WOBBLcast_motor_mount.step
Does the cast motor mount last ten years of starts and running without a fatigue crack?
MARGINAL
10 runs of the duty cycle use 62.7% of the fatigue life on the 3.99 mm inside fillet (face 65) at (43.9, 8.96, 79.6) (Miner's rule), so it can run about 16 times before cracks start, and the stresses could rise 1.03 times before the required 10 runs would start a crack (target 2). Confidence is medium.
life
16 repeats
safety factor
1.03
required
10 repeats
target
2
Confidence medium
- strength result: the stresses come from a result Wobbl rated medium confidence (load introduction: the off part loads enter through a distributing coupling (the attached part adds no stiffness))
- the S-N line comes from a rule of thumb for this material, not a fatigue test
Simulation
Checks
| status | check | result |
|---|---|---|
| warn | strength result | the stresses come from a result Wobbl rated medium confidence (load introduction: the off part loads enter through a distributing coupling (the attached part adds no stiffness)) |
| pass | first cycle yield | the highest stress in a cycle, 124 MPa, stays under the 165 MPa yield (factor 1.21), so the part stays elastic and the stress life method applies |
| info | S-N range | a block lasts 1.66e11 cycles, past the 500 million the data reach, so that life extends the line |
| pass | mean stress model | the mean stress models give safety factors of Goodman 1.03, Gerber 1.29 and Soderberg 1.03. The answer uses Goodman, and the verdict is the same with each |
| pass | other peaks | 2 stress peaks were tracked away from supports and sharp corners, and under the same cycling none starts a crack sooner than the one judged |
How to fix it
Cut the cyclic stress by 49%
A safety factor of 2 needs every stress 1.94 times lower. In bending the stress falls with the square of the wall, so about 1.39 times the thickness at the peak gets there.
Switch to Aluminum 6061-T6
Its fatigue strength is 2.3 times that of Custom material (cast), which covers the 1.94 needed if the part is otherwise the same. It is a wrought alloy, so the part would be made that way instead.
Load blocks
Stresses are at the notch, after the notch sensitivity, and the life is how many such cycles alone would start a crack.
| block | amplitude MPa | mean MPa | cycles | life |
|---|---|---|---|---|
| 100% x 3650 R=0 | 61.8 | 61.8 | 3,650 | 58,800 |
| 70% x 1e8 R=0.714 | 12.4 | 74.2 | 100 million | 1.66e11 |
Marin factors
| factor | value | why |
|---|---|---|
| surface ka | 1.000 | as-cast surface (Shigley Table 6-2, as cast taken as hot rolled) |
| size kb | 1.047 | equivalent diameter 4.85 mm, taken as 0.808 times the 6 mm wall |
| load kc | 1.000 | bending load |
| temperature kd | 1.000 | room temperature |
| reliability ke | 0.897 | 90% of parts survive |
Assumptions
- Wobbl guessed: required life not given, so Wobbl assumed 1e7.
- Stress life method for crack initiation, as Shigley sets it out, with an S-N line from f Sut at 1000 cycles to the corrected endurance strength. It does not cover crack growth after a crack starts, which is a separate damage tolerance question.
- S-N data for Custom material (cast) give 42.7 MPa at 500 million cycles, smooth specimens, fully reversed (estimated as 0.26 Sut at 5e8 cycles, the ratio of sand cast A356-T6).
- The Marin factors are surface as-cast (1.00), size 1.05, load 1.00, reliability 90% (0.897), with the part at room temperature.
- Mean stress by Goodman, with no credit for a compressive mean.
- Fatigue lives scatter by a factor of three to ten at the same stress, so the life is an estimate and the safety factor on stress is the steadier number.
- Each block scales the studied load and cycles it between R times its peak and the peak (or at the study's load ratio when the block gives none), and the stress field keeps its shape, as it does for a linear part.