Fatigue-cracked Specimens Research Articles

A new ultrasonic angle-beam technique for sensitive evaluation of closed cracks

This paper is on the ultrasonic nondestructive evaluation of smaller cracks in the size range of 1–7 mm. Sensitive evaluation of the crack that was originally present or that developed during operation is imperative for assessing the integrity of structural components. In order to develop a method for the enhancement of sensitivity of ultrasonic evaluation of closed cracks, the principle of a new angle-beam approach is proposed, where the use of an oblique longitudinal wave with a small angle of incidence upon the specimen surface is emphasized. The crack treated exists nearly vertically on the back surface of the specimen. In contrast with the normally incident wave beam, the effect of angular incidence on the ultrasonic response due to a crack is investigated in an attempt to predict the optimum angle of evaluation. The sensitive feature of the present angle-beam technique is discussed in light of comparison made with the usual methods of normal incidence and large-angle beam technique. Ultrasonic measurements performed on the closed fatigue-cracked specimens show that the method offers bright prospects for the sensitive evaluation of small, closed cracks.

Cyclic fatigue behavior and fracture toughness of silicon nitride ceramics sintered with rare-earth oxides

The cyclic fatigue behavior and fracture toughness of silicon nitride ceramics sintered with selected rare-earth oxides were investigated to determine whether crystallization of grain boundaries affected room temperature crack-growth behavior under cyclic and monotonic loads. Stable crack propagation under cyclic loads was observed in all compositions, and shown to be a distinct cyclic fatigue phenomenon rather than a manifestation of stress corrosion cracking in ambient air. Extensive wear debris on the fatigue fracture surfaces of the rare-earth oxide specimens provided evidence of a frictional wear crack-growth mechanism. Compact-tension fatigue tests yielded crack-growth rates which were power law dependent upon the applied stress-intensity range, with exponents between 15 and 31. Subsequent fracture toughness testing of fatigue-cracked specimens yielded values of the critical stress intensity K c of ∼ 9 MPa✓m . Comparison of fatigue crack-growth rates and fracture toughness values obtained in this study with those for a reference commercial silicon nitride (with amorphous grain-boundary phases) indicated that crystallization of grain boundary phases does not appear to degrade cyclic fatigue resistance or fracture toughness. This observation may be explained by considering that impurity-stabilized amorphous regions tend to be preserved at interfaces between adjacent silicon nitride grains and between silicon nitride grains and crystalline grain boundary pockets. Since these interfaces form the preferred crack propagation paths in both stable and fast fracture, the immediate crack tip environment may not be affected by crystallization of the bulk grain boundary phases.

単一過大ひずみ負荷による塑性疲労表面き裂の加速・遅延挙動

The surface crack propagation behavior after single overstraining during constant amplitude tests in low-cycle fatigue was studied by using the small holed and through-crack specimens of low carbon steel. The tests were carried out with various kinds of overstrain waveforms and ratios.The results obtained are summarized as follows;(1) During the subsequent crack growth after overstraining the retardation-after-acceleration phenomenon was observed on the surface fatigue crack specimen in the both tension and compressiontension waveforms as well as on the through-crack specimen, and the duration of acceleration for the small holed specimens was longer than that of the through-crack one. The acceleration-after-retardation phenomenon was also observed in the both compression and tension-compression waveforms.(2) The durations during acceleration and retardation increased linearly with increasing overstrain ratio.(3) The stress amplitude after overstraining differed in the tension side and the compression side in such a way that the compression side was larger when the final peak of overstrain was in the tension side, whereas the tension side was larger when in the compression side. But the difference between the stress gradually decreased with increasing cycling, and approached to the saturation value of the constant amplitude tests. In the case of the tension waveform, the decreasing duration was almost the same as the duration during accelation of crack propagation.(4) The value of (ΔJhmax/2σy), evaluated by the J-integral range at the tension side overstraining, was related to the affected retardation and acceleration crack lengths as given by ld/2, la/2=α(ΔJhmax/2σy).

Fatigue properties of specimens similar to components deep rolled under optimized conditions

A method is presented for determining the rolling load that produced optimum fatigue strength improvement in deep-rolled specimens. Based on the surface stresses calculated using Hertzian theory and von Mises distortion energy failure criterion, the method yields an equivalent stress that is shown to be a suitable parameter for describing deep-rolling conditions. In addition, fatigue tests have been carried out on deep-rolled smooth and notched specimens of 37CrS4 steel. Specimens with optimized deep rolling are found to attain higher bending fatigue strengths than deep-rolled smooth samples, thus fully removing the notch effect. The greater fatigue strength improvement observed for notched samples is explained on the basis of fatigue behaviour. Adequate static strengths are demonstrated by fatigue-cracked specimens, particularly for ductile material conditions.

Fracture Toughness of Hot-Pressed Beryllium

Abstract This paper presents the results of an investigation into the fracture toughness, sustained-load flaw growth, and fatigue-crack propagation resistance of S200E hot-pressed beryllium at room temperature. It also reviews the literature pertaining to the influence of various factors on the fracture toughness of hot-pressed beryllium determined using fatigue-cracked specimens.

Fatigue-crack-tip plastic strains by the stereoimaging technique

The stereoimaging technique is an accurate, high-resolution means of measuring the in-plane displacements resulting from the deformation of a specimen so that the corresponding components of the strain tensor can be computed independently of the stresses. The example used in this paper is a fatigue-cracked specimen of a microscopically homogeneous experimental powder-metallurgy aluminum alloy, analyzed to determine the displacement and strain fields accompanying the opening of the fatigue crack. The displacement measurements are processed by a computer program which compensates for measurement fluctuations in the displacement data by smoothing, and derives the strain magnitudes. The principal strains and the maximum shear strain are determined using Mohr's circle, and the latter strain is then used to estimate the plastic-zone size. The crack-opening mode may be inferred from the displacement map, and the state of stress (plane stress or plane strain) inferred by applying the in-plane compatibility equation.

Effect of austenitizing temperature on toughness of martensitic steels

"Effect of austenitizing temperature on toughness of martensitic steels." Metals Technology, 3(1), pp. 208–209

Notch-Ductility Transition of Structural Steels of Various Yield Strengths

The notch-ductility transition of six structural steels, A36, ABS-Class C, A302-Grade B, HY-80, A517-Grade F, and HY-130, ranging in yield strength from 36 to 137 ksi, was studied with the use of 5/8 and 1 in. dynamic-tear (DT) test specimens. The results were compared with previously published data for V-notch and fatigue-cracked Charpy tests and dynamic fracture-toughness (KID) tests. Energy, lateral-contraction, and fracture-toughness values were compared. The results of this study showed that the full-shear upper energy shelves in the Charpy V-notch and DT specimens are the products of constant average plastic energy densities for each steel and the plastic volume estimates for the fracture of the different specimens. The transition from ductile to brittle fracture behavior is essentially the same in the fatigue-cracked Charpy and DT specimens since, for each steel, the same lateral contraction was measured in each specimen broken at a given temperature. This lateral contraction increased exponentially with temperature until a full-thickness shear fracture developed. However, the maximum lateral contraction increased with increased test-specimen thickness, suggesting that the Kc values corresponding to full-shear fracture should also increase with thickness. Using the proportionality found between the lateral contraction and the values of KID2/σYDE for the brittle-fracture behavior of these steels, the Kc values are estimated to be as much as 4.5 times greater than the KIc values at the same temperatures. In general, the notch-ductility transition can best be quantitatively characterized by the lateral contraction through KID and Kc values, whereas upper shelf energies are related by constant plastic energy densities and plastic volumes which develop during fracture.

Kic Transition-temperature behavior of A517-F Steel

Linear-elastic fracture mechanics has been widely used to obtain K ic values on very-high-strength steels (yield strengths > 200 ksi) that generally do not exhibit a ductue-to-brittie transition in failure mode as a function of temperature. However, as the use of the K ic test approach is extended to those steels that do exhibit a ductile-to-brittle transition, information on the K ic transition-temperature behavior of steels is required. Therefore, to establish general relations between K ic and Charpy test results, slow-bend K ic fracture tests and various Charpy tests were conducted on A517-F steel at temperatures between −320 and +80°F. The results indicated that a plane-strain K ic température transition does exist for A517-F steel. Furthermore, this transition occurred in the same temperature range (−150 to −50°F) as the transition denned by slow-bend Charpy test results for fatigue-cracked specimens. In both the K ic tests and the Charpy tests, the transition-temperature behavior appeared to be related to a gradual change in the microscopic fracture mechanism. The upper shelf, as denned by slow-bend Charpy tests, appeared to be a region in which K ic values cannot be obtained, regardless of specimen geometry, because of general yielding and crack blunting. A procedure is proposed in which the dynamic K ic behavior of a material can be predicted from static K ic test data by shifting the static K ic values along the temperature axis by the same amount as the static Charpy energy values are shifted by impact testing. In general, the results of this investigation have demonstrated that a transition in K ic behavior of A517-F steel does exist as a fution of temperature and that that transition is independent of the K ic to K c stress-state transition.

Static and Dynamic Low-Temperature KIc Behavior of Steels

To determine the crack-toughness behavior of steels subjected to high strain rates, a study was undertaken to establish the effects of strain rate and temperature on the KIc-values of steels. As an initial step, a procedure was developed to obtain dynamic KIc-values of steels by impact loading a fatigue-cracked bend specimen in a drop-weight machine. Steel plates having widely different yield strengths (40, 140, and 250 ksi) were studied initially to establish testing techniques. The results indicate that the procedures and analysis developed were satisfactory for determining static and dynamic KIc-values. For the 250 ksi yield strength steel [18Ni(250) maraging] the KIc linearly increased with increased temperature over a 400 F temperature range with no significant effect of strain rate. The strain rate had no effect on the KIc-values of the 140 ksi yield strength steel [HY-130(T)] between −320 and −150 F. However, in the 1-in-thick specimens the increase in strain rate which occurred in the dynamic test increased the temperature range for which plane-strain crack extension occurred. The KIc behavior of the 40 ksi yield strength steel (ABS-C) showed a marked sensitivity to strain rate with the same crack toughness being measured dynamically at a temperature 190 F higher than for the static test. Correlations of ABS-C steel KIc-values with its yield strength and a rate parameter, T In A/ε demonstrated the equivalency between decreasing temperature and increasing strain rate as influencing the crack toughness. The agreement of an estimate of KId from the dynamic yield strength at the NDT with the measured dynamic KIc-value showed that the six orders of magnitude increase in strain rate for this dynamic test appears sufficient to give KIc-values within 10 percent of the crack-toughness value estimates for a running crack in rate and temperature-sensitive steels. In general, the results of this investigation showed that a dynamic KIc test has been developed which can be used to establish the strain-rate sensitivity of the crack toughness of steels as well as corresponding design information.

The Static and Low Cycle Fatigue Strength of Notched and Fatigue-Cracked Specimens of Low Alloy Steels

Two kinds of low alloy steel of about 100kg/mmmm2 in tensile strength were put to tests: their specimens, notched and unnotched respectively as well as fatigue cracked, to the fatigue test of low pulsation cycle of tensile, and their fatigue cracked specimens in the range of 16 to 60mm in diameter to the static tensile test. The results of the tests are given as follows.(1) The fatigue strength of the unnotched specimens scarcely drops lower than their tensile strength till the pulsation of 104 cycles is reached, whereas of the specimens, both notched and cracked, the static strength, though higher than their tensile strength, decreases linearly with increasing number of pulsation cycles.(2) Fatigue strength of notched specimens after inititating crack on notch root coincides with that of fatigue cracked specimen.(3) Critical stress intensity factor KIC is constant independently of apecimen size and crack depth, when the external diameter of specimens is above 25mm.(4) Thefi nal fracture of low cycle fatigue occurs in the fatigue cracked specimens, within the diameter range of constant KIC, when their stress intensity factor KI reaches KIC. But when the diameter of are below this range the fracture condition of the low cycle fatigue does not coincide mechanically with that of Fracture Mechanics.

Stress-Corrosion Susceptibility of High-Strength Steel, in Relation to Fracture Toughness

The stress-corrosion susceptibility of several ultrahigh-strength ferrous alloys is described on the basis of fracture-mechanics parameters. Two general conditions were considered: First, the susceptibility of a material at various levels of tensile yield strength and fracture toughness; and second, the susceptibility as a function of applied subcritical stress-intensity levels. Experimentally, a notched and fatigue-cracked bendbar specimen configuration was used. Under a subcritical load, fracture of the specimen occurs upon sufficient environmentally influenced crack extension. The susceptibility criterion adopted in this investigation was the time period to catastrophic fracture of the specimen. The results are evaluated in terms of strength level and fracture toughness, and the usefulness of the data generated is examined in terms of material evaluation and application.

Effect of Section Size on Notch Strength

The effect of section size on the room-temperature notch strength of H-11 steel, 2014-T6 and 7075-T6 aluminum alloys, and Plexiglas was investigated in three different series of notched cylindrical tensile specimens and notched rectangular bend specimens. The three test series permit the separation of the influences of the geometrical parameters, which determine the stress field near the base of the notch, on the notch strength as the size of the specimen increases. The notch strength decreases with increasing size for all series investigated. The decrease in strength is most pronounced for the sharp machine-notched and fatigue-cracked specimens but it is also observed, to a much smaller degree, in the other series of notched specimens for all materials. An analysis of the results of this study indicates that the loss in strength with increasing section size is due to at least two factors: An increase in stress concentration factor and an increase in critically stressed volume with increasing size. With a superposition of stress concentration and volumetric effects, a loss in strength with increasing size greater than that predicted by existing fracture concepts for a brittle material may be anticipated and was actually observed for sharply notched H-11 steel specimens. Insight into the size effect and experimental scatter observed in notched specimens of real materials is gained from a consideration of the behavior of a model of an inhomogeneous material in the presence of external notches. The constant-fracture stress concept applied to this model yields a size-effect prediction on notch strength and an expression for experimental scatter in terms of the interflow spacing and the notch root radius.