s16.1 Experimental Stress Analysis 431 until final rupture occurs.As this often requires several thousand repeated cycles of load under service conditions,full-scale production is normally well under way when failure occurs.Delays at this stage can be very expensive,and the time saved by stress analysis techniques in locating the source of the trouble can far outweigh the initial cost of the equipment involved. The main techniques of experimental stress analysis which are in use today are: (1)brittle lacquers (2)strain gauges (3)photoelasticity (4)photoelastic coatings The aim of this chapter is to introduce the fundamental principles of these techniques, together with limited details of the principles of application,in order that the reader can appreciate (a)the role of the experimental techniques as against the theoretical procedures described in the other chapters,(b)the relative merits of each technique,and (c)the more specialised literature which is available on the techniques,to which reference will be made. 16.1.Brittle lacquers The brittle-lacquer technique of experimental stress analysis relies on the failure by crack- ing of a layer of a brittle coating which has been applied to the surface under investigation. The coating is normally sprayed onto the surface and allowed to air-or heat-cure to attain its brittle properties.When the component is loaded,this coating will crack as its so-called threshold strain or strain sensitivity is exceeded.A typical crack pattern obtained on an engineering component is shown in Fig.16.1.Cracking occurs where the strain is greatest. Fig.16.1.Typical brittle-lacquer crack pattern on an engine con-rod.(Magnaflux Corporation.)$16.1 Experimental Stress Analysis 43 1 until final rupture occurs. As this often requires several thousand repeated cycles of load under service conditions, full-scale production is normally well under way when failure occurs. Delays at this stage can be very expensive, and the time saved by stress analysis techniques in locating the source of the trouble can far outweigh the initial cost of the equipment involved. The main techniques of experimental stress analysis which are in use today are: (1) brittle lacquers (2) strain gauges (3) photoelasticity (4) photoelastic coatings The aim of this chapter is to introduce the fundamental principles of these techniques, together with limited details of the principles of application, in order that the reader can appreciate (a) the role of the experimental techniques as against the theoretical procedures described in the other chapters, (b) the relative merits of each technique, and (c) the more specialised literature which is available on the techniques, to which reference will be made. 16.1. Brittle lacquers The brittle-lacquer technique of experimental stress analysis relies on the failure by crack￾ing of a layer of a brittle coating which has been applied to the surface under investigation. The coating is normally sprayed onto the surface and allowed to air- or heat-cure to attain its brittle properties. When the component is loaded, this coating will crack as its so-called threshold strain or strain sensitivity is exceeded. A typical crack pattern obtained on an engineering component is shown in Fig. 16.1. Cracking occurs where the strain is greatest, Fig. 16.1. Typical brittle-lacquer crack pattern on an engine con-rod. (Magnaflux Corporation.)