The results of an experimental and analytical study of the structural response and failure characteristics of selected bead-stiffened thermoplastic shear-webs are presented. Results are given for specimens with one stiffeneer, with two stiffeners, and different stiffener geometries. Selected analytical results that were obtained with the Computational Structural Mechanics (CSM) Testbed computer code are presented. Analytical results that describe normal and transverse shear stress are also presented

Rouse, Marshall

English

NASA Technical Reports Server

h93-30873
STRUt__ RESPONSE OF BEAD-S_NED THERMOPLASTIC SHEAR WEBS
¢
_-- o_ /
Marshall RouseNASA Langley Research Center /
"\ Hampton, VA / " '%
'..INTRODUCTION
Advanced composite materials offer an attractive potential for reducing the mass of modern
aircraft structural components. To achieve this p0"_ential, the ability to provide reliable structural
designs that can safely carry the desired loads must _'_e developed. Current design practices allow
some metallic structural components (e.g., fuselage _nd stabilizer panels) to buckle under various
loading conditions and, as a result, are said to have po.,stbuckling strength. The corresponding
structural response characteristics of graphite-thermopl_astic composites must be evaluated before
such composites can be considered for application to cVvil transport aircraft structures. One
aspect of the design of aircraft structural components is tthe structural behavior of panels loaded
"_-__-_':_-"_.:_Experimental results have been presented that describe buckling and postbuckling
behavior of graphite-epoxy and graphite-thermoplastic sipecime0s under variot s loading conditions
(References 1 and 2). An advanced concept for stiffened' _,oraphite-thermopl_s:ic panels is a
thermoformed bead-stiffener. The thermoforming process is a potentially cost-effective
manufacturing technique for these bead stiffeners. The postbuckling respo,_se of bead-stiffened
graphite-thermoplastic shear webs has not been fully described. The influences of local effects,
such as bead-stiffener geometry and orientation, on the postbuc.klipg response of graphite-
thermoplastic shear webs has also not been adequately described.
The present paper describes the results of an experimental and analytical study of the structural
response and failure characteristics of selected bead-stiffened thermoplastic shear webs.
Experimental results are presented for specimens with one stiffener, two stiffeners, and different
stiffener geometries. Selected analytical results that were obtained with the Computational
Structural Mechanics (CSM) Testbed (Ref. 3) computer code are presented. Analytical results that
describe normal and transverse shear stress resultants are also presented.
969
https://ntrs.nasa.gov/search.jsp?R=19930021684 2020-03-17T05:12:00+00:00Z
TEST SPECIMEN DESCRIPTION
The graphite-thermoplastic specimens tested in this investigation were P'aoricated from
commercially available unidirectional tapes of Hercules Incorporated ASs, graphite fiber and ICI
ACP PEEK resin. All of the specimens tested in this investigation were'_ made from 16-ply
laminates with a [+452/0/+45/90] s stacking sequence. The specimew,s had a 12-inch by 12-inch test
section and the edges were reinforced with fiberglass remforceme .,_ts bonded to the specimens with
a room-temperature-cure adhesive. A total of three specimens ; were tested in this investigation.
All of the specimens were tested had bead stiffeners oriente,a,a parallel to one side of the specimen.
The bead stiffeners had an 1 l-inch length, 0.5-inch helg _t, and a stiffener wadth b s of either 2 or
3 inches. One specimen was tested with one 3-inch-w!,de bead stiffener located at'the center of
the test section. Two specimens were tested with two bead stiffeners located 3.5 inches from the
edges of the test section. One of the two-stiffener ,_pecimens had 2-inch-wide bead stiffeners and
the other specimen had 3-inch-wide bead stiffen er,s. Photographs of specimens with one and two
bead stiffeners are shown in Figure 1.
Edge,
reinforcement
11 in, [3ead
stiffener
Test
• :......... section
12 in.
Material
AS4 PEEK
16-pty laminate
[+4520 +4590] s
Figure 1
970
SUMMARY OF TEST RESULTS FOR BEAD-STIFFENED WEBS
A summary of results for all of the bead-stiffened shear webs that were tested is shown in
Figure 2. All of the specimens were tested using a picture-frame fixture which is composed of
two back-to-back steel rails bolted to the edge reinforcements on each specimen edge. The rails
are connected with high strength pins, and the pins are positioned with their center lines
coincident with the comers of the test section of the specimen. A uniaxial loading condition was
used to test each specimen. The uniaxial loading condition consisted of applying a tension load P
along one diagonal of the specimen. The sketch on the left of Figure 2 illustrates the applied
load P. The applied shear flow q was calculated from the applied load by imposing equilibrium
conditions on the test section of the specimen and by assuming that the picture-frame fixture and
edge reinforcements are rigid and pinned at the comers.
Applied shear flow q as a function of extension along the diagonal _i in the direction of the
applied load is shown on the right of the figure. The theoretical shear flow at buckling for the
specimen with one bead stiffener (one-stiffener web) is indicated in the figure by the open circle.
Only the specimen with one centrally located stiffener failed at an applied load greater than the
theoretical buckling load. Failure of the specimen with one bead stiffener is indicated by the
filled circle. Failure of the specimens with two bead stiffeners (two-stiffener web) is indicated by
the filled squares. The specimen with one 3-inch-wide bead buckled at an applied shear flow of
approximately 1,000 lb/in, and failed at an applied shear flow of approximately 1,500 lb/in. The
specimens that were tested with two bead stiffeners failed prior to buckling. The two-stiffener
specimens with 2-inch-wide bead stiffeners and 3-inch-wide stiffeners failed at a value of applied
shear flow of approximately 1,400 lb/in, and 1,900 lb/in., respectively.
P, Ib
b=12in. bS
20001"- b$ = 3 in. _/
,.oo y
,b I b==2,n.q='°°°r,'S=uc ,,no
500_ _/" ° One-stiffener web
0 .04 .08 .012 .016
8, in.
Figure 2
971
OUT-OF-PLANE DEFLECTION OF BEAD-STIFFENED WEBS
Out-of-plane deflection w was measured at the center of a bead normalized by the laminate
thickness t is shown in Figure 3 as a function of applied shear flow q for all of the specimens
that were tested. Failure of the one-stiffener specimen is indicated by the filled circle. Failure
of the two-stiffener specimens is indicated by the filled squares. All of the specimens deformed
out-of-plane when loaded. Photographs of typical moire-fringe patterns for a one-stiffener and a
two-stiffener specimen are shown on the right of the figure. The one-stiffener specimen with a
3-inch-wide bead stiffener exhibited the most out-of-plane deflection during loading with a
maximum value of almost 10 times the laminate thickness. The two-stiffener specimens with 2-
and 3-inch-wide bead stiffeners had a maximum out-of-plane deflection of approximately two and
one times the laminates thickness, respectively. The change in slope of the curves shown in
Figure 3 suggests that the bead-stiffened webs have a nonlinear response when loaded to failure.
2000 -
1500
Ih
q, _ 1000
in.
5O0
0
-bs= 3.0 in.
/- bs = 2.0 in.
• One-stiffener web
• Two-stiffener web
t= Laminate thickness
I t I 1 I
2 4 6 8 10
w/t
W
q
q
q q
q
W
Figure 3
972
BACK-TO-BACK SURFACE STRAIN RESULTS FOR WEBS WITH 3-INCH-WIDE
STIFFENERS
A comparison of surface strain results for one- and two-stiffener specimens with 3-inch-wide
stiffeners is presented in Figure 4 as a function of applied shear flow q. The surface strains e
were recorded from back-to-back strain gage rosettes located at the center of a bead stiffener.
Results for the one-stiffener specimen are indicated by the filled circles. Results for the two-
stiffener specimens are indicated by the filled square. Surface strain measurements e. from strain
gages oriented parallel to the direction of applied load are shown in the upper left of the figure.
The one-stiffener specimen had a maximum tensile strain approximately of 0.006 in./in, at failure.
The two-stiffener specimen had a maximum tensile strain value of approximately 0.004 in./in, at
failure. Surface strain measurements e _ from strain gages oriented normal to the direction of
applied load are shown in the upper ngfit of the figure. The one- and two-stiffener specimens
had a maximum compressive strain of approximately -0.001 in./in, and -0.004 in./in., respectively.
The change in slope of the curve for the one-stiffener specimen suggests a nonlinear load-strain
response. The divergence of the back-to-back strain gage results also suggests that bending strains
occur at the center of the bead stiffener in the direction of applied load and increase as the
specimen is loaded to failure. Surface strain measurements e x from back-to-back strain gages
oriented parallel to the longitudinal direction of the stiffener are shown in the lower left of the
figure. The one-stiffener specimen had a maximum strain value in the x-direction of
approximately 0.004 in./in, at failure. The two-stiffener specimen had a maximum strain value of
approximately 0.001 in./in, at failure. Also, the change in slope of the curves suggests that the
load in the stiffeners was redistributed as the specimen was loaded to failure.
1500 1500
q,l_-lO00 Ib
• q,[_..lO00
500 500
0
t-direction c-direction
2000 - 2000 -
2000
1500
q, I,_.b1000
In.
50O
0 I I
-.002 0 .002 .004 .006 -.004 -.002 0 •002
_t £c
x-direction • One-stiffener web .4 ,,=
- _ " Two-stiffener web ./_X.
_.rl .. _,_ %/ _q_strain gage
I I I I l_-_]_c
0 .001 .00_ .003 .004 "X_/ t _ "_ x-
Figure 4
973
BACK-TO-BACK SURFACE STRAIN RESULTS FOR WEBS WITH TWO STIFFENERS
A comparison of surface strain results for two-stiffener specimens with 2- and 3-inch-wide (bs)
stiffeners are presented in Figure 5 as a function of applied shear flow q. The surface strains E
were recorded from back-to-back strain gage rosettes located at the center of a bead stiffener.
Results for a two-stiffener specimen with 2.0-inch-wide stiffeners are indicated by the filled
circle. Results for the two-stiffener specimens with 3-inch-wide stiffeners are indicated by the
filled square. Surface strain measurements E t from strain gages oriented parallel to the direction
of applied load are shown in the upper left of'the figure. Back-to-back surface strain results in
the t-direction indicate that two-stiffeners specimens with 2- and 3-inch-wide stiffeners had the
same load-strain response up to failure. However, the two-stiffener specimen with 3-inch-wide
stiffeners failed at higher values of applied shear flow and tension strain. Surface strain
measurements _ e from strain gages oriented normal to the direction of applied load are shown in
the upper right of Figure 5. Back-to-back surface strain results in the c-direction indicate that
the two-stiffener specimen with 2-inch-wide stiffeners had higher values of compressive strain at
corresponding values of applied shear flow than the two-stiffener specimen with 3-inch-wide
stiffeners. Surface strain results _ .. from back-to-back strain gages oriented parallel to the
longxtudmal direction of the stiffener are shown m the lower left of the figure. Back-to-back
surface strain results in the x-direction indicate that the two-stiffener specimen with 3-inch-wide
stiffeners had higher bending strains than the specimen with 2-inch-wide stiffener. The change
in slope of the curves for back-to-back surface strain in the x-direction suggests that a
redistribution of load occurred in the bead stiffener as the specimens were loaded to failure.
Also, the difference in back-to-back surface strain results shown in the figure indicmes that
significant bending strains occurred as the specimens were loaded to failure.
2000- t-direction c-direction
1500
q, _ 1000
in.
5OO
_t
0 -- t i
-.004 0 .004 .008 .012
_t • Location A
_/__ _A__ • Location B
Failure
2000 -
1500
q, lb 1000
IW.
500
G
-.012 -.006 0 .006 .012
EC
Failed specimen
Figure 5
974
TYPICAL FAILURE CHARACTERISTICS OF BEAD-STIFFENED WEBS
Typical failure characteristics of the bead-stiffened shear webs that were tested are shown in
Figure 6. The surface strain results for a one-stiffener specimen are presented in Figure 6 as a
function of applied shear flow. The surface strain results were recorded from back-to-back strain
gage rosettes at two locations. Surface strain results from strain gage rosettes at Location A,
which is near the center of the stiffener, are indicated by the filled circles. Surface strain
results from strain gage rosettes at Location B, which is near the end of the stiffener, are
indicated by the filled squares. Surface strain measurements _. from strain gages oriented parallel
to the t-direction _ t are shown in the upper left of the figure. _urface strain measurements _ c
from strain gages in the c-direction are show in the upper right of the figure. The surface strata
results indicate that much higher tensile and compressive strains occur near the end of the bead
stiffener. Also, divergence of the back-to-back strain gage results indicate that higher bending
strains occur near the ends of the stiffener than at the center as the specimen was loaded to
failure. The bending strains near the end of a stiffener is caused by the eccentricity of the
stiffener with respect to the midplane of the test section. A photograph of the failed specimen
with one stiffener is shown in the lower right of Figure 6. The failure of the specimen initiated
at the end of bead stiffener. The failure was probably caused by the high strains that occur near
the end of the stiffener.
2000- t-direction
1500
q, Ib 1000
500 -
0
-.004
I •
m .,_1[ t,
t
\
\ 1 I I
0 .004 .008 .012
_t
• Location A
q q • LocationB
Failure
2000 -
c-direction
1500
q, _ 1000
In.
500
0 I
-.012 -.006 0 .006
_Sc
Failed specimen
Figure 6
I
.012
975
STRESS RESULTANTS DISTRIBUTION IN A BEAD-STIFFENED WEB
Stress resultant contours for a one-stiffener specimen calculated from a geometrically nonlinear
finite element analysis at an applied shear flow value of 1553 lb/in, which included transverse
shear deformation effects are presented in Figure 7. Contour plots of the inplane normal stress
resultants N_ in a direction parallel to the longitudinal direction of the stiffener are shown in the
upper left otXthe figure. The inplane normal stress resultant contour results show that regions of
high normal stress resultants exist near the ends of the bead stiffener where failure initiated
during the tests Contour plots of the inplane shear stress resultants N are shown in the upper
• " X , ,
right of the figure. The shear stress resultant contour results show that _eglons of high shear
stress resultants exist near the ends of the bead stiffener and away from the stiffener in the test
section area Contour plots of the transverse shear stress resultants Q and Q are shown in the
• • . X
lower left and right of the figure, respectively. The transverse shear stress resYultant Qy contours
show regions of high transverse shear stress resultants occur near the ends of the bead'_tiffener.
The transverse shear stress resultant Qv contours show regions of high transverse shear stress
resultants occur near to and along the ddges where the bead stiffener intersects with the skin.
The stress resultant contour results suggest that the stiffener eccentricity causes significant local
transverse shear stress resultants in a bead-stiffened shear web.
Nmax
I
N min
amax
i i
1
Qmin Qmin
Figure 7
976
CONCLUDING REMARKS
An experimental and analytical investigation has been conducted to study the structural response
and failure characteristics of bead-stiffened graphite-thermoplastic shear webs. Results are
presented for specimens with one and two stiffeners. Results are presented for specimens with
different bead stiffener widths. Results from a nonlinear finite element analysis are also
presented that describe inplane and transverse stress resultant distribution of a bead-stiffened
composite plate loaded in shear. Experimental results suggest that bead-stiffened shear webs
have a nonlinear response when loaded to failure. Surface strain results suggest that bending
strains occur near the bead stiffener when loaded to failure. Analytical results suggest that
stiffener eccentricity causes significant local transverse shear stress resultants to occur in bead-
stiffened webs. The bead-stiffened webs failed near the end of the stiffener where analysis
indicates that significant inplane stress resultants exist.
REFERENCES
, Jegley, Dawn C.: Compression Behavior of Graphite-Thermoplastic Panels with Circular
Holes or Impact Damage. 8th DOD/NASA/FAA Conference on Fibrous Composites in
Structural Design, November 28-30, 1989. NASA CP-3087, September 1990.
o Rouse, Marshall: Effect of Cutouts or Low-speed Impact Damage on the Postbuckling
Behavior of Composite Plates Loaded in Shear. AIAA Paper NO. 90-0966; Proceedings of
AIAA/ASME/ASCE/ASH/ASC 31st Structures, Structural Dynamics and Materials Conference,
Long Beach, CA, April 1990.
o Stewart, C. B.: The Computational Structural Mechanics Testbed User's Manual. NASA TM-
100644, October 1989.
977



Structural response of bead-stiffened thermoplastic shear webs

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server