Paper: Polished Rod Loads and Their Range of Stress

Practically all sucker rod breaks are fatigue failures. These breaks are brittle type fractures which occur at a stress much below either the ultimate strength or yield strength of the metal after a great number of stress reversal cycles. This type of failure is common to all metals and all kinds of metallic structures subjected to cyclic loading. To design a structure so that such failures cannot occur, we must determine the fatigue endurance limit of the metal we are using, then limit the working stresses to a safe value below this figure.