抄録
The measurement of residual stress by X-rays is utilized nowadays in many laboratories in consequence of the considerable improvement made in the accuracy of measurement brought about recently by the remarkable development of the X-ray diffraction apparatus and the accessory measuring devices. It is becoming one of the powerful methods of residual stress measurement.
One of the authors has investigated the change in residual stress due to fatigue stressing, and found that a linear law of fading of residual stress holds in the range except in the early stage of repeated stressing. In this paper, the change in residual stress due to repeated impact is reported, and the application of the linear law of residual stress fading to the present case is examined.
Two sorts of plate specimens were used, the one was of notched type using a low carbon steel (0.08% C), and the other was of unnotched type employing a high carbon steel(0.55% C). With the former, residual stress was introduced by plastic stretching, and with the latter by low temperature quenching. The specimens were put into a groove cut on the surface of a hard wood so as to be supported at both ends. Impact loading was given to the central parts of the specimens by a simple device of dropping a weight. The speed of cyclic impact was made to be 22 blows per minute by using a disc attached with a claw, which was rotated at 22rpm by the motor through reduction gears.
Stress measurement by X-rays was carried out by the film method using the Glocker's technique of vertical and 45° oblique incidences. CoKα beams were used and the distribution curves of diffraction intensity were drawn by an automatic recording microphotometer. The residual stresses were calculated by the following formula:
σ=K(cosecθψ-cosecθ⊥)
where K=sinθ0E/1+ν·1/sin2ψ
The residual stresses were measured at three stages of blow numbers of 500, 1000 and 2000 as well as at the initial state. As was expected, residual stresses decreased gradually with the increase of blow numbers for both the notched and unnotched specimens. It may supposed that the specimens were subjected to a fatigue stressing of pulsating bending of a low cyclic speed. The fading of residual stresses in the present experiment might be explained by the above fatigue phenomenon. Besides, another fatigue phenomenon due to the propagation of stress waves of tension and compression through the thickness of the specimen might be taken into consideration. The duration of impact in the present experiment, however, is infinitesimally small, therefore, the fatigue phenomenon of this sort is considered to play little role in diminishing the residual stress.
Taking the ratio of residual stress σ/σ0 (σ0 and σ are the initial and the current value, respectively) as ordinate and the ratio of blow numbers n/N (n is the current number and N is taken as the final number of 2000) as abscissa, a linear relationship was obtained for both specimens, as in the previous case of fatigue stressing. It is a noticeable fact, however, that the linear law held in the earlier period of impact stress repetitions. Be the matter as it may, it may be said that the linear law of residual stress fading holds also for the case of impact repetitions.
Experiments were made under another impact load using a larger weight, and as was naturally expected, the fading of residual stress was greater. This tendency was especially remarkable in the earlier stage of blow repetitions. In this case also, the linear law of residual stress fading was seen to hold.
