Investigation of Fatigue Cracks in B737 Lap Joints

[kgwilson]

This is interesting. Nothing we fly will ever be subject to the stresses encountered in commercial aircraft but it is fascinating to see what could be seen as an insignificant issue can be responsible for a possible catastrophic failure. It all started from the lessons learnt from the DH Comet disasters of the 1950s.

 

Joe Rakow's specialty is composites, but he is also pretty knowledgeable about failures in more traditional structures.

 

Without jumping to conclusions I suspect that the NTSB investigation into the most recent Southwest 737 incident will confirm Joe's earlier findings.

 

KGW

 

-----Original Message-----

 

From: Joseph Rakow <[email protected]>

 

To: Joseph Rakow <[email protected]>

 

Sent: Tue, Apr 5, 2011 8:49 am

 

Subject: Investigation of Fatigue Cracks in B-737 Lap Joints

 

Dear Clients and Contacts:

 

Three years ago, Exponent completed an investigation of fatigue cracks in lap joints of a fleet of ten 737 aircraft. An abstract of the investigation is provided below. The full investigation is currently summarized in a white paper, available upon request.

 

Abstract

 

In March 2003, two through-wall cracks were discovered in fuselage lap joints of Boeing 737-200 aircraft operated by a major commercial airline. The airline suspected the cracks were associated with scribe marks created by an unapproved metal sealant removal tool employed by their painting contractor during a repainting process in the mid-1990’s. The aircraft was retired, along with nine other aircraft that had been purchased and repainted in the same time period, and had exhibited scribe marks in their lap joints. Exponent’s investigation had two goals: 1) Identify the type(s) of sealant removal tool (metal, plastic, wood, etc.) that likely created the scribe marks found on the subject aircraft, 2) Estimate the number of cycles required for a scribe to grow into a through-thickness crack, and compare that estimate to the service history of the subject aircraft.

 

Through a series of experiments, Exponent’s investigation demonstrated that unapproved metal tools produced scribe marks with physical characteristics (depth and shape) consistent with the scribe marks found on the subject aircraft, while approved plastic and wood tools produced much shallower and broader marks than those produced by the metal tools and, in some instances, with nearly undetectable depths. Only metal tools produced scribe marks with depths sufficient to initiate a fatigue crack (greater than 0.0026 inch, as determined by fracture mechanics), and only metal tools produced gouges with depths as large as those measured on the subject aircraft (0.005 inch, nearly twice as deep as the threshold). Fatigue analysis indicates that a scribe mark of the depth measured on the subject aircraft requires approximately 23,000 flight cycles to propagate a crack from the time of scribing to a through-thickness crack. The subject aircraft had accumulated approximately 22,000 flight cycles since the repainting process in the mid-1990’s.

 

This investigation highlights the sensitivity of aircraft structures to mechanical damage, and emphasizes the extreme care that is required when performing maintenance and other services such as repainting.

 

For additional information contact:

 

Joseph F. Rakow, Ph.D., P.E.

 

Senior Managing Engineer

 

Exponent Failure Analysis Associates

 

149 Commonwealth Drive

 

Menlo Park, CA 94025

 

[email protected]

 

Office: 650-688-7316

 

Cell: 415-608-9215

 

Fax: 650-688-7231

 

http://www.exponent.com/joseph_rakow/

 

Exponent is the nation’s premier scientific and engineering consulting company. With over 90 scientific and engineering disciplines, Exponent’s staff of approximately 800, located in 19 offices throughout the nation and 4 international offices, combines unparalleled technical expertise with the ability, when necessary, to focus this knowledge in extremely short time frames. Our multidisciplinary team of scientists, physicians, engineers, and regulatory consultants will perform either in-depth scientific research and analysis, or very rapid-response evaluations, to provide our clients with the critical information that both day-to-day and strategic decisions can require.