First off, a Tensometer. A device that used to measure the amount of stress or strain acting on an object in a given environment. It does so by applying a tensile load to a sample of material, and measuring the corresponding change in length. This can be plotted on a stress strain curve below:
The elastic region is where a load can be placed onto a material but the material is still able to reform it original shape when the load is removed.
The plastic region is the point where the load on the material is too great so that it cannot reform to its original shape.
The point at which this happens is its Yield Point and the point at which the material will snap is known as the Failure Point.
The Yield Point can be calculated using Hooke’s law, that dictates that the extension of a material is proportional the force applied. Put simply Stress = E x Strain. Where E is a constant known as Young’s Modulus of Elasticity.
Put in a practical situation: the higher the value of E the stronger the material
If we have a 3mm diameter steel wire supporting a mas of 50k?
- What is the tension in the wire?
Tension = mass x acceleration
T = 50 * 9.81 = 490.5 N
- What is the stress in the wire?
Stress = Load/Area
Area = pi r2 = 3.142 x 1.52x10-6 = 7.07×10-6
Stress = 490.5 / 7.07×10-6
= 69387466 (N/m2)
Stress = 69.39 (MN/m2)
- What level of strain would you expect?
E = Stress/Strain
Strain = Stress / E
E can be found in a table of averages
Stress we calculated in previous question
Strain = 69.39 / 210,000
So, if we assume that the wire is 1m then we say that the wire will stretch by 0.33mm when a 50kg weight is applied to it.
For design purposes this needs to be considered so that you can achieve a perfect product. For instance, how much weight you can put onto wire or cable before it can no longer hold that object. Also how much the wire or cable stretches may affect the overall shape of the design, therefore the design may need to change to suit the stretch of material.
Something to consider on a tension and compression – when it is stretched the center will become thinner, the cross-sectional area with become smaller and vice versa with a compression.
This is known as Poisson Ratio
This can be calculated by = Transverse/longitudinal strain
Longitudinal strain = the change in length
Transverse strain = the change in the thickness of the area.
A practical use would be using cork in a wine bottle. Because its Poisson value is minimal, compared to rubber that has a higher value, so cork will not expand as much as rubber so will not get stuck in the bottle.
Materials will a lover value with not expand or compresses as much as a material with a higher value of poisson.
Shear stress and strain
The Same principles apply
Shear stress can be calculated by: Shear stress = F/A (Force/area)
and Shear strain = x/l (extension (amount is moved)/length)
Shear stress and strain will always act on the parallel axis.
For shear forces their constant is known as modulus of rigidity (G) and a materials table will be able to show you the modulus of both elasticity and rigidity and its poisson ratio.
This diagram shows you the effect of shear force on a parallel plane
The poisson value of each material can tell you how much it will move when a force is applied, as well as its tension and compression.
Strength of materials: These define what properties materials have and how they are either a strengthening property or a weakening.
- Malleable meaning it can be deformed easily under compression without cracking
- Ductile meaning it can be deformed easily under tension without fracturing. These can easily be drawn into wires
- Tough since is can be bent to and fro before it will fracture.
- Brittle materials can’t be bent to and fro without cracking or fracturing happening, small amounts of bending can be applied but only to an extent (e.g. glass)
Photo-elastic Stress Analysis
This is a simple and effective way of analyzing different stresses in product design and can be done like this:
- Making a scale model of a product in a transparent plastic and placing the model in a beam or polarizing light.
- Then apply a load to the model and observe the color (interference) patterns forming on the model.
These light patterns can determine the amount of stress or strain on a product. A polarised lenses will block light coming in from a certain angle, for instance in car mirrors to stop light from shining in your face.
Different colors appear in different patterns when a force in applied to it. For instance, the colors will get brighter the more tension that is applied.
A polariscope is used to read the change in light patterns received, when a force in applied to the transparent plastic model. A special coating can be used to emphasize the light going through the model.
- Can highlight possible failures due to unknown stressed by showing problems areas in a design. In practical use you can the areas in a design that need stronger materials.
- Does not give you any numerical values to work with so the accuracy is limited.
Finite Element Analysis (FEA):
FEA consist of a computer model of a product that is stressed and analysed for specific results. They can test new products to see if it performs to the required specification prior to manufacture.
How it is done:
- Create geometry 2D or 3D using a CAD package.
- Create a meshing around he materials to analyse (looks like a grid). This contains all the structural properties to define how the structure will react to different load (force) conditions.
- Assign a material, which will automatically assign values based on those material properties.
- (They can also be used to read temperature analysis).
Structural Analysis: Simple linear models assume materials is not plastic-ally deformed. More complicated non-linear models consist of stressing the materials so that it can deform plastic-ally.
A similar test is Vibrational Analysis.
- It is a type of analysis used to test a material against vibrations, shock, and impact.
Fatigue Analysis: Helps designers to predict the lift of a materials or structure by showing the effect of constant loading on the design. For instance, how long can a bell last when it is constantly being struck by something?
Heat transfer analysis:
- Takes much less resources and time by jut altering the geometry on the computer software’s (ansys mechanical).
- It is quick and accurate, being able to design and optimize it quickly.
- Can predict a failure due to unknown stress loads being able to see hidden problem areas.
- Allow designer to see all theoretical stresses.
- If incorrect geometry or material in used then the calculations can be way off, causing problems I the future of the design
This example of ‘ansys mechanical’ is showing stress on this object. The blue areas have no stress where the red areas have stress at the given example.
What I have learned from this week is that all designs, no matter how big or small can be accurately tested to provide the best outcome for the structure of an object. This means that products can be made more durable and more efficient. When i go forward i my design practice I will consider these forces, especially when designing something that had a lot of weight. Doing so I will be able to calculate the exact materials to use that will provide maximum support and strength.