Fatigue Crack Growth Behavior Of 10Ni5CrMoV High Strength Steel Welded Joints

Welding has been applied as a commonly used technique in the connection of marine structures such as submarines,aircraft carrier and destroyers.It is generally accepted that fatigue life of welded structures can be assessed by fracture mechanics based method only,because crack initiation life is only a small portion and can be neglected due to the inevitably existing crack-like weld imperfections.Understanding of the fatigue crack growth behavior of their welded joints is of great importance to the damage tolerance design and assessment of marine structures since these welded structures are usually exposed to harsh marine environments.This dissertation was mainly concerned with the long fatigue crack growth(FCG)behavior of a new generation N i-Cr-Mo-V high strength steel,10Ni5CrMoV,and its welded joints employed in modern marine structures of C hina.Various factors that have an influence on the FCG behavior are scrutinized critically.Since elastic-plastic mechanical properties of the material is essential in computational welding mechanics,crack tip stress-strain response and welding residual stresses redistribution analysis during fatigue crack growth,the cyclic visco-plasticity behavior of 10Ni5CrMoV high strength steel is investigated first.Uniaxial tests of monotonic loading at different strain rate,symmetrical and asymmetrical strain cycling and stress controlled cyclic loading are conducted.Results reveal that the material exhibits very limited rate dependency and cyclic softening continues to evolve with nonlinear-linear-nonlinear feature until failure without a saturated value under strain cycling.Three-stage asymptotic ratcheting with a long steady-state period in the second stage is experimentally detected under stress cycling and no shakedown occurs.O n the basis of the experimental findings a new cyclic visco-plasticity constitutive model is proposed aiming to describe the non-saturated cyclic softening and asymptotic ratcheting behavior of material.The identification procedure of the material parameters used in the proposed model is scrutinized critically by quantifying the contributions of the isotropic hardening and kinematic hardening on cyclic softening.The accurate determination of welding residual stress(WRS)is a prerequisite for studying its effect on fatigue crack growth rate(FCGR).To this end WRSs in a single bead-on-plate weld as well as in a multi-pass multi-layer weld of 10Ni5CrMoV high strength steel are investigated by experimental tests and numerical simulation.Effects of material's constitutive model,phase transformation and yield stress change during heating and cooling are studied.Compared with experimental measurements,the recommended thermal-metallurgical-mechanical model for FE calculation of WRSs in 10Ni5 Cr Mo V high strength steel welded joints is determined as using the combined kinematic-isotropic hardening rule along with consideration of solid-state phase transformation and yield stress change.After that,in order to establish a basic database for the FCG property of 10Ni5 Cr Mo V high strength steel welded joint,FCGR measurements are performed for the base material(BM)and weld metal(WM).Effects of material's microstructure and stress ratio on FCGR are investigated and results and results are analyzed with the help of fractography and fatigue trajectory map.Results show that WM possesses better resistance to crack propagation compared with BM at each stress ratio.The carbides in tempered sorbite microstructure gave rise to the high brittleness.In the WM the crack zigzagged its way through the basket-weave structure of acicular ferrite packets with chaotic arrangement of ferrite plates and the resulted tortuous paths leaded to the favorable toughness and consequently,better crack growth resistance.It is indicated by the fractography and the trajectory map that the crack growth mechanism changes from fatigue dominated to monotonic fracture-dominated,not only with increasing crack growth rate,but also with increasing R-ratio.Moreover,fairly reasonable correlation of the R-effect is obtained for mode I long crack growth of the BM using either the crack closure model or the two-parameter model.The consolidation of the WM data is not as good as the BM data due to the existence of retained WRSs in the extracted WM sample.Mixed mode I-II FCG behavior with close proximity to realistic in-service conditions is investigated for 10Ni5CrMoV high strength steel welded joint after its mode I FCG property has been clarified.The partly retained compressive WRSs in the extracted CTS specimens noticeably increase the crack closure level in the WM.Calibrated by the force-compliance offset data of all tested specimens under mode I and mixed mode I-II loading,the 4% compliance offset criterion in the ASTM procedure is proved to be suitable for determination of the crack opening force of N i-Cr-Mo-V high strength steel weldment.The effective stress intensity factor range parameters proposed by Tanaka,Richard and Borrego et al.are found to give a poor correlation of the tested FCGR data under mixed mode I-II load condition for both BM and WM.A unified effective driving force model for mixed mode I-II loading that combines the crack closure model and the two-parameter model is proposed and is found to obtain a better correlation of the mixed mode I-II loading FCGR data.Finally,effect of WRSs on FCGR of 10Ni5CrMoV high strength steel welded joint is investigated in detail on the basis of the above achievements.N umerical work on influence of WRSs on residual stress intensity factor Kres and crack opening stress ?op is undertaken.It turns out that the new plastic strains induced by the superposition of WRSs and applied load have a certain influence on the calculation of Kres when the crack is propagating in a compressive WRS field.The effects of tensile and compressive WRSs on plasticity induced crack closure are also investigated.Taking advantage of these outcomes,we propose the concept of elastic redistribution and plastic redistribution of WRSs during FCG and the latter should be explicitly considered if the superposition model is employed to evaluate the effect of WRSs on FCGR.Modified superposition model and modified unified effective driving force model taking into consideration of plastic redistribution of WRSs are subsequently proposed.Better correlations of WRSs-effect in FCG are achieved using these modified models relative to the conventional superposition model which only consider elastic redistribution and crack closure model.At last the damage tolerance assessment procedure of structures containing defects in WRS field is proposed.