Page 333 - Mechanical Behavior of Materials

8

Fracture of Cracked Members

8.1 INTRODUCTION
8.2 PRELIMINARY DISCUSSION
8.3 MATHEMATICAL CONCEPTS
8.4 APPLICATION OF K TO DESIGN AND ANALYSIS
8.5 ADDITIONAL TOPICS ON APPLICATION OF K
8.6 FRACTURE TOUGHNESS VALUES AND TRENDS
8.7 PLASTIC ZONE SIZE, AND PLASTICITY LIMITATIONS ON LEFM
8.8 DISCUSSION OF FRACTURE TOUGHNESS TESTING
8.9 EXTENSIONS OF FRACTURE MECHANICS BEYOND LINEAR ELASTICITY
8.10 SUMMARY

OBJECTIVES

• Understand the effects of cracks on materials and why the fracture toughness, K Ic ,isa
measure of a material’s ability to resist failure due to a crack. Explore trends in K Ic with
material and with variables such as temperature, loading rate, and processing.
• Evaluate the effects of cracks in engineering components, using linear-elastic fracture
mechanics and applying the stress intensity factor, K, to combine stress, geometry, and crack
size to characterize the severity of a crack situation.
• Analyze the effects of plasticity in cracked members, including plastic zone sizes, constraint
effects due to plate thickness, and fully plastic limit loads, and briefly introduce advanced
fracture mechanics methods.

8.1 INTRODUCTION

The presence of a crack in a component of a machine, vehicle, or structure may weaken it so that
it fails by fracturing into two or more pieces. This can occur at stresses below the material’s yield
strength, where failure would not normally be expected. As an example, photographs from a propane
tank truck failure caused in part by pre-existing cracks are shown in Fig. 8.1. Where cracks are
difficult to avoid, a special methodology called fracture mechanics can be used to aid in selecting
materials and designing components to minimize the possibility of fracture.

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