Chapter 6 Materials Flashcards
What is the opposite of compressive forces?
Tensile forces
What is a tensile force?
It is a force that produces extension
What is hooke’s law?
The extension of a spring is directly proportional to the force applied
What featurea of a force-extension graph can be identified and what doe they mean?
The point where the line is no longer straight is the elastic limit which is the point at which the object will not go back to its original shape
The gradient of the graph is the force constant
What is the force constant? What are its units?
This is a constant for an object and is a measure of the stiffness of an object. Nm^-1
What is an experiment that can be conducted to find the force constant?
If you hang a spring from a clamp stand and measure its length, then add weight and measure the extension. this is then graphed. The gradient is the force constant?
What is the equation for work done on springs?
Worok = Foce x Extension
What does the force-extension curve for rubber look like? what does this show?
The loading curve is higher than the unloading curve. The gap between the two shows the amount of energy (work) that has been dissipated by the rubber
What does the force-extension graph for a polymer look like?
The loading curve starts from the origin and then the unloading curve does not return to the origin because the energy has been put into permanently plastically deforming the polymer
What is tensile stress?
This is a measure of the force per cross sectional area.
Tensile stress = Force / Cross-sectional area
Tensile stress has the symbol: σ
What is tensile strain?
This is extension divided by the original length.
Tensile strain = Extension / Original Length
Tensile strain has the symbol: ε
What features of a stress strain graph can be worked out?
Limit of proportionality: When the line on the graph is no longer straight
Elastic limit: This is the point where after it no longer obeys hookes law and permanently plastically deforms
Ultimate tensile stress: This is the highest point on the graph where there is the maximum stress through the object
Breaking point: this is the point where the material breaks
Youngs modulus: The gradient up to the limit of praportionality
What is youngs modulus?
Youngs modulus = Tensile stress / Tensile strain
It is unique to a material. The larger the number the stiffer the object
Units: Nm^-1 or Pa
How can the youngs modulus of a wire be determined?
The wire is held at one end of the table and then curved around a wheel off the other end of the table. The diameter of the wire is measured with a micrometer and a marker is placed on the wire. The length of the wire is then measured with a ruler from the end where the wire is clamped to the marker. Weights are added to the end of the wire and the distance the marker moved is noted. Calculations for stress and strain are graphed. this is then plotted onto a graph. the weight is translated to a force using F = mg
What does the stress strain graph for brittle materials look like?
The loading and unloading graph follow the same line and the breaking point is the same as the ultimate tensile stress
