Block-shear test conditions

An adhesively bonded structure is considered in shear when the applied loads act in the plane of the adhesive layer. The loads tend to produce sliding of the adherends and this results in sliding or shearing of the adhesive. These shear stresses should not be confused with pure shear stresses, which are typical of uniform blocks of a homogeneous material. Interactions between adhesive thickness, adherend thickness and yield strength, and bond geometry produce nonuniform stresses. When this occurs, the shear bond may actually be dominated by tensile stresses rather than shear stresses. However, in service, adhesive structures rarely encounter these pure shear conditions. Therefore, standard shear tests provide adequate duplication of conditions which may exist in an actual structural adhesive application. 

D-3024. Specification for Protein-Base Adhesives for Structural Laminated Wood Products for Use Under Interior (Diy Use) Exposure Conditions. Employs the D-0>905 block shear, D-0906 plywood shear, and D-4300 mold resistance tests to evaluate mostly casein-base adhesives for use in interior grade laminated beams. 

Block polymer B differs substantially in its failure characteristics from BP A polycarbonate. For the block polymer a mixed failure mode predominates in three-point bend tests of notched specimens from —100°-90°C. In the mixed mode craze breakdown and plane strain fracture occur first inside the specimen subsequently shear failure occurs in the surface regions of the specimen. Shear lips (11) are formed as a result. Shear lips are also found on the notched Izod impact fracture surfaces of block polymer B, implying that the same mixed mode of failure occurs under high speed loading conditions. 

Automatic Bonding Evaluation System (ABES) The ABES consists of a small press and a tiny testing machine in a single unit. It enables bonds to be formed under highly controlled conditions the joints that contain the bonds which are to be measured are pressed against heated blocks for a certain time, cooled within a few seconds, and pulled immediately thereafter in shear mode. Repetition of this procedure at different curing times and temperatures yields the points (a point for each specimen) of a near-isothermal strength development curve. 96,97... 

The basic principle on which this test, given by the ASTM D950 standard, relies is to apply an impact-loading condition, mainly in shear, by means of a test rig similar to that used for Izod resilience measurement. O Figure 21.1 shows the experimental arrangement. An upper block (usually of steel) is bonded onto another larger block which, in turn, is fixed to the base... 

An estimated 75 % of the mastic products sold in North America indicate compH-ance to stringent AFG-01 (i.e. Adhesives for Field Gluing ) requirements. ASTM D 3498 is quite similar to the AFG-01 standard originally written by the American Plywood Association. These methods define a series of six test specifications (Fig. 8-28), five of which involve shearing plywood to lumber wood block specimens in compression mode after various carefully controlled material conditioning and specimen curing protocols (Fig. 8-29). 

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