The Experimental Research On Several Points In Unified Fracture Theory Of Ductile Material

The fracture of ductile material is popular in the engineering structures, however they have not been solved completely. In the paper, the fracture phenomena of metal materials in the process of conventional destructive test and mixed mode fracture test are systematically investigated. From the facets of fracture test, fracture criterion, stress field and fracture mechanism, undermentioned progresses were made:1. Affected by stress status and material characteristic, fracture mechanism may change. With the decreasing of the stress triaxiality, the fracture mechanism of metal materials may change from trans-granular brittle fracture to the coalescing and expansion of cavity and localized shear band under different loading condition. Then, five fracture modes may be occurred in materials, including cleavage and quasi- cleavage fracture, traction fracture with cavity, shear fracture affected by cavity, plastic shear fracture not affected by cavity and seam. There exist four boundery stress tri-axicity on abovementioned five fracture mechanisms, according to which, the five fracture mechanisms are made judgement in the material in sequence.2. Departing from the two objectivities that cracked body has the same fracture mechanism with uncracked body and stress or strain is limited in blunted crack tip, strangeness theory in fracture mechanics should not become the foundation of fracture criterion. The fracture criterion of material with crack and without crack should be correspondent.3. Based on those phenomena listed above, several fracture controlling parameters for different fracture forms have been discussed, and several new parameters which affect various fracture forms have been proposed. Then a new theory electing stress tri-axicity as fracture parameter was presented, unified fracture theory is perfect.4. The requirements for thickness and other dimension in fracture test were denied, two dominant factors-- stress status and material characteristic affecting fracture toughness are proposed. On the grounds of which, several conclusions obtained. Plane strain fracture toughness KIC reflects exclusively the indicator of traction fracture, while different thickness only express variant status. On that account, it is unreasonable that make materials traction fracture through controlling the thickness.