Fatigue notch factor and short crack propagation

This paper addresses the problem of high cycle fatigue at notches and the role of short crack propagation in the fatigue notch factor k_f. Ahead of a V-notched feature, the stress field is characterized by two parameters, i.e. the stress concentration factor k_t and the normalized notch stress intensity factor k_n. Whether fatigue strength at a given life is controlled by crack initiation (k_f = k_t) or by short crack propagation (k_f < k_t) depends on k_t, k_n and the material resistances to crack initiation and to short crack propagation. The analysis accounts for the effects of notch acuity, notch size, material and fatigue life on the fatigue notch factor k_f. It opens the door to a new method for predicting fatigue life using two S-N curves for a given material; one being measured from a smooth specimen, the other from a severe V-notch.     

Fatigue of V-notched members: Short crack behavior and endurance limit

A previous experimental investigation showed that the short crack behavior in severe V-notched geometries is essentially characterized by an initial transient variation of the crack opening level within the notch plastic zone. This result is exploited to derive a general formulation of the endurance limit of V-notched members. Analytical solutions for the stress intensity factor and the notch plastic zone extent are obtained to compute the effective crack driving force ΔK_eff as a function of notch geometry, crack length and nominal stress. Then, the endurance limit is determined as the nominal stress below which an initiated crack becomes non-propagating, i.e. by equating the minimum ΔK_eff value at the end of the plastic zone to a threshold value. By using Irwin's redistribution principle which allows us to make relations between the V-notch elastic and plastic fields, the following results are obtained. The endurance limit can be computed only from the V-notch “stress intensity factor” and the material threshold and cyclic yield stress. The endurance limit dependence on the threshold diminishes as the notch angle increases from zero and vanishes in the limit case of a smooth surface. When the notch contains a pre-existing crack, a Kitagawa-type endurance diagram is obtained. The uncracked notch endurance limit, the critical crack length and the slope of the decrease beyond this length, increase with increasing notch angle, and there is a total coincidence with the Kitagawa diagram in the limit case where the notch angle tends to 180°. This result shows that a global theory should govern at one and the same time the short crack behavior in smooth and severe geometries.     

FATIGUE LIFE PREDICTION OF WELDED JOINTS—A RE-ASSESSMENT

Abstract— Fatigue life prediction of welded joints needs an accurate and exhaustive theoretical Fracture Mechanics characterization of weld toe crack propagation. The method proposed by Albrecht et al. leads rapidly to accurate solutions of the LEFM δK-parameter. However, non-LEFM short crack behavior within the notch (weld toe) plastic zone must be taken into account. Available information on notch fatigue is surveyed, and practical cases where short crack growth is likely to occur are identified. Based on an elastoplastic finite element analysis, the LEFM validity limits and errors resulting from the misuse of LEFM in fatigue life prediction are quantified.     

EARLY DEVELOPMENT OF FATIGUE CRACKING AT MANUAL FILLET WELDS

Abstract— An experimental study within the Canadian Offshore Corrosion Fatigue Research Programme was performed on the early development of fatigue cracking along the wavy toe of manual fillet welds between structural steel plates. Stress relieved and as-welded cruciform joints were tested under R = −1 and R = 0 loading at different stress amplitudes. The depth and the opening level of cracks as small as 10–20 μm were monitored using miniature strain gauges installed along the toe apex, in combination with beach marking. Most of the "initiation life" (25% to 50% of total life), conventionally defined by a crack depth of 0.5 mm, is consumed in short crack propagation. Three types of short crack development for different combinations of local mean stress and stress range are identified and analyzed. Growth rates in as-welded specimens are faster than in stress relieved specimens, which results in shorter "initiation lives". This is associated with a higher effective stress range, particularly under R = - 1 loading where cracks are open over nearly the full stress range. The V-notch stress intensity factor is a promising parameter to rationalize the crack "initiation life". It takes into account the thickness effect experimentally observed. Under R = - 1 loading of as-welded joints, using R = 0 data and taking the whole stress range gives a reasonably conservative approximation of the crack "initiation life".     

Assessment of Mode I and Mode II Stress Intensity Factors Obtained by Displacement Extrapolation and Interaction Integral Methods

Journal of Failure Analysis and Prevention - A numerical study of the parameters influencing the accuracy of computed stress intensity factor (SIF) values is presented in this article. Mode I SIFs...     

MECHANICALLY SHORT CRACK GROWTH FROM NOTCHES IN A MILD STEEL

Abstract— Two L-notched specimens made of mild steel (average grain size =30 μm) and having root radii of 0.1 mm and 3 mm, and also a smooth surface specimen were cyclically loaded at different stress levels at R =−1 and at R = 0. A technique based on miniature strain gauges was successfully used to monitor the depth and the opening level of mechanically short cracks of depths from 0.015 mm to 0.5 mm. Three dimensional FEM computations were made to obtain appropriate calibration curves for varying crack aspect ratios and gauge eccentricities as well as notch plastic strain distributions. The fracture of L-notched specimens having a root radius of 0.1 mm was characterized by an early and multiple crack initiation phase (defined by a crack depth of 30 μm), and the short crack growth rates showed a mechanical behaviour different from that of long cracks (large discrepancies at the same Δ K -value, crack deceleration at R =−1 even beyond the notch plastic zone). For smooth surface specimens both the initiation and the propagation of a single short crack represented important fractions of the total life; the short crack growth rates were high and continuously increasing. The notch influence was highly reduced when the stress singularity is truncated by a 3 mm radius. The cracking behaviour was, in several aspects, close to that at smooth surfaces. The evolutions of crack closure were analyzed in each condition (transient decrease and stabilized value of the closure ratio U =Δ K _eff/Δ K ) and were shown to have a strong influence on short crack growth. Most of the short crack growth rates obtained in the various geometry/loading conditions are well consolidated with LEFM long crack growth rates using the Δ K _eff parameter.