stress and strain calculations

  • Measure A0 and L0 for 2 specimens (steel and plexiglass).
  • Perform tensile tests on the two specimens and generate curves of Load (N) vs. distance (mm).
  • Convert this data to plots of engineering stress (MPa) vs. strain using the original lengths and x-sectional areas.
  • Determine the experimental material properties in the table below (use 0.002 offset for yield stress/strain) and answer the questions.
  • Compare your results to tabulated textbook values (plexiglass = poly methyl methacrylate in Table 15.1 on pg 572 and steel 1020 in Appendix B of your text).

Questions:

  • Determine the following experimental (based on your plots) and theoretical values for each:
sample E(GPa) σy(MPa) εy σuts(MPa) σf(MPa) εf Book E(GPa) Book   σy(MPa) Bookσts

(MPa)

% Elongation at failure Ur(MPa)
Steel 21.4 196.5 0.011 321 173 0.383 207 210 380 25% 1.081
Plexiglass 1.09 45.5 .0435 45.6 45.6 .0435 2.24-3.24 48.3-72.4 53.8-73.1 2-5.5% 0.99

 

  • What is the optimal shape geometry for a tensile specimen? Why?
  • Which material was stiffer? Why?  Did the values for E match the theory?  If not, why?
  • Which material was more resilient? (show calculation)
  • Which material had higher strength? Did this match the textbook’s prediction?
  • Describe the deformation and fracture surface of each material and what this conveys about the material (crack propagation speed, roughness, necking, angle). Use concepts discussed in the lecture.
  • Based on the stress-strain data, which material is most ductile? Which is most brittle? Does this match the fracture surface outcome (6)?  What was the percent elongation of the steel specimen vs. the plexiglass specimen?
  • Which polymer curve on VMSE is most similar to the plexiglass sample (HDPE, Nylon, Bakelite, or Rubber)? How so?
  • What does it mean when σf is much lower than σuts?  Did you observe this in either sample?  If we were to plot True Stress vs. True Strain, how would the curve look different?
  • Why might our experimental results be different than the textbook data? (Think about statistical variation and major sources of error or uncertainty in our measurement). Give at least 3 reasons.

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