A Tensile strength and deformation

When materials are exposed to forces, such as tension (stretching forces ←□→) and compression (crushing forces →□←), they deform - that is, they change shape. The type of deformation depends on the type of force that is applied.

When a material is subjected to tension, its length will increase by a certain amount. This is called extension or elongation. It is especially important to understand the performance of materials in tension, as their tensile strength (ability to resist tension) is usually lower than their compressive strength (ability to resist compression).

 

B Elasticity and plasticity

Some materials can extend significantly, but still return to their original shape. A material’s ability to do this is called elasticity. Rubber is an example of a very elastic material - it can be elastically deformed to a considerable extent.

If a material has very low elasticity, and is strong, engineers say it is stiff. If a material has low elasticity and is weak, it is described as brittle - that is, it fractures (breaks, due to tension) very easily. Glass is an example of a brittle material.

Some materials can change shape significantly, but do not return to their original shape. We say these materials are plastic. Often, plasticity is described in specific terms. A material that can be plastically deformed by hammering or rolling - for example, lead (Pb) - is malleable. A material that can be drawn out (stretched) into a long length - for example, copper (Cu) - is ductile.

 

C Stages in elastic and plastic deformation

The graph below shows the typical extension behaviour of ductile materials in tensile testing - where a sample bar is subjected to a progressively increasing tensile force.

Points 0-1	The extension of the bar is proportional to the increase in tension. For example, when tension increases by 10%, length increases by 10%.
Point 1	The bar reaches the limit of proportionality. Beyond this point, length begins to increase at a slightly greater rate than tension.
Point 2	The elastic limit is reached. Beyond this point, the bar will no longer return to its original length. In many materials, the elastic limit occurs almost immediately after the limit of proportionality.
Point 3	The bar reaches its yield point. Once it yields, it continues to increase in length, even without a further increase in tension.
Point 4	This is the ultimate tensile strength (UTS) of the material. Beyond this point, a waist (a narrower section) appears at a point along the length of the bar, signalling that it is about to fracture.
Point 5	This is the fracture point, where the bar breaks in two.

18.1 Complete the sentences using the words in the box. You will need to use one word twice. Look at A opposite to help you.

 

compression

deformation

elongation

extension

tension

 

1. A stretching force is called........

2. A crushing force is called........

3. Extension is also called..........

4. Tension causes......or.......

5. Tension or compression cause......

 

18.2 Match the two parts to make correct sentences. Look at B and C opposite to help you.

1. If a material is stiff

2. If a material is brittle

3. If a material is plastic

4. If a material yields

5. If a material fractures

6. If a material is elastically deformed

 

a. it is malleable and/or ductile

b. it has low elasticity and low tensile strength

c. it has low elasticity and high tensile strength

d. it has been extended to a point before its elastic limit

e. it has been loaded beyond its ultimate tensile strength

f. it has been significantly plastically deformed, but not broken

18.3 Complete the magazine article about springs using words from A, B and C opposite.

How are the springs used in car suspension made springy? It sounds like a silly question, but think about it for a moment. In order for a spring to compress or extend, then return to its original shape, it must be (1)............... But springs are made from wire, and wire is made from very (2)........ metal (often cold drawn carbon steel). When the wire is manufactured, it is not only stretched beyond its (3)........................ -meaning it will no longer return to its original length - but also beyond its (4),........ where significant, irreversible (5)...... occurs.

The metal from which springs are made has therefore been (6)..... deformed and, consequently, needs to have its springiness put back.

To do this, once a spring has been formed into a coil, it is tempered - a process in which it is heated and kept at a high temperature for a sustained period. This 'resets' the atomic structure of the ^— metal (partly, at least), so that after tempering, the spring will behave as it should - it can be (7)...... deformed and will subsequently return to its original shape.

 

Over to you

Think about a device, vehicle or structure you're familiar with, and the materials used to make it. What properties do the materials have? Which properties are strengths in this situation? Which properties are weaknesses, and how are these weaknesses overcome?

 

Material properties 2

A Hardness

The hardness of a material affects its durability - that is, how long it will last. Generally, hard materials are more durable than soft materials, because they are better at resisting wear - progressively worsening damage - to their surfaces. Hardness can be defined in two main ways:

- Scratch hardness describes a material’s ability to resist being scratched. Materials with a high degree of scratch hardness are said to have good abrasion resistance - they are good at resisting damage due to abrasion (the action of two surfaces being rubbed together).

- Indentation hardness describes a material’s ability to resist indentations - that is, compressions in the surface of a material caused by impacts

 

B Fatigue, fracture toughness and creep

The article below is from an aviation magazine.

In aircraft construction, special attention must be paid to two materials problems that are well understood by mechanical and structural engineers.

One is fatigue, often called metal fatigue in metals. This problem is caused by cyclic loads - forces that continually vary. In aircraft, the wings are affected by cyclic loading as they frequently flex, continually bending up and down due to air turbulence. The consequence of fatigue is micro-cracking - the formation of cracks too small to see with the eye, and which worsen over time. The speed at which fatigue cracking progresses depends on the material’s fracture toughness. This is a measure of how easily cracks that have already formed continue to open up and increase in length.

Another problem is creep - where components become permanently deformed (stretched, for example), due to loads. Creep increases over time. The problem is made worse by heat, so is a major issue in engines, where both loads and temperatures are high.

 

C Basic thermal properties

Some materials conduct (carry or transmit) heat better than others. Therefore, thermal conductivity varies, depending on the material. Copper, for example, is an excellent thermal conductor. Polystyrene, on the other hand, is an excellent thermal insulator (and so a very poor thermal conductor).

As temperature increases, most materials expand (increase in size due to heating), and as temperature falls, they contract (decrease in size due to cooling). The extent to which expansion and contraction occur is measured by a material’s coefficient of thermal expansion - that is, its change in size for a given change in temperature. The coefficient for aluminium, for example, is 0.000023. This means that for an increase in temperature of one degree Celsius, a one-metre length of aluminium will increase in length by 0.000023 metres. This figure can also be referred as the coefficient of linear expansion, since it describes change in length (a linear measurement).

 

19.1 Complete the design brief for parts of a cutting machine using four of the words in the words in the box. Look at A opposite to help you.

 

abrasion

durability

durable

hard

indentation

scratch

soft

 

The cutting wheel will be surrounded by transparent guards. These will allow the operator to see the cutting wheel at all times, and will shield the operator from flying metal fragments. The guards must therefore be constructed from material with a high degree of (1)....... hardness, to protect it from impacts. As the guards will require regular cleaning, the action of wiping away metal fragments will result in (2)........ The guards must, therefore, have sufficient (3)........ hardness in order to retain their transparency and ensure adequate (4)......

 

19.2 Match the descriptions (1-4) to the technical terms (a-d). Look at B opposite to help you.

1. the cause of fatigue

2. the consequence of fatigue

3. a material property that helps to slow down cracking

4. permanent changes in shape due to the action loads over time

 

a. creep

b. cyclic loads

c. micro-cracking

d. fracture toughness

19.3 Complete the extract from an electrical design handbook using words and expressions from C opposite.

When comparing copper and aluminium as materials for electrical wires, it is necessary to consider their thermal properties. For instance, in situations where high temperatures are involved, it is important to

understand how quickly wires (1)....... heat along their length - for example, away from hot parts, such as motors, towards heat-sensitive electrical components. In this regard, the (2)...... of copper is roughly 40% greater than that of aluminium, so copper is a much more effective (3)......

In the example above, a designer might therefore prefer aluminium wiring over copper wiring.

Another issue is thermal movement - the extent to which the metals (4)...... when heated, and (5)..... as they cool. In situations where temperature continually rises and falls, the resulting (6)..... and (7)....... can be problematic, as it can cause mechanical electrical connections to loosen over time, in this regard, copper has a (8)....... approximately 40% lower than that of aluminium. Copper therefore has the advantage in this respect, as it is less susceptible to movement.

 

Over to you

For a product you know about, say what the designer needed to consider with regard to:

§ abrasion

§ indentations

§ fatigue

§ creep

§ thermal issues.

What materials were chosen as a result of these considerations?