TECHNICAL FIELD

The present invention relates to a method for measuring stress in a target object.

BACKGROUND ART

During these years, as can be found in PTL 1 and the like, two-dimensional detection methods (so-called cos a methods) employing a two-dimensional detector are widely used as nondestructive methods for measuring stress (residual stress) in a target object composed of a metal. In these methods, stress is measured on the basis of a diffraction ring of a diffracted X-ray generated as an X-ray incident on a target object at a certain incidence angle Ψ is diffracted from the target object. The measurement accuracy of the two-dimensional detection method is substantially proportional to sin 2Ψ and decreases as the incidence angle Ψ of an X-ray incident on a target object changes from 45°. For this reason, the incidence angle Ψ of an X-ray on a target object is usually set within a range of 25° to 65° in the two-dimensional detection methods. In PTL 1, the incidence angle Ψ is set at 30°.

Although stress in relatively flat parts of a target object can be accurately measured using the two-dimensional detection methods, it is difficult to accurately measure stress in concavities of a target object. More specifically, when a part of a target object to be measured is a concavity that recesses from surfaces of parts therearound, an incident X-ray and a diffracted X-ray interfere with the target object, and a clear diffraction ring is not detected or, even if a diffraction ring is detected, accuracy decreases in parts of the diffraction ring obtained by a diffracted X-ray that has passed near the target object. It is therefore difficult to secure a sufficiently large incidence angle Ψ of an incident X-ray. As a result, it is difficult to accurately measure stress in concavities using two-dimensional detection methods. In other words, when stress in a concavity needs to be accurately measured, it is usually difficult to employ a two-dimensional detection method.

CITATION LIST

Patent Literature

PTL 1: Japanese Unexamined Patent Application Publication No. 2011-27550

SUMMARY OF INVENTION

An object of the present invention is to provide a method for measuring stress capable of accurately measuring stress in concavities of a target object using a two-dimensional detection method.

A method for measuring stress according to an aspect of the present invention is a method for measuring stress in a concavity of a target object, which is composed of a metal and includes a surface and the concavity, the concavity being provided in the surface and extending like a groove. The method includes the steps of causing X-rays to be incident on the concavity and detecting, using a two-dimensional detector, diffraction rings of diffracted X-rays generated as the X-rays are diffracted from the concavity and calculating the stress in the concavity on a basis of a result of the step of detecting. In the step of detecting, the X-rays are caused to be incident on a plurality of parts of the concavity of the target object and the two-dimensional detector detects the diffraction rings generated as the X-rays are diffracted from the concavity.

In addition, a method for measuring stress according to another aspect of the present invention is a method for measuring stress in a concavity of a target object, which is composed of a metal and includes a surface and the concavity, the concavity being provided in the surface and extending like a groove. The method includes the steps of causing X-rays to be incident on the concavity and detecting, using a two-dimensional detector, diffraction rings of diffracted X-rays generated as the X-rays are diffracted from the target object and calculating stress in the concavity on a basis of a result of the step of detecting. In the step of detecting, the X-rays are caused to be incident on a single part of the concavity of the target object at a plurality of different incidence angles and the two-dimensional detector detects the diffraction rings generated as the X-rays are diffracted from the concavity.

