Ductile Shear Fracture Research Articles

焼もどしぜい化材料のき裂発生強度について

The effects of temper embrittlement on the crack initiation properties of 3.5 Ni-Cr-Mo-V steel for large rotor forgings have been studied by using the burst tests with application of internal pressure on thin tubular specimens. The results obtained are as follows.(1) Temper embrittled steels with severe segregation show premature fracture before the point of plastic instability, and the fracture appearance has a very brittled grain boundary fracture mode.(2) The strength against crack initiation, which can be measured by the critical circumferential strain at bursting, decreases remarkably in the materials with fracture appearance transition temperature, FATT, above 100°C.(3) Temper embrittled steels with light segregation, however, fracture at the point of plastic instability and show a more ductile shear fracture mode.(4) The materials having higher yield strength show a noticeable decrease in crack initiation strength.(5) The metal resistance against crack initiation depends upon the nature of defects and properties of surrounding matrix. Temper embrittlement enhances the notch-sensitivity of the matrix.

Déformation plastique et rupture du beryllium coulé et lamine sous pression hydrostatique inférieure à 15 kbar

Lower than 15 kbar hydrostatic pressure effect on mechanical properties (plastic behaviour and fracture) of casted rolled beryllium gauge specimens strained by tension parallel and perpendicular to the rolling direction was analysed. Room temperature and constant strain rate tests were carried out. Load and lengthening of gauge specimens were directly measured. Furthermore, metallography and electronic microscopy studies were made. Plastic behaviour was not affected by pressure, but fracture lengthening and load increased, leading to a partial striction. Evidence of ( c + a) slip could not be established, but probably such a slip exists in regions where strain concentration is high. Fracture begins by an intragranular cracking. Then a cleavage cracking occurs, giving a suitable theory verification. At high pressures, ductile shear fracture occurs and leads to a larger ductility; as high is cold work coefficient, larger is ductility.

On the Fracture of Metals

In Part I, a phenomenological fracture criterion of polycrystalline metals under conventional loading conditions is presented on the premise that there exist generally two modes of fracture ; ductile shear fracture and brittle cleavage fracture. The two major effects on fracture ; that is, triaxiality of stress and plastic strains preceding fracture are formulated mathematically in this criterion. The experimental results under combined stresses on ductile metals are well explained on the basis of this criterion. In Part II, the tensile fracture tests of mild steel cylindrical bars having circumferential hyperbolic notches are performed, and the plastic stress and strain distributions at fracture necessary for an application of this criterion to practical problems are analysed approximately. The various fracture behaviors in these tests are well explained and a view as to the notch brittleness are given.

On the Fracture of Metals (1st Report)

A phenomenological fracture hypothesis or criterion of metals is presented on the premise that there exist generally two fracture stress surfaces for ductile shear fracture and brittle tension fracture in metals. Two major factors which effect on fracture, that is, triaxiality of stress and plastic strain up to fracture are formulated in this criterion using linear invariant of stresses and quadratic invariant of strains respectively. The fracture stresses obtained by the method presented for predicting those of various materials under combined stresses agree well with previous experimental values, also with the fracture modes. The problems of the low temperature brittle fracture and the effect of prestrain are explained qualitatively from a consideration of temperature characteristics of fracture stress surfaces. An analysis of stress and strain distributions which is to be necessary for an application of this criterion to notch brittleness will be given in the 2nd Report.