Common Causes of
Fastener Failure
Explained
Is It Really the Fastener’s Fault?
When a bolt or nut fails in service, the instinctive assumption is often that the fastener itself was defective — the wrong grade, a bad batch, a manufacturing flaw. In practice, failure analysis across the industry consistently finds that the fastener itself is the root cause in only a minority of cases. Far more commonly, the actual cause is one of: incorrect installation torque, a joint design that didn’t account for cyclic loading, a material grade genuinely mismatched to the service environment, thread galling from an unlubricated stainless-on-stainless joint, or vibration that the joint’s design never adequately resisted.
This distinction matters because the correct fix is completely different depending on the actual root cause. Replacing a fatigue-fractured bolt with an identical one, without addressing the cyclic loading or stress concentration that caused the original fatigue crack, simply guarantees a repeat failure on the same timeline. Understanding which of the mechanisms below actually applies to a specific failure is the only way to prevent it recurring.
This page walks through the most common genuine failure mechanisms, how to recognise each, and — because Akbar Fasteners manufactures the part itself — where correct material selection, manufacturing process control, and documentation genuinely reduce the risk of each one.
Common Fastener Failure Mechanisms
Overload (Tensile Failure)
Load simply exceeds the fastener’s tensile strength in a single event — a straightforward, one-time exceedance rather than progressive damage.
Fatigue Failure
Repeated cyclic loading below the material’s tensile strength initiates and grows a crack over time, typically starting at a thread root or other stress concentration. Covered in detail below.
Stress Corrosion Cracking
Tensile stress and a specific corrosive environment acting together cause cracking that neither factor alone would cause. Covered in full depth below.
Hydrogen Embrittlement
Atomic hydrogen absorbed during electroplating causes delayed brittle fracture in high-strength fasteners. Full mechanism explained on our Coated Fasteners page.
Thread Galling
Friction during tightening causes localized cold-welding and tearing of mating threads, especially common in unlubricated stainless-on-stainless joints.
Vibration-Induced Loosening
Cyclic vibration causes the joint to gradually lose clamp load and back off, distinct from fracture — a loose joint often precedes a later fatigue failure.
Improper Installation Torque
Under-torque leaves insufficient clamp load, permitting joint movement; over-torque can yield the fastener or reduce its effective clamp load before service even begins.
Material Substitution or Counterfeit Parts
A fastener supplied with an incorrect or substandard grade marking fails at a load the genuine specified grade would have safely carried.
Fatigue vs Overload Fracture
Distinguishing these two correctly is the first step in any real failure investigation.
Overload Fracture
Occurs when a load applied once exceeds the fastener’s tensile strength. The fracture surface typically shows a single, relatively uniform ductile or brittle break with no progressive crack growth pattern — the failure happens at the moment the load is applied.
Fatigue Fracture
Occurs from repeated loading well below the material’s tensile strength, with a crack initiating at a stress concentration point — commonly a thread root — and slowly growing with each load cycle until the remaining cross-section can no longer support the load and fractures suddenly.
Stress Corrosion Cracking — Why the Combination Matters
Stress corrosion cracking (SCC) is a genuinely distinct failure mode from ordinary corrosion. It requires three factors present simultaneously: a susceptible material, sustained tensile stress (which can be from applied load, or simply residual stress left in the material from manufacturing), and a specific corrosive environment the material is vulnerable to. Remove any one of these three factors, and SCC does not occur — a stressed fastener in a benign environment is fine, and an unstressed fastener in the same corrosive environment is also fine. It is specifically the combination that causes cracking.
This is what makes SCC a genuinely dangerous failure mode from an inspection standpoint: the fastener can show little to no visible general corrosion before failing, because the damage is a fine, often branching crack propagating through the material rather than broad surface metal loss. A part can appear largely sound right up until sudden fracture.
Prevention requires addressing the actual combination, not just one factor — selecting a material genuinely resistant to SCC in the specific service environment (chloride-bearing environments are a classic SCC trigger for standard austenitic stainless, which is one reason duplex and super austenitic grades exist), reducing residual stress through correct manufacturing and heat treatment, and avoiding unnecessary sustained tensile stress in the design where practical. See our Super Duplex and SMO 254 pages for material options specifically selected for superior chloride resistance in demanding environments.
How Our QC Process Addresses Several of These Modes
Not every failure mode is preventable by the manufacturer alone — installation and design matter enormously — but several genuinely are.
PMI and NABL-accredited testing on every applicable lot directly prevents material substitution failures by confirming actual grade matches certification.
Honest material selection guidance across our site — chloride resistance, PREN, temperature range — reduces the risk of a genuinely mismatched material being specified in the first place.
Documented post-plating baking per ASTM F1940 on high-strength electroplated fasteners directly addresses this specific failure mode at the manufacturing stage.
First article inspection on custom parts and standard dimensional control on catalogue parts reduce the risk of a thread or fit issue contributing to premature failure.
Related Pages on This Topic
Several failure modes on this page are covered in additional depth elsewhere on our site.
Coated Fasteners
Full explanation of hydrogen embrittlement risk in electroplated high-strength fasteners and how baking prevents it.
View Coated Fasteners →Stainless Steel Screws
Why an otherwise-correct stainless grade can still show surface rust if the passivation step is skipped or rushed.
View Stainless Steel Screws →Super Duplex Fasteners
PREN and chloride pitting resistance explained in detail — directly relevant to SCC prevention in aggressive environments.
View Super Duplex →Washers
An honest look at whether split lock washers actually prevent joint loosening under vibration.
View Washers →Investigating a
Failure?
Share the details and, if possible, photographs of the failed part. Our team can help identify the likely root cause.
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