Mechanics of Materials Lab Lecture 18 Impact Test

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

Strain Energy Modulus of toughness & modulus of resilience Increasing the strain rate increase strength, but decrease ductility 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

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

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

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

Ductile-Brittle Transition Jiangyu Li, University of Washington

Brittle Failure Jiangyu Li, University of Washington

Failure Criteria • Materials assumed to be • Materials have flaw or crack in them: perfect: – Linear Elastic Fracture – Brittle Materials Mechanics (LEFM) • Max Normal • Stress intensity factor (K) Stress describes the severity of the existing crack – Ductile Materials condition • Max Shear Stress • If K exceeds the Critical • Octahedral Shear stress intensity (Kc), then failure will occur Stress Jiangyu Li, University of Washington

Stress Concentration Jiangyu Li, University of Washington

Crack Tip in Real Materials Jiangyu Li, University of Washington

Effect of Crack Length Jiangyu Li, University of Washington

Brittle vs. Ductile Behavior Jiangyu Li, University of Washington

Jiangyu Li, University of Washington

Assignment • Mechanical Behavior of Materials 4. 34, 8. 1, 8. 3 Jiangyu Li, University of Washington