Reference

Fundamentals

The core engineering knowledge tested in every product design interview. Quick reference formulas at the top, expandable deep dives below. Grounded in real consumer electronics, not textbook abstractions.

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Statics & Strength of Materials

Every PD interview starts here. If you cannot draw a free body diagram and calculate bending stress, nothing else matters.

Quick Reference

Bending Stress

\sigma = \frac{M \cdot c}{I}

M = bending moment, c = distance from neutral axis, I = second moment of area

When: Evaluating whether a beam or bracket will yield under load

Deflection (Simply Supported, Center Load)

\delta = \frac{P \cdot L^3}{48 \cdot E \cdot I}

P = force, L = span, E = elastic modulus, I = second moment of area

When: Phone sit-test, display deflection check, bracket stiffness

Deflection (Cantilever, End Load)

\delta = \frac{P \cdot L^3}{3 \cdot E \cdot I}

Same as above; cantilever is 16x more flexible than simply supported

When: Snap fit deflection, button cantilever, PCB flex during assembly

Deflection (Fixed-Fixed, Center Load)

\delta = \frac{P \cdot L^3}{192 \cdot E \cdot I}

Fixed-fixed is 4x stiffer than simply supported for the same span

When: Laptop lid supported at both hinges, phone midframe with bonded display

Normal Stress

\sigma = \frac{F}{A}

F = applied force, A = cross-sectional area

When: Tension or compression in a fastener, standoff, or strut

Shear Stress

\tau = \frac{V \cdot Q}{I \cdot b}

V = shear force, Q = first moment of area, b = width at section

When: Shear in adhesive joints, snap fit bases, thin walls

Safety Factor

SF = \frac{\sigma_{yield}}{\sigma_{applied}}

Consumer electronics: SF 1.5-2.0 on yield; safety-critical: SF 3.0+

When: Every structural design decision. Below 1.5 requires justification.

Moment of Inertia (Rectangular Section)

I = \frac{b \cdot h^3}{12}

b = width, h = height. Height cubed means doubling height gives 8x stiffness.

When: Quick estimation of beam or bracket stiffness from cross-section dimensions

Fundamentals

Statics & Strength of Materials

Quick Reference

Deep Dives