WOBBLenclosure.py

Does the PC shell survive a 120 N thumb press, and how far does the top dent?

Strength report
2026-10-13 10:46
HOLDS

Peak 8.77 MPa on face 29 at (23.8, 12.8, 25), safety factor 7.07 against yield (target 2). Confidence is medium.

peak stress
8.77 MPa
safety factor
7.07
max deflection
0.217 mm
target
2

Confidence medium

  • singularities: excluded from the answer: 20.6 MPa at a sharp inside corner (the 7 mm boss (face 35)) (a sharp corner: singular in theory, so the value depends on the mesh: 15.2 -> 20.8 -> 20.6 MPa); 18.2 MPa at a sharp inside corner (face 29) (a sharp corner: singular in theory, so the value depends on the mesh: 13.1 -> 18 -> 18.2 MPa); 9.85 MPa at a sharp inside corner (face 21) (a sharp corner: singular in theory, so the value depends on the mesh: 9.33 -> 9.55 -> 9.85 MPa). Wobbl reports the stress 2 mm or more away from these spots

Stress

von Mises stress with the governing point marked (8.77 MPa). Color scale tops out at the reported value.
von Mises stress with the governing point marked (8.77 MPa). Color scale tops out at the reported value.
Close up of the governing point
Close up of the governing point
Safety factor (red is below 1, green is comfortable)
Safety factor (red is below 1, green is comfortable)
Deflected shape, exaggerated so it is visible
Deflected shape, exaggerated so it is visible

Checks

statuscheckresult
passmesh convergencevon Mises stress settled: 8.67 -> 8.82 -> 8.77 MPa over 3 meshes, last change 0.6%
passforce balancereactions balance the loads to 1.7e-05% (forces) and 3.6e-06% (moments)
skiphand calculationno simple beam model applies (the governing point is not between the fixtures and the load, so no single section carries it)
warnsingularitiesexcluded from the answer: 20.6 MPa at a sharp inside corner (the 7 mm boss (face 35)) (a sharp corner: singular in theory, so the value depends on the mesh: 15.2 -> 20.8 -> 20.6 MPa); 18.2 MPa at a sharp inside corner (face 29) (a sharp corner: singular in theory, so the value depends on the mesh: 13.1 -> 18 -> 18.2 MPa); 9.85 MPa at a sharp inside corner (face 21) (a sharp corner: singular in theory, so the value depends on the mesh: 9.33 -> 9.55 -> 9.85 MPa). Wobbl reports the stress 2 mm or more away from these spots
passunits and sanitypart 112 mm across, peak strain 0.0037, max deflection 0.217 mm (0.19% of size): all plausible

How to fix it

Round the sharp inside corner to a 1.5 mm radius
Stress piles up in the sharp inside corner. A fillet of 1.5 mm (about half the 3 mm wall) spreads it out.
Switch to PLA (3D printed)
Stress stays about the same for the same shape and load, so PLA (3D printed) (50 MPa) gives safety factor 5.70. It is 1.5x as stiff, so deflection scales by 0.671. Other options: PETG (3D printed) (5.36), ABS (3D printed) (3.76).

Mesh convergence

Peak stress at each refinement level
Peak stress at each refinement level
levelnodesglobal size mmfinest mmgoverning MPachangemax deflection mmsolve
0114,0142.70.7178.670.21342.4 s
1142,5712.290.4288.82+1.7%0.21866.2 s
2165,8451.950.2668.77-0.6%0.21787.3 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.

#wherex, y, z (mm)MPa by levelstatus
1at a sharp inside corner (the 7 mm boss (face 35))35.4, 22, 24.615.2 → 20.8 → 20.6excluded
2at a sharp inside corner (face 29)37.8, 19.7, 2513.1 → 18 → 18.2excluded
3at a sharp inside corner (face 21)25.3, 27, 24.69.33 → 9.55 → 9.85excluded
4at a sharp inside corner (face 21)20.5, 27, 24.69.04 → 9.08 → 9.89excluded
5on face 2924, 13.2, 258.67 → 8.82 → 8.77counted

Setup

materialPolycarbonate
limit62 MPa
target safety factor2
mass48.55 g
load120 N down @ top and within 8 of 25,15,28
fixturefixed z=min
part4 holes, 1 inside fillets, 30 sharp inside corners, walls about 3 mm

Assumptions

  1. Linear elastic material and small deflections (no yielding redistribution, no buckling).
  2. Polycarbonate: E 2350 MPa, Poisson 0.37, von Mises vs yield 62 MPa (Covestro Makrolon 2405 datasheet).
  3. Loads are static and applied all at once; no impact, vibration or fatigue.
  4. Molded part treated as isotropic and uniform; weld lines and the area near the gate can be weaker than the datasheet.
  5. Fixed faces are perfectly rigid; real mounts flex a little, which lowers peak stress next to them.
  6. Contact between separate parts, friction and preload are not modeled.

Notes