The standard method ASTM D4027 describes the test and equipment for measuring the shear modulus and shear strength of adhesive layers between rigid adherends. The equipment described in this test method may be applied to test the adhesive’s shear creep compliance and strain history effects like cyclic loading impact upon shear properties. Biaxial stress situations such as shear combined with tension or compression can be evaluated for potential joint failure. ASTM D4027 specifies high density wood as the preferred substrate, though it is permitted to test other construction materials such as wood-base composites, metal, plastics, and reinforced plastics. In practice, the maximum shear modules that can be measured are limited by the shear modulus of the adherends as well as the strain measurement device. For high density wood, the maximum adhesive shear modulus that can be measured is about 690 MPa (1 × 105 psi).
The ASTM D4027 method employs thick rigid adherends with a short overlap length to minimize peel stresses and ensure a uniform distribution of shear stresses across the adhesive bond in the overlap area. The specimen is configured as two rectangular thick adherend bars bonded together with a layer of adhesive, with an orthogonal zig-zag notch splitting the bars into two lap-jointed pieces, enabling their movement in opposite directions when pulled longitudinally in opposite directions. The shear stress gets exerted on the adhesive layer in the overlap area between the two staggered vertical notches. Wooden specimens for ASTM D4027 may be prepared by sawing and then planed into two smooth laminates of required thickness for adhesive bonding and curing, after which the notches can be cut. Alternatively, pre-cut thick pieces with beaks to form the lap joint can be bonded together. Properly conditioned wood needs to be used for the specimens.
In ASTM D4027, the tensile load is applied to the thick lap-jointed specimen by a modified-rail shear tool, connected to a standard testing machine. The adherend is clamped vertically, with the upper rails gripping the top part of the sample and the bottom rail gripping the bottom half of the sample. When the upper half is pulled up by the upper, movable rail, shear stress acts on the adhesive at the lap-joint area. The rigid rails limit undesired adherend deformation during testing. The results of using this shear tool are nearly uniform stress and strain distributions and the reduction of normal stress in the adhesive layer under load. The testing machine used in ASTM D4027 testing should have a minimum load capacity of 8900 N (2000 lbf), and a range of crosshead speed from 0.317 to 10.16 mm/min (0.0125 to 0.40 in./min). The displacement of the adherend pieces under load is measured using a dual transducer slip gauge or an extensometer.
Video 01: Adhesive Testing
The standard specimen dimensions for ASTM D4027 testing are: width 3.18 mm (0.125 in.), length 203 mm (8 in.), and adherend thickness 19 mm (0.75 in.). Adhesive bondline thicknesses can be from 0.15 to 3.18 mm (0.006 to 0.125 in.). The range of specimen sizes can vary as specified in the ASTM D4027 standard.
The ASTM D4027 adhesive strength and shear modulus is expressed in megapascals (Mpa).
The standard method ASTM D4027 describes the test and equipment for measuring the shear modulus and shear strength of adhesive layers between rigid adherends.