Answer Keys
Solutions for all section knowledge checks. Keep this page bookmarked — do not share the URL with students.
Section 1 — The Physics You Need
Knowledge Check: Three Calculation Scenarios
Given: A bar with a cross-sectional area of 40 mm² has a 200 N force applied.
Find: Stress
Solution: σ = F / A = 200 N / 40 mm² = 5.0 MPa
Given: A 100 mm bar stretches to 100.3 mm under load.
Find: Strain
Solution: ε = ΔL / L₀ = 0.3 mm / 100 mm = 0.003 (or 0.3%)
Given: A part made from FDM PLA (yield strength = 40 MPa) experiences a maximum stress of 25 MPa.
Find: Factor of Safety. Will it survive?
Solution: FoS = Yield Strength / Applied Stress = 40 / 25 = 1.6
FoS > 1.0, so the part is predicted to survive the load with a 60% margin before yield.
Section 2 — What FEA Actually Does
Knowledge Check: Workflow Labeling
| Step | Name | Answer |
|---|---|---|
| 1 | Geometry | Human Decision |
| 2 | Material | Human Decision — this is where errors hide |
| 3 | Fixtures | Human Decision |
| 4 | Loads | Human Decision |
| 5 | Mesh | Semi-automatic — computer generates, human can refine |
| 6 | Solve | Computer Calculation |
| 7 | Interpret | Human Decision |
Key takeaway students should articulate: 5 of the 7 steps involve human decisions. The computer only does Step 6. The quality of the output depends entirely on the quality of Steps 1–4.
Section 3 — SolidWorks Simulation Setup
Knowledge Check: Annotated Screenshots
Students submit screenshots showing four things. Verify:
| Screenshot | What to Look For | Common Mistakes |
|---|---|---|
| Material assignment | Custom "FDM PLA — 100% Infill" with E = 3,300 MPa, UTS = 50 MPa, Yield = 40 MPa, ν = 0.35, ρ = 1,240 kg/m³ | Using built-in PLA instead of custom; leaving a field blank; wrong units (GPa vs MPa) |
| Fixture placement | Two faces or edges fixed at the support locations (bottom of bar, near each end) | Fixing the entire bottom face; fixing only one support; fixing the top face |
| Load application | Single downward force (~50N) at the midpoint of the top face | Load on wrong face; load pointing up; load value in wrong units |
| Results summary | Max Von Mises stress, max displacement, min FoS values visible | Screenshot too small to read values; showing displacement plot instead of stress |
These will vary based on exact fixture placement, but reasonable values are:
- Max Von Mises stress: 15–25 MPa
- Max displacement: 0.05–0.15 mm
- Min FoS: 1.5–2.5
If a student gets stress > 50 MPa or FoS < 1.0 at 50N, their setup likely has an error.
Section 4 — Reading Results
Knowledge Check: Interpret a Results Screenshot
Given values for the sample part: Max Von Mises = 35 MPa at fillet near fixed end. Yield strength = 40 MPa. Max displacement = 0.15 mm at free end. Min FoS = 1.14 at the fillet.
- Max stress location: At the fillet near the fixed end
- Max displacement: 0.15 mm at the free end (farthest from the fixtures)
- Minimum FoS: 1.14 at the fillet near the fixed end
- 2-sentence summary (example acceptable answer): "The part will probably survive this load because the minimum Factor of Safety is 1.14, which is above 1.0. However, the margin is small — the fillet near the fixed end is close to yielding, and any increase in load or material weakness could cause failure at that location."
Section 5 — Print the Specimen
Knowledge Check: Print Documentation
Verify students provide:
- Screenshot from Bambu Studio showing 100% infill, 0.20mm layer height, correct temperatures
- Photo of labeled specimens with correct naming convention (PLA-100-XY-02-[Initials]-[Number])
- Minimum 3 specimens
Section 6 — Break It
Knowledge Check: Data Sheet + Calculations
These ranges assume 100% infill, 0.20mm layers, flat orientation, on a Bambu Lab P1S with PLA:
- Failure force: 60–100 N (depends on specific filament and print quality)
- Flexural stress at failure: 35–60 MPa
- Flexural modulus: 2,500–3,800 MPa (CNC Kitchen reference: 3,300 MPa)
Values outside these ranges aren't automatically wrong — but warrant investigation of test setup and print quality.
Worked example to verify student math:
If F = 85 N, L = 64 mm, b = 10 mm, d = 4 mm:
σ_f = (3 × 85 × 64) / (2 × 10 × 16) = 16,320 / 320 = 51.0 MPa
Section 7 — Compare
Knowledge Check: Comparison Table + Reflection
Grading the reflection (3–5 sentences):
| Criteria | Full Credit | Partial Credit |
|---|---|---|
| States percent error | Correct calculation shown | Calculation attempted but with arithmetic error |
| Identifies causes | Names 2+ specific, plausible sources of error | Vague ("it was different") or names only 1 source |
| Proposes improvements | Suggests specific, actionable changes | Suggests vague improvements ("be more careful") |
Section 8 — The ABS Trap
Knowledge Check: Side-by-Side Comparison + Reflection
Expected outcome: The default (injection-molded) ABS profile produces significantly larger percent error (25–50%) compared to the PLA comparison (typically 10–20%).
Key insight students should articulate in their reflection:
- The default ABS profile is for injection-molded ABS, not 3D-printed ABS — they have different properties
- ABS is more affected by print conditions (layer adhesion, chamber temp) than PLA
- The simulation wasn't wrong — the material input was wrong
- Engineers must verify that material properties in the simulation match the actual manufacturing process
- The corrected FDM-ABS profile should produce smaller error, proving the input was the issue