Average Integral Residual Stresses Research Articles

AN INFLUENCE OF ROLLER STRENGTHENING ON MULTCYCLIC FATIGUE OF SPECIMENS WITH CUTS OF VARIOUS DEPTH AND A PRESSURIZED HUB

The influence of roller strengthening on a residual stresses distribution and multicyclic fatigue of solid cylindrical specimens of 50 mm diameter made of steel 20 with cuts of various depths and a pressurized hub has been examined. As a result, an endurance limit of roller strengthened specimens diminishes. In order to preserve the effect of hardening with a cuts depth increase, its necessary to increase the thickness of a layer with compressive residual stresses. Surface hardening influence on an endurance limit of specimens has been estimated by two criteria: axial residual stresses on a concentrators surface and average integral residual stresses, calculated through the dangerous sections surface layer thickness that equal a critical depth of a non-propagating fatigue crack. Its been stated that its more reasonable to use the average integral residual stresses criterion for an evaluation of roller strengthening influence on an endurance limit of specimens with cuts of various depth and a pressurized hub.

AN EVALUATION OF PARTS’ FATIGUE RESISTANCE AFTER SURFACE THERMO CHEMICAL MACHINING

In order to raise a fatigue resistance the calculation-experimental method of parts surface thermo chemical machining efficiency has been carried out. The employment of two types of reference specimens in experiments has been substantiated. The employment of smooth reference specimens for a determination of residual stresses in a surface layer and of corset reference specimens for fatigue tests has been recommended. The results of experimental study of two types of thermo chemical machining (cementation and nitriding) influence on a fatigue resistance of corset specimens made of high-strength allows ВКС-5 и ВНС-17 have been used in the work. . The calculations of the study have been carried out by the Finite Elements Modeling method using the calculation complexes ANSYS and PATRAN/NASTRAN. The residual strained state modeling is carried out by thermo elasticity method. On grounds of conducted calculation its been stated that an axial residual stresses distribution in a dangerous (smallest) section of a corset specimen doesnt practically differ from a similar distribution in smooth specimens. An endurance limit increment due to corset specimens hardening was determined with use of the average integral residual stresses criterion. Carried out results let also maintain that the optimal regimes of thermo chemical machining are the regimes under which average integral residual stresses are maximum.

THE INFLUENCE OF CUT’S DEPTH ON RESIDUAL STRESSES DISTRIBUTION AND AN ENDURANCE LIMIT OF SPECIMENS UNDER SURFACE HARDENING

The influence of cuts depth on residual stresses distribution and an endurance limit under bending of hollow cylindrical specimens made of steel 20 with external diameter 50 mm and interior diameter 40 mm after outstripping superficial plastic deforming by roller strengthening of two regimes has been examined. Its been stated that the value of compressive residual stresses in a dangerous section of specimens decreases on cuts depth increase. As a result, an endurance limit increment of hardened specimens with cuts diminishes. In order to preserve the effect of hardening by increasing the endurance limit under outstripping superficial plastic deforming with an increase in a cuts depth, it is necessary to increase the thickness of a smooth part with compressive residual stresses layer. Its been shown that in order to estimate the increase in the endurance limit of hardened specimens, one should use not residual stresses at the bottom of a cut, but average integral residual stresses through the parts dangerous section surface layer thickness that equal a critical depth of a non-propagating fatigue crack.

THE INFLUENCE OF HYDRO BLASTING ON THE EN-DURANCE LIMIT OF SPECIMENS WITH A CUT UNDER BENDING AND STRETCHING-COMPRESSION

The hydro blasting influence on an endurance limit of cylindrical specimens with circular cuts of a semicircular profile of 0,3 mm radius under bending and stretching-compression in a case of symmetric cycle has been examined. Specimens of 10 mm diameter for fatigue tests were made of steels 30ХГСА, 12Х18Н10Т, ЭИ961,45 and aluminum alloys В93, Д16Т.The influence of surface hardening on an endurance limit of specimens with cuts was valuated by two criteria: residual stresses on the concentrator surface and the average integral residual stresses calculated through the parts dangerous section surface layer thickness equal the critical depth of a non-propagating fatigue crack. 
 On results of residual stresses and specimens with cuts endurance limits determination its been stated that coefficients of surface hardening influence on an endurance limit increase under bending and stretching-compression by two criteria are approximately equal. Besides, the coefficient of the influence by the average integral residual stresses criterion changes within essentially lesser limits than the coefficient of the influence by the residual stresses on the concentrator surface criterion. So the average integral residual stresses criterion should be used for the evaluation of a surface hardening influence on an endurance limit of parts with concentrators.
 On an example of specimens within and without peening in a dangerous section its been shown that peening doesnt influence the endurance limit of a surface hardened part. The compressive residual stresses only influence the endurance limit of these parts.

THE ESTIMATION OF ROLLER STRENGTHENING INFLUENCE ON THE ENDURANCE LIMIT OF SHAFTS WITH PRESSURIZED HUB

The influence of roller strengthening on endurance limit under bending of the 25 mm and 50 mm diameter shafts made of steel 20 and 25 mm diameter shafts made of steel 40X with a pressurized hub has been examined. Its been stated that the endurance limit of roller strengthened shafts with a pressurized hub depends not only on a value of compressive residual stresses in their dangerous section but on the character of its distribution. The valuation of a surface hardening influence on the endurance limit of shafts by the surface residual stresses criterion and the average integral residual stresses criterion has been shown that the average integral residual stresses criterion calculated through the parts dangerous section surface layer thickness equal the critical depth of the non-propagating fatigue crack can be recommended for the hardened shafts with a pressurized hub endurance limit increase prediction. Its been shown that on a diameter of a shaft with a pressurized hub raising it is necessary to increase a thickness of a hardened surface layer with compressive residual stresses.

The influence of initial radial deformations on the residual stresses distribution in the hardened layer

The study of the initial radial deformations influence on the residual strained state components distribution through the hardened layer thickness under various types of surface hardening has been conducted. Chemical thermal machining (CTM) and shot blasting (SB) have been examined on the example of smooth solid steel cylindrical specimens with diameters D=10 mm and D=25 mm under a linear distribution of initial deformations through the hardened layer thickness (the maximum value is on the surface and zero on the maximum depth of hardening). The calculation has been conducted with use of the program complex PATRAN/NASTRAN. The solution has been carried out in axial symmetric positing by thermo elasticity method. On study results its been stated that the radial stresses through the hardened layer thickness under CTM and SB are positive. Incidentally the radial component of initial deformations under chemical thermal machining and shot blasting doesnt exercise essential influence on the radial and axial residual stresses distribution through the hardened layer thickness. Hence the initial radial deformations havent much significance at the evaluation of the residual strained stress state influence on the hardened parts endurance limit increment calculated by the average integral residual stresses criterion.

Dependence of the limit endurance of specimens with notches from the depth of hardened surface layer

It is shown that, growth of the layer with compressive residual stresses at advanced surface plastic deformation of smooth samples leads to an increase in the endurance limit of the notched specimens due to an increase in the compressive residual stresses in their dangerous section. The study was conducted on the specimens with outer diameterDequal to 10 and 25 mm and on a 25 mm specimen with inner diameterd= 15 mm which were subjected to air shot blasting and roll-in. Residual stresses in smooth specimens were determined by the mechanical method. Circular incisions (R= 0.3 and 0.5 mm) were applied on hardened smooth samples with a shaped cutter. Residual stresses in notched specimens were determined by a numerical method calculating the redistribution of the residual stresses upon the incision. As the diameter of the samples increases with the same strengthening surface treatment,the depth of the layer with the compressive residual stresses increases due to an increase in the rigidity of the samples. The effect of surface hardening on the endurance limit of notched specimens was evaluated according to the criterion of the average integral residual stresses, which takes into account both the magnitude and completeness of the residual stresses. This criterion was calculated from the depth of the non-propagating fatigue crack that occurs in the surface layer in the dangerous section of the specimen during the fatigue test at the limit endurance. Fatigue tests are carried out in symmetrical cycle, the test base being 3 × 106cycles of loading. Estimation of the increment of the limit endurance of surface-hardened cylindrical specimens with semicircular notches gave the results acceptable for practice. The use of the proposed method can significantly reduce long-term and expensive fatigue tests.

Prediction of endurance limit of hardened parts with account of operational factors

Fracture endurance limit is one of the main parameters used in calculating the strength of surface hardened parts. The influence of various operational factors along with hardening should be taken into account in predicting this parameter. Prediction of the endurance limit under stress concentration was carried out according to the criterion that takes into account the influence of residual stresses on the surface of the dangerous section of a hardened part, as well as the criterion of the average integral residual stresses over the thickness of the hardened surface layer equal to the critical depth of a non-propagating fatigue crack. The effect of surface hardening on the endurance limit was analyzed taking into account such operational factors as the type of deformation, working temperature, the asymmetry of the loading cycle of specimens made of steels and aluminum alloys. Fatigue tests of cylindrical specimens with circular notches of a semicircular profile were carried out under torsion, bending and tension-and-compression. It was established that the use of the criterion of the average integral residual stresses is quite a good indicator of the effect of surface hardening on the fracture endurance limit taking into account the operational factors studied.

Residual stresses and fatigue resistance of toothed wheels

Residual stresses in the dangerous section of toothed wheels made of materials and according to practices accepted in aviation engine construction have been determined using a mechanical method. The influence of the technology of manufacturing of toothed wheels, thermo-chemical treatment, hardening and coating on the residual stresses has been examined. Current technologies of manufacturing toothed wheels lead to significant dispersion of residual stresses, especially after thermo-chemical treatment. Therefore, the results have been statistically processed for each batch of parts and the average values are shown on the residual stress diagrams. We demonstrate the possibility of determining residual stress distribution in the dangerous section of a wheel tooth by the initial deformations of the reference specimen, which makes it possible to keep the part under examination unimpaired. A plate made of the same material as the toothed wheel and hardened simultaneously with the part under examination was used as a reference specimen. The influence of residual stresses on the fatigue resistance of toothed wheels by the average integral residual stresses criterion that takes into account both the value of residual stresses and the nature of their distribution in the dangerous section of toothed wheels was examined.

Relation between characyeristics of low-cycle and high-cycle fatigue of threaded parts and residual stresses

The dependence of low-cycle and high-cycle fatigue characteristics of threaded parts on the criterion of average integral residual stresses is examined in the article. These characteristics play an essential role in ensuring reliability of aeronautical equipment. The study was carried out using titanium-alloy M6 bolts with various ways of producing residual stresses. In all the cases the dependences are quadratic. Further analysis showed linear dependence between the increment of cycle life and the limiting cycle amplitude under changes of residual stresses. This conclusion holds for all examined ways of producing residual stresses. The increments mentioned are linear functions of the criterion of average integral residual stresses. A procedure for determining the increment of the limiting cycle amplitude with the use of the criterion of average integral stresses is presented. Special attention is given to the method of finding the coefficient of residual stress influence on the limiting cycle amplitude for the constant mean stress. The approach to determining the increment of cycle life by a known value of the increment of the limiting cycle amplitude calculated with the use of the criterion of average integral residual stresses is validated.

Analysis of the mechanism of fatigue crack arrest in a cylindrical notched specimen

The phenomenon of fatigue crack arrest in surface-hardened cylindrical specimens with stress concentrators is examined from the perspective of linear fracture mechanics. The stress intensity coefficient (SIC) is considered as the criterion, its scope determines the speed of fatigue crack growth in the case of cyclic loading. The calculations of the study were carried out by the Finite Elements Modeling method using the calculation complex ANSYS. The calculation complex ANSYS that uses the Finite Elements Modeling method in the form of displacements was applied to determine the stress intensity coefficient under bending of cylindrical surface-hardened and non- hardened specimens with the diameter of 10 mm with a semicircular notch with the radius of 0,5 mm. It is established that the stress intensity coefficient has two extremes in the initial area of the fatigue crack development: the minimum and maximum values. The results of the calculation indicate that the minimum value of SIC is lower than its threshold value below which the crack does not develop in the case of a cylindrical surface-hardened notched specimen. This fact is in good agreement with the results of tests of these specimens as non-propagating fatigue cracks were revealed in these specimens only. It is also established that the depth of a crack is approximately 0,02 of the minimum size of the section of a cylindrical specimen for the maximum value of the stress intensity coefficient. The study carried out validates the use of the criterion of average integral residual stresses to calculate the increase of the endurance limit of surface-hardened specimens and parts with stress concentrators due to compressive residual stresses.

Selection of reference specimens for assessing the efficiency of part surface nitriding

The use of corset reference specimens for carrying out fatigue tests and smooth reference specimens for determining residual stresses in a surface layer to assess the sufficiency of nitriding (thermochemical processing) is substantiated in the paper. The calculations are carried out by finite element simulation using the ANSYS and PATRAN/NASTRAN software complexes. The ANSYS complex is used to determine the stress concentration factor for the case of corset specimen bending. The employed models with harmonic axisymmetric finite elements make it possible to find the stress concentration factors in the case of tension- and- compression and bending of body-of-rotation parts. The obtained value of the influence of the coefficient of residual stresses on the endurance limit increase is in quite good agreement with its experimental values for nitriding and other types of thermochemical treatment of corset specimens. The PATRAN/NASTRAN complex is used to calculate the stress-strain state in corset specimens depending on residual stresses arising on a part surface under nitriding. Modeling is performed by the thermoelasticity method, with experimental diagrams of residual stresses in smooth specimens used as initial data. It is established that the distribution of axial residual stresses in the dangerous (smallest) section of a corset specimen practically does not differ from the distribution in smooth specimens. The results obtained provide the basis for determining average integral residual stresses by the distribution of residual stresses in smooth specimens and using them to calculate the endurance limit for hardened corset specimens. The results of the study allow making a conclusion that the most optimal nitriding practices are those wherein the average integral residual stresses are maximal.

Influence of crack depth on the stress intensity coefficient in notched and smooth plates

The paper presents an analysis of variations in the stress intensity coefficient (SIC) in plates with different sizes of the cross-section, with and without concentrators. We also analyze the dependence of the SIC maximal value on the size of the smallest section of a plate with stress concentrator. SIC of smooth specimens and specimens with stress concentrator in the form of a semicircular notch are calculated using the ANSYS finite- element analysis program system. The static elastic problem is solved, with the strained state taken to be biaxial. The material of the plate is considered to be isotropic. The plates under consideration with a concentrator in the initial area of the crack size increase show maximum stress intensity coefficient. In plates without concentrator the SIC changes smoothly over the whole area of the crack size increase. It is established that the crack size is equal to 0.02 of the plate smallest size at the maximal value of the stress intensity coefficient. The study conducted confirms the feasibility of using the criterion of average integral residual stresses to calculate the increase of the endurance limit of surface- hardened specimens and parts with concentrators due to compressive residual stresses.

Estimation of residual stress influence on the low-cycle fatigue of threaded parts

The possibility of using the criterion of average integral residual stresses to estimate the cyclic life of threaded parts in the area of low-cycle fatigue is examined in the article. The peculiarities of fracture under high-cycle and low-cycle stresses are pointed out. The problems of using equations and criteria of linear mechanics in the low-cycle fatigue area are specified. These problems are mostly related to the origination of plastic deformation regions; the sizes of the latter may be significantly greater than those of the cracks. Technological factors affecting the origination and distribution of residual stresses are considered. It is noted that the diameter of the workpiece for thread rolling, heat treatment, the force and time of rolling are of the greatest importance. The connection between the number of cycles prior to fracture and the residual stress value, expressed in terms of the average integral residual stress criterion under the influence of different technological factors is illustrated. According to the results of low-cycle fatigue tests of threaded parts it has been shown that the dependence of the number of cycles prior to fracture on the value of the average integral residual stress criterion is approximated closely by equations of the quadratic form. The appropriate magnitudes of approximation reliability are not lower than 0.9163.

Assessing the influence of surface hardening on the endurance limit of specimens made of steel 40X using residual stresses of reference specimen

The possibility of the employment of reference specimens hardened simultaneously with the parts has been examined for the hydroblasting influence estimation on the parts with concentrators endurance limit on the example of hollow cylindrical parts made of steel 40 with circular cuts of semicircular profile with the radius 0,3 mm. The investigation has shown that the difference between residual stresses values found by calculating and experimental methods doesnt exceed 8% in smooth specimens, in specimens with cuts 6%. Under the hydroblasting influence on the endurance limit estimation in conditions of bending in the case of symmetric cycle for hollow cylindrical specimens made of steel 40Х with cut of radius 0,3 mm its been established that the employment of residual stresses distribution calculated by the initial deformations of reference specimen let predict the specimens endurance limit increase under stress concentration conditions to an approximation of 8%. It has been shown that its reasonable to use the average integral residual stresses criterion calculated through the dangerous section surface layer thickness equal the non-propagating fatigue crack critical depth for the prediction of the endurance limit increase of hardened by hydroblasting specimens made of steel 40 with stress concentrators.

Forecasting an increment in the parts’ endurance limit due to surface hardening

Rolling with various forces influence on the endurance limit of specimens made of steel 20 of 50 mm diameter with cuts of various radiuses and pressurized bush has been examined. The estimation of the surface hardening influence on the specimens endurance limit has been carried out using two criterions, namely: residual stresses on the dangerous section surface and the average integral residual stresses through the dangerous section thickness, equal the fatigue non-propagating crack critical depth. On base of carried out experiments its been established that the employment of the average integral residual stresses criterion is more rightful for the endurance limit prediction of surface hardened cylindrical specimens made of steel 20 with diameter 50 mm with cuts and pressurized bush taking into account the concentration degree. The residual stresses on the concentrator surface is not acceptable as a criterion for the endurance limit prediction of surface hardened specimens made of steel 20 with cuts and pressurized bush because the corresponding coefficient of the criterion influence on the endurance limit has a essential dispersion. Besides, the dependence of the fatigue non-propagating crack critical depth on the specimen (part) dangerous cross-section sizes has been confirmed.

Dependence of the endurance limit on the character of residual stresses distribution in the surface layer of hardened parts

The influence of the character of compressive residual stress distribution on the endurance limit of surfacehardened specimens made of various steels and alloys under bending has been examined. Specimens madeof ВНС40 and 38Х2МЮА steels with V-shaped notches were hardened by nitriding, foundation specimensmade of ВКС-5 alloy by cementation and ionic cementation, flat specimens made of ЭИ698ВД alloy by airshot-blasting. The influence of surface hardening on the endurance limit was assessed using two criterions,namely: residual stresses on the dangerous section surface and the average integral residual stresses. It was establishedthat the endurance limit of surface-hardened specimens (parts) is determined not by the value of compressiveresidual stresses on the surface but by the completeness of the diagram of the residual stress over the partsurface layer thickness on condition that the surface layer thickness is equal to the critical depth of the nonpropagatingfatigue crack. The average integral residual stress criterion calculated over the thickness of a surfacelayer of the dangerous section equal to the critical depth of the non-propagating fatigue crack gives a rather accurateestimation of the influence the character of compressive residual stress distribution has on the endurancelimit of hardened specimens (parts). In practice the estimation of the increment of surface- hardened specimens(parts) endurance limit by the criterion of residual stresses on the dangerous section surface may lead to a significanterror.

Forecasting the endurance limit of surface-hardened cylindrical parts made of steel 20 by the initial deformations of reference specimen

The possibility of forecasting the increment of endurance limit for solid and hollow cylindrical parts made of steel 20 with semicircular notches after outstripping superficial plastic deforming has been analyzed using the residual stresses (initial deformations) of a reference specimen hardened simultaneously with the part . It is established that the compressive residual stresses calculated in smooth parts by the initial deformations of the reference specimen differ insignificantly (less than 6%) from those found experimentally. The increments of endurance limits of hardened notched parts have been calculated by a well-known method using the average integral residual stresses criterion. Its established that the design values of endurance limit increments differ from the experimentally found ones by no more than 15%.