Fatigue in the “Real World”
Posted on 27. Apr, 2010 by Rob in Failure Analysis
In the “real world” fatigue usually – that’s usually, not always – initiates at a location that acts as a stress concentration, or focal point, to the stresses imposed on a component. Stress concentrations take a wide variety of forms. They include geometric features (such as holes, slots, corners and radii), rough areas of surface finish, welds, corrosion pits, and microstructural defects such as inclusions.
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
