Fatigue Failure

Bolts and screws in vehicles and aircraft almost invariably fail by fatigue under repeated loading. Fatigue cracks usually initiate at stress concentrators (often the first thread engaged by the nut or a sharp root) and grow slowly with each load cycle. According to experts, insufficient preload or lost clamp force is a key cause: if a bolt isn’t tightened enough (or loses tension), cyclic loads create tensile stress fluctuations that initiate cracks[1][2]. Once a crack reaches a critical size, the bolt breaks suddenly (often without warning) under normal service loads. In aircraft and cars, one failing fastener can trigger a “zipper” effect: the extra load on remaining bolts causes a cascade of failures[3].

• Why it occurs: Cyclic (repeated) loads cause alternating stress in the fastener. Any feature that concentrates stress – such as sharp thread roots, notches, or surface defects – can become a crack origin. Low or lost preload means the bolt bends or stretches more under load, accelerating fatigue[1][2]. High-strength bolts (often used in automotive/aerospace) can also fatigue if overloaded relative to their rated endurance.
• Consequences: A fatigue failure usually appears as a brittle fracture surface with characteristic “beach marks.” The fastener suddenly fractures, potentially causing loss of clamping and catastrophic disassembly of joints (e.g. engine parts separating or airframe components detaching). A single broken bolt in a critical joint can lead to cascading failures of adjacent fasteners[3].
• Prevention: Ensuring high preload (proper torque) and stiff joint members greatly reduces fatigue. Preload ties most of the external load into clamp load, reducing the cyclic stress range in the bolt[2]. Design joints with long grip lengths (long bolt stretch) and use bolts just long enough so most of their shank stretches under load[4]. Use smooth, rolled threads (with generous root radii) to minimize stress risers. Follow correct torque values (using calibrated torque wrenches) and retorque after any relaxation period. Regular inspection (e.g. X-ray or dye penetrant) in critical applications can catch early cracks. In aerospace, fatigue monitoring and predictive maintenance are standard; in automotive, torque-to-yield bolts and proper assembly procedures help avoid fatigue risk.

Overload & Over-Tightening

Excessive torque or an unexpected load spike can instantly overload a fastener. Over-tightening a bolt stretches it beyond its elastic limit. The result may be yielded (permanently elongated) or broken threads, bolt neck, or head. Mechanical overload from external forces (crashes, impacts, or over-pressurization) can shear or pull the bolt apart. Industry sources warn that overtightened fasteners can suffer permanent deformation or fractures[5][6]. For example, Qewit Fasteners notes that “over-tightening fasteners can result in structural damage, reduced load-bearing capacity, and material deformation”[5]. ASMC Industrial similarly cautions that excessive torque “can strip threads, distort bolt heads, or weaken the material,” leading to failure[6].

• How it happens: Installing a bolt with too much torque (or sudden excessive load) forces the steel beyond yield, causing necking or shear at the weakest section (often at the shank under the head or at the thread root). A soft joint material (like aluminum or mild steel plate) may deform or crack around the fastener. In the worst case, the bolt head snaps off or the threaded portion shears out. Over-torquing can also “gall” threads (cold-weld them together), making removal impossible.
• Consequences: Overloaded fasteners lose clamping force or break immediately. In a critical assembly (engine block, landing gear, suspension), this can cause misalignment, leaks, or catastrophic joint separation. Even if the bolt doesn’t immediately fail, plastic deformation may leave it permanently weaker and prone to fatigue. The surrounding joint can also be damaged. These effects increase maintenance costs and safety hazards[5].
• Prevention: Always use the manufacturer’s specified torque values and tools. A properly calibrated torque wrench (and torque-angle method if specified) ensures bolts are tightened to just the right tension[7][8]. Regularly calibrate torque tools to maintain accuracy[7][8]. Train installers to turn bolts steadily and stop at the torque limit. Use thread lubricants or coatings only as specified (lubrication can reduce required torque, so specs assume either dry or lubricated condition). For critical joints, some industries use torque / tension monitoring. If over-tightening is suspected (e.g. wrench “strips” or head deforms), replace the bolt and nut rather than reusing them. Also design joints so that a shear failure (bolt breaking visibly) is preferred over hidden thread stripping[9] – a broken bolt is an obvious failure that can be corrected.

Loosening and Preload Loss

Bolts can gradually lose tension and back out, even if properly tightened. Vibration is a prime culprit: cyclic transverse movements can cause the nut or bolt to turn loose (the well-known Junker’s self-loosening effect)[10][11]. Thermal cycling also shakes bolts loose. Temperature changes cause different expansion of metals, so as parts heat and cool the clamp force fluctuates. If the nut and bolt expand at different rates, preload can drop enough for the joint to loosen[12][13]. Embedment and Settling: Contact surfaces have microscopic roughness that yields under pressure, plus coatings (paint, plating) can deform. As these layers flatten, bolt stretch shortens and preload drops (often 5–50% soon after assembly[13]). Over time, soft or uneven joint materials (composites, worn parts) can creep or settle, further reducing tension. Improper Torque: Under-tightening simply leaves inadequate preload, and even correctly torqued bolts often lose tension over time and need re-torquing. Corrosion and Wear: Rust or galling can reduce grip on the threads, allowing slow unwinding[14].

Key causes of bolt loosening include:

– Vibration/Shock: Constant shaking allows threads to slip unless prevented[11].
– Thermal Expansion: Repeated heating/cooling changes clamping force, especially in dissimilar materials[12].
– Creep/Embedment: Material yielding and surface flattening cause tension loss soon after tightening[13].
– Improper Torque: Too little torque leaves preload low; even correct torque can diminish with load or use.
– Corrosion/Wear: Oxidation or debris on threads reduces friction and holding strength, so bolts unwind more easily[14].

Prevention of Loosening

• Locking Hardware: Use lock washers (spring, split, or Nord-Lock wedge washers) that maintain tension under vibration. Prevailing- torque nuts (e.g. nylon-insert or distorted threads) provide friction to resist backing off[15]. In aerospace, castellated nuts with cotter pins or locking wire are common for critical joints. Double-nutting (jam nuts) or safety wire can also secure bolts.
• Thread-locking Adhesives: Apply anaerobic threadlocker (Loctite) to secure threads. These compounds harden between threads and prevent rotation. Various strengths are available, from removable (low-strength) to permanent (high-strength) grades[16].
• Proper Torque & Sequencing: Always tighten bolts in the correct pattern and to the specified torque. A torque wrench ensures the nut is not under-tightened – which is as dangerous as over-tightening. After initial assembly, bolts should be rechecked (retorqued) after the first run-in or thermal cycle to compensate for embedment losses[17][13].
• Washers and Preload Distribution: Use flat washers under nuts or heads to spread the clamping force and reduce surface embedment[18]. Sealing washers or Belleville (disc spring) washers can help maintain tension over time.
• Design Considerations: In design, avoid placing fasteners in high-vibration zones without locks. Keep parts well-aligned and surfaces clean (dirt or corrosion in the joint can encourage loosening). Selecting slightly longer bolts (to stretch more) or including spring washers can help maintain clamping force.
• Regular Inspection: Periodically check critical fasteners for tightness. Many automotive and aircraft maintenance schedules include torque checks for safety-critical bolts (engine mounts, wheel lug nuts, etc.).

Corrosion and Chemical Attack

Environmental corrosion severely undermines fastener integrity. General Corrosion (Rust): Exposure to moisture, salt, acids or alkalis causes oxidation and pitting. Corrosion reduces cross-sectional area and introduces crack-like pits that concentrate stress. ASMC notes that corrosion “weakens bolts and the surfaces they fasten, reducing their holding strength,” and rust buildup can prevent full thread engagement, accelerating loosening[14]. Stress Corrosion Cracking (SCC): Bolts held in tension in a corrosive environment (especially warm, humid, or chemical atmospheres) can crack suddenly. SMRP explains that high-strength steels under constant tensile stress are prone to SCC if exposed to certain agents (carbon steels crack in ammonia or sulfides; stainless steels crack in chlorides)[19]. Over time such bolts can develop fine cracks that “eventually fail” without visible warning.
– How it occurs: Corrosive agents penetrate microscopic surface films on a bolt under load, causing brittle crack growth. Hydrogen from corrosion can enter the steel lattice. Localized corrosion (pitting) creates high stress at pit bases, initiating cracks. In sacrificial-coated fasteners (zinc, cadmium), breaches in coating allow active corrosion of the underlying steel.
– Consequences: Corrosion fatigue and SCC lead to sudden bolt failure under load. A corroded bolt may appear damaged or oxidized; even if not yet broken, its load capacity is greatly reduced. For assemblies containing fluids (fuel, oil, coolant), corroded fasteners can cause leaks. In aerospace, any crack in a fastener is treated as a critical defect, and automotive recalls have occurred due to rusted bolts failing in use.
– Prevention:

• Material Selection: Use corrosion-resistant materials where needed. Stainless steel (e.g. 304, 316) resists rust, though it can still SCC in chlorides. In highly corrosive environments (marine, chemical plants) use exotic alloys or plated steel. Avoid galvanic couples (e.g. aluminum bolt against steel), or insulate them.
• Protective Coatings: Apply protective plating or coatings. Common finishes include zinc (electroplated or galvanic), cadmium (in aerospace, with mandatory baking), nickel, and specialized corrosion-resistant finishes[20]. For example, aerospace fasteners often use nickel plating (excellent corrosion resistance and wear/galling protection) and silver plating (anti-galling and corrosion prevention over a wide temperature range)[20].
• Avoiding SCC: For critical high-tensile bolts, use plating processes that bake out hydrogen or choose hydrogen-compatible finishes (silver or trivalent coatings). Ensure any cadmium- or zinc-plated high-strength bolt is properly baked to remove hydrogen (per ASTM standards). Stainless bolts should avoid chlorinated cleaners or atmospheres to reduce stress corrosion risk.
• Sealants and Lubricants: In automotive, underbody fasteners often get rubber boots or sealants to keep out salt water. Using anti-seize grease on threads can inhibit corrosion (but adjust torque for lubrication).
• Design Drainage: Where possible, design joints so water drains away from the bolt shank. Vented (hollow) fasteners and vented washers are used in aerospace to prevent fluid trapping.
• Regular Maintenance: Inspect and replace corroded fasteners before failure. In corrosive environments, shorter replacement intervals are prudent. For example, wheel studs and suspension bolts on vehicles in winter service should be checked yearly.

Hydrogen Embrittlement

High-strength steel fasteners (typically ≥ 150 ksi tensile) are susceptible to hydrogen embrittlement: a material failure mode induced by hydrogen rather than load cycling. Hydrogen can enter the steel during plating (especially zinc or cadmium plating of hard bolts) or even during corrosion. SMRP notes that high-strength bolts with hardness over 39 HRC are at high risk: “pickling and plating processes … can introduce hydrogen, and if not properly baked, entrapped hydrogen can result in delayed cracking days or weeks after installation”[21]. The bolt will then crack in a brittle manner, often near the thread or under the head, after it has been in service (sometimes after long delays).

• How it happens: Hydrogen atoms diffuse into the steel’s crystal lattice and accumulate at stress sites (grain boundaries, inclusions). Under sustained tensile load (even the preload itself), microscopic cracks form and grow without the ductility typical of steel. The fracture surface is usually granular and brittle.
• Consequences: A bolt may appear intact after tightening but will shatter unexpectedly later, often catastrophically in a critical joint. For example, an engine manifold stud might break days after assembly with no additional external load. Since it is invisible until failure, hydrogen cracking is very dangerous.
• Prevention: Use proper post-plating baking (per AMS or ASTM specs) to drive out hydrogen before use. Many aerospace-grade bolts are steam-cleaned and baked after zinc plating to eliminate hydrogen. Alternatively, use hydrogen-embrittlement-resistant coatings (silver plating is one choice) or unplated corrosion-resistant alloys where feasible[20]. Design with a safety factor: avoid keeping high-strength bolts at sustained high tension for long periods. If a fastener shows any crack, scrap the entire lot, as hydrogen damage is often batch-related.

Thread Stripping and Installation Errors

Thread stripping (shearing off the bolt or nut threads) is a common failure, often due to installation issues. According to technical sources, nut threads typically strip before the bolt breaks when properly chosen. However, stripping occurs if the nut is too hard, too thin, or not fully engaged[22]. For example, a common cause is using a hard, fine-threaded bolt with a softer thin nut – the nut’s threads cut out first. A related issue is “short bolting”: if the bolt does not protrude beyond the nut by at least two full threads, the engaged thread length is insufficient, and the joint will strip gradually under load[22][9].

• How it happens: Excess torque, heavy load, or vibration gradually shears threads. Thread stripping is progressive: the first engaged thread fails, then the next, until the bolt pulls through[23]. Unlike brittle fracture, stripping gives little warning – the nut may spin freely once failed, but only at the very end. Using a nut made of too-high grade (harder than bolt) can even cause the bolt’s threads to strip[24]. Cross-threading during assembly (misalignment) can nick threads, greatly reducing load capacity and causing early stripping or galling.
• Consequences: A stripped thread means zero clamping force. In service, the joint loses preload and separation or leaks can occur. Sometimes a stripped bolt spins without tension, which often causes self-loosening (vibration-induced). Critical joints (e.g. brake caliper bolts, aircraft engine nuts) cannot tolerate thread stripping, so this must be avoided.
• Prevention: Match bolt and nut grades: always use a nut of equal or lower hardness than the bolt (so the bolt, if anything, will yield first). Provide full thread engagement: after tightening, the bolt end should stick out at least two threads beyond the nut[9]. Use coarse threads rather than fine for greater durability (coarse threads have larger root radius and more shear area). During installation, avoid cross-threading by hand-starting the bolt, ensuring alignment before wrenching. Use anti-seize or appropriate lubricants on threads to prevent galling (see below), but adjust torque accordingly. When in doubt, replace stripped or damaged parts; do not re-tap or overtighten a damaged thread. Regular inspection after heavy loads or vibration can catch initial stripping before complete failure.

Figure: Thread geometry of a typical bolt (major diameter, minor diameter, crest and root). Proper thread engagement and profile are crucial – the thread roots are where fatigue cracks start and where stripping initiates[9].

Galling and Thread Seizure

Galling is a cold-welding phenomenon where mating metal threads (typically stainless steel or titanium fasteners) seize together under high friction. According to aerospace sources, “when two or more metal objects rub together, material transfer will occur” and the parts lock up[25]. Galling often happens during installation: if a stainless bolt is turned too forcefully against a stainless nut without lubrication, the soft material can tear and weld, preventing further tightening or removal.

• How it happens: High friction plus pressure causes microscopic junctions in the metal to bond. Over-torquing exacerbates this: Monroe Aerospace warns that “the tighter you make a fastener when installing it, the greater the risk of galling”[26]. Once galling starts, turning the fastener spreads the metal and deepens the seizure.
• Consequences: A galled nut can no longer be loosened without destroying it. Threads are typically ruined, requiring replacement of both bolt and nut. In-situ galling repair is time-consuming (heating or cutting out fasteners may be needed). Besides maintenance headaches, a seized fastener may mask underlying preload loss in a joint.
• Prevention: Use lubricants or special anti-galling compounds on stainless or other gall-prone fasteners. Zinc or cadmium plating and certain dry-film lubricants can reduce friction[27]. Aerospace-grade parts often come coated (e.g. silver or nickel) to prevent seizure. Do not overtighten stainless fasteners; tighten them slowly and carefully[26]. Consider using dissimilar materials judiciously (e.g. a stainless bolt with a coated nut, or silver-plated hardware). Finally, use high-quality fasteners: softer, cheaper metals gall more readily under load[28].

Conclusion

Bolts, screws, and nuts may seem simple, but their failure modes are many and can have severe consequences in automotive and aerospace systems. In summary, the most common issues are:

• Fatigue: Prevent by proper preload, joint stiffness, and good thread design[1][2].
• Overload/Over-Torque: Prevent by strict torque control and training[5][6].
• Loosening: Prevent by locking devices, adhesives, proper torque, and maintenance[29][13].
• Corrosion (including SCC and Hydrogen Embrittlement): Prevent by using suitable materials, coatings, and environment control[19][21].
• Thread Damage (Stripping/Galling): Prevent by correct assembly practice, full thread engagement, and surface treatments[9][26].

For engineers and technicians, the key recommendations are: follow torque specifications with calibrated tools; inspect and maintain joints regularly; use locking hardware or threadlocker where vibration is present; select materials and coatings appropriate for the environment; and treat any damaged fastener as suspect and replace it. By understanding each failure mode and applying best practices — from design through installation to maintenance — reliability of bolted joints in vehicles and aircraft can be maximized.

Sources: Authoritative maintenance and fastener engineering references have been consulted, including SMRP industry articles[1][19], fastener supplier guidelines[5][29], and technical resources[9][26].

[1] [19] [21]  Bolt Failures‎– Why Learn to Recognize Mechanical Failure Modes

https://smrp.org/News/Solutions-Archive/Member-Written-Articles/Bolt-Failures-Why-Learn-to-Recognize-Mechanical-Failure-Modes

[2] [4] [10] [13] Threaded Fasteners — DesignJudges.com

https://www.designjudges.com/articles/threaded-fasteners

[3] Fatigue Failure in Bolts: How to prevent it? | RVmagnetics

https://www.rvmagnetics.com/threaded-fasteners-smartify-the-bolted-structure-with-microwire-sensors-makes-self-detection-of-fatigue-loosening-and-vibrations-possible-132

[5] [7] [8]  Understanding Fasteners Consequences of Over-Tightening

https://www.qewitfastener.com/blog/understanding-fasteners-consequences-of-over-tightening/

[6] [11] [12] [14] [15] [16] [17] [18] [29] Why Do Bolts Loosen And How To Keep Them Tight – ASMC Industrial LLC – ASMC Industrial LLC

https://www.asmc.net/blog/how-to-keep-bolts-from-loosening/

[9] [22] [23] [24] Bolt and Nut Failures | Boltmasters Pty Ltd

https://www.boltmasters.com.au/boltandnutfailures

[20] What you should know about fasteners for Aerospace applications – UC Components, Inc.

https://www.uccomponents.com/what-you-should-know-about-fasteners-for-aerospace-applications/

[25] [26] [27] [28]  Galling: Understanding Why Fasteners Stick Together | Blog- Monroe Aerospace

https://monroeaerospace.com/blog/galling-understanding-why-fasteners-stick-together/?srsltid=AfmBOoqtUAZVB1l76agGpln11WZkZRLlhjf8tw0QTFkI-WYT8Z5FbMvq