Qual Dos Dois Materiais Apresenta Maior Tenacidade

Introduction to mechanical testing

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Contents

Main pages

Aims

Introduction

Theory 1: Tensile testing

Theory 2: Precipitation hardening in alloys

Method

Results 1

Theory 3: Heat treatment of copper

Results 2

Notes on tensile testing

Results 3

Summary

Questions

Going further

Additional pages

Definitions of stress and strain

Resistance to motion of dislocations

The effects of grain size on yield strength

Aims

On completion of this tutorial you should:

understand the general shape of the tensile stress-strain curve

understand how the microstructure of a material can effect its properties, in particular the yield-strength and stiffness

understand some ways that the microstructure can be controlled

Introduction

While it is very common to associate properties with a metal, it is possible to produce a wide range of properties from the same material, by varying the composition slightly, or by varying its processing. The different ways of processing a metal include how it has been cooled and worked, and any heat treatments it has been subjected to.

These processes and compositional changes affect the microstructure of the material, in particular the grain shape and size and the dislocation density, as well as phase structure and precipitates.

In this package changes in microstructure will be studied for two different materials. The first is aluminium, which will demonstrate how the phase structure and the precipitates caused by the addition of small amounts of copper can effect mechanical properties, in particular in the tensile test.

The second material is copper, which will demonstrate how the grains and the dislocation density are affected by work hardening and by heat treatments. This will be shown with both the tensile test and with three-point bending.

Theory 1: Tensile testing

In a tensile test, a sample is extended at constant rate, and the load needed to maintain this is measured. The stress (σ) (calculated from the load) and strain (ε) (calculated from the extension) can either be plotted as nominal stress against nominal strain, or as true stress against true strain (definitions). The graphs in each case will be different:

Graph of nominal stress against nominal strainGraph of true stress against true strain

Graphs illustrating the difference between nominal stress and strain and true stress and strain.

There are two main types of strain - elastic strain and plastic strain. Elastic strain is the stretching of atomic bonds, and is reversible. Elastic strain can be related to the stress by Hooke's law :

σ = Eε

where E is the Young's modulus .

Plastic strain, or plastic flow, is irreversible deformation of a material. There is no equation to relate the stress to plastic strain.

Several points on the graph can be defined:

A - limit of proportionality - the point beyond which Hooke's Law is no longer obeyed. This is the point at which slip (or glide ) due to dislocation movement occurs in favourably oriented grains. The graph is linear up to this point, and begins the transition from elastic to plastic deformation above this.

B - yield stress - the stress at which yielding occurs across the whole specimen. The stress required for slip in a particular grain will vary depending on how the grain is oriented, so points A and B will not generally be coincident in a polycrystalline sample. At this point, the deformation is purely plastic.

C - proof stress - a third point is sometimes used to describe the yield stress of the material. This is the point at which the specimen has undergone a certain (arbitrary) value of permanent strain, usually 0.2%. The stress at this point is then known as the 0.2% proof stress. This is used because the precise positions of A and B are often difficult to define, and depend to some extent on the accuracy of the testing machine.

D - ultimate tensile strength (UTS) - the point at which plastic deformation becomes unstable and a narrow region (a neck) forms in the specimen. The UTS is the peak value of nominal stress during the test. Deformation will continue in the necked region until fracture occurs.

E - final instability point - the point at which fracture occurs, ie the failure point

F - fracture stress - The stress at which fracture occurs - only obtainable from the true stress-strain curve. See fracture toughness .

Theory 2: Precipitation hardening in alloys

Duralumin is an aluminium alloy containing 4wt% copper, as well as smaller amounts of other elements. The impurities in the material changes its properties by changing the microstructure, and since the distribution of the copper atoms can be varied using heat treatments, a variety of microstructures, and hence properties can be produced.

In the samples used in this experiment, the copper forms precipitates of CuAl2 within an aluminium