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Failure





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  1. Introduction

Failure of materials may have huge costs. Causes included improper materials selection or processing, the improper design of components, and improper use.

  1. Fundamentals of Fracture

Fracture is a form of failure where the material separates in pieces due to stress, at temperatures below the melting point. The fracture is termed ductile or brittle depending on whether the elongation is large or small.

Steps in fracture (response to stress):

· track formation

· track propagation

Ductile vs. brittle fracture

  Ductile Brittle
deformation extensive little
track propagation slow, needs stress fast
type of materials most metals (not too cold) ceramics, ice, cold metals
warning permanent elongation none
strain energy higher lower
fractured surface rough smoother
necking yes no

· Ductile Fracture

Stages of ductile fracture

· Initial necking

· small cavity formation (microvoids)

· void growth (elipsoid) by coalescence into a crack

· fast crack propagation around neck. Shear strain at 45o

· final shear fracture (cup and cone)

The interior surface is fibrous, irregular, which signify plastic deformation.

· Brittle Fracture

There is no appreciable deformation, and crack propagation is very fast. In most brittle materials, crack propagation (by bond breaking) is along specific crystallographic planes (cleavage planes). This type of fracture is transgranular (through grains) producing grainy texture (or faceted texture) when cleavage direction changes from grain to grain. In some materials, fracture is intergranular.

    1. Principles of Fracture Mechanics

 

Fracture occurs due to stress concentration at flaws, like surface scratches, voids, etc. If a is the length of the void and rthe radius of curvature, the enhanced stress near the flaw is:

sm » 2 s0 (a/r)1/2

where s0 is the applied macroscopic stress. Note that a is 1/2 the length of the flaw, not the full length for an internal flaw, but the full length for a surface flaw. The stress concentration factor is:

Kt = sm/s0 » 2 (a/r)1/2

Because of this enhancement, flaws with small radius of curvature are called stress raisers.

    1. Impact Fracture Testing

Normalized tests, like the Charpy and Izod tests measure the impact energy required to fracture a notched specimen with a hammer mounted on a pendulum. The energy is measured by the change in potential energy (height) of the pendulum. This energy is called notch toughness.

Ductile to brittle transition occurs in materials when the temperature is dropped below a transition temperature. Alloying usually increases the ductile-brittle transition temperature (Fig. 8.19.) For ceramics, this type of transition occurs at much higher temperatures than for metals.

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