INDUSTRIAL SECTION

E X P E R I M E N T A L I N V E S T I G A T I O N O F T H E

S T R E S S O F P R E S S U R E C H A M B E R S

E . S . U m a n s k i i , I . A . K o z l o v , T . S h . S h a g d y r , I . V . L e b e d e v , A . P . P o l y v y a n y i , a n d A . I . Y a h

S T A T E O F

UDC 539.3

Surgery and therapy in oxygen-enriched gaseous a tmospheres at high p r e s su re s have been widely used in recen t yea r s in medical prac t ice . The p re s su re chambers used in this case have a quite complex configuration and regions in which considerable s t r e s s concentrat ions can ar ise . In many cases theoret- ical calculat ions do not permi t determining s t r e s s e s in p ressu re chambers with a sat isfactory accuracy . The purpose of the presen t investigations was to evaluate the state of s t r e s s of var ious components of p r e s s u r e chambers , to f indzones of high concentrat ion of s t r e s ses , and to establish the permiss ible magni- tude of the working p r e s su re .

The surgica l p r e s su re chamber , designed for working at a p re s su re of 3 gauge atm (Fig. 1), consists of two pa r t s : a large operating chamber and a smal l fo rechamber (or entrance lock). The hull of the p re s - sure chamber is an al l-welded vesse l made up of two cylindrical shells of aifferent d iameters with bottoms. The front, intermediate , and r e a r bot toms have the shape of ellipsoids of revolution. The r e a r bottom is solid, whereas the intermediate and front bot toms have a rectangular groove into which is welded the r igid door. f r ame .

The therapeutic p r e s s u r e chamber , designed for working at a p r e s s u r e of 5 gauge atm (Fig. 2), con- s is ts a lso of two pa r t s : a t reatment par t and a forechamber . The front, intermediate, and r e a r bottoms have the same profile as for the fo rechamber of the surgical p r e s su re chamber , with the same doorways.

The wall thickness of all a reas is 12 mm and the wall thickness of the bottoms is 16 mm.

On the cyl indrical shells and elliptical bot toms are places with grooves to which are welded the locks, por tholes , and var ious connecting pipes for communicat ion with the outside environment, introduction of light, and observation of the inside.

R D

9 r

Fig. 1.

Y

Sketch of surgical p r e s s u r e chamber .

Institute of Strength of Materials , Academy of Sciences of the Ukrainian SSR, Kiev. Transla ted f rom Problemy l>rochnosti, No.4, pp.83-88, April , 1970. Original ar t ic le submitted October 25, 1969.

�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 of the publisher. A copy o[ this article is available [rom the publisher for $15.00.

379

�9 , ~ "t .... /

11 II | !

/7 72oO

Fig. 2. Sketch of therapeutic chamber .

Fig. 3. Measurement scheme.

Both p re s su re chambers operate in the same manner . Their main components are the operating chamber for the surgical p r e s s u r e chamber and the t rea tment chamber for the therapeutic. During su rge ry or t rea tment a constant p re s su re of the necessa ry magnitude is maintained in them. The fo rechambers (locks) of both p res su re chambers are the same and have a common p u r p o s e - entrance to the oper - ating (treatment) chamber and exit of the serv ice personnel during work, and therefore the p re s su re in them changes f rom atmospher ic to the working p r e s s u r e with each change.

The selection of the places to investigate s t r e s s was based on available experience in the most hazardous sections. These include f i r s t of all places which are s trong s t r ess r a i s e r s , for example, the corner a reas of the doorways in the bot toms, places where the fo rechambers join the main chamber , and a reas adjacent to the portholes . Par t i cu la r attention was given to a study of the s t r e ss in the corner a reas near the doorways, where the complexity of the shape is made even worse by additional loads due to the p re s su re of the doors on the f rame .

Co-workers of the Ukrainian Resea rch and Design Institute of Chemical Machinery in Kharkov made calculations of the described p re s su re chambers , but they were based on simplified schemes which did not permi t an evaluation of the s t r e s s e s in the zones of concentrat ion.

The main components of the p re s su re chambers are made of steel St.3 (~y = 26-34 kgf/mm 2, ~t = 44/50 kgf/mm2). The safety factor was established with respec t to the yield point:

ny= ~Y,= 1.5. [Oy!

The state of s t r e s s of the p r e s s u r e chambers was investigated by means of wire s t ra in gauges placed on the outside surface of the p re s su re chambers and on their inside par t s at those places which are not amenable to r igorous calculation and at which the occur rence of large s t r e s s e s could be expected.

The gauges were ar ranged in pa i rs in the direct ion of the principal s t r e s s e s . At places where it was impossible to establish beforehand the direct ion of the principal s t r e s s e s , we cemented rose t t es of three gauges, two of which were ar ranged meridionally and c i rcumferent ia l ly and the third at an angle of 45 ~ to them.

Strain gauges of types PKV-10-100 and PKV-5-100, having bases respect ive ly of 10 and 5 ram, were cemented onto precleaned places with 192T glue, which does not require hot drying. After drying, connect- ing wires were soldered to the gauges and then the gauges and soldered junctions were coated with a c o m - posit ion of gun grease (70%) and ros in (30%) for waterproofing. The connecting wires were laid on the

380

Ore d , kg/cm o red, I 8or,

~~ V \

a 0 250 l, m m 0 500 1000 L, m m 0 000 I000 1500 L, m m b c d

Fig. 4. Sketch of the p r e s s u r e chamber (a) (view through DD, Fig. l ) and s t r e s ses in c ross sections aa {b), bb (c), and cc (d).

sur face of the p r e s s u r e chamber and fastened by foil (steel 1Kh18N9T) b racke t s welded to the chamber by means of a capaci tor d ischarge welder .

The measuremen t s were taken with compensat ion s t ra in gauges which were placed inside the p r e s - sure chamber and which were under the same tempera ture conditions as the working gauges. Compen- sat ion gauges f r o m the same group as the working gauges were cemented onto a plate made of the same ma te r i a l as the hull of the chambers and placed inside the forechamber .

To eliminate the effect of the tempera ture increment of res i s tance on the measurement resu l t s , we used c i rcu i t compensation in which the working and compensation gauges were connected into adjacent a r m s of an outside (extended) haf t -br idge. Since it was neces sa ry to take measurements at many points we used a group sys tem of connecting the gauges in which there was one compensat ion gauge per group of working gauges.

~

Figure 3 shows the measu remen t scheme. The al ternately connected working gauges Ral, Ra2 . . . . . Rai . . . . . Ran f rom the active b ranch of the extended haft-br idge and the compensat ion gauge R c r ep resen t s the compensat ion branch. The switching device 1 se rves for al ternate switching of the working gauges into the measur ing c i rcui t . The second half of the input br idge is formed by two standard r e s i s t o r s and a slide wire built into e lectronic s t ra in me te r 2. As the measur ing ins t rument we used an A I - l m type s t ra in me te r with automatic balancing designed for measur ing static and slowly varying s t ra ins . The ins t rument is an e lect ronic automatic balanced br idge with the measur ing bridge supplied with a 50-Hz, 4- or 2-V al ternat ing cu r r en t operating on the bas is of the ze ro - read ing method.

To measure s t ra ins at many points the ins t rument was used together with the switching device. For the la t ter we used a 104-channel switch made up of telephone switches, the intermediate res i s tance of wh~h had a sufficiently high stabili ty.

According to the manufac tu re r ' s cer t i f ica te , the e r r o r of the wire gauges was 1.37% and e r r o r of the measur ing ins t rument 1%. Taking into account the cal ibrat ion e r r o r s and measu remen t e r r o r s ar is ing as a consequence of the g rea t length of the connecting lines and part ia l unsealing of cer tain s t ra in gauges which were under water , we could consider that the total measurement e r r o r was within 5-10%.

The magnitudes of the s t r e s s e s along the appropriate direct ions w e r e calculated on the bas is of the measured s t ra ins by the formulas

E ~ = ~ (sx + I~%); (1)

E (% + ~8x) , (2) Oy = 1 - - [z 2

where # is Po i s son ' s rat io, # = 0.3; E is the modulus of elast ici ty, E = 2" 10 kgf/cm 2.

The principle s t ra ins at the places of installing the rose t tes were determined by the fo rmulas

8 : + St/ + [ ( ' , - + - 's ~'~- - - " - - ' ~ - - -

(3)

~' 8~+~u V - - ~ = ~ - - [(8~ - - ~45) ~ + (~45 - - ~u)~], (4)

381

where ex, ay are s t rains along mutually perpendicular direct ions; E4s is s t ra ins in a direct ion 45 ~ to the x and y axes.

The strength was checked on the basis of the energy theory of distort ion (fourth theory of strength) and the reduced s t r e s ses were calculated by the formula

o..i, + co.,- + (o.,- o.0,,. (5}

The p re s su re chambers prepared for the tests were filled with water and loaded by an inside hydro- static p ressure , which was measured by p re s su re gauges to within +1.5%.

Originally the main volume of the p r e s s u r e chamber (without the forechamber) was filled with water . After this all gauges in this par t of the chamber and in sections adjacent to it were in terrogated in turn. Then the chamber was loaded with a p r e s su re of 3 gauge arm and the readings of the gauges recorded. After re leas ing the p re s su re we again made a control recording of the readings of the gauges at zero p r e s - sure and filled the forechamber with water . Since owing to the large volume filling took several hours, during which some zero drift of the gauges was possible, all s t rain gauges were again in terrogated i m m e - diately before loading the entire volume of the chamber .

Following this the entire chamber was loaded with a p ressu re of 3 gauge atm, the s t ra in gauges were again interrogated in a loaded state, af ter which the p res su re was re leased, and the gauges were again interrogated in an unloaded state for checking the stability of the zero readings.

The places passing f rom the intermediate floor to the door f rame (Fig. 4) proved to be the most dangerous when loading the operating chamber of the p re s su re chamber , shown in Fig.1. We point out that the centers of the gauges were at a distance of 20-25 mm from the middle of the joint, and the magni- tudes of the s t r e s ses obtained are the average for the section corresponding to the base of the gauge. The maximum s t r e s ses reached 2563 kgf /cm 2. The s t r e s s e s near the portholes were ve ry high, (TIrVed = 2493 kgf /cm 2. Unexpected was the high compress ive s t r e s s in a direct ion radial to the opening (2212 kg-f/cm2), whereas in an annular direction the tensile s t r e s s was 489 kg f / cm 2.

After unloading, the readings of the electronic br idges for the most loaded places re turned to zero, consequently residual s t rains did not ar ise .

W kg/cm z Ored, 2006

iOOO

a

0 400 000 Z, mm 0 b $00 ~ mm

Fig.5

a a

IV olV kg/cm z Ored,IV kg/cm z ~ kg/cm 2 red, 2001 MOO ~0~

1.'-',,, . . / 000 800 /,mm 0 200 400 l, m m 0 400

c c d

Fig.6

Fig. 5. Sketch of the p r e s s u r e chamber Ca} (view through G, Fig. 1} and s t r e s s e s in c ro s s sections aa (b) and bb (c).

Fig. 6. Sketch of p r e s s u r e chamber af ter strengthening: a,b) respect ively view through AA and DD (see Fig.l}; c,d} s t r e s se s respect ively in c ross sections aa and bb.

382

olV t,./cm 2 red' "~"

400, 200 " ~ 806

C

IV 2 Ore d, kg/cm

0 ,~ObO l, m m 0 /00 c 200 l, m m

IV Ored, kg/cm 2 orlV d, kg/cm z

2.O0 '~ /r

0 / 00 200 /,mm 0 2 0 0 @OOl, m m d e

Fig. 7. Sketch of the therapeutic p r e s - sure chamber (a) (view through AA, see Fig. 2) and s t r e s se s in c ro s s sections aa (b), bb (c), cc (d), and dd (e).

Upon loading the entire p r e s su re chamber the maxi- mum s t r e s se s arose in the region of the jdnction of the shell of the forechamber with the intermediate bottom (Fig. 5) of the operating chamber (points II and HI). The s t r e s ses at these points were 1685 and 1680 kgf /cm ~, respectively. If we take into account that the gauges were located at some distance from the contour of the junction and the possible measurement e r r o r , such s t r e s ses could be dangerous.

The concentrat ion of s t r e s ses in the corner points of the rectangular supporting door f r a m e was not unexpected. It is known [1] that in an elliptical shell with a square ape r - ture having rounded edges, the coefficient of concentration of s t r e s se s obtained analytically in the annular section on the contour of the co rne r s can reach 10. For reinforced rectangular aper tures the coefficients of concentration of s t r e s se s obtained experimental ly on cylindrical shells in tension [2] reaches 8.

Calculation of the design in question is very complex f rom the theoret ical point of view. The elliptical shell of the intermediate bottom, on one hand, is reinforced by two concentr ic cyl indrical shells, one of which, being the shell of the operating chamber , is "welded from the inner (concave) side to the outer contour. The other cylindrical shell, being the shell of the forechamber , is welded from the outer (convex) side around the annular section between the doorway and the outer shell. On the other hand, the p re s su re t r ans - mitted by the door of the large chamber along the profile of the rectangular groove through the door f rame has a definite effect.

The elliptical shell of the intermediate bottom during the f i rs t loading is loaded, besides by the inside hydros ta t ic p r e s su re , by the forces t ransmit ted f rom the shell of the operating chamber by the reactive fo rces in the zone of the welded junction of the shell.

During the second loading the elliptical shell of the intermediate bottom experiences, in addition to the hydrostat ic p r e s s u r e applied in the region between the shells of the operat ing chamber and forechaml~er, also the load f rom the shells themselves .

In this case there is no p r e s s u r e of the door on the f rame, but the uniformly distributed axial force acting in this direct ion f rom the forechamber in the region of the junction with the intermediate bottom is added. In both cases of loading in the region of the junction there are pronounced edge effects. The region of the junction of the fo rechamber shell with the intermediate bottom was also under unfavorable conditions, although the calculated s t r e s s e s in it do not exceed 460 kgf /cm 2.

Owing to the large s t r e s se s , the indicated places were strengthened, as shown in Fig.6. Lobes were welded in the co rne r regions of the door f rame and a much l a rge r lobe was welded at the corner adjacent to the porthole. In the region of the welded junction of the forechamber shell with the intermediate bottom we welded, uniformly around the c i rcumference , 24 gusset plates made of 20-mm-th ick steel of the same grade f rom which the elliptical bot toms were manufactured.

After strengthening, the p r e s s u r e chambers underwent new tests , which showed that the s t r e s ses dec reased markedly in the dangerous places detected ea r l i e r . The s t r e s s e s at point IV (Fig.6b) reached only 1257 kgf /cm 2. On extrapolating the curve (Fig. 6c) to the junction of the bottom with the door f rame, the magnitude of the s t r e s s e s is r dIV = 1450 kgf /cm 2. Thus, as we see f rom the graphs in Fig.6, the s t r e s s e s in the p r e s s u r e chamber after strengthening it do not exceed the permiss ib le .

In the therapeutic p r e s s u r e chamber (Fig. 2) we investigated the state of s t r e ss mainly in the in te r - mediate bottom with the doorway and in the regions of the shells of the forechamber and treatment chamber adjacent to it. For an additional check gauges were also installed in one corner section of the doorway of the front bottom and forechamber .

383

Designwise the therapeut ic p r e s s u r e chamber di f fers f rom the surgica l in that it has the same d i a m - e t e r of the shells of the t r ea tment chamber and f o r e c h a m b e r . This improves the working conditions of the therapeut ic p r e s s u r e chamber when loaded by an inside p r e s s u r e . However , the in te rmedia te el l ipt ical bot tom with the rec tangula r doorway separa t ing the t r ea tmen t chambe r f rom the f o r e c h a m b e r is fas tened to the hull by a r igid in te rmedia te ring. This type of connection leads to the appearance of substant ia l edge effects at the junction of the t r ea tmen t chambe r and f o r e c h a m b e r during loading. Calculat ions show that in the regions of the shel ls adjacent to the in te rmedia te r ing the s t r e s s e s a re a r e dIV = 5000-8000 k g f / c m 2 at loads of 5 gauge a tm.

Taking into considera t ion the exper ience gained f rom studying the s ta te of s t r e s s of the surg ica l p r e s s u r e chamber , we p re l imina r i l y s t rengthened the region of the junction of the in te rmedia te bot tom with the shell by means of 12 gusse t p la tes uniformly welded around the c i r c u m f e r e n c e .

F igure 7 shows a sketch of the p r e s s u r e chamber and some r e su l t s of the invest igat ion during the f i r s t loading.

We note that the local s t r e s s e s near the suppor ts of the p r e s s u r e c h a m b e r s , as calcula t ions showed, a re smal t and the re fore we re not checked exper imenta l ly .

The invest igat ions es tab l i shed the following.

The mos t s t r e s s e d zones in the surg ica l p r e s s u r e chamber are :

regions of the in te rmedia te bot tom immedia te ly adjacent to the c o r n e r s of the r igidly f r a m e d

1~

a) doorway;

b)

c)

the region of the in te rmedia te bot tom adjacent to the porthole;

the region of the in te rmedia te bot tom adjacent to its junction with the f o r e c h a m b e r shell .

After s t rengthening the p r e s s u r e cham be r the s t r e s s e s in these places dropped cons iderably .

2. Tes t s of the therapeut ic p r e s s u r e cha mbe r p r e l im ina r ly s t rengthened by t r i angula r gusset p la tes welded between the in te rmedia te bot tom and the shell f rom the side of the f o r e c h a m b e r revea led the following dangerous zones:

a) regions of the in te rmedia te bot tom adjacent to the c o r n e r s of the r igidly f r a m e d doorway;

b) these s ame regions but for the front bot tom of the f o r e c h a m b e r .

1.

2.

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

G. N. Savin, Concentra t ion of S t r e s s e s around Holes [in Russian] , Gostekhizdat , Moscow--Len ingrad (1951). A. Ya. Aleksandrov, M. Kh. Akhmetzyanov, and A. S. Rakin, "Invest igat ion of e las top las t ic deformat ion of shel ls with notches and r e in fo rcemen t s by the photoelas t ic coat ing method, " Pr ik ladnaya Mekhanika, No.2 (1966).

384