Decrease In Fatigue Strength Research Articles

The effect of plasma electrolytic oxidation on the mean stress sensitivity of the fatigue life of the 6082 aluminum alloy

In this work the mean stress influence on the high cycle fatigue behavior of the plasma electrolytic oxidized (PEO) 6082 aluminum alloy (AlSi1MgMn) is investigated. The present study is focused on the fatigue life time and the susceptibility of fatigue-induced cracking of the oxide coating and their dependence on the applied mean stress. Systematic work is done comparing conditions with and without PEO treatment, which have been tested using three different load ratios. For the uncoated substrate the cycles to failure show a significant dependence on the mean stress, which is typical for aluminum alloys. With increased load ratio and therefore increased mean stress, the fatigue strength decreases. The investigation confirms the well-known effect of PEO treatment on the fatigue life: The fatigue strength is significantly reduced by the PEO process, compared to the uncoated substrate. However, also the mean stress sensitivity of the fatigue performance is reduced. The fatigue limit is not influenced by an increasing mean stress for the PEO treated conditions. This effect is firstly shown in these findings and no explanation for this effect can be found in literature. Supposedly the internal compressive stresses and the micro-cracks in the oxide film have a direct influence on the crack initiation and growth from the oxide film through the interface and in the substrate. Contrary to these findings, the susceptibility of fatigue-induced cracking of the oxide coating is influenced by the load ratio. At tension-tension loading a large number of cracks, which grow partially just in the aluminum substrate, are present. With decreasing load ratio to alternating tension-compression stresses, the crack number and length increases and shattering of the oxide film is more pronounced due to the additional effective compressive part of the load cycle.

Thickness Effect on Fatigue Strength of Welded Joint Improved by HFMI

Fatigue strength decreases by thickness effect, therefore the improvement treatments of the fatigue strength are important as the structure consisting of the thick material. Post weld treatments are one of useful method to improve fatigue strength. However, the thickness effect on welded joint improved by post weld treatments is not clear because the experimental data is insufficient for quantitative evaluation.The object of this study is to evaluate thickness effect on the fatigue strength which is improved by post weld treatments to reflect it in fatigue design guideline; therefore in transverse no-load carrying cruciform joint which is one of the main welded joints of the ship structure, the improvement effect of the fatigue strength by post weld treatments is evaluated. In this report, the result of HFMI (High Frequency Mechanical Impact treatment) and Shot peening are evaluated with the result of the as-welded condition and conventional Burr grinding by nominal stress approach. In addition, the fatigue strength is evaluated using the equivalent stress approach by the modified MIL-HDBK-5 method that both the stress concentration of weld toe and the weld residual stress are separated from nominal stress.Good correlation between the fatigue test results was achieved using modified version of the equivalent stress approach defined in MIL-HDBK-5. With the equivalent stress that was adopted in this study, both the stress range and the effect of maximum stress including the residual stress are considered. It is considered that both number of cycles to crack initiation and that to failure are evaluated by equivalent stress approach integrally.

Fatigue properties of high-strength steel in diesel oil

This paper describes the fatigue properties of chromium-molybdenum steel in air and diesel oil. 4-point bending fatigue tests were conducted at a stress ratio of R=0.05. The corrosion mark was not confirmed on the specimens tested in diesel oil, and the fatigue strength at Nf < 3×104 improved and that at Nf > 3×104 declined compared to in air. The fatigue crack initiation life accounted for about 90 % of the fatigue life Nf. The results of the elemental analysis of the specimen surface tested in diesel oil which were broken at Nf > 3×104, O, Si, and C had been concentrated. The reduction of fatigue strength at Nf > 3×104 is considered to be caused by some influence of deterioration of the diesel oil winhich progress with the fatigue test. On the other hand, the fatigue test at Nf < 3×104 is usually finished within one day, so diesel oil does not have the time to deteriorate. Then, a few fatigue tests were conducted in the deteriorated diesel oil at a relatively high stress range, in order to break the specimen at Nf < 3×104. As a result, fatigue life tends to shorten. Therefore, it was found that the diesel oil deterioration should be considered as one of the important factors to explain the decrease of fatigue strength.

キンク損傷を有する高弾性率タイプPBO繊維の疲労強度

In this study, fatigue strength of high-modulus type PBO (Poly-p-Phenylene Benzobisoxazole) fiber with kinking damage was investigated in monofilament fatigue tests. PBO fiber is high polymer material and has the highest tensile strength and tensile modulus of elasticity in any polymeric fiber. Kinking damages was created on surface of virgin fiber by winding bundle fibers to steel rod. Diameter of steel rod was set to 0.625 mm. Average value of compressive strain on surface of fiber determined based on the diameter of steel rod was -1.71 %. Load-controlled fatigue tests were carried out using a sinusoidal load with frequency 10 Hz and stress ratio of R = 0.5, and the load cell capacity was 10 N. It was found that fatigue strength of PBO fiber with kinking damage was lower than that of PBO fiber without kinking damage. Fatigue strength decrease of PBO fiber with kinking damage in a low cycle area was larger than that in a high cycle area.

Artificial Aging Time Effect on Fatigue stress for Friction Stir Welded AA6061T6

This work involves studying the effect of heat treatment on the fatigue stress of friction stir welded joint of 6061T6 aluminum alloy. The microstructure,micro hardness , tensile and fatigue test of the welds and base alloy were examined before and after heat treatment which includes solution heat treatment at 500ºC and artificial aging at 190ºC for 2, 4, and 8 hrs.The friction stir welding was carried out by using CNC milling machine at constant tool rotational speed of (1000 rpm) and welding speed of (20mm/min),the fatigue test is investigated for all specimens to obtain the S-N curve. The results show that the heat treatment decreased fatigue strength for all elements comparing with base metal, while ageing at 2 hrs gives the best fatigue strength due to participated phases.

Effect of Thermal Exposure on Tensile and Fatigue Properties of Clay Reinforced Nylon Nanocomposites

In this paper, both nylon 6 and 2 wt% clay reinforced nylon 6 matrix nanocomposite were used for thermal exposure tests at temperatures of 80 oC and 120 oC and 150 oC, respectively. Then, the tensile tests and fatigue tests of the exposed specimens were conducted at room temperature. It was shown that the tensile strength in both nylon 6 and NCH-2 decreased with an increase in thermal exposure temperature. The brittle fracture occurred in the specimens exposed at 120 oC and 150 oC. After pre-oxidation treatment at 80 °C for 100 hours, the fatigue strength decreased 14% in nylon 6, and 8% in NCH-2. From this result, it was understood that the addition of clay in nylon 6 could suppress the decrease of fatigue strengths.

Material characterization of high-frequency mechanical impact (HFMI)-treated high-strength steel

Fatigue strength improvement due to high-frequency mechanical impact (HFMI) is based on compressive residual stresses, improved weld toe geometry and strain hardening of the treated surface region. In order to understand the combined effects of these factors on the weld toe elastic–plastic response and residual stress stability, the material properties in the HFMI-treated weld toe region need to be determined. In this study, five different material conditions were studied: high-strength steel base material, Gleeble simulated heat-affected zone, and three HFMI-treated material conditions. The local material characteristics were determined by microhardness measurements, electron backscatter diffraction and mechanical testing. The specimens were fatigue tested under fully reversed strain-controlled loading. Surface residual stresses in the fatigue test specimens were measured with X-ray diffraction. The results showed that the heat-affected zone had lower fatigue and yield strength than the base material due to larger grain size. HFMI-treatment increased the steel yield strength up to 1–1.5mm. This observation was supported by change in grain shape in the treated surface region. This lead to an increase in fatigue strength at low strain amplitudes and a decrease in fatigue strength at high strain amplitudes. Continuous cyclic softening was observed for all material conditions.

Effect of welding parameters on the fatigue properties of dissimilar AISI 2205–AISI 1020 joined by friction welding

In this study, AISI 2205 duplex stainless steel, most commonly used in its class and economical AISI 1020 steel couple with low carbon content, were connected using different operation parameters through friction welding. Tension test and rotary bending fatigue test were applied to the welded connections, and the impact of the welding parameters on fatigue strength was examined. It was discovered that when the welding parameters used in connecting AISI 2205 and AISI 1020 steel couple through friction welding were selected correctly, fatigue strength of the connection would increase compared to the main material, and incompliant parameters decreased fatigue strength.

Fatigue behavior of bulk β-type titanium alloy Ti–15Mo–5Zr–3Al annealed in high temperature nitrogen gas

β-type titanium alloy, Ti–15Mo–5Zr–3Al, was annealed in nitrogen gas at the temperature of 1200°C for 0.5 or 1h. Subsequently, four-point bending fatigue tests were carried out using the annealed specimens. Based on the results obtained, the effect of the solid solution of nitrogen on fatigue properties was discussed. The hardness of specimen surface was increased markedly from 311HV to about 480HV by the annealing in nitrogen gas. The increase of hardness was due to the solid solution of nitrogen, formation of titanium nitride (TiN) and precipitation of α-phase resulting in Widmannstaetten (needle-like α- and β-phases) layer near the specimen surface. In spite of the improvement of hardness at the specimen surface, the fatigue strengths of the annealed specimens were lower than that of the untreated specimen. EDX analyses revealed that the brittle TiN was formed along grain boundaries near specimen surface. Though the re-solution treatment at 850°C for 1h dissolved TiN, the fatigue strength of re-solution treated specimen was not recovered compared to the untreated specimen. It was considered that the decrease in fatigue strength was mainly attributed to the grain coarsening during the annealing.

高湿度中における高強度Al合金の疲労特性に及ぼすショットピーニングの影響

Fatigue properties of shot-peened high strength Al-Cu-Mg alloys, extruded and drawn 2017-T4 alloys, and effect of humidity on the properties were investigated in relative humidity environments of 25% and 85% under rotating bending. Fatigue strength was increased by shot-peening in each humidity. Although fatigue strength was largely decreased by high humidity in the electro-polished specimen, the decrease in fatigue strength was very small in the shot-peened one. In the electro-polished extruded alloy, the crack propagation changed from a shear mode to a tensile one with decreasing in stress level in high humidity, while the propagation in the shot-peened one was in a tensile mode only in both humidity environments similar to the ones in both of the electro-polished and the shot-peened specimens in low humidity. The propagation mode in the drawn alloy was a tensile one irrespective of stress level and humidity. Most of fracture surface in low humidity was occupied by striations in both of the electro-polished and the shot-peened specimens in both alloys. On the other hand, those in high humidity were covered with slip planes in the shear mode crack in the electro-polished extruded alloy, while a few granular facets in the extruded alloy or brittle striations in the drawn one were observed at the fracture surface in high humidity in addition to striations in the tensile one irrespective of stress level and shot peening.

Influence of Stress Ratios and Residual Stress on the Fracture Pattern of Ti-6Al-4V Alloy in High Cycle Regime

Experiments are conducted on the high cycle fatigue behavior of Ti-6Al-4V in air at room temperature. Fatigue strengths are determined at various stress ratios for specimens prepared by hard turning plus polishing and residual stress relieved in vacuum after polishing. For various stress ratios and stress amplitudes investigated, the relaxation of residual stresses is measured during the interrupted fatigue test. The relationship of residual stresses relaxation with the cycles and the effects of residual stress as long as stress ratios on fatigue strength are analyzed. Results indicate that the improvement of residual compressive stress on fatigue strength decreases and vanishes finally with the increase of stress ratio. Compared to fatigue crack originating from surface for annealed specimens, the fatigue crack initiation sites are located in the interior of the specimen due to the effect of residual stress at lower stress ratios. At R=-1.0, the residual stresses are relaxed after several cycles and then maintained stable. The improvement of fatigue strength is attributed to surface crack closure induced by compressive residual stress. For R=-0.6-0.1, in contrast, the relaxation of residual stress is insignificant and fatigue failure is dominated mainly by the internal defect and local stress concentration.

G0301403 球状化剤中のレアアース含有量における疲労強度への影響

Mechanical properties of the spheroidal graphite cast iron are dependent on the spheroidized graphite. In the field of casting, rare earth (RE) is used in a spherical shape of the graphite. However, domestic production of RE is small, I rely on imports. Therefore, reduction of the RE elements is an issue. In this experiment, as compared to the previous studies were produced spheroidal graphite cast iron by using a sphering agent unified element other than RE, we were subjected to fatigue tests. From the results, RE in spherical agents is I have investigated the effect on the test specimen. Test specimen the RE content of 0%, 0.3%, 0.5%, using a 1.0% four types of smooth test specimen were subjected to fatigue tests by a plane bending fatigue tester. Decreased fatigue strength is about 20-33% in the RE0% test specimens were compared with RE-containing test specimens. Result of cross-sectional observation of the fractured test specimen by the test, a large casting defects were observed inside the RE0% specimens. Thus the fatigue strength it is believed to have significantly reduced.

OS2140 Ti-6Al-4V合金の環状切欠き丸棒のねじり疲労特性評価

Torsion fatigue tests were conducted for smooth bars and circumferentially notched bars of Ti-6Al-4V alloy. The purpose of this work is to evaluate fatigue strength and notch sensitivity under stress concentration factors of Kt=2.0 and 4.0. Fatigue life and fatigue strength decrease with increasing stress concentration. To investigate the crack initiation and propagation, the crack length were calculated with Hooke's law. When compared at the same fatigue life, the crack initiation life for Kt=4.0 was much shorter than that for Kt=2.0. Difference of the fatigue fracture mechanisms between blunt and sharp notch is considered to cause this phenomenon. Under low stress amplitude, the crack propagates under tensile mode toward a 45° to the axial direction. The crack propagated all over the blunt notch, and the crack propagated in the cross section of the sharp notch root under low stress amplitude. As a result, the fracture surface of blunt notch specimen under low stress amplitude was flat, and that of sharp notch specimen showed factory-roof pattern. The formation of factory-roof increases contact areas in the fractured surface and lengthens crack propagation time.

Effect of Laves Phase Precipitation on Fatigue Properties of Niobium-containing Austenitic Stainless Steel Type 347 in Laboratory Air and in 3%NaCl Solution

The effect of Laves phase precipitation on the fatigue properties of niobium-containing austenitic stainless steel, type 347, was investigated. When this material is exposed to the temperature range of 700 ∼ 900°C, the precipitation of Laves phase (Fe2Nb) occurs, resulting in the embrittlement of the material which is known as the Laves phase embrittlement.In the present study, rotary bending fatigue tests have been conducted in laboratory air and in 3%NaCl solution using type 347 aged at 700, 800 or 900°C. In laboratory air, the fatigue strength was improved by the aging at 700°C due to the fine Laves phase precipitation in γ-phase. Whereas the aging at 900°C decreased fatigue strength, which could be attributed to the coarsening of Laves phase. In 3%NaCl solution, the fatigue strength of 700°C-aged specimen became lower than that in laboratory air. It is considered that the sensitization occurred due to the formation of chromium-depleted zone along grain boundaries during the heat treatment.

Fatigue Strength of Aluminum Cast Alloy in Plant Oil

Effect of plant oil on the fatigue strength of a squeeze cast Al-Si-Mg alloy was investigated. Crack growth rate was higher in plant oil than in air, inducing a decrease in fatigue strength. The reason included is explained from the viewpoint of high water absorbency of plant oils, on basis of hydrogen enhanced localized plasticity.

Effect of anodizing treatment on the very high cycle fatigue behavior of 2A12-T4 aluminum alloy

The effects of chromic acid anodizing (CAA) and sulfuric acid anodizing (SAA) treatments on the very high cycle fatigue behavior of 2A12-T4 aluminum alloy were investigated through ultrasonic fatigue tests. The results showed that CAA and SAA treatments resulted in an appreciable reduction in the fatigue strength and crack initiation from the interface between the anodizing film and substrate, whereas the fatigue crack of substrate specimens initiated from the specimen subsurface over 5×108cycles. The decrease in fatigue strength of the CAA specimens was associated with the residual tensile stress arising from the elastic mismatch between the anodizing film and the substrate. The defects in SAA film, such as segmentation cracks and overgrowth of the anodizing film into the substrate, contributed to the reduction of fatigue strength and multi-crack initiations. The predicted fatigue limit for SAA specimens well agreed with the experimental results.

OS1305 アルミニウム合金A2017-T4およびADC12の超音波疲労強度に及ぼす湿度の影響

Ultrasonic fatigue tests were conducted for aluminum alloy, A2017-T4, and aluminum die casting material, ADC12, under stress ratio, R, of-1 at a cyclic frequency of 20 kHz in low and high humidity environments in order to investigate the effects of humidity on fatigue strength. The relative humidity tested was 20 % and more than 90 % For A2017-T4 fatigue strength was decreased by high humidity. The reason for the decrease in fatigue strength was attributed to the different fracture morphology. Tensile fracture was observed in low humidity, while it changed as shear fracture in high humidity. Replica observation showed that cracks grew rapidly just before fracture. SEM observation of ADC12 at the fracture origin revealed that crack initiation caused by casting defect. Stress intensity factor range, ΔK, and the number of cycles to failure, N_f, relationship showed that large casting defect produced the most remarkable effect on the fatigue strength reduction. In addition, fracture mechanics approach showed that the fatigue strength of ADC12 was decreased by high humidity.

Corrosion Fatigue Of Welded Aluminium Vehicle Structures Under Constant And Variable Amplitude Loadings

The structural durability design of welded vehicle structures of aluminium also comprises the aspect of corrosion fatigue, especially for the experimental and numerical proof of chassis components that are submitted to salt corrosion in winter. Investigations carried out with specimens and components show that the decrease of fatigue strength under cyclic salt spray corrosion is not as pronounced under spectrum loading as under con- Qstant amplitude loading. This effect on spectrum fatigue strength and life, in turn, allows lightweight design of welded aluminium structures; knowledge and application of constant amplitude fatigue behaviour only would result in thicker cross-sections in design. Recommendations are given for the consideration of the different fatigue strength reductions arising from cyclic salt spray corrosion under constant and variable amplitude loadings and for the assessment of fatigue life under spectrum loading by cumulative damage calculations.

Effect of the intermediate layer on the properties of a coating of 50KhFA steel produced by electrical resistance welding

The results of investigations of deposition of a coating of 50KhFA steel (0.46–0.54% C, 0.80–1.10% Cr, and 0.15% V) on shafts of steel 45 through an intermediate layer produced from a powder material and an amorphous strip brazing alloy are presented. It is shown that at the optimum combination of the parameters of electric resistance surfacing the bonding strength of the coating with the component is identical with that of the parent metal, the microhardness of the coating equals 6900–8100 MPa, the length of the zone of the thermomechanical effect does not exceed 0.35–0.6 mm, and the impact toughness and fatigue strength decrease by no more than 10% in comparison with steel 45 in the initial condition.

Fatigue behavior of a structural steel coated with a WC–10Co–4Cr/Colmonoy 88 deposit by HVOF thermal spraying

The fatigue behavior of a SAE 4340 steel, coated with a 50% WC–10Co–4Cr/50% Colmonoy 88 deposit, by high velocity oxygen fuel (HVOF) thermal spray, has been investigated. The change in the maximum alternating stress with the number of cycles to fracture has been described by means of the relationship advanced by Stromeyer. A fractographic analysis has been carried out on some representative fracture surfaces, by means of scanning electron microscopy (SEM) techniques. The mechanical properties of the coating were characterized by means of nanoindentation tests. The results indicate that the coating is highly heterogeneous. Its deposition gives rise to a decrease in the fatigue strength of the substrate of ∼30%, in comparison with the uncoated substrate. The decrease in fatigue strength is due to the presence of stress concentrators at the substrate–coating interface, as well as the intrinsic characteristics of the coating.