Comparing the failure load and fatigue life of wire thread inserts

Wire thread inserts play a critical role in ensuring the longevity and strength of bolted joints, particularly in lightweight or high-load environments where stress concentration and fatigue are significant design concerns. Understanding how different types of inserts perform under cyclic loads and extreme conditions is essential when selecting the right solution for a specific application.     

Let’s explore and compare the failure load and fatigue life of various wire thread insert types, with a focus on how factors like insert design, material composition and application environment influence performance. 

Understanding failure load and fatigue life

Failure load refers to the maximum load a joint can sustain before permanent deformation or fracture occurs. This is especially relevant when inserts are used in soft parent materials like aluminum or magnesium, where the surrounding substrate is the limiting factor.

Fatigue life measures how many stress cycles a joint can withstand before failure. It is particularly important in aerospace, automotive, and dynamic mechanical applications where components are subject to vibration, thermal cycling, or continuous operation.

Comparing insert types - tanged vs. tangless

The design of a wire thread insert has a significant impact on how it handles load and fatigue over time. Two primary types, tanged and tangless, offer distinct performance characteristics that suit different engineering environments and maintenance requirements. 

Tanged inserts

Tanged inserts feature a break-off tang used during installation. Once installed, they exhibit strong mechanical performance, particularly in static load environments. However, if the tang is not properly removed, it can pose a risk of foreign object debris (FOD) and introduce stress risers that can reduce fatigue performance in sensitive assemblies. This makes it necessary for tang removal procedures to be in place during installation. 

Tangless inserts

Tangless inserts eliminate the tang, reducing the risk of FOD and simplifying installation and maintenance. These inserts offer a bi-directional design and allow for non-destructive removal. Because they remove stress points and ensure even load distribution, tangless designs are ideal for fatigue-critical applications like aerospace and motorsport.

Material comparison and influence on performance

Each material provides a different balance of tensile strength, wear resistance and longevity under repeated stress. Selecting the right one depends on both the expected static load and the number of cycles a joint must endure.

Influence of insert length on performance

Insert length (typically ranging from 1D to 3D) plays a pivotal role in joint durability.

• Shorter inserts (1D, 1.5D): Suitable for low-load, compact applications. Reduced surface area limits load distribution, increasing stress per thread.
• Longer inserts (2.5D, 3D): Provide superior load transfer and fatigue resistance, especially in soft materials. The additional thread engagement allows for more even stress dissipation and higher resistance to stripping.

Performance in aerospace fatigue scenarios

In aerospace testing environments, wire thread inserts must pass rigorous fatigue testing under NASM8846 and MA1565 specifications. 

• Torque cycle testing to 15+ insertions/removals
• Vibration resistance under simulated flight conditions
• Corrosion and thermal cycling tests

Tangless inserts with dry film lubricants or cadmium/silver coatings typically perform best, demonstrating longer fatigue lives and reduced wear on both inserts and bolts. 

Selecting the right insert for durability

Failure load and fatigue life are two of the most critical metrics for evaluating the reliability of wire thread inserts. Whether your application is aerospace-grade or geared toward industrial machinery, the combination of material, length, insert type and finish must align with the joint’s mechanical demands.

KATO® wire thread inserts and particularly Tangless® designs in high-performance materials like Inconel and Nitronic consistently outperform traditional options in both static and cyclic loading environments.

To maximise the reliability of your assemblies, consult with a KATO® Advanex engineer to find the optimal insert configuration for your specific performance requirements. To find out more about the line of inserts we offer, download our brochure below.