Test Method for Impact Strength of Adhesive Bonds ASTM D950

Scope:

The Standard Method ASTM D950 describes how to obtain comparative impact strength of adhesive bonds in shear, utilizing standard specimens under specified pretreatment and test conditions. Impact failure of adhesives may occur under suddenly applied loads such as from dropped objects or collision. Impact strength is the energy absorbed by the specified test specimen when it fails from a single blow of a testing-machine hammer. The ASTM D950 impact test is performed on adhesively bonded wood to wood or metal to metal specimens. The metal specimens can be ferrous or non-ferrous metals and alloys and the recommended grades are described in section 6.1 of ASTM D950. The materials covered include Brass, Copper, Aluminium, Steel, Phosphor Bronze, Magnesium, Nickel Silver. The accuracy of adhesive impact strength tests depends on the specimen preparation and bonding process and each standard specimen must follow the manufacturer’s recommended process in this regard. Specimens will need to be conditioned as specified in a conditioning chamber at a relative humidity of 50 ± 2 % and temperature of 23 ± 1.1°C, or in desiccators filled with a saturated salt solution (e.g Calcium Nitrate) to give a relative humidity of 50 ±2 % and temperature of 23 ± 1.1°C.

Procedure:

The ASTM D905 test is performed on block shear specimens in a pendulum type impact machine, using a hammer with a flat striking face. Each test specimen is configured as a small adherend block placed on top of a larger base block of the same material and adhesively bonded along the face of contact. The base block of the specimen is held firmly in a robust steel jig. The jig is bolted to the base of the testing machine. The jig is provided with a screw to tighten the specimen. Figure 1 of ASTM 950 provides a sketch of the jig and lists the various sizes depending on test machines and vises available. It must be ensured that the specimen cannot move when struck by the testing hammer. After the specimen is fastened, the impact head is raised to the predetermined height and released. The head should move at a velocity of about 3.5 m/s. The smaller adherend block is stuck by the striking face of the test hammer, just at the adhesive bond line, preferably within 0.79 mm (1⁄32 in.). Direct reading of impact energy should be possible in the machine.

Specimen size:

The smaller adherend is a square block of  maximum dimensions 25.4 mm × 25.4 mm × 9.5 mm thick (1 inch ×1 inch ×3/8 inch). In cases where this size is too large to be fractured by the available testing machine, then the top adherend block size may be appropriately reduced. The larger adherend block should be 44.5 mm×25.4 mm×19 mm thick (1³/4 inch ×1 inch ×3/4 inch). In the case of metal to metal specimens, at least ten specimens are required. Wood to wood bonded specimens should have the grains in the top and bottom blocks aligned in the same direction.

Result:

The ASTM D950 impact strength is reported in joules per square metre (or ft-lbf per sq. inch).