SU1552064A1 - Method of fatigue testing of materials - Google Patents

Abstract

The invention relates to the testing of materials, and in particular to methods of fatigue testing. The purpose of the invention is to simplify the method. The first sample is loaded cyclically until failure at a constant rate of growth of the load level, and the breaking stress is determined. The second sample is loaded cyclically at a constant load level equal to the failure voltage of the first sample, the durability of which determines the total operating time before failure, taking into account the endurance limit of the first sample. 1 il.

Description

The invention relates to material testing, to fatigue testing methods.

The purpose of the invention is to simplify the method.

The drawing shows a voltage diagram — the number of cycles with a destructive voltage applied to it and a total time to failure.

A device for the implementation of the method is a test machine, which allows to smoothly change the level of cyclic loading, equipped with a counter of the number of cycles and means of measuring the load.

The method is carried out as follows.

in a way.

Cyclically load the sample or structural element from the level (5, not above the endurance limit 6d

at a constant rate of voltage growth before unloading. The breaking stress is determined. B. . To improve accuracy, a batch of samples is tested under the same conditions and the mean value of failure voltage is determined.

The second sample (or a batch of samples, respectively) is tested at a constant voltage level Op and the durability N "or the average durability Np in the second batch of samples is determined, after which the total operating time until the first sample is destroyed, corresponding to endurance:

ate to

about

About 4

P

CR

Nl

where g is the exponent in the Pro equation (for many materials, g is 0.5).

After that, the endurance limit is determined by the formula

6R (Sp -6inCR

(one)

The progressive accumulation of fatigue damage to an object occurs at stresses exceeding the endurance limit. Therefore, when an object is tested at an increasing voltage from any initial level taken not above the endurance limit, destruction occurs at the same destructive stresses (disregarding scattering). Therefore, the dependence 3р (пЈ) (curve 1) degenerates into the horizontal of h - (one point is enough for its construction).

In order to determine the time to failure, according to a known method, the second section is constructed by testing additional samples before destruction with increasing load, starting from levels above endurance limit. It is simpler to determine this value, since when describing the fatigue curve 2 by the Weibull equation, the total durability to destruction Np corresponding to the voltage G by the equation

R

expressed

ten

with

 "Sp e

(2)

P,

(3)

where mw and cw are parameters.

The total durability, corresponding to the limit of endurance to failure, when tested monotonously increasing load is determined from the expression

6p-6R

 c Find the ratio-ps,

M „

about

10Cw

Converting the expression (6) with regard to formula (4), get

i.

Ik U (shi / i M {(shJ 1) .G

(7)

N

P or

10.Cw

mw +

one

(eight)

Enter the notation 1

# +

one

(9)

20

A - (pun). (YU)

and substituting (9) and (10) into equation (4), one obtains equation II p0

b

R

6G.

(eleven)

Regardless of the rate of increase of the load and the initial abscissa of the Baseball fatigue curve with the nc / N ratio. depends only on the parameter mw and is the inverse of the exponent of the equation PV

4ro

HT-v 4

or n;

Nl.

g

(12)

Take g 0.5, i.e. mw 1, get nt / N "2.

Example. Ten blocks of hydraulic cylinders are tested, of which five are at a voltage increasing from an initial level of (3, 100 MPa, at a rate of Pa / cycle, and five at a constant voltage equal to the average breaking voltage obtained from test results of the first five objects;,

The difference (5p - Au can be expressed in terms of the parameters of the fatigue curve, based on the Weibull equation and the linear hypothesis of damage summation:

-i- br ob (mw-H) - 10tw (4)

The operating time before destruction (3) in view of (4) can be represented as

By . () (five

(5R - Zl6n./n at 143 MPa.

 i e 1

According to the test results with a constant voltage, the average durability of objects is determined.

0

N

R

1 Np./nCS 3.07 f (

10 cycles.

According to the dependence Np / nc r, where r is the exponent in the equation Pro, the total operating time up to

destruction of an object tested under normal stress, increasing from the initial level (5, equal to the endurance limit. It is

at

P"

Np / r

tgg-wp / t. - 6,110-10 CYCLES.

The endurance limit is calculated from the dependence of the destructive stresses on the total operating time to failure:

V 3p ncf 112 MPa, which satisfactorily coincides with the endurance limit defined by standard methods.

Claims (1)

Invention Formula The way materials are tested for fatigue is that Compiled by D. Pospelov Editor I. Derbak Tehred M. Khodanich Proofreader A. Obruchar Order 325 Circulation 498 VNIIPI State Committee for Inventions and Discoveries at the State Committee on Science and Technology of the USSR 113035, Moscow, Zh-35, Raushsk nab. 4/5 A sample of the material is loaded cyclically with growth at a constant rate of voltage level, starting from a level not exceeding the endurance limit, the breaking stress is determined, the second sample is cyclically loaded until destruction and damage, taking into account that the ultimate strength of the material is determined by the failure stress of the first sample, characterized in that, in order to simplify the method, the second sample is loaded at a constant load level equal to the first load destructive voltage ruin court t for durability second sample. / g with ps ps.N Subscription