Two buzzwords flying around the engineering community are "plastic analysis." It sure sounds good, but what good does it do you when you are working out in the plant, factory or production unit? Well, first, let's get on some common ground as to what plastic analysis is all about. Plastic analysis is a numerical method for the determination of the stress and strain in a mechanical component when certain portions of the component are above the yield stress. Usually, the numerical method used for plastic analysis is the finite element method. Maybe that still does not help you understand plastic analysis much, but let's keep going.
Plastic analysis
Plastic analysis is a higher-level structural mechanics analysis that one can apply to static and rotating equipment when lower-level elastic analysis does not "cut the mustard." This means it is possible to revisit a design or failure analysis to get a better idea of what happens under its operating or design load. You might ask, "What makes it so good?" Well, the answer is simple. It closely predicts the actual response of the mechanical component. However, if the equipment operates completely in the elastic region, this analysis is not required.
Elastic analysis
Remember that elastic analysis means the stress is directly proportional to the strain. In reality, many mechanical components have locations that yield very slightly and the conditions are just local. In this local case, load is transferred to other sections of the mechanical component. The stresses described here would be secondary, strain-controlled stresses. Another case is where the local area goes plastic and the rest of the component keeps straining with time and eventually yields. These would be called load-controlled stresses. In this case, the stress would be primary stress and it would most likely cause failure.
Applications
In the Pressure Vessel Code, much of the analytical work is elastic. In an elastic analysis, the stresses calculated are often higher than yield. Stresses are classified, acceptance criteria are applied and a determination is made about whether the component meets code. Sometimes it is difficult to classify the stresses in a mechanical component. Experience is always a big help here. Since a plastic analysis simulates what is actually going on in the material, there is no need to classify the stresses. Sometimes a plastic analysis is used to aid in classifying stresses.
A few examples of where a plastic analysis would be used are:
- Actual nonmetal components.
- Pressure vessels for a "fit-for-service" analysis.
- Pressure vessel design.
- Failure analysis.
- High-temperature and high-pressure mechanical equipment.
- Rotating equipment.
- Cyclic load to determine if a component experiences "shakedown" or "ratcheting."
- Reminders
Here are some key points to remember about plastic analysis:
- Use it when the solution or result of an elastic analysis costs money and when the cost of a plastic analysis will have a return on investment.
- The material properties that define the elastic and plastic behavior need to be strong. Sometimes materials testing has to be performed to get good data.
- Run the elastic model first to determine areas of plastic flow. This helps in the design of the plastic model.
- Have an experienced professional engineer review the results.
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