The accuracy of stress measurement by X-rays has recently been remarkably improved, and some studies on the accuracy of measurement are reported. In this study, for the case of the film method, several problems on the accuracy of stress measurement were investigated.
First, the effect of errors in the determination of the radius of diffraction ring on the measured stress values was examined for each case of the Glocker method, Δ method and sin2
ψ method. In the Glocker method, let the error in the vertical incidence and the oblique incidence of 45° be e
' respectively, then the incorrect stress value is given by
'=radius of diffraction ring for the specimen and the standard substance, respectively. In the case of the Δ method, the erroneous stress value is expressed as
The erroneous stress values were calculated for four possible cases of combination of the signs of e
' by taking the values of 0, ±0.03, ±0.05, ±0.07, ±0.09mm for e
and 0, ±0.05, ±0.09mm for e
' in Eqs. (1) and (2). These calculations were carried out for two beams of CoKα and CrKα by selecting two arbitrary stress levels of 8kg/mm2
as correct values. For the sin2
ψ method, erroneous values of cosecθ'ψ
can be calculated from the correct values of cosecθψ
by introducing the error e
. Thus two incorrect values of cosecθ'ψ
were plotted as the ordinates of corresponding sin2
ψ, and from these points the maximum and minimum gradients were determined. For the purpose of comparison of accuracy among the three methods, the errors e
of opposite signs were introduced into the two extreme cases of ψ=0° and 45° for the sin2
ψ method. Under the same values of the error as in the Glocker and Δ methods, the incorrect stresses were determined.
Secondly, in the sin2
ψ method, the influence of errors in the setting angle was studied. Assuming the error in the setting angle as ±1° in each incidence and plotting the erroneous values of cosecθ'ψ
as the ordinates of corresponding correct sin2
ψ, the maximum and minimum values of σ' were calculated.
Thirdly, as experimental study, the effect of several ways for the determination of the peak position of diffraction profile was investigated. In addition to the ordinary half-breadth method at half height (1/2H), those at the heights of 3/8H and 5/8H were experimented, and the accuracy was compared among these three ways. Besides, the efficacy of the separation of Kα doublet was examined.
Lastly, the problem on the grain size of the film was dealt with as a possible factor affecting the accuracy. Two sorts of films, Fuji type 100 and 200, were used for this purpose. Adjusting the exposure time so as to obtain the same degree of blackening of diffraction ring between the two films, the values of cosecθ'ψ
and stress were compared for a stress level of 10kg/mm2
From the results of calculation and experiment, the following conclusions were obtained.
(1) The effect of errors in the radius of diffraction ring on the measured stress values may, in some instances, be appreciably large, especially when the errors are of opposite signs between vertical and oblique incidences.