In a previous article I discussed the degradation of materials due to exposure to stressors (use conditions) and how to identify stressors. Cracks form and grow in axles and shafts due to cyclic stress, steel screws corrode when exposed to water, some plastics become brittle when exposed to sunlight, and coatings on surfaces can wear away. When too much degradation occurs, components and joints fail, leading to product failure.

Things to consider during design

If you’re someone who likes to design reliable products, you must think about the stressors and their effects. When designing a product, we must identify the following things [Read more…]

Fatigue is a common degradation and failure mechanism. It involves localized, permanent damage to metals exposed to cyclic stress. The stress can be uniaxial, bending, or torsional resulting from a variety of sources including an applied force, vibration, acceleration and deceleration, and differences in thermal expansion between mating components exposed to heating and cooling cycles. Localized means the damage is confined to a small portion of a component or joint.

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Stainless steel is known for its corrosion resistance in many environments, with different alloys having different levels of corrosion resistance. Also, stainless steels are available with a wide range of strengths. Understanding the reasons for the corrosion resistance is helpful for selecting alloys based on the required strength and environment to which the steel will be exposed.

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Fatigue cracks that originate at inclusions. Stainless steel intergranular corrosion due to chromium carbide precipitates. Low steel toughness because martensite not tempered enough. Low aluminum strength because of excessive grain boundary precipitation. Orange peel due to large grains.

These are examples of how problems with a metal’s microstructure lead to reliability and performance problems. Of course, there are thousands of examples of microstructures that lead to good reliability and good performance.

One hurdle to understanding metallurgy is being able to think small – very small. Less than a millimeter. Less than a micron. And sometimes on the scale of atoms.

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When designing components consider fatigue or stress corrosion cracking. It’s important to be cognizant of the residual stresses in the component. Understanding residual pressure and its sources is important when making decisions about a component’s shape, features, alloy, and fabrication process.

Fatigue and stress corrosion cracking require the presence of tensile stresses on a component. When residual presures are tensile they add to the applied tensile pressure, reducing the life of a component. In fact, components sometimes fail due to stress corrosion cracking when residual stress is the only source of tensile stress.

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One failure mechanism that I’m frequently asked about is hydrogen embrittlement of carbon and low-alloy steel. So, in this article I’ll discuss that topic.

Hydrogen embrittlement is the result of the absorption of hydrogen by susceptible metals resulting in the loss of ductility and reduction of load bearing capability. Sustained stress on an embrittled material can result in cracking and fracture at stresses less than the metal’s yield strength.

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In a recent Accendo podcast, Chris Jackson and Fred Schenkelberg discussed who is responsible for producing a reliable product, which included designers and suppliers. I’m going to weigh in.

The reliability of any product depends on the reliability of the individual components and joints within the product. That is, the ability of the components and joints to withstand exposure to stressors without degrading to the point that they fail, resulting in the product no longer performing as required. Stressors, which include corrosion conditions, fatigue, and wear, were discussed in an earlier article.

Whether individual components and joints have the reliability required boils down to two basic aspects of engineering – selection and control. The appropriate form (i.e. shape, dimensions, features) and materials for components and joints must be selected during product design. Then, systems must be put in place to control fabrication of components and joints, ensuring their form and materials are as specified. This will enable the components and joints to consistently meet performance and reliability requirements.

So, who’s responsible for this selection and control?

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Metal strength and fracture toughness are important mechanical properties for components exposed to fatigue conditions and components with stress concentrations. Optimization of the two properties through alloy selection and component fabrication must be considered when designing components for these situations.

For structural components, strength and fracture toughness are two important mechanical properties. Yield strength is the stress a metal can withstand before deforming. Tensile strength is the maximum stress a metal can support before starting to fracture. Fracture toughness is the energy required to cause a material that contains a crack to fracture.

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Here’s an example of how a metallurgical failure analysis led to identification of the root cause of a failure, and to identification of the corrective actions needed to prevent the failures from recurring.

Failure analysis

As I discussed in my previous article, metallurgical failure analysis can be used to improve product reliability. The information from failure analysis of a failed component is used to determine the root cause of the failure. Once the root cause is identified, the failure analysis data and findings is used to help identify the corrective measures required to prevent the failure from recurring.

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Failures during product testing and use are a fact of life. Even with the most robust design we can develop an overly aggressive reliability test or find users that dish out punishing treatment, causing product failures. And for designs that are less robust, standard reliability tests and normal users will cause failures, occasionally or frequently depending on the design robustness.

When a product fails, its related to failure of individual components and/or joints between components. When a component or joint fails, it’s because their materials degraded to the point that the component or joint could no longer perform as required.

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