Table of Contents

Seminar on Brittle and Ductile Fracture Report PDF Download

Contents

Introduction
Mechanism of Ductile Fracture
Mechanism of Brittle Transgranular Fracture (Cleavage)
Intergranular Fracture
Ductile to Brittle transition
Notched-bar Impact Tests
Ductile to Brittle Transition-Temperature Curve (DBTT)
Criterion for Transition Temperature
Metallurgical Factors affecting Transition Temperature
Conclusion

Introduction

Since the World War II there has been great progress in understanding the ways in which the materials fracture. Nevertheless, it is still not possible to use this knowledge, together with other material properties, for predicting fracture behaviour in engineering terms with a high degree of confidence. Metallic materials, especially alloys are highly complex. An indication of this complexity is given by the figure below, which shows various microstructural features (not all of which need to be present in a material) and also the two main types of fracture path, transgranular and intergranular. Of fundamental importance is the fact that almost all the structural materials are polycrystalline, i.e. they consist of aggregate of grains, each of which has a particular crystal orientation. The only exceptions are single crystal turbine blades for high performance jet engines.

Metals fail by two broad classes of mechanisms: Brittle and Ductile failure

The Brittle fracture has following characteristics:
• There is no gross plastic deformation of the material and failure occurs with low energy absorption.
• The surface of the brittle fracture tends to be perpendicular to the principal tensile stress although other components of stress can be factors.
• Characteristic crack advance markings frequently point to where the fracture originated.
• The path the crack follows depends on the material’s structure. In metals, transgranular and intergranular cleavage are important. For energy related reasons, a crack will tend to take the path of least resistance.

The Ductile fracture has the following characteristics

• There is considerable deformation before failure and lot of energy is required compared to ductile failure.
• The fracture surface is dull and fibrous. The appearance of a ductile fracture at a high magnification is a surface with indentation as if marked by an ice-cream scooper. This surface morphology is appropriately called dimpled
• Rupture by total necking is very rare because most metals contain second phase particles that act as initiation sites for void. However high purity metals such as copper nickel gold and other very ductile material fail with very high reduction in areas.
• Most structural material exhibit considerable strain before reaching the tensile or ultimate strength.

comparison of stress stain curve of brittle and ductile Material

Metallurgical Factors affecting Transition Temperature.

The shape and position of DBTT curve is important as it determines the transition temperature, which indicates where it is safe to use for the given application. There are several factors affecting DBTT curve.
• Crystal structure
• Interstitial atom
• Grain size
• Heat treatment
• Specimen orientation
• Specimen thickness