Guest Lecture PE Part 1 Flashcards
What are 2 main uses of automated photoelastic stress analysis?
1) Design validation - allows for fast and easy correlation of FE analysis
2) Determining assembly stresses - load paths - stress concentration
- In this series we are looking at Monitoring of Automotive glass residual stresses
Why do designs need validation? (3 reasons)
1) Fit for purpose - will it withstand service loads
2) Avoid failure - what is the durability/fatigue life
3) Design optimisation - Weight reduction/cost effectiveness
Often a compramise is needed - light, efficient and durable yet cost effective
Product Development Cycle
How has early FEA validation combined with rapid prototyping and manufacturing had an impact on product development cycles? (3 points)
- Experimental analysis can be carried out at the concept stage
- Builds confidence in designs
- Reduces the number of working prototypes needed
What 4 design aspects need to be controlled and understood for accurate FEA?
1) Number of elements - More elements = greater accuracy
2) Material properties - must be known and understood
3) Part geometry - potential stress concentrations
4) Loading and boundary conditions - difficult to represent sometimes
What are the basic principals behind Potoelastic stress analysis?
1) Polarised light is passed through a stressed birefringent body
2) During the propagation of polarised light, interference fringes are formed at lines of shear stress
What does a Grey Field Polariscope do?
(5 points)
1) Delivers circularly polarised light
2) Uses a CCD camera to measure bi-refringence
3) Interprets data into full field shear stress or strain map
4) Fully automated
5) Suitable for static loading
(Works most efficiently in the sub fringe order i.e. Below 0.5 fringes; So thin coatings are used which are easier to apply)
What is the benefit of getting digital experimental data from automated photoelasticity?
The digital data from automated Photoelasticity can be compared to the equivalent digital data from the FEA
and correlation can be looked for
Why would a kaleidoscope lens be used rather than a regular lens?
A kaleidoscope lens splits an image into 4 separate images allowing for the testing with 4 separate polarities of light with one test - useful when looking for stress conentrations in dynamic fatigue tests
What are the 6 steps involved in applying a photoelastic coating?
1) Clean surface
2) Spray surface with reflective paint
3) Mix two part epoxy resin
4) Brush mixed resin onto surface to be investigated
5) Spread to obtain an even coating thickness
6) Allow 4-6 hours for the coating to cure before loading and testing
What is an advantage to using a thin coating regarding part geometries?
Complex geometries can be coated including welds
Why should assembly stresses be looked at?
Assembly stress can be significant, identification of these stresses can avoid the need for re-design
e. g.1 Excessive stresses on a single spot weld caused premature cracking withing a trick cab - photoelasticity highlighted the problem, the spot weld was moved by five mm and the problem was eliminated
e. g.2 A problem was found caused by the tourquing up sequence. Changing the sequence reduced the stress and avoided the need for re-design
Why is photoelasticity helpful in welded structures?
High stress concentrations at the end of welds or in basic geometry changes can be easily identified using photo elasticity
Photoelasticity Case Study 1
- Dynamic PSA was used to determine the real stress distribution on a vehicle suspension tower
- Dynamic images were taken from a multi axis test rig
Results were used to set up a simple single actuator test rig to test local geometry changes
Why was the PSA video helpful and why would a single actuator be used
- The PSA video allowed the single actuator to be calibrated in order to create the same stress pattern so loading could be replicated exactly
- A single actuator test rig would be used so that when testing a re-design, the use of sn expensive 4 poster rig can be avoided and new designs can be tested before being incorporated into the final body shell
