Methods of Test for Wood-based Structural Sandwich Construction
1979 Edition

Per IS 9307 Part 1-8, standard specimen dimensions for wood-based sandwich panel testing are as follows: the cross-section should be rectangular at 10 cm by 10 cm (width x depth), with thickness equal to that of the sandwich panel itself. The specimen width must be at least twice the total thickness, at least three times the core cell size, and no more than half the span length. The length should be the span length plus an additional 5 cm or half the sandwich thickness, whichever is larger. Measurements should be precise to within 0.01 cm. These criteria ensure consistent and reliable test results.

According to IS 9307, flexural stiffness (D) is computed using the formula: D = [E × (h³ - c³) × b] / [12 × (1 - ν²)], where E is the modulus of elasticity of facings, h is total sandwich thickness, c is core thickness, b is specimen width, and ν is Poisson's ratio of facings. The core shear modulus (G) is derived via shear stiffness (N), given by N = G × (h + c) × b. Using deflection data from two quarter-span loading tests, G is calculated by a specific relation involving load and geometric parameters. These calculations enable assessment of bending behavior and shear response of sandwich panels.

IS 9307 Part VIII prescribes two types of weathering exposure cycles, each comprising six complete iterations with a maximum 30-minute interval between cycles. Cycle 1 involves immersing specimens horizontally in water at 49 ± 2°C for 1 hour, spraying steam at 93 ± 3°C for 3 hours, storing at -12 ± 3°C with 65 ± 5% relative humidity for 20 hours, heating in dry air at 99 ± 2°C, additional steam spraying, and final dry heating. Cycle 2 follows a similar pattern at milder temperatures: water immersion at 49 ± 3°C, hot water spraying at 71 ± 3°C, storage at -40 ± 3°C, and heating in dry air at 71 ± 3°C. Post-exposure, specimens are conditioned at 65 ± 5% RH and 27 ± 2°C until constant weight, then inspected for damage and tested for mechanical properties.

The adhesive bond strength between the core and facings is primarily evaluated through the flatwise tensile test described in IS 9307 Part V, where steel or aluminum blocks are bonded to the specimen faces and pulled apart perpendicularly to the sandwich panel plane using a universal testing machine. This test quantifies tensile bond strength by recording maximum load and failure mode. Additionally, shear tests assess shear strength parallel to the facings, and flexural tests provide indirect information on bond integrity by measuring flexural stiffness and shear modulus.

Creep behavior in sandwich panels is measured by supporting the specimen on two rollers and applying a central load equal to 30% of the maximum flexural load (per IS 9307 Part I). Deflection at the mid-span is recorded using a cathetometer with 0.01 mm accuracy at intervals: every 10 minutes for 2 hours, every 30 minutes for the next 3 hours, hourly for 5 hours thereafter, and twice daily for approximately a week or until failure. Data is plotted as deflection versus time to analyze creep characteristics. Reports include sandwich construction details, applied tensile stresses, failure type and location, and individual properties of facings and core.