Rotating Bending Fatigue Testing Machine Research Articles

OS0714 アルミニウム合金の長寿命疲労挙動に対するアルマイト処理および微細組織の影響(構造用材料の疲労挙動と寿命評価,オーガナイズドセッション)

In order to investigate the effects of niicrostructure and alumite surface treatments on the fatigue behavior, fatigue tests were caffied out on aluminum alloy specimens with two different alumite treatments by means of a dual-spindle rotating bending fatigue testing machine. Tests were also conducted on untreated specimens for the sake of comparison. Fracture surfaces of all the failed specimens were examined by a scanning electron microscope (SEM). Fatigue fracture mechanisms are discussed in the light of fractography.

Influence of machining parameters on fatigue endurance limit of AISI 4140 steel

The general purpose of this research is to study the influence of commercial machining parameters on fatigue limits of steels. Specifically in this work, the influence of cutting speeds, depth of cut, feed rate and residual stresses of turned surfaces of AISI 4140 steel specimens on fatigue strength were analyzed. In some specimens, the residual stress was eliminated by heat treatment. The fatigue experiments were carried out at room temperature, applying a cyclical frequency of 58Hz, with mean stress equal to zero (R=-1), on a rotating-bending fatigue testing machine of the constant bending moment type. It was used the staircase (or up-and-down) method to determine the fatigue limit of the specimens.

Influence of Plasma Radical Nitriding on Fatigue Properties of SCM435 Steel

The effect of plasma radical nitriding treatment on fatigue properties of SCM435 steel in super long life region was investigated. Fatigue tests were carried out using a dual-spindle rotating bending fatigue-testing machine at room temperature in air for the specimens nitrided at 773 K and 823 K for 3 hrs. The fatigue strength of nitrided specimen was greater than that of un-nitrided specimen and the crack initiation mode changed from the surface cracking of un-nitrided specimen to the subsurface cracking of nitrided specimen. Hardening layer and compressive residual stress were formed by nitriding, which resulted in the improvement of the fatigue strength. The stress intensity factor was calculated using facet area in Fish-eye fracture mode. As a result, the stress intensity factor indicated almost constant value, ~ 3-4 MPa·m1/2, regardless of the number of cycles to failure.

Coaxing Effect in Stainless Steels and High-Strength Steels

The effects of prestrain and strength level on the coaxing behavior were studied in austenitic stainless steels and high strength steels, respectively. The materials used were austenitic stainless steels, SUS304 and SUS316, and high strength steels, SCM435, SNCM439 and SUJ2. Stress incremental fatigue tests were performed using cantilever-type rotating bending fatigue testing machines. It was found that the steels except for SUJ2 showed a marked coaxing effect. Non-propagating cracks were not detected in all the steels examined. Based on hardness test, X-ray diffraction measurement and EBSD analysis, it was indicated that the coaxing effect occurred due to work hardening and strain-induced martensite transformation in austenitic stainless steels and to strain-aging in high strength steels.

Effect of Postweld Aging Treatment on Fatigue Behavior of Pulsed Current Welded AA7075 Aluminum Alloy Joints

This article reports the effect of postweld aging treatment on fatigue behavior of pulsed current welded AA 7075 aluminum alloy joints. AA7075 aluminum alloy (Al-Zn-Mg-Cu alloy) has gathered wide acceptance in the fabrication of light weight structures requiring high strength-to weight ratio, such as transportable bridge girders, military vehicles, road tankers, and railway transport systems. The preferred welding processes of AA7075 aluminum alloy are frequently gas tungsten arc welding (GTAW) process and gas metal arc welding (GMAW) process due to their comparatively easier applicability and better economy. Weld fusion zones typically exhibit coarse columnar grains because of the prevailing thermal conditions during weld metal solidification. This often results inferior weld mechanical properties and poor resistance to hot cracking. In this investigation, an attempt has been made to refine the fusion zone grains by applying pulsed current welding technique. Rolled plates of 10 mm thickness have been used as the base material for preparing multipass welded joints. Single V butt joint configuration has been prepared for joining the plates. The filler metal used for joining the plates is AA 5356 (Al-5Mg (wt.%)) grade aluminum alloy. Four different welding techniques have been used to fabricate the joints and they are: (i) continuous current GTAW (CCGTAW), (ii) pulsed current GTAW (PCGTAW), (iii) continuous current GMAW (CCGMAW), and (iv) pulsed current GMAW (PCGMAW) processes. Argon (99.99% pure) has been used as the shielding gas. Rotary bending fatigue testing machine has been used to evaluate fatigue behavior of the welded joints. Current pulsing leads to relatively finer and more equi-axed grain structure in GTA and GMA welds. Grain refinement is accompanied by an increase in fatigue life and endurance limit. Simple postweld aging treatment applied to the joints is found to be beneficial to enhance the fatigue performance of the welded joints.

Effects of Roller Working and Ion Nitriding on Fatigue Properties of Eutectoid Steel

In this study, the authors have investigated and compared the effects of roller working and ion nitriding on fatigue properties of eutectoid steel. Five kinds of roller worked and two kinds of ion nitrided specimens were used in this test. The fatigue test was performed using a rotating bending fatigue testing machine to evaluate the fatigue strength of roller worked and ion nitrided eutectoid steel. The fatigue test result shows that roller working is more effective on improving the fatigue strength of the material than ion nitriding. On the other hand, the ion nitriding can much greatly increase the surface hardness than the roller working. In the case of roller working, the fatigue properties are improved by the three main factors which are compressive residual stress, work hardening and fiberized micro-structure.

Effect of pulsed current welding on fatigue behaviour of high strength aluminium alloy joints

High strength aluminium alloys (Al–Zn–Mg–Cu alloys) have gathered wide acceptance in the fabrication of light weight structures requiring high strength-to weight ratio, such as transportable bridge girders, military vehicles, road tankers and railway transport systems. The preferred welding processes of high strength aluminium alloy are frequently gas tungsten arc welding (GTAW) process and gas metal arc welding (GMAW) process due to their comparatively easier applicability and better economy. Weld fusion zones typically exhibit coarse columnar grains because of the prevailing thermal conditions during weld metal solidification. This often results inferior weld mechanical properties and poor resistance to hot cracking. In this investigation, an attempt has been made to refine the fusion zone grains by applying pulsed current welding technique. Rolled plates of 6 mm thickness have been used as the base material for preparing single pass welded joints. Single V butt joint configuration has been prepared for joining the plates. The filler metal used for joining the plates is AA 5356 (Al–5Mg (wt%)) grade aluminium alloy. Four different welding techniques have been used to fabricate the joints and they are: (i) continuous current GTAW (CCGTAW), (ii) pulsed current GTAW (PCGTAW), (iii) continuous current GMAW (CCGMAW) and (iv) pulsed current GMAW (PCGMAW) processes. Argon (99.99% pure) has been used as the shielding gas. Fatigue properties of the welded joints have been evaluated by conducting fatigue test using rotary bending fatigue testing machine. Current pulsing leads to relatively finer and more equi-axed grain structure in gas tungsten arc (GTA) and gas metal arc (GMA) welds. In contrast, conventional continuous current welding resulted in predominantly columnar grain structures. Grain refinement is accompanied by an increase in fatigue life and endurance limit.

P-14 Fatigue Properties of Ion Nitrided Tool Steel

In this study, the authors have investigated and clarified that ion nitriding was able to improve the fatigue properties of tool steel. Three kinds of specimens under the different heat treatment conditions were used in this study. The fatigue test had been performed using a rotating bending fatigue testing machine to investigate the effects of ion nitriding on fatigue properties of tool steel. The fatigue fracture surface was analyzed using a scanning electron microscope (SEM), and hardness distribution was also investigated using a microhardness tester. The fatigue strength and hardness of the heat treated specimen were much greater than those of the non-heat treated specimen. It is considered that the micro structure of the heat treated specimens was refined; and the refined structure reduced fatigue failure and increased hardness of the specimens. According to the results of the fatigue test, the most effective treatment method on improving the fatigue limit was determined in this study. The hardness of the specimens remarkably increased after ion nitriding processing.

Fatigue Strength of Ion Nitrided Tool Steel

The authors have studied and clarified that ion nitriding was able to improve the fatigue properties of tool steel. Five kinds of ion nitriding methods (ion nitriding condition is different) were used in this study. The fatigue test had been performed using a rotating bending fatigue testing machine to investigate the effects of ion nitriding on fatigue properties of tool steel. The fractography was analyzed using a scanning electron microscope (SEM), and hardness distribution was also investigated using a microhardness tester. As a result, the fatigue strength and hardness of the ion nitrided specimen increased after ion nitriding processing. It is considered that the compressive residual stress which produced by ion nitriding processing in the layer reduced fatigue fracture, and the altered surface composition improved surface hardness. According to the results of the fatigue test, the optimal ion nitriding method on improving the fatigue limit of tool steel was determined. The hardness of the specimens remarkably increased after ion nitriding processing.

Fatigue properties of Cu–Cr in situ composite

The new electrical conductive material, Cu–15 mass%Cr in situ composite, has been developed for the application to lead frame, high field magnet and trolley wire. The Cu–Cr composite was fabricated by casting method in vacuum and forged, solution-treated and cold drawn. The value of the cold drawing strains, η, for the composite were 4.66 and 6.94. η is defined by the value of ln( A 0/ A), where A 0 and A are virgin area and final area of material, respectively. The cold drawn Cu–Cr composite was tested using a rotating bending fatigue testing machine and axial loading fatigue testing machine ( R = 0.1, where R is stress ratio of minimum stress to maximum stress) at room temperature in comparison to the data for pure copper. The fatigue stresses decreased with increasing number of cycles to failure. The double-knees, which have been found in other face centered cubic materials, such as aluminum alloys, were not seen on the S– N curves for both alloys. The fatigue strength of Cu–Cr composite was twice higher than that for pure copper. The fatigue strength of Cu–Cr composite cold drawn up to η = 6.94 was higher than that drawn up to η = 4.66 when tested under an axial loading. In the fatigue fracture surface, the fatigue crack initiated from the un-dissolved chromium particle situated beneath the specimen surface and the crack direction was changed along the chromium fiber. It was suggested that, in order to improve the fatigue strength of Cu–Cr composite, it is very important to reduce the size and amount of un-dissolved chromium particles.

Two-Step Multiple Block Loading Tests for a High Strength Steel Considering the Transition of Failure Mode in High Cycle Regime

Fatigue behavior under variable amplitude loading was studied on a high carbon chromium steel, JIS SUJ2. Special attentions were paid on the evaluation of fatigue damage by the Palmgren-Miner rule and the damage mechanisms under variable amplitude loading accompanied by the failure mode transition in high cycle regime. Tests were performed using cantilever-type rotating bending fatigue testing machines in laboratory air. Load sequences employed were two-step multiple block loadings (low ↔ high). When two stress levels, under each of which the failure mode was the same, were applied, the failure mode was subsurface-type. Fatigue damages were evaluated successfully by the Palmgren-Miner rule. On the contrary, when the failure modes under both stress levels were different, the failure mode was surface-type. In this case, the cumulative damage at each stress level was independent of each other. Based on SEM observation, the morphologies of fish-eye formed under two-step multiple block loadings were the same as those under constant amplitude loadings.

Evaluation of Fatigue Strength in Very-long Life Regime of SNCM439 Steels

Nickel chromium molybdenum steels (SNCM439) were fatigued with the rotating bending fatigue testing machine and the ultrasonic fatigue-testing machine (20 kHz) in laboratory air. The duplex S-N curve could not be observed for the ultrasonic fatigue tests. The origins of crack initiation in the long life regime were internal inclusions. The fish-eyes were observed in the specimens broken from internal inclusions. When the number of stress cycles to failure was longer than 8.1×106, the facet region was observed around inclusions. The fracture surface could be divided into three regions. The first region surrounding inclusion was a facet area. The crack propagation in the facet region was Stage A and the crack propagation outside the facet region was Stage B. When the fatigue cracks reached the specimen surface, the fatigue cracks propagated as surface cracks. The crack propagation rate in each region was estimated to be a power function of the stress intensity range. Fatigue life was calculated by integrating with the crack propagation laws determined from the experimental data. The predicted fatigue life agrees with the experimental result.

Morphology of step‐wise S–N curves depending on work‐hardened layer and humidity in a high‐strength steel

ABSTRACTThe present paper describes the effects of work‐hardened layer and humidity on the fatigue response of high strength steel, JIS SNCM439 (AISI4340). Tests were conducted in laboratory air and in dry air using cantilever‐type rotating bending fatigue testing machines. The dew point of dry air was −60 °C. Specimens with two different surface conditions were prepared, i.e., buff‐finished and electro‐polished specimens. In the buff‐finished specimens, a stepwise S–N curve was seen in both environments, but the transition stress below which subsurface crack initiation occurred shifted to a higher stress level in dry air than in laboratory air. In the electro‐polished specimens, stepwise S–N curve was not observed in laboratory air, but seen in dry air. At stress levels where cracks initiated at the surface, the fatigue lives of the electro‐polished specimens were shorter than those of the buff‐finished specimens. At stress levels where subsurface cracks initiated, the fatigue lives of both specimens were the same, irrespective of surface finishing condition and humidity.

高強度鋼の長寿命域における破壊機構の遷移を考慮した変動応力試験

Fatigue tests under variable amplitude loading were conducted on a high carbon chromium steel, JIS SUJ2. The aim in the present study is to clarify the damage mechanisms under variable amplitude loading for a high strength steel showing the transition of failure mode in long life regime. Tests were performed using cantilever-type rotating bending fatigue testing machines in laboratory air. Load sequences employed were two-step block loading (high→low and low→high) and two-step multiple block loading (high↔low). Fatigue damages were estimated by the Palmgren-Miner rule. When failure mode was the same at two stress levels employed, i. e. subsurface crack initiation, fatigue damages were evaluated properly by the Palmgren-Miner rule. Also when the cycle ratio at low stress level became large and multiple block loading was applied, the estimation of fatigue damages was made successfully. According to SEM analysis, it was found that the morphology of fish-eyes formed under variable amplitude loading was the same as that obtained under constant amplitude loading.

Effect of Strain Aging on Fatigue Properties of Pre-Strained Carbon Steel.

Fatigue test has been performed to investigate the effect of strain aging on fatigue properties of pre-strained carbon steel (S45C) using Ono-type rotary bending fatigue testing machine. Main results summarized in this study are as follows : (1) It is considered that both of the pre-strain and annealing condition influence the fatigue limit of pre-strained specimen. (2) Both the fatigue limit and hardness are increased by annealing. (3) Under the same annealing condition, the fatigue limit of pre-strained specimen increases with increase of pre-strain. (4) The optimal heat treatment temperature for obtaining the maximum value of fatigue limit decreases with increase of pre-strain.

湿度によって助長された高強度鋼の二段折れ曲がりＳ‐Ｎ曲線

The present paper describes the effect of humidity on the fracture mechanisms and associated morphology of the S-N curve in a high strength steel, JIS SNCM439. Fatigue tests were conducted on cantilever-type rotating bending fatigue testing machines operating at a frequency of 3150rpm in laboratory air and in dry air up to long life region around 109 cycles. The dew point of dry air was -60°C. In laboratory air, fatigue failure occurred at stress levels below the conventional fatigue limit, leading to a step-wise S-N curve. Also in dry air, similar step-wise S-N curve was obtained, but the horizontal part of the S-N curve shifted to a higher stress level than in laboratory air. Fatigue lives were the same in both environments at stress levels above the horizontal part in dry air and below the horizontal part in laboratory air, because the same fracture mode operated in those regions, surface-related initiation in the former and subsurface crack initiation with a fish-eye in the latter. At stress levels between the horizontal parts in both environments, surface-related initiation and subsurface crack initiation were seen in laboratory air and in dry air, respectively. Fatigue lives were significantly shorter in laboratory air due to humidity-assisted crack initiation and small crack growth. Based on the experimental results, the environment dependence of fracture mode and associated morphology of S-N curve is discussed.

237 環状切欠き材の疲労強度向上(OS 軽量化・高強度化と材料代替・材料進化)

More than 90% of failures of machines and mechanical parts are caused by fatigue. In addition, approximately 90% of the fatigue cracks initiate at a stress concentration part of structural component. Therefore, it is important to improve the fatigue limit of a notched specimen. In this study, the fatigue properties of specimens with circumferential notch have been investigated using Ono-type rotating bending fatigue test machine. The main results obtained in this study are as follows: (1) The fatigue strength of a notched-specimen considerably increases by roller-working. (2) The improvement of fatigue limit by roller-working would be attributed to work-hardening and compressive residual stress. (3) Furthermore, the non-propagating cracks are observed in the specimen subjected to the stress amplitude of fatigue limit by 1 × 10^7 cycles.

Effect of Shot-Peening Process on Fatigue Strength of Austenite Stainless Steel.

It is well known that the shot peening process is often used industrially for the purpose of improvement of the fatigue strength of many kinds of steels. Generally the surface of these steels subjected to this process is hardened and addition to it, the compressive residual stress is also induced on the surface layer. In this study, the fatigue tests of shot-peened steel SUS 304 were conducted using the rotating-bending fatigue test machine, and residual stresses were detected by X-ray stress measurement. And surface characteristics related to residual stress σR, hardness and microstructure are also investigated to clarify the influential factors on the bending fatigue strength of stainless steel SUS 304 by shot peening. As a result, the surface layer was hardened by the shot peening process and the strain-induced transformation was ascertained by X-ray analysis. And residual stresses were almost released at the initial time during fatigue cycles. Concerned about work hardening, it is also studied precisely. We found that the increments of hardness was attributed to both martensitic transformation and hardness of austenite itself, and then the bending fatigue strength of austenitic stainless steel with shot peening process was improved by twenty percents. Consequently, it is found that the increments of hardness by the shot peening are mainly due to hardness of austenite structure itself, and it affects the improvement of the fatigue strength of the shot peened austenite stainless steel.

536 高強度ばね鋼の高サイクル疲労挙動に及ぼす非金属介在物の影響(OS 加工による材料の機能創製)

The primary aim of present study is to clarify the fatigue fracture mechanism of high strength steel at the high cycle region. Oil-tempered high strength steel wire (SWOSC-V) is prepared for fatigue test by using Nakamura rotating-bending fatigue testing machine. The S-N diagram had a knee point at about 10^5cycle. From scanning electron microscopy (SEM) observation of the fracture surface at the long cycle fatigue range, nonmetallic inclusion was identified at the origin of fracture and it formed "fish-eye" failure. On the other hand, a constitution of the nonmetallic inclusion is measured by using electron probe micro analyzer (EPMA). As a result, Aluminum, calcium and oxygen were detected mainly at the inclusion. Based on these results, finite element analysis (FEA) was conducted for analyzing the stress field around the neighborhood of the nonmetallic inclusion. Then, the effect of mechanical property of inclusion on fatigue failure behavior was discussed.

Effect of Structure Stability on Fatigue Properties of Austenitic Stainless Steels.

The fatigue properties of austenitic stainless steels with different Cr/Ni equivalent values have been investigated in the high-cycles region using Ono-type rotating bending fatigue testing machine. The materials used in this test were three kinds of representative austenitic stainless steels, i. e. typical austenitic stainless steel, SUS304 and nitrogen-contained SUS304 (SUS304 N and YUS170). The main results obtained in this test are as follows ; (1) Though each material has the different δ-ferrite value according to Ni/Cr equivalent value, there are no relation between δ-ferrite value and fatigue limit. (2) The fatigue cracks do not necessarily initiate at the segregated portion of chemical content. (3) Considering Ni/Cr equivalent, the austenitic structure in YUS 170 is more stable than those in SUS 304 and SUS 304 N. Therefore, the increase of Micro-Vickers hardness due to deformation induced martensite transformation in YUS170 is lower than those in SUS 304 and SUS 304 N. (4) The lower fatigue strength, the larger the crack tip opening displacement (CTOD) of each material which has same fatigue life.