Lecture 18 Impact Test and Stress Concentration

1. Mechanics of Materials Lab Lecture 18 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. Strain Energy Increasing the strain rate increase strength, but decrease ductility Modulus of toughness & modulus of resilience Jiangyu Li, University of Washington

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

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

5. 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 Trend in Impact Behavior Jiangyu Li, University of Washington

6. Effect of Temperature Decrease temperature increase strength, but decrease ductility Jiangyu Li, University of Washington

7. Ductile-Brittle Transition Jiangyu Li, University of Washington

8. Brittle Failure Jiangyu Li, University of Washington

9. 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 Jiangyu Li, University of Washington

10. Stress Concentration Jiangyu Li, University of Washington

11. Crack Tip in Real Materials Jiangyu Li, University of Washington

12. Effect of Crack Length Jiangyu Li, University of Washington

13. Brittle vs. Ductile Behavior Jiangyu Li, University of Washington

14. Jiangyu Li, University of Washington

15. Assignment • Mechanical Behavior of Materials 4.34, 8.1, 8.3 Jiangyu Li, University of Washington