| Energy steels including high-grade pipeline steels,high strength ship steels and vessels steels used at extremely low temperature,are key materials that are actively involved in the national strategy of "Silk Road Economic Belt and Maritime Silk Road" in the field of energy transportation.Concerning these steels are commonly working under the rigorous service conditions like high inner pressure,low temperature,large deformation and dynamic loading,therefore the excellent resistance performance to the ductile fracture is crucial to these energy-related material development,product application and engineering design.However,it is still a technical challenge to appropriately evaluate the dynamic fracture properties and cracking behavior of the full-thickness and large-scale energy steel plates in laboratory,due to the lack of testing equipments and standardized methods across the world as far.In the present research,high-capacity instrumented pendulum impact system in accordance with the technical requirements of drop-weight tear testing(DWTT)was established to study the dynamic fracture toughness and ductile cracking of full-scale high strength structural steel plates like energy steels,under their simulating service conditions,with the attempt to satisfy the demand on the fracture control of these steels during the engineering design and further application.Main conclusions from the present study were summarized:(1)On the basis of energy conservation law and theorem of impulse,a method to dynamically calibrate the force coefficient of the load-cell after each test was proposed,by introducing the measured absorbed DWTT energy from the pendulum machine.So that,the traceability and reliability of instrumented force-displacement curves could be assured.(2)By performing DWTT onto the large-scale and full-thickness specimens,the influences of the temperature,thickness and notch configuration on the material toughness and ductile-brittle transition behavior of energy steel plates at low temperature were investigated.The method to determine the arresting performance of high strength ship steels to brittle crack,e.g.index of Kca,was also proposed through the low-blow DWTT loading approach.(3)Derived from the Martinelli-Venzi kinematic model of ductile fracture,two analytical regression algorithms to estimate CTOAc,including energy-force method and displacement-ligament method were developed upon the acquired instrumented force-displacement relations.(4)By using key-curve method,the dynamic crack extension(?a)of the standardized pressed V-notch DWTT specimen was researched.The calculated crack growth behavior from the force-displacement was verified and correlated to the morphology of real fracture surfaces of DWTT specimens.As a result,an experimental solution to evaluate the material based parameters(A*?f),usually required by some other CTOAc algorithms,was also established and given in the present study.(5)By performing the optical full-field 3D strain analysis and interior hardness determination on the bending deformed low-blow DWTT specimens,it was observed that the distribution of the total strain along the residual ligament exhibited a characteristic of V-shape.The true rotation center was identified as a transition from the necking ahead of the crack tip into the swelling nearby the hitting area,which also suggested the location of the neutral strain axis without work hardening.(6)On the principle basis of Gabor filter-cluster analysis widely used in the image processing area,an automatic machine-vision technique for rating shear area percent(SA%)from fractorgraphy image of DWTT specimens was proposed in the present work and an original testing machine was also developed and industrially operated in Baosteel. |
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