Johnson, Donald F

Meyers, Andre J , Jr

English

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RM E54I7a 
NACA 
RESEARCH MEMORANDUM 
PRELIMINARY INVESTIGATION OF THE STRENGTH AND 
ENDURANCE OF PLASTIC -IMPREGNATED 
FIBERGLASS COMPRESSOR BLADES
By Donald F. Johnson and Andre J
.
 Meyer, Jr. 
Lewis Flight Propulsion Laboratory
Cleveland, Ohio 
NATIONAL ADVISORY COMMITTEE 
FOR AERONAUTICS 
WASHINGTON 
January 10, 1977

Dcc1aif1cd November 8, 19nj7
https://ntrs.nasa.gov/search.jsp?R=19930088481 2020-06-17T07:26:35+00:00Z
NACA RM E54127a
NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS

RESEARCH MEMORANDUM 
PRELIMINARY INVESTIGATION OF THE STRENGTH AND ENDURANCE OF 
PLASTIC -IMPREGNATED FIBERGLASS COMPRESSOR BLADES 
By Donald. F. Johnson and Andrg
 J. Meyer, Jr. 
SUMMARY 
Compressor blades made of fiberglass impregnated with phenolic 
resins were subjected to several strength tests including tensile, 
cantileve± bending, cantilever torsion, fatigue, and damping tests. 
The modulus of elasticity was calculated using the standard formula 
for bending deflection of a cantilever beam and utilizing tensile-test 
data. The result of the formula method was E = 4.37xlO 6
 psi, and of 
the data method., E = 3.03xlO 8
 psi. The dynamic modulus of elasticity 
was calculated to be 3.63X1O 6 psi. 
The modulus of rigidity was O.92 XlO6 psi. The ultimate tensile 
strength was 17,000 psi, the ultimate bending strength was 43,000 psi, 
and the fatigue endurance limit at zero mean stress was 15,000 psi. 
The third-stage rotor of a jet engine equipped with a full set of 
fiberglass blades was run for 105 hours, 28 of which were at rated 
speed. After 105 hours running time, no damage to the blades was 
apparent.
INTRODUCTION 
The present trend in axial-flow compressor design is toward higher 
aerodynamic blade loading and longer blades of higher aspect ratios, 
both of which greatly increase the blade vibration problem. Compressor 
diameters are also being increased so that the compressor can handle 
more air; as a result, it becomes heavier and requires larger quantities 
of critical materials. New materials are being sought to alleviate 
these pending problems as well as the existing vibration problem. One 
of the new materials, laminated fiberglass bonded with plastics, is par-
ticularly promising. It is lightweight (one-fourth the weight of steel), 
has very high internal damping to reduce vibrations, and is noncritical, 
easily formed, and readily available. The laminated material has been
NPCA RM E54127a 
suggested for the rotor and for the compressor casing, but this report 
is concerned only with its use for rotor blading. 
According to the available literature, laminated fiberglass blades 
have not yet been run successfully in an engine. The object of this 
investigation was to endurance-test in a J47 compressor a full third-
stage rotor of laminated blades impregnated with a phenolic resin. In 
addition, physical properties of the new material were established by 
bending, twisting, rupturing, and fatiguing individual blades identical 
to the blades used in the engine. 
SYMBOLS 
The following symbols are used in this report: 
A	 amplitude of vibration 
a	 mode constant 
d	 deflection 
E	 modulus of elasticity 
G	 modulus of rigidity 
i(i) function of distribution of moment of inertia 
p(i) function of distribution of moment of inertia 
P(K) polar moment of inertia function 
7.	 length 
n	 number of cycles 
P	 load 
T	 torque 
5	 logarithmic decrement 
e	 angular displacement 
mass per unit length 
fundamental bending frequency
NACA RN E54127a	 3 
Subscripts: 
n	 number of cycles 
p	 plastic blade 
s	 steel blade 
0	 start of decay curve 
APPARATUS AND PROCEDURE 
Fabrication of Fiberglass Blades 
The compressor blades with which this investigation is concerned 
were made of approximately 56 laminations each of fiberglass cloth 181-
114 impregnated with CTL 91LD phenolic resin. The impregnated cloth was 
supplied by Cincinnati Testing and Research Laboratories to Thompson 
Products, Inc., who fabricated the blades according to specification 
TAP-MS-210. The blade airfoil was built up of usually 20 laminations 
each about 0.008 inch thick with laminations on the outside surface 
equal in width to the chord of the blade and decreasing in width toward 
the center. The base of the blade flared out to a bulb shape, neces-
sitating about 36 additional laminations. The whole was covered with a 
sheet of phenolic-impregnated 120-114 glass cloth designated TAP-MS-211. 
The blade was then cured in a die at approximately 300 psi at 320 0 to 
3300
 F for 5 minutes. The die-cast aluminum roots were cemented in 
place on the blades with an epoxy adhesive. The assembly is shown in 
figure 1, and a comparison of the cross-sectional properties of steel 
and fiberglass blades is shown in figure 2. The bakelite-impregnated 
blades mentioned later in the report differed in that the laminations 
were widest at the center and became narrower toward the surface. 
Determination of Physical Properties 
Because physical properties of plastic-impregnated fiberglass de-
pend on the shape of the test specimen, tests were performed on actual 
blades rather than on test panels. 
Bending test. - Weights were attached to the end of a blade using 
a clamp made of a low-melting-point alloy. This clamp fit the airfoil 
profile perfectly from leading edge to trailing edge and spanwise 1/2 
inch from the tip. Figure 1 shows a fiberglass blade and the clamp. 
Weights from 1 to 8 pounds in 1-pound increments were suspended 
from the tip clamp in a direction which imposed compression on the
4
	
NACA RM E54127a 
concave side of the blade, and the deflection for each weight was meas-
ured with a dial gage at the tip. This procedure was repeated with each 
of five fiberglass blades and four standard 403 martensitic stainless-
steel blades. The last fiberglass blade was loaded in 1-pound increments 
until failure, and the tip deflection was measured as before with a dial 
gage. In addition, the stress at the point of maximum thickness on the 
convex side at the base of the blade was measured up to failure with a 
bakelite, wire-resistance strain gage. 
The formula for the deflection at the tip of a cantilever blade of 
arbitrary distribution of moment of inertia along the span and with a 
concentrated load at the tip is
PI 3d=—f(I) 
where f(I) is a function depending on the distribution of moment of 
inertia along the span. Comparing the deflections obtained from the 
steel blades with those from the fiberglass blades and assuming the 
lengths and the moment of inertia distributions to be the same gave an 
expression for the bending modulus of elasticity of the fiberglass 
blade in terms of the known modulus of elasticity of steel. 
E=E 
The value used for the modulusof elasticity of 403 stainless steel was 
28x106 
Torsion test. - The end of the blade tip clamp was supported by a 
ball bearing at the approximate center of torsion so that there could 
be no bending of the blade. A torsional lever arm and a pointer were 
attached to the clamp. Several torques ranging from 0 to 120 inch-
pounds were applied by hanging weights on the lever arm. Angular de-
flections were obtained using the pointer and a fixed scale. 
The formula for the angle of twist of a cantilever blade due to a 
moment at the tip is
e=jF(K) 
where F(K) is a function of the polar moment of inertia. Comparing 
the angular deflections obtained from the steel blades with those from 
the fiberglass blades and assuming the lengths and the torsional spring 
constant functions to be the same yielded an expression for the modulus 
of rigidity of the fiberglass blade in terms of the known modulus of 
rigidity of steel.
NkCA EM E54127a	 5 
G = G5 es 
The value used for the modulus of rigidity of the steel blade was 
G = 10 X106 psi. 
Tension test. - Approximately 2 inches of the end of a blade was 
immersed in a low-melting-point alloy. In this way a contour-fitting 
form similar to the tip clamp in figure 1, only longer, was obtained. 
This form was in two halves which were cemented with bakelite to the 
blade. This clamp along with the metal blade root at the other end 
supplied the necessary gripping surface for the grips of a conventional 
tensile machine. The blade was pulled in tension to failure, the strain 
being measured by a bakelite wire-resistance strain gage mounted at the 
base of the blade. 
Damping test. - It was desired to obtain blade damping values at 
high base bending stresses. Because of the low modulus of elasticity 
of plastic, the strain is much larger in plastic for a given stress than 
in steel, and it was found that strain gages would not stand up in the 
desired stress range. Since the stress and hence the strain decreases 
spanwise toward the tip, two strain gages were mounted on a blade, one 
at the base and one about halfway between the base and the tip. The 
blade was deflected by a triggering mechanism and rapidly released, and 
the decay curve was recorded on film. Data were taken at progressively 
larger deflections of the tip of the blade using both strain gages 
until the gage at the base failed, and then from the gage at midspan 
only. Thus, the stress at the location of the outer gage was calibrated 
against the stress at the base. This calibration was used to compute 
the stress at the base for large deflections. 
The damping was calculated by measuring the amplitude of the decay 
curve at the start and n cycles from the start and applying the 
formula
! 
n 
This procedure was followed for a new blade impregnated with a phenolic 
resin made by the Bakelite Corporation and for a blade impregnated with 
the CTL resin and which had vibrated for 1.75 X107 cycles at a stress of 
±15,000 psi. The fundamental bending frequency was determined from the 
damping curve to be 185 cps. 
Fatigue test. - A small air exciter was used to excite blades to 
vibration (fig. 3). Fatigue tests were run by exciting a blade at its 
natural frequency at a given tip amplitude until failure.
6	 NACA RN E54127a 
The formula for the frequency of the fundamental bending mode of a 
cantilever blade is
U) 
= anV_ P(I) 
Comparing the frequencies obtained for a steel blade with that for a 
fiberglass blade and considering that 1, (p(I), and an are the same 
for both blades gave an expression for the dynamic modulus of elasticity 
of the plastic blade in terms of the known dynamic modulus of elasticity 
of the steel blade.
P
lip 
 s(s)
2
The values used for the density and dynamic modulus of the steel blade 
were 0.277 pound per cubic inch and 32x10 6 psi, respectively, and the 
density of the plastic-impregnated fiberglass blade was experimentally 
determined to be 0.067 pound per cubic inch. 
Engine test. - The third-stage rotor of a J47 was completely out-
fitted with CTL 91LD impregnated fiberglass blades. The engine was run 
under static, sea-level conditions as shown in figure 4. The first 
28 hours were at rated speed (7950 rpm); then the speed was reduced to 
7800 rpm to decrease the number of breakdowns of engine components 
other than the compressor. This resulted in a decrease in centrifugal 
stress of only 4 percent. The engine was run for two 8-hour shifts a 
day, being shut down, or throttled down to idle, at various times as 
shown in figure 4 to add oil, change oil filters, and make repairs. 
After a total running time of about 105 hours, the compressor was 
disassembled and the fiberglass blades were examined for damage. As a 
detector of any internal separation of laminations, two weights were 
hung on each blade and the deflections were measured using a dial 
indicator.
RESULTS AND DISCUSSION 
The flexural strength of a fiberglass blade in bending is compared 
with the flexural strength of a steel blade in figure 5(a). The bending 
modulus of elasticity of the fiberglass blades was calculated on the 
basis of these data to be 4.37 XlOS psi, and the ultimate bending strength 
was 43,000 psi. Only the data for blade 5 are plotted, as the fiber-
glass blades were sufficiently uniform that the data for all blades fell 
on the straight line shown. Figure 5(b) shows that the tip deflection
NACA RM E54127a
	 7 
at failure for the fiberglass blade was 0.899 inch. The load at this 
point was 31.56 pounds. 
The dynamic modulus of elasticity calculated by comparing the 
natural frequencies of a fiberglass and a steel blade was E = 3.63*106 
psi.
The torsional stiffness of fiberglass blades is compared with that 
of steel blades in figure 6, and the modulus of rigidity was calculated 
on the basis of this comparison to be G = 0 . 92*106 psi or approximately 
the value for steel. 
The results of the tensile test of a fiberglass blade are shown in 
figure 7. There possibly was some eccentricity of loading, but the 
ratio of ultimate tensile strength to ultimate bending strength agrees 
favorably with ratios reported for plastic specimens. The straight-
line variation of stress with strain to failure is to be expected for a 
rather brittle material. 6 The tensile modulus of elasticity was calcu-
lated to be E = 3.03X10 psi, and the ultimate tensile strength was 
17,000 at a load of 2100 pounds. 
A rough S-N diagram (stress against fatigue cycles) showing the 
fatigue strength of fiberglass blades is given in figure 8. Many more 
blades would have to be tested in fatigue in order to establish accur-
ately the shape of the curve. However, it can be seen that at a stress 
above ±15,000 psi, the life of a plastic-impregnated fiberglass blade 
is limited to lO XlOS cycles or less. In all cases after 1 or 2xl06 
cycles, an outside lamination at the base appeared to separate from the 
blade. This separation did not progress any farther at a given stress. 
Results from the fatigue tests of two fiberglass blades impregnated 
with phenolic resin supplied by the Bakelite Corporation are also 
plotted on figure S. They apparently have about the same fatigue 
strength as those impregnated with CTL 91LD phenolic resin. 
A comparison of the flexural strength in bending of 50 blades after 
the engine endurance run of 105 hours is shown in figure 9. The uni-
formity of the blade deflections indicates that no apparent internal 
dartage was sustained by any of the blades. Figure 10 shows damping 
curves obtained for fiberglass blades impregnated with CTL 91LD and 
Bakelite resins as compared with those for 403 stainless-steel blades. 
The damping of a plastic-impregnated fiberglass blade is about 12 per-
cent, with little change due to stress. In comparison, stainless steel 
has about 3 percent damping.
8
	
NACA RM E54127a 
SUMMARY OF RESULTS 
An investigation was made of third-stage J47 compressor blades made 
of fiberglass impregnated with CTL 91LD phenolic resin and similar blades 
impregnated with Bakelite resin. The following results were obtained: 
Bending modulus of elasticity, psi 	 .............. 4.37xl06 
Dynamic modulus of elasticity, psi .............. 3.63xlO 
Tensile modulus of elasticity, psi 	 .............. 3.03xl06 
Modulus	 of rigidity,	 psi	 ................... 0.92xl0 
Maximum tip load for failure (bending), lb ........... 31.56 
Tip deflection at failure (bending),	 in .	 .	 .	 .	 .	 .	 . .	 .	 .	 .	 .	 .	 0.899 
Ultimate bending strength, psi .	 .	 .	 .	 .	 .	 .	 .	 .	 .	 . .	 .	 .	 .	 .	 .	 43,000 
Maximum tensile load, 	 lb	 ..................... 2100 
Ultimate tensile strength, psi 	 .	 .	 .	 .	 .	 .	 .	 .	 .	 .	 . .	 .	 .	 .	 .	 .	 17,000 
Damping,	 logarithmic decrement	 .................. 0.12
The J47 jet engine with its third-stage rotor completely equipped 
with fiberglass blades ran 105 hours, 28 of which were at rated speed, 
with no apparent damage to the blades. 
Lewis Flight Propulsion Laboratory 
National Advisory Committee for Aeronautics 
Cleveland, Ohio, October 1, 1954 
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N&CA PM E54121a
Figure 1. - Fiberglass blade showing tip clamp and die-cast root insertion.
10
	
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Preliminary investigation of the strength and endurance of plastic-impregnated fiberglass compressor blades

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