Mechanics unit 4 - stress analysis deck 3

Question 1

What is photoelasticity ?

Answer 1

It is an experimental technique used for stress analysis, which particularly useful for structures with complicated geometry and complicated loading conditions

Question 2

Describe how the stresses can be measured in the technique of photoelasticity using the transparent plastics used to make the models of the structures being analysed

Answer 2

• When the model (made of the transparent plastic material) is subjected to force, some of their optical properties change in direct proportion to the stresses developed.
• The material becomes bifringent and a colourful interference pattern is observed.
• Polarised light passing through the material splits into 2 beams creating a fringe pattern
• Stress can be quantified and localised by counting the number and of fringes and their density. The closer the fringes, the steeper the gradient, indicating an are. a of stress concentration

Question 3

Describe how the method of photoelasticity works

Answer 3

A transparent polymer copy (certain plastics used) of the structure under study is made and then placed between polarizing filters.

Forces are then applied to the model and the resulting fringe patterns are observed. Allowing stress concentrations and weak points to be idenitified

Question 4

What other stresses can the technique of photoelasticity be used to measure ?

Answer 4

• The residual stress (due to manufacturing process) of a material
• Also a structure can be coated in a photoelastic resinm allowing the stress present at the surfaces of real structures to be quantified.

Question 5

What is the disadvantage of strain gauges and photoelasticity compared to the finite element method?

Answer 5

Strain gauges and photoelastic techniques only indicate surface stresses and rely on physical measurements and modelling

Question 6

How does the finite element method work?

Answer 6

It works by splitting a structure into small sub-regions called elements, with each element associated with a number of points called nodes, a series of equations are made for each node point and for some nodes assumptions are made which allow all the equations for the rest to be solves.

The end result is an image with different colours representing the levels of stress

Question 7

Do SAQ 10 pg. 21 stress analysis unit

Answer 7

Ans in workbook