WB627, stumbled upon this purely by accident and thought it may interest you. From page 12 of the link.
Anson A4-5 crashed at Glenbrook, NSW on 29 January 1941 while flying from Parkes to Sydney. It was immediately obvious that the port wing had failed in flight and the detached wing sections were sent to ARL for expert examination. As designed, the one-piece wooden wing of the Anson followed standard Fokker practice. Two box beam spars, internally braced, were built up from laminated booms and plywood webs, and connected by plywood ribs. The whole was then covered by a load-carrying plywood skin locally thickened by cushion strips where it passed over the spar booms.
At ARL many test specimens were cut from the wing sections and subjected to tensile,shear and impact tests. The results obtained were generally satisfactory but specimens cut from the skin-main spar joint showed a shear strength less than half the specified value. Visual examination of this joint disclosed extensive areas of poor adhesion,delamination and excessive glue thickness. ARL concluded that quality control during manufacture had been poor with insufficient pressure applied to the joint during curing of the casein glue used, S & M Report 6.
It is interesting to compare the ARL conclusion on the port wing of A4-5 with that reached by the AAIC with respect to the starboard wing of A4-8; see Section 6. For the latter, the AAIC concluded that the wing 'apparently exploded' and let it go at that. It is also interesting to note that both aircraft were drawn from the first RAF production batch of 174 aircraft of which K6212 -K6223 were shipped to Australia to become A4-1 to A4-12.
Concurrently with the work on A4-5, ARL carried out similar inspections and tests on the main spar of Anson N1331 which had been damaged in a ground collision. This wing had been manufactured during September 1938 as part of the fifth RAF production batch of 98 aircraft. By this time, modern synthetic resins were replacing casein and N1331made extensive use of urea formaldehyde (UFD). This work suggested that the strength of UFD deteriorates with age and was one of the first indications of this adverse property.
This early work on Anson wing failures generated a comprehensive program of research.The static strength of Anson wings was investigated in a full-scale structural test, Fig. 3. Improved structural analysis techniques were formulated for wooden box beams while the strength and stiffness characteristics of plywood panels and shells were investigated.The ageing properties of UFD were quantified leading, ultimately, to the final grounding,in Australia, of all aircraft constructed with this resin. The effect on bonding strength of extreme temperature and humidity cycles was examined; work which continues to this day in connection with modem fibre reinforced composite materials.
https://apps.dtic.mil/dtic/tr/fulltext/u2/a267086.pdf