The phenomenon of cavitation flow damage is discussed. The initial phases of damage and the effect of external stresses on the extent to which damage is incurred are analyzed. Three experimental facilities were used to procure the data required: (1) a water loop with venturi, (2) a mercury loop with venturi, and (3) a vibratory facility (stationary specimen, nonflow system). A description of each system is provided for the clarification of test conditions. Photographs of typical cavitation damage instances are included

Hammitt, F. G.

Kemppainen, D. J.

English

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362 CAVITATION 
the initial damage period the inhibiting effect of the compressive stress 
decreases and then becomes negative. Thus the compressive stress appears 
to be detrimental once the surface of the material has been broken. 
These same trends were not seen in the copper. However, no measure- 
ments for less than 0.1 mil XlDP were made. 
Photographic Investigation of Short-Duration Damage Tests 
The results of a photographic investigation of specimens under the 
three load conditions of tcnsion, compression, and zero applied load are 
shown in figure 10. These three series of photographs are a composite of 
10-degree segments of a cylindrical specimen of type 304 stainless steel 
(fig. 5) run in the mercury loop (fig. 2) .  The test duration was 1.5 minutes 
with a throat velocity of 30 ft/sec and a pressure drop across the venturi 
of 71 psi. The pitting density changes which are evident from sector to 
sector are due to  the distribution of the cavitation field which surrounds 
the specimen in this configuration (ref. 6) .  Figure 10a shows a polished 
304 stainless-steel specimen tested with zero applied stress. Figure 10b 
shows the same material tested under a compressive load parallel to its 
a.-No applied force. 
b.-Compressive force. 
c.-Tensile force. 
FIGURE lO.-E.fect of surface stress on damage for short-duration tests. 
APPLIED STRESS ON EARLY STAGES OF CAVITATION DAMAGE 363 
axis of 88 percent of its yield strength, while figure 1Oc is for an impressed 
tensile load of 88 percent of the yield strength. 
Comparing the photo for the imposed compressive force (fig. lob)  to 
that for zero force (fig. loa) shows a very significant reduction in the 
number of smaller size pits and some reduction for all pits for compressive 
pressive specimen. The tensile specimen (fig. 1Oc) and the zero-force 
specimen (fig. loa) appear to be quite similar in pit-number densities 
of all sizes. 
Three highly polished brass (65/35) specimens were run in the vibra- 
tory rig under conditions of tensile, zero, and compressive external applied 
load. The test duration in each stress mode was 10 seconds. The results 
are shown in figure 11. As in figure 10, there is markedly less pitting for 
compressive applied load than for other conditions. Under microscopic 
!oad. There Ere, h"weVer, ati!! some very large pits visible in the ? O m -  
c.-Compression (1OOx). 
FIGURE ll .-cavitatwn damage in brass vibratory specimm after 10-second exposure. 
1 
364 CAVITATION 
examination, all specimens showed approximately uniform radial damage 
distribution over the damaged area. For comparison, figure 12 shows a 
heavily damaged specimen. Proficorder traces across its damaged area 
indicate that damage depth is again approximately uniform (ref. 2). 
Comparing the short-duration specimens in detail, the pit-number 
density is greatest for the tension specimen (fig. l l a ) .  There is a greater 
number of small pits than of large ones, as is typical in cavitation tests. 
The specimen with zero applied load (fig. l l b )  has a somewhat larger 
number of small pits than the tension specimen, but the number and size 
of large pits are about the same. The compressive specimen (fig. l l c ) ,  
however, shows a definite decrease in the number of both small and large 
pits. 
Comparing the results from the vibratory facility with those from the 
mercury loop, the effects of externally applied stress are about the same 
even though specimen shape, material and fluid properties, and cavitation 
field differed drastically. In both tests the effect of the compression on 
damage in the early stages is to strongly decrease the number of small 
pits, with less effect on larger pits. Even in the early stages, however, the 
compressive force does not totally alleviate the damage. Rather, it  
appears to increase the threshold bubble energy required for creating a 
pit, thereby decreasing most strongly the number of small pits. The 
material is still vulnerable to higher energy bubble collapses, which cause 
the larger pits. It thus appears that the “incubation period’’ is increased 
by a compressive preload. 
D” 
FIGURE l2.-Center damage area of a specimen with a reduced cross section prior lo 
tensile pul l .  

366 CAVITATION 
high-velocity impacting liquid jet. Selected pictures of this type have 
been published previously by this laboratory (ref. 9) in support of this 
theory. 
Metallurgical Examination of Damage Pits 
A metallurgical examination of the region of the damaged surfaced area 
of the tension and compression specimens was made. SAE 660, a cast 
FIGURE 13.-Eflect of increasing cadmium coating thickness of stainless steel tubular pin 
specimens on water-loop cavitation pits. 
APPLIED STRESS ON EARLY STAGES OF CAVITATION DAMAGE 367 
bearing bronze, and O.F.H.C. copper were tested for this purpose in the 
vibratory rig. Test duration was sufficient to provide relatively large 
MDP. Figure 15 illustrates the method of sectioning. The specimens were 
polished, etched, and then photographed at  400 diameters. 
Figure 16 is four photomicrographs of a damaged SAE 660 specimen, 
showing both transverse and iorigitudinai aeciions. The inierior “hoies” 
are porosity in the cast material. This specimen had been tested under 
a.-Coating thicknesi: 2.5X10-5 
inches (enlargement of central pit) 
(SOOX). 
I I 
c.-Coating thickness: 5 X 1 0 - 6  inches 
(note difference in area around p i t s )  
(1 20 x 1. 
I 
I 
I 
FIGURE 14.-Cauitation pits in coaled specimens showing material removal. 
368 CAVITATION 
Transv rse 
163 _c Longitudinal 
FIGURE 15.--Cutting directions on damaged specimens. 
a,-Transverse (400 X ) .  
c.-Longitudinal (400 X). 
b.-Transverse (400x1. 
d.-Longitudinal (400 X). 
FIGURE 16.-Photomicrographs of cavitation damage area of zero-load SAE 660 vibratory 
specimens. 
APPLIED STRESS ON EARLY STAGES OF CAVITATION DAMAGE 369 
zero applied load. In the section in the transverse direction (fig. 16a, b) 
only a few cracks are visible. Slip lines, however, can be seen in photograph 
a at the base of the pit on the top right. Slip lines are also visible in 
photographs b, c, and d, indicating that the material adjacent to the pits 
has experienced considerable cold work. In photographs a and b, the slip 
iuies appear almost perpendicular to the pit apex. 
The longitudinal photographs (fig. 16c, d )  show some evidence of 
microcracks a t  the base of the pits. In the center of photograph d, a large 
pit has a t  its apex a number of microcracks emanating in different direc- 
tions. There are also slip lines on either side of the pitted region, particu- 
larly to the right of the apex, indicating that considerable cold work was 
done in this region. In figure 17, a and b are photomicrographs of SAE.660 
1. 
b.-Compresswn-transverse (400 X). 
a.-Conapressum-longitudinal (400 X): 
e.-Tension-transverse (400 X). d.-Tenshn-longitudinal (400 X 1. 
FIGURE 17.-Photomicrographs of cavitation damage area of tension and compression 
SAE 660 vibratory specimens. 
370 CAVITATION 
specimens tested under compressive load, while c and d are for a tensile 
specimen. In both stress modes the longitudinal pictures (a, d )  show the 
microcracks from the area of the pit apex running vertically to the sur- 
face, and the transverse pictures ( b ,  c )  show the microcracks angled off 
from the pitted surface. None of these photos show slip lines, and there is 
no apparent difference between specimens tested under compressive or 
tensile load. 
In figure 18, photos a and b show a copper specimen damaged with zero 
applied force, and photos c and d show a copper specimen damaged under 
a.-No force-tranaverse (4OOX). 
c.-Tmion-transverse (40OX). d.-Tension-longitudinal (400 X). 
FIGURE 18.-Photomicrographs of cavitation damage area of tension O.F.H.C. copper 
vibratory specimens (@OX). 
APPLIED STRESS ON EARLY STAGES OF CAVITATION DAMAGE 371 
tensile load. In a and b there appears to be no difference between the 
transverse direction ( a )  and the longitudinal direction ( b ) .  There are, 
however, a few small microcracks at or near the apex of the damage pits. 
There are no discernable slip lines in these photographs, perhaps in- 
dicating little or no work hardening of the damaged surface. 
transverse ( c )  and longitudinal ( d )  directions show larger, more promi- 
nent microcracks emanating from the base of the damage pits. As for the 
SAE 660, the cracks in the longitudinal direction appear to  come away 
from the pits at  an angle almost perpendicular to the centerline of the pit. 
Figure 19 shows both the transverse ( a )  and longitudinal ( b )  directions 
of a copper specimen cavitated in compression. Close examination of the 
photographs reveals a number of microcracks a t  the base of the damage 
pits. These seem to start at  the apex of the pit and continue until they 
intersect a grain boundary, at which point they terminate. The longitu- 
dinal picture again shows the direction of the cracks to  be perpendicular 
to the surface. 
Photomicrographs for brass specimens (not shown) give results similar 
to those for copper. There were, in all cases, microcracks in the region of 
the damage pits, but no slip lines. The damage is thus accompanied by 
two possible effects: (1) microcracks emanating from the base of the 
damage pits and (2) cold work or strain-hardening, depending on the 
material just below the damage surface. There appears to be no difference 
in the number of microcracks or their appearance in specimens tested 
. .  Flioicjs c aiid d of Sgiie 18 ~ S G  S ~ C X  i l ~  slip ! i ~ .  HGT~V~Z, S ~ t h  the  
a.-Transverse ($00~). b.-Longitudinal (4OOX). 
FIGURE 19.-Photomicrographs of cavitation damage area of compression O.F.H.C. 
copper vibratory specimens. 
. 







Some effects of applied stress on early stages of cavitation damage

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