Lecture 12 Thermoelasticity Flashcards
The principles of thermoelasticity:
Under _________, __________ (elasticity not plasticity) conditions, a _____________ loaded, _________ structure experiences (detectable) __-_____ ___________ variations that are proportional to the sum of _________ ______.
Under adiabatic, reversible (elasticity not plasticity) conditions, a cyclically loaded, isotropic structure experiences (detectable) in-phase temperature variations that are proportional to the sum of principal stress.
The principles of thermoelasticity depend upon the thermoelastic effect. A football pump is a good (gas) example of the effect where the substance is compressed and it heats up . Give another example.
Fire extinguisher:
Expand a substance - cools down
Or aerosol - same as above
What assumptions are made in the theory which results in us being able to relate temperature fluctuations with dynamic stresses for a cyclically loaded body
Energy Equation wrt gas:
1st law of thermodynamics
Adiabatic
Ideal Gas
Principle of virtual work to relate to elastic solids
(change in work = change in strain energy)
(see early slides in lecture for derivation of equations)
What is used to observe something like this, and what is this anyway?
Thermoelastic Stress Analysis uses an infrared radiometer to measure these small local temperature fluctuation signals from the surface of a cyclically loaded body.
What you were looking at is called a thermoelastic signal
Why can we reduce all the theory to simply ‘change in sum of stress components’ = calibration constant multiplied by signal from infra-red detector
INfra red detector takes signals from THE SURFACE aka where or atleast very near to where failures are most likely to occur.
Name 5 major experimental issues with this method
Apparatus (and commercial availability)
Adiabatic conditions
Specimen preparation
Material response
Calibration
Annotate the components with regard to what they are for.
Applications please (6)
Stress / Strain Visualisation
Small scale applications
Experimental Stress Separation -
Thermo-Photo-elasticity
Fracture Mechanics
High Temperature Stress Analysis
Residual Stress Analysis
Disadvantages please (4)
Cyclic loading needed
High capital cost
$90K to $160K
Edge data poor
Liquid Nitrogen required (for some models)
Advantages please (6)
Non-contacting measurements
Full-field analysis
Can be applied to most materials
Resolution similar to strain gauges
Range of applications
Fast stress visualisation
(using Delta Therm system)
What are 3 things you could do to create assimilate adiabatic conditions?
Increase the scale of the model (heat conduction occurs in high stress gradients)
If you are testing a certain shape, you could use a differnet material.
Increasing the frequency of the machine, decreases thermal diffusion length.
What 4 things do temperature changes depend on?
Material properties: 1)density
2)specific heat
3)coefficient of thermal expansion
(these 3 make up the thermoelastic constant)
4)ambient temperature
How must the specimen be prepared?
Why?
Why are edge effects important?
Uniform emissivity is ideal. Use thin coating of matt black paint.
Approximates the surface to a black body, increasing photon flux (temperature change)
Movement of the body when loading is most obvious at the edges.
