Effective Threshold Stress Intensity Range Research Articles

The effects of thermo-mechanical processing on fatigue crack propagation in commercially pure titanium with a harmonic structure

Commercially pure (CP) titanium having a bimodal harmonic structure, in which coarse grains are enveloped by a network consisting of fine grains, was obtained by consolidating mechanically milled powders to enhance both strength and ductility. We have examined the fatigue properties of materials having harmonic structures to achieve sufficient performance for practical applications in the engineering fields. In this study, thermo-mechanical processing (TMP) was performed for the CP titanium having a bimodal harmonic structure to improve its fatigue properties. Four-point bending fatigue tests and fatigue crack propagation tests were conducted under ambient conditions to investigate the fatigue crack initiation and propagation in CP titanium having a harmonic structure following TMP. Applying TMP to CP titanium having a harmonic structure increased its fatigue life due to greater resistance to the initiation of fatigue crack as a result of grain refinement. In contrast, CP titanium having a harmonic structure shows lower threshold stress intensity range, ΔKth, values following TMP as a result of reduced the degree of crack closure and lower effective threshold stress intensity range, ΔKeff,th, values due to grain refinement.

Effect of TiB Orientation on Near-Threshold Fatigue Crack Propagation in TiB-Reinforced Ti-3Al-2.5V Matrix Composites Treated with Heat Extrusion.

TiB-reinforced Ti-3Al-2.5V matrix composites, in which TiB whiskers are oriented parallel to the direction of heat extrusion, were fabricated via mechanical alloying and hot isostatic pressing (HIP). To investigate the near-threshold fatigue crack propagation in TiB-reinforced Ti-3Al-2.5V matrix composites, stress intensity factor K-decreasing tests were conducted for disk-shaped compact specimens having two different orientations of TiB whiskers at force ratios from 0.1 to 0.8 under ambient conditions. The crack growth rates, da/dN, for the composites incorporating TiB whiskers oriented perpendicular to the direction of crack growth were constantly lower than those obtained in the case where the orientation was parallel at the same stress intensity range ΔK, while the threshold stress intensity range, ΔKth, was higher. This effect can be explained by the increase in the degree of roughness-induced crack closure resulting from the perpendicular TiB, because fatigue cracks preferentially propagated across the boundaries between the matrix and the TiB in certain regions. In contrast, the effective threshold stress intensity range, ΔKeff,th, for composites was unaffected by the TiB orientation at low force ratios.

Evaluation of Fatigue Properties under Four-point Bending and Fatigue Crack Propagation in Austenitic Stainless Steel with a Bimodal Harmonic Structure

Austenitic stainless steel (JIS-SUS304L) with a bimodal harmonic structure, which is defined as a coarse-grained structure surrounded by a network of fine grains, was fabricated using powder metallurgy to improve both the strength and ductility. Four-point bending fatigue tests and K-decreasing tests were conducted in air at room temperature under a stress ratio R of 0.1 to investigate fatigue crack propagation in SUS304L. The fatigue limit of this harmonic-structured material was higher than that of the material with a homogeneous coarse-grained structure. This is attributable to the formation of fine grains by mechanical milling and to the suppression of pore formation. In contrast, the threshold stress intensity range, DKth, for the harmonic-structured material was lower than that for the homogeneous coarse-grained material, while the crack growth rates, da/dN, were higher at comparable DK. These results can be attributed to a reduction in the effective threshold stress intensity range, DKeff,th, due to the presence of fine grains in the harmonic structure.

Evaluation of near-threshold fatigue crack propagation in harmonic-structured CP titanium with a bimodal grain size distribution

To examine near-threshold fatigue crack propagation in commercially pure (CP) titanium with a bimodal harmonic structure, which is defined as a coarse grained structure surrounded by a network of fine grains, K-decreasing tests were conducted. The crack growth rates for harmonic-structured CP titanium were higher than those for homogeneous material with coarse grains at comparable stress intensity range, while the threshold stress intensity range was lower. This phenomenon was attributed to a reduction in the extent of crack closure and the effective threshold stress intensity range, resulting from the presence of fine grains in the harmonic structure.

Fatigue thresholds of discontinuously reinforced aluminum alloy correlated to tensile strength

The fatigue strength of reinforced and unreinforced aluminum alloys with silicon carbide particles or whiskers increases in proportion to the ultimate tensile strength. The proportional constant is slightly larger for composites than for unreinforced alloys. The threshold stress intensity range Δ K th tends to decrease with increasing tensile strength. When compared at the same tensile strength, Δ K th tends to be larger for composites than for unreinforced alloys. The effective threshold stress intensity range, Δ K effth, is fairly constant irrespective of the tensile strength. The characteristic defect sizes, a ′ 0 and a 0, tend to be smaller with increasing tensile strength. When compared at the same tensile strength, a ′ 0 is larger for composites, which indicates a low notch sensitivity of composites.

A Comparison of the Near-Threshold Corrosion Fatigue Crack Propagation Rates in Mil S-24645 HSLA Steel and Its Weld Metal

Near-threshold corrosion fatigue crack propagation studies have been conducted using compact tension specimens of Mil S-24645 HSLA steel and a compatible weld metal in air, ASTM seawater at the free corrosion potential, and ASTM seawater with −0.8V and −1.0V (SCE) cathodic protection. Tests were conducted at 10 Hz and a stress ratio, R = 0.1. The weld metal exhibited slower crack growth rates and higher threshold stress intensity ranges in both air and ASTM seawater than the base metal. However, the weld metal itself exhibited similar behavior in air, ASTM seawater and under cathodic protection, giving a threshold stress intensity range of 10 to 12.7 ksi in1/2 (11 to 14 MPa m1/2). With the exception of the weld metal tested in air, after correction for crack closure, the effective threshold stress intensity range was reduced to the range 3.2 to 5.0 ksi in1/2 (3.5 to 5.5 MPa m1/2). Metal wedges, approximately 5 to 10 μm (1–2 grain sizes), which developed during the tests, were observed to contribute to crack closure in the −1.0V (SCE) cathodically protected specimens.

A CRACK CLOSURE MODEL FOR PREDICTING THE THRESHOLD STRESSES OF NOTCHES

AbstractA model is proposed to estimate the threshold stress range of a notched component. The model considers the variation of crack closure with crack length in the presence of a notch. The threshold stress range was found from the condition that the minimum value of effective threshold stress intensity range of a crack emanating from a notch equals the effective threshold stress intensity range of a “long” crack. The effects of notch depth, of notch acuity, of notch and specimen type, of load ratio, and of material properties on the threshold stresses were considered. Experimental data reported in the literature were used to assess the validity of the model. It was found that the model correctly predicts the behavior of cracks in notched components.

Near-threshold fatigue crack growth properties at elevated temperature for 1Cr-1Mo-0.25V steel and 12Cr stainless steel

Near-threshold fatigue crack growth properties were investigated for a low-alloy steel 1Cr-1Mo-0.25V and a stainless steel SUS403 (13Cr) in the temperature range from 25 to 550°C. Fatigue tests were conducted at frequencies of 0.5, 5, and 50 Hz, in a manner designed to avoid crack closure. The effective value of threshold stress intensity range increased with increasing temperature and with decreasing frequency for the Cr−Mo−V steel, whereas the effective threshold stress intensity range was independent of temperature and frequency in the case of the SUS403 steel. At a given ΔK value, the fatigue crack growth rates accelerated with increasing temperature and with decreasing frequency for the Cr−Mo−V steel. However, although the rate of fatigue crack growth was independent of frequency at a given temperature for the SUS403 steel, the rate did increase with temperature. The observed threshold levels and crack growth behavior were closely related to the oxidation process of the bare surface formed at the crack tip during each load cycle.

アルミニウム合金７０７５‐Ｔ６の微小疲労き裂成長挙動の応力比依存性 主として第Ｉ段階き裂成長過程に注目して

The growth behaviour of small fatigue cracks was investigated on an aluminum alloy 7075-T6 at the stress ratios R of 0, -1 and -2. The effect of stress ratio was discussed on the basis of the detailed observation with special interest in the stage I region of small crack growth.Cracks initiated at R=-1 and -Z, grew to a certain depth by the stage I growth mechanism, and the stage II crack growth followed. The stage I to stage II transition occurred under a constant maximum stress intensity factor which was approximately consistent with the effective threshold stress intensity range, ΔKeff, th, for large cracks. At R=0, on the other hand, the stage I crack growth was not observed because of crack initiation at inclusions.At all the stress ratios, small cracks grew more rapidly than large cracks in the same nominal stress intensity range, and grew below the threshold stress intensity range, ΔKth, for large cracks. Particularly, the stage I cracks showed much higher growth rate and grew below ΔKeff, th for large cracks. It is suggested that the stage II crack growth rate can be characterized in terms of the effective stress intensity range, while a micromechanics approach is successful to evaluate the stage I crack growth rate.

A notch stress-strain analysis approach to fatigue crack growth

A model of fatigue crack growth based on elastic-plastic notch stress-strain analysis has been presented. It was assumed that the fatigue crack can be analysed as a notch with the tip radius ρ ∗ . It was also assumed that the material was composed of elemental blocks of a finite linear dimension ρ ∗ . The fatigue crack growth was regarded as an increment of successive crack reinitiation over the distance ρ ∗ from the crack tip. The number of cycles N ∗ necessary to fracture the material over the distance ρ ∗ was calculated from the Manson-Coffin strain-life relation. The near-crack tip elasticplastic stress and strain were calculated on the basis of the elastic solution for blunt cracks and energy density criterion derived earlier by the author. Both the near-crack tip mean stress and the fatigue damage accumulation in the material beyond the process zone were included into the analysis. The calculations were based on effective stress intensity range. It was shown that the fatigue crack growth rate in the linear and near-threshold region of the da dN vs ΔK curve can be deducted on the basis of the proposed model and the smooth specimens material data. The effective threshold stress intensity range can be determined also. The theoretical results were compared with the experimental data obtained from three different alloy steels. Good agreement was achieved in both the near-threshold and linear range of the da dN vs ΔK curve.

Crack closure: An explanation for small fatigue crack growth behavior

The deceleration and/or acceleration of small fatigue crack growth rates with increasing stress intensity range, Δ K, can be explained by crack closure. Crack closure additionally appears to offer an explanation as to why some small cracks reach a point at which they cease to propagate. Moreover, the different growth rate behavior of small and large cracks can be rationalized by crack closure in both linear-elastic and elastic-plastic fracture mechanics. In linear-elastic fracture mechanics, it is suggested that there exists a unique effective threshold stress intensity range, Δ K th, eff , for both small and large cracks. The effects of load ratio, yield strength and microstructure on small crack growth behavior are predicted.

Effect of Overload on Fatigue Crack Growth Threshold

In order to investigate the effects of overload history on the behavior of crack growth threshold and the crack closure at the threshold, fatigue crack propagation tests under multi-overload and single overload of four different load patterns were carried out using 784MPa class high-tensile-strength steel bending specimens for the case of R=-1 and R=0.The results obtained were as follows;(1) For the multi-overload tests, the greater the overload stress intensity KOL was, the greater the threshold stress intensity Kthv was, both for the case of R=-1, and R=0.(2) As the result of single overload tests, in the case of R=0, the effect of the overload pattern appeared remarkably and the effect of the tension overload pattern was similar to that of multi-overload. However, in the case of R=0, the effect mentioned above was little.(3) The fatigue crack growth threshold was expressible only by the effective threshold stress intensity range ΔKeffth, independent of the load history tested in this investigation.

Near-threshold fatigue crack growth behavior in metals

Near-threshold fatigue crack growth rate (FCGR) behavior in six alloy systems (iron, aluminum, copper, magnesium, nickel and titanium) is extensively reviewed and compared. It is suggested that a unique effective threshold stress intensity range, ΔK th, eff , exists for each alloy system in room temperature air. The value of ΔK th, eff was found to be directly proportional to Young's modulus, E Furthermore, Young's modulus normalizes the near-threshold fatigue crack propagation behavior in the various alloy systems investigated. The present findings are consistent with the existing threshold FCGR theories that relate ΔK th, eff to E. Additional research is required, however, to incorporate crack closure phenomena in these near-threshold FCGR theories to assess the influence of microstructure, load ratio and environment.