There are three stages in the life of a fatigue failure; 1. Initiation, 2. Crack Growth (propagation), and 3. Final Fracture. These stages are illustrated in the SEM image of a fractured rectangular wire above. The Initiation is indicated by the large red arrow at the lower left. The area of progressive Crack Growth extends from this arrow to the line indicated by the three smaller red arrows. Final Fracture, the point at which the remaining intact cross section of the wire could not sustain the next cyclic stress load – “the straw that breaks the camels back” – and complete fracture occurred, is the light area above the three arrows. This fracture is an example of bending fatigue (in one direction) initiating from a single point of origin.

The fractured crane lifting hook above is an example of reverse bending fatigue (back and forth in two directions). In this case, the major bending stress was applied from the bottom of the fracture as oriented in this photo, and a minor stress from the top. The darker gray area indicates final fracture in a single stress cycle. The thin horizontal band at mid fracture indicates a significant “jump” in the fracture progression that occurred in the cycle proceeding final fracture which almost, but not quite, resulted in complete fracture. This fatigue fracture initiated from multiple origins. Multiple origins are indicated by the steps, or “ratchet marks”, at the outer diameter of the fracture indicated by the arrows. Ratchet marks occur when multiple fatigue cracks initiate at slightly different planes on a component’s surface. As these multiple cracks progress into the component, they eventually join into a single fracture plane as show above.

Rachet marks resulting from multiple fatigue origin locations are shown at high magnification in these images taken on our Scanning Electron Microscope (SEM). Fatigue cracking penetrated only a short distance into this NASCAR Racing suspension component before it failed completely in a single load cycle. As a result, the multiple origin fatigue cracks never progressed far enough to coalesce into a single fracture plane. Several of the individual origin sites are indicated by arrows.

The diagonal bands exhibited by the fatigue fracture of this compressor connecting rod are commonly called “arrest lines”. These indicate a change in the frequency of cyclic stresses, such as “stop-start” sequences, changes in RPM, or variations in load. The initiation site at the crankshaft journal bore (arrow) is heavily damaged. This is not uncommon in fatigue failures. As the first location to crack, the initiation site is exposed to potential relative movement of the two sides of the crack during propagation. This presents a significant challenge to the analyst in determining the root cause of fatigue cracking.

Fatigue fractures exhibit distinct features, called striations, when viewed at high magnification using Scanning Electron Microscopy. Striations appear as relatively evenly spaced parallel lines. Each striation is actually a shallow crack that results from a single load, or stress, cycle. Repetition of these cycles produces an advancing repetition of shallow cracks as shown above in this fatigue fracture in a hydraulic valve body. This process is characterized by the term, “fatigue crack propagation”.

The appearance, or morphology, of fatigue fracture striations varies depending on the magnitude and frequency of the applied load and the physical characteristics of the affected component such as hardness, microstructure and chemical composition of the alloy. These SEM images illustrate fatigue striations in an aluminum valve body (left) and an alloy steel high pressure hydraulic cylinder (right).

In some cases, the root cause of a fatigue failure can only be discovered by an analysis of internal characteristics of a component at the crack location. In this example, a metallographic cross section revealed decarburization (dark phase at arrow) of the surface of a steering arm due to faulty heat treating. This carbon depleted layer has significantly reduced hardness and strength, as well as residual tensile stress, conditions highly conducive to fatigue crack initiation.

Other types of internal defects which act as initiation sites for fatigue are apparent on the fracture surface. Examination of this brake return spring by SEM revealed fracture features which radiate from a single initiation point. Viewed at higher magnification, this initiation point exhibits a void containing a non-metallic inclusion which acted as a stress concentration.

The exception to “usually”, the cases where fatigue fractures initiate from component surfaces that are free of stress concentrations, typically result from one of two causes; under-design of the component, or abusive service conditions. Just as all materials have an ultimate tensile strength, they also have a fatigue strength, sometimes called the fatigue limit or endurance limit. Once a component is subjected to cyclic stresses that exceed this limit, fatigue fracture occurs.

Fatigue failures of this type are less common than fatigue failures initiating from stress concentrations. Usually components are intentionally over-designed to deal with stresses several times greater than what they would be subjected to in service as a safety margin.

Fatigue Crack Initiation – The Critical Event

If the initiation stage can be prevented, fatigue fracture will not occur.  It sounds so obvious and simple. It’s not. As noted above, initiation is the most complex stage of fatigue fracture. A low magnitude load, which would have no effect whatsoever on a component in a single application, can be devastating when repeatedly applied as thousands or millions of cycles. The cumulative effect of these cyclic loads are microscopic “shifts” in the material’s structure which ultimately produce a “dislocation” – at this scale it is too small to be called a crack – and the focal point of stress concentration is born. Corners, holes, rough surface finish, welds and other features only accelerate the process.  To further complicate the issue, vibration harmonics, dampening of the system and the environment in which the component functions add a large unknown factor. Collectively, these affects become difficult to predict in the design stage.

Confronting Fatigue – Attack and Defense

From a practical standpoint, fatigue failures present a danger to you, the manufacturer, at three points in a components life. These are the design stage, the manufacturing process, and the service environment.

Design

The design engineer is the first line of defense against fatigue fracture.  He or she can’t prevent failures originating in the manufacturing process or service environment, but the designer lays the foundation of prevention.

In an ideal world, each design would be subjected to extensive stress calculations and fatigue testing.  In the real world this is rarely cost effective for non-critical components. Instead, accepted and “proven” parameters are applied.  These typically include safety margins which are more than adequate.  Typically, but not always.

Computer Aided Design (CAD), Finite Element Analysis (FEA) and a variety of other computer driven design and predictive technologies can greatly enhance the fatigue resistance of a component at the design stage.  But they can not prevent fatigue failures.  That’s because the next two threats of fatigue failure are beyond the designer’s control.

The Manufacturing Process

Manufacturing processes are a rich, though unintended, source of stress concentrations from which fatigue cracks can initiate.  The list is almost endless, and includes rough machined surfaces from dull tooling or excessive feeds and speeds, burrs from cutting or drilling operations, and insufficient chamfers or corner radiuses. Welds, even when technically faultless, provide geometric stress concentrations.  Defective welds and welding procedures may result in porosity and high hardness heat affected zones from which fatigue can initiate. Mechanical fasteners – bolts, screws, studs, and rivets- are highly prone to fatigue failure, either due to defects in the fastener itself, or to insufficient tightening torque during the assembly stage of the manufacturing process.

Care in manufacturing and a good quality control program will avert many of these potential sources of fatigue initiation.  However, despite the best quality control program, the manufacturer is often at the mercy of their raw material supplier. These suppliers may open the door to fatigue failure through castings which contain excessive porosity or  microstructural defects, mill products which are work hardened, forgings with undetected laps or seams, or gross non-metallic inclusions in any of these products. Appropriate specifications on outsourced stock and components are vital in guaranteeing their quality, but as with so many aspects of production, they are a compromise. Loose specs solicit low cost bids, but a potentially high percentage of defective products, while tight specs limit the number of vendors capable of meeting them and drive costs higher, cutting into profits.

Read part 3 – The Service Environment