Fatigue testing machines (1) may be classified as to: 1. Type of load: constant load or constant displacement. 2. Type of stress: bending, torsion, etc. 3. Design characteristics; mechanical, hydraulic, magnetic, etc. 4. Operating characteristics: resonant or non-resonant. 1 The most general classification designates machines as being either of constant load or constant displacement types. Distinguishing characteristics of these two types are: Constant Load Type: (a) Applied load or amplitude of loading is constant throughout the test. (b) After the fatigue crack initiates its rate of propagation usually increases. Constant Displacement Type: (c) Applied deformation or amplitude of deformation is constant throughout the test. (d) Load on specimen is reduced after fatigue crack initiates and rate of propagation of crack is usually retarded. As a result of conditions existing in (b) and (d) above, the shape of the S-N curves may be different when obtained under constant loading as compared with constant displacement. Influence of type of loading can only be evaluated by introducing the time at which the fatigue crack initiates. An important consideration in any machine is the means used to measure and maintain the forces acting on the member under test. Constant loading machines of the mechanical type may use inertia forces, dead weights or a spring system having a low spring constant, which permits convenient evaluation of thel force on the specimen. In a displacement-type machine the deflection, shortening or lengthening of the specimen itself may be measured with wire-type strain gages, mirrors, or micrometer devices. Often a dynamometer is connected in series with the specimen to ascertain the force acting but careful consideration of inertia forces is required.