Fatigue Strength of Welded Joints Under Multiaxial Loading: Comparison Between Experiments and Calculations

Abstract

In reality most welded components are loaded with a combination of different variable forces and moments which often cause a state of multiaxial stress in the fatigue critical areas of the weldment. If the multiaxial loading is nonproportional, conventional hypotheses are not able to give a satisfying lifetime prediction. This investigation is a cooperation of three German research institutes to build an experimental database for the verification of different concepts of lifetime prediction. In accordance with former investigations, a flange-tube connection made of steel P460 is used. The test program is divided into constant amplitude and variable amplitude tests. The ratio between the nominal bending and shear stress is 1. The ratio between the local shear and normal stress at the critical point is 0.6. For the variable amplitude tests, a Gaussian standard is used. A lifetime prediction software for multiaxial state of cyclic stress was developed. The software has a modular structure and allows calculations with different hypotheses and methods. The calculations are based on the local elastic stresses. This is an acceptable method for high-cycle fatigue. The failure criteria “critical plane approach” and “integral damage approach” are used in the software. Lifetime predictions for the flange-tube connection were performed using various hypotheses and concepts. In this work, one type of calculation, the integral approach with elementary Miner's Rule and damage sum D = 1, is illustrated. The calculations correspond well with the experimental results.

Multiaxial fatigue of welded joints under constant and variable amplitude loadings

“…Although uniaxial variable amplitude bending and variable amplitude torsion tests reveal different real damage sums ( D real,b = 0.08, D real,t = 0.38) the value of the real damage sum for cumulative multiaxial fatigue assessments was found to be D real = 0.35–0.38. However, a systematic investigation of multiaxial random fatigue, as it is presently performed, 2 , 25 , 26 is necessary, because the real damage sum obtained cannot yet be generalized. Despite conservative recommendations concerning the use of existing knowledge for the assessment of multiaxial random loading of welds, validation tests must be carried out in critical cases.…”

“…9), and then extended to multiaxial random loading. Further research projects 25 , 26 have also investigated these topics.…”

“…Although not all necessary clarifying knowledge from research presently performed on multiaxial random fatigue 2 , 25 , 26 is yet available (i.e. the obtained real damage sums for multiaxial random loading cannot yet be generalized) some recommendations can be given for the dimensioning of welded structures against such complex loading situations.…”

Lebensdauer von Schweißverbindungen unter mehrachsigen Belastungen mit variablen Amplituden - Schadensakkumulation und Hypothese der wirksamen Vergleichsspannung

“…(Diese Frage wird auch in einer kurz vor dem Abschluû stehenden und von der Deutschen Forschungsgemeinschaft (DFG) unterstu Ètzten Gemeinschaftsarbeit vertieft [10,11,12].) Im folgenden werden die Ergebnisse des zweiten EGKS-Projektes [7] im Hinblick auf die Schadensakkumulation und Spannungskonzentration sowie die Bewertung des mehrachsigen Span-nungszustandes mit Hilfe der WVS unter zufallsartiger kombinierter Belastung dargestellt und diskutiert.…”

“…8. Fatigue test results obtained with as-welded specimens under pure torsion diejenigen in[10,11,12] zur Ermittlung der tatsa Èchlichen Schadenssummen der nicht bearbeiteten und bearbeiteten Flansch-Rohr-Schweiûverbindungen, unter einachsiger Zufallsbelastung durch Biegung bzw. Torsion sowie anschlieûend unter u Èberlagerter mehrachsiger Zufallsbelastung durchgefu Èhrt.Tabelle 1 und Abb.…”