# Sandwich Beam

**Type:** Primary structural component
**Material:** Steel (AISI 1005) — see [steel-aisi.md](../materials/steel-aisi.md)
**Status:** Confirmed from KBS session (Gen 002)

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## Description

Sandwich I-beam serving as primary load-bearing member in a test fixture assembly. Cross-section features a core layer flanked by face sheets on top and bottom flanges. Web contains a pattern of lightening holes (circular cutouts) to reduce mass. Beam is extruded from an I-beam sketch profile.

## Geometry — NX Expressions

| Parameter | NX Expression | Baseline Value | Units | DV? | Range | Notes |
|-----------|--------------|----------------|-------|-----|-------|-------|
| Beam half-height | `beam_half_height` | ❓ TBD (Gap G12) | mm | No (currently) | — | Half total I-beam height |
| Beam half-width | `beam_half_width` | ❓ TBD (Gap G13) | mm | No (currently) | — | Half flange width |
| Face thickness | `beam_face_thickness` | 20 | mm | **Yes (DV2)** | 10–40 | Flange thickness |
| Core half-thickness | `beam_half_core_thickness` | 20 | mm | **Yes (DV1)** | 10–40 | Half web height |
| Beam length | `beam_length` | 5,000 | mm | No | Fixed | ✅ Confirmed (KBS) |
| Hole count | `hole_count` | 10 | — | **Yes (DV4)** | 5–15 (integer) | ✅ Confirmed (KBS) |
| Hole diameter | ❓ (Gap G14) | 300 | mm | **Yes (DV3)** | 150–450 | ✅ Confirmed (KBS) |
| Hole span | `p6` | 4,000 | mm | Potential (G15) | TBD | ✅ Confirmed (KBS) — total span for hole distribution |
| Hole start offset | (fixed) | 500 | mm | No | Fixed | Requirement — not parametric |
| Hole end offset | (fixed) | 500 | mm | No | Fixed | Requirement — not parametric |

## Mass

| Parameter | NX Expression | Value | Units | Source |
|-----------|--------------|-------|-------|--------|
| Mass (baseline) | **`p1`** | **11.33** | kg | ✅ KBS session 20260210-163801 |
| Density | (material card) | 7.3 | g/cm³ | ✅ KBS session — Antoine stated directly |

> ⚠️ **Previous value of ~974 kg (expression `p173`) is superseded.** See Gen 002 mass discrepancy resolution.

## Construction Method

1. **Sketch:** I-beam cross-section profile with 4 key expressions (`beam_half_height`, `beam_half_width`, `beam_face_thickness`, `beam_half_core_thickness`)
2. **Extrusion:** Sketch extruded to `beam_length` (5,000 mm) → solid body
3. **Holes:** `hole_count` circular holes of specified diameter, distributed over `p6` span (4,000 mm), centered in web, starting 500 mm from each end
4. **Idealization:** Promote body → mid-surface extraction (pair mid-surface function) → thin shell sheet bodies

## Hole Geometry Constraints

- **Span:** Holes are distributed over 4,000 mm (expression `p6`)
- **Fixed offsets:** First hole center at ≥500 mm from beam start, last hole center at ≤4,500 mm from beam start
- **Spacing:** At baseline: 10 holes in 4,000 mm → ~444 mm center-to-center, 300 mm diameter → ~144 mm ligament
- **Collision risk:** At extremes (15 holes × 450 mm dia in 4,000 mm): spacing = 267 mm, ligament = −183 mm → **HOLES OVERLAP** → infeasible
- **Feasibility formula:** `ligament = (hole_span / (hole_count - 1)) - hole_diameter > 0`
  - Must also check: `hole_diameter < web_height` (hole fits in web vertically)

## Structural Behavior

*From Technical Breakdown (Gen 001), confirmed by KBS session:*

The beam is a **cantilever** (left side fixed, right side free with 10,000 kgf downward load). It is **bending-dominated**:

| Behavior | Governing Parameters | Notes |
|----------|---------------------|-------|
| Bending stiffness (EI) | Face thickness, core thickness | Faces carry bending stress, core carries shear. Stiffness scales ~quadratically with distance from neutral axis |
| Mass | All four variables | Core and face add material; holes remove material from web |
| Stress concentrations | Hole diameter, hole spacing | Larger holes → higher SCF at edges. Closely spaced holes can interact |
| Shear capacity | Core thickness, hole count, diameter | Holes reduce shear-carrying area |

## Design Variable Interactions

1. **Core × Face** — classic sandwich interaction. Optimal stiffness balances core depth (lever arm) vs face material (bending resistance)
2. **Hole diameter × Hole count** — both remove web material. Large + many could leave insufficient ligament width
3. **Face × Hole diameter** — thicker faces reduce nominal stress, allowing larger holes
4. **Core × Hole diameter** — core thickness determines web height, constrains max feasible hole diameter

## History

- **Gen 001** (2026-02-09): Initial documentation from intake + technical breakdown
- **Gen 002** (2026-02-10): Updated with confirmed values from KBS sessions. Mass corrected from 974 kg → 11.33 kg. Beam length confirmed 5,000 mm. Hole parameters detailed. Expression names confirmed.