Fastener Failure Analysis: Causes, Fracture Modes & Testing Methods
Why Fastener Failure Analysis Matters (Simple Guide)
Fasteners like bolts, screws, and nuts are small but critical. A single failed bolt can cause a bridge to collapse, a plane to crash, or dangerous leaks. Failure analysis finds out why a fastener broke — so you can fix the real problem and prevent it from happening again.
Common Ways Fasteners Fail
1. Fatigue Fracture (Most Common)
Cause: Repeated small loads (even well below the bolt's strength) slowly crack the fastener over time.
Appearance: Smooth fracture surface with "beach marks" (concentric rings).
Prevention: Proper tightening (preload) reduces stress changes.
2. Hydrogen Embrittlement (HE)
Cause: Hydrogen gets into high-strength steel (e.g., during electroplating) and makes it suddenly snap.
Appearance: Grainy, intergranular fracture (cracks along grain boundaries).
Prevention: Bake fasteners after plating (ASTM F1941).
3. Stress Corrosion Cracking (SCC)
Cause: Tension + corrosive environment (e.g., chlorides for steel, marine air for stainless).
Appearance: Branching cracks (like tree roots) under microscope.
Prevention: Choose the right material for the environment.
4. Overload Fracture
Cause: Load exceeds bolt strength (e.g., over-tightening or design error).
Appearance: Necking (stretched metal) or flat, featureless break.
Prevention: Use torque wrenches and correct design.
5. Corrosion
Cause: Rust, galvanic corrosion (different metals touching), or pitting.
Effect: Weakens the fastener and creates spots where cracks start.
Prevention: Coatings, material selection, and isolating dissimilar metals.
How Failure Analysis Works (Step by Step)
| Step | What It Finds |
| Visual exam | Where it broke, rust, thread damage, improper installation |
| SEM (microscope) | Fatigue, HE, or SCC patterns |
| EDS (chemical analysis) | Corrosion products, plating layers |
| Hardness test | Heat treatment quality, strength grade |
| Thread inspection | Size and shape defects |
Why It Matters by Industry
Aerospace – Mandatory for safety. One failed bolt can ground a fleet.
Automotive – Prevents costly recalls (engine, suspension, wheels).
Infrastructure – Bridges, pressure vessels, and buildings need regular checks.
Electronics – Small screws fail from wrong torque or corrosion.
Simple Takeaway
Fastener failure analysis answers one question: Why did this bolt break?
• Fatigue? → Tighten it better.
• Hydrogen embrittlement? → Bake after plating.
• Corrosion? → Change material or coating.
• Overload? → Redesign or use correct torque.
Without analysis, you might fix the wrong thing — and the same failure will happen again.
Final Tip
Always investigate failed fasteners. A few hours of analysis can save millions in repairs, recalls, or accidents.