Stress measurement photographic method

Until about 1950, x-ray stress measurements were made only by photographic methods. Today the diffractometer is preferred because it is faster and more precise. The photographic method, which still has its place, will be described later. 

This method of stress measurement is not often used today, because it is slower and less precise than the diffractometer method. However, the photographic method still has some advantages. The required apparatus is very simple a small back-reflection camera fixed to the head of a shockproof x-ray tube energized through a shockproof cable. This apparatus is smaller, lighter, more robust, and much cheaper than a mobile diffractometer. It is also more mobile and better suited to work in a confined space. 

Fig. 16-14 Photographic method of stress measurement (single-exposure technique). After Norton [16.13].

The small diameter of the incident beam from a stress camera or a Fastress unit is an advantage when one wishes to measure stress variations from point to point on a surface, as in the region near a weld. The stress distribution shown in Fig. 16-16, determined by a photographic method, simulates the residual stresses due to spot welding. The specimen was a steel strip 10 x 3 x inch (25 x 8 x 0.6 cm). A circular area of about I inch (1 cm) diameter, whose size is indicated on the graph, was heated locally to about 700°C for a few seconds by clamping the strip at its center between the two electrodes... 

The elastic stress in a given direction can be obtained from Equation (67). by making measurements at a range of incident angles. If a photographic method is used, objects such as bridge piers can be tested nondestructively in situ. 

Prior investigations into the behavior of notched specimens of materials have used the approach of recording a load and displacement and then photographing the fracture surface immediately after the test to obtain the ultimate axial true stress [26]. This method is acceptable in metals, but UHMWPE shows substantial strain relaxation upon fracture (Figure 31.3), which leads to inaccuracies in the calculated true ultimate stress. Also, this method does not provide information as to what is the deformation behavior of the notch during the test itself. Additionally, any method that uses a form of measurement that involves contacting extensometry risks premature fracture of a UHMWPE specimen due to the creation of a stress riser at the point of contact of the extensometer. Therefore, we developed a video-based system that could capture the stress-strain behavior throughout the duration of the test and avoid specimen contact [3]. 

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