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Use of SR-4 strain gages for determining stresses in cast iron requires a different approach than in the case of steel. Within the elastic limit of steel, an increase in strain is accompanied by a proportional increase in stress, since the modulus of elasticity of the steel is constant up to the yield point. The strain in steel can be measured by SR-4 gages. The stress then can be calculated from the modulus of elasticity. In the case of previously unstressed cast iron, at even low loads, the strain is a combination of plastic and elastic deformation. For this reason, the strain is not directly proportional to the stress. As shown in Fig. 1, the stress-strain curves for ferritic and austenitic cast irons are curves whose moduli change with change in stress. Therefore, strain measurements on cast iron obtained by the use of SR-4 strain gages cannot be used in the usual manner to calculate stresses. A technique using SR-4 strain gages for calculating stresses in cast-iron valve parts and in laboratory experiments has been developed. This technique has been proved accurate by both commercial and laboratory studies. It involves obtaining a stress-strain curve to fracture and a complete set of stress-strain curves for different load levels within a working range of loads. A modulus of elasticity can be assigned to a curve at each load level. These curves are then used to evaluate the strains determined with SR-4 strain gages.
The Lunkenheimer Co., Cincinnati, Ohio