Plastic Strain Components Research Articles

22Cr-1Cuフェライト系および18-8オーステナイト系不銹鋼の熱疲労

The problem of failure of structures subjected to cyclic thermal stresses is of interest both from fundamental and also practical points of view. The thermal stress arises because of thermal strain which is produced in consequence of constraint of free thermal expansion or contraction of structures. Thus, the thermal strain is an independent variable in the present problem.The authors, as well, have been interested in the problems of lifetime of metals subjected to varying temperature or combined variation of temperature and stress, and have published some reports in this line. The present study is undertaken to obtain information regarding fundamental problems of thermal cycling of heat resistant alloys. For this purpose, a thermal cycling test apparatus was manufactured and the tests were performed, using 22Cr-1 Cu ferritic and 18-8 austenitic stainless steels. The followings are the results from our studies:(1) The apparatus for thermal cycling give good patterns of both temperature cycling and thermal stress cycling for the thermal cycling test.(2) Temperature dependency of failure life of thermal cycling on AISI 318 type stainless steel is not so remarkable as compared with the case of creep rupture life under temperature cycling for the same material.(3) From a series of thermal cycling tests for mean temperature of 650°C on 318 type stainless steel, it is found that variation of wall thickness of hollow specimen from 0.5mm to 1.5mm has no significant effect on the failure life.(4) If mean temperature is adopted as a basis of test temperature, the data of tests where minimum temperature is taken during temperature cycling as a basis of test condition can be estimated as well, provided that temperature dependency of failure life of thermal cycling is known.(5) In thermal cycling tests, adoption of total strain amplitude which is to be composed of elastic and plastic strains is practically more useful than that of plastic component of strain.(6) From the present experiments some pertinent problems which are to be developed are clarified.

Low Temperature Attenuation Measurements in Lead Single Crystals and their Bearing on Acoustic Losses Due to Dislocation Motions

An analysis has been made of the attenuation measurements in lead single crystals, carried out by H. Bömmel, down to liquid helium temperatures. Above the superconducting temperature, relaxations have been found which verify the measurements of Bordoni made at lower frequencies. An analysis of the variation of attenuation with temperature and frequency shows that at low temperatures, i.e., up to 50°K, all the attenuation is accounted for by relaxation mechanisms having activation energies from 100 to 1500 calories per mole. Above this temperature, a hysteresis type mechanism enters which produces a loss proportional to the frequency. This has an activation energy similar to the highest energy relaxation mechanism and at room temperature accounts for most of the loss. It is shown that measurements of this type require that dislocations be tied down not only by impurity atoms but also by residual stresses along the dislocation so that there are no freely vibrating dislocations. The dislocation motions are then temperature-induced jumps, biased by the applied stresses, occurring over activation energy barriers from 100 to 1500 calories per mole. The hysteresis component is thought to be a successive step process in which one motion triggers off another at a given part of the cycle. This process causes the plastic component of strain to lag behind the stress, thus producing a hysteresis loop in the stress-strain curve whose area is independent of the frequency up to a frequency limited by the activation energy of the lowest components.

The theory of combined plastic and elastic deformation with particular reference to a thick tube under internal pressure

In certain problems of plastic flow, for example, a thick tube expanded by internal pressure, it is important to consider changes in the elastic strain of material which is flowing plastically in order to deduce the correct stress distribution and deformation. The usual plastic theory which neglects elastic strains in the plastic region may lead to considerable errors in certain cases. In this paper we review the theory of the deformation of a material under combined stresses which involves both elastic and plastic components of strain. The relationship between stress and strain is represented on a plane diagram, the reduced stress-strain diagram, which facilitates discrimination between the elastic and plastic components of strain and aids considerably the solution of certain problems. The diagram can also be used to express the relationships governing the dissipation of energy during plastic flow under combined stresses. The theory is applied to the deformation of a long thick tube under internal pressure with zero longitudinal extension. The solution is compared with that based on the usual theory which neglects elastic strains in the plastic region, revealing an error which reaches a maxi­mum of over 60% in the longitudinal stress distribution. The significance of the differences between the two solutions is discussed in detail.