The MWCNT vacuum gauge area was connected with a pair of ITO electrodes on the glass plate of cathode to measure the electrical parameters.
Firstly, a local strain measurement, based on the following of three dots drawn on the gauge area, gives the evolution of the strain tensor eigenvalues during the test.
The DIC method provided similar results compared to the strain gauge measurements, but with reduced scatter due to higher gauge area, making the technique less sensitive to the wood heterogeneity (earlywood and latewood layers).
It is shown, by comparing the theoretical kinematics of simple shear with a slightly perturbated one, that the strain state is close to the ideal one in the center of the gauge area.
The geometry is optimized for maximum stress using several parameters, ensuring that in the gauge area the stress distributions on the loading directions are uniform and maximum with two limit phase shift loading conditions (δ = 0° and δ = 180°).
Three conditions are defined that ensure the validity of the experimental procedure: the cracks appear in the middle of the specimen, the strain field is homogeneous in the gauge area, and buckling must be avoided.
Three measuring electrodes are placed in selected positions over the gauge area of a specially designed shear specimen and their readings are associated with the actual position of the crack tip using Finite Element Analysis FEAA).
This specimen was optimized for the test machine in order to achieve the maximum uniform stress at the centre gauge area, while keeping the remaining stresses at least 20% lower than the stress at centre.
This sample has the merit, as detailled in a previous study [1], of enhancing simultaneously: (i) the heterogeneity of the strain field within the whole gauge area of the sample, (ii) the diversity of strain-paths, (iii) and the sensitivity of strain maps to the hardening parameters.
The main goal of this paper is to present the methodology on how to obtain an optimal cruciform specimen geometry, with thickness reduction in the gauge area, appropriate for fatigue crack initiation, as a function of the base material sheet thickness used to build the specimen.
