Jiangyu Li, University of Washington Lecture 18 Impact Test and Stress Concentration Mechanical Behavior of Materials Section 4.8, 8.1, 8.2 Jiangyu Li University of Washington Mechanics of Materials Lab

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 perfect: –Brittle Materials Max Normal Stress –Ductile Materials Max Shear Stress Octahedral Shear Stress 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 (K c ), then failure will occur

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

Assignment Mechanical Behavior of Materials 4.34, 8.1, 8.3