1. Concepts of Stress and Strain and their Measurement

Question 1

What are the three MEC405 course objectives?

Answer 1

1. Knowledge of common experimental stress analysis techniques.
2. Understand the principles, advantages and disadvantages of modern techniques
3. Selection of the most appropriate technique/combination for use in a particular situation.

Question 2

Experimental Stress Analysis.
What are the 4 reasons we need it?

Answer 2

1. Check design calculations
2. Analyse designs not solved confidently theoretically
3. Provide information enabling theoretical and numerical analysis
4. Provide rapid and economical results for one-off problems

Question 3

Is the title of this course a misnomer?

Answer 3

No. Stress cannot be measured in experiments - but strain can (change in dimension)
Therefore ,using elastic theory , convert strain to stress. E.S.A is not a misnomer

Question 4

Describe strain energy

Answer 4

• Tension applied to material
• Atoms are displaced away from one another
• Net attractive force produced between atoms
• Leads to energy storage (strain energy)

Question 5

Describe plastic strain

Answer 5

• permanent rearrangement of atoms
• permanent deformation,almost no residual stress

Question 6

How do you measure the movement of atoms? How would you measure larger strain

Answer 6

• e.g X-ray diffraction ,neutron diffraction
• measure large samples of atoms , strain analysis

Question 7

Define normal strain & shear strain

Answer 7

normal strain = change in line length/original line length

shear strain = angular change of two originally perpendicular lines

Question 8

what is principle strain?

Answer 8

• max and min strains at a point
• occur on planes with no shear

Question 9

write the strain equations for small displacements

Answer 9

Question 10

write the equation for mohr’s circle of strain

Answer 10

Question 11

Stress/Strain Relationship

How are principle strains converted to principle stresses?

What assumptions are made?

Answer 11

If E and v are known, strains can be converted into stresses

Assume plane strain conditions therefore epslon₃ = 0

Question 12

what are 5 relevant issues regarding E.S.A?

Answer 12

1. Types of problem (surface strains and internal stresses)
2. Full field vs Point analysis
3. Accuracy - gauge length
4. Sensativity
5. Cost

Question 13

Accuracy of strain measurement depends on ……..?

Hence

what are the differences between strain gauges and optical methods ?

Answer 13

gauge length

ε= δ/l (where l is gauge length)

Optical methods have small gauge lengths

Strain gauges have large gauge lengths

Question 14

What are 5 E.S.A methods?

Answer 14

1. Photoelasticity (2D & 3D methods, Reflection photoelasticity)
2. Interferometry (Moiré, Digital imaging correlation)
3. Strain gauges
4. Thermoelasticity
5. Combining the methods

Question 15

What is the technique and the material used in this experiment?

Answer 15

Photoelasticity

Birefringent gelatine

Question 16

What E.S.A technique is used here ?

What is it used for?

Answer 16

Moiré test

Frame behavior:
Out of plane displacement after removal the frame:

The deformations and the curvatures are largest close to the crack and also in the lower part of the panel

Question 17

Aircraft wing dent identification

What E.S.A technique is used here ?

Answer 17

Shadow Moiré

Question 18

A380 upper skin panel - compression by bending analysis

What E.S.A technique is used here ?

Answer 18

Digital Image Correlation

Question 19

Automotive Door Slam Test

What E.S.A technique is used here ?

Answer 19

Thermoelastic Stress Analysis