ADVANCED MATERIALS & PROCESSES | JANUARY 2026 27 bending fatigue, torsional fatigue, thermal fatigue, and corrosion fatigue. Brittle Fracture. A brittle fracture is characteristic of a fracture that has little to no plastic deformation of the material. Brittle fractures occur rapidly as the loading exceeds the tensile strength of the material. The fracture surface of a brittle fracture is always perpendicular to the loading. The fracture surface will usually have chevron or herringbone features that will point to the origin of the fracture. A brittle fracture determination does not indicate that the material or component itself was brittle as a brittle fracture can occur in a normally ductile material. There are several factors that can result in the transition of a normally ductile material to exhibit a brittle failure, one of which is temperature. Ductile Fracture. A ductile fracture is characteristic of a fracture that has a considerable amount of plastic deformation of the material. Ductile fractures occur relatively slowly and generally exhibit necking of the material prior to fracture. The fracture surface is usually dull and fibrous. Ductile failures occur when the loading exceeds the tensile strength of the material. Wear Failures. Wear failures are the result of material being removed from the surface of a component by the interaction of another material or component. There are four types of wear failures and each has subgroups. The four wear failure subcategory modes are adhesive wear, abrasive wear, erosion wear, and contact stress fatigue. Wear failures are often observed in bearings and other components that experience loading and relative motion between two or more components. Wear failures may be due to the introduction of foreign material, lack of proper lubrication, or other mechanisms introduced from the source of the failure. Some examples of the subcategories of wear failure include abrasive wear, adhesive wear, fretting, corrosion, and surface fatigue. Corrosion Failure. Corrosion failures are due to the unacceptable degradation of a material or component by the interaction with the environment the material or component is exposed to. Corrosion failures are due to the chemical or electrochemical reaction of a material with its environment. A search on corrosion failure causes reveals that there are many types and sources for corrosion failures. Corrosion can lead to other failure initiation mechanisms such as fatigue. When analyzing failures the presence of corrosion products on the surface of the failed component or material must be evaluated as the possible source of the failure initiation. Buckling Failure. Buckling failures are caused by loading a structural member or component at a level that causes deflection and a change of shape in the loaded member. The loading may be compressive or sheer, and occurs below loads or stresses that would cause material fracture. Stress Corrosion Cracking. Stress corrosion cracking failure is a result of crack initiation and propagation by corrosion on a material or component under a tensile stress. This mode of failure is sometimes listed under corrosion and wear failures. It is considered a bimodal failure as two mechanisms are acting simultaneously on the material. The stresses involved in the initiation and propagation of the fracture may be externally applied or due to residual stress within the material. Stress corrosion cracking is similar to a fatigue fracture. Thermal Fatigue. Thermal fatigue failure is the result of stresses created in the material due to expansion and contraction from repeated heating cooling cycles. As the material is heated and then cooled, tensile stresses build up and can initiate a fatigue crack. As the heating and cooling cycles continue the fatigue cracks can join together causing complete failure of the component. Brake and engine components are prone to failure via thermal fatigue. Erosion Failure. Erosion failure is due to the loss of a material or components surface from the interaction of the material with a fluid that may or may not contain abrasive particles. Erosive wear failure is common in pumps impellers and pipping. Yielding Failure. Yielding failures are the result of loading ductile materials or components to levels of stress that causes plastic deformation but are below the level for fracture. ABRASIVE WEAR FAILURE Visual Examination: All three pins, shown here, were essentially in the same condition and had two areas of very heavy wear in locations of the bearing contact. The surface in the areas of wear had been worn away for 50% of the entire circumference and worn to a depth of approximately 0.070 in. at deepest point. There was evidence that the pins were “walking” toward the serrated end as the pin material had serrations worn into the body of the pin past the original length of the serrations (Reference area “A”). In this photograph of a typical abrasive wear failure, the planet gear pins failed due to excessive loading. Courtesy of Auburn Gear LLC Metallurgical Lab Report MLR 7399.
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