Bolted Joint Analysis

---
name: bolted-joint-analysis
description: "Use when analyze load transfer, bearing stress, bypass ratio, preload,
  and failure modes in a bolted metallic or composite joint per ECSS-E-ST-32C clause
  4.6.2.13: distribute applied load among fasteners by relative stiffness, compute
  bearing stress and bypass ratio at each fastener location, check the linear bearing-bypass
  interaction criterion against material allowables, derive bolt preload from installation
  torque, and identify governing failure modes including bearing, net-section tension,
  shear-out, pull-through, fastener shear, fastener tension, and fatigue. Trigger:
  ecss, e-st-32-structures-scope, bolted-joint, bearing-stress, bypass-ratio, preload,
  metallic-fastener-joints, composite-bolted-joints, lug-joint-analysis."
license: Apache-2.0
compliance: STANDARDS-REF
standards:
  - id: ecss
    reference-only: true
gated: false
domain: space-systems
pack: space-systems
compatibility: "agentskills.io SKILL.md; any SKILL.md host (Claude Code, Hermes, OpenClaw)"
metadata:
  domain: space-systems
  subdomain: ecss
  tags: [ecss, e-st-32-structures-scope, bolted-joint, bearing-stress, bypass-ratio, preload, metallic-fastener-joints, composite-bolted-joints, lug-joint-analysis]
  version: 0.1.0
  author: Aero Agent Skills
---

# ECSS Structures — Bolted-Joint Analysis (space-systems/ecss/bolted-joint-analysis)

Use when the task is the bolted-joint analysis required by ECSS-E-ST-32C
clause 4.6.2.13 -- determining load transfer among fasteners, computing
bearing stress and bypass ratio, checking the bearing-bypass interaction
criterion, deriving bolt preload from installation torque, and assessing
each governing failure mode for metallic-fastener joints, composite bolted
joints, and lug joints.

## Domain quick reference

- Clause 4.6.2.13 requires that every bolted joint be assessed for load
  transfer (the fraction of the total applied load carried by each
  fastener), bearing stress (the compressive contact stress at the
  fastener hole wall), and the bypass load (the portion of total load
  that passes through the plate section past a given fastener rather than
  being transferred by it).
- Load transfer among fasteners in a multi-fastener joint is distributed
  proportionally to each fastener's axial stiffness: a stiffer fastener
  attracts a larger share of the load. For a joint with uniform fasteners,
  each carries an equal fraction.
- Bearing stress is computed as the fastener load divided by the product
  of fastener diameter and plate thickness (units: N/(mm × mm) = MPa). It
  must be checked against the material's allowable bearing stress.
- Bypass ratio is the fraction of the total applied load that bypasses a
  given fastener: (total load − fastener load) / total load. A fastener
  that transfers all of the load has zero bypass; one at the end of a
  long joint may have a bypass ratio approaching 1.
- The bearing-bypass interaction criterion combines both effects: the
  bearing ratio (bearing stress / allowable bearing stress) and the bypass
  ratio are summed (linear interaction); the result must be ≤ 1.0 for
  compliance. More conservative interaction curves exist for specific
  material and laminate families.
- Bolt preload is generated by the installation torque. The simplified
  relationship is F_preload = T / (K × D), where T is applied torque
  (N·m), K is the nut factor (dimensionless, approximately 0.2 for
  standard metallic fasteners), and D is the nominal bolt diameter (m).
  Preload affects joint stiffness, the friction-carried portion of shear
  load, and fatigue performance.
- Recognized failure modes for bolted joints: bearing (hole wall
  crushes), net-section tension (plate fails across the reduced section
  at the fastener hole), shear-out (plate shears along two planes from
  the hole edge to the free edge), pull-through (fastener head or nut
  pulls through a thin sheet), fastener shear (bolt shears at the
  shear plane), fastener tension (bolt breaks under combined preload
  and external tension), and fatigue (cyclic load initiates a crack at
  the hole stress concentration).

## Workflow

1. Verify the joint type: categorize it as metallic-fastener, composite
   bolted, or lug joint. Reject any unrecognized type before analysis
   proceeds.
2. Identify all fasteners in the joint, collect each fastener's nominal
   diameter, plate thickness at the fastener hole, axial stiffness, and
   material allowable bearing stress. Flag any fastener whose allowable
   bearing stress has not been set — absent allowables block the margin
   calculation and must be resolved before compliance can be declared.
3. Distribute the total applied load among the fasteners proportionally
   to their stiffnesses.
4. For each fastener, compute the bearing stress and the bypass ratio from
   the distributed load and the total applied load.
5. Compute the margin of safety for bearing: (allowable bearing stress /
   computed bearing stress) − 1. A negative margin is a finding.
6. Check the bearing-bypass interaction: bearing ratio + bypass ratio ≤
   1.0. An interaction value exceeding 1.0 is a finding.
7. Where installation torque is specified, derive the preload from the
   torque, nut factor, and diameter. Note that the preload model here is
   the simplified scalar formula; a friction-based model is needed when
   the thread friction coefficient is separately characterized.
8. Review all findings. A joint is compliant only when every fastener has
   a positive bearing margin, no bearing-bypass interaction exceeds 1.0,
   and all allowable-bearing entries are populated.

## Pitfalls

- Applying the total load as the bearing load on every fastener without
  distributing it — this overstates the bearing stress on each fastener
  and masks the correct bypass ratio, producing a conservative but
  incorrect interaction check.
- Ignoring the bypass ratio contribution when the bearing ratio is low —
  a fastener at the end of a splice joint can have a very high bypass
  ratio; even with low bearing stress the interaction criterion may
  exceed 1.0.
- Treating an absent allowable bearing stress as a zero violation —
  a missing allowable means the material data reference has not been
  consulted, which is itself a non-compliance finding, not a free pass.
- Using the linear interaction criterion for carbon-fibre reinforced
  polymer laminates without checking whether the programme's composite
  structural analysis plan specifies a non-linear or laminate-specific
  interaction curve — clause 4.6.2.13 references the bearing-bypass
  envelope from the composite materials data; the linear sum is the
  default fallback, not the governing criterion for all laminates.
- Omitting fatigue from the failure-mode list when the fastener is
  subject to cyclic loading — clause 4.6.2.13 includes fatigue as a
  required check; it cannot be waived by showing adequate static margin.

## Behavior contract (gate 3)

The joint-type categorization, load distribution, bearing stress, bypass
ratio, bearing-bypass interaction, preload, margin-of-safety, and
joint-compliance logic is exercised by the gate 3 contract test:
scripts/test_bolted_joint_analysis.py against
scripts/bolted_joint_analysis_logic.py (stdlib unittest, offline). Run:
python3 scripts/test_bolted_joint_analysis.py

## Compliance

- ECSS standards are freely downloadable (ESA); cite the source and
  paraphrase per standards-map.yaml.
- compliance: STANDARDS-REF, gated: false.