Research On Fatigue Behavior Of High Strength Steel Undermatched Cruciform Welded Joints

With excellent mechanical strength,ductility and fracture resistance,high strength steel and ultra-high strength steel are widely used in the manufacturing of engineering structures,such as ships,automobiles,bridges and nuclear power.Due to the limitations of evenmatched weldments developing technology for high-strength steel,the related welded joints occur insufficient toughness or hydrogen-induced cracking,which leads to structural load-carrying capacity reduction.The undermatched austenite welding consumables with strong hydrogen absorption capacity are an effective way to solve the above problems.However,the yield stress of weldments is lower than the base metal.The research on the load-carrying capacity of undermatched welded joints under cyclic loading is limited,especially for Low Cycle Fatigue(LCF)and multiaxial fatigue under cyclic loading.Therefore,the fatigue behavior of undermatched high-strength steel welded joints has become an important part of the urgent need for explicit study of welded structural integrity design.On the other hand,the calculation of fatigue characteristic parameters greatly reduces the work efficiency of fatigue life assessment.It is also worthy to study to establish an effective analytical model for joint fatigue assessment.In this dissertation,the fatigue behavior of mismatched welded joints provides some feasible design schemes for quickly predicting joint fatigue life and improving joint fatigue strength from different perspectives.This study provides an important scientific basis for structural integrity assessment.The main contents include the following aspects: research on fatigue properties of materials in mismatched highstrength steel welded joint,HCF and LCF behavior of Load-carrying Cruciform Welded Joints(LCWJ),and multi-axial fatigue behavior of notched components.The effect of strength discrepancy on the LCF properties of the materials was systematically investigated.The low-cycle strain-controlled fatigue test under different amplitudes was carried out on 10CrNi3 MoV high strength steel and its mismatched weldments.The fatigue discrepancy between base metal and mismatched weldments was comprehensively evaluated according to different fatigue life prediction theory,such as strain-life curves,plastic strain energy,Coffin-Manson equation,SWT-fatigue life prediction model and the fatigue crack growth rates.The results show that the base metal and welds demonstrate the cyclic soften behaviors.According to the hysteresis curves of half fatigue life,the base metal shows the Masing fatigue behavior,while the undermatched welds shows the non-Masing behavior.Meanwhile,the undermatched welds exhibits better strain-controlled fatigue properties than base metal.The HCF behavior of undermatched CLWJ was discussed on the basis of notch mechanics and fracture mechanics.The effects of joint geometry on the notched stress intensity factor and average Strain Energy Density(SED)for different failure modes were studied.The analytical model of SED in CLWJ was established.The model can effectively reflect the characteristics of different failure modes and evaluate transformation relationship of different failure modes accurately.According to the fracture mechanics theory,different defect types and sizes were defined.The relationships between different fracture parameters,failure modes,geometric differences,defect sizes and joint fatigue strength were studied.The quantitative conditions for the evaluation of fatigue failure of CLWJ were determined.The variations of local mechanical responses under elastoplastic mechanics conditions considering geometrical characteristics and effective notch energy were studied.Combining with proposed analytical solutions,the NLCWJ and LCWJ models with different match ratios were established.The fatigue initiation point model provides a new perspective vision for judging the failure modes under low cycle fatigue.The relationships among the size of the fillet,the penetration size and matched ratio and failure modes were analyzed,and the basis for determining the failure transition is quantitatively determined.The LCF tests of 10CrNi3 MoV mismatched LCWJ were completed under strain-controlled cyclic loading.The results show that LCF of root failure mode of undermatched welded joints is lower than the evenmatched welded joints.Meanwhile,the HCF and LCF of CLWJ is uniformly characterized based on the effective notch energy values.The HCF and LCF behaviors of notched specimens of base metal and undermatched welds under multiaxial cyclic loading were studied.The evolutions of stress and strain and energy gradient at the notch tip were studied.The notch energy model was proposed to characterize the notch multiaxial fatigue life combining with the degree of notch stress concentration,material non-proportional hardening behavior,and the SWT damage parameters.Meanwhile,it incorporates the interactions among the notch geometry characterizations,material elasto-plastic properties and non-proportional loadings.Different notched multiaxial fatigue prediction models were used to compare and analyze the experimental data,and the fatigue life prediction results under different models were quantitatively compared.The results show that the proposed model has high fatigue life prediction accuracy for notched specimens,and can be further applied to the fatigue life prediction of welded structures under multi-axial load conditions in engineering.