1. Mechanics of Materials Lab
Lecture 11
Impact Test and Stress Concentration
Mechanical Behavior of Materials Section 4.8, 8.1, 8.2
Jiangyu Li
University of Washington
Jiangyu Li, University of Washington

2. Jiangyu Li, University of Washington
Impact Test
Charpy V-notch & Izod tests most common
Energy calculated by pendulum height difference
Charpy – metals, Izod - plastics
Jiangyu Li, University of Washington

3. Different heat treatments of AISI 4140 steel – harder on left
Fracture Surface
Different heat treatments of AISI 4140 steel – harder on left
Jiangyu Li, University of Washington

4. Trend in Impact Behavior
Toughness is generally proportional to ductility
Also dependent on strength, but not so strongly
Brittle Fractures
Lower energy
Generally smooth in appearance
Ductile Fracture
Higher energy
Rougher appearance on interior with 45° shear lips
Jiangyu Li, University of Washington

5. Ductile-Brittle Transition
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Brittle Failure
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Stress Concentration
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Stress at Crack Tip
Plane Stress:
Thin
Poisson Contraction
Plane Strain:
Thick
Constrained in z direction:
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9. Plane Stress and Strain
Plane Stress Mixed Plane Strain
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10. Crack Tip in Real Materials
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11. Jiangyu Li, University of Washington
Failure Criteria
Materials have flaw or crack in them:
Linear Elastic Fracture Mechanics (LEFM)
Stress intensity factor (K) describes the severity of the existing crack condition
If K exceeds the Critical stress intensity (Kc), then failure will occur
Materials Assumed to be perfect:
Brittle Materials
Max Normal Stress
Ductile Materials
Max Shear Stress
Octahedral Shear Stress
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Effect of Crack Length
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13. Brittle vs. Ductile Behavior
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14. Jiangyu Li, University of Washington
Assignment
Mechanical Behavior of Materials
4.34, 8.1, 8.3
Jiangyu Li, University of Washington