Influence of the preparation technique of a stress raiser on the mechanical characteristics of...

I N F L U E N C E O F T H E P R E P A R A T I O N T E C H N I Q U E O F A

S T R E S S R A I S E R ON T H E M E C H A N I C A L C H A R A C T E R I S T I C S

O F S P E C I M E N S IN H I G H - S P E E D T E N S I L E T E S T S U N D E R

L O W - T E M P E R A T U R E C O N D I T I O N S

S. Y a . Y a r e m a , Z . G. D u t s y a k , Z. M. M a n y u k , a n d Yu . I . B a b e i

UDC 620.172.25:669.14

An analysis of the f rac tu re of par ts of machines and equipment at low tempera tures has shown that, as a rule, the f rac tu re s ta r t s at places of various fo rms of s t r e s s r a i s e r s [1]. There is not mere ly a local inc rease in s t r e s s e s around these s t r e s s r a i s e r s but a complex s t r e s sed state is also produced, which sup- p re s se s plastic deformation and embri t t les the metal. Under normal conditions (ordinary tempera tures , static loads), the r e s e r v e of plast ici ty in the mater ia l is usually sufficient to level out the s t r e ss peaks. The strength of specimens with s t r e s s r a i s e r s is not only not reduced but is even increased since owing to the absence of necking, the strength r e s e r v e s of the c ros s section can be more fully utilized [2, 3]. A reduction in the test t empera tu re leads to an inc rease in they ie ld s t r e ss and a reduction in the c r o s s - s e c - tional contradiction. Thus, the plast ici ty r e s e r v e of the mater ia l , in neutral izing the influence of s t r e ss r a i s e r s , is exhausted and the s trength of the mater ia l is reduced. At the present time, for many cons t ruc- tional mater ia l s , the t empera tu re dependence of the s trength of specimens with s t r e ss r a i s e r s has been established experimental ly [4]. However, little attention has been devoted to the technique of preparing the s t r e s s r a i s e r s , although it is just this that determines the physicomechanical state of the surface layers which play a leading par t under s t r e s s -concen t ra t ion conditions. The influence of the ra te of s t ress ing in the p resence of s t r e s s concentrat ion has also been inadequately investigated, although it is known that an inc rease in this rate , like a reduct ion in tempera ture , leads to an increase in the yield s t ress . The work descr ibed in this ar t ic le dealt with these problems.

E X P E R I M E N T A L M E T H O D

Cylindrical specimens in batches of five (d = 9.8 mm) were made f rom normal ized rods (D = 22 mm) of steel 35. An annular groove having a c ro s s section corresponding to the c ross section of a sc rew thread M10 • 1.5 (radius of curvature at the bottom 0.25 mm, depth 0.9, aper ture angle 60 ~ was applied by two methods, i.e., grinding and roll ing [5]. The theoret ical tensile s t r e ss concentration coefficient according to Neuber [6] for such a groove is 3.5. The specimens were tested to rupture on a machine [7] with a con- stant ra te of movement of the active grip which was adjustable in the range of 10-~-103 m m / s e c in a t e m - pera ture interval of 20 to -196~ [8]. During the application of tensile s t r e s s s t r e s s - t i m e and active grip movement - t ime record ings w e r e m a d e on a type 8SO-4 high-speed loop oscil lograph, and were subsequently converted to s t r e s s - s t ra in d iagrams. The tensile strength ~B, the upper yield s t r e ss aTU and lower yield s t r e s s CrTL were determined f rom the diagrams. The relat ive reduction in a rea r was calculated f rom the resu l t s of measurements of the specimen diameter at the bottom of the groove. The values of the mechan- ical proper t ies givenbelow are the mean values of tests on not less than three specimens.

R E S U L T S

The tensile s t r e s s d iagrams of specimens with ground-in grooves (GG specimens) and rolled grooves (RG specimens) differ f rom the d iagrams of plain specimens [8] in that yield s t r e ss "tooth" inflection in

Physicomechanical Institute of the Academy of Sciences of the Ukrainian SSR, L'vov. Transla ted f rom I>roblemy Prochnost i , No. 10, pp. 84-87, October, 1970. Original ar t ic le submitted February 26, 1970.

�9 1971 Consultants Bureau, a division o[ Plenum Publishing Corporation, 227 West 17th Street, New York, N. Y. 10011. All rights reserved. This article cannot be reproduced [or any purpose whatsoever without permission o[ the publisher. A copy o[ this article is available [rom the publisher for $15.00.

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o, kg/mm z ~176 kg/mm2

.. - /0

12o~\ ,~..,

8O

60 O0 50 ,o/,.A.o 30

20

, -200 -/50 -100 -50 0 t?C

Fig. 1

o B, kg/mm z

...-a--"'-'�82 __ _..o!

~,% 5O

4O 3O

3O 4,4

_ . - ~ o n o 20

. . . . . - ~ - . . - - - w . . . . . ~ /0

~ I i i I

6 I000 2500 6125 15312 6, kg/mm z. sec

Fig. 2 Fig. 1. Tempera tu re dependence of the mechanical proper t ies of plain specimens and specimens with a ground-in s t r e s s r a i s e r (A) and rol led s t r ess r a i s e r (B): 1, I) ~B (tensile strength); 4,IV) ~b (reduction in area); 5, V) ~ y u - ~ Y L (upper yield s t r e ss - l o w e r yield s t ress) for minimum s t ress ing ra te (6 kg /mm 2. sec) and maximum s t ress ing ra te (1.53.104 k g / m m 2. sec).

Fig. 2. Dependence of mechanical proper t ies of plain specimens and specimens with ground-in s t r e ss concent ra tors (A) and rolled s t r e s s concentra tors (B) on the loading ra te at 20~ 1) cr B (tensile strength); 2) ~YU (upper yield point); 3) cryL (lower yield point); 4) r (reduction in area) (shaded portion shows the size of the yield s t r e s s deflection).

them occurs only at high ra tes of loading or low tempera tures and the yield s t r e s s platform is sloping, the slope being less than the slope of the hardening zone. With decrease in tempera ture , the hardening zone disappears f rom the d iagrams and the lat ter assume a form typical of bri t t le f rac ture . This t ransi t ion is found in GG specimens in the range -120 to -150~ and for RG specimens in the range -100 to -120~ the lower l imit corresponding to the maximum s t ress ing ra te (1.5-104 k g / m m 2. sec), while the upper l imit corresponds to the minimum s t ress ing ra te (6 k g / m m 2. sec). The picture descr ibed is i l lustrated to some extent by the size of the yield point inflection a y u - - ~ y L shown in Figs. 1 and 2 (Cryu , ~YL are the upper and lower yield s t r e s ses , respectively).

The variat ion in static tensile strength ~B of specimens with decrease in tempera ture may be ex- plained by the usual relationship: the strength at f i rs t inc reases , attaining a maximum, and then falls (see Fig. 1). However, for this it is neces sa ry to suppose that the maximum for RG specimens is lower than -200~ since only GG specimens have a maximum in the investigated range. With maximum s t ress ing rate , the intensity of the inc rease in s trength increases and then the reduct ion in s trength increases .

The reduction in area r charac ter iz ing the capacity of the mater ia l for plastic deformation at the bottom of the s t r e ss r a i se r , diminishes smoothly with reduction in temperature . With inc rease in the s t ress ing rate , the value of ~. var ies insignificantly, except in the case of GG specimens, for which below -50~ ~b clear ly decreases . The value of r for specimens with rol led s t r ess r a i s e r s is always lower than for the specimens with ground-in s t r e s s r a i se r s .

The dependence of the mechanical proper t ies of specimens with s t r e s s r a i s e r s at a t empera tu re of 20~ on the s t ress ing ra te (see Fig. 2) is g rea te r than for plain specimens. The values of the tensile s trength and yield s t r e s ses increase in proport ion to the logari thm of the s t ress ing rate. The upper field s t r e ss aYU is the more sensit ive to s t ress ing rate. The relat ive reduction in area var ies little while for RG specimens it falls somewhat w i t h i n c r e a s e in s t r e ss ing rate. With dec rease in test tempera ture , the sens i - tivity of the specimens to s t ress ing ra te increases , attaining a maximum in the range -100 to -120~ (see Fig. 1).

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K, K d, Ktech

u

/.0

-2oo -]50 -1oo. -5o Q t?c Fig. 3 Fig. 4

Fig. 3. F r ac tu r e s of specimens with ground-in s t r e ss r a i s e r after testing at 20~ with a s t ress ing ra te of 2.5.103 k g / m m 2 �9 see (a) and at a t empera - ture of -40~ with a s t r e s s ing ra te of 1.53.104 k g / m m 2 �9 sec (b).

Fig. 4. Tempera tu re dependence of re lat ive strength k (1, I) dynamic s t r ess ing coefficient 1%l (2) and the coefficient of the influence of prepa- rat ion technique ktech (3, III) of specimens with ground-in (A) and rol led (B) s t r e s s concent ra tors (arabic numera ls - static stressi~ig, roman numera ls - h igh- ra te s t ress ing) .

�9 In flat f r ac tu res of specimens with s t r e ss r a i s e r s , two zones are to be seen (Fig. 3); an outer fibrous zone, evidence of tough f rac ture , and an inner crysta l l ine zone, corresponding to brit t le f racture . With increase in the s t r e ss ing ra te and especial ly with decrease in temperature , the crysta l l ine zone increases . The intensity of its increase is g rea te r in RG specimens, in the f rac tures of which only one zone is ob- served at t empera tu res below -40~ while for GG specimens, the disappearance of the fibrous zone com- mences at a t empera tu re o f - 7 0 ~

D I S C U S S I O N

The influence of s t r e s s r a i s e r s on the strength proper t ies at different s t ress ing ra tes may be char - ac ter ized by its re la t ive s t rength k, determined as the ra t io of the tensile strength of the specimen with a s t r e s s r a i s e r CrBR and the plain specimen aB (k = aBR/ORB), tested under identical conditions. This value (Fig. 4) var ies insignificantly (in static tests it inc reases , and in dynamic tests it decreases) ; with fur ther dec rease in the tempera ture , however, it rapidly increases , reaching a maximum at -70~ Com- mencing with a t empera tu re of -120~ the variat ion of k depends substantially on the technique of s t r e s s r a i s e r preparat ion. For GG specimens it continues to decrease rapidly (curves 1A, IA); for RG concen- t r a to r s for the maximum s t r e s s ing rate (curve IB) this decrease is smoother , for static s t ress ing ra tes (curve 1B), on the contrary , an increase in strength is found. The strength of RG specimens throughout the entire t empera tu re range investigated and the s t rength of GG specimens in the range - 2 0 to -160~ is higher than the strength of plain specimens (k > 1).

The dependence of the s t rength of specimens with a s t r e ss r a i s e r on s t ress ing ra te was determined by the variat ion in the dynamic coefficient kd, i.e., the rat io of the yield s t r e s se s of identical specimens tested at the maximum s t r e s s ing ra te (1.53.104 k g / m m 2 �9 sec) and minimum s t ress ing ra te (6 kg /mm 2. sec). As can be seen (see Fig. 4, curves 2A, 2B), with increase in the s t ress ing rate , the s trength increases (k d > 1), and only at very low tempera tu res (t < -190~ does it fall. The s t ress ing ra te has its maximum influence in the range --100 to -130~ in which kd(t ) reaches a maximum. RG specimens are more .sen- sit ive to a variat ion in s t r e s s ing ra te than GG specimens.

The influence of the technique of prepar ing the s t r e s s r a i s e r was a s sessed by means of the coefficient ktech, the ra t io of the yield s t r e s s e s of specimens having a ground-in groove and those having a rolled groove for identical test conditions. The static s trength of the specimens (see Fig. 4, curve 3) down to a t empera tu re o f - 1 2 0 ~ is pract ical ly independent of the preparat ion technique of the s t r e s s ra i se r , after which there is a sharp decrease in the strength of GG specimens compared with the s trength of RG speci- mens (ktech < 1). At high s t r e s s ing ra tes (curve III), the super ior i ty of RG specimens is apparent even

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at 20~ and the rapid fall in ktech begins ear l ie r (at -100~ At a tempera ture o f -196~ rol l ing the groove increases the strength of the specimens almost twice compared with grinding the groove.

The considerable difference in the values and charac te r of the variat ion in the tensile strength of specimens having ground-in and rolled s t ress r a i s e r s at low tempera tures is evidently due to the dif fer- ence in the s t ruc ture and mechanical condition of the metal. The metal on the bottom of the groove was investigated by means of microhardness measurements and also by x - r a y s t ruc ture analysis (using the URS-55 apparatus and back reflection method) for determining residual s t r e s s e s and dislocation density.

After t reatment by grinding, no variation in mic rohardness was found, and residual tensile s t r e s se s appeared in a very thin layer on the groove bottom. The bottom of the rol led groove showed quite a different picture. At a distance of 0.04 mm f rom the surface, the microhardness was higher by a factor of 1.8; residual compress ive s t r e s ses were found; the level of s t r e s se s of the second kind increased by a factor of 2.5, the dislocation density increased by a factor of 5, and the dimensions of the c rys ta l blocks de- creased considerably compared with the ground-in groove. In addition, the metal in the rol led groove was compacted, the grains were elongated along the profile of the bottom, and small sc ra tches caused by ab ra - sive were absent f rom the surfaces . All this occur red in a thin surface layer of a thickness of less than 0.5 ram.

The changes enumerated above led to an improvement in the strength proper t ies of the mater ia l in the surface layer of the rolled groove. It still remains obscure, however, why this improvement should appear only at t empera tures below -100~ An analysis made by G. V. Uzhik [1, 9] of the s t r e s sed condition in the elastoplastic deformation stage in the c ross section of a specimen weakened by a s t r e s s r a i s e r shows that the maximum tensile s t r e s ses occur at some distance f rom the tip of the s t r e s s ra i se r . This distance is greater , the more plastic is the mater ia l , other conditions being the same. Consequently, under normal testing conditions, the s t r e s s peaks are located in the region of the metal which has not been subjected to the t reatment technique; only with a reduction in the capacity of the mater ia l for plastic deformation down to some value, to which there corresponds a definite low tempera ture , do the s t r e s s peaks move into the s u r - face layer. If we assume that f rac ture (starting of a crack) commences where the s t r e s ses are a maximum, it becomes clear why, down to a certain tempera ture (in our case to -100~ the s trength of the specimens depends little on the t rea tment technique, and only below this tempera ture does the influence of that tech- nique become vital. Special investigations are essential for fully disclosing the mechanism of the influence of the technique employed for prepar ing the s t ress r a i se r .

1.

2. 3. 4.

5. 6. 7. 8. 9.

L I T E R A T U R E C I T E D

G. V. Uzhik, The Strength and Plast ici ty of Metals at Low Tempera tu res [in Russian], Izd. Akad. Nauk SSSR, Moscow (1957). Ya. B. Fridman, Mechanical Proper t ies of Metals [in Russian], Oborongiz, Moscow (1952). A. Nadai, Plast ici ty and the F rac tu re of Solids [Russian translation], IL, Moscow (1954). P. F. Koshelev and S. E. Belyaev, Strength and Plast ici ty of Constructional Materials at Low T e m - pera tures [in Russian], Mashinostroenie, Moscow (1967). Yu. I. Babei and Z. G. Dutsyak, FKhMM, No. 4 (1967). G. Neuber, Stress Concentration [Russian translation], Gostekhizdat, Moscow (1957). V. V. Panasyuk, S. Ya. Yarema, et al., FKhMM, No. 4 (1967). S. Ya. Yarema and M. Z. Manyuk, FKhMM, No. 2 (1970). G. V. Uzhik, Resis tance to Rupture and the Strength of Metals [in Russian], Izd. Akad. Nauk SSSR, Mos c o w - Leningrad ( 1950).

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