Common Fastener Failures and How to Prevent Them

Fasteners play a crucial role in holding together countless products and structures. However, faulty or incorrectly selected and installed fasteners can lead to catastrophic joint failures. By understanding the most prevalent fastener failure modes, quality managers and engineers can take proactive steps to choose optimal fasteners and prevent issues from occurring. This in-depth guide covers common fastener failure mechanisms, root causes and best practices for prevention.

Fatigue Failures

One of the most common causes of premature fastener failure is fatigue. When a fastened joint or component is subjected to fluctuating or cyclic loads, the repeated variability causes metal fatigue cracks and fractures. Even when peak loads remain within design limits, the cumulative damage from load variability can initiate cracks and propagate them until sudden failure occurs. 

Fatigue failures typically initiate at regions of high-stress concentration, such as thread roots, followed by propagation through the bolt or nut. Cracks then lead to complete fractures. Preloading fasteners only delays the onset of fatigue but does not prevent the failure mode. Detecting fatigue damage before failure is difficult, requiring techniques like penetrant testing or magnetic particle inspection.

Steps to prevent fatigue failures include:

- Selecting fastener materials with high fatigue strength and endurance limits, such as alloy steels. Consulting with knowledgeable fastener suppliers in Dubai can guide optimal material selection for the expected load characteristics.

- Maintaining proper preload and torque throughout the fastener service life to minimize cyclic stresses.

- Avoiding unnecessary side or bending loads that amplify stresses.

- Using larger fastener sizes and designs with reduced stress concentrations.

- Regular inspection for cracks and replacing components at defined service intervals.

Corrosion Failures

For fasteners exposed to outdoor environments, chemicals, moisture or high temperatures, corrosion is a common cause of degradation and failure. As corrosion damage accumulates on bearing surfaces and threads, it causes pitting, reduced clamp load, loss of preload, and compromised fastener strength. This leads to insufficient grip, joint loosening, and potential catastrophic failure.

Corrosion attacks all common fastener materials to varying degrees based on service conditions. Low alloy and carbon steels corrode readily when unprotected. Stainless steel fasteners provide excellent corrosion resistance in most applications. Where stainless steels are unsuitable, other alloys or protective platings like zinc and cadmium provide sacrificial corrosion protection.

Recommended steps to avoid corrosion failure include:

- Selecting suitable corrosion-resistant materials or protective finishes based on the environmental conditions. Consulting quality fastener suppliers in Dubai for guidance is advised.

- Using appropriate sealants, greases or threadlocking compounds to exclude moisture and prevent galvanic corrosion between dissimilar metals.

- Routine inspections to identify corrosion issues in the early stages before they compromise fastener integrity.

Overload Failures

When a fastener is subjected to loads exceeding the design capacity, either through unexpected increased loads or incorrect fastener sizing/specification, overload failures occur. Gross plastic deformation, shank elongation, stripping of threads and shear fracture can result. 

Overload failures are especially dangerous because they happen suddenly without warning. Root causes include inadequate engineering analysis of expected loads, unanticipated high loads, insufficient fastener margins, and installation errors. Using fasteners above their rated capacity leads to immediate or eventual overload failure.

Recommendations for avoiding overload failures include:

- Thorough engineering analysis to anticipate peak dynamic and static loads within an adequate design margin.

- Closely partnering with reputable fastener suppliers in Dubai during specification and procurement to match fastener strength ratings to expected loads.

- Robust quality control and validation testing to verify procured fastener physical properties and rated capacities meet system needs prior to installation.

- Regular inspection and planned maintenance programs to detect signs of overload or loosened joints before catastrophic failure occurs.

Loosening Failures  

Under high levels of vibration or cyclic loading, fasteners can progressively loosen over time even when properly tightened initially. Insufficient torque, variable thermal expansion, and dynamic loads may further contribute to gradual loosening. As fasteners lose preload, joints have increased clearances, reduced grip, and compromised structural integrity that may lead to detachment.

Solutions for preventing loosening involve:

- Using self-locking fastener configurations, such as nuts with nylon inserts or bolts with distorted threads to resist loosening.

- Properly torquing fasteners during initial installation and periodically re-torquing as needed to maintain adequate preload. Using calibrated torque wrenches is advised.

- For critical connections, implementing redundant fastening methods or safety wiring.

- Routinely checking for loosened joints, especially after equipment operation. Early detection allows re-tightening before hardware detaches.

Embrittlement Failures

Prolonged exposure to high temperatures, certain chemicals or corrosive environments, irradiation, or heavy strain can damage the crystalline structure of metals, causing embrittlement. Embrittled fastener materials lose ductility and toughness, becoming prone to sudden fracture at loads below the rated capacity.

Steps to prevent embrittlement issues include:

- Selecting fastener alloys with excellent environmental stability for the expected exposure conditions. Nickel-based alloys typically offer superior high temperature performance.

- Testing samples of installed fasteners for signs of embrittlement like reduced ductility/impact values or increased hardness. Replace any suspect fasteners proactively.

- Controlling chemical exposures to minimize contact with embrittling substances. Use protective platings or coatings when needed.

- Keeping temperatures and strains within design limits through controls and monitoring.

Installation Errors

Even high quality fasteners made from premium materials can still fail when installed incorrectly. Cross-threading, uneven torquing, overtightening, undertightening, and using uncalibrated or improper tooling during installation often lead to problems. Insufficient joining torque prevents adequate preload for the loads. Excess torque can overstress the fastener. Either scenario risks joint failure.

Good installation practices are essential:

- Operator training on proper techniques, joint inspection, following documented procedures.

- Using calibrated torque wrenches and tooling sized for the fasteners.

- Careful alignment and threading control to prevent cross-threading or thread damage.

- Torque monitoring during tightening to ensure optimal preload is achieved.

- Post-installation validation checking for any issues.

By understanding the most likely failure modes, engineers can make informed fastener selections, engage quality suppliers, and institute controls that improve reliability and product life.