抄録
It is very important to clarify the creep strength of the members which have non-uniform stress field, for example, notched members.
In connection with this, the creep deformation of cold-worked aluminium specimen of 40mm width subjected to eccentric tension at 200°C is treated in this paper. In this experiment, the creep tests under eccentric loading were carried out under a series of condition of eccentricity, e=2.5, 5.0, 7.5, 10.0, 12.5 and 15.0mm. The load is applied to the specimen in the longitudinal direction, and the loading axis is kept apart from the center axis of the specimen. A technique of photograting method previously proposed by the authors is applied here in order to know the plastic strains produced in the specimen during the creep.
By using the values of strain obtained at 60, 180, 300 and 420min. of elapsed time, the corresponding stress distributions are determined with a simple assumption that the stress is constant during each period divided by the times when the strains are measured. The calculations are carried out in both cases, one case is under the assumption of uniaxial stress, and the other is of biaxial stresses. The additional calculation results with the consideration of time-hardening and strain-hardening theory are also obtained.
The results obtained are as follows:
(1) In the transverse direction to the specimen axis, the strain distribution is linear regardless of the eccentricity and the elapsed time.
(2) In the transverse direction, the point, where the strain is zero, is not fixed with the elapse of time. But the movement of the point is considered to be negligible in the case of large eccentricity.
(3) The relation between the above-mentioned zero strain point and eccentricity and the amount of eccentricity with transition of time can be explained quantitatively using the experimental fact described in (1).
(4) Except the case of e=2.5mm, the stress distribution in the specimen is almost unchanged during the test.
(5) Both the stress distributions calculated with the assumption of uniaxial stress field and of biaxial stress field agree with slight variations with one another. It is presumably possible to treat the creep deformation of the specimen subjected to eccentric tension as under uniaxial stress.
(6) The tensile load calculated from the stress distributions obtained from the measurement of strain is slightly higher than the externally applied load. The creep deformation of the notched specimen has the contrary tendency. This means that the essential cause of notch strengthening lies not in the stress gradient in the transverse direction, but in other factors, such as the steep stress gradient in the specimen axis or the biaxial stress field.
