Abstract This paper contributes to the understanding of a stress – strain diagram (SSD) in the region beyond the elastic limit (Viscoelastic and Plastic region). According to our results, the SSD in this region can be approximated by a second degree parabola. The advantage of such an approach is the possibility to simply calculate the value of stress if the strain is known and vice versa. Another advantage is in the possibility to simply calculate the tangent modulus. In the experimental part we determined the tangent and chord moduli for beech ( Fagus sylvatica L.) and aspen ( Populus tremula L.) “ WS ”. We also investigated the influence of the following factors on their value: material thickness “ MT ” (4, 6, 10, and 18 mm), degree of densification “ DOD ” (10% and 20% of the original thickness), and the number of loading cycles “ NC ” (0 versus 10,000). The paper describes the behavior of the tangent modulus in the whole force – deflection diagram.