I was trolling the NTSB's DMS system for interesting reports, and I encountered this forensic report on the fatal in-flight failure of a Schleicher K7 sailplane wing in Texas in 2013:
http://dms.ntsb.gov/public/54500-54999/54600/544197.pdf
The urea formaldehyde glues used had pretty much disintegrated, and investigators were able to separate the components of the sample by hand with little effort. Of course, this is on a glider that was built in 1959, so the glue was 54 years old.
One interesting aspect here is that it looks like the critical rupture was where scarf splices in the lower spar cap separated. The lower spar cap is made of two plies of pine; it looks like each ply is about 13mm deep. Each ply of pine is spliced together out of two shorter pieces with a scarf joint that looks to be about 19:1, which is a perfectly reasonable scarf angle. However, the two scarf joints are pretty much at the same spanwise location (see Figures 5 and 6). They appear to be staggered by about an inch, but that isn't much considering that each splice is about 9" long. I'd think that it would be better practice to have the splices staggered by at least their length, so that they do not overlap. That would yield more bond area and slightly reduce the stress on the adhesive in each splice joint.
However, all that is pretty much made moot by the fact that the glue had deteriorated seriously over its half-century lifespan, and that inspections recommended as a result of another similar failure had not been performed. I have no doubt that the wing design is perfectly sound, and would have held together if the glue had not been allowed to become extremely degraded by age.
Thanks, Bob K.
http://dms.ntsb.gov/public/54500-54999/54600/544197.pdf
The urea formaldehyde glues used had pretty much disintegrated, and investigators were able to separate the components of the sample by hand with little effort. Of course, this is on a glider that was built in 1959, so the glue was 54 years old.
One interesting aspect here is that it looks like the critical rupture was where scarf splices in the lower spar cap separated. The lower spar cap is made of two plies of pine; it looks like each ply is about 13mm deep. Each ply of pine is spliced together out of two shorter pieces with a scarf joint that looks to be about 19:1, which is a perfectly reasonable scarf angle. However, the two scarf joints are pretty much at the same spanwise location (see Figures 5 and 6). They appear to be staggered by about an inch, but that isn't much considering that each splice is about 9" long. I'd think that it would be better practice to have the splices staggered by at least their length, so that they do not overlap. That would yield more bond area and slightly reduce the stress on the adhesive in each splice joint.
However, all that is pretty much made moot by the fact that the glue had deteriorated seriously over its half-century lifespan, and that inspections recommended as a result of another similar failure had not been performed. I have no doubt that the wing design is perfectly sound, and would have held together if the glue had not been allowed to become extremely degraded by age.
Thanks, Bob K.
