Experimental Study On Shear Fatigue And Dynamic Fracture Toughness Of Steel Structure Joints At Low Temperature

Steel structures and joints such as bridges and offshore platforms may be subject to cyclic and impact loading.When exposed to low temperature,these structures are more vulnerable to brittle fracture.At present,there are no clear regulations regarding the fatigue problem at low temperature in the design codes of various countries.The regulation of anti-impact fracture is mainly reflected as the constraint of the Charpy energy.Therefore,in order to investigate the fatigue and dynamic fracture properties of steel structures under low temperature conditions,low fatigue tests were conducted on 60samples of two structural details welded by Q345B.Meanwhile,ambient and low temperature instrumented impact tests on 60 samples made of Q235B and Q345B and their welds were conducted.The numerical simulation of low temperature dynamic fracture test was carried out.The effect of dynamic stress fracture factor amplification was discussed.The main research contents are as follows:?1?The low temperature and high-cycle fatigue tests were carried out on the side fillet welded joints and front fillet welded joints.The S-N curve of two structural details at 0?,-20?,-40?temperature were obtained and compared with test results at room temperature and the curve specified in the specification.The difference of failure modes of these two structural details were analyzed.The results show that the fatigue life of the side fillet welds is slightly improved and more stable at low temperature.However,the fatigue performance of front fillet welds at low temperature is mainly controlled by the welding process,and fatigue life may decrease at low temperature.?2?The instrumented impact tests were carried out on the Q235B,Q345B,E4303and E5015 welds at 20?,0?,-20?,-40?,-60?.The dynamic fracture toughness of these materials were obtained at these five temperature.The results show that the dynamic fracture toughness does not decrease monotonously with temperature.The dynamic fracture toughness shows a significant decrease under-20?for all of the four materials.The ductile-brittle transition temperatures of four materials are given according to Boltzmann function fitting.?3?The dynamic fracture toughness of Q235B at low temperature was calculated by ABAQUS explicit kinetic analysis combined with displacement extrapolation.The J_Id-t curve was obtained by ABAQUS implicit kinetic analysis.The obtained dynamic fracture toughness agrees well with the experimental results at-60?and-40?.?4?The dynamic stress intensity factor response of a two-dimensional plate with a central crack under an ideal step load was calculated by ABAQUS.The relation between the amplification factor of the dynamic stress intensity factor and the geometric dimensions of the plate was analyzed.The dynamic response of the two cracked beam applied in actual structures was also calculated.The results show that the central crack is more sensitive to the dynamic loads than the edge through crack.The case of vehicles hitting traffic signal poles was simulated by ABAQUS,giving the allowable impact velocity for a given crack length.The variation of allowable velocity at low temperature was also dicussed.