Mechanical fasteners (bolts, screws, nuts, rivets) often fail for a few predictable reasons. Industry studies agree that fatigue is by far the leading cause – roughly 85% of bolt/screw failures stem from cyclic loading. Other common failure modes include static overload (yield or shear), thread stripping, stress corrosion cracking, and metallurgical issues like improper heat treatment or hydrogen embrittlement. Failures typically initiate at stress concentrators – for example, in bolts the first engaged thread or the fillet under the head is often the weakest point. Understanding these mechanisms helps engineers prevent costly joint failures by proper design and manufacturing controls.

Fatigue Failures (Cyclic Loading)

Fatigue cracks in bolts/screws grow under repeated (often small) cyclic loads. Even if each load is well below the material’s strength, the alternating stress causes tiny cracks to initiate and grow with each cycle. These micro-cracks usually start at stress concentrators – e.g. the thread root, sharp notches, or any inclusion in the steel – and then propagate until the fastener fractures.

Bolts in vibrating or cyclic service (engines, presses, etc.) are especially at risk if not properly preloaded. If a bolt is under-tightened, the external fluctuating load can exceed the clamp force and induce alternating stress in the shank – quickly causing fatigue cracks. Indeed, “insufficient pre-load (under-torquing) is the leading cause of fatigue failures in bolts”. Conversely, very high preload (overtightening) also raises the mean stress and can shorten fatigue life. In practice, fatigue cracks often start at the first loaded thread or under the bolt head and then spread through the cross-section.

• Prevention: Ensure correct preload/torque so the bolt stays in the elastic range. Use rolled (cold-formed) threads, which create compressive surface stresses and continuous grain flow – these can be ~30–100% stronger in fatigue than cut threads. Specify high-quality materials free of inclusions, and use processes like shot-peening or surface polishing to reduce stress risers.

Tensile and Shear Overload

When a fastener sees a load exceeding its capacity, it fails in an overload (ductile) mode. For example, applying too much torque during installation or imposing an extreme external load can push the combined stress above yield, causing the bolt to stretch, neck, and finally pull apart. In shear, the shank may literally shear off along a plane. One source notes that excessive tightening torque under high friction can combine axial and torsional stresses to exceed yield. A related analysis summarizes that “ductile or shear failure can occur from (a) over-torquing during installation, (b) an excessive loading event during service and/or (c) severely understrength bolts”. Visually, overload failures show classic ductile features (e.g. cup-and-cone fracture) or clean shear surfaces, unlike the striations of fatigue.

• Prevention: Avoid loads beyond design limits. Follow torque specs carefully – too much torque can itself cause yield. Use bolts of adequate strength (grade/class) for the application. Inspect failed bolts: necking indicates tension overload, whereas flat shear faces indicate a pure shear overload.

Thread Stripping and Engagement Failures

Thread stripping occurs when the mating threads (in a nut or tapped hole) shear off instead of the bolt. This often happens if the engaged thread length is too short or the tapped material is weak. Stripping is usually gradual – partial failure of threads may go unnoticed until a load suddenly pulls the nut off the bolt. For instance, a study noted that to fully shear threads, a displacement of at least half a thread pitch is needed; partial shearing can hide failure until a significant load is applied. In a well-designed joint, the bolt (or insert) should fracture in tension before any threads strip. That means providing sufficient thread depth and matching material strengths. “Nuts…are designed so that the bolt can be expected to fail by tensile fracture before thread stripping occurs” – the key is using enough thread engagement.

• Prevention: Ensure adequate thread engagement: in soft materials, use more than one diameter’s worth of depth. Always use full-height nuts or inserts rated for the bolt class. Rolled threads, with continuous grain flow, resist stripping better than cut threads. Avoid under-torquing (which leaves the joint loose) or over-torquing (which can damage threads during assembly). When tapping holes, minimize “bell-mouth” taper and use proper drill sizes to achieve at least ~60% thread height in the hole.

Stress Corrosion Cracking (SCC)

Stress corrosion cracking is a brittle failure caused by the combination of tensile stress and a corrosive environment. Although SCC involves an environmental factor (e.g. chlorides or acids), the mechanical key is the sustained tensile stress from preload. SCC typically produces a sudden, brittle fracture with little deformation. As one review notes, SCC is “a type of localized corrosion characterized by fine cracks… that propagate quite rapidly leading to failure of the component”. In fasteners, this might appear as a flat, branching crack through the shank, often along grain boundaries.

Preventing SCC requires removing at least one of its factors: use corrosion-resistant alloys (e.g. austenitic stainless steels or coatings) and avoid exposing stressed fasteners to aggressive media. In some cases designers can use sealants or inhibitors. From a mechanical viewpoint, minimizing stress (not over-tightening) and using materials with good SCC resistance are the controls. (For example, hardened carbon steel bolts are very susceptible to chloride SCC, whereas suitable stainless grades would not crack under the same conditions.)

Hydrogen Embrittlement and Improper Heat Treatment

High-strength fasteners (e.g. grades 10.9, 12.9) are sensitive to hydrogen embrittlement. During processes like acid pickling or electroplating (zinc, cadmium, etc.), hydrogen atoms can diffuse into the steel. If not baked out, the trapped hydrogen causes delayed brittle cracks under residual stress. One expert notes that such cracks often occur 1–24 hours after tightening if a post-plating bake is omitted. That is why standards (ISO 4042) require that plated high-tensile bolts be heat-treated (baked) soon after plating to drive out hydrogen. Without this step, bolts can fail in a brittle manner far below their expected load.

Beyond hydrogen, any deviation in heat treatment can ruin a fastener. Over-hardening or incomplete tempering leaves the steel brittle; under-hardening or decarburization leaves it too weak. For example, improperly quenched bolts may have non-uniform hardness and will fail in low-strain conditions. In short, faulty heat treatment produces fasteners that either fracture easily (if too brittle) or yield under normal loads (if too soft).

• Prevention: Strictly follow heat-treat and plating procedures. Verify fastener hardness and tensile strength meet specification (e.g. ISO 898-1 or ASTM grades). Always bake high-strength plated bolts within the recommended time (e.g. within 4 hours of plating at ~190 °C). If embrittlement is suspected, perform a bend test or check fracture surfaces (hydrogen cracks are often intergranular). Use coatings that don’t trap hydrogen (mechanical zinc flake vs. electroplated zinc, for example).

Manufacturing Defects and Material Issues

Hidden defects from production can be failure seeds. Non-metallic inclusions (slag, oxides) in the steel create local weak spots. Indeed, the fatigue image above shows a crack starting precisely at an internal inclusion. Likewise, poor forging or machining can leave micro-cracks or voids. Even a slight burr or scratch on a thread acts as a stress raiser. Metallurgical problems also matter: e.g. coarse grain size, segregation, or an unintended hardness gradient through the shank. The net effect is a fastener with significantly reduced strength or fatigue life.

• Prevention: Use reputable fastener suppliers with tight quality control. Require material certifications and, for critical applications, non-destructive testing (ultrasonic or X-ray) to detect internal flaws. Inspect threads and bearing surfaces for defects before assembly. Choose grades with proven microstructure (spheroidized carbides for alloy steels, fine ferrite-pearlite for carbon steels). Enforce clean manufacturing processes: control sulfur content, ensure proper die lubrication to avoid cracks in cold heading.

Rivets and Other Fasteners

Rivets (solid or blind) and similar one-piece fasteners are usually loaded in shear and bearing. Their failure modes mirror bolts, but in shear rather than tension. A rivet will shear off if its shear capacity is exceeded, or the surrounding sheet may tear out in bearing. Rivets can also fatigue under cyclic shear: cracks often start at the rivet head edge or at stress concentrators in the sheets. Like bolts, improper installation (too soft a rivet, over-driving, or holes too large) can cause slipping or fatigue issues. Prevention involves correct rivet sizing, ensuring full clamping force, and avoiding eccentric loads.

Prevention and Best Practices

To minimize fastener failures, engineers should follow these guidelines:

• Correct Preload: Apply the specified torque or tension. Under-torquing leads to joint slippage and fatigue; over-torquing causes yielding and shear. Use calibrated tools, consider friction (lubrication or dry) and use multiple torque steps.
• Quality Threads: Use fully-formed (rolled) threads. Rolled threads have compressive surface stresses and better grain flow, giving ~30–100% more fatigue resistance than cut threads. Ensure nuts and taps are undamaged and correctly matched (e.g. no mixing coarse & fine pitch).
• Adequate Engagement: For bolts into tapped holes, provide sufficient depth (typically >1× diameter) so that tensile failure is far more likely than thread shear. Use hardened nuts/inserts when possible, especially in softer materials.
• Material Selection: Choose the right alloy/class. Don’t over-strengthen beyond needed: higher-grade steel is more embrittlement-prone. For corrosive environments, use corrosion-resistant grades or coatings rated for that environment.
• Proper Heat Treat and Plating: Verify each batch of fasteners with hardness/tensile tests. Follow post-plating bake schedules to eliminate hydrogen (ISO 4042 compliance). If coating, ensure it is uniform and free of cracks.
• Avoid Stress Concentrators: Smooth out any sharp corners or edges around fastener joints. Chamfer holes, deburr threads, and use washers if needed to spread load.
• Inspection: Before assembly, inspect fasteners for visible flaws. In service, periodically check critical joints for loosening or cracks (e.g. magnetically inspect high‑strength bolts). Perform destructive testing on samples for very high-risk applications.

By addressing both mechanical stresses (load, preload, geometry) and manufacturing quality (materials, heat treatment, coating), most fastener failures can be prevented. Each failure mode – fatigue, overload, stripping, or embrittlement – has specific design and process remedies that fit within good engineering practice.