Lecture 18 Impact Test and Stress Concentration
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Transcript of 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
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Strain Energy
Modulus of toughness & modulus of resilience
Increasing the strain rate increase strength, but
decrease ductility
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Impact Test
• Charpy V-notch & Izod tests most common
• Energy calculated by pendulum height difference
• Charpy – metals, Izod - plastics
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Fracture Surface
Different heat treatments of AISI 4140 steel – harder on left
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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
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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
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Assignment
• Mechanical Behavior of Materials 4.34, 8.1, 8.3