Abstract
The susceptibility of several types and thicknesses of mild steel of ship-plate and pressure-vessel qualities and of samples of welds to development of cleavage or brittle fractures has been determined by a new test, termed the “cleavage-tear” test, in which a notched tensile-bend type of test coupon is used. The effects of notch and coupon geometries, load eccentricity, rate of loading, testing temperature, and of heat-treatments were investigated.
At temperatures below the transition-temperature range, the presence of a notch of proper geometry and orientation was found to create a state of multiaxial stress which appears to expand the elastic range of steel and to raise its proportional limit almost to coincide with its breaking point. The breaking strength of rectangular sections containing a notch of the foregoing type was found to decrease, and susceptibility to cleavage fracturing to increase as the notch root radius was decreased. Within the limits investigated, the breaking strength of steel was found to be modified by the notch geometry, this modification being practically independent of the size effect if the section was 6 in. or greater in depth. The transition temperatures for the steels investigated in the as-rolled condition were found to be about 100 deg F higher than those indicated by the conventional Charpy impact test, and are considered to be more nearly indicative of behavior of steels in service. Nevertheless, the transition temperature of a steel cannot be defined for service conditions unless the state of stress which prevails under the same conditions is also defined.
Two types of fracture were encountered, one being the brittle or cleavage type exhibiting no appreciable deformation and lower nominal breaking strength, and the second the shear type exhibiting normal ductility and higher breaking strength. Fractures above the transition range were of the shear type and below that temperature range of the cleavage type. On the basis of the test data presented, reconsideration of our present factors of safety is suggested for monolithic structures with the object of safeguarding against the possibility of development of large cleavage fractures from small discontinuities, notches, or cracks. Other remedial measures for minimizing the damage of cleavage fractures, including modifications in design and material of construction, are suggested.
