lecture 18 impact test and stress concentration

Jiangyu Li, University of Washington

Lecture 18Impact Test and Stress Concentration

Mechanical Behavior of Materials Section 4.8, 8.1, 8.2

Jiangyu LiUniversity 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

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Ductile-Brittle Transition

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Brittle Failure

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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 (Kc), then failure will occur

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Stress Concentration

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Crack Tip in Real Materials

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Effect of Crack Length

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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