Strain rate has a significant effect on the fracture properties of metals like structural steels. Classical material dynamic fracture models assume the effect of strain rate and those of other influencing factors, e.g. stress state, to be decoupled, but past experiments showed otherwise. Hence, fracture models that can reflect the coupled effects are still lacking, and the microscopic mechanisms remain poorly understood. In this paper, over 200 coupon tests were performed, covering three materials in a welded joint (base metal, weld, HAZ), six different coupon types (taking over a wide stress state space) and five different strain rates (0.001s-1~5000 s-1). Fracture strain tended to increase under higher strain rates, and the strain rate effect was more notable for higher stress triaxiality and Lode angle parameter. Microstructural characterizations were conducted on nine fracture surfaces of base metal (i.e. Q355 steel). Synergistic mechanisms of dislocation multiplication, grain refinement and crystal texture evolution contribute to the coupled effects of strain rate and stress state on the fracture properties. A new microscopic mechanism-based coupled fracture model is then proposed, by adding a term that reflects the strain rate-stress state coupled effect to the LMVGM quasi-static fracture model. The new dynamic fracture model has the advantage of concise form (five independent constants) and exhibits higher predictive accuracy than the other models proposed in the existing literature for different metallic materials. The fracture behaviour of the weld metal and HAZ were also evaluated.