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A listing in reverse chronological order of articles by:
Fatigue involves localized, permanent damage to metals exposed to cyclic stress. In a previous article I discussed the fatigue mechanism. This article covers factors that can be addressed to improve high-cycle fatigue life
Factors that influence fatigue life
Several design, material, and fabrication factors influence component and joint fatigue life, including the following: [Read more…]
Once word got out that I was taking graduate-level courses in statistics, I dreaded the knock on the door. Colleagues, some of which I knew and others from some far reach of the company, would ask if I could take a look at their data. I didn’t learn the necessary first steps with a stack of data in class.
I’ve lost count of the number of data sets I’ve reviewed and analyzed. I know there are important considerations and questions before creating the first plot. Let’s review the essential first steps you should take when presented with data.
[Read more…]
I was recently asked by a product design engineer why their organization struggles with reliability even though they have a very ‘robust’ design process that seemingly has lots of different ‘good’ reliability engineering activities embedded in it. And when I say ‘good’ reliability engineering activities, I mean activities that have (in the past) shown to have a really good impact on reliability.
When I looked at some of this engineer’s processes as summarized by a ‘process flow chart,’ the reason his organization struggled with making reliable products was quite obvious.
The process was so complex that it became the ‘product.’
[Read more…]
After 30 minutes of being on hold, I wasn’t sure what to expect from customer service for a product reliability issue. The scratchy soundtrack didn’t foretell a great experience either.
Once connected to a company representative, we resolved the issue quickly and satisfactorily. Unfortunately, that was a pleasant surprise. All too often, the frequently repeated “Your call is important to us.” (an Amazon affiliate link) just isn’t true, in my experience.
Did you know that customer service can add significant value to an organization, especially when product reliability doesn’t meet customers’ expectations? Besides providing the team with valuable product reliability performance information for past products, the service team can improve customer loyalty.
[Read more…]
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.
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…]
While I’m not much of a New Year’s Resolution guy, I guess I’ve set a resolution. I need to restart writing weekly articles for Accendo Reliability.
The thing is, I struggle to write. Plenty of other interests and tasks keep me away from the keyboard. Yet, as I explained to a few new authors how writing and posting on Accendo Reliability is a good thing, I realized I have been putting off hitting the keyboard again.
Over the past few weeks, I’ve explained to those interested in writing articles about the virtuous circle created by having many authors contribute articles, which increases interest, engagement, and traffic to those articles, which further increases the reach of those articles. Of course, this is just one reason, and there are others, plus plenty of hurdles to overcome.
[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.
The Arrhenius Equation is widely used and accepted by reliability and validation engineers from many industries to develop accelerated life tests (ALT) for thermal aging environments.
The general form of the equation describes the reaction rate of a process as a function of temperature (K), the Boltzmann constant (8.617E-5 eV/K) and two empirical factors. It can be written as:
[Read more…]
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.
[Read more…]
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.
[Read more…]
An example of an automotive validation test reliability target is R97C50. In this case, the objective of the validation effort would be to demonstrate at one life on test, with 50% confidence, that the product has a reliability of 97% or better.
Does this mean that a field failure rate of 3% can be expected?
[Read more…]
As you begin design validation (DV) and product validation (PV) testing you are entering the more formal phase of a test program. Usually, you are working to internal and/or customer test specs with timelines that are critical and little margin for error. If you’ve conducted adequate design verification testing and given your parts a chance to fail on test exposures that are similar to those your product will see in the field, you have reason to expect to be successful in validation [see previous article outlining accelerated test methods: Expect to Pass Validation].
[Read more…]
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.
[Read more…]
Product Validation Testing is a critical and expensive endeavor. The part build process must align with the latest prototype process (for Design Validation (DV)) or production process (for Product Validation (PV)) and be fully documented for posterity. Depending on the product and application, the validation test plan consists of a battery of tests, some of which are lengthy – often six months or more. Because of this, DV and PV test plans are invariably on the critical path for a customer program. Test failures that require fixing the design and repeating DV or PV jeopardize project timing and company profits. Further, they jeopardize the customer program along with your company’s reputation.
[Read more…]
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.
[Read more…]