1. Concepts of Stress and Strain and their Measurement Flashcards
What are the three MEC405 course objectives?
- Knowledge of common experimental stress analysis techniques.
- Understand the principles, advantages and disadvantages of modern techniques
- Selection of the most appropriate technique/combination for use in a particular situation.
Experimental Stress Analysis.
What are the 4 reasons we need it?
- Check design calculations
- Analyse designs not solved confidently theoretically
- Provide information enabling theoretical and numerical analysis
- Provide rapid and economical results for one-off problems
Is the title of this course a misnomer?
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
Describe strain energy
- Tension applied to material
- Atoms are displaced away from one another
- Net attractive force produced between atoms
- Leads to energy storage (strain energy)
Describe plastic strain
- permanent rearrangement of atoms
- permanent deformation,almost no residual stress
How do you measure the movement of atoms? How would you measure larger strain
- e.g X-ray diffraction ,neutron diffraction
- measure large samples of atoms , strain analysis
Define normal strain & shear strain
normal strain = change in line length/original line length
shear strain = angular change of two originally perpendicular lines
what is principle strain?
- max and min strains at a point
- occur on planes with no shear
write the strain equations for small displacements
write the equation for mohr’s circle of strain
Stress/Strain Relationship
How are principle strains converted to principle stresses?
What assumptions are made?
If E and v are known, strains can be converted into stresses
Assume plane strain conditions therefore epslon3 = 0
what are 5 relevant issues regarding E.S.A?
- Types of problem (surface strains and internal stresses)
- Full field vs Point analysis
- Accuracy - gauge length
- Sensativity
- Cost
Accuracy of strain measurement depends on ……..?
Hence
what are the differences between strain gauges and optical methods ?
gauge length
ε= δ/l (where l is gauge length)
Optical methods have small gauge lengths
Strain gauges have large gauge lengths
What are 5 E.S.A methods?
- Photoelasticity (2D & 3D methods, Reflection photoelasticity)
- Interferometry (Moiré, Digital imaging correlation)
- Strain gauges
- Thermoelasticity
- Combining the methods
What is the technique and the material used in this experiment?
Photoelasticity
Birefringent gelatine
What E.S.A technique is used here ?
What is it used for?
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
Aircraft wing dent identification
What E.S.A technique is used here ?
Shadow Moiré
A380 upper skin panel - compression by bending analysis
What E.S.A technique is used here ?
Digital Image Correlation
Automotive Door Slam Test
What E.S.A technique is used here ?
Thermoelastic Stress Analysis
