Equal Load Carrying Capacity Design For Butt Joint Of High Strength Steel Under Impact Load

Impact load has the characteristics of high loading rate and high strain rate,under which circumstances materials are more likely to be damaged due to changedconstitutive relation. Therefore it s unreasonable to design welding joint applied toimpact load with the Equal Load Carrying Capacity (ELCC) principles derived fromstatic conditions. This article puts forward the ELCC design principles for lowmatched joints under impact load, which applies impact energy to characterize theimpact load carrying capacity; the aim of this design is to obtain the minimumthickness of joint to make sure it will absorb same impact energy with base metal infracture. In addition, the corresponding design guideline is also presented in thispaper.Cracking moment is marked by the critical equivalent plastic strain. Thestandard V-notch Charpy impact test of A921high strength steel is simulated byMarc., stress and strain distributions of V-notch front area at fracture moment areobtained,plane stress coefficient distribution is calculated. On the basis of the datacollected and fracture mechanics, a centre cracking and propagating prioritymechanism is proposed, which depicts reasons and process of shear lips generation.This theory is also applied in forecasting the width of shear lips and explaining twoproblems raised by previous scholars about shear lips, which further proved theeffectiveness of the fracture mechanism proposed in describing fracture process ofCharpy impact test and in providing theoretical basis for subsequent analysis ofimpact toughness changing law with geometry size.Thickness Change Group(10mm width, thickness variable from5mm to15mm)and Width Change Group(10mm thickness, width variable from5mm to15mm)impact tests of A921high strength steel were conducted respectively. Impacttoughness was found to rise with the increasing thickness or width by analyzing theimpact energy and impact toughness of the tests; however the effect of thicknesswas obviously bigger. Width of shear lips were found to increase as thicknessincreased in the Thickness Change Group,whereas width of shear lips showed noobvious difference in Width Change Group.With SEM, more and deeper toughnessnests were found in wider or thicker fractures, which represents bigger specimen hasbetter toughness.Causes and regularities of variation of toughness with geometry size werestudied from two aspects. On one hand the effects of stress state and strain rate were analyzed by obtaining stress, strain, stress state distributions in the front area of Vnotches of specimens of different sizes, by using finite element method to simulatetheir impact tests. Combined with the proposed fracture mechanism, it is found thatdifferent with fracture toughness K, the stress state distribution change caused bygeometric sizes has little effect on the impact toughness. On the other hand, basedon mechanics of materials, effects of bending capacity on specimen s mechanicalresponse under impact load were analyzed. Bending capacity of geometryrepresentation (H2B, H3B, ect) and impact toughness was fitted. Fitting results foundimpact toughness and bending capacity parameters H3B have good linear relation,with application of the two groups of test results, the fitting R is greater than0.9,which testified that bending capacity change is the main cause of impact toughnesschange. This regularity was also testified by thickness change specimen impact testsof another material Q345, which can certify that the rules above can be generalizedto other materials. The found of this linear relation between specimen s impacttoughness and its geometric size H3B will lead to a great deal of time and expensessave, permitting an accurate forecast of mechanical response under impact load witha small amount of tests or results of previous studies.According to research above and ELCC principle proposed, in the design ofELCC joint under impact load, minimum thickness can be found applying theimpact toughness-H3B curves of base metal and weld metal to make their impactenergy the same. Further considerations should be made to optimize joint s othergeometrical parameters in order to achieve as good carrying capacity of other loadsas possible.