Failure Analysis

Failure Mechanisms

All sucker rod, pony rod, and coupling failures are either tensile or fatigue failures. Tensile failures occur when the applied load exceeds the tensile strength of the rod. The load will concentrate at some point in the rod string, create a necked-down appearance around the circumference of the rod, and a fracture occurs where the cross-section is reduced. This rare failure mechanism only occurs when pulling too much load on the rod string-such as attempting to unseat a stuck pump. To avoid tensile failures, the maximum weight indicator pull for a rod string in "like new" condition should never exceed 90% of the yield strength for the known size and grade of the smallest diameter sucker rod. For unknown sucker rod conditions, sizes, or grades a sufficient de-rating factor should be applied to the maximum weight pulled. All other sucker rod, pony rod and coupling failures are fatigue failures.

Fatigue failures are progressive and begin as small stress cracks that grow under the action of cyclic stresses. The stresses associated with this failure have a maximum value that is less than the tensile strength of the sucker rod steel. Since the applied load is distributed nearly equally over the full cross-sectional area of the rod string, any damage that reduces the cross-sectional area will increase the load or stress at that point and is a stress raiser. A small stress fatigue crack forms at the base of the stress raiser and propagates perpendicular to the line of stress, or axis of the rod body. As the stress fatigue crack gradually advances, the mating fracture surfaces opposite the advancing crack front try to separate under load and these surfaces become smooth and polished from chafing. As the fatigue crack progresses, it reduces the effective cross-sectional area of the sucker rod until not enough metal remains to support the load, and the sucker rod simply fractures in two. The fracture surfaces of a typical fatigue failure have a fatigue portion, tensile portion, and final shear tear.

Fatigue failures are initiated by a multitude of stress raisers. Stress raisers are visible or microscopic discontinuities that cause an increase in local stress on the rod string during load. Typical visible stress raisers on sucker rods, pony rods and couplings are bends, corrosion, cracks, mechanical damage, threads, and wear or any combination of the preceding. This increased stress effect is the most critical when the discontinuity on the rod string is transverse (normal) to the principle tensile stress. In determining the stress raiser of a fatigue failure, the fatigue portion opposite the final shear tear (extrusion/protrusion) must be carefully cleaned and thoroughly examined. Fatigue failures have visible or macroscopic identifying characteristics on the fracture surface, which help to identify the location of the stress raiser. Ratchet marks and beach marks are arguably two of the most important features in fatigue failure identification. Ratchet marks are lines that result from the intersection and connection of multiple stress fatigue cracks while beach marks indicate the successive position of the advancing fatigue crack. Ratchet marks are parallel to the overall direction of crack growth and lead to the initiation point of the failure. Beach marks are elliptical, or semi-elliptical rings radiating outward from the fracture origin and indicate successive positions of the advancing stress fatigue crack growth.

Figure 1 is an example of tensile and fatigue failure mechanisms. The two examples on the right are tensile failures. A tensile failure is characterized by a reduction in the diameter of the cross-sectional area at the point of fracture. Typical tensile failures have cup-cone fracture halves. The second example from the right in Figure 1 is typical in appearance for tensile failures. Fractures from tensile failures rupture, or shear, on 45A angles to the stresses applied. A good example of the shear is the characteristic cup-cone fracture surfaces of a typical tensile failure. The rod body on the right in Figure 1 is an excellent example of needing to look past the obvious for the not so obvious. A fatigue failure is primarily responsible for this failure even though fracture occurred while trying to unseat a stuck pump. Visual examination of the fracture surface reveals a small, semi-elliptical, stress fatigue crack. This sucker rod had pre- existing, transverse stress fatigue cracks, from in-service stresses. One of the stress fatigue cracks opened during the straight, steady load applied in attempting to unseat the pump, and fracture occurred. The tensile failure is secondary and results in the unusual appearance of the fracture surface-with the small fatigue portion, large tensile portion and unusually large 45A double shear tears.

The remaining examples are fatigue failures on: casehardened sucker rods; normalized and tempered sucker rods; and quenched and tempered sucker rods. The example on the far left is a torsional fatigue failure from a progressing cavity pump. Ratchet marks found in the large fatigue portion, and originating from the surface of the rod body, completely encircle the fracture surface with the small tensile tear portion shown slightly off middle-center. The second rod body on the left is a casehardened fatigue failure. The case encircling the rod body diameter carries the load for this high tensile strength sucker rod and if you penetrate the case, you effectively destroy the load-carrying capability of this type of manufactured sucker rod. The stress fatigue crack advances around the case and progresses across the rod body. A fatigue failure on a casehardened sucker rod generally exhibits a small fatigue portion and a large tensile tear. The third rod body from the left is typical in appearance for most fatigue failures. Typical fatigue failures have a fatigue portion, tensile portion and final shear tear. The width of the fatigue portion is an indication of the loading involved with the fracture. Mechanical damage can prevent or hinder failure analysis by destroying the visual clues and identifying characteristics normally found on a fatigue fracture surface. Care must be exercised when handling the fracture halves. It is very important to resist the temptation to fit the mating fracture surfaces together since this almost always destroys (smears) microscopic features. To avoid mechanical damage, fracture