ing stirrers (FD; Fb, Z0), as' these govern the hydrodynamic setting at the walls and enable one to provide the required rates and product quality. The working results for apparatus of other designs will not differ basically from those given here. LITERATURE CITED i. 2. 3.

GOST 24000-80: Enameled Apparatus Containing Mechanical Mixing Devices: Types, Basic Parameters, and Dimensions. OST 26-01-104-78: Mixing Stirrer Devices for Enameled Equipments: Types, Parameters, and Basic Dimensions. L. N. Briginskii, V. I. Begachev, and V. M. Barabash, "Liquid mixers," in: Physical Principles and Engineering Methods in Calculations [in Russian], Khimiya, Leningrad

(1984). 4.

5. 6. 7.

RD 26-01-90-85: Mechanical Mixing Devices: Calculation Methods. "A hydrodynamic contactless propellor," Information Letter, Series Protecting Air and Water from Pollution [in Russian], KhTsNTI, Kharkov (1976). A. S. Ginevskii, Turbulent-Jet Theory: Integral Calculation Methods [in Russian], Mashinostroenie, Moscow (1969). E. Marshall, Biophysical Chemistry: Principles, Techniques, and Applications, u 1 [Russian translation], Mir, Moscow (1981).

METHOD OF ANALYZING FACTORS DETERMINING THE FAILURE OF WELDED STRUCTURES UDC 624.014.25

V. S. Milanchev

According to the materials of the International Welding Institute all external and internal factors, determining the failure of welded structures (structural, metallurgical, technological, and operational) can be reduced to three: stress concentrators, level of tensile stress, and resource of metal plasticity. This conclusion has been obtained by the methods of qualitative statistics without establishing the quantitative relations between these factors. The nonuniformity of the structure and properties of metal, their own stresses, and the great variety of stress concentrators of various origin and type (general, local, and microscopic) cause the complexity of calculation assessment of the stressed state. Here the redistribution of stresses and strains in actual welded structures are so complicated that for their quantitative assessment, taking into account all outer and inner factors, at present no analytical nor approximate calculation methods are available. On the other hand, for the assessment of factors determining the failure of welded structures, it is necessary to carry out a quantitative analysis. In this paper for the assessment of the strength of welded structure elements working under static load an equation is used which has been obtained in [i] by an analysis of the stressed-strained state of plates from a perfect elastic-plastic material with a stress concentrator according to which the conditions of crack initiation in tension are linked with the following relation

~=(~e +~p)EC~,

(i)

where o r is the nominal tensile stress during crack initiation; Ce is the elastic component of the relative strain, Ep = in [I/(I - ~)] is the maximum relative plastic strain in the concentrator center (source of metal plasticity); ~ is the relative contraction of the metal during break; E is the elasticity modulus; C t = I/Kt is the theoretical coefficient of the specimen quality (element); K t is the theoretical coefficient of stress concentration; in the case of coincidence of local and general stress concentrators K t = Ks KZ, Kg are the local Translated from Khimicheskoe i Neftyanoe Mashinostroenie, No. 2, pp. 7-9, February, 1991.

0009-2355/91/0102-0059512.50

9 1991 Plenum Publishing Corporation

59

Ou, MP_a ~O00

30O0 20O0

1000

Ou, MPa

7000

Or,

MPa 3000

2000

1000

o 9 0

i 10

0,2 i ZO

0,~ 0,6 i ,,, I I 30 O0 50

0,8

~,0 ; 60

Ep i r176

Fig " i. Effect of various factors on ep , C a n d o r : A, 9 9 ~ are e x perimental data; .... is the calculation; a) C t = 0.017 is the crack in the low-carbon steel plate after aging; C t = 0.08 to 0.4 is the notch in a plane 300M steel specimen at test temperatures of 77 (I), 200 (II), and 293 K (III); C t ~ i is the dislocation in the threadlike iron crystal when the specimen diameter changes from i to 30 ~m; b) i) the content of pores in 40Kh steel (0.5-1.5%) [5]; 2) the content of nonmetallic inclusions (0.1-0.5%) [5]; 3) the hydrogen content 0.5-1.5 cm3/100 g [6]; 4, 5) 65-100 mm thick sheet of 09G2S steel in tests across and along the sheet thickness correspondingly [7]; 6) the low-cycle metal fatigue near the weld zone of the welded joint of St3 steel in the case of up to 2000 cycles [8]; c) i) carbon content of 0 to 2.7% [5]; 2) annealing temperature up to 600~ after hardening of 40Kh steel [511 3) electrode weld metal UONI-13/45 (C r = 0.022), UONI-13/65 (C r = 0.03), UONI-13/85 (C r = 0.04), NAIT-3 after hardening by annealing (C r = 0.09); 4) cooling rate of 42Kh2GSNM steel in welding from 0.i to 600~ [9]; 5) thermomechanical processing of high-carbon steel [I0]. Arrows indicate the growth direction of characteristics. and general stress concentration coefficients respectively; m = 2/(n + i); n is the indicator of the metal deformation strengthening stage (for structural steels n = 0.25 at ~ > 50%; n = 0.15 at ~ = 30 to 50% and n = 0.05 at ~ < 30%). Since for structural steels the elastic component of strain in the concentrator center is small compared with the plastic component, it can be neglected. Then c r can be represented as a function of two variables ep and Ct: ~r = epECt m. In tensile testing of smooth plastic metal specimens

"'=~u =~pECL

(2)

where ou is the tensile strength of metal; C r = I/K r is the theoretical coefficient of metal quality (indicates structural perfection); K r is the critical value of K t for the given metal (can be determined from the results of testing for tension the metal of smooth standard specimens). Equation (2) permits one to use numerous results of tests of metal for tension in various conditions.

66

Taking into account the identical (geometrical) nature of C r and C t the general case can be written as

~

(3)

where C is the theoretical coefficient of quality reflecting the shape, the macro- and microstructure of the specimen or element. At a constant C value for a plastic metal relation (i) takes the following form: o,= apEC"'=A%. where A is a constant. Relation (3) can be represented as a diagram (Fig. i). For analyzing the dependence of ep, C, and ar on various factors: structural (notch, element thickness), technological (presence of pores, nonmetallic inclusions, gases), metallurgical (carbon content, alloying, thermal and thermomechanical processing, reaction of steel on the thermal cycle of welding, directed crystallization), operational (recooling, low-cycle fatigue, aging) the experimental data of Soviet and foreign scientists are used. It also represents the limiting range of stress concentrators (from crack to dislocation). The linear dependence of o r on Sp for a constant value of C t is confimrmed for example in [2] in testing low-carbon steel plates with a 10 mm long crack. Here the value of Or min is practically zero (Fig. la). Figure la also shows the results of testing plane 300M steel specimens with a notch [3]. The dependence of o r on ep at a constant C value can in this case also be characterized as being linear. As the strength of metal increases a big role is played by the elastic component of strain which for the threadlike iron crystal with the removal of dislocations reaches the maximum value: (ge + ep) = 0.048. The plastic component in this case is hardly noticeable. The dependence of the threadlike iron crystal on the specimen diameter has a hyperbolic nature [4] as also the dependence of o r on K t of any other specimens. Here o r changes from 200 to 13,400 MPa approaching the theoretical value of the strength of iron. Within the range C = 1 we obtain a linear relation of the following form: ~

e E.

The results given in Fig. 1 show that the values of C r obtained in tensile testing of smooth standard specimens and the values of C t obtained in testing specimens with a notch and calculation from the Neuber equation reveal a surprising agreement with the values calculated from Eq. (i). For C t = C r the stress concentrators in smooth and notched specimens are equivalent. The results show that different factors affect variously the criteria ~p, C, and ar. Figure 2 shows the scale of C values for individual welding defects plotted on the basis of data of [ii]. With decreasing s the metal sensitivity to stress concentrators increases, in brittle state (gp < 0.05) the metal is sensitive even to dislocations. At s < 0.25 metal is sensitive to all technological defects, therefore in order to avoid the causes of crack formation in the production of welded structures the metal must have a plasticity of Ep > 0.25 as shown, for instance in [9]. In Fig. ic hyperbola 5 is the curve of metal plasticity maximum values gpmax for each strength level and reflects the maximum work of crack initiation. It carries points characterizing the limiting structural state of the metal from normal to threadlike iron crystal (for the ordinary iron crystal au = 160 MPa, and ~ = i00%). Together with the axes the hyperbola coordinate limits the region of the possible critical state of the metal during failure under any conditions. On the graph ep, C, and Sr change actually from zero to the limiting values. The graph makes it possible to determine any value of the three criteria from the two known values. Any point of the graph characterizes the critical state of the specimen (element), depending on numerous external and internal factors. The graph can be used for determining in the first approximation the state of the element with any stress concentrator.

61

N,*/.

at, l,l P a

if zoea' l I I

/ " I l/YI //.r' I /L':-.'I .,,/

\\/ ' /" " "'

" ///I -.~C ~"

,.o.

o,., 0

o

o,e

o,~

o,a

o,s

&o

~p

Fig. 2

30

38

06

Fig. 3

Fig. 2. Graph of E--C-or: A) dislocations; B) pores; C) slags; D) welding failures and cracks; E) cuts; F) weld shape; G) notches [II]. Fig. 3. Statistical data [13] on the cases of failure of main tubes IN) the number of cases of failure (total number of failures)-100%]. It is known that however well a weld has been made, it has separate pores and slag inclusions, i.e., the theoretical quality coefficient is C t = 0.17 (Kt = 6). If in this case e D equals approximately 0.25 then the nominalstress of crack formation is about 2000 MPa (point-M, Fig. 2). According to [12] this value is the upper limit for welded joints, which is confirmed by experience in the manufacture of high-strength steel thin-walled vessels, whose use required a high quality of manufacture (use in the structure of only direct joints, special assembly devices, only automatic welding, and general thermal treatment of the product). For most welded structures (which include angle joints, combination of general and local stress concentrators, with the existing technology of hand welding with permitted holes and absence of thermal treatment) the theoretical coefficient of quality is less than 0.03, which requires the use of high-plasticity metal (Ep > 0.7, ~ > 50%). This guarantees absence of cracks when using steels even with a strength of o u < 500 MPa (point N, Fig. 2). This requirement for the plasticity of metal is confirmed for example, by statistical data on the mains tube failures in service (Fig. 3). At ~ > 50% cases of tube failure have not been noted. At ~ < 50% the nominal breaking stress of tubes decreased to 0.15 cu [13]. Thus, an analysis of factors determining the failure of welded structures carried out by two methods [by the method of qualitative statistics and by means of Eq. (i)] permitted one to reduce the quite numerous factors to three criteria, Ep, C and o r , to draw a conclusion on the interdependence of the three criteria, and to show the effect of the external and internal factors on them. The values ep and C can change within a wide range even at the same level of metal strength. In order to attain an even strength of the welded joint, it is necessary to maintain the condition o r ~ Ou, i.e.,

(~C')~ >! %/f, where ~D' C, and m are the characteristics in each zone of the welded joint (from the first to the ~-th); here C is the lower value from C r and Ct; whereo u is the ultimate strength of the base metal. Since various factors have a different effect on Ep and C, the evaluation of the element strength must be carried out according to both criteria:

~,EC~ (~u/p= - . 7 - =eC~(~:r,/p) =~pe(C'~ /p), where p is the strength reserve. The reserve of an element strength can be maintained only with corresponding reserves from two of the three criteria. Such an approach permits one to carry out better the assessment of the welded element strength.

62

When the proposed analysis method of factors in a case of failure of the welded structure is used, then it is necessary to determine the element where failure took place, the source of failure, and the stress concentrator which caused it; all factors involved (those taken into acocunt by the project and those not taken into account); the initial data (gp, Cr, ou) for the base metal as well as for the welded joint in the fracture zone (point O in Fig. 2) and C t of the element in this zone; these characteristics after failure (point R); ~ep and AC (Fig. 2) in the failure source; and factors causing a reduction of the effectiveness of the structural element and its failure. This work is becoming much simpler since there exists a crack formation diagram for the given steel. The described method can also be used in solving many other scientific and practical problems, including the problems of quality of manufacture, of metal content, and of the service life of welded structures. LITERATURE CITED i. 2. 3. 4.

5.

V. S. Miianchev, "Evaluation of the workability of tubes in the presence of stress concentration," Stroit. Truboprovodov, No. 2, 23-25 (1984). U. D. Hall, H. Kihara, V. Zud, and A. A. Wells, Brittle Fracture of Welded Structures [Russian translation], Mashinostroenie, Moscow (1974). V. Weiss, Analysis of Failure under Conditions of Stress Concentration Failure [Russian translation], Part 3, Mir, Moscow (1976). Yu. A. Osip'yan, "Results of investigation of mechanical properties of NK," in: Problems in Metallurgy and the Physics of Metals, [in Russian], Metallurgiya, Moscow (1964), pp. 108-111. N. T. Gudtsov, M. L. Bernshtein, and A. G. Rakhshtadt (eds.), Metals Science and Heat Treatment of Steel and Cast Iron: Reference Book [in Russian], Metallurgizdat, Moscow

(1956). 6. 7.

8. 9. i0. ii. 12. 13.

L. S. Moroz and B. B. Chechulin, Hydrogen Brittleness of Metals [in Russian], Metailurgizdat, Moscow (1965). I. P. Medinskaya, Yu. I. Rubenchik, T. A. Pisarenko, and E. A. Afanasenko, "Effect of properties of anisotropy on the technology of steels to be welded," Khim. Neft. Mashinostr., No. ii, 18-20 (1980). V.A. Vinokurov (ed.), Welding in Machine Construction: Reference Book [in Russian], Vol. 3, Mashinostroenie, Moscow (1979). M. Kh. Shorshorov, Metallurgy of Welding Steel and Titanium Alloys [in Russian], Nauka, Moscow (1965). A. G. Rakhshtadt, Spring Steels and Alloys [in Russian], Metallurgizdat, Moscow (1982). G. P. Karzov, V. P. Leonov, and B. G. Timofeev, Welded High-Pressure Vessels [in Russian], Mashinostroenie, Moscow (1982). S. A. Kurkin, Strength of Thin-Walled Welded Vessels Working under Pressure [in Russian], Mashinostroenie, Moscow (1976). M. P. Anuchkin (ed.), Strength of Tubes of Mains According to Data of Investigations Carried Out in the Soviet Union and the USA [in Russian], TsNTIGazproma SSSR, Moscow (1965), pp. 9-89.

63