Search

In this study, we investigated long fatigue crack growth perpendicular to a soft-hard material interface comprising the same Young’s moduli but different yield strengths. We determined the degree of fatigue crack growth retardation attributable to the interface under the constant stress intensity factor range (Δ K ) using the plasticity-induced crack closure analysis with the Dugdale model. We subsequently determined the threshold stress intensity factor range (Δ K th ) under a constant stress amplitude (σ a ). Under a constant Δ K , the retardation degree was primary dependent on the two materials’ yield strength ratio. Under a constant σ a , Δ K th was dependent on both the yield strength ratio and the distance between initial crack tip and interface. [ABSTRACT FROM AUTHOR]

An investigation was conducted into the influence of statically-applied, mode I, crack-opening load on the threshold condition for propagation of a shear-mode fatigue crack in a bearing steel. Torsional fatigue test was carried out at an R of −1 using a hollow cylindrical specimen into which a semi-elliptical, small slit was axially introduced. A static axial compressive stress was simultaneously applied to suppress crack branching. A coplanar, shear-mode, non-propagating fatigue crack emanating from the slit was attained by appropriate control of shear stress amplitude. Internal pressure was then applied to generate a hoop stress as a static crack-opening stress, σ θ static . Consequently, the threshold shear-mode stress intensity factor range, Δ K τth , was significantly decreased with increase of the static mode I stress intensity factor. To further understand the contribution of σ θ static to the reduction in Δ K τth , microstructural observations for the cross-sections of a non-propagating crack were conducted using a scanning electron microscope in conjunction with the electron backscatter diffraction analysis. The results revealed that the excess loading of σ θ static accounts for the change in the crack-path, resulting in a further reduction in Δ K τth . [ABSTRACT FROM AUTHOR]

• Fatigue crack growth of hydrogen-charged 17–4 PH steel was investigated. • Threshold stress intensity factor range was not degraded by hydrogen. • Frequency dependence of crack growth acceleration by hydrogen was predicted. • A newly model was proposed as a substitute for ordinary acceleration model. • Its model incorporated an interaction of cycle- and time-dependent crack growth. Effect of hydrogen on the fatigue crack threshold and fatigue crack growth (FCG) behavior of 17–4 PH steel was investigated using a circumferentially notched specimen with a stress concentration factor of 6.6 precharged in a 100 MPa hydrogen gas environment. Fatigue life tests were performed in air at room temperature at a stress ratio of −1 and test frequencies of 0.001–10 Hz. Although the FCG rate was accelerated, it was discovered that the fatigue crack threshold was not degraded by the hydrogen. The H-induced FCG acceleration ratio was predicted by a new model considering cycle- and time-dependent FCGs. [ABSTRACT FROM AUTHOR]

Abstract: The stress and displacement fields near the bonding edge, sharp notch, and contact edge show singularity behaviors, so methods of evaluating the strength of these points using maximum stresses calculated by a numerical stress analysis, such as the finite element method, are generally not valid. We have previously presented a new method of evaluating the strength of these singular points using two stress singularity parameters H and λ. The difficulty with this method was in obtaining the critical value of the intensity of the stress singularity parameter H c for each order of stress singularity λ. Then we have developed a method of formularizing critical value of stress singularity parameter H c for each order of stress singularity λ by utilizing critical distance stress theory. Firstly we apply this method to the delamination strength evaluation. The estimated delamination criteria H c(λ) agrees well with the experimental results. Then we apply this method to general sharp notch corner and contact edge fatigue problems. In these cases the fatigue-crack initiation criteria can be derived from two typical strength parameters, namely, fatigue limit σ w0 and threshold stress intensity factor range ΔK th. These estimated critical H th(λ) value agrees well with the experimentally measured value. Using this simple critical distance stress approach we estimated the fretting fatigue-crack initiation criteria for any contact edge angle and optimized the contact edge geometry. Moreover, we can apply this new strength criterion to stress singularity fields for general stress-concentration structures. [Copyright &y& Elsevier]