RU1796986C - Method for determining fatigue failure of material - Google Patents

Abstract

The invention relates to mechanical tests, fatigue testing methods. The purpose of the invention is to increase the reliability of determining damage by eliminating errors associated with the influence of reversible changes in the structure of the material and changes due to the training factor under cyclic loading. The material sample is cyclically loaded and the phase shift between the zero stress cycle and the deformation cycle, the average and minimum deformations, by which the change in the stiffness characteristics of the material are determined as characteristics of damage: 3 ill.

Description

The invention relates to mechanical tests, fatigue testing methods. Y ..:. . ; „.-; -; -:,

Fatigue testing methods are known in which a batch of material samples is cyclically loaded and the degree of damage to the material k is determined: the time function according to the residual static strength of the samples.

The disadvantage of such methods is that they require the destruction of test objects. As a result, such methods do not allow one to obtain individual dependences of damage accumulation, by, -: since at each operating time it is necessary to destroy the corresponding sample, which prevents its reuse, /;

Closest to the proposed is the fatigue test method, in which the accumulation of fatigue damage is judged by reducing the shape of the cycle

one of the characteristics of a stress-strain state. For this, the sample is loaded in a soft mode, maintaining a constant amplitude of the stress, the spectral characteristics of the deformation cycle are determined, and fatigue damage is accumulated from phase shifts of the harmony of the center of the deformation cycle relative to the harmony of the stress cycle.

The disadvantage of the prototype is that it is extremely difficult to select changes in the physical and mechanical characteristics of the material with it, since all changes are interpreted as damage. At that; At the same time, when loading samples from composite materials, there is both a change in properties, which is a consequence of the development of damage - cracking, delamination, slippage of layers, etc., and is a consequence of the training of the material.

ate

with

XI O CN YU 00

The aim of the invention is to increase reliability by eliminating errors due to the influence of reversible changes in the structure of the material and changes due to the training factor.

In FIG. 1-3 show typical time dependences: in FIG. 1 - stresses; FIG. 2 - strains; FIG. 3 - moduli of elasticity of the material.

The device for implementing the method is a testing machine that allows cyclic loading of samples in soft mode, equipped with means for recording the parameters of the deformation cycle and stress and phase shift between them.

The method is implemented as follows.

The sample is installed in a testing machine and loaded in a soft mode with a pulsating cyclic load. The maximum voltage in the cycle o, then the voltage is canceled according to the law.

. a (t) Ob (1 -cos cot),

0)

n (t) П0е

-t / te

(4)

where Po is a constant, t0 is the relaxation time. In this case, 5, by direct substitution of (1) and (4) in (3), it is easy to obtain the law of strain

10

e (t) Ser - a soph; t - $.

(5)

where Esr is the average strain: a is the strain amplitude: V is the phase shift.

Under conditions a) to 1 and it is easy to obtain the relations

Јav

 .5a + Ob over t0,

 2 SHOW

About to

tg ip (a to.

(6)

taking into account which we obtain relation (2), in which the deformation parameter of the material

.

where w is the load frequency curve;

t-time

The time dependence of the strain amplitude a, the average strain ЈCp and the phase shift t / of the delay of the deformation cycle relative to the stress cycle is determined. As a characteristic of damage, determine the change in the value of A, determined from the ratio

 Ser-4- tg2V (2)

Change A, as shown by direct experiments, characterizes a change in inverse stiffness. To theoretically substantiate relation (2), we give the following calculations. Let the stress in a material exhibiting viscoelastic properties change according to the law (1). We accept the law of the relation of stresses to strain in the form

E (t) (t-r) cr (T) dr, (3)

with about

where ((t) is the creep core.

As a relaxation core for a composite polymer material, exponential functions are widespread, therefore, we take the core in the form

Since the change in I is connected with the change in E, A can serve as a measure of damage. In FIG. 3, section I corresponds to the training of the material - on it the damage does not increase, although the characteristics of the shape of the deformation cycle vary. In section II, a stable accumulation of damaged ™ occurs - gradually decreases. In section 111, the stiffness decreases sharply, indicating the occurrence of macrodefects. Thus, the inventive method allows to increase the reliability of the result due to the fact that, unlike the prototype, it allows to determine the boundary between sections I and II and to judge the damage of the material according to a characteristic that takes into account the difference in causes that distort the shape of the deformation cycle.

Fiberglass samples (PPN + EDT - 10) were subjected to cyclic loading, the time was up to 7 s, with a frequency of 10 Hz based on the order of 100 thousand cycles at 300 ° С. In section I, the stiffness of the samples increased by approximately 8–10%. Some of the samples were preliminarily destroyed, some after the operating time of the order of 1000 cycles, which corresponded to section I. The residual strength of the samples subjected to such cyclic loading - training, increased by 5-7%, i.e. talk about

there is no reason to increase its damaged ™, displacement of deformation cycles relative to stress has occurred. When cyclic loading was interrupted in section II, where parameter A increased by about 4% (i.e., module E decreases by about 4%), the residual strength of the samples turned out to be less by 4-5%. When loading was interrupted in section IV, macrodefects in the form of delaminations, shells, or macrocracks were clearly visible on thin sections.

FORMULA AND SECTION

A method for determining material fatigue failure, in which a material sample is cyclically loaded with a constant voltage cycle, determine the phase displacement of the deformation cycle relative to the stress cycle, which determines the deformation parameter of the material, and the damaged ™ is judged by the specified parameter, characterized in that, in order to increase reliability by eliminating errors associated with the influence of reversible changes in the structure of the material and changes due to the training factor 4Y / l /

0

Thus, but on the basis of an example illustrating the method, it can be seen that the method improves the reliability of the result, since it more reasonably allows us to conclude that damage develops or changes in the characteristics of the material without changing the damage.

The method can be used for non-destructive testing of cyclically deformable structures during their operation without interruption of operation.

During cyclic loading, loading is carried out at a zero stress cycle and the minimum and average deformations in the cycle are determined, and the deformation parameter is determined by the following ratio:.

A Ser -f- tg2 V where A-deformation parameter;

ECP - average strain in the cycle;

a is the strain amplitude;

ip is the mixing of the phase of the deformation cycle relative to the stress cycle.

FIG. 2.

FIG. 3.