Common Fastener Failures and Preventive Measures

In industries such as machinery manufacturing, construction, and new energy, fasteners are the core components connecting parts. Their performance stability directly determines equipment safety and service life. Whether ordinary machine bolts or high-strength fasteners for wind turbines and solar panels, failure can cause equipment shutdown, production interruption, or major safety accidents with significant economic losses and casualties. Therefore, understanding common fastener failure modes, their causes, and preventive measures is essential for fastener professionals, equipment maintenance personnel, and related industry practitioners.

Based on industry experience, common fastener failure modes fall into six categories: fracture, loosening, thread stripping, corrosion, galling, and deformation. Each has distinct characteristics and root causes, requiring targeted prevention.

Fracture

Fracture is one of the most dangerous and common failure modes, occurring mainly in high-strength bolts and screws under load. It includes overload fracture, fatigue fracture, and hydrogen embrittlement fracture.

Overload fracture occurs when the load exceeds the fastener's tensile strength limit, causing sudden breakage. The fracture surface is usually flat without fatigue marks. Causes include improper selection (using a lower strength grade than required) or equipment overload and uneven loading. For example, substituting 10.9 grade bolts with 8.8 grade in steel structures can easily cause overload fracture under heavy loads.

Fatigue fracture is a progressive fracture caused by long-term alternating loads and vibration. It is the most common fracture mode in industrial equipment. The fracture surface shows distinct fatigue marks originating from stress concentration points. Common causes include prolonged vibration, frequent load changes, insufficient installation precision, and thread surface defects. For wind turbine tower bolts under alternating wind loads and vibration, thread burrs or misalignment during installation can easily lead to fatigue fracture, threatening equipment safety.

Hydrogen embrittlement fracture is unique to high-strength fasteners and is highly dangerous and difficult to detect. Hydrogen atoms penetrate the metal during pickling or electroplating, then gather under stress, forming bubbles that reduce toughness and increase brittleness, causing delayed fracture. This typically occurs sometime after installation, not immediately. It often affects grade 10.9 and above bolts, especially electroplated ones. Without effective hydrogen baking, the risk increases significantly. For automotive chassis high-strength bolts, hydrogen embrittlement could cause sudden fracture while driving, leading to accidents.

Loosening

Loosening may seem less dangerous than fracture but can trigger a chain of failures. It occurs when vibration, shock, temperature changes, or creep of connected parts reduces preload, causing loss of fastening. Loosened fasteners allow relative displacement between parts, increasing wear and generating noise. In severe cases, parts may detach or equipment may disintegrate. For PV mounting structures, loosening can cause tilting and module displacement, reducing sunlight capture efficiency or even causing collapse in severe weather.

Causes of loosening include insufficient preload from improper tightening torque; unstable friction coefficient due to oil, debris, or damaged surface coating; and environmental factors such as prolonged vibration, high temperatures, creep of connected parts, or thread wear. Lack of effective anti-loosening measures, such as washers or nylon-insert nuts, also increases loosening risk.

Thread Stripping

Thread stripping refers to wear or deformation of thread surfaces, preventing proper engagement between bolt and nut. Causes include insufficient thread manufacturing precision, improper material selection, improper installation, and corrosion wear. Rough thread surfaces, excessive pitch deviation, burrs, or cracks cause uneven loading accelerating wear. Insufficient hardness or material mismatch leads to easy wear and deformation. Excessive force, improper tightening angle, or wrong tools during installation directly damage threads. In damp or corrosive environments, thread corrosion also causes stripping.

Stripping prevents proper use and can make disassembly impossible, increasing maintenance difficulty and cost. For fasteners in chemical equipment exposed to corrosive media, corrosion stripping can prevent normal disassembly for inspection, requiring destructive removal and increasing downtime and costs.

Corrosion

Corrosion failure is common in harsh environments where fasteners react with water, oxygen, and corrosive media, causing oxidation, rust, and reduced mechanical properties. Types include uniform corrosion, pitting, and crevice corrosion. Uniform corrosion causes overall rusting, reducing cross-section and strength, common in untreated or improperly treated carbon steel fasteners. Pitting concentrates in local areas, forming small pits that deepen, eventually causing perforation or fracture, often on stainless steel surfaces in chloride-containing environments. Crevice corrosion occurs in gaps between fasteners and connected parts where water accumulates and oxygen is limited, forming a corrosion cell that accelerates local corrosion, common at flange connections and washer contact areas.

Corrosion not only reduces strength and service life but can also seize threads, prevent disassembly, or trigger secondary failures like loosening or fracture. Seaside PV mount fasteners continuously exposed to seawater and salt spray are highly susceptible to corrosion failure without effective protection.

Galling

Galling, also known as cold welding or thread seizure, occurs when thread surfaces adhere and wear during tightening or loosening, preventing relative rotation. It mainly affects stainless steel and high-temperature alloy fasteners under high temperature, pressure, or vibration. Causes include insufficient or improper lubrication, causing excessive friction; material properties leading to adhesion under friction; excessive tightening torque causing plastic deformation and adhesion; and high-temperature oxidation where oxide layers detach and exacerbate adhesion.

Galling prevents disassembly or installation, greatly inconveniencing maintenance. Severe cases require destructive methods like cutting or grinding, increasing costs and potentially damaging connected parts. High-temperature fasteners in wind equipment, if improperly lubricated or poorly selected, are prone to galling, affecting normal maintenance.

Deformation

Deformation failure occurs when fasteners undergo plastic deformation under load or temperature changes, altering their dimensions and shape, losing fastening function. Common forms include bolt bending, head deformation, and thread deformation. Causes include loads exceeding yield strength; uneven loading or angular misalignment during installation causing localized overloading; high-temperature reduction in material strength causing thermal deformation; and material defects such as coarse grains or insufficient toughness.

Deformed fasteners cannot be reused. If not detected and replaced promptly, they cause loosening and uneven loading, leading to other failure modes. For PV mount connection bolts, bending deformation causes uneven loading, potentially leading to tilting or collapse over time.

Preventive Measures

In response to these failure modes, effective prevention includes proper selection, standard installation, enhanced corrosion protection, and regular maintenance. During selection, choose appropriate strength grade, material, and surface treatment based on actual operating conditions and loads. During installation, tighten strictly to specified torque, ensure precision, and apply effective anti-loosening measures. For corrosion and hydrogen embrittlement risks, select suitable surface treatments such as Dacromet or hot-dip galvanizing, with hydrogen baking for high-strength fasteners. Regularly inspect and maintain fasteners, replacing aged or damaged ones promptly to reduce failure probability and improve equipment safety and stability.