high strength aircraft plywood
1968 Edition

The standard permits specific timber species for manufacturing high-strength aircraft plywood as per Clause 3.1 and Table 1. The main species include Dysoxylum malabaricum Bedd. (known commercially as White cedar, symbol WCE) and Palaguium ellipticum (Dalz) Engles (trade name Pali, symbol PAL). Additionally, other species such as Rosewood, Sissoo, Walnut, Padauk, Teak, Mullilam, Sandan, Padri, Debadaru, and Hollock may be utilized provided they meet performance requirements and have proven satisfactory in service. It is recommended to use a single timber species consistently throughout all layers, especially for the face and back veneers, to ensure compatibility and optimal strength.

IS 4859 focuses on quality control, bonding, and manufacturing protocols rather than prescribing explicit mechanical strength values. However, typical strength parameters for high-strength aircraft plywood include tensile strength parallel to grain around 70-90 MPa, bending strength (modulus of rupture) approximately 80-120 MPa, shear strength across glue lines near 8-15 MPa, and density ranging from 600 to 700 kg/m³. Thickness tolerances for plywood less than 6 mm are ±10%. Manufacturers must submit samples for testing to verify compliance. For precise mechanical properties, detailed test data or updated amendments to the standard should be consulted.

Moisture content control is critical and specified between 8% and 12% for test pieces (Clause 9.1) and 10% to 14% for specimens conditioned prior to testing (Clause 2.3). Moisture is determined by oven drying the sample at 103 ± 2°C until a constant weight is achieved, then calculating moisture percentage using the formula: Moisture Content (%) = ((Wet Weight - Oven-Dry Weight) / Oven-Dry Weight) × 100. Proper conditioning in controlled environments is essential to maintain these ranges before mechanical testing, ensuring accurate and consistent results.

The standard mandates stringent veneer quality criteria. Face veneers must be free from open defects such as insect holes, splits, pitch pockets, and pinworm holes. Large, loose, or dead knots are not allowed; however, small live knots up to 6 mm diameter are acceptable, with the total dead knot area not exceeding 10 mm diameter per 1000 cm². Core veneers may have closed splits with widths up to 0.25 mm. Defects such as brash wood, sap stain discoloration, ingrown bark, and any form of filling or inlaying are prohibited to ensure high quality and durability.

Glue adhesion is evaluated using specimens approximately 25 x 10 cm, cut with grain direction parallel to the veneer face. After immersion in boiling water for three hours, specimens sized 150 mm x 25 mm are tested by forcibly separating the plies using a tensile testing machine, with load applied perpendicular to the grain of the central ply. The standard requires an average failing load of at least 90 kg, with no individual specimen below 70 kg. Alternatively, a knife test per IS 709-1957 Appendix A may be used, requiring average load ≥135 kg and minimum individual load ≥100 kg. Shear strength tests involve applying axial load at a rate of approximately 455 kg/min on loading blocks with specific dimensions, measuring ultimate load, and calculating shear strength using the prescribed formula.