Failure Analysis

Bent Rod Failures

Bending fatigue failures account for a significant number of all mechanical failures. It is a fact that all bent sucker rods eventually fail. New sucker rods are manufactured to a body straightness of no less than 1/16 inch in any twelve inches of rod body length. Sucker rods within this tolerance of straightness will roll easily on a level rack with five supports. Any degree of bend greater than this will cause an increase in local stress at the point of the bend during applied load. When the bent rod body is pulled straight during load, the ultimate strength of the material is quickly reached. The cycle of continually exceeding the ultimate material strength is repeated during the pumping cycle and causes stress fatigue cracks on the concave side of the bend. These stress fatigue cracks progress across the bar, during load, until not enough metal remains in the bar to support the load, and fracture occurs.

Straightening the raw bar stock is the first step in the process of manufacturing sucker rods. Cold straightening the bar deforms the grain structure below its re-crystallization temperature, putting a strain in the bar that is accompanied by a work hardening effect. During the manufacturing process, the function of heat treatment is to stress-relieve the residual and induced stresses caused by bar rolling, bar straightening processes and from forging the rod upsets. Heat treatment changes the metallurgical structure of the forged ends to match that of the rod body and also controls the mechanical properties of the sucker rod. Any rod body bend created after heat treatment causes work hardening, which creates an area of hardness different than the surrounding surfaces. This condition is referred to as a "hard spot" and is a stress raiser to load. Mechanical processing, such as passing the finished sucker rod through a system of rollers, will attempt to remove the bend so it appears to be straight. However, reconditioning processes are not capable of stress relieving bent sucker rods. A bent sucker rod is permanently damaged and should not be used because all bent sucker rods will eventually fail.

Figure 6 andd inset of Figure 7 are examples of bending fatigue failures. Bending fatigue failures can be identified by the angled fracture surface, which will be at some angle other than 90A to the axis of the rod body. The example on the left illustrates a fracture caused by a long radius bend, or gradual bow in the rod body as in Figure 7. The fracture surface is normal in appearance, but has a slight angle when compared to the axis of the rod body. The middle example is a short radius bend in Figure 7. The fracture surface is at a greater angle to the axis of the rod body with a small fatigue portion and a large tensile tear portion. The example in figure 7 is the result of a corkscrewed sucker rod. Notice how convoluted the fracture surface is in appearance. As a general rule, the greater the bend in the rod body, the more convoluted the fracture surfaces appear. In operation, the time for the rod to fracture is greatly shortened. Poor care and handling procedures usually cause bent rods.