Generally speaking, these shear tests can be performed in both the tensile and compression directions, however tensile tests are customary due the fact that they are simpler to conduct. The results of this test is a shear strength in the plane shear.
When testing relatively thin and flexible laminates, angular errors in the stress plane can occur in single lap shear specimens due to an acting bending moment, which can lead to an uneven stress distribution and thus a significant measurement error. A symmetrical, double lap shear specimen, in which the acting bending moments compensate for one another and the stress plane remains precisely aligned, is best suited for this.
The shear deformation can be measured with a contacting extensometer in both cases.
Horizontally adjustable grips are needed to test simple single lap shear specimens. These can be wedge screw, screw, or pneumatic specimen grips.
Slotted single lap shear specimens can be tested with simple, symmetrically closing grips. The EN and DIN standards specify the use of a support to prevent bending.
Double lap shear specimens may always be tested with simple grips.
Slotted lap shear specimen allow the same grip opening on both grips.