Materials 3.4 Flashcards
(13 cards)
What is elastic and plastic deformation?
-Elastic: when the object returns to its original shape once the forces are removed e.g a cushion
-Plastic: The material has exceeded its elastic limit(the force which the atoms can’t return to their original shape) and the object will remain permanently deformed
What will compressive and tensile forces cause a wire to do?
-Two equal and opposite tensile forces will cause a wire to extend
-Two equal and opposite compressive forces will causes the wire to decrease in length
This is a graph of force against extension, describe each point?
-A: limit of proportionallity, point where extension is no longer directly proportional to force, object will still behave elastically but not for much more load
-B: elastic limit, beyond this point material won’t behave elastically
-C: plastic behaviour, material won’t return to original shape
-D: fracture, at this point the material will break
What is the gradient of a force extension graph?
The force constant
What is hookes’ law and the equation for it?
Hookes’ law- the extension of an object is proportional to the force applied, provided the elastic limit isn’t exceeded
What is the equation for hookes’ law (including work done) and how and why can you work out work done from a force extension graph?
-Work done can be found by finding areas under a force-extension graph
-This is because when a wire is compressed or stretched all its energy is stored as elastic potential energy so work done=elastic potential energy
Describe an experiment investigating stretching a wire?
- Slowly add weight and measure the extension of the wire by looking at the marker above the ruler (avoid parallax error)
- Force is found by multiplying mass of hanger(kg) and g (9.81)
- Plot a graph of force against extension
- Wear eye protection as the wire could snap and don’t stand directly under the weight
Define stress and strain?
-Stress is the force per unit cross sectional area, it has the units n/m^2 or Pa
-Strain is the extension per unit length, It therefore doesn’t have a unit
What is the equation for youngs modulus?
Describe an experiment for calculating the youngs modulus of a wire?
- Measure the original length of the wire and the cross-sectional area by finding diameter of a wire using a micrometer
- Increase tension by adding weight and record the extension of the wire at each weight
- Use this to find stress and strain at each point and plot a graph, gradient will be equal to youngs’ modulus
- or use the equation youngs modulus=stress/strain
Do objects which are stiff have a large or small Youngs modulus?
-Stiff objects will have a large young modulus as a lot of stress(force per unit area) will cause less strain(extension)
Define ductile, malleable, brittle, hard, stiffness?
-Ductile material can be drawn into wires and will show plasctic deformation under stress before breaking
-Malleable materials can be hammered into shape and will show lots of plastic deformation under stress before breaking
-Brittle material will break with little or no plastic deformation
-A hard material will resist plastic deformation by surface indentation
-Stiffness is the ability to resit a tensile force
Draw the stress-strain graph of brittle, ductile and polymeric materials?
