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Don’t Be Afraid Of A Little Creep (Strain)

Vince Farrell

One of the most interesting projects I’ve worked on was one that involved deformation due to creep effects. For those who aren’t familiar with this phenomenon, click here for a quick refresher.

Looking back, the most difficult part of that project was determining the material properties, particularly the creep constants. In this blog I wanted to share how we developed those constants in the context of the project, and get you thinking about creep effects in your own products.

Background

A customer of ours designed an assembly that needed to stay latched together for a long period of time. Their primary concern was that over time, the latch would deform due to sustained load application and “unlatch” itself. The design of the assembly was set, but they were trying to decide which material to use.

Obtaining Creep Constants

Where did we get them? In general, there are two ways:

  1. The material manufacturer provides the constants.
  2. Someone would have to derive the constants from manufacturer-supplied creep data.

The worst case scenario would be that both the material data and constants aren’t available; in that case the material would have to get tested, and then you would have to choose one of the two options above.

In our project, we had the manufacturer-supplied creep curves (method #2); all we had to do was develop the constants and run the analysis. Let’s break down how we developed the creep constants to tackle this project:

Step 1: Picking the Correct Curve The first step was picking the correct curve from the manufacturer-supplied data (Fig. 1).

Strain vs Time graph

[Figure 1. Experimental creep data.]

Each curve in Fig. 1 represents the material behavior at a specific stress level and ambient temperature. To pick the appropriate curve, we needed to know the ambient temperature and what maximum von Mises stress levels the model was going to see. The appropriate ambient temperature was easy to find; but in order to determine the stress levels, we needed to either:

  1. Use hand calculations.
  2. Set up an analysis.

We ended up using method #2. An added benefit to this method was that we could kill two birds with one stone; we could run a nonlinear analysis with the creep effects turned off to find applied stress and then use the same study later with the creep effects off to obtain the results after creep.

Step 2: Understanding the Equation

The material constants we needed are used in the Bailey-Norton equation. The Bailey-Norton equation is a power law-type equation that is commonly used to create creep curves based on experimental data. The equation has the form: Mathematical formula

where Ec is the creep strain, σ is the stress level in MPa, t is the time in seconds, and A1, A2, and A3 are the material constants.

Step 3: Developing Constants

Our next step was to take the provided experimental data and develop constants for the Bailey-Norton equation that would match that data.

This is where things got a bit hairy. If you look at the equation in step 2, you’ll notice that we had one equation with 3 unknowns (A1, A2, and A3). There were probably more elegant ways for solving this, but what we ended up doing was we:

  • Reviewed the SolidWorks Knowledgebase (article S-018620) to see what range of values was expected for A1, A2, and A3 and what values for A2 and A3 we should start with.
  • Developed an Excel spreadsheet that took those starting values for constants A2 and A3, to develop A1.
  • Compared our “best guess” curve to the experimental data.
  • Continued to adjust A2 and A3 (which in turn would change A1) until our Bailey-Norton curve matched the experimental data (Fig. 2).

Bailey-Norton Equation vs. Experimental Data

[Figure 2. Comparison of Bailey-Norton Equation with best guess constants to experimental data.]

Step 4: Adjusting the Constants

We weren’t quite done yet. Once these three constants were found, we had to divide the A1 constant by 100 because the constants that we developed predicted creep strain as a percent strain; SolidWorks needed the A1 constant to be in actual (length/length) strain. Constants A2 and A3 were unit-less so they were left as is.

Step 5: Setting Up and Running the Analysis

With constants in hand, we duplicated our original nonlinear analysis (remember, in step 1?), turned on the creep effects option, updated our material properties, and ran our analysis. As a double check, we looked at our area of interest to see what our stress levels were. Our stress levels still matched with the stress curve we picked in step 1 (which was a good thing); if they didn’t match we would have gone back to step 1, choose a more appropriate curve, and adjust the creep constants.

Conclusion: Don’t be Afraid of the Creep

Based on the results we provided, the customer was able to determine which material was best suited for their application. Another successful project in the bag!

Think about your own projects and designs. Are they affected by creep? Are you considering it in your analyses? Let me know your questions, thoughts, and comments below!

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    Jim Clark

    Thanks for the blog entry.

    Some day I may need this.