Smooth Notch Research Articles

Application of continuum damage mechanics in fatigue assessment of A516 steel specimens considering residual stresses

The present study evaluates the effects of tensile residual stresses on fatigue behaviour of ASTM A516 pressure vessel steel specimens. In this regard, fatigue specimens containing a smooth notch region are studied. A damage coupled elastic-plastic constitutive material model is developed to investigate effects of residual stresses on fatigue life. Isotropic and kinematic hardening parameters of the material are experimentally determined. Additionally, the damage model parameters are specified from fatigue experiments conducted on standard specimens in different stress ratios. Tensile residual stresses are introduced into the smooth notched specimens through employing four-point-bending method. The hole drilling technique is utilized to measure the residual stress magnitudes. The results indicate that about two third of fatigue life of the specimens is decreased due to the existed tensile residual stresses. Furthermore, fatigue crack initiation and propagation behaviour in the specimens containing residual stress are studied based on the developed continuum damage model.

Theoretical local buckling and bulging analysis on miniature buckling-restrained brace with corrugated core bar

An all-steel miniature buckling-restrained brace with corrugated core bar, termed as CMBRB, which consists of a metallic core bar with smooth notches and a circular restraining tube, was proposed, and it is proved to have enhanced ductility and stable hysteretic behaviors under seismic events. In this study, 12 specimens were tested under cyclic loading to investigate effects of slenderness ratio, segment spacing-to-notched cross section height ratio and Teflon spacers on mechanical behaviors of CMBRBs, and the first 2 parameters were found to have great effects on strength, stiffness, energy dissipation and ductility, while the spacers contribute no significant effect. 3 different buckling cases were defined for CMBRBs with different core bar geometries, and corresponding theoretical buckling analyses were conducted to obtain lateral thrust generated by contact between core bar and restraining tube. The proposed buckling models have high accuracy as the relative error between theoretical and numerical results is only 3.9%, and it was found that CMBRBs are more susceptible to local bulging failure than conventional miniature buckling-restrained braces. Finally, a simple design procedure was developed to prevent local bulging failure of CMBRB, and the proposed design criterion showed good agreement with both experimental and numerical results in this study with a safety factor of 1.13.

Stress-Strain State Of Elastic Plate With An Arbitrary Smooth Notch

Abstract The paper contains comparing calculations of the stress fields in an elastic plate with notch along the arc of a circle, ellipse or parabola obtained by analytic method based on complex Kolosov-Muskhelishvili potentials and by numerical variation-difference method. These fields differ by no more than 2%, which, in particular, indicates the reliability of such numerical implementation. This discrepancy can be explained by the fact that in the analytical solution domain is unbounded, while the numerical calculation was carried out, obviously, for a finite field. The given stresses at the top of the notch along the arc of an ellipse or a parabola significantly increase with increasing of the relative depth of the notch (while increasing its depth or decreasing width).

Microstructure and Strain-Based Fatigue Life Approach for High-Performance Welds

Microstructure and pre-existing surface flaws in smooth notch geometries significantly affect the fatigue life of welded joints. Traditionally, a welded joint is assumed to incorporate crack-like defects and the crack propagation dominates the total fatigue life. For a smooth weld notch geometry, the macro crack initiation period becomes more significant, and this difference cannot be modelled with the existing stress or fracture mechanics ‑based approaches. In this paper, a microstructure and strain ‑based fatigue life approach is presented. In the approach, the fatigue damage process is modelled as a repeated crack initiation process within a material volume related to the microstructure. The novelty of the developed approach is that the size of the damage zone is defined from the grain size statistics without using fracture mechanics. The approach is able to consider the changes in the stress gradient, stress triaxiality and plasticity during the fatigue crack initiation and growth. The developed approach has been validated with experiments on submerged-arc and laser-hybrid welded joints. The predicted fatigue life, crack growth path and rate showed good agreement with the experiments. For a welded joint with smooth and favourable notch shape, the short crack growth, i.e. macro crack initiation period is dominant and it has a significant influence on the fatigue life.

Strain-based approach to fatigue crack initiation and propagation in welded steel joints with arbitrary notch shape

In this paper, a novel strain-based approach for the fatigue strength modelling of welded steel joints is introduced. The actual weld notch geometry and the variation in the microstructure characteristics of the material are considered, and thus, the approach enables the fatigue crack growth simulation from the crack initiation to the critical crack length before the final fracture. The predicted fatigue strength is in line with the experimental results. By considering the crack tip plasticity and stress triaxiality, the approach is able to describe the different crack growth periods of the fatigue life: the short crack, long crack, and tearing-related long crack growth periods. For a welded joint with a smooth notch shape, the short crack growth period is observed to be dominant and to have a significant influence on fatigue life.

Effect of the Conditions of Laser Hardening on the Initiation and Propagation of Fatigue Cracks in Notched Specimens

UDC 620.194 Laser hardening is widely used for the strengthening of elements of machines and mechanisms, first of all, in order to enhance their surface hardness, wear resistance, and resistance to fatigue fracture. The effect of laser heating on changes in the structure and properties of metals and alloys was studied mainly on smooth specimens and specimens with smooth notches or fillet junctions. At the same time, the efficiency of laser hardening of articles with acute notches or cracks has seldom been considered [1, 2]. In this case, laser hardening does not always lead to the desired results because, as a rule, a clearly pronounced structural inhomogeneity appears in the zone of stress concentration, and, hence, the fatigue characteristics of an article depend on the spatial location of the crack tip with respect to locally hardened metal and the residual thermal stresses arising in the course of hardening. In the present work, we consider the effect of laser hardening on the time to fatigue crack initiation in specimens with acute notches and on the rate of crack growth depending on its location relative to the zones of hardened low-alloy chromium-molybdenum-vanadium steel.

A numerical study of the elastic stress field arising from sharp and blunt V-notches in a SENT-specimen

The elastic stress field arising from a V-notch in a Single Edge Notched Tension (SENT) specimen is studied using the Sherman–Lauricella integral equation. Accurate values of the generalized stress intensity factor, here denoted QI, are determined and compared with values from the literature. A QI-dominated distance ahead of the notch tip is defined and determined for several notch angles and different notch depth to specimen width ratios. It is found that for depth to width ratios over 0.45 the QI-dominated distance is approximately constant while below this value the distance can show a great variation with the notch angle. Additionally, a general formula for the maximum stress in a V-notch with a finite root radius is derived. Its applicability on the SENT-specimen is studied and presented as a family of curves. If used within these curves, the formula can accurately predict stress concentrations, even for very smooth notches.

An experimental-analytical hybrid model for the elasto-plastic stresses at a crack

The primary obstacle preventing the analytical determination of physically sensible stresses at a crack tip is the presence of a mathematical singularity there. This singularity is best known in its elastic form; however it persists even in elasto-plastic crack-tip stresses. To overcome the difficulty we adopt the following strategy: we attempt to capture initial elastic stresses experimentally, than track subsequent elasto-plastic stress distributions analytically. We infer a finite stress at a crack tip from the experimental behaviour of cracked specimens at fracture when the specimens are made of a truly brittle material. Given a size-independent result, we argue that the crack-tip stress at fracture must equal the ultimate stress for such a material; thus dividing by the applied stress at the same point gives a measure of the stress concentration factor, K T . The approach is checked for size independence and against hole configurations with known theoretical, yet physically reasonable, K T . Then the effective experimental K T are taken as inputs for the second phase of the study in which we model the crack as being a smooth notch having the same stress concentration factor as found experimentally. In this way our configuration initially shares the same stresses at the crack tip as we inferred physically. Next we track effects of incremental plastic flow on a set of finite element grids. Satisfactory resolution in return for modest computational effort is obtained by employing a substructuring method. The accuracy in both the elastic and the elasto-plastic regime is checked against trial problems with exact solutions. Thereafter, physically interpretable stress distributions ahead of the crack are determined for a range of materials and for varying load levels.