WOBBLmount.py
The same mount without its ribs
MARGINAL
Peak 41.3 MPa on the 3 mm inside fillet (face 3) at (-5.09, 5.11, 7.2). It survives the load, but the safety factor 1.14 is below the 2.5 target. Confidence is medium.
peak stress
41.3 MPa
safety factor
1.14
max deflection
2.6 mm
target
2.5
Confidence medium
- mesh convergence: von Mises stress (layer pull scaled by layer strength) still moving: 44.4 -> 41.4 -> 45.4 -> 41.3 MPa, last change 9.7% (stopped by level limit); expect about that much error
- singularities: excluded from the answer: 60.4 MPa where the rigid attachment load1 grabs the part (face 13) (set by the idealized support: 56.6 -> 75.2 -> 75.1 -> 60.4 MPa); 53.1 MPa at a sharp inside corner (face 5) (a sharp corner: singular in theory, so the value depends on the mesh: 54.9 -> 52.5 -> 53.5 -> 53.1 MPa); 59.4 MPa where the rigid attachment load1 grabs the part (face 13) (set by the idealized support: 54.4 -> 55.3 -> 57.2 -> 59.4 MPa). Wobbl reports the stress 1.85 mm or more away from these spots
Stress
Checks
| status | check | result |
|---|---|---|
| warn | mesh convergence | von Mises stress (layer pull scaled by layer strength) still moving: 44.4 -> 41.4 -> 45.4 -> 41.3 MPa, last change 9.7% (stopped by level limit); expect about that much error |
| pass | force balance | reactions balance the loads to 0.00011% (forces) and 2.7e-05% (moments) |
| skip | hand calculation | no simple beam model applies (the governing point is not between the fixtures and the load, so no single section carries it) |
| warn | singularities | excluded from the answer: 60.4 MPa where the rigid attachment load1 grabs the part (face 13) (set by the idealized support: 56.6 -> 75.2 -> 75.1 -> 60.4 MPa); 53.1 MPa at a sharp inside corner (face 5) (a sharp corner: singular in theory, so the value depends on the mesh: 54.9 -> 52.5 -> 53.5 -> 53.1 MPa); 59.4 MPa where the rigid attachment load1 grabs the part (face 13) (set by the idealized support: 54.4 -> 55.3 -> 57.2 -> 59.4 MPa). Wobbl reports the stress 1.85 mm or more away from these spots |
| pass | bolt bearing | worst hole of fix1 (d 5.5 mm) carries 81 N sideways and 52.6 N along its axis; bearing stress 2.93 MPa, safety factor 16.04 on bearing |
| pass | units and sanity | part 102 mm across, peak strain 0.02, max deflection 2.6 mm (2.5% of size): all plausible |
| pass | large deflection | deflection is large next to the wall thickness, so Wobbl solved the first mesh with large deflection theory too: the peak moves +7.1% and the deflection +6.3%; linear theory holds |
| info | load introduction | the off part loads enter through a rigid attachment (the attached part does not flex). With a distributing coupling (the attached part adds no stiffness) instead, the governing stress converges to 73.6 MPa on face 13 at (-13.8, 4.86, 15.5) (1.8x, safety factor 0.64) over 3 meshes refined around that peak: 70.7 -> 73.1 -> 73.6 MPa, last change 0.7%. The verdict is the same either way |
How to fix it
Round the sharp inside corner to a 2.71 mm radius
Stress piles up in the sharp inside corner. A fillet of 2.71 mm (about half the 5.41 mm wall) spreads it out.
Thicken the wall at the peak from 5.41 to 8.02 mm
The wall through the peak is 5.41 mm. To reach safety factor 2.5 it needs 8.02 mm (+48%), because bending stress falls with thickness squared.
Add a triangular gusset across the inside corner
The peak sits where two walls meet and bend. A gusset as thick as the wall, with legs about half as long as the shorter wall, ties them together and usually cuts the corner stress by half or more.
Mesh convergence
| level | nodes | global size mm | finest mm | governing MPa | change | max deflection mm | solve |
|---|---|---|---|---|---|---|---|
| 0 | 16,507 | 4.5 | 1.1 | 44.4 | 2.55 | 3.9 s | |
| 1 | 22,520 | 3.82 | 0.895 | 41.4 | -6.8% | 2.55 | 5.7 s |
| 2 | 26,660 | 3.25 | 0.889 | 45.4 | +9.6% | 2.58 | 7.1 s |
| 3 | 31,532 | 2.76 | 0.932 | 41.3 | -8.9% | 2.6 | 8.3 s |
Stress peaks
Every peak Wobbl found, with its value at each mesh. Peaks at sharp corners and idealized supports are excluded from the answer because their value depends on the idealization, not the part.
| # | where | x, y, z (mm) | MPa by level | status |
|---|---|---|---|---|
| 1 | where the rigid attachment load1 grabs the part (face 13) | 7.92, 5, 8.62 | 56.6 → 75.2 → 75.1 → 60.4 | excluded |
| 2 | at a sharp inside corner (face 5) | -25, 7.3, 5.08 | 54.9 → 52.5 → 53.5 → 53.1 | excluded |
| 3 | where the rigid attachment load1 grabs the part (face 13) | -9.74, 5, 8.72 | 54.4 → 55.3 → 57.2 → 59.4 | excluded |
| 4 | at a sharp inside corner (face 2) | 25, 7.3, 5.08 | 53.5 → 51.8 → 51 → 53.2 | excluded |
| 5 | where the rigid attachment load1 grabs the part (face 13) | 1.39, 5, 8.8 | 48.1 → 60.8 → 57.4 → 62.4 | excluded |
Setup
| material | PETG (3D printed) |
| limit | 47 MPa |
| target safety factor | 2.5 |
| mass | 34.95 g |
| load | 250 N down at 0,-15,31 @ normal +y and box -21,4,10,21,6,52 rigid |
| fixture | bolt holes d=5.5 |
| part | 9 holes, 1 inside fillets, 2 sharp inside corners, walls about 5 mm |
Assumptions
- Linear elastic material and small deflections (no yielding redistribution, no buckling).
- PETG (3D printed): E 2100 MPa, Poisson 0.38, von Mises vs yield 47 MPa (Prusament PETG technical data sheet).
- Loads are static and applied all at once; no impact, vibration or fatigue.
- Printed layers stack along z; pull across layers is judged against 70% of the in-plane strength.
- Bolted holes are rigidly held over the hole and a washer sized ring (twice the hole radius).
- Remote loads reach the part through a rigid attachment (the attached part does not flex).
- Contact between separate parts, friction and preload are not modeled.
Notes
- Built from mount.py with thickness=5, rib=0, fillet=3, height=55.
- 2 curved elements folded on thin walls, so this mesh uses straight sided second order elements.