Fatigue Fracture Characteristics Research Articles

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.

Fatigue Crack Length Real Time Measurement Method Based on Camera Automatically Tracking and Positioning

Fatigue crack propagation test is a very important testing way to study the metallic engineering material fatigue fracture characteristics by on-line measuring crack length under the cyclically varying loads. Crack length has been obtained through crack image capture, image processing and image analysis. In order to improve the crack measurement precision when the fatigue crack keeps growing, a new crack length real time measurement method based on the image processing and camera automatically tracking techniques was presented in this paper. This method includes the crack image processing algorithms and camera tracking and positioning movement control. The crack length measurement errors based on the camera tracking and fixed camera are respectively 0.076mm and 0.148mm.The experiment results show the proposed method has greatly improved the crack length measuring precision with the ordinary optical device.

Research on fatigue behavior evaluation and fatigue fracture mechanisms of cruciform welded joints

Abstract The applications of infrared thermographic method and the relationship between the microstructures and mechanical properties of materials were reviewed. Experimental tests, using the thermographic method, have been carried out in order to evaluate the fatigue behavior of cruciform welded joints subjected to a high mean stress. A good agreement was achieved between the predicted values and those obtained by the traditional procedures. The fatigue life of the welded joint was significantly decreased since the high mean stress accelerated the fatigue microcrack initiation and propagation. The residual fatigue life was well predicted using the energy model of Miner's law advanced in the present paper. Though both the microstructures were tempered martensite along with secondary phase particles, the mechanical properties of the base metal were better than that of the weld seam due to the unfavorable influence of the thermal welding. The welded joints, undergoing fatigue loading, failed at the weld toe or the weld seam, indicating typical fatigue fracture features. Two fatigue mechanisms, involving the crack initiation at persistent slip bands and inclusions, were found by use of SEM.

A Comparison of Fatigue Properties of Austempered Versus Quenched and Tempered 4340 Steel

This study was conducted to determine if austempered 4340 steel had different fatigue resistance compared to quench and tempered (Q&T) 4340 steel with an identical hardness of nominally 45 HRC and an identical yield strength of nominally 1340 MPa (194 ksi). Strain-life and stress-life fatigue testing was conducted at room temperature under identical test conditions. The standard array of strain-life and stress-life regression constants was obtained. The two heat treatments produced virtually identical total strain-life curves and fatigue limits at 5 million cycles. However, the two materials exhibited different trends in the elastic and plastic strain regimes. The austempered steel exhibited greater high cycle fatigue (finite) lives than the Q&T samples at comparable elastic strain amplitudes in strain-life fatigue testing and at comparable stress amplitudes in stress-life fatigue testing. However, the Q&T samples exhibited greater low cycle fatigue lives than the austempered samples at comparable plastic strain amplitudes in strain-life testing. Although both materials generally exhibited similar fatigue fracture characteristics, the overload regions of the Q&T samples were composed entirely of dimple rupture, whereas the austempered samples exhibited both dimple rupture and quasicleavage.

Influence of curvature location along an artificial canal on cyclic fatigue of a rotary nickel-titanium endodontic instrument

This study evaluated the effects of curvature location along an artificial canal on cyclic fatigue (CF) of an Mtwo rotary instrument, verifying the number of cycles to fatigue fracture (NCF) and morphologic characteristics of the fractured instruments. CF testing of instruments was performed in artificial canals with curvature radii of 10 mm and arc lengths of 11 mm. Mtwo rotary instruments size 40, 0.04 taper were used in 2 groups (n = 10): group A, curvature positioned on middle part; group B, apical curvature. All instruments were rotated until fracture. The number of cycles to failure was registered. Data were analyzed by independent sample t test. Fractured surfaces and the helical shafts of the instruments were analyzed by scanning electron microscopy. NCF for groups A and B had significant statistic differences (P < .05). The highest values were found in the group where the curvature was positioned on the apical part of the canal. No plastic deformation was observed on the helical shafts. The number of cycles to fracture of the Mtwo instruments increased when the arc was changed from the middle to the apical part of the canal. The morphologic characteristics of the fractured surfaces were of the ductile type.

High cycle fatigue characteristics of 2124-T851 aluminum alloy

The fatigue crack growth rate, fracture toughness and fatigue S-N curve of 2124-T851 aluminum alloy at high cycle fatigue condition were measured and fatigue fracture process and fractography were studied using optical microscopy (OM), X-ray diffraction (XRD) technique, transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The results show that at room temperature and R = 0.1 conditions, the characteristics of fatigue fracture could be observed. Under those conditions, the fatigue strength and the fracture toughness of a 2124-T851 thick plate is 243 MPa and 29.64 MPa · m1/2, respectively. At high cycle fatigue condition, the higher the stress amplitude, the wider the space between fatigue striations, the faster the rate of fatigue crack developing and going into the intermittent fracture area, and the greater the ratio between the intermittent fracture area and the whole fracture area.

Fatigue limits and SEM/TEM observations of fracture characteristics for three Pd—Ag dental casting alloys

The fatigue limits and fracture characteristics for three Pd-Ag dental casting alloys (Super Star, Heraeus Kulzer; Rx 91, Pentron; W-1, Ivoclar Vivadent) were studied. Specimens meeting the dimensions for ADA Specifications No. 5 and 38, and having the as-cast surface condition, were subjected to heat treatment simulating dental porcelain firing cycles and fatigued in air at room temperature under uniaxial tension-compression at 10 Hz. A ratio of compressive stress amplitude to tensile stress amplitude (R-ratio) of -1 was used. Alloy microstructures and fracture surfaces were examined with a scanning electron microscope and a transmission electron microscope. Fatigue limits for the three alloys had low values of approximately 15% of the yield strength for 0.2% permanent tensile strain. Complex fracture surfaces with characteristic striations were observed for all three fatigued alloys. Planar slip of dislocations occurred in the Pd solid solution matrix, along with dislocation-precipitate interactions and dislocation networks in the interfaces between the precipitates and surrounding matrix. Twinning occurred in the Pd solid solution matrix of Rx 91, and within discontinuous precipitates in Super Star and Rx 91. The low fatigue limits for these alloys are attributed to their complex microstructures and perhaps to casting defects.

Mechanical properties of Friction Stir Processed 2618/Al2O3/20p metal matrix composite

The effect of Friction Stir Processing (FSP) on the mechanical properties of 2618 aluminium alloy reinforced with 20% of alumina particles aluminium alloy has been studied in the present paper. The material was processed into the form of sheets of 7 mm thickness after T6 treatment and was tested in tension and fatigue at room temperature. Tensile tests were also performed at higher temperatures and different strain rates in the nugget zone, in order to analyse the superplastic properties of the recrystallized material and to observe the differences with the parent materials as a function of the strong grain refinement due to the Friction Stir Process. The high temperature behaviour of the material was studied, in longitudinal direction, by means of tensile tests in the temperature and strain rate ranges of 400–500 °C and 10 −3 –10 −1 s −1 , respectively. Fracture surfaces of the deformed fatigue test specimens were comprehensively examined in a scanning electron microscope equipped with field emission gun to determine the macroscopic fracture mode and characterize the fine-scale topography and microscopic mechanisms governing fatigue fracture. The mechanisms governing fatigue life, cyclic deformation and fracture characteristics are analysed in function of magnitude of applied stress, intrinsic micro structural evolution and material deformation behaviour.

Influence of Material State at a Crack Tip on the Process of Fatigue Striations Formation for Al-Based Alloys

Fatigue fracture features such as pseudo- and fatigue striations were investigated for the Al-alloys AL-Si-Mg and Acoustic Emission (AE) was used to analyze plastic deformation processes ahead of a crack tip. Dramatic change of fatigue striations spacing was demonstrated in accordance with AE-signals evolution. Specimens with fatigue crack have not macroscopic significance in plastic deformation process under tension up to the fast fracture when striation spacing is over 2.1x10-7 m. Mechanisms of material deformation and fracture around of a crack tipduring fatigue striations formation process are discussed on the bases of the synergistic approach.

Cyclic plastic strain response and fracture behavior of 2009 aluminum alloy metal-matrix composite

In this paper the influence of discontinuous ceramic particulate reinforcements on cyclic stress response, cyclic stress versus strain response, cyclic strain resistance, deformation and fracture behavior of 2009 aluminum alloy discontinuously reinforced with silicon carbide particulates are presented and discussed. The cyclic strain amplitude–controlled fatigue properties and fracture characteristics of the 2009/SiC composite specimens are discussed for a range of cyclic strain amplitudes and at two different temperatures. The conjoint influence of test temperature and strain amplitude on cyclic stress response, cyclic stress versus strain response, and cyclic strain resistance is highlighted. The intrinsic mechanisms governing stress response, cyclic deformation and fatigue fracture characteristics are presented and discussed in light of the competing and mutually interactive influences of intrinsic composite microstructural effects, deformation characteristics of the composite constituents, cyclic strain amplitude and concomitant response stress, cyclic ductility, and test temperature.

Effect of Moisture Damage on Material Properties and Fatigue Resistance of Asphalt Mixtures

Dynamic mechanical analysis (DMA) has been used successfully to evaluate complex characteristics of fatigue damage and fracture of asphalt binders and mastics by measuring fundamental viscoelastic properties and damage characteristics. DMA was used to define the effect of moisture on fatigue damage and to concentrate on the fatigue damage susceptibility of the sand and asphalt mixture mastic fraction. Dynamic frequency sweep and time sweep tests were performed on cylindrical sand-asphalt samples in a dry state and after being subjected to moisture saturation. Test results clearly indicate that moisture reduces viscoelastic stiffness, fatigue resistance, and eventually fatigue life of sand-asphalt. The mechanistic role of moisture in fatigue was analyzed and quantified by using nonlinear viscoelastic theory based on pseudovariable concepts and a continuum damage fatigue model. The effect of material surface energies, which is strongly related to fracture and damage, is further discussed by using DMA fatigue test results and varying surface energy characteristics of individual mixture constituents. The DMA experimental procedure and analysis is an efficient way to identify the influence of moisture and to compare sand-asphalt mixtures in terms of moisture susceptibility.

The fatigue and final fracture behavior of SiC particle reinforced 7034 aluminum matrix composites

Abstract In this research paper, the cyclic stress-amplitude-controlled fatigue response and fracture behavior of aluminum alloy 7034 discontinuously reinforced with silicon carbide particulates (SiCp) is presented. In view of the limited ambient temperature ductility, test specimens of the 7034/SiCp composite, in both the under-aged and peak-aged conditions, were cyclically deformed under stress-amplitude-control at an elevated temperature corresponding to the aging temperature of the alloy. The cyclic fatigue tests were conducted at two different load ratios with the objective of documenting the conjoint influences of intrinsic composite microstructural effects, nature of loading, and magnitude of cyclic stress amplitude on cyclic fatigue life and fracture characteristics. The final fracture behavior of the composite is discussed in light of the concurrent and mutually interactive influences of composite microstructural effects, deformation characteristics of the composite constituents, nature of loading, and resultant fatigue life.

The cyclic fatigue and final fracture behavior of aluminum alloy 2524

This paper presents and discusses the results of a study on the influence of test temperature on high-cycle fatigue properties and fracture characteristics of aluminum alloy 2524. Specimens of the alloy, in the T351 temper, were cyclically deformed over a range of stress amplitudes at cryogenic, ambient and elevated temperatures. An increase in test temperature was found to have a detrimental influence on cyclic fatigue life of specimens taken from both the longitudinal and transverse orientations of the wrought plate. At a given test temperature, the macroscopic fracture mode was essentially identical, regardless of the orientation of the test specimen with respect to the wrought rolled plate. The mechanisms governing high-cycle fatigue life, cyclic deformation and fracture characteristics are discussed in light of the mutually interactive influences of magnitude of applied stress, test temperature, intrinsic microstructural effects and matrix deformation characteristics.

Fatigue studies of high-palladium dental casting alloys: Part I. Fatigue limits and fracture characteristics.

The fatigue limits and fracture characteristics for a Pd-Cu-Ga alloy and a Pd-Ga alloy were studied. The alloys were cast into tensile test bars with gauge diameter of 3 mm and gauge length of 15 mm, and the surfaces of the castings were neither air-abraded nor polished after removal from the investment. Specimens were prepared from all-new metal (not previously melted), a combination of 50% new metal and 50% old metal (previously melted one time) and 100% old metal. The cast bars were subjected to heat treatment simulating the complete firing cycles for dental porcelain, and fatigued in air at room temperature under uniaxial tension-compression stress at 10 Hz and a ratio of tensile stress amplitude to compressive stress amplitude (R-ratio) of -1. The alloy microstructures and fracture surfaces were examined with a scanning electron microscope (SEM). Results showed that the fatigue limits at 2 x 10(6)cycles of the Pd-Cu-Ga and Pd-Ga alloys were approximately 0.20 and 0.15 of their 0.1% yield strength (YS) in tension, respectively. The fatigue resistance for specimens from both alloys containing 50% old metal and 50% new metal was comparable to that of specimens containing all-new metal, although this decreased dramatically for Pd-Cu-Ga alloy specimens containing all-old metal. The fatigue resistance of the Pd-Cu-Ga alloy subjected to heat treatment simulating the porcelain firing cycles was not adversely affected by remnants of the original as-cast dendritic microstructure that remained in the relatively large test specimens. A longer heat treatment than recommended by the manufacturer for the porcelain firing cycles is needed to completely eliminate the as-cast dendritic structure in these specimens. The Pd-Cu-Ga alloy exhibited superior fatigue resistance to the Pd-Ga alloy, which has an equiaxed-grain microstructure and lower yield strength.

The characteristics of vibration fracture of Pb-Sn and lead-free Sn-Zn eutectic solders

This work investigated the fatigue fracture characteristics of Sn-7∼11wt.%Zn and Sn-30∼50wt.%PPb solder alloys under resonant vibration. For the alloys containing proeutectic Sn-rich grains, the results show stratum appearance on the deformation of coarse proeutectic grains. This stratum-type deformation will reduce the vibration resistance of Pb-Sn solder by inducing cracks. For the Sn-Zn system, fine eutectic structure will cause cracks to form and coalesce easily in the regions concentrated with small fibrous deformation grains, whereas hypereutectic structure shows coarse proeutectic Zn-rich particles as the crack initiation sites. Sn-t-Zn solders have better damping capacity than Pb-Sn solders. With hypoeutectic composition to induce stratum-type deformation and under lower vibration strain, these lead-free solders tend to exhibit superior crack propagation resistance, and, in that respect, are possible to replace Pb-Sn solders.

Fatigue Fracture Features of Pressure Vessel Steel in Simulated Light Water Reactor Environment

To evaluate fatigue crack growth of reactor pressure vessel steel at reactor operating conditions, tests were performed in air saturated hot water. The main test parameter was a loading frequency. Crack growth rate was increased with decreasing frequency up to a critical value. It was assumed through fractographic study and surface analysis that the enhancement of crack growth rate was environmentally assisted due to hydrogen embrittlement. In the low stress intensity factor range, cracks grew through crystallographic facets and tear ridges. Moreover, cleavage occurred at MnS inclusions. Larger cleavage occurred near MnS inclusions in the high stress intensity factor range. Brittle facets combined with voids and stair or step-like cracks with arrest marks appeared at sites some distance from the MnS inclusions. It was suggested that the environmentally assisted cracks were related to interactions of hydrogen with oxide film and also to dislocation motion at the crack tip.

Fatigue Fracture Features of Pressure Vessel Steel in Simulated Light Water Reactor Environment.

Fatigue Fracture Features of Pressure Vessel Steel in Simulated Light Water Reactor Environment.

Studies on the stress distribution and fatigue behavior of weld-bonded lap shear joints

The construction and hardness distribution of weld-bonded lap joints were investigated, and a computational model for weld-bonded joints was established on this basis, which consisted of the four parts of the base metal, the heat-affected zone, the nugget and the adhesive layer. Stresses in weld-bonded joints were computed and compared with those in spot-welded and adhesive-bonded joints. Fatigue tests were carried out on these three types of joints to study the fatigue behavior and fracture characteristics of the joints. Results showed that stresses from numerical analysis agree well with experimental results and observed phenomena. The application of adhesives in spot welding greatly improves the joint fatigue performance, while the presence of weld spots in adhesive-bonding has a negative effect on the joint fatigue performance.

Fatigue crack propagation and fracture characteristics of in-situ titanium-matrix composites

In this paper, the fracture and fatigue crack growth behavior of in-situ titanium-matrix composites reinforced with whiskers of titanium boride, are discussed. The influence of boron additions on the cyclic fatigue crack growth behavior of several titanium-base alloys is elucidated. The improved crack growth resistance of certain compositions is explained by considering the effects of crack–microstructure interactions and the limited effects of crack-tip shielding due to ligament bridging.

Cyclic Crack Resistance of Austenitic Steels for Equipment of Nuclear Power Plants

We investigate the influence of the mode of thermal treatment, the structure and properties of a material, the direction of propagation of cracks, temperature, and the parameters of a loading cycle and a corrosion-active medium on the cyclic crack resistance of chromium–nickel austenitic stainless steels of a Du-500 pipeline, the main gate valve G33 Du-500, and the header of a PGV-1000U steam generator. We show that water with high parameters, the stress ratio, and the temperature have the strongest effect on the growth rate of cracks. By using metallographic and electron-fractographic methods, we revealed specific features of fatigue fracture of steels in air and water with high parameters. On the basis on these data and a generalization of the well-known results, we propose the rated dependences for the determination of the growth rate of fatigue cracks. These dependences are compared with the rated curves of the M-02-91 normative documentation and the ASME Code and recommended for their correction.