Compressive Macrostress Research Articles

Nitriding Response of Chromium-Alloyed Steels

Hardness-increasing, coherent, chromiuin-nitride particles of submicroscopic size develop from chromium initially dissolved in the ferritic matrix of FeCr and FeCrC alloys. Transformation of existing chromium-carbide particles leads to coarse, incoherent, chromium-nitride particles and precipitation of cementite at grain boundaries. Due to the long-range strain fields surrounding the coherent precipitates, the ferritic matrix can be supersaturated with nitrogen. The tendency of the case, as a whole, toward volume expansion leads to development of compressive residual surface macrostress. The misfit between secondphase particles and the matrix induces residual microstresses. At latter stages of nitriding discontinuous precipitation, loss of excess nitrogen leads to void development and the relaxation of compressive residual surface macrostress.

The role of gas pressure and lateral stress on blistering

Both gas pressure in bubbles and lateral stress have been suggested as primary causes of blistering. An analysis of both mechanisms is presented, and the conditions for blistering are examined. To realistically predict the gas pressure in bubbles, a recently derived high-density equation of state for helium is utilized. It is shown that the formation of overpressurized gas bubbles leads to a state of stress in the surface layer which is a superposition of a tensile microstress surrounding the bubbles and a lateral compressive macrostress across the bombarded layer. When the microstress reaches a value of 0.003 μ for fcc metals or 0.009 μ for bcc metals, interbubble fracture occurs. These critical values are reached for an implanted helium concentration between 20 to 40%. The exact value depends to some degree on the bubble density, the temperature, and the helium to vacancy ratio. The lateral stress is shown to saturate prior to the onset of blistering, but it remains the driving force for blister dome formation once a sufficiently large area of the bombarded layer is detached.

Connections between strength and microproperties in granular composites

The micro stress in a granular composite is investigated by means of a model consisting of elastic spherical inclusions embedded in an elastic matrix. For a uniaxial compressive macro stress state the properties on both the micro and macro levels are calculated by means of an approximate method. The result shows of course, that considerable compressive normal stress concentrations occur; it also shows however, normal tensile stress concentrations at the equatorial zones of the interface between matrix and inclusions. This tensile stress, which depends on the elastic properties of the materials, can be of greater importance for the strength of the composite than the compressive stress. This is in good agreement with the observed fact that, e.g. in concrete, cracks often start in the equatorial zone.

X-ray diffraction study of residual macrostresses in shot-peened and fatiqued 4130 steel

A study has been made of the effects of shot peening and fatigue cycling on the residual macrostresses determined by X-ray methods in an austenitized and tempered AISI 4130 steel (150–170 ksi). The results show that the effect of shot peening is to produce a residual compressive macrostress layer 0.014-in. deep. The residual-stress profile (stress vs. depth) exhibits a small negative stress gradient at and near the surface and a large positive stress gradient in the interior. Stress relaxation (due to fatique cycling) which occurred early in the fatigue history of the specimen was found greater at the surface than in the subsurface layers. Stress gradients of the stress profile increased with continued cycling and varied with depth. A correlation appears to exist between stress relaxation and stress gradients at the surface.