**2. Materials and Methods**

A structural Al-Mg alloy was chosen as the material for the study. The chemical composition, according to GOST 4784-97, is shown in Table 1. Specimens were made from a rod with a diameter of 20 mm as delivered (without additional heat treatment) on a lathe.



In tensile tests with the attachment having a variable stiffness, solid cylindrical specimens were used, a scheme of which is shown in Figure 1. On one of the gripping parts, a thread is cut to fix it to the tooling rod. This sample geometry meets the requirements of GOST 1497-84 "Metals. Tensile Test Methods ".

**Figure 1.** Scheme of a solid cylindrical sample for testing with fixtures with variable stiffness.

Mechanical uniaxial tensile tests are carried out at the Instron 8850 biaxial servohydraulic testing system, which allows tensile-compression tests with a maximum load of 100 kN and torsion with a maximum torque of 1000 Nm. Kinematic loading is realized with a traverse speed of 2.4 mm/min, which corresponds to a material strain rate of 6.67·10−<sup>4</sup> <sup>s</sup><sup>−</sup>1. The stiffness of the loading system is changed by using specialized equipment with a variable stiffness.

The equipment shown in Figure 2 consists of the following elements: body—1, consisting of two parts connected by means of a threaded connection, a package of plate springs—2; a guide rod—3; compensation rings—4; hardened washers—5; fastening nuts for fixing the package of springs—6; a rod to install the equipment in the grips of the testing machine—7; and a threaded hole for fastening in the equipment of the test specimen—8. Elements 3 and 5 are hardened to prevent damage and bite to the springs when the package is loaded. There is a patent (RU 153985 U1) for the rigging with a variable stiffness used in the work for tensile tests with various loading parameters.

**Figure 2.** A scheme containing the main elements (**a**), a general view (**b**) and elements of attachment with a variable stiffness (**c**).

By changing the stacking order and the number of plate springs (see Table 2) we provided the required stiffness of the loading system in the range of 5 MN/m to 120 MN/m.

In total, four values of the rigidity of the loading system are implemented during the work: 120 MN/m (nominal value), 50 MN/m, 18 MN/m and 5 MN/m (Figure 3). The maximum stiffness of the tooling coincides with the stiffness of the Instron 8850 testing machine when using hydraulic grips.


**Table 2.** Setting up of plate springs inside the attachment with a variable stiffness.

**Figure 3.** Stiffness characteristics for various setting-ups of plate springs corresponding to the values: 1—120 MN/m, 2—50 MN/m, 3—18 MN/m, 4—5 MN/m (**a**), testing equipment set-up (**b**).

Plate springs are installed according to the required scheme. Further, the tooling is installed in the grips of the testing machine in series with the sample. The sample is fixed to the tooling rod by means of a threaded connection. Then a constant velocity displacement is applied to the sample, the change in loading is recorded, and the evolution of the deformation field on the surface of the working part of the sample is recorded. The test system controller allows synchronization with the Vic-3D non-contact video system based on the digital image correlation (DIC) method.
