Mechanical Properties of Materials & Structures Flashcards
What is a structure?
What is a structure material?
Structure - an arrangement of one or more materials in a way that is designed to sustain loads
Structure Material - any material that may be used to construct a structure
What is a mechanical property?
The basic mechanical behaviour of a material, measured using simple, idealised tests and divorced from the material’s shape and size
In materials/structures, what do the following symbols mean:
- sigma?
- epsilon?
- eta?
sigma - stress
epsilon - strain
eta - coefficient of viscosity
What is deformation?
Change in shape or size of a structure, or any part of it
What does knowledge of mechanical properties allow for?
Prediction of what will happen to a body when it is loaded
Formula for stress?
stress = force / cross-sectional area
sigma = F/A
Relationship between stress and materials?
Stress is independent of the shape and size of a material.
If there are 2 bars made from the same material, and one has a cross-sectional area double that of the other, it will take twice as much force to break the wider one.
Therefore, the stress in the materials will be the same
stress 1 = F/A
stress 2 = 2F/2A
SI units of stress?
N m^-2 (same as pressure or Pascal)
Formula for strain?
Strain = change in length / original length
strain = epsilon
Relationship between strain and material?
Elongation is dependent on the length of the structure.
If there are 2 bars with the same cross-sectional area but one is 2x the length of the other, the strain in the longer bar will be 2x that of the shorter bar.
What are axial loads?
Tension or compression
Load applied along a geometric axis of a structure (not across i.e. bending or twisting)
What does a stress-strain curve illustrate?
Why else is it useful?
How a material deforms as it is loaded
It can also be used to compare different materials to see if one is relatively more or less stiff, tough, ductile and/or brittle
Although some materials may predominantly exhibit one of these characteristics, all materials will have all of these behaviours to a certain degree depending upon the magnitude of the load to which they are subjected
6 regions and points of particular interest in a stress-strain curve?
Proportional limit Elastic limit Yield point Strain hardening Ultimate strength Rupture point
What is the proportional limit (P)?
Between the origin and the proportional limit, P, the stress-strain curve is in a straight line.
i.e. the stress is directly proportional to the strain (thus if the stress is doubled so is the strain)
What is the elastic limit (E)?
The greatest stress which can be applied to a material without causing permanent deformation (elastic behaviour).
If this point is passed and the material is unloaded, it will not return to its original shape and size (plastic behaviour)
What is elastic behaviour?
How does this appear on a stress-strain curve?
When a material deforms instantaneously upon loading and returns to its original shape and size once it is unloaded (e.g. rubber)
In the elastic region, therefore, the curve follows back down the same path to the origin
What is plastic behaviour?
How does this appear on a stress-strain curve?
When a material deforms instantaneously upon loading, but retains its new shape and size when unloaded (e.g. putty).
The material may partially recover some of its original shape (elastic recovery), but not completely, and there will remain a residual strain
Past the elastic limit, E, the material will not follow back down the same path to the origin, and will not return to the origin
What is the yield point (Y)?
Once passed, the material will undergo considerable elongation (yielding) without a corresponding increase in stress.
This is indicated by the flatness of the region following the yield point. In this region, the material may exhibit perfectly plastic behaviour (no elastic recovery). The stress at the yield point is called the yield stress
What is strain hardening?
After undergoing the large strain that can occur during yielding, the material begins to strain/work harden. In this region, the material undergoes changes in its atomic and crystalline structure resulting in increased resistance to further deformation
What is ultimate strength (U)?
What happens after this point?
This occurs at the highest point of the stress-strain curve. It is the maximum stress the material can withstand before beginning to fail.
After this point, stretching occurs with an actual reduction in stress, resulting in NECKING/waisting in the material, whereby the cross-sectional area of the material is reduced
The stress the bar can withstand after the ultimate strength point is reduced, but not due to loss of material strength, purely due to reduction in cross-sectional area. The thinnest part of the ‘neck’ is used to calculate this.
What is rupture point (R)?
What is stress at this point called?
When the material breaks
Stress at this point is called rupture stress
What is a brittle material?
What is a ductile material?
Brittle - a material that can only sustain a limited strain before breaking e.g. glass
Ductile - a material which plastically deforms before breaking e.g. copper
What is Hooke’s Law?
Up to a certain level of stress, the strain produced is proportional to the applied stress
(this limit is the proportional limit, what it refers to is the linear region of the graph)
What is the stiffness of a material?
How is it measured?
How difficult it is to deform under loading
Measured by Young’s Modulus (modulus of elasticity)
