SU1744585A1 - Method of estimating fatigue damage of composite material - Google Patents

Abstract

Invention from: w.tsch test area, to methods for determining fatigue damaged ™ composite materials. The purpose of the invention is to improve the accuracy by taking into account the difference in the properties of the filler and the binder. Cyclically loaded in a hard mode with a nebulized cycle sample material. Reduce the frequency and determine the maximum maximum loop voltage. Increase the frequency and determine the maximum amplitude of the voltage cycle. Taking into account the changes in these two values, changes in the stiffnesses of the binder and filler are determined by calculation. 3 il.

Description

The invention relates to mechanical tests, to methods for determining fatigue damaged ™.

Methods are known for determining the fatigue-damaged ™, in which a batch of samples is cyclically loaded and the residual strength of the samples is determined at various operating times, by which material fatigue damage is judged depending on the operating time.

The disadvantage of such methods is that they require the destruction of samples. As a result, such methods do not allow to obtain individual dependencies of the fatigue-damaged ™ test objects.

Closest to the present invention is a method for determining the fatigue damage of a composite material, in which the material is cyclically loaded, periodically determined

stiffness characteristics according to which the court r is about fatigue damaged. ™.

The disadvantage of the prototype is that with its help the integral hardness of the material is determined, which makes it impossible to judge accumulated damage reasonably when testing samples of composite materials, because the rigidity of composite materials changes as a result of changes in elastic characteristics and development of relaxation processes. .

The purpose of the invention is to improve accuracy by taking into account differences in the characteristics of the filler and the binder.

Fig, 1 shows the dependence of the strain on time; in fig. 2 — relative arrangement of strain and stress cycles at frequencies close to zero; FIG. 3 shows the relative arrangement of the deformation and stress cycles at high frequencies;

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The device for implementing the method is a standard testing machine for cyclic loading in hard mode with adjustable loading frequency, equipped with means for measuring the parameters of the voltage cycle parameters.

The method is implemented as follows.

The sample (or structural member) is loaded in a rigid mode with a pulsating cycle before it acquires some damage, and in the state when it has acquired fatigue damage. Reduce the frequency and determine the ratio of maximum stress and maximum strain. The limit of this ratio is determined when the frequency tends to zero. In reality, the oscillation period should exceed the relaxation time of the material, in this case the indicated ratio is close to the limiting one. Increase the frequency and determine the ratio of the amplitude of the stress cycle. The limiting ratio is determined when the frequency tends to infinity. In reality, the ratio will be close to the limit when the condition is fulfilled, where to is the time of material oxidation; w - circular load frequency. The damage of the filler is judged by the change in the magnitude

but

F °

00 ..

and lim

a (o)

yy)

where a is the amplitude of the cycle

and about damage sv

change in magnitude

Omax

In A-

where is Omaks lim Omaks (“), yuyu

Omax - maximum cycle voltage.

The proposed method is based on the desirable properties of the composite material. It is known that composite materials exhibit rheological properties. In the hard mode, the dependence of the stress o (t) on the strain e (t) can be described by the relations of linear viscosity using the following relation:

CT (t) Ee (t) -J R (t-r) e (T) dr, (3)

about

where R (t) is the relaxation core;

E is the modulus of elasticity.

The relaxation core can be represented as

H (t. No 1 / g% (4)

where Ro is the material constant;

to - relaxation time.

When a material is loaded in a rigid mode with a neutral cycle of deformation, the deformation depends on time according to the law.

s (t) Ј0 (1 - cos w t), (5)

where is the circular loading frequency.

Substituting (5) and (4) into (3) allows to obtain the dependence

cr (t) Her0 -Rot0Јo

1- (1 1

one

) +

RO to

+ (°, 0, -E) fibCOSQ t +

1 + o} to

+ RofiotoO, e | nu, t

one

With increasing frequency, ratios ratios

Alo 1; t to, relation (6) will take the form

o (t) Е е0 - R0 t0 Оо - Е Ј0 cos ш t. Amplitude of voltage fluctuations

a °° EЈ0. or

00

p j

To

35

40

45 50 55

With decreasing frequency, with (Y to "1,

relations (6) takes the form 7 (t) Ee0 -RO t0Ј0 - (E EC, - RO t0 fio) COS tt t (E - Ro to) (t).

It follows that

Omax, - p

7, - 1О

with

Thus, the change in the value of A, determined by relation (1), characterizes the change in the elastic part of the specimen rigidity, which, as shown by numerous studies, characterizes, first of all, the damage to the filler. At high loading frequencies, some heating of the material occurs (as shown by the studies performed, heating amounts to 30–40 K), which has practically no effect on the change in E, but significantly (more than 2 times) changes the relaxation characteristics. Subtracting from the value of A the value of B, determined from formula (2), will remain the value of B, the change in which, as studies have shown, characterizes the damage of the binder. Thus, the proposed method

allows to separately determine the damage of the filler and the binder.

Example. Fiberglass samples (PPN + EDT-Yu) were subjected to cyclic loading, s, the circular frequency of loading was 1 Hz. Cyclic damage was created by loading for 104 cycles.

Before and after this, the frequency increased to 10 Hz () and decreased to 0.02 Hz (, 088 «1). The change in the value of A and B was determined. The change in the value of A was 2%, and the value of B changed by 35%, which indicates that the filler had a slight accumulation of damage, and the binder was significant. After additional loading for 2-104 cycles, the operations for determining changes in A and B were repeated. In this case, the change in A was 7%, and the value of B was 40%, which means that the filler started the damage accumulation process, and in the binder the growth rate stopped, because the binder cracked - the damage growth saturated.

Thus, the proposed method allows the selection of the filler and binder damage.

0

five

0

five

Claims (1)

Claims The method for determining the fatigue damage of a composite material is that the sample of a material is cyclically loaded, the cycle parameter is changed, and the change in the stiffness characteristic of the material is determined, by which its damage is judged, characterized in that differences in the properties of the filler and the binder, loading is carried out in a hard mode with neural deformation cycles at varying loading frequencies, and as a stiffness characteristic determine the parameters A and B of the rigidity, respectively, of the filler and binder from the following relationships: And a °° / Ј °; At A-OYax / Ј °, 00 where a is the maximum amplitude of the voltage cycle at the maximum loading frequency; e ° - maximum cycle deformation; Omax - maximum maximum cycle voltage at the minimum loading frequency. FIG. one & & Cf} + u S cp tfcp-in FIG. 2 . FIG. H