How to Choose the Right Material 1E10 Lecture by David Taylor Mechanical Engineering Dept What’s It About? • This lecture is about the mechanical properties of materials… • …how to measure them and use them. • It’s important for any material which is going to be subjected to mechanical forces in use. • These forces cause the material to deform (i.e. change shape) and may cause it to fail (i.e. break). Mechanical Properties in Design • Designers need to know about mechanical properties… • …to choose the right material for a given component (e.g. a car’s crankshaft) or structure (e.g. a dam). • They need to make sure that there won’t be too much deflection under load, and that the forces won’t be high enough to cause failure. Examples of Failures A 737 engine; one of the turbine blades broke away and exited through the engine casing, nearly taking someone’s head off! Examples of Failures X-ray showing an artificial hip joint, made of metal, which broke in two whilst inside someone’s leg. As an engineer, you don’t want to be famous for designing a component that failed. Defining and Measuring Mechanical Properties • There are lots of mechanical properties; in this lecture we are just going to look at the simplest (and most important) ones, which we call the “static” properties. • These can be measured using very simple tests, conducted on samples of the material. • The most important test is the tensile test… The Tensile Test FORCE F • Take a sample of material • Pull on the ends to stretch it Original • Measure the force Length Lo needed • You can also apply other types of loads, such as compression or torsion, but we’ll stick to tension here. Stretch to new length L Some Practicalities • You can use any size and shape of sample provided it has parallel sides… • …so the cross section is the same throughout. • The shape of the cross section doesn’t matter, it can be rectangular (as here), square, circular, etc. Its area is A. • Normally we make the ends of the specimen bigger so it’s easy to grip in the testing machine Cross Section of Sample, area A The Stress/Strain Curve • We want to see how much the sample stretches for a given applied force. • So we could plot the force, F, against the stretch (L-Lo). • But it’s better to normalise these quantities, so that the overall size of the sample doesn’t matter. • We do this by calculating the STRESS, s, which is F/A and the STRAIN, e, which is (L-Lo)/Lo. Typical Results (units N/m2 = Pa) Stress s • The stress/strain curve has different shapes in different materials; below are some examples. • As strain increases, stress can go up or down • X indicates the point at which the sample breaks X X X X Strain e (no units) The stress/strain curve in more detail… • First Stage: Elastic Deformation Stress s In this part of the curve, the material behaves like a spring. X If you remove the stress, the strain goes back to zero. Stress is (usually) proportional to strain. Strain e Stiffness • The material’s “stiffness” is the slope of the stress/strain curve in the elastic region. • Called Young’s modulus (or the elastic modulus), symbol E. • If the line is straight then E = stress/strain at any point on the line. Using Stiffness • You can use E to calculate the strain for a given stress, and therefore work out how much the structure will deflect under load… • …e.g. how much a car’s suspension will move when six people get in. • Also used to find the stress in the material for a given amount of deformation… • …if this stress is too high it may fail. Using Stiffness • Also used to prevent buckling. • Buckling is what happens when you have a long, thin, structure loaded in compression… • …like a straw or a drinks can when you push on the ends. • It suddenly “gives” – this is buckling. • The analysis of buckling is complex – the important thing is that the only material property it depends on is E. Elastic Energy Stress s • If you load up a material in its elastic region, to some stress s… • …then the area under the line is a measure of the energy you used to do it. This area is actually the energy per unit volume of material in the sample • This energy is stored in the material and will X be released if you unload it. • This is very useful in a mangonel, for example! Strain e Non-Linear Elasticity,Hysteresis • In some materials (e.g. some polymers) the stress/strain line is curved in the elastic region… • …and sometimes the loading and unloading lines are different. Loading Unloading Non-Linear Elasticity,Hysteresis • In that case E is not constant… • …and some energy is lost, (given by the area between the lines). This is called hysteresis. Loading Unloading Plastic Deformation, Damage and Failure • Above a certain stress, sy, the stress/strain line becomes flatter and curved, and unloading gives you a permanent deformation. A sy Stress s X e.g. if you load up from O to point A and then unload, you get back to B, not O. The distance OB is the plastic strain left in the material O B Strain e Plastic Deformation, Damage and Failure • Why does this happen? • It depends on the material. • In metals, plastic strain occurs because the atoms in the material can flow over each other. • In other materials, such as concrete and wood, it’s due to damage in the material; small cracks and splits which weaken it. • The important thing is that we can’t use the material at a stress greater than sy, so for engineering purposes it’s the maximum allowable stress. Plastic Deformation, Damage and Failure • Two other points from the stress/strain curve: • The maximum point in the curve is called the Ultimate Tensile Strength (UTS). We used to use this as a measure of the strength of a material but these days we normally use sy. • The strain and stress at the failure point X (ef, sf) are also useful to know. ef is called the “ductility”; it tells you how much deformation the material can take without failing, which is useful for manufacturing operations like forging and wire-drawing. • The area under the whole stress/strain curve is the energy (per unit volume) needed to make it fail. • But you get some energy back in elastic recoil (the black triangle). • The remaining area is the energy absorbed by the material in failing. This is one measure of the “toughness” of the material. Stress s Energy Again, Toughness X O Strain e More About Toughness • Toughness is a property which is difficult to define. • One definition is the energy to failure (as above)… • …but these days we normally use the so-called “fracture toughness” which is a measure of how easily the material cracks. • You’ll learn more about toughness (and other mechanical properties) in the materials course next year. Summary • We’ve seen that the simple tensile test can tell you a lot about how a material performs under load… • …how much it deforms, both temporarily (elasticity) and permanently (plasticity)… • …how much energy it can store and release… • …and how much stress and energy are needed to break it. What’s the stress? • This material information is useful only if you know what stress (or strain) is going to arise in the component or structure that you’re designing. • In the tensile test the stress is simply found by F/A, but that’s not the case for a real structure, where the stress will depend on the shape and forces it sees, and will vary from place to place in the structure. • Stresses can be calculated for any structure; these days we mostly use computer models (such as finite element analysis) but you can use analytical equations for simple structures like beams and arches, and to get a rough estimate in more complex cases. • You will learn more about stress analysis in other lectures and other courses. More Information • If you want to learn more, try… • Textbooks by Ashby & Jones: Engineering Materials books 1 and 2. We use these books in courses in 2nd and 3rd year. • Materials by Ashby, Shercliff and Cebon • Lots of information on line, in Wikipedia, company databases, etc