Nonproportional Fatigue of Welded Structures

Abstract

Bending and shearing stresses in many structures are out-of-phase. A typical example is the evaluation of a fixed position in a bridge beam as a moving load crosses it. Traditional fatigue design approaches for welded structural details are generally based on inphase laboratory data. Thus the effect of nonproportional stress histories on the fatigue life of welded structural details is usually not considered. Stress-relieved tube-to-plate weldments were subjected to multiaxial in-phase and out-of-phase stress states using combinations of bending and torsion. The fatigue test results were evaluated using three local stress damage criteria modified for multiaxial loading. Finite element techniques were used to calculate the local stresses and strains. The local shear-stress based approach of Findley provided the best correlation of all test data for the loading paths considered. The unique aspects of nonproportional loading are reflected by the Findley model fatigue damage calculations, and the test data were correlated to within a factor of two. Since the scatter in the experimental data was also a factor of two for identical tests, further improvements in predictive ability are not possible. Also, the predicted direction of fatigue damage for the Findley model coincided with surface cracking characteristics.

Fatigue resistance of pipe-to-plate welded joint under in-phase and out-of-phase combined bending and torsion

“…R ¼ À1 and R ¼ 0). As opposite to what reported in [3,4,[8][9][10] failures mainly originated from the weld root where a severe notch, typical of fillet welding, was present (only few failures from the weld toe were observed in some of bending tests). In [15] it was observed that for the examined joint the load ratio had a significant effect on the fatigue endurance only in case of bending loading, while no appreciable effect was present in case of torsion loading.…”

“…As reported in many papers (see e.g. [2][3][4]), the use of the maximum principal or the equivalent stress range can overestimate the fatigue life of more than an order of magnitude if applied to the case of non-proportional loading.…”

“…In the present paper, the fatigue resistance of a typical pipe-to-plate welded joint was investigated under in-phase and out-of-phase multiaxial loading conditions. The fatigue resistance of similar joint with fillet welding under pure and combined tension and torsion has been discussed in [3,4,8], while the fatigue resistance of a similar joint with bevel butt weld has been discussed in [9,10].…”

“…It is shown that design methods based on principal stresses or equivalent stresses were not adequate to predict the fatigue life under combined out-of-phase loading and how their use can be unsafe. In [4] the results of a series of tests under in-phase and out-of-phase combined bending and torsion were analyzed by three different local stress damage criteria: max principal stress range, von Mises equivalent stress range and Findley's local equivalent shear stress range. Local stresses were obtained by FE analyses.…”

Stress-Based Multiaxial Fatigue Analysis

“…As a time instant progresses from A to M, the maximum principal stress (σ 1 ) axis rotates with respect to the local x axis in an angle of θ, as illustrated in Figures 5.2 through 5.4. Nonproportional loading causes equal (Archer, 1987), or present more damage (Siljander et al, 1992;Sonsino, 1995) than, proportional loading, based on the same von Mises stress range. The cause of this phenomenon has been explained by an additional nonproportional strain hardening due to slip behavior of the material (Itoh et al, 1995;Socie & Marquis, 2000).…”