8/13/2019 Ovalitate tevi

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C O N S I D E R A T I O N O F T H E C R O S S S E C T I O N A LO F P I P E B E N D S W H E N C A L C U L A T I N G T H E I RL O W - C Y C L E F A T I G U E S T R E N G T H

V . A . N a k h a l o v , R . Z . S h r o n ,R . K h . B a r a z , a n d R . K . B a l a s h o v a

F O R M

UDC 620.178.3:621.774

P i p e s o p e r a t i n g u n d e r c y c l i c l o a d i n g c o n d i t i o n s a r e d a m a g e d m o s t o f t e n o n b e n t s e c t i o n s w h e r e h i g hl o c a l s t r e s s e s o c c u r . T h e p i p e c r o s s s e c t i o n d e f o r m e d w h e n m a n u f a c t u r i n g t h e b e n d t r i e s t o t a k e a r o u n df o r m u n d e r t h e e f f e c t o f i n t e r n a l p r e s s u r e a n d s e c o n d a r y b e n d i n g s t r e s s e s o c c u r i n t h e w a l l o f t h e b e n da l o n g w i t h t h e p r i m a r y t e n s i l e s t r e s s e s .

T h e m a g n i t u d e o f t h e l o c a l s t r e s s e s f r o m i n t e r n a l p r e s s u r e i s u s u a l l y d e t e r m i n e d b y a s s u m i n g t h ep i p e s e c t i o n t o b e e l l i p t i c . I n t h i s c a s e t h e m a x i m u m s t r e s s e s a c t a t t h e v e r t i c e s o f t h e e l l i p s e p o i n t s A ,B , a n d C i n F i g . i ) a n d c a n b e f o u n d b y t h e e q u a t i o n s g i v e n i n [ I, 2 ]. M e a n w h i l e , m e a s u r e m e n t s s h o w t h a tt h e a c t u a l s e c t i o n s o f b e n d s h a v e a n i r r e g u l a r s h a p e w h i c h o f t e n d i f f e r s c o n s i d e r a b l y f r o m a n e l l i p s e . A sa c o n s e q u e n c e o f t h i s t h e m a x i m u m s t r e s s e s f r o m i n t e r n a l p r e s s u r e c a n d i f f e r a p p r e c i a b l y f r o m t h e v a l u e sc a l c u l a t e d o n t h e b a s i s o f a n e l l i p t i c f o r m o f t h e s e c t i o n .

T o e v a l u a t e t h e e f f e c t o f t h e c r o s s s e c t i o n a l f o r m o n s t r e s s e s , w e m e a s u r e d a b o u t 4 0 0 s e c t i o n s o f p i p eb e n d s m e a s u r i n g 3 2 x 4 m m r a d i u s o f t h e b e n d R = 6 0 m m ) a n d 1 0 8 x 4 . 5 m m R - - 5 0 0 m m ) m a d e w i t h a n dw i t h o u t t h e u s e o f a m a n d r e l . F r o m t h e m e a s u r e m e n t d a t a w e d e t e r m i n e t h e o v a l i t y o f t h e s e c k i o n

Drnax-- Dmin 100 , (1)== Dnand by means of the method given in [3] we calculated the stresses from the internal pressure in the bendwall. In Eq. (1) Dmax, Dmin are the max imu m and min imu m outside dia met ers of the section; Dn is thenominal outside diameter of the pipe.

A comparis on of the maximum bending str ess es Aa in the zone of point A of the measur ed pipes withthe st re ss es Aae l at point A of a bend of elliptic section having the same ov ality is shown in Fig. 2. As

F i g . i . C h a r a c t e r i s t i c p o i n t s a n d d i m e n -s i o n s o f b e n d .

we see from the figure, the st res ses in bends of the realform can exceed considerably those in the bend of el-liptic section, especi ally in the case of small ovality whenlocal irr egul ari tie s of the cro ss sectional profile becomeessential. In the case of ovality to 8 the stres sescalculated with consideration of the actual form of thesection can exceed by a factor of two and more the valuesobtained on the assump tion of an elliptic profile .

The upper lim it of the 99 confidence inte rval inFig. 2 is appr oxim ated by the equation

~ - - 0 , 9 9 . 2 )h lThe known results of strain gauge measurements of thebends of pipes with a diam ete r f rom 32 to 550 mm [1, 4,5, etc. ] also fall within the limits of this region.

C h e l y a b i n s k . T r a n s l a t e d f r o m P r o b l e m y P r o c h n o s t i , N o . i 0 , p p . 1 2 1 - 1 2 4 , O c t o b e r , 1 9 7 3 . O r i g i n a la r t i c l e s u b m i t t e d F e b r u a r y 1 7 , 1 9 7 2 .

9 J974 C onsu l tan t s Bureau , a d iv i s ion o f P lenu m P ubl i sh ing Corpora t ion , 227 Wes t ]7 th S t r ee t , New York , N . Y . 10011.No par t o f t h i s pub l i ca t ion m ay be r eproduced , s tored in a r e t r i eva l s ys t e m , or t ransm i t t ed , i n any form or by any m eans ,e l ec t ron ic , m ec hanic a l , pho tocopyin g , m icro f i lm ing , r ecord ing or o ther w ise , w i thou t wr i t t en perm is s ion o f the publ i sher . .4copy o f th i s ar t i c l e i s ava i lab le f rom the publ i sh er for $15 . 00 .

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