08Fatigue Failure Of Bolts

FATIGUE FAILURE OF BOLTS

Albert K. Fletcher
CEO/PM Consultant (Dataman System Consultancy)

This article was written by Bill Eccles. Bill is a Chartered Engineer with specialty in Bolting Technology. His company Bolt Science writes and markets bolted “joint analysis” software with complete consultancy assignments and training courses.

Fatigue is Progressing Cracking

The crack can start at some existing defect, such as an inclusion in the metal, or at point of high stress, such as a notch, and slowly grow in length at each loading. It may take millions of loadings and unloadings (known as load or stress cycles) before the crack is actually detectable.

As the length of the crack increases, the material remaining is placed under increasing stress because there is less area to sustain the loading. When the crack actually reaches a critical length it progresses all the way through the material resulting in complete failure.

A fatigue crack can take years to progress through a bolt.

The term fatigue damage is frequently used to describe the damage to a part that has been sustained as a result of loading. Such fatigue damage is nowadays equated to crack length. In some critical applications there is a requirement that bolts are periodically crack detected using dye penetrant or even by x-rays to ensure that there are no detectable cracks present. (Cracks may be present on the microscopic scale, that is, below the detection threshold of the measurement technique.)

Fatigue failure of nuts is practically unknown since fatigue cracks propagate as a result of a fluctuating tensile stress — nuts are under a compressive stress and so any pre-existing cracks tend not to propagate.

Factors Affecting a Bolt’s Fatigue Strength

1. Surface Finish

The quality of the surface finish of the threads is known to have an effect on fatigue life: the smoother the surface, the higher is the fatigue life. Hence, in general terms, rolled threads have a higher fatigue life than cut threads.

2. Thread Roots and Stress Concentration

Fatigue cracks usually initiate in the roots of the threads due to high stress concentration. Thread rolling after heat treatment induces localized compressive stresses in the thread roots which help to prevent crack initiation. Rolling before heat treatment does not provide this benefit.

Bolts with threads rolled after heat treatment can have double the fatigue strength of ground threads, although manufacturing costs increase.

3. Nut Face Distance from Thread Run-Out

Tests show that placing the nut face too close to the thread run-out region results in premature failure. The run-out area is poorly formed and highly stressed. Keeping a distance of two or more thread pitches between the nut face and thread run-out eliminates this issue.

4. Increasing Root Radius

Increasing the root radius reduces stress concentration and improves fatigue strength. Thread profiles like MJ threads (used in aerospace) have larger root radii.

5. Size Effect

Larger parts have lower fatigue strength for the same stresses. This applies to bolt threads as well.

6. Under-Head Cracking

Fatigue cracks can also initiate under the bolt head due to inadequate under-head radius or mounting on an inclined surface. Even a small joint angle (e.g., two degrees) can create severe fatigue problems, especially in welded structures.

7. Uneven Load Distribution in the Nut

Most load is carried by the first few threads in the nut, causing fatigue failures in the thread just under the nut. Improving load distribution increases fatigue strength.

8. Asymmetric Thread Profiles

Asymmetric thread profiles improve load distribution. Examples:

  • Spiralock thread (for nuts)

  • SPS Technologies asymmetric profile (20% fatigue life improvement)

9. Modifying the Nut Profile

Placing a groove near the nut face and making the face concave can improve fatigue life by 25%. Other methods:

  • Using nuts with different modulus of elasticity

  • Different pitch between bolt and nut

  • Tapered threads

10. Proper Tightening

The most effective way to ensure fatigue resistance is to properly tighten the bolt.

A preloaded bolt in a typical joint sustains only 5% or less of the applied load — the rest reduces clamp force on the joint.

Thus, a properly tightened bolt is highly resistant to fatigue loading.

Fatigue failures often result from:

  • Inadequate tightening

  • Loosening

  • Bending stresses

A con-rod bolt, for example, would fail rapidly if not properly tightened.

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