WOBBLenclosure.py
Does the PC shell survive a 120 N thumb press, and how far does the top dent?
HOLDS
Peak 9.77 MPa in the wall under face 2 at (23.9, 12, 26.5), safety factor 6.34 against yield (target 2). Confidence is medium.
peak stress
9.77 MPa
safety factor
6.34
max deflection
0.23 mm
target
2
Confidence medium
- solid cross check: a coarse solid (21,774 nodes) gives 0.208 mm of deflection against the shell's 0.222 mm (+6%), and 8.32 MPa within 6 mm of the shell's governing point against 9.7 MPa (+17%), both on first meshes. The shell idealization moves the answer that much here, so read the shell numbers with that margin
Simulation
Stress
Checks
| status | check | result |
|---|---|---|
| pass | mesh convergence | von Mises stress settled: 9.7 -> 9.53 -> 9.77 MPa over 3 meshes, last change 2.5% |
| pass | force balance | reactions balance the loads to 3.6e-10% (forces; shell supports also hold rotations, whose reaction moments are not compared) |
| skip | hand calculation | no simple beam model applies (the part is not slender) |
| info | singularities | excluded from the answer: 10.4 MPa at (34.5, 22.2, 26.5), within 3 mm of a support, boss joint or rigid attachment, where the idealization sets the stress. Wobbl reports the stress away from these spots |
| pass | shell model | the shell holds 92% of the part's material and 4 bosses as beams bring it to 100%; 17 rim or edge faces are not walls |
| warn | solid cross check | a coarse solid (21,774 nodes) gives 0.208 mm of deflection against the shell's 0.222 mm (+6%), and 8.32 MPa within 6 mm of the shell's governing point against 9.7 MPa (+17%), both on first meshes. The shell idealization moves the answer that much here, so read the shell numbers with that margin |
| pass | units and sanity | part 112 mm across, peak strain 0.0042, max deflection 0.23 mm (0.21% of size): all plausible |
Mesh convergence
| level | nodes | global size mm | finest mm | governing MPa | change | max deflection mm | solve |
|---|---|---|---|---|---|---|---|
| 0 | 6,165 | 3.72 | 3.72 | 9.7 | 0.222 | 8.1 s | |
| 1 | 9,295 | 2.61 | 2.61 | 9.53 | -1.8% | 0.227 | 12.2 s |
| 2 | 17,785 | 1.86 | 1.86 | 9.77 | +2.6% | 0.23 | 19.3 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 | at a support, boss joint or rigid attachment | 34.5, 22.2, 26.5 | 10.4 | excluded |
Setup
| material | Polycarbonate |
| limit | 62 MPa |
| target safety factor | 2 |
| mass | 48.78 g |
| load | 120 N down @ top and within 8 of 25,15,28 |
| fixture | fixed z=min |
| part | 4 holes, 1 inside fillets, 30 sharp inside corners, walls about 3 mm |
Assumptions
- Linear elastic material and small deflections (no yielding redistribution, no buckling).
- Polycarbonate: E 2350 MPa, Poisson 0.37, von Mises vs yield 62 MPa (Covestro Makrolon 2405 datasheet).
- Loads are static and applied all at once; no impact, vibration or fatigue.
- Walls are shells on their midsurface with the thickness the CAD gives at each point; stress is taken on both faces. Through the thickness stress under a load and at sharp inside corners is not modeled.
- Fixed faces hold the walls they touch against moving and turning.
- 4 bosses are beams with their own round or pipe section, joined to the wall they meet by a stiff spider over their outline; their own weight is left out.
- Rims, edges and chunky features that are not walls are left out of the shell, so their stiffness and stress are not in the model; the solid cross check shows what that costs.
- Molded part treated as isotropic and uniform; weld lines and the area near the gate can be weaker than the datasheet.
- Points within 3 mm of supports, boss joints and rigid attachments are left out of the answer.
Notes
- Built from enclosure.py with wall=3, length=90, width=60, height=28, corner=6, boss=7.
- The shell model holds 92% of the part's material in 21 wall faces.
- 17 faces that are rims, edges or chunky features such as bosses are not walls and are left out of the shell.