Identifying cracks in gun barrels

Testing system extends the in-service life of barrel without compromising safety


Gun barrels have a variety of failure modes that must be monitored over the life of a barrel. Fatigue cracking is one of the more critical failure modes that must be prevented as it will result in a catastrophic barrel rupture.

This is typically prevented by assigning a maximum rounds count to the barrel set. This mandatory replacement point is the worst-case crack growth rate with an appropriate safety factor applied. However, most barrel sets do not see these limiting conditions and more life could be gleaned from them if a means of accurately determining cracks could be developed. Typical military practice to identify cracks is visual and magnetic particle inspection during manufacture and only visual inspection once the gun is fielded. However, visual inspections will not disclose insipient cracks internal to the barrel material and automated systems that scan the inside diameter of gun barrels – during manufacture and in the field – are complex, bulky, expensive, and relatively slow.

The Army has developed a non-destructive test that will accurately and reliably assess fatigue cracking in gun barrels and compare it against a maximum allowable crack length. The test utilizes pattern recognition of changes in acoustic vibration and uses a Fourier transform to decompose the filtered signal into its constituent major frequencies. The decay coefficient of the time series data is calculated and compared to a defect-free sample.

If relative to a defect-free sample, the vibration pattern of the decay envelope is that of a series of hi-low waves; or the envelope of the vibration pattern decays more quickly; or if there is a shift or splitting of the fundamental modes of vibration relative to a defect-free baseline, then a defect has been identified within the subject thick-walled cylindrical geometry.

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