Publication:Navy Engineering Bulletin March 2006/Just an Innocent Scratch???
Scratches or small nicks in service come from a variety of sources including ground support equipment, environmental debris and aircraft tooling used in maintenance. Commonly, scratches originate from the process of paint or sealant removal, and scratches in the order of 100µm depth can be common during fabrication and assembly procedures.
Scratches, whilst obviously damaging the surface protection and promoting corrosion, also act as points of stress concentration in loaded structures, such as pressurised fuselages.
Whilst the majority of aircraft structure and components are designed such that their static strength would not be compromised by such a seemingly insignificant scratch, the fatigue implications can be somewhat more alarming.
The subject of fatigue has gained increasing attention in the past decades, with a number of aircraft accidents attributed to fatigue failures of flight critical components. As the age of aircraft fleets around the world increases at a rapid rate- so too does the risk of fatigue related accidents.
Contemporary aircraft design incorporates damage tolerance methodology which involves analysis and testing to determine how quickly fatigue cracks grow. The time taken to reach a 'critical' length before causing structural failure can be subsequently determined and inspection regimes can be scheduled accordingly.
Minor damage, such as scratches, can hasten fatigue crack growth and invalidate this design methodology, both by causing fatigue cracks where they are not expected and by propagating fatigue cracks faster than expected.
In 2003, two Boeing 737 aircraft were found with cracks emanating from scribe lines, one of which had cracks ranging from 5-10 inches in length. A Boeing inspection regime that followed identified six older 737-200 aircraft with similar damage to a significant portion of the joints along the fuselage, including critical lap joints.[1]
This damage was most likely caused by the use of a sharp metal instrument (such as a razor or screwdriver), instead of the prescribed wooden or plastic spatulas, for removal of the paint and/or sealant at structural skin joints.
The un-repaired scribe marks in the pressurised skin were identified as initiation sites for the fatigue cracks found on the aircraft, creating potential for widespread multisite fatigue damage.
Undetected damage such as this could result in catastrophic failure of an aircraft fuselage, similar to Aloha Airlines Flight 243 in April 1988. Only one fatality resulted from this incident, but it could have easily been more. Whilst this incident was not specifically attributed to fatigue cracks emanating from scratches or 'scribe lines'; the basic premise is no different.
Whilst well understood, fatigue, inherently is a difficult process to predict. There are numerous parameters that influence the fatigue life of a plain, undamaged specimen, which include applied loads, material properties, surface condition and environmental conditions. Add scratch damage to the mix, and you have yourself a real problem.
Aspects such as scratch depth, scratch width, scratch tip radius of curvature and local microscopic plasticity changes all influence the fatigue life of a structure which has scratch damage.
It is reported that for Aluminium 2024-T3 with a scratch depth of 100µm, a reduction in fatigue life of up to 95% can be experienced compared to the undamaged pure specimen[2]
The depth of a scratch is by far the most dominating effect on fatigue life. As the scratch depth increases, the fatigue life, intuitively, is shown to decrease accordingly. Increasing the scratch depth from 100µm to 225µm decreases the fatigue life by up to 71%.[3]
It has also been shown that the deeper the scratch, the higher the crack growth rate. For scratch depths of 20 µm, 50 µm and 100µm, the fatigue crack growth rates were approximately 1.5, 4 and 10 times the rate of an unscratched specimen.[4] So how much is 100µm??? - 0.1mm, or the average width of a human hair!!!
With composite airframes and structures on the rapid increase, the day will come when fatigue of metal structures is an issue for only vintage aircraft operators. However, for the foreseeable future at least, we will be dealing with metal structures in some form on all aircraft and we need to be cognisant of the fatigue implications for structures when subject to these seemingly innocent scratches.
For operators of aircraft, and in particular maintenance personnel, this means being observant for damage during routine inspections, and also maintaining an emphasis on the use of correct tooling for the conduct of maintenance tasks - in particular, paint and/or sealant removal.
LEUT Cliff Kyle Royal Australian Navy Rotary Wing Section - DGTA
Footnotes
- ↑ Flight Standards Information Bulletin for Airworthiness, FSAW 03-10B, 20 Nov 2003.
- ↑ Flight Standards Information Bulletin for Airworthiness, FSAW 03-10B, 20 Nov 2003
- ↑ Talia, M. Crack Propagation Modelling for Surface Generated Scratches in Al 2024-T3 Clad Alloy, University of Kansas, 1997.
- ↑ Talia, M. Crack Propagation Modelling for Surface Generated Scratches in Al 2024-T3 Clad Alloy, University of Kansas, 1997.
