Significance and Use
3.1 The strength and modulus of rigidity of wood structural panels in shear through-the-thickness obtained by these test methods are required for the rigorous design of many lumber-panel structural components such as trusses with panel gussets, box beams, folded plate roofs, and space plane structures, as well as floor and roof diaphragms, and shear walls. These properties are of secondary importance in typical roof deck and sheathing applications, and in crates and shipping containers.
3.2 Veneer produced by slicing or rotary peeling may contain fine checks or separations parallel to the grain on the knife side of the veneer that are produced as the knife is forced through the wood. These checks are termed “knife checks” to distinguish them from occasional checks that may be formed on the opposite side of the veneer by forces at the compression bar, and from checks caused by drying. Average depth of knife checks has been found to strongly influence shear properties in plywood panels and may be of significance in veneer incorporated in composite panels. Measurement of depth of knife checks is recommended in these test methods.
3.3 To control or define other variables influencing shear properties, these test methods require determination of moisture content and elapsed time to failure. The conditioning of test material in controlled atmosphere and determination of specific gravity are recommended.
1.1 These test methods determine the shear through-the-thickness properties of wood structural panels associated with shear distortion of the major axis. Wood structural panels in use include plywood, oriented strand board, and composites of veneer and of wood-based layers. Three test methods are included which differ somewhat in their application:
A. Small Panel Shear Test
B. Large Panel Shear Test
C. Two-Rail Shear Test
The choice of test method will be determined in part by the purpose of the tests, characteristics of test material, and equipment availability. In general, Test Method B or C for large specimens is preferred when equipment, amount of test material, and experimental plan permit.
1.1.1 Test Method A: Small Panel Shear Test—This test method is suitable for testing small samples of uniform material including investigations of the effects of grain direction or orientation and of many raw materials and manufacturing process variables which influence shear properties uniformly throughout the specimen. The test method is unsuited for determining effects of grade and manufacturing features such as density variations, knots, and core gaps within the specimen.
1.1.2 Test Method B: Large Panel Shear Test—This test method is regarded as giving the most accurate modulus of rigidity and is therefore recommended for elastic tests of materials to be used in stress analysis studies of test structures. This test method also yields excellent shear strength values for clear material. However, in spite of the large size of the specimen, failures generally occur only in narrow zones at the perimeter of the test area. This characteristic, a result of the heavy perimeter framing, causes this test method to be generally unsuited for determining grade and manufacturing effects such as density variations, core gaps, and knots that are not uniformly distributed throughout the panel. Generally, only in cases where effects of these factors under conditions of heavy perimeter framing are desired, should the test method be applied.
1.1.3 Test Method C: Two-Rail Shear Test—This test method is applicable to a wide variety of materials and problems. The specimen fabrication and test procedures are somewhat simpler than in Test Methods A and B. The specimen is free to shear parallel to its 24-in. (610-mm) length dimension anywhere within the 8-in. (203-mm) width between rails. Thus, the test method is well suited for determining grade and manufacturing effects such as core gaps and knots occupying and affecting small areas. The test method is not so ideally suited for determination of modulus of rigidity, but when adjusted for strain distribution effects, values approximating those obtained by Test Method B result. The test method simulates effects of heavy framing when expected planes of weakness are oriented perpendicular to rails and no framing at all when parallel to rails.
Note 1: A smaller scale version based on the principles of this two-rail shear method is contained in Test Methods Section 27. The results from Test Methods Section 27 may not be equivalent to the results from Test Methods Method C.
1.2 Significant differences, moderate to small in magnitude, among the three test methods have been found to exist when these test methods are applied to plywood of clear straight-grained veneers. Therefore, when comparisons are made among test results, it is recommended that the same test method be used throughout.
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
1.4 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.