Difference In Fatigue Strength Research Articles

高強度鋼の長寿命疲労挙動に及ぼす切欠きと表面粗さの影響

In order to evaluate the effects of notch and surface roughness on fatigue fracture morphology of high strength steels in long life region, cantilever-type rotary bending fatigue tests have been performed using notched specimens with different stress concentration factors, α, of 1.16, 1.51 and 2.0, and specimens with roughened surface of the maximum height, Rz, of approximately 10μm, 16μm and 19μm. The smooth specimen and the notched specimen with α=1.16 showed subsurface fracture in long life region. However, subsurface fracture was not seen in the notched specimens of α=1.51 and 2.0 because of much higher maximum stress at the notch root. In the surface fracture region, fatigue strength decreased with increasing stress concentration factor, while in the subsurface fracture region, there was no discernible difference in fatigue strength between the smooth specimen and the notched specimen with α=1.16. Furthermore, it was indicated that the high strength steel studied had a very high sensitivity to notch. All specimens with roughened surface exhibited subsurface fracture, where the transition stress at which fracture mode changed decreased in the specimens with rougher surface. In the surface fracture region, fatigue strength decreased with increasing surface roughness, while in the subsurface fracture region, it did not depend on surface roughness. The √area parameter model gave a reasonable estimation of the transition stress of the surface-roughened specimens.

Inclusion-controlled fatigue properties of 1800 MPA-class spring steels

Fatigue tests were conducted on a series of 1800 MPa-class spring steels whose fatigue properties were inclusion controlled. The fatigue tests were conducted on billets and on hot-rolled bars, taking into account the elongation of the oxide-type composite inclusions that were deformed during hot rolling, i.e., controlled inclusions. Anisotropy of the fatigue properties due to the slender shape of the elongated inclusions was also discussed. Fatigue tests were then conducted for both the rolling direction (RD) and transverse direction (TD) in the case of the billets. The fatigue test results in the RD showed a slight difference between billets and bars. The inclusions that were deformed during hot rolling were sufficiently elongated, even for the billet specimens, and differences in the effective inclusion sizes between the billets and their hot-rolled bars for the RD were small. However, there were marked differences in fatigue strength between the RD and TD in the billet specimens: the fatigue strength was almost half in the TD due to the presence of fish-eye fractures originating in large and slender MnS inclusions. In these fatigue tests, the two types of deformable inclusions revealed remarkably different effects on fatigue strength: the deformable oxide-type inclusions never caused fish-eye fracture, although the MnS inclusions found in the billets were extremely detrimental to fatigue strength when stress was applied in the TD.

Influence of Frequency on High-Temperature Fatigue Behavior of 17-4 PH Stainless Steels

The effect of frequency (2 and 20 Hz) on the high-cycle fatigue and fracture behavior was investigated at 573-773 K for 17-4 PH stainless steels in three conditions: Condition A (unaged), H900 (peak-aged) and H1150 (overaged). S-N results indicated that at 573 and 673 K, there was generally no difference in fatigue strength between 2 and 20 Hz, except for H900 at 673 K where the fatigue strength at 2 Hz was lower than that at 20 Hz. At 773 K, the fatigue strength of each condition at 2 Hz was lower than that at 20 Hz due to the occurrence of creep mechanism at this low frequency. At 773 K and 2 Hz, the fatigue fracture mode exhibited a mixed mode involving transgranular and intergranular cracking and the grain boundary cavities were also observed. At a given temperature and frequency, the fatigue strength for the three conditions generally took the following order: H900 > Condition A > H 1150, except for Condition A at 773 K in the long life regime where the fatigue strength was close to that of H 1150 due to a precipitate-coarsening effect. With the exception of Condition A tested at 673 K, the fatigue strength of each condition was decreased with increasing temperature as a result of a reduction in yield strength. At both frequencies, the fatigue strength of Condition A at 673 K was greater than that at 573 K as a result of an in-situ precipitation-hardening effect. Fractography observations indicated that the fatigue crack initiation site, crack propagation path and fracture surface morphology were functions of testing temperature, loading frequency and applied cyclic stress level.

Effect of suture locking and suture caliber on fatigue strength of flexor tendon repairs.

The objectives of this cadaveric study were 2-fold: to determine the effect of different locking configurations on the cyclical fatigue strength of flexor tendon repairs and to assess the differences between each repair when a 3-0 or 4-0 suture is used. One hundred twenty flexor digitorum profundus tendons were cut and repaired using nonlocked, simple locked, and cross-stitch locked variations of 2- and 4-strand flexor tendon repairs. Using an incremental cyclical loading protocol we performed 10 trials of each repair with both 3-0 and 4-0 sutures and analyzed the number of Newton-cycles to failure using a 3-way ANOVA. The use of a 3-0 suture led to a 2- to 3-fold increase in fatigue strength in all repairs tested and the fatigue strength of the 4-strand repairs was significantly greater than the 2-strand repairs. All repairs performed with 4-0 suture failed by suture rupture. Of the 3-0 suture repairs, the three 2-strand repairs and the 4-strand cross-stitch locked repair failed by suture rupture. In contrast, 6 of 10 of the 4-strand simple locked and nonlocked repairs failed by suture pullout. There was no significant difference in fatigue strength between the 2 locked and the nonlocked 2-strand repairs using either 3-0 or 4-0 suture. There also was no significant difference in holding capacity or fatigue strength between the simple locked or nonlocked 4-strand repairs. However, the 4-strand cross-stitch locked repair with a 3-0 suture had significantly improved fatigue strength and holding capacity compared with the other repairs tested. Based on the consistently inferior biomechanical performance of 4-0 suture, we recommend that 3-0 suture be considered for 2- or 4-strand tendon repairs when early active motion is planned. The orientation of the transverse and longitudinal components of simple locked repairs did not significantly influence their holding capacity or fatigue strength. The cross-stitch type of locked repair provides better holding capacity and fatigue strength compared with simple locked or nonlocked 4-stranded flexor tendon repairs.

201 予ひずみを受けたTRIP型複合組織鋼板の疲労信頼性評価

Pre-straining effects on fatigue strength of TRIP-aided multiple phase steel was studied from microstructural viewpoint in order to evaluate fatigue reliability of mechanical product after plastic working process. Effects of pre-strain were examined by comparing two types of specimen: one was cut from 10% pre-strained TRIP steel sheet, the other was untreated virgin material. The pre-strained material ensured a 20% improvement in the fatigue strength over the virgin material. Both of pre-strained and virgin specimens were broken with ductile fracture appearance as well as uniaxial tensile fracture. Difference of fatigue strength between two types of materials was closely related to the deformation behavior during cyclic loading. The improvement of fatigue strength could be explained in the following factors associated with the microstructure of TRIP steel: (a) connected configuration of hard phase in microstructure, (b) γ-phase transformation induced plasticity by pre-straining, and (c) origination of compressive residual stress in α-phase by pre-straining.

Fatigue strength of welded joints made of high‐strength steels

AbstractBy rearranging existing fatigue test results, fatigue performance of typical welded joints, (longitudinal, cruciform and out‐of‐plane gusset welded joint) are discussed with respect to the dependence on steel strength and on joint size. The relationship between fatigue strength and steel strength was not clear for longitudinal welded joints of small specimens and large‐scale girder specimens. For cruciform welded joints and out‐of‐plane gusset welded joints, fatigue strength of small joint specimens is independent of the steel strength, but that of girder specimens decreases with increase in steel strength. In addition, fatigue strengths of these two types of welded joints decrease with increase in joint size, especially for girder specimens.In order to discuss these relationships in detail, fatigue tests with out‐of‐plane gusset welded joint specimens and large‐scale girder specimens, including web gusset joints made of JIS SM570 and 900 MPa class steel were carried out. From the fatigue tests, similar tendencies were found to those obtained from existing test results. Although fatigue tests of joint specimens showed no difference in fatigue strength, crack initiation life and crack propagation rate between the steel strengths, fatigue strength of girder specimens made of 900 MPa class steels is much lower than for JIS SM570. In addition, fatigue strength of the girder specimens is considerably lower than that of the joint specimens. This difference results mainly from an increase in stress concentration due to joint geometry, and combination of nominal stress and shear stress in the girder specimens. It was found that the influence of the combination can be approximately evaluated from the nominal principal stress.

The Fatigue Properties of Ferrite-Pearlite Steel Modified by WPC Process.

Rotational bending fatigue tests were carried out using ferrite-pearlite steel modified by the WPC process, which is one of the shotpeening processes. To clarify the high cycle fatigue properties, measurements of residual stress and hardness distribution were performed. Fatigue strength of the specimen with a WPC-treated surface was higher than that of an untreated one. This is because (i) the WPC process forms an extremely shallow hardened layer with high compressive residual stress, and (ii) the compressive residual stress generated by the WPC process was very stable under cyclic loading ; after 107 cycles of stress loading, the reduction of compressive residual stress was very small. A comparison with fatigue properties of the specimen hardened by conventional shotpeening was also conducted with special focus on the difference in fatigue strength and fatigue crack initiation behavior.

Tensile Behavior of Filament-Wound Composites in Medium Vacuum

In order to design the most efficient flywheel energy systems using fiber-reinforced polymer composite rotors, design allowables for the rotor material in a realistic environment and under realistic stresses must first be established. As a first step, this investigation characterizes the 0-deg tensile behavior of unidirectional T700 carbon and E-glass reinforced epoxy composites both as received and after conditioning at a pressure of 15 mtorr and a temperature of 20 to 25°C. Constant-amplitude tension-tension cyclic fatigue tests were carried out on half the specimens in a similar medium vacuum environment. Weights of specimens were tracked with immeasurable changes noted due to vacuum conditioning for up to 300 days' duration. E-glasslepoxy specimens had a higher quasi-static strength following conditioning, but this difference was not evident on an absolute stress basis in the fatigue tests. At the baseline comparison point of 10 5 cycles to failure, there was little difference in fatigue strength of as-received and conditioned E-glasslepoxy specimens. In the case of T700/epoxy, little difference between conditionings was noted in quasi-static tests, but as-received specimens had slightly higher fatigue strength at a lifetime of 10 5 cycles. Microscopic inspection of fracture surfaces following quasi-static and fatigue failures revealed no significant differences due to the type of loading or conditioning.

The Estimation of Temperature Rise in Low-Cycle Fatigue of TiNi Shape-Memory Alloy.

A difference in fatigue strength depending on the rotational speed between air and water was found for the rotating-bending fatigue characteristics of a TiNi shape-memory alloy wire in our previous work. The heat transfer characteristics around a shape-memory alloy wire were measured using Reynold's analogy. The rise in temperature of the metal during the rotating-bending cycles was calculated considering the heat transfer result. As a result, the rotating-bending fatigue characteristics of a shape-memory alloy wire were explained well from the viewpoint of the heat transfer characteristics.

827 複合表面性質処理を施したチタン合金 (SP-700) の疲労特性

The purpose of this study is to investigate the effect of the hybrid surface modifications on the fatigue properties of titanium alloy (SP-700). Rotational bending fatigue tests were performed on five types of specimens with different surface treatments. The results are summarized as follows : 1) The fatigue strength of the WPC treated specimen was larger than that of the untreated specimen. This is because, WPC process generates the hardened layer with compressive residual stress. 2) There was no difference in fatigue strength between the untreated and the physical vapor deposited specimen. At high stress levels, however, the fatigue lives of the specimen with TiN film were longer than that of the untreated ones. 3) The fatigue strength of the hybrid surface modified specimen was decreased less than that of the WPC treated specimen. This is because, the heat treatments during PVD process reduce the compressive residual stress.

The Estimation of Temperature Rise on Low Cycle Fatigue of TiNi Shape Memory Alloy.

Difference in fatigue strength depending on rotational speed between in air and in water at the rotating-bending fatigue characteristics of a TiNi Shape-memory alloy wire was found in our previous work. The heat transfer characteristics around a shape-memory alloy wire was measured using Reynold's analogy. The temperature rise of metal during the rotating-bending cycles were calculated considering heat transfer result. As a result, the rotating-bending fatigue characteristics of a shape-memory alloy wire were well explained from the viewpoint of heat transfer characteristics.

Fatigue Strength and Subsurface Crack Growth Properties of Plasma Assisted Duplex Surface Treated Tool Steel (SKD61).

Numerous methods of surface treatment have been developed for purpose of the increase in high cycle fatigue strength and extension of lifetime of the engineering materials in various kinds of environment. In this study, cantilever-type rotating-bending fatigue test in air was conducted using the smooth specimens of alloy tool steel, SKD 61, with three kinds of surface treatment ; that is, TiN coating by PVD method, plasma assisted ion-nitriding and duplex treatment with the ion-nitriding followed by the TiN coating. Fatigue strength of the surface treated specimens was improved as compared with untreated specimen. The highest fatigue strength was the duplex surface treated specimen, followed by the ion-nitriding one and the lowest was the TiN coating one. This tendency was obvious at the region of high stress amplitude and disappeared at low stress amplitude region. Increase of fatigue strength of specimen by surface treatment was due to transition of fatigue crack initiation site from surface to subsurface by an effect of compressive residual stresses occurred at surface layer by ion-nitriding, and barrier to the formation of surface crack nuclei beneath the TiN coating film. The difference in fatigue strength between the duplex surface treated specimen and the ion-nitriding one, of which crack initiation site was subsurface, was discussed by the effect of TiN coating film on subsurface crack growth behavior. It was found that TiN coating film acts as barriers to formation of plastic zone at the front of subsurface crack and retards crack growth twoard surface.

Comparative strengths of metal framework structures for removable partial dentures

Statement of problem. The framework of removable partial dentures often breaks where the metal retention for the resin denture base attaches to the clasp and major connector. Purpose. To study these failure factors, various types of three-dimensional simulated denture frameworks were designed to measure their ultimate strengths and stiffnesses under static load. This study also sought to determine which could be recommended for long-term clinical service. Material and methods. Three groups of three-dimensional frameworks (double, T-shaped, and trussed structure) were designed, and two groups of classic framework designs (uniform and nonuniform skeleton) were used with an acrylic resin block as a control ( n = 5 per group). Fatigue strength, stiffness and amount of strain, and free-end displacement under dynamic loads were measured for each framework. The fatigue strength was determined with a constant-loading fatigue test by counting the number of loading cycles required to induce catastrophic failure. Results. With 20 kgf of load, four T-shaped samples and three trussed structure samples failed at 30,000 to 60,000 loading cycles. No samples of double structure group failed until 100,000 loading cycles. Conclusions. Three-dimensional frameworks, especially the double structure, have significant differences in fatigue strength and can provide long-term clinical service. (J Prosthet Dent 1997;78:302-8.)

快削ステンレス鋼ＳＵＳ３０３同種摩擦圧接継手の疲労特性

Fatigue tests were conducted on the free cutting stainless steel SUS303/SUS303 friction welded joints under pulsating zero-tension load to study the effect of microstructure produced by friction welding process. The specimens with circumferential notch which was machined near the welding interface were used in the fatigue tests. Fatigue crack initiation and propagation were examined by using DC potential drop method.The fatigue limit of the friction welded joints was almost the same as that of the base metal. But the difference in fatigue strength between the base metal and the joints increased with increasing stress level. The crack initiation strength of the joints was lower than that of the base metal and the non-propagating cracks were found in the specimens of the joints which did not fracture above 107 stress cycles. The fatigue crack growth rate of the joints decreased and reached the minimum value near crack length of 0.2mm. The crack growth rate of the joints was smaller than that of the base metal in the low stress intensity factor range, while it increased and gained upon that of the base metal. The degradation of fatigue strength of the joints, especially for the fatigue crack initiation strength, was mainly caused by the shape, size, direction and distribution of non-metallic inclusions which differed from those of the base metal.

Microstructure dependence of fatigue strength and fatigue crack propagation in titanium aluminide

Abstract This paper describes the fatigue strength and fatigue crack propagation (FCP) at ambient temperature in a titanium aluminide, TiAl. In order to study the effects of microstructure on fatigue properties, two different microstructures were prepared: an as-cast material consisting predominantly of lamellar ( α 2 + γ ) grains, and a heat-treated (homogenized) material composed of equiaxed γ grains and lamellar grains. The as-cast material showed higher fatigue strength than the heat-treated material. As fatigue cracks were initiated at more than 90% of fatigue life, the observed difference in fatigue strength was attributed to the differences of the crack initiation resistances of the microstructures. The FCP rates were faster in the heat-treated material than in the as-cast material in the whole stress intensity factor range, ΔK , studied, and when the FCP data were plotted in terms of the effective stress intensity factor range, ΔK eff , the heat-treated material still exhibited faster FCP rates. The SEM observation revealed transgranular cleavage of γ grains in the heat-treated material, thus leading to lower FCP resistance.

球状黒鉛鋳鉄固相接合材のねじり疲労強度に及ぼすニッケルインサートの効果 球状黒鉛鋳鉄の固相接合（第６報）

The authors have studied the tensile strength, rotational bending fatigue strength and Charpy impact property of the joints of spheroidal graphite cast iron bonded by diffusion bonding and found that the properties of the joints were comparable to the base metal. In this study, spheroidal graphite cast irons were bonded by diffusion bonding and the effects of nickel foil as an insert metal on the torsional fatigue strength of the joints have been investigated. Main results are as follows ;(1) For the directly bonded specimens after ferritizing, an S-N curve could not be obtained because of large scatters of the fatigue lives at a given stress amplitude due to the presence of fine precipitates of graphite on the interface. However, no failure was occurred for specimens tested for 107 cycles at stress amplitudes below 160 MPa.(2) When the specimens bonded with insert metal of a 10μm thick nickel foil were ferritized, the torsional fatigue limit was 170 MPa, which was lower by 20 MPa than that of the base metal. The difference in fatigue strength at the cycles between 10% and 90% of the failure probability was about 38 MPa ; this is higher by about 16 MPa than that of the base metal.(3) The torsional fatigue limit of the joints having a ferrite and pearlite structure bonded with insert metal of a 10μm thick nickel foil was 210 MPa which was lower by 10 MPa than that of the base metal. However, the difference in fatigue strength at the cycles between 10% and 90% of the failure probability was nearly equal to that of the base metal.(4) The torsional fatigue limit of the joints bonded using a 10μm thick nickel foil increased with increase in the tensile strength of the joints and was about 43% of the tensile strength.

β型チタン合金Ｔｉ‐１５Ｍｏ‐５Ｚｒ‐３Ａｌの疲労挙動に及ぼす組織の影響

The fatigue behaviour has been studied under rotating bending using the materials with three different microstructures (β grain sizes) of a Ti-15Mo-5Zr-3Al alloy which were solution-treated at 735°C (STA735), 850°C (STA850) and 1000°C (STA1000) followed by aging at 500°C. It was found that STA735 showed the highest fatigue strength, and in the materials solution-treated above the β transus, STA850 and STA1000, the fatigue lives decreased with increasing solution temperature i.e. β grain size but the fatigue limit was almost the same in both materials. Fatigue cracks were initiated as a consequence of slip deformation in STA735, while at β grain boundaries in both STA850 and STA1000. In particular, STA850 exhibited an interesting initiation behaviour in which cracks were initiated at the specimen surface at high stress levels, but subsurface crack initiation was observed at low stress levels. The growth rate of small cracks was almost identical for all the materials, indicating no influence of solution temperature. Therefore, the observed difference in fatigue strength was attributed to the resistance to crack, initiation of the microstructures studied.

Fatigue Strength and Fatigue Crack Propagation in Titanium Aluminide TiAl.

Fatigue strength and fatigue crack propagation (FCP) of titanium aluminide TiAl have been studied at ambient temperature for two microstructural conditions ; an as-cast material consisting predominantly of lamellar (α2+γ) grains and a heat-treated material (homogenized) composed of equiaxed γ grains and lamellar grains. It was found that fatigue strength of as-cast material was superior to that of heat-treated material. Since fatigue cracks were initiated at more than 90% of fatigue life, the observed difference in fatigue strength was attributed to crack initiation resistance of both microstructures. The FCP rates were faster in heat-treated material than in as-cast material, and when the FCP data were plotted in terms of the effective stress intensity factor range, the heat-treated material still exhibited faster FCP rates. The SEM observation revealed predominantly transgranular cleavage of γ regions in the heat-treated material, thus leading to lower FCP resistance.

破壊力学 Ａｌ‐１５Ｚｎ粉末冶金合金における表面微小疲労き裂の発生および進展挙動

Reversed plane bending fatigue tests were conducted on extruded Al-15Zn P/M (Powder Metallurgy) aluminum alloy using two types of specimens with different notch dimensions. The initiation and growth of small surface fatigue cracks were continuously monitored by the replica technique and investigated in detail.A large difference in crack initiation strength was observed between the extruded (L-C orientation) and transverse directions (C-C orientation). It was found that the strength of shallow notched specimen, when evaluated by true stress at the notch root, was lower than that of deep notched one. If the stress intensity range of inclusion at which the fatigue crack initiated was calculated, the difference in fatigue strength depending on specimen orientation and notch size could be eliminated. And also, a prediction method of fatigue strength in smooth specimen was proposed on the basis of the extreme value statistics. The macroscopic fatigue crack growth rate could be expressed by the Paris equation in terms of the maximum stress intensity factor irrespective of stress level and the notch size, and the growth rate was found faster in C-C orientation specimen than that in L-C orientation specimen.

高張力鋼の腐食疲労とフラクトグラフィ

Considering that the corrosion fatigue is especially time-dependent, it is necessary to confirm the long-life fatigue properties of structural steels to be adoptable to the practical design curve. Fatigue tests have been performed on three kinds of high tensile steels for offshore structures in air or by dropping fresh water and sea water to the midsection of each specimen under rotating bending load at various cyclic speeds including lower frequencies. In addition, fractographic observation has been carried out on the crack initiation and fracture surface of each specimen.The main results obtained on the basis of the long-time corrosion fatigue tests (maximum period; 1.45 years/specimen) are as follows:(1) The fatigue strength decreases with higher cyclic numbers and lower frequencies. There exists a good linear relation between stress amplitude and cyclic frequency. The relation can be expressed by a linear logarithmic equation.(2) The effect of corrosion liquid on fatigue strength is remarkable in long period tests and the fatigue strength in sea water becomes lower than that in fresh water in the frequency range between 1500 and 4rpm.(3) The fatigue strength is a little dependent on tensile strength in the low cycle region and the difference in fatigue strength among high tensile steels becomes reversed in the high cycle region. This phenomenon appears not only in fresh water but also in sea water.(4) The crack initiates from pits and propagates mainly by transgranular fracture. Both quasi-cleavage fracture and intergranular fracture are observed in the heat-treated steels under low cyclic frequencies in the high cycle region. These fracture patterns would be caused by corrosive effect and accelerate the crack propagation rate of the heat-treated steels.