PSI - Issue 65

A.N. Romanov / Procedia Structural Integrity 65 (2024) 200–208

201

A.N. Romanov / Structural Integrity Procedia 00 (2024) 000–000

2

decreases; when loaded with a given strain range in a cycle (hard loading), the stresses increase with an increase in the number of loading cycles. Unstrengthening materials are characterized by active growth of the width of the hysteresis loop under soft loading conditions and a load drop under hard loading. In cyclically stabilizing materials, the width of the hysteresis loop and the loads under the specified loading conditions remain virtually unchanged or change insignificant. All metallic materials were generally strengthened during the first loading cycle under the delivery condition. The same material can be cyclically hardened, de-strengthened, or stabilized depending on its initial structural state (on the type of heat treatment). The nature of material behavior can be determined by the shape of the static fracture curve: the greater the value of uniform strain (strain at the level of the tensile strength), the more inclined the material is to hardening, and vice versa; the smaller the value, the more the material will unstrengthen. For a cyclically stabilizing material, this value has an intermediate value (tentatively, it is equal to or close to 0.5 of the total design strain). This division of materials is valid under loading (operation) conditions in which materials do not undergo significant structural changes. Steel 22k at room temperature can be considered a cyclically stable material. In the investigated temperature range, the most intensive aging was observed at 270 0 C (ψ = 27% ). At 150 0 , deformation aging (ψ = 43% ) was insignificant compared to deformation aging at room temperature (ψ = 45% ). At 350 0 C, the strain aging effect decreased (ψ = 40% ). When the temperature was increased to 450°C, the ductility (ψ = 47% ) exceeded the room temperature value. It was found that at these temperatures, the assessment of the accumulated damage and the limit states using the strain kinetic criterion of fatigue failure is valid. This assessment was carried out with the involvement of experimental static and cyclic fracture data. The effect of strain aging is known to depend on the level of plastic deformation and loading time. In static fractures, aging is induced by large deformations and short periods. Under cyclic loading: smaller deformations but longer times. During operation, products and various technical systems undergo multiple temperature changes (heating, cooling) associated with temperature transitions, and short-term deformation aging processes take place and can significantly contribute to changes in the ductility of materials. The aim of the present study was: on the basis of experimental data on the kinetics of deformation and strength characteristics to evaluate the possibility of describing the kinetics of damage accumulation and limit states under low-cycle loading conditions under intensive strain aging. 2. Material and method of research Experimental studies of 22k steel were carried out at a temperature of 270 0 C on corset specimens with a minimum diameter of 10 mm.

Table 1. Chemical composition of steel 22k (in %) С Mn Si P S

Cr

Ni

Cu

Ti

0.24

0.84

0.29

0.014

0.019

0.13

0.15

0.24

0.23

Mechanical properties:

20 0 С: ϭ 270 0 С: ϭ

0,2 = 330 MPa, σ b = 620 MPa; ψ = 45%.

0,2 = 240 MPa, σ b = 610 MPa; ψ = 27-29%. Mechanical loading was performed via tension compression at a frequency of 3-5 cycles per min. The transverse strains were measured and converted to longitudinal strains using transverse strain coefficients (0.5 for plastic strain and 0.3 for elastic strain). Heating was carried out by passing a current through the specimen. A specially designed water-cooled strain gauge with an error of no more than 0.5% of the measured value was used to measure the strain [1]. The deformation diagrams were recorded on a two-axis device. The above-mentioned character of the change in deformation characteristics (Fig.1) determines the nature and rate of damage accumulation η. For quasi-static fracture (limit state - loss of plastic deformation stability), when the damaging role of elastic deformation can be neglected, the accumulated damage for any number of cycles and in