FRACTURE and TOUGHNESS DESIGN 11/21/2018 11/21/2018 FRACTURE and TOUGHNESS DESIGN © 2007 Microsoft Corporation. All rights reserved. Microsoft, Windows, Windows Vista and other product names are or may be registered trademarks and/or trademarks in the U.S. and/or other countries. The information herein is for informational purposes only and represents the current view of Microsoft Corporation as of the date of this presentation. Because Microsoft must respond to changing market conditions, it should not be interpreted to be a commitment on the part of Microsoft, and Microsoft cannot guarantee the accuracy of any information provided after the date of this presentation. MICROSOFT MAKES NO WARRANTIES, EXPRESS, IMPLIED OR STATUTORY, AS TO THE INFORMATION IN THIS PRESENTATION. 1

Strength Toughness Resistance of a material to plastic flow Resistance of a material to the propagation of a crack

This type of test provides a comparison of the toughness of materials However, it does not provide a way to express toughness as a material property

Remote stress applied to a cracked material The local stress is proportional to the number of lines of force which rises steeply as the crack tip is approach

Cracks concentrate stress At the tip of a crack the stress can be very high: Metals: the zone near the crack tip is plastic Ceramics: the zone near the crack tip has micro-cracks Composites: near the crack tip, delamination and debonding

Cracks propagate when the stress intensity factor exceeds a critical value The critical value is known as the fracture toughness K1c

A plastic zone forms at the crack tip where the stress would otherwise exceed the yield strength

A material transitions from yield to fracture at a critical crack length

Critical crack lengths are a measure of the damage tolerance of a material Tough metals are able to contain large cracks but still yield in a predictable, ductile manner

Values range from 0.01 – 100 MPa√m Contours show the toughness, Gc

Transition crack length plotted on chart Values can range from near-atomic dimensions for ceramics to almost a meter for ductile metals

Characteristic of ceramics and glasses Local stress rises as 1/√r toward the crack tip If it exceeds that required to break inter-atomic bonds, they separate, giving a cleavage fracture

Materials contain inclusions which act as stress concentrations when loaded The inclusions separate from the matrix causing voids to nucleate and grow, causing fracture

If a material is ductile, a plastic zone forms at the crack tip Within the plastic zone, voids nucleate, join, and link to cause fracture The plasticity blunts the crack tip, reducing the severity of the stress concentration

At low temperatures some metals and all polymers become brittle As temperatures decrease, yield strengths of most materials increase, leading to a reduction in the plastic zone size Only metals with an FCC structure remain ductile at the lowest temperature

Impurities in an alloy are normally found in grain boundaries This leads to a network of low-toughness paths that can lead to brittle fracture

Increasing the yield strength of a metal decreases the size of the plastic zone surrounding a crack This leads to decreased toughness

Fillers, impact modifiers, and fiber reinforcement can significantly alter the fracture toughness of polymers

When a crack grows in a matrix, the fibers remain intact and bridge the crack

SUMMARY Toughness is resistance to the propagation of a crack Tough materials yield rather than fracture Toughness is a material property By choosing materials with sufficient toughness, design can be immune to small cracks Many metals and some composites are tough “enough” for this kind of design Polymers and ceramics are not tough enough in most cases