Creep Experiment

Introduction

Q: What happens when we attach a very heavy weight to a wire at relatively low temperature?
A: The wire snaps right away!

Q: What happens when we hang a lighter weight from the wire at a higher temperature?
A: At first, seemingly nothing—the weight just hangs there. Or does it? Under some circumstances, we may return several hours (…or severals days… or several weeks…) later and find that the weight has gradually stretched the wire.

This deformation at elevated temperature under constant load is called creep. It depends on the wire material and the temperature of the room. Let's explore creep in more detail, and then look at the results of a real creep experiment. We will look at a fractured wire too!

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Procedure

If we hang a small weight (4.8 kg = 10.6 lb) on a long 1/8” diameter wire made of 60/40 solder, at first we will not notice significant elongation. This load is well below that required to plastically deform the material rapidly, and only a very small amount of elastic elongation has occurred. The animated video below shows the experiment. (See a screenshot.) An indicator bar is attached to the bottom of the weight. A centimeter scale is shown in the background so that we can monitor the elongation of the wire over time.

Experimental Conditions

Summary of Creep Test Conditions
  • Wire Material: Solder = 60% Sn - 40% Pb
  • Melting Temperature: 183°C = 456 K
  • Wire Diameter: 1/8" = 0.125" = 3.175 mm
  • Load: 4.8 kb = 10.6 lbs

Experiment

The load applied to the wire (4.8 kg) is only about one tenth of the load required to get immediate plastic deformation. The experiment shows, however, that there is slow, continuous deformation (creep deformation) over a period of several days. The graph below shows a plot of the wire length versus time during the creep test. The wire is initially 30 cm long, but after more than 90 hours, it has stretched to around 140 cm.

Final Data

  • Original wire length: 12" = 1 ft = 30.5 cm
  • Initial data indicator: 47 cm
  • Final data indicator: 156 cm
  • Final wire length: 54.9" = 4.5 ft = 139.5 cm
  • Test temperature: 77°F
  • Test duration: 90.8 hours = 3 days, 18 hours, 48 seconds

Conclusions