Initial Stress Intensity Factor Range Research Articles

Fatigue and fracture of small cracks in superelastic Nitinol

The present study develops a hybrid total life and damage-tolerant fatigue methodology for the determination of a small crack growth threshold in Nitinol. Axial tension–tension/Mode I fatigue testing was performed on superelastic Nitinol wire specimens with starter cracks milled into the specimens using focused ion beam (FIB). Fatigue testing was performed in displacement control at various strain amplitudes, typical for Nitinol fatigue characterization given its superelastic behavior. The simplistic geometry of the test specimen and axial loading conditions also allowed for the calculation of stress amplitude and initial stress intensity factor range. A logistic regression analysis was used to determine strain/stress-based fatigue limits. The small crack growth threshold for superelastic Nitinol was obtained for the first time from physically small cracks with sizes comparable to native inclusions. Lastly, a fractographic analysis was performed to examine crack initiation and propagation from the FIB-milled starter cracks and to identify relevant fractographic features. While the proposed methodology is centered around the use case of Nitinol medical devices, it could be adapted for other materials and applications where small crack initiation and propagation play an important role in fatigue lifetime prediction.

Influence of Mean Stress and Stress Variation on Fatigue Strength of Stainless Cast Steel SCS5 for Hydraulic Turbine Runner

In hydraulic turbine runners, accidents due to cyclic loading often occur. In this study, in order to clarify the fatigue properties and fracture mechanism of stainless cast steel SCS5 cut out from the hydraulic turbine runner vane for about 22 years, fatigue tests under constant amplitude with positive mean stress and variable amplitude loading were carried out by using tension-compression specimens. The results are summarized as follows : (1)The S-N curves of aged SCS5 used in this study have wide scatter. (2) From SEM observation of the fracture surface, fatigue crack initiation site were found to be the casting defect which consists of dendritic phase and shrinkage cavity. (3) It is possible to evaluate the fatigue life of aged SCS5 by using the defect size √area and the initial stress intensity factor range calculated by casting defect size. The value of the threshold stress intensity factor range obtained in this study were 3.8 MPa·m1/2. (4) Under repeated two-step loading pattern, a fatigue crack was found to grow when the stress intensity factor range was lower than the threshold stress intensity factor range of under constant amplitude loading.

The Effects of Fracture Origin Size on Fatigue Properties of Ductile Cast Iron with Small Chill Structures

Reducing the weight of a machine structure is an increasingly important consideration both for the conservation of resources during production and for the energy saving during operation. With these objectives in mind, thin-walled ductile cast iron has recently been developed. Because rapid cooling could result in brittle microstructure of cementite (chill) in this cast iron, it is necessary to investigate the effect of cementite on the fatigue properties. Therefore, fatigue tests were carried out on a ductile cast iron of block castings which contained a relatively small amount of cementite. Fracture surface observation indicated that the fracture origins were located at graphite clusters and cast shrinkage porosity, not at cementite. It appears that when the size of the cementite is smaller than that of the graphite, the cementite does not affect the fatigue properties of ductile cast iron. Not surprisingly, the fatigue lives were found to increase with decrease in the size of the fatigue fracture origin. The threshold initial stress intensity factor range ΔKini,th for fatigue failure was found to be about 3-4MPa√m, independent of microstructure.

Fatigue Crack Propagation Behavior of Thermally Degraded SiCp/Al Composites

Silicon carbide particle reinforced aluminum 6061 composites (SiCp/Al) with 5 and 10% SiC particle volume fractions as well as aluminum 6061 alloy (AA6061) were used to examine their fatigue crack propagation behavior after degraded by thermal cycling and isothermal exposure. Fracture surfaces were observed under a scanning electron microscope. Thermally cycled specimens (TC-SiCp/Al and TC-AA6061) were prepared through 100 thermal cyclings between 200 C and 300 C and isothermally exposed specimens (IE-SiCp/Al and IE-AA6061) was prepared by isothermal exposure at 300 C for 80 h. Difference of fatigue crack propagation behavior in the IE- and TC-AA6061 was small with increase of stress intensity factor range, iK even if da/dN of the both types of AA6061 were different at the initial stress intensity factor range. Fatigue crack propagation behavior of the IE-SiCp/Al was similar regardless of SiC particle volume fractions. While for the TC-SiCp/Al, fatigue crack propagation behavior was different in the SiC particle volume fractions and to the TC-AA6061. Besides, it notes that there was good relation between exponent m and coefficient C of the Paris equation regardless of thermal cycling and isothermal exposure and SiC particle volume fractions. The relation between m and C could be described as C 1/4 AΘm where constants A and B were 1.668 E-5 and 0.165, respectively.

Effects of oxidation on the fatigue crack growth resistance and the interfacial properties of a Ti-MMC laminate composite

Fatigue crack growth rates in a [0/90]2s Ti-6Al-4V/SCS-6 cross-ply laminate, correlated with push-out tests, have been measured to assess the effects of varying test temperature, environment, load ratio (R), and initial stress intensity factor range (ΔK). The fatigue crack growth resistance is degraded in tests at 450 °C in air, but tests carried out at test temperatures of up to 450 °C under vacuum, both at R=0.1 and R=0.5, have shown crack arrest/catastrophic failure transitions (CA/CF), which are similar to those observed for specimens studied at room temperature and at 300 °C in air. Moreover, for such [0/90] composites, the critical role of intact 0 deg fibers bridging in the crack wake, in promoting fatigue crack growth resistance, has been confirmed. Sudden increases of fatigue crack growth rate can be attributed to individual fiber failure(s), which were detected by acoustic emission techniques. The effect of the experimental conditions (environment, test temperature, and duration) on the mechanical behavior (fatigue crack growth rate, push-out tests, and broken fibers pull-out lengths) of this laminate may be explained by the modification of the interfacial zone (decrease in the carbon layer thickness due to oxidation and formation of TiO2).

Fatigue crack growth behavior of off-axis Ti–6Al–4V/SCS-6 metal matrix composites

The fatigue crack growth (FCG) behavior of five different laminate orientations ([0/90] 2s, [90/0] 2s, [±45] 2s, [150/60] 2s and [60/150] 2s) of cross-ply Ti–6Al–4V/SCS-6 has been investigated at room temperature in air. All these tests were performed on testpieces with a similar notch depth of 0.5 mm under three-point bending (3PD). The FCG behavior was investigated as a function of initial stress intensity factor range (Δ K ini), fiber orientation, surface condition and composite architecture to determine the crack arrest/catastrophic failure (CA/CF) transition of these laminates. For a given Δ K ini, the laminates containing 0° (or on-axis) plies to the loading direction (e.g. [0/90] 2s and [90/0] 2s) show higher FCG resistance than the laminates containing off-axis plies (e.g. [±45] 2s, [150/60] 2s and [60/150] 2s). The damaging of fibers on the surface ply due to polishing also reduces the CA/CF transition. Regarding crack propagation behavior, laminates with 0° fibers show mode-I cracking, whereas, the laminates with off-axis fibers show fatigue crack propagation at an angle to the mode-I direction. It is also found that the dominant direction of crack propagation in these laminates is controlled by the angle of the central double plies with respect to the mode-I direction.

Effects of Surface Finishing and High Vacuum Environment on Surface- and Interior-Originating Fatigue Properties of High Strength Steel.

Uniaxial tension fatigue tests were carried out in air and high vacuum environments using pieces made of high strength steel (SNCM 439) with various finishing conditions. Based on the results of fatigue tests and the data arranged by the initial stress intensity factor range (ΔKini), (i) effects of surface finishing and high vacuum environment on surface- and interior-originating fatigue properties, (ii) effects of high vacuum and inner environment of materials on the growth limit of the fatigue crack, were considered. The following results were obtained; (1) Interior-originating fractures were not affected by surface finishing conditions and surrounding environments. (2) The shape of the "Duplex S-N curve" depended on surface finishing conditions and surrounding environments. (3) Fatigue lives of surface-originating fractures in high vacuum and interior-originating fractures were much longer than surface-originating fractures in air. Surface cracks in high vacuum and interior cracks of materials grew from smaller defects whoseΔKini were lower than the growth limit of surface cracks in air. A lower growth limit of fatigue cracks and a longer fatigue life of interior-originating fractures were inferred from the effects of the inner environment of materials, which were similar to those of high vacuum environment.

OS11W0355 Effects of high vacuum environment on high cycle fatigue properties of Ti-6Al-4V alloy

Uniaxial tension fatigue tests were carried out in air and high vacuum environments using pieces made of Ti-6Al-4V alloy with several surface finishings. Effects of high vacuum environment on high cycle fatigue were investigated based on results of fatigue tests and the data arranged by initial stress intensity factor range (ΔK_<ini>). Considering results of fatigue tests that interior-originating fractures occurred near yield stress in high vacuum environment, it is necessary to pay attention to the interior-originating fracture in case of using this material in vacuum environment like space. Fatigue lives in high vacuum strongly depended on cyclic stress compared with those in air as stress decreased. As for data arranged by ΔK_<ini>, the dependence of the threshold stress intensity factor range (ΔK_<th>) on crack size in high vacuum is expected to be stronger than that in air.

Influence of weld size on fatigue life prediction for flux cored arc welded cruciform joints containing lack of penetration defects

Fracture mechanics equations have been developed to predict the fatigue life of flux cored arc welded cruciform joints between pressure vessel grade steels, containing lack of penetration defects. These equations have been developed by combining the Paris law and ΔKi (initial stress intensity factor range) endurance equations. The initiation life and the propagation life of the joints were accounted for, to obtain the total fatigue life. The initiation life was obtained experimentally and the propagation life was evaluated using the numerically developed equations. The accuracy of the developed equations was tested by comparing the predicted data with the experimental data and the correlation was found to be fairly close. Further, the influence of weld size on fatigue life has been analysed in detail.

Mean stress dependence of fatigue strength and subsurface crack initiation in Ti–15Mo–5Zr–3Al alloy

This paper describes the effect of mean stress on fatigue strength and subsurface crack initiation in β Ti–15Mo–5Zr–3Al alloy. Axial fatigue tests were performed at stress ratios, R, of −1, 0 and 0.5 using smooth specimens of two materials solution treated at different temperatures of 735°C (STA735) and 850°C (STA850). Fatigue strength decreased with increasing stress ratio in both materials and at a given stress ratio, STA735 exhibited a higher fatigue strength than STA850. The crack initiation site was the specimen surface at all stress ratios in STA735 and at R=−1 and 0.5 in STA850, while the interior at R=0 in the latter material, thus subsurface crack initiation was found to be both microstructure and stress ratio related phenomenon. The mean stress dependence of fatigue strength could be evaluated by the Goodman relationship, except for unconservative estimation of the fatigue strength at R=0 in STA850, which was attributed to the different crack initiation mode. A smooth facet was always seen at the crack initiation site in STA850 regardless of whether cracks were initiated at the specimen surface or internally. It was found that there was a good correlation between the initial stress intensity factor range calculated using the facet size and fatigue life.

疲労 高炭素クロム軸受鋼のＳ‐Ｎ曲線と内部き裂発生挙動

In order to investigate high cycle long life fatigue characteristics, cantilever-type rotary bending fatigue tests were conducted in an open environment at room temperature using hourglass shaped specimens of high carbonchromium bearing steel. The S-N curve obtained was clearly classified fracture modes into two groups of different crack origin. One was governed by a crystal slip on the specimen surface which occurred in the region of short fatigue life and high stress amplitude level. The other was governed by nonmetallic inclusion at subsurface which occurred in the region of long fatigue life and low stress amplitude. The inclusion governed fish-eye fracture mode was distributed in a wide range of stress amplitude not only below the fatigue limit defined as the threshold of fracture due to the slip governed mode but also above the fatigue limit. This remarkable shape of the S-N curve was different from a step-wise one reported in previous literature and was characterized as duplex S-N curves which composed of two different S-N curves corresponding to the respective fracture modes. An examination of the initial stress intensity factor range of fracture origin, ΔKini, showed that the ΔKini of the slip governed fracture mode was larger than the threshold stress intensity factor range of the surface crack growth, ΔKth. On the other hand, cracks originate and propagate from inclusion at subsurface with smaller ΔKini than the slip-originating crack and the ΔKth.

An Observation of Fish-eye Fracture Process in High Strength Steel SUJ2.

Long-term fatigue tests have been conducted on a high strength steel, SUJ2, in order to clarify the fatigue behaviour in long life region, more than 107 cycles. The fatigue failure at the stress level under the ordinary fatigue limit occurred in long life region. Fracture mechanism in this region is fish-eye type, i. e. inner defect induced failure, while surface slip type fracture is found in short fatigue life region. At the center of fish-eye non-metallic inclusion is recognized, which is surrounded by an indefinite and grainy region. However, the fish-eye without grainy region was also observed exceptionally. Therefore, a fish-eye is divided usually into three parts, i. e. inclusion, grainy region and inner crack. Most of the inclusions generating the fish-eye were located within 0.1mm from the specimen surface. The sizes of inclusions calculated by the √(area) method are almost constant to the stress level and the number of cycles. The initial stress intensity factor range, ΔKini, at which crack was generated from the inclusion, is almost the same with increasing fatigue life, i, e. 2.3∼3.5 MPa √(m). On the other hand, the sizes of fish-eyes calculated by the √(area) method are scattered to the stress level and the number of cycles. Based on the results, a model of the fatigue failure at the stress level under the fatigue limit is proposed.

Effects of control mode in a cross-ply Ti-MMC at ambient and high temperature in air and in vacuum

Fatigue crack growth rates from unbridged defects in a [0/90°] 2s Ti–6Al–4V/SCS-6 cross-ply laminate composite have been measured to assess the effects of the control conditions under a stress ratio of R=0.1 and at a frequency of 10 Hz, whilst varying the initial stress intensity factor range (Δ K ini.), the test temperature and the environment. Initial fatigue crack growth rates in air were found to increase with increasing test temperature. Δ K ini. transition values leading to fatigue crack arrest compared with eventual specimen catastrophic failure are, at ambient temperature, approximately equal to 14 MPa√m in load control but are approximately 40% higher in position control. In addition, at a temperature of 450°C in air, the limiting value ofΔ K ini. promoting crack arrest decreases to 8 MPa√m in load control. In contrast, the Δ K ini. transition in vacuum at a temperature of 450°C is identical to that at ambient temperature in air under load control. Under position control at the elevated test temperature, a period of constant nominal stress intensity factor range (Δ K nom.) occurs. The crack growth rate at a test temperature of 450°C, in vacuum, is approximately 4×10 −7 mm/cycle in the constant Δ K nom. region, and this is somewhat less than that observed in air at the same test temperature (5×10 −6 mm/cycle), while crack arrest at ambient temperature in air is observed. Finally, it is suggested that position control experiments are of some advantage for the studies of the underlying crack growth mechanisms.

Relation between High Cycle Fatigue Characteristics and Fracture Origins in Low-temperature-tempered Cr-Mo Steel.

Uniaxial tensile fatigue tests were carried out on high strength steel to investigate the relation between fatigue characteristics and fracture origins. The examination compared two surface finishing conditions, one grinding polished by the emery paper and the other electro-chemically polished. The emery paper finished specimens showed a stepwise S-N diagram while the electro-chemically polished specimens had a straight S-N diagram. The sites of fracture origins changed from surface to interior of test pieces as the stress level was reduced on both kinds of test piece. An examination of the initial stress intensity factor range ΔKini of fracture origins was interpreted to show that large surface defects in the grinding condition lowers the fatigue strength of surfaceoriginating fractures, resulting in the stepwise S-N diagram. Interior-originating fractures showed that cracks originate and propagate from inclusions with much smaller ΔKini than the surfaceoriginating cracks.

Simple approximations for bridged cracks ion fibrous composites: Cox, B.N. and Lo, C.S. Acta Metall Mater40 7 (July 1992) pp 1487–1496

Fatigue crack growth rates from unbridged defects in a [0/90°]2s Ti–6Al–4V/SCS-6 cross-ply laminate composite have been measured to assess the effects of the control conditions under a stress ratio of R=0.1 and at a frequency of 10 Hz, whilst varying the initial stress intensity factor range (ΔKini.), the test temperature and the environment. Initial fatigue crack growth rates in air were found to increase with increasing test temperature. ΔKini. transition values leading to fatigue crack arrest compared with eventual specimen catastrophic failure are, at ambient temperature, approximately equal to 14 MPa√m in load control but are approximately 40% higher in position control. In addition, at a temperature of 450°C in air, the limiting value ofΔKini. promoting crack arrest decreases to 8 MPa√m in load control. In contrast, the ΔKini. transition in vacuum at a temperature of 450°C is identical to that at ambient temperature in air under load control. Under position control at the elevated test temperature, a period of constant nominal stress intensity factor range (ΔKnom.) occurs. The crack growth rate at a test temperature of 450°C, in vacuum, is approximately 4×10−7 mm/cycle in the constant ΔKnom. region, and this is somewhat less than that observed in air at the same test temperature (5×10−6 mm/cycle), while crack arrest at ambient temperature in air is observed. Finally, it is suggested that position control experiments are of some advantage for the studies of the underlying crack growth mechanisms.

Influence of Frequency on Corrosion Fatigue Crack Propagation in Squeeze-Cast Al-Si Aluminum Alloy.

Corrosion fatigue crack propagation tests of squeeze-cast Al-Si aluminum alloy, AC 8 A-T 6, were performed under the conditions of the stress ratio of 0.1, triangular waveform and testing frequency, f, of 0.1, 1 and 5 Hz in 3.0% saline solution using compact tension specimens with side grooves. The experimental results obtained in this investigation were compared with those reported previously under the tests of f=10Hz. Corrosion fatigue crack propagation behavior under f=0.1 and 1 Hz was also affected by the initial stress intensity factor range, ΔKi, as the test was started, the same as that under f=10 Hz. It was observed that the retardation and complete arrest of crack propagation occur within a short period in low ΔK, due to the wedge effect induced by the corrosion products. This behavior was explained uniquely by the contributory stress intensity factor range, ΔKcont, considering the viscoelasticity of the corrosion products formed within a crack. The enhancement in da/dN occurred immediately after starting the test because the preferential dissolution around the eutectic spheroidal silicon was not dependent on ΔKi, or f. Corrosion fatigue cracks were apt to propagate along eutectic spheroidal silicon, as compared with that in air, and this behavior was independent of f.

Fatigue of spot-welded lap joints

The fatigue properties of spot-welded lap joints under a constant mean load made from 1.2 and 3 mm sheet thickness stainless steel with one, two or three-spot welds in series are reported. A log plot of cyclic load range versus fatigue life shows that for given sheet thickness and fixed load range, fatigue life increases with the number of spot welds. Oil has a beneficial effect by increasing the fatigue life of the welded joints. A fracture mechanics analysis is carried out on the data by treating the spot weld as a crack. A log plot of initial stress intensity factor range versus fatigue life is a straight line which is independent of the number of spot welds. However, increasing the sheet thickness shifts the straight line upwards in the log plot, thus indicating a longer fatigue life for the same applied initial stress intensity factor range.