Fastener and Bolting Failure Analysis

The Four Failures That Account for Most Broken Bolts

Threaded fasteners fail in a small number of characteristic ways, and the four below cover the large majority of cases that reach a laboratory. They are separated by the fracture mode and by hardness, not by the appearance of the broken bolt.

• Hydrogen embrittlement: delayed, brittle, intergranular fracture in a high-strength fastener, typically within hours or days of being tightened. The hydrogen comes from electroplating, pickling, phosphating, cathodic protection or a corroding service environment. Hardness above roughly 39 HRC is the threshold at which susceptibility becomes the governing risk.
• Stress corrosion cracking: branched, largely brittle cracking that needs a susceptible alloy, a sustained tensile stress and a specific environment together. In bolting it is most often found at the first engaged thread, which is where the tensile stress peaks.
• Fatigue: beach marks and a final fast-fracture zone, almost always starting at the thread root or the head-to-shank fillet. The usual root cause is under-tightening, because a joint that separates puts the cyclic load into the bolt instead of into the clamped members.
• Overload: ductile necking with dimpled rupture, or brittle cleavage in a material below its transition temperature. The question that follows is whether the load was outside design or the bolt was below grade.

Galling, thread stripping, decarburisation and improper heat treatment make up most of the remainder, and each is settled by a specific measurement rather than by inspection.

The Measurements That Decide It

Fastener investigations are unusually well served by standards, because the acceptance criteria for hardness, microstructure and mechanical properties are published for every common grade.

Chemical analysis, mechanical testing, metallography and hardness are performed under NABL certificate NABLT0726MH18640. For aerospace fasteners the same bench is NADCAP AC7101 accredited, PRI certificate 29415245997, under scope codes AC7101/3 mechanical, AC7101/4 metallography and AC7101/5 hardness and conductivity.

Hydrogen Embrittlement, and Why It Is a Process Question

Hydrogen embrittlement is the failure that most often ends in a commercial dispute, because the fastener met every specification on the certificate and broke anyway. The hydrogen was introduced after the material was made, and usually by a process step someone else owned.

• Internal hydrogen embrittlement comes from manufacturing: acid pickling, electroplating, phosphating or an electrochemical cleaning step. It is controlled by baking within a defined window after plating, and the failure often traces to a bake that was late, short or omitted.
• Environmental hydrogen embrittlement comes from service: cathodic protection, a corroding surface, or a sour environment generating hydrogen at the metal surface. It has no manufacturing fix and is a material selection and hardness question.
• The evidence: intergranular fracture at the first engaged thread or the head fillet, delayed failure under a sustained static load, no corrosion product at the origin, and hardness at the top of or above the grade range.
• The prevention: ASTM F519 qualifies whether a plating process embrittles, and ASTM F1940 controls it in production. Both are process tests, and both are run before the fastener is installed, not after it breaks.

Where a batch failure is suspected, the investigation extends to the unfailed fasteners from the same lot. That is what separates a single defective item from a consignment that has to be recalled.

Where Bolting Failures Are Decided

The consequence of a fastener failure is out of all proportion to the cost of the part, which is why the investigation is usually urgent and usually contested.

• Flanged joints in process plant: a stud failure is a containment failure, and the investigation runs alongside the leak and the incident report.
• Structural and infrastructure connections: high-strength friction-grip bolting, where hardness, coating and installed tension are the three things that get checked.
• Rotating and reciprocating machinery: connecting rod, coupling and foundation bolting, where fatigue from an under-tightened joint is the usual mechanism.
• Aerospace and defence fasteners: run on the NADCAP AC7101 accredited bench, against the standing 21-item aerospace test menu.
• Wind, marine and offshore bolting: where the environment and the cathodic protection system are part of the mechanism, not part of the background.

Where the failure raises a question about the joint rather than the bolt, the work continues as a root cause failure analysis of the assembly, and where it raises a question about a population, as a knowledge-based risk assessment.

Related services and industries

• Root Cause Failure Analysis
• Mechanical Testing
• Heat Treatment Verification
• Coating Evaluation
• Corrosion and Sour Service Testing
• Aerospace and Defence

Related insights

Hydrogen is the fastener failure that ends in a dispute, and these four cover both the post-mortem and the process test that prevents it.

• ASTM F606 Embrittlement Testing for Coated Fasteners
• Hydrogen Embrittlement Test at TCR Engineering in India
• Fastener Testing That Actually Matters: NABL Accredited Validation for Critical Connections
• Material Testing of Fasteners as per IS 1367 Standards