Stressing and Straining

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:stressstraincurve

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

Example:
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
    = 0.33×10-3

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.

Poisson Ratio
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.

sheer

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.

fea

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.

Advantages:

  • 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.

Disadvantage:

  • 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.
Simply put:

  • 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:

Advantage –

  • 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.

Disadvantage –

  • If incorrect geometry or material in used then the calculations can be way off, causing problems I the future of the design

heat

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.

 

 

 

Upon Reflection:

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.

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Moving into the Future

Here is our final ‘map’ of ideas. Including all of our research into how the Hertzian pace could effect us in the future.

16409762_1679948342022760_1796492900_o

Close ups show the how we have developed some of our ideas already:

The top left shows the way technology already interacts with the world around us. Human kind has created games that physically interact with the world, for instance “Pokemon go”. This games very objective is to get people out and about by having the collectible items placed around the ares that they live. In some instance the collectibles are only available in certain areas meaning people used technology to visit the world in search of their next Pokemon.

This led us onto the reading of phone signals, simply playing games and connecting with others requires at phone or computer that connects to the internet or satellite communication. So, where is the best place to access these thing in the CSAD building. The images in center right is a floor layout of the CSAD building, including coloured markers of what signal each phone network was able to get on each floor of the building. We found out that the best phone signal were revived on the side of the building that was next to the river. We thought this odd as the nearest telephone mast was on the other side of the building, did this mean that the river was effecting the phone signals?….
Turns out no, it was just the fact that there was a lot of concrete running through the building. But either way it was a good way to see what effects certain things have of the technology around us.

The bottom picture is a more blown up version of the ones above, but this one also shows a graphically altered image of a city, that depicts what the world might look like if we could see the hertzian space around us.

At the moment we have a lot to think about, and i look forward to carrying this project on.

I am leaving this week with the question, where do I or We belong in the Dunne and Raby Cone of Preferable Futures.

Don’t Break our Stool…

What a great start to our mechanical engineering project this term. First things first, mechanical engineering can be defined as the basis of how things work. This can be in all fields: chemical, biological and physical. the main focus is understand and study mechanisms.

Firstly the difference between something static and something dynamic. a static object has every force around it are acting in equilibrium so it remains stationary. Where a dynamic object has a force not acting at equilibrium so causes a movement or acceleration.

The resulting force of an object is the sum of all the forces acting on an object. This was quite intriguing to think as designer you would need to think about any forces that would act on a product to realise the best way to manufacture it.

We learned that the best way to define a force is that it is a push or pull caused by an interaction with an object. A great analogy (we were taught) for this is thinking of a door opening. When it is shut it is a static object, however when a push or pull (a force) is applied to it, it becomes dynamic, therefore it opens in the direction that the force was applied to.

door-opening

Direction being the key word in the previous statement, that because something can be defined by 2 categories, a Scalar or a Vector.

A scalar is something that has a quantity of magnitude (temperature or mass)

A vector has a quantity of magnitude, but also a direction. (Acceleration, force or velocity)

Force being a Vector, is measured in KgN, meaning Kilograms per Newton.

Moving onto to the late great Sir Isaac Newton.
issaac-newton

He was the first to define forces in a mathematical sense during the 1670’s.

We’ve all heard the story of the apple hitting him on the head and open reflections he realised that this falling force can be quantified and eventually calculated a 9.81m/s/s,

Using this we can calculate the weight of an object acting on the earth with the equation

W = mg, where m is the mass of the object and g is the gravity constant.

Isaac Newton created and lived by his 3 laws of Motion.

Laws 1 and 2 can be summed up by:

F = ma (mass x acceleration (rate of change of velocity)) where a = v/t (velocity/time)
A force occurs when a mass is accelerated.
Upon reflection of this it can be said when designing a car, it needs to have a light mass so that the acceleration can be higher.
If the force must be high to combat a heavy car, then there will be a high waste of energy that makes the car more expensive.

Law 3 can be simplified to this example: When throwing a ball at a wall then the wall will exert the same force back on the ball in the opposing direction so it will bounce back at the same rate it was thrown (Any reaction of force will always have an equal or opposite reaction or force).

These can be summarized with the diagram below:
3-laws

From all of this it can seen the Weight is a force so the two equations W = mg is the same as F = ma where a as acceleration is a variable that can be changed, instead of g for gravity that is a constant.

Moving on from this, all forces acting on the structures around them.

There are 4 primary ways that forces can be transmitted within a structure.

Tension – like a wire hanging something from a ceiling (to prevent on object acceleration the string holing it mustard pull in the opposite direction with an equal and opposite force.) What is the tension strength it can reach before it snaps apart?

Compression – forcing two objects together. (a brick on a column, because the column cannot move it pushes back with an equal and opposite force to the weight of the brick.) What is the compression strength of dick it can reach before is breaks?

Torsion – the rotation of an object. Attaching a steel bar to a wall, then using a grip at the other end, will rotate the bar making it look like a corkscrew. How much can you rotate an object before it snaps?

Shear – objects sliding against each other (when looking at glue, you see the shear strength of how much force it can take before it breaks.) e.g. a pin or rivet is subjected to a shear across a plan surface or axis.4-ways-of-force

These can all be seen in the diagram here:

 

 

 

 

From this we started looking at materials:
They will always behave differently depending on force and temperature and other factors
E.g. a steel rood is good a resisting a tensile force, where concrete is very good at resisting compression force.

More terms…

Stress – When a force is acting on object that causes a stress (Stress = force applied/area it is applied to). If we can calcite the maximum stress of a component we can work out if it will be strong enough. How much area would need to hold a force of so much before the stress is too much. For instance, how much material is needed to carry a load and what’s the maximum load that material can take. Cross sectional area, in a cylinder that can be calculated by (piR2).

Example – crane wire – mass of 1 tonnes suspended from a wire rope from the crane. How thick does the wire rope have to be if it had a cross sectional area of 100mm2?

So, 1000 x 9,81 (N)/100×10-6 (m2)

=98.1 MN/ m2 (MN = Mega Newtons stands for 10x6)

(10-6 changes the units to meters, because there are 1,000,000 mm2 in 1m2)

How do we know if it will be strong enough? Strain can tell us!

Strain – change in length/original length. Stress can be both tensile and compression. So we know materials react to stress creating a strain by distorting or changing shape.
How much does a material change shape for a given stress level? Depends on…
type of material
shape of the component
type of stress that is applied to it (tension, compression)
A thermos is round because the shape is very good at taking the forces from the vacuum created inside the product.
This information is critical for a product designer to realise how to make a new product for it to look good, as well as work well and is strong enough for purpose.
Strain = change in length/original length (ΔL/L)

In Summary:

Force = Mass * Acceleration
Newtons laws
Stress = Load/Area
Strain = change in length/original length.

Finally to the title of this blog!! – Don’t break our Stool…

This was in reference to the practical assignment we were set in the last hour of our class. We were given a large sheet of card and asked to make a 25cm tall stool, that could support the weight of a our lecturer. This as to be done in groups of 3.

We were given tape to keep the card together and the information that the lecturer as approximately 80 Kg in mass. At first we thought about calculating the weight that the lecture would put onto the stool and the amount of strain he the material would need to take to support this. Quickly we realised that this was not getting us anywhere and deiced that the cylinder was the perfect shape for the stool due to its strong shape and large surface area it could create. Here was our product, that was able to support the lecturer.

As you can see we gathered together lots of cylinders and compressed them together using a large cover. The cover also doubled as the stand for the lecturer. This would offer maximum spread of weight over all of the strongly shaped cylinders below.

Quick Summary:

I have thoroughly enjoyed this mornings session and look forward to progressing with project. I have learned that all kinds of things have to be considered with the mechanics of a design. Including the way a person uses a design and the forces implemented, that may cause strain to it structure. 

‘Hertzian Space’

In this first week of the futures project, we were introduced to this idea of the Hertzian space, as far as I understand it is the space that connects the human function with that of electronic devices. “Although we cannot sense much of this space, we are affected by it, both physically and psychologically” This is how Anthony Dunne had phrased it. This tells me that is is more undiscovered space, that needs more exploring and that it would be a good place for my self and my group to research to further this project.

The whole aim of this week and next week is to map out ideas of projects about this space and what areas of futuristic technology we could research into.

We had to think:

  • What if questions
  • Create stories
  • Think realistically
  • Use the Hertzian Space
  • Think Critically
  • Read about the old
  • Watch about the new

These were all comprised in our wall of research so far, that I will be showing the complete body of research next week!

For now we have thought about Kinetic architecture, Reading phone signals, futuristic phones, visual sing radio signals.

So far the ideas stage is all we have obtained, next week we will finish the ideation and mapping of the project and we will see where we can go, based on the information we have gathered.