Fastener failures can cause equipment damage, production downtime, and safety incidents. Understanding failure modes helps prevent them. This guide covers the most common failure types and how to avoid them.
1. Tensile Overload Failure
What It Looks Like
- Bolt breaks with necking (reduction in area)
- Fracture surface appears cup-and-cone
- Usually occurs at thread root (weakest point)
Causes
- Applied load exceeds bolt strength
- Wrong grade selected
- Undersized bolt for application
- Incorrect torque (over-tightening)
Prevention
- Calculate actual loads including dynamic factors
- Select appropriate grade with safety factor
- Use proper torque control methods
- Consider using larger diameter or higher grade
2. Fatigue Failure
What It Looks Like
- Beach marks on fracture surface (progressive cracking)
- Final fracture zone appears rough
- Crack usually starts at stress concentration (thread root, under head radius)
Causes
- Cyclic loading below static strength
- Insufficient preload (joint separates under load)
- Stress concentrations from poor design
- Surface defects or damage
Prevention
- Ensure adequate preload to prevent joint separation
- Use roll-threaded bolts (better fatigue life)
- Avoid sharp corners and stress risers
- Consider using larger bolts to reduce stress amplitude
- Regular inspection for early crack detection
3. Stripping (Thread Shear)
What It Looks Like
- Threads torn from bolt or nut
- Bolt remains intact but nut spins freely
- Or bolt threads stripped, nut remains
Causes
- Insufficient thread engagement length
- Soft material in nut or tapped hole
- Over-torquing
- Cross-threading during assembly
Prevention
- Ensure minimum 1.5x diameter thread engagement
- Match nut grade to bolt grade
- Use proper torque specifications
- Start threads by hand to avoid cross-threading
- Inspect threads before assembly
4. Hydrogen Embrittlement
What It Looks Like
- Brittle fracture with little or no necking
- Delayed failure (hours to days after installation)
- Usually occurs in Grade 10.9 and higher
Causes
- Hydrogen introduced during electroplating
- Insufficient baking after plating
- High-strength steel susceptible to hydrogen
Prevention
- Bake plated fasteners within 4 hours (190-220°C for 8-24 hours)
- Use Dacromet or mechanical plating (no hydrogen)
- Consider hot-dip galvanizing for large fasteners
- Avoid electroplating for Grade 12.9 and above
5. Corrosion Failure
What It Looks Like
- Red rust on carbon steel
- White corrosion products on aluminum or zinc
- Pitting, general attack, or stress corrosion cracking
Types
- Uniform corrosion: General surface attack
- Pitting: Localized deep attack (chlorides on stainless)
- Galvanic: Dissimilar metals in electrolyte
- Stress corrosion: Combined stress and corrosive environment
- Crevice: In gaps, under washers, in threads
Prevention
- Select appropriate material for environment
- Use protective coatings (zinc, galvanized, Dacromet)
- Avoid galvanic couples (or insulate)
- Design to avoid crevices and water traps
- Regular inspection and maintenance
6. Loosening
What It Looks Like
- Joint becomes loose over time
- Bolt can be turned by hand
- Vibration marks on fastener or joint
Causes
- Vibration causing rotation
- Embedding of surfaces
- Thermal cycling
- Insufficient initial preload
Prevention
- Use locking mechanisms (lock nuts, Nord-Lock washers)
- Apply thread-locking adhesive
- Ensure proper preload
- Re-torque after initial service
- Regular inspection and maintenance
7. Stress Corrosion Cracking (SCC)
What It Looks Like
- Brittle fracture in normally ductile material
- Branching crack pattern
- Often occurs in stainless steel with chlorides
Causes
- Combined tensile stress and corrosive environment
- Chlorides + stainless steel + stress
- Caustic environments + carbon steel
Prevention
- Use SCC-resistant materials (duplex stainless, nickel alloys)
- Reduce stress levels
- Control environment (remove chlorides)
- Stress relieve after cold working
Failure Investigation Process
- Document: Photograph failure in situ before disassembly
- Collect: Preserve failed fastener and mating parts
- Examine: Visual inspection, measure dimensions
- Analyze: Fractography, metallurgical examination
- Test: Hardness, tensile, chemical analysis
- Determine: Root cause and contributing factors
- Correct: Implement preventive measures
When to Seek Expert Help
- Safety-critical applications
- Multiple failures of same type
- Unclear failure mechanism
- Legal or insurance implications
- Need for formal failure report
Chaoshuo Trading – We help customers analyze fastener failures and recommend solutions. Our technical team can assist with material selection, design review, and failure prevention. Contact us for support.