Bending Fatigue Specimens Research Articles

High-cycle fatigue properties of curved-surface AlSi10Mg parts fabricated by powder bed fusion additive manufacturing

PurposeThis study aims to uncover the multiscale relations among geometry, surface finish, microstructure and fatigue properties of curved-surface AlSi10Mg parts fabricated by powder bed fusion (PBF) additive manufacturing.Design/methodology/approachThis paper investigated the high-cycle tensile and bending fatigue behaviors of PBF-built AlSi10Mg parts with curved surfaces. Besides, the surface finish, porosity and microstructure around various curvatures were characterized. Meanwhile, the stress distributions of the fatigue specimens with curved surfaces under the dynamic tensile/bending loading were analyzed via theoretical analysis and ANSYS simulation.FindingsThe results showed that the as-built specimens with the smallest curvature exhibited the best surface quality, smallest grain sizes and thinnest grain boundaries. In addition, the tensile fatigue fracture occurred around the largest curvature position of fatigue specimens, which was consistent with the simulated fatigue safety factor results. Moreover, the bending fatigue specimens with the largest curvature presented the shortest fatigue life due to the highest bending and shear stresses along the loading direction.Originality/valueSo far, most studies have focused on the fatigue behavior of as-built AlSi10Mg parts with planar structures only. The investigation on fatigue properties of as-built AlSi10Mg parts with curved surfaces remains unexplored. This study provides new insights into the characterization and quantification of the fatigue performance of PBF-built metal parts with complex geometries, the knowledge of which can promote their adoption in real industries.

Fatigue Cycles and Performance Evaluation of Accelerating Aging Heat Treated Aluminum Semi Solid Materials Designed for Automotive Dynamic Components

The A357-type (Al-Si-Mg) aluminum semi solid casting materials are known for their excellent strength and good ductility, which make them materials of choice, preferable in the manufacturing of automotive dynamic mechanical components. Semi-solid casting is considered as an effective technique for the manufacturing of automotive mechanical dynamic components of superior quality performance and efficiency. The lower control arm in an automotive suspension system is the significant mechanical dynamic component responsible for linking the wheels of the vehicle to the chassis. A new trend is to manufacture this part from A357 aluminum alloy due to its lightweight, high specific strength, and better corrosion resistance than steel. This study proposes different designs of a suspension control arm developed, concerning its strength to weight ratio. Furthermore, this study aims to investigate the effect of accelerating thermal aging treatments on the fatigue life of bending fatigue specimens manufactured from alloy A357 using the Rheocasting semi-solid technology. The results revealed that the multiple aging cycles, of WC3, indicated superior fatigue life compared to standard thermal aging cycles. On the other hand, the proposed designs of automotive suspension control components showed higher strength-to-weight ratios, better stress distribution, and lower Von-Mises stresses compared to conventional designs.

Inconel 718 Superalloy Controlled Surface Integrity for Fatigue Applications Produced by Precision Turning

An original approach is introduced to discriminate the effects of residual stress and surface roughness on turned Inconel 718 alloy parts. This could be used for assessing or studying the performance of these specimens during fatigue tests. This method consists of building surface integrity maps in terms of surface roughness parameters and residual stress in order to select specific turning conditions with adequate machining parameters. The surface integrity maps were generated by using response surface methodology based empirical models and precision turning tests on coupons. The proposed approach was therefore successfully applied to manufacture rotating bending fatigue specimens with the surface axial residual stress varying from − 239 to 446 MPa while keeping the arithmetic average roughness and the surface hoop residual stress constant. The effect of the surface axial residual stress on the fatigue life could thus be discriminated from the other surface integrity characteristics effects. The proposed methodology is independent of the manufacturing process and can be extended to other machining processes such as milling, grinding, and broaching.

Low-cycle fatigue behaviour of laser welded high-strength steel DOMEX 700 MC

The article presents the results of research on low cycle fatigue strength of laser welded joints vs. non-welded material of high-strength steel DOMEX 700 MC. The tests were performed under load controlled using the total strain amplitude ɛac. The operating principle of the special electro-mechanic fatigue testing equipment with a suitable clamping system was working on 35 Hz frequency. Fatigue life analysis was conducted based on the Manson-Coffin-Basquin equation, which made it possible to determine fatigue parameters. Studies have shown differences in the fatigue life of original specimens and laser welded joints analysed, where laser welded joints showed lower fatigue resistance. In this article a numerical analysis of stresses generated in bending fatigue specimens has been performed employing the commercially available FEM-program ADINA.

Coverage and Peening Angle Effects in Shot Peening on HCF Performance of Ti-6Al-4V

Shot peening of titanium alloys is known to enhance the HCF strength by inducing in near-surface regions residual compressive stresses which can drastically retard the growth rates of surface cracks. In addition, the induced high dislocation densities may increase the resistance to fatigue crack initiation while the typically accompanied high surface roughness has the opposite effect.The present work aimed at studying the effect of coverage and peening angle effects in shot peening on fatigue crack initiation and micro-crack growth on Ti-6Al-4V.The coverage was varied to a wide extent ranging from 20 % to 1200 % and the peening angle from 90 to 30 degrees. Fatigue life of shot peened rotating bending hour-glass shaped and flat bending fatigue specimens was studied and compared to an electrolytically polished reference.The results indicate that low (20%) coverage peening leads to a loss in HCF strength, presumably caused by insufficient residual compressive stress fields which cannot compensate the early crack initiation caused by the high roughness. In contrast, full (100 %) up to a high (1200 %) coverage was found to result in a marked increase in HCF strength.The variation of peening angles resulted in a significant beneficial effect on the fatigue strength of the flat bending fatigue samples in direction to increasingly flat angles and high coverages (1200 %) at constant Almen intensities of 0,20 mmA. Beside the deeper compressive stress layer at flat impact angles especially the increased dislocation density and the creation of a material texture orientated 90 degrees to the cracking direction seems to be responsible for the increased fatigue strength.These results are even more interesting as those peened surfaces resulted in the highest roughness values and a topography with a starting waviness on the surface.

A Plastic Strain and Stress Analysis of Bending and Torsion Fatigue Specimens in the Low-cycle Fatigue Region Using the Finite Element Methods

The assessment of a reliable S-N curve requires an accurate determination of stresses involved. If the stress state is complicated like in a plastic field of deformation the stress for a neck specimen may not always be calculated using formulae available in standard handbooks. Furthermore, the stress may vary from one point to another in a manner which is difficult to predict. This situation arises for instance during bending, torsion and also combined of fatigue specimens loading. In this article a numerical analysis of stresses generated in bending and torsion fatigue specimens has been performed employing the commercially available FEM program ADINA. Computer simulation has been performed on fatigue specimens from high-strength steel DOMEX 700 MC D.

Analysis of Stress and Strain of Fatigue Specimens Localised in the Cross-sectional Area of the Gauge Section Testing on Bi-axial Fatigue Machine Loaded in the High-cycle Fatigue Region

Fatigue is a progressive, localized, permanent structural change that occurs in materials subjected to fluctuating stresses and strains that may result in cracks or fracture after a sufficient number of fluctuations. Fatigue fractures are caused by the simultaneous action of cyclic stress, tensile stress and plastic strain. If one of these three acting phenomena is not present, fatigue cracking will not initiate and propagate. Small geometrical changes in the specimens, or specifically, test conditions by invariable angular displacement cannot significantly allow analytical calculation of stresses and strains. Therefore a numerical analysis of stresses generated through bending and torsion fatigue specimens has been performed using available FEM-program ADINA. Computer simulation has been performed on fatigue specimens made of high-strength steel DOMEX 700 MC D.

Fatigue modelling for gas nitriding

The present study aims to develop an algorithm able to predict the fatigue lifetime of nitrided steels. Linear multi-axial fatigue criteria are used to take into account the gradients of mechanical properties provided by the nitriding process. Simulations on rotating bending fatigue specimens are made in order to test the nitrided surfaces. The fatigue model is applied to the cyclic loading of a gear from a simulation using the finite element software Ansys. Results show the positive contributions of nitriding on the fatigue strength. KEYWORDS. Nitriding; Fatigue; Lifetime.

冷間工具鋼の破壊じん性と疲労強度

In this study, the investigation and discussion of the fracture toughness and fatigue strength of cold work tool steel were carried out. The materials under test were rotary bending fatigue specimens of SKD11, and fracture toughness specimens made that specimens. Fatigue strength, fracture toughness and hardness were measured by the rotary bending fatigue test, fracture toughness test of SEPB method and hardness test. From the results of rotary bending fatigue test, the variations were observed in the fatigue strength. From the results of fracture toughness test and hardness test, if the variation of the S-N property is small, fracture toughness and hardness were almost the same values.

Effect of Austenizing and Tempering Heat Treatment Temperatures on the Fatigue Resistance of Carburized 16MnCr5 (ASTM 5117) Steel

The present investigation deals with the study of the effect of austenizing and tempering heat treatment temperatures on the fatigue resistance of carburized 16MnCr5 steel. Rotating bending fatigue specimens were machined from 16MnCr (ASTM 5117(steel rod, and pack carburized at 900°C for 2 hours soaking time. Carburized specimens were then austenized at 900°C for one hour, water quenched, reaustenized at temperatures 750°C, 800°C and 900°C for one hour, then tempered at 200°C temperature. Other carburized specimens were tempered by heating to 760°C temperature, water quenched to room temperature, then tempered at temperatures 200°C, 300°C, and 400°C for one hour. Austenized and tempered steel specimens after carburization as well as uncarburized steel specimens were then tested by rotating bending fatigue machine up to fracture under different stress levels (200, 250, 300, 350, 400) Mpa. Experimental results showed that fatigue resistance of austenized steel specimens after carburization process has been increased, and the crack length developed on the specimen surfaces was decreased with an increase in austenizing temperature up to 800°C, due to lath martensitic microstructure formation, beyond this temperature fatigue resistance was decreased and crack lengths were increased due to the grain coarsening of the lath martensite. It was also concluded that fatigue resistance of steel specimens that have been tempered after carburization process was increased, while crack lengths due to fatigue have been decreased with an increase in tempering temperatures due to the formation of tempered martensite and troostitic microstructure. The results also revealed that uncarburized steel specimens showed a lower fatigue resistance and a higher crack lengths than those austenized and tempered specimens after carburization.

Detection of Small Cracks for Rotating Bending Fatigue Specimens (The First Report; A Method with Measuring Small Displacement during Fatigue Test)

A replica method is commonly used to detect a crack initiation and propagation for rotating bending fatigue cracks. But in this method, a length of crack on the surface can be detected at only the certain cycles, and can't detect the crack initiation accurately or observe the change of fatigue crack length continuously. In this report, small fatigue cracks are detected by measuring the displacement at a notch root of specimens during fatigue test with a laser displacement meter. As a result, we can observe the propagation of fatigue cracks by measuring the amount of displacement change in real time.

The effect of high temperature gas carburizing on bending fatigue strength of SAE 8620 steel

The bending fatigue performance of high temperature gas carburized SAE 8620 steel was evaluated and compared to conventional temperature gas carburization. To vary the case depth, different carburizing times were applied. The typical times were: 3 and 5 h at a temperature of 940 °C and 1 and 2 h at a temperature of 980 °C. To cause failure in the rotating bending fatigue specimens, the applied load was chosen as the equivalent load at 106 cycles when the material is subjected to bending fatigue. The characterization of residual stress and percent retained austenite in the specimens was performed using X-ray diffraction techniques; optical metallographic examination was also carried out. The fracture surfaces were cleaned in an ultrasonic bath and examined using a scanning electron microscope (SEM) equipped with EDX to evaluate the crack initiation and growth characteristics of the materials in the core and carburized case regions of the specimens. The results indicate that the fatigue performance of the high temperature gas carburized specimens was relatively poor compared to the conventionally gas carburized specimens.

The relationship between case depth and bending fatigue strength of gas carburized SAE 8620 steel

The fatigue performance of gas carburized SAE 8620 steel was evaluated as a function of case depth. To vary the case depths, different carburizing times were applied. The typical times were: 45 min, 3 and 5 h at the temperature of 940 °C. To cause failure in the rotating bending fatigue specimens, the applied load was chosen as the equivalent load at 10 6 cycles for the material when is subjected to bending fatigue. The characterization of the specimens was carried out using X-ray diffraction technique for stress measurement and retained austenite and optical metallographic examination. The fractured surfaces were cleaned in an ultrasonic bath and examined using scanning electron microscope (SEM) equipped with EDX to evaluate the crack initiation and growth characteristics of the materials in the core and carburized case regions of specimens. The results showed that the fatigue limit was associated with the microstructure, the case depth, the distribution of retained austenite, the depth of the internal oxidation and the compressive residual stresses near the surface. The bending fatigue strength of gas carburized specimens was showed to decrease with the increasing case depths caused by the increasing of internal oxidation and nonmartensitic transformation present at the surface.

EFFECT OF AN ELASTIC‐PLASTIC SHOCK WAVE PART A: ON THE FATIGUE STRENGTH OF WELDED JOINTS

AbstractFatigue data for welded joints subjected to an explosion treatment (ET) were obtained using rotary bending fatigue specimens. The fatigue fracture surfaces were observed by SEM and the dislocation morphologies by TEM. Mechanical properties have been quantitively studied by considering elastic and plastic shock wave characteristics. The test results indicate that the fatigue strength of welded joints subjected to ET is apparently improved due to the action of elastic or plastic stress waves while the ductility of the welded joints, i.e., reduction in area, is greatly increased.

Development of miniature bending fatigue specimens

Two new miniaturized bending fatigue specimens have been designed and developed to aid in the scoping of materials for fusion first-wall and blanket structural applications. One of these is rectangular in shape with a gauge section 6.35 mm in length, while the other is cut from a 3 mm transmission electron microscopy (TEM) disk and has a gauge length of 1.5 mm. Test results for unirradiated annealed type 316 stainless steel tested at room temperature, 550°C and 650°C are presented. A good correlation between miniature and standard subsize fatigue specimen results was obtained. The miniature specimen results show the same dependence of strain range on cycles to failure as the standard subsize specimens with the miniature-disk specimen results falling below all the other results. The results indicate that these specimens provide reliable data that can be used to scope fatigue properties for fusion applications.

Mechanism maps for cyclic plasticity and fatigue of single-phase materials : Pedersen, O.B.Acta Metall. Mater. July 1990 38, (7), 1221–1239

The bending fatigue performance of high temperature gas carburized SAE 8620 steel was evaluated and compared to conventional temperature gas carburization. To vary the case depth, different carburizing times were applied. The typical times were: 3 and 5 h at a temperature of 940 °C and 1 and 2 h at a temperature of 980 °C. To cause failure in the rotating bending fatigue specimens, the applied load was chosen as the equivalent load at 106 cycles when the material is subjected to bending fatigue. The characterization of residual stress and percent retained austenite in the specimens was performed using X-ray diffraction techniques; optical metallographic examination was also carried out. The fracture surfaces were cleaned in an ultrasonic bath and examined using a scanning electron microscope (SEM) equipped with EDX to evaluate the crack initiation and growth characteristics of the materials in the core and carburized case regions of the specimens. The results indicate that the fatigue performance of the high temperature gas carburized specimens was relatively poor compared to the conventionally gas carburized specimens.

回転曲げ疲労を受けたＶＣ被覆鋼の破壊点近傍の表面き裂について

Cracks often appear in the surface of the VC coated steel specimens subjected to rotating bending fatigue tests. In order to investigate the cause of formation of these cracks, detailed observation was made on the surfaces of the rotating bending specimens after the fatigue test and during the static test, and of the two dimensional model specimens during the plane bending fatigue test.The results obtained are as follows:(1) In the rotating bending fatigue test, wide aperture cracks were observed on the fracture origin side, while narrow cracks and sometimes netty cracks were observed on the opposite side. These cracks became more significant with increasing ductility of the substrate steels.(2) Even in the last stage of the fatigue process, no such surface crack appeared in the unbroken specimens. Therefore, it is considered that the surface cracks are generated by the deformation around the fracture point in the separation process of fracture.(3) The static bending test results suggest that the wide cracks on the fracture origin side are formed by compressive stress, while the narrow and netty cracks on the opposite side are formed by tensile stress. Consequently, it is considered that the cracks mentioned at (1) are formed through the following process: During the final fracture of the rotating bending fatigue specimens, separation occurs by tensile stress, which creates narrow and netty cracks. Then, the cracked parts of the origin side strike each other, producing compressive stress, which creates wide cracks.

ADINA analysis of large deflections and stresses in bending fatigue specimens

Laboratory tests of flapper valve steel bending fatigue specimens are compared to large deflection ADINA analyses based on the nine-node shell element. Three standard thicknesses, 1 100 in. (0.254 mm), 3 200 in. (0.381 mm) and 1 50 in. (0.508 mm), are investigated. Both the longitudinal and the anticlastic curvatures could be brought into good agreement with experiment through proper modelling of the grip section by bakelite brick elements in sandwich form. The calculated stress maxima are observed in the specimen edge region. This result is consistent with observations that fracture initiation usually occurs in that region.

Difference in Low Cycle Fatigue Strength by Strain Controlled Axial Load and Deflection Controlled Bending Load

The ratio of fatigue strength of deflection controlled bending load cycling to that of diametral strain controlled axial load cycling was investigated in the low cycle fatigue life range by testing four kinds of steel. Cyclic characteristics of surface strain at the test section of a bending fatigue specimen were measured by strain gauges, which provided good results about strain amplitude. Fatigue test results were plotted in the relation between total strain amplitude and visible crack initiation life, and a general tendency was observed that the bending fatigue strength was the same order of or higher than the axial fatigue strength. The fatigue strength ratio was increased with the increasing crack initiation life, showing that the maximum value of the ratio was 1.45 for a mild steel, 1.17 for a HW 50 steel, 1.26 for a HW 70 steel and 1.56 for a 9% Ni steel. By applying a concept of cyclic hysteresis energy to an effective volume from a surface of a bending specimen, an analysis of the fatigue strength ratio was made with an assumption that bending and axial load fatigue specimens should be cracked at an equal number of cycles when equal hysteresis energy is accumulated in both specimens.

The role of inclusions in fatigue crack initiation in an aluminum alloy

A literature survey gave preliminary evidence that large second phase particles were cracked under static loading and that these might subsequently be sources of fatigue crack initiation. In order to determine more fully the effect of this type of inclusions on crack initiation bulk samples of 2024-T3 material were strained to 6, 8, 10 and 12% and subsequently reheat-treated. Reverse bending fatigue specimens were then prepared for crack initiation studies. Observations by optical microscopy showed that the prestrain had caused several voids at the inclusions partly by cracking of the inclusions while the more probably event was failure of the matrix-inclusion interface. It was shown that the voids were sources of fatigue crack initiation. Tests on as received material indicated initiation to occur also at inclusion clusters. Repeated polishing revealed the complex three-dimensional nature of the inclusions and its significance for the local direction of crack growth.