SU1353833A1 - Method of retardation of cracks in metal articles - Google Patents

Abstract

The invention relates to the hardening of elements of metal structures by plastic deformation, in particular, to methods for increasing the residual durability and service life of the metal products of the engineering industry. The purpose of the invention is to increase the durability of the product; LIA by increasing the crack growth delay. The method allows to obtain the maximum possible delay. ku growth of cracks in the product regardless of their size due to the fact that. The values of the stress intensity coefficient corresponding to the maximum delay of crack growth are preliminarily determined on the reference samples. The magnitude of the load under static loading of the product is determined taking into account the length of the existing cracks from the ratio P & aiA, where PC. load-with static loading Tox is the stress intensity factor corresponding to the maximum delay of crack growth in the reference samples; G, g - the surface area of the gross section of the product, perpendicular to the action of the working load; 1 „zda - half the length of the crack growing in the product; q, d — the crack length function known for the product. c & cl

Description

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The invention relates to the field of strengthening of elements of metal structures by plastic deformation, in particular, to methods for increasing the residual durability and service life of metal products of a machine-building industry.

The purpose of the invention is to increase the durability of the product by increasing the crack growth delay time.

The essence of the invention is ToMj that test reference samples of the same material as the product in the words of its operation. During the test, different

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its direction of action, i.e. carry out tensile overload. Find the value of the stress intensity factor (CII) according to

static loads P higher initial (working) load

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load at static loading of the sample; gross surface area of the sample; the length of the crack in the specimen; the crack length function known for the reference sample.

later, the corresponding growth factors for treina growth are determined. Find the value of the CIN K corresponding to the maximum delay time. The static load P that is applied to the product in the working direction is determined from the condition of equality of the CIN in the reference sample and the product, i.e.

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the gross section of the product; 21ig, and the length of the crack in the product; Y |, is a crack length function known for a particular product.

During a short-term load, the load is higher than the operational one. there is an increase in the radius of the apex - original technological defects such as cracks, as well as cracks formed during the operation of the product. The deformed material in front of the crack tip is exposed to residual compressive stresses. These two phenomena cause the growth process to slow down.

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cracks or stop them completely. As the load level increases, this effect will increase. However, as the CIN approaches, with this static loading to the fracture toughness, there is a significant damage to the material in front of the crack tip, which consists in exhausting its ability to further plastic deformation. This leads to an increase in the speed of the destruction process after some of its delay. The probability of stalling increases, and its length increases. This proves the existence of a maximum delay in the growth of a crack at a CIN value less than the fracture toughness.

Thus, the proposed method provides the maximum growth retardation of a trashdny regardless of its length after applying a static load during the further operation of the product. Repeating this operation periodically as the crack grows through a local deformed zone in front of its tip, the durability can be significantly increased by

An example, the Method is implemented on the AIMA-5-2 serial unit. The tests were carried out under conditions of long-term static loading in air at a temperature of 923 K, DL observation of the process of propagation of cracks using flat samples of steel of type X18H10T of size M mm and of type X16H15MZB of size Jub; The original sharp central incision with a tip radius of less than 0.08 mm is created by the electroerosion method in the middle part of the sample. 10 mm notch length for samples of steel Х18Н10Т and 4 Mi-i from CTaj-ш Х16Н15МЗБ Tracking the crack growth is carried out using an MBS-1 microscope with an accuracy of 0.015 mm and an electric potential pickup method. Potential contacts are located on 5 (j notch banks along the centerline of the sample.

When the total length of cracks grown from the notch is reached, a certain value (0.9 mm for 515 samples from Х18Н10Т and 0.7 mm from Х16Н15МЗБ) produces a short-term increase in the load to the required level. At this point, the top of the growing crack is out of the original

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a notch, which is determined by the change in the slope of the curve, which reflects the dependence of the crack propagation velocity on the stress intensity factor. After reducing the load to the original level, it remains unchanged until the end of the test.

To register the load, a power meter in the form of a hollow cylinder with load cells connected to it in a bridge circuit and a two-coordinate recorder PDP 4-002 are used. The error is less than 1% of the load. To determine the effect of either an overload level on a static one; loading on the braking of growing cracks is carried out in the entire possible range of CIN: from zero, when stationary testing occurs, to the value at which the fracture toughness is reached, i.e. The cracked trap moves at maximum load. Some time after the overload, the crack does not move, after which its slow growth begins.

In order to quantify the effect of load increase, a parameter is introduced - crack crack time T, for which the growth rate becomes equal to its value before overload. The test results of the two studied steels can be presented as a dependency. from K / K,., where the crack deceleration time in case of overload to the level of fracture toughness

Neither K. The values of CIN are calculated by the formula K P-D Y / W-H, where P is the load; 1 - half the length of the crack; W is the width of the sample; H is the sample thickness. The maximum effect of overload is observed at K = 0.65 K for samples of steel X18H10T and K 0.75 Kj. for samples of steel H16N15MZB. Compared with the known method, the braking time is increased by 1.8 times for steel X18H10T and 2.1 times for steel X16H15MZB. So-

Editor V. Petrash

Compiled by A. Kulemin Tehred I. Veres

Order 5985

Circulation 550

VNIIPI USSR State Committee

for inventions and discoveries 113035, Moscow, Zh-35, Raushsk nab., 4/5

Production and printing company, Uzhgorod, st. Project, 4

By giving a stress state in the product at the moment of overload, which characterizes K, determined by means of samples, the longest crack retardation time is obtained. The load is then determined as follows:

sg

 K .R ,,,, / ,,,,

Thus, the method allows to produce the maximum possible delay in the growth of cracks in metal products, which increases their durability.

Claims (1)

Invention Formula The method of braking growing cracks in metal products, including the creation of a local deformed zone in front of the crack tip, by applying a static load to the product, coincides in direction with the working load, which is different in order to increase the durability of the product. by increasing the crack growth delay time, previously on the reference samples made of the same metal as the product, the dependence of the crack growth delay time on the stress intensity factor at static load, this dependence determines the value of the stress intensity factor K, which corresponds to the maximum crack growth delay time, and the static load to the product is determined by the dependence P K F st mgax . Y and A, F .. is the gross surface area of the product, perpendicular to the action of the working load; 1 „, d - half the length of the growing in cracked products; NEA function of crack length for a specific product. Corrector S.Cherni Subscription