, S45C)) in order to investigate the material structure and fatigue fracture surface. After fatigue crack growth tests of CT specimens, three-dimensional surface roughness of the fracture surface was studied using a laser confocal microscope (LCM). An image analysis method of Homology theory was applied to the LCM’s data. The Betti numbers, b1 were calculated at four points on the fracture surfaces in order to study the effect of asperity heights on the b1 values.Based on these analyses, we found the relation between the structure and fatigue cracks from the viewpoint of Homology.

Fractal Geometry in Metallurgy of Welding and Coatings

Application of new methods for the description of structure of metals and alloys, the identification of new quantitative structure-morphology tours are especially important to establish the relationship "structure - property" and predicting the properties of weld metal on the basis of mathematical modeling. In our work we applied a fractal approach to the analysis of following objects: structure of metal during welding, fatigue fracture surface of metal of welded joints, structure of coatings in microarc oxidation, macrostructure of the heat-resistant steels during electron beam welding

Effect of Multi-Pass Ultrasonic Surface Rolling on the Mechanical and Fatigue Properties of HIP Ti-6Al-4V Alloy.

The main purpose of this paper was to investigate the effect of a surface plastic deformation layer introduced by multi-pass ultrasonic surface rolling (MUSR) on the mechanical and fatigue properties of HIP Ti-6Al-4V alloys. Some microscopic analysis methods (SEM, TEM and XRD) were used to characterize the modified microstructure in the material surface layer. The results indicated that the material surface layer experienced a certain extent plastic deformation, accompanied by some dense dislocations and twin generation. Moreover, surface microhardness, residual stress and roughness values of samples treated by MUSR were also greatly improved compared with that of untreated samples. Surface microhardness and compressive residual stress were increased to 435 HV and −1173 MPa, respectively. The minimum surface roughness was reduced to 0.13 μm. The maximum depth of the surface hardening layer was about 55 μm. However, the practical influence depth was about 450 μm judging from the tensile and fatigue fracture surfaces. The ultimate tensile strength of the MUSR-treated sample increased to 990 MPa from the initial 963 MPa. The fatigue strength of the MUSR-treated sample was increased by about 25% on the base of 107 cycles, and the lifetime was prolonged from two times to two orders of magnitude at the applied stress amplitudes of 650–560 MPa. The improved mechanical and fatigue properties of MUSR-treated samples should be attributed to the combined effects of the increased microhardness and compressive residual stress, low surface roughness, grain refinement and micro-pore healing in the material surface-modified layer.

Corrosion Fatigue Studies on a Bulk Glassy Zr-Based Alloy under Three-Point Bending

Corrosion fatigue (CF) tests were carried out on bulk glassy Zr52.5Cu17.9Al10Ni14.6Ti5 (Vitreloy 105) samples under load-controlled three-point bending conditions with a load ratio of R = 0.1 in 0.01 M Na2SO4 + 0.01 M NaCl electrolyte. During cyclic testing, the bar-shaped specimens were polarized in situ at constant potentials and the current was monitored. Three different anodic potentials within the interval between the pitting potential EP and the repassivation potential ER, and three different load amplitudes were applied. In some cases, in situ microscopic observations revealed the formation of black corrosion products in the vicinity of the crack tip during anodic polarization. Fractographic analysis revealed a clear distinction between two modes of crack growth characterized by smooth dissolution induced regions on the one hand and slim fast fracture areas on the other hand. Both alternating features contributed to a broad striated corrosion fatigue fracture surface. Moreover, further fatigue tests were carried out under free corrosion conditions yielding additional information on crack initiation and crack propagation period by means of the open circuit potential (OCP) changes. Thereby, a slight increase in OCP was detected after rupture of the passive layer due to bare metal exposed to the electrolyte. The electrochemical response increased continuously according to stable crack propagation until fracture occurred. Finally, the fracture surfaces of the corrosion fatigue samples were investigated by energy dispersive X-ray with the objective of analyzing the elemental distribution after anodic dissolution. Interestingly, anodic polarization at a near repassivation potential of -50 mV vs. SCE (Saturated Calomel Electrode, E = 0.241 V vs. SHE, Standard Hydrogen Electrode) led to favorable effects on the fatigue lifetime. In conclusion, all results are conflated to a corrosion fatigue model for bulk glassy Vitreloy 105 under anodic polarization in chloride-containing electrolyte and compared to the previously proposed stress corrosion mechanisms under similar conditions.

Effect of heat treatment on tensile and fatigue deformation behavior of extruded Al-12 wt%Si alloy

This study investigated the effect of heat treatment on tensile and high-cycle fatigue deformation behavior of extruded Al-12 wt%Si alloy. The material used in this study was extruded at a ratio of 17.7: 1 through extrusion process. To identify the effects of heat treatment, T6 heat treatment (515 °C/1 h, water quenching, and then 175 °C/10 h) was performed. Microstructural observation identified Si phases aligned in the extrusion direction in both extruded alloy (F) and heat treated alloy (T6). The average grain size of F alloy was 8.15 °C, and that of T6 alloy was 8.22 °C. Both alloys were composed of Al matrix, Si, Al2Cu, Al3Ni and AlFeSi phases. As T6 heat treatment was applied, Al2Cu phases became more finely and evenly distributed. Tensile results confirmed that yield strength increased from 119.0 MPa to 329.0 MPa, ultimate tensile strength increased from 226.8 MPa to 391.4 MPa, and the elongation decreased from 16.1% to 5.0% as T6 heat treatment was applied. High-cycle fatigue results represented F alloy’s fatigue limit as 185 MPa and T6 alloy’s fatigue limit as 275 MPa, indicating that high-cycle fatigue properties increased significantly as heat treatment was conducted. Through tensile and fatigue fracture surface analysis, this study considered the deformation behaviors of extruded and heat treated Al-Si alloys in relation to their microstructures.

Fatigue Fracture Surface Analysis by Local Lattice Rotation Measurement at FIB-milled Areas

Fatigue Fracture Surface Analysis by Local Lattice Rotation Measurement at FIB-milled Areas

Fatigue behaviour of dissimilar Al 5052 and Mg AZ31 resistance spot welds with Sn‐coated steel interlayer

AbstractThe fatigue property of dissimilar spot welds between an aluminium alloy (AA5052) and a magnesium alloy (AZ31) was studied in this research. The AA5052 and AZ31 coupons were resistance spot welded together by using an interlayer of Sn‐coated steel between the two coupons. The fatigue test results revealed that the Mg/Al joints had the same level of fatigue strength as Mg/Mg resistance spot welds. It was found that within the life range of Nf < 105 cycles, Mg/Al welds degraded faster than Mg/Mg joints. This was attributed to the larger bending moment on the plane of fatigue failure in the Mg/Al welds. Three failure modes were observed under different cyclic loading regimes: Al/steel interfacial failure, Mg coupon failure and Al coupon failure. Fatigue fracture surface of Mg/Al welds consisted of two distinct regions: crack propagation region with brittle morphology and final rupture with ductile morphology.

Fatigue strength improvement of GMAW T-welds in AA 5083 by friction-stir processing

The aim of this study is to verify to what extent the friction stir processing (FSP) can improve the fatigue behaviour of GMAW welds in aluminium alloys. In fact these welds often have problems such as: formation of porosity, distortion, loss of mechanical strength and deterioration of their fatigue behaviour. In this study the FSP technology is applied to GMAW T-fillet welds performed on 5083-H111, 6mm thick aluminium alloy plates. The FSP effects were studied by metallographic analysis as well as hardness, tensile and fatigue tests and the fatigue-fractured surfaces were analysed using scanning electron microscopy (SEM). The fatigue tests were done under constant loading for two stress ratios, R=0 and R=−1, and variable loading with spectrum shape exponent of ν=2 and ν=5 with the stress ratios R=0 and R=−1. The FSP slightly increases the hardness and Rp0.2 of toe microstructures, but reduces the elongation on fracture of tensile specimens. However, the fatigue strength of the welded joints is significantly improved because of the grain refinement, porosity removal and reduction in stress concentration promoted by the weld toe radius increase.

High Cycle Fatigue Life of Ti6Al4V Alloy Produced by Direct Metal Laser Sintering

High cycle fatigue life of Ti6Al4V alloy specimens manufactured by Direct Metal Laser Sintering (DMLS) was experimentally determined. The DMLS fabrication process was characterized by a 400 W laser power and 50 μm layer melted thickness. Post-fabrication heat treatment consisted in stress relieving for 3 h at 720 °C in vacuum with subsequent cooling in argon atmosphere. Fatigue testing of specimens oriented in three different directions with respect to the material build direction was performed with the aim to examine the influence of the layered microstructure on the fatigue behavior. Results of measurement of surface roughness, metallographic examinations of the layered material and fractographic investigation of the fatigue fracture surfaces were employed in the discussion of fatigue crack initiation in DMLS fabricated Ti6Al4V alloy.

Effects of ultrasonic impact treatment combined with the electric discharge surface alloying by molybdenum on the surface related properties of low-carbon steel G21Mn5

Abstract To enhance the fatigue, corrosion and wear behaviors of low-carbon casting steel 20GL (G21Mn5 in EN classification) it was subjected to the effects of severe plastic deformation induced by ultrasonic impact treatment (UIT) and the electric discharge surface alloying (EDSA) with molybdenum. The structural formations in the sub-surface layers were characterized by means of light microscopy, X-ray diffraction analysis and transmission electron microscopy (TEM). Several intermetallic phases were observed in the EDSA-induced layer. TEM studies of transition area between the EDSA layer and steel substrate (at a depth of 15–30 μm) demonstrate that the combined EDSA + UIT process induces the formation of a dislocation-cell structure in the alloyed ferrite grains. Both the cell boundaries and separate dislocations are often blocked by fine intermetallic and/or carbide particles. SEM analysis of fatigue fracture surfaces reveals the subsurface crack nucleation in the UIT-processed specimens instead of superficial crack initiation observed in the pristine and EDSA-processed ones. This provides enhanced fatigue strength and prolonged lifetime of the steel specimens after UIT and EDSA + UIT processes. High molybdenum content supports the improved corrosion resistance and fine intermetallic precipitates in the Fe-Mo protective layer provide the increased hardness and wear resistance of the EDSA + UIT-treated specimens. Thus, the specimens underwent combined EDSA + UIT treatment were shown to exhibit simultaneously increased fatigue strength (by ~ 5%) in low cycle fatigue regime, anti-corrosion properties (by ~ 85%) and wear resistance (by ~ 43%) in comparison with those of the pristine steel G21Mn5.

High frequency cycling behaviour of three AZ magnesium alloys – microstructural characterisation

Abstract Three Mg–Al–Zn alloys, namely AZ31, AZ63 and AZ91, were subjected to high frequency cyclic loading with the aim of revealing their fatigue behaviour. AZ63 and AZ91 alloys contained discontinuous and continuous precipitates and also the intermetallic compound Mg17Al12. Samples were loaded step by step at increasing stress amplitudes at room temperature. Three characteristics of the cycled samples were investigated: the sample surface, the fatigue fracture surface and the fracture surface after the static fracture of samples. Details of the fatigue and fracture behaviour were examined using scanning electron microscopy. The significant roles of twinning and dislocation movement during the high frequency cycling are discussed. The limited plasticity of alloys estimated during the high frequency cycling is due to a decrease in the moving dislocation density entering into the thermally activated process.

Fatigue Properties of Welded Butt Joint and Base Metal of MB8 Magnesium Alloy

The fatigue properties of welded butt joint and base metal of MB8 magnesium alloy were investigated. The comparative fatigue tests were carried out using EHF-EM200K2-070-1A fatigue testing machine for both welded butt joint and base metal specimens with the same size and shape. The fatigue fractures were observed and analyzed by a scanning electron microscope of 6360 LA type. The experimental results show that the fatigue performance of the welded butt joint of MB8 magnesium alloy is sharply decreased. The conditional fatigue limit (1×107) of base metal and welded butt joint is about 69.41 and 32.76 MPa, respectively. The conditional fatigue limit (1×107) of the welded butt joint is 47.2 % of that of base metal. The main reasons are that the welding can lead to stress concentration in the weld toe area, tensile welding residual stress in the welded joint, as well as grain coarsening in the welding seam. The cleavage steps or quasi-cleavage patterns present on the fatigue fracture surface, indicating the fracture type of the welded butt joint belongs to a brittle fracture.DOI: http://dx.doi.org/10.5755/j01.ms.22.3.9132

Synergy of crack closure, near-tip residual stress and crack-tip blunting in crack growth under periodic overloads – A fractographic study

Fatigue crack growth tests were performed on an Al-Cu alloy under specially designed load sequences with a periodic overload cycle. These overloads were either immediately followed by or preceded by underloads of controlled magnitude. Baseline loading magnitude was designed to induce near-threshold conditions well below the Paris Regime. The fatigue fracture surfaces obtained from the tests were subject to fractographic analysis. The study shows that irrespective of their magnitude, underloads preceding tensile overloads do not affect crack growth rate after the overload. However, if the underload follows an overload, it carries a deleterious effect on retardation, whose magnitude is determined by the extent to which the underload drops below baseline cycling level. In either case, observed effects do not seem to have anything to with crack closure, but rather, are attributable to the effect of underloads on near-tip residual stress during baseline cycling that in turn controls threshold stress intensity range. Post-overload crack closure is reduced by crack-tip blunting, but rapidly recovers.

Fractography of Fatigue Fracture Surface in Silumin Subjected to Electron-Beam Processing

The surface modification of the eutectic silumin with high-intensity pulsed electron beam has been carried out. Multi-cycle fatigue tests were performed and irradiation mode made possible the increase in the silumin fatigue life more than 3.5 times was determined. Studies of the structure of the surface irradiation and surface fatigue fracture of silumin in the initial (unirradiated) state and after modification with intense pulsed electron beam were carried out by methods of scanning electron microscopy. It has been shown, that in mode of partial melting of the irradiation surface the modification process of silicon plates is accompanied by the formation of numerous large micropores along the boundary plate/matrix and microcracks located in the silicon plates. A multi-modal structure (grain size within 30-50 μm with silicon particles up to 10 μm located on the boundaries) is formed in stable melting mode, as well as subgrain structure in the form of crystallization cells from 100 to 250 μm in size). Formation of a multi-modal, multi-phase, submicro- and nanosize structure assisting to a significant increase in the critical length of the crack, the safety coefficient and decrease in step of cracks for loading cycle was the main cause for the increase in silumin fatigue life.

Analysis on Low Cycle Fatigue Life and Fatigue Fracture Surface Morphology of TC25 Titanium Alloy

At room temperature, the low cycle fatigue tests for smooth specimens of TC25 titanium alloy under various stress ranges are operated at a CSS280I-20w Electro Hydraulic Servo Universal Testing Machine with a microscopic observation system, and the low cycle fatigue lifetimes are measured. Based upon the analysis of stress-strain hysteresis loop of low cycle fatigue of TC25 titanium alloy, a simplified Manson-Coffin formula is derived according to both the experimental characteristics and the stress-strain constitutive model, the fatigue lifetimes are plotted against stress ranges, and a stress-fatigue life curve for TC25 titanium alloy is obtained by the linear regression analysis method. Finally, the fracture surface morphologies of TC25 specimens are investigated using a JSM-6360 Scanning Electron Microscopy, and the fatigue fracture mechanisms of low cycle fatigue are studied. It shows that the plastic deformations are mainly formed at the accelerated fracture stage, and various shear lips can be observed on the fracture surfaces, which demonstrates that the shear stress results in the final rupture of TC25 titanium alloy. During the fracture of low cycle fatigue, the cleavage nucleation leads to the formation of fatigue crack initiation region, the fatigue crack growth exhibits a mixed transgranular and intergranular crack growth mode, and in the final rupture region, the fracture surface of low cycle fatigue of TC25 titanium alloy appears as a typical semi-brittle fracture mode.

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.

Fatigue crack growth resistance of 7075 Al alloy after equal channel angular pressing

AbstractThis work presents experimental results on effects of severe plastic deformation (SPD) and subsequent natural ageing on tensile mechanical properties and fatigue crack growth resistance of fine‐grained 7075 Al alloy. The alloy was subjected to equal channel angular pressing (ECAP) after solution treatment. Fatigue crack propagation tests were conducted in room condition, at load ratio R = 0.1 and different load ranges on small disk shaped compact tension specimens. Fatigue fracture surface is also investigated using scanning electron microscopy observations and showed more ductile fatigue crack growth in the unECAPed specimen. Despite the increased tensile strength after ECAP, the ductility that controls low‐cycle fatigue behaviour has decreased. It is found that ECAP has resulted in a remarkable change in Paris regime parameters and a significant increase in fatigue crack growth rate. The decrease in fatigue crack growth resistance and ΔKc after ECAP can be attributed to the decrease in alloy's ductility.