RU1816996C - Method of fatigue tests of composite materials - Google Patents

Abstract

The invention relates to mechanical testing, and in particular, to methods of proactive testing of composite materials. The purpose of the invention is to increase confidence by refining the normalization of the damaged ™ curve. Two samples are cyclically loaded at the same initial stress level, one in soft, and the other in hard conditions. The ratio of the durability of the samples indicates the ultimate decrease in the elastic modulus of the material. As a defective ™ characteristic, the ratio of the reduction in the elastic modulus to the limit value is determined. 3 ill.

Description

The invention relates to mechanical tests, to methods for carbon-electric tests of composite materials.

The purpose of the invention is to increase the reliability of determining a damaged ™ curve by refining its normalization.

In FIG. Figure 1 shows the law of variation of voltage under soft mode; in FIG. 2 - law of variation of relative stiffness; in FIG. 3 is a corresponding hard loading mode.

The device for implementing the method is a standard test machine with a soft and hard cyclic load loader, equipped with a cycle counter and means for recording the tensile diagram to determine the current stiffness of the sample. Such testing machines are widely known and are used for fatigue tests (see L. M. Shkolnik. Fatigue Test Methodology, M .. Metallurgists, 1978, see 160-210).

The method is implemented as follows.

Previously, the characteristics of the fatigue curve are determined for the test material by known methods. These characteristics can be taken from the reference literature, or preliminary tests are carried out and the voltage – durability relationship is built. The sample is loaded in soft mode and

the dependence f (n) is determined, where

to

n is the number of cycles; AE is the magnitude of the decrease in elastic modulus; E0 is the initial value of the elastic modulus (corresponding to intact material). Since the limit value of damage v is 1, this dependence is subjected to calibration

ate

WITH

00

Oh Yu

Yu

about

v -

AL

SW

(D

where k

limiting value of elastic modulus reduction.

Using the long-term sample constant in the hard MF mode and in the soft NM mode, one can determine the limiting value of AEPr / E0, i.e. the t & calibration factor can be determined. For example, in the case of linear damage accumulation according to the Palmgren-Miner law, the indicated value can be determined with a sufficient degree of accuracy by the formula

where m is the exponent in the approximation of the fatigue curve

N

(3)

The essence of the claimed method consists in the following.

In a fairly general case, the law of summing damage is presented in the form

-v-g

1

dn

where a is a constant.

we use the expansion in the Mikloren series with retention of terms quadratic in K. In this case

Bj NM

km 2

10

whence formula (2) follows.

Example. Consider an orthogonally reinforced epoxy, cyclically on

loaded at a frequency of 10 Hz with a maximum cycle voltage of 0.5 of the limiting static voltage.

The maximum relative change in the elastic modulus when measured in accordance with the prototype is about 18%. It is quite difficult to directly measure this value directly, since the change curve

20 stiffness at the last stage of loading changes quite sharply, therefore, the indicated value is underestimated. The approximation of the fatigue curve allows one to find the exponent m 12.6 in this range of stresses. With the indicated voltage level of 100 thousand cycles, and L-270 thousand cycles. Then the specified value:

thirty

 - (2.7-1) 0.27.

DE

o o0 (1) (1 -kv), (5)

then, taking into account conditions (1-3) and (5), from equation (4) we obtain an equation of the form

1st

,- and

dv v-4r (1 I-) -Ј-Ooncln

and

 -Ј-dn.

NM

(6)

Integrating equation (6), we obtain an equality of the form

± v ± - «- (kr NM a J0 a

)

In the case of a linear summation of damage 1. get

N 1M ft -

N7) 11 n kj J

Since the elastic modulus decreases in known composite materials by 15–25%, the quantity k «1, therefore, the specified value of the ultimate characteristic of damage 35 to which it is necessary to normalize is 27%.

0

5

0

5

Claims (1)

SUMMARY OF THE INVENTION A method of fatigue testing of composite materials, namely, that a sample of material is cyclically loaded in soft mode and the characteristics are determined (damage to the material by reducing its elastic modulus, characterized in that, in order to increase reliability by refining the normalization of the curve damage, an additional sample, similar to the main one, is cyclically loaded in a hard mode at an initial stress level equal to the stress level of the main sample in a soft mode, and ate dissolved limit decrease elastic modulus in view of durability of the material ratios of samples under mild conditions and corresponding hard and n as determined characteristics damage reduction ratio of elastic modulus to its limiting value. F "g