The importance of textile composite materials in aerospace structural applications has been gaining momentum in recent years. With a view to better understand the suitability of these materials in aerospace applications, an experimental program was undertaken to assess the mechanical properties of these materials. Specifically, the braided textile preforms were infiltrated with suitable polymeric matrices leading to the fabrication of composite test coupons. Evaluation of the tensile properties and the analyses of the results in the form of strength moduli, Poisson's ratio, etc., for the braided composites are presented. Based on our past experience with the textile coupons, the fabrication techniques have been modified (by incorporating glass microballoons in the matrix and/or by stabilizing the braid angle along the length of the specimen with axial fibers) to achieve enhanced mechanical properties of the textile composites. This paper outlines the preliminary experimental results obtained from testing these composites

Avva, V. Sarma

Lyon, Malcolm

Sadler, Robert L.

English

NASA Technical Reports Server

fN93"
Tensile Properties of Textile Composites
V. Sarma Awa, Robert L. Sadler and Malcolm Lyon
Mars Mission Research Center
Department of Mechanical Engineering
North Carolina A & T State University
Greensboro, NC
Abstract
14754
The importance of textile composite materials in aerospace structural ap-
plications has been gaining momentum in recent years. With a view to better
understand the suitability of these materials in aerospace applications, an ex-
perimental program was undertaken to assess the mechanical properties of
these materials. Specifically, the braided textile preforms were infiltrated with
suitable polymeric matrices leading to the fabrication of composite test
coupons. Evaluation of the tensile properties and the analyses of the results
in the form of strength, moduli, Poisson's ratio, etc., for the braided compos-
ites are presented. Based on our past experience with the textile coupons, the
fabrication techniques have been modified (by incorporating glass microbal-
loons in the matrix and/or by stabilizing the braid angle along the length of
the specimen with axial fibers) to achieve enhanced mechanical properties of
the textile composites. This paper outlines the preliminary experimental re-
suits obtained from testing these composites.
Introduction
The advantages in using the engineered laminated composites having
high specific strength and specific modulus in the design of payload sensitive
spacecraft have been known for some time. As the range of applications using
these laminated composites is increasing, other design limiting problems such
as the interlaminar strength, material toughness, fabrication and tooling costs
need to be addressed critically. By virtue of the inherent geometrical layups of
laminated composite structural components, non-uniform stresses in the vari-
ous layers and directions take place under loads. These stresses in turn induce
interlaminar shear and normal stresses at the 'free' boundaries or edges [1] of
composite laminates resulting in damage and or premature failures. While de-
sign modifications or techniques can be incorporated to suppress the delami-
nations, nevertheless newer approaches in developing alternate and cost-effec-
tive technologies are needed. As a result, the existing technologies from the
textile industry are being explored and developed [2,3] in the design of
aerospace composite components.
PRECEDING PAGE BLANK NOT FILMED
145
https://ntrs.nasa.gov/search.jsp?R=19930005565 2020-03-17T09:00:03+00:00Z
Many of the textile manufacturing processes such as braiding, knitting,
and weaving will reinforce through the "thickness" of the composite structural
components, thereby virtually eliminating or minimizing the effect of delamina-
tions as well as giving rise to additional strength, if necessary, between the lay-
ers. Further, it's believed that the textile fabrication technologies could con-
tribute to lower costs. As a result, the research and developmental activi-
ties in the use of textile technologies for structural composites are gaining mo-
mentum in developing multiaxial fabrics, integrally woven structures with stiff-
eners, and near-net-shape preforms [4,5]. These textiles preforms assume the
near net structural shapes after impregnating and curing with suitable matrix
with very little, if any, further processing operations. In addition, the textile
preforms in their near-net-shape may be dry, prepregged or impregnated with
(matrix) powder or commingled with matrix filaments. The latter two tech-
niques by-pass the (liquid) resin transfer molding operation. One of the attrac-
tive features in using the net-shape preform technologies is its amenability for
automation such as pultrusion, where production quantities justify, thereby
maintaining quality control and lowering the cost of fabrication.
Materials, Processing, and Fabrication
146
The graphite textile prefonns were braided with tows of Celion G30-500
(BASF Structural Materials, Inc.) fibers. These fibers measure 7 microns in di-
ameter and have a modulus of 30 Msi. The preforms were braided by a 4-step
process where each tow was interlocked with the other tows to form a true 3-D
structure. The braid angle was varied between 17 and 30 degrees.
The preforms were fabricated using a vacuum/compression molding con-
solidation process. The matrix system was comprised of two components-
Epon 828 (Shell Chemical Co.) and Jeffamine T-403 (Texaco Chemical Co); 100 ....
parts-by-weight to 42 parts-by-weight, respectively. This resin system was cho-
sen because it had a relatively long pot life and low viscosity which was neces-
sary for this method of molding. The two components were heated separately
at 600 C for ten minutes to reduce the viscosity which in turn reduces the time
required to evacuate the catalyzed resin. The two components were then com-
bined and evacuated for ten minutes after the vacuum pressure reached one
Torr. The bottom of the stainless steel mold cavity was completely covered to a
depth of about 1/16" with the evacuated resin. The textile preform was then
placed into the mold cavity and was covered with the remaining resin, and
again evacuated for ten minutes after the vacuum pressure reached one Torr.
Once the evacuation process was complete, the plunger was placed in the mold
cavity and the mold was placed in a heated press where the excess resin was
squeezed out in order to obtain a desired composite thickness. The nominal
specimen dimensions are 0.100 x 0.750 x 10 inches. The composites Were
cured for three hours at i00 ° C. The fiber volume of the specimens was found
to vary between 45% and 55%. The tensile coupons were tabbed using 1/8"
fiberglass tabs. The tabs were attached with an adhesive film, FMi23-5
(American Cyanamid), which required a cure cycle of 90 minutes at 100 ° C and
f50 psi. The geometry and dimensions of four different types of braided test
specimens are shown in Figure 1.
In an effort to lower the density of the composite specimens, glass mi-
croballoons were introduced into the matrix of some of the test coupons.
Glass microballoons can be thought of as controlled voids which will displace
some of the resin. Thus, the idea was to use controlled voids to reduce the
density of the samples without any significant loss in many of the properties.
The glass microballoons used in the study had a diameter range from 7 to 70
microns. The resulting density reduction was found to be about 2%. The spec-
imen without microballoons had a density of 1.51 g/cm 3 and with microbal-
loons, it was 1.48 g/cm 3 . The test results indicate that the resin displaced by
the glass microballoons (5% by weight) did not significantly affect the tensile
strength of the composite specimen. However, a difference in the strain behav-
ior was observed. Further studies are suggested to optimize the strength and
density with respect to the percentage of microballoons that can be incorporat-
ed in the composite materials.
Experimental Results and Analyses
Numerous factors affect the performance of braided textile composites. It
is not uncommon to observe significant variations in the mechanical properties
of braided composites. At the laboratory coupon-level fabrication, the quality
control of textile composites becomes even more critical if one is assessing the
relative merits of these composites for their mechanical properties. Several fac-
tors such as the quality of braided preform, stabilizing the braid angle through
the molding process, extent of curing, compaction of the preform, maintaining
precise fiber volume ratio, size effect, uniform density, location of the specimen
cut from a finite length of the preform, complete removal of voids and tiny
pockets of trapped air bubbles, uniform distribution of fiber tows and the sur-
rounding matrix throughout the length of the test specimen, symmetry of the
composite, dimensional variations in the as-molded specimen, etc., may
adversely affect the mechanical properties.
Preliminary experimental data of the tensile properties of four different
types of braided composites are shown in Table I. In presenting this initial
data, no attempt has yet been made to censor the data based on the factors
mentioned in the preceding paragraph that may adversely affect the mechanical
properties. Further analyses of the raw data are underway to refine and com-
pare the properties of the four types of specimens with the factors that influ-
ence the results in the background.
The objective in introducing the glass microballoons in the composite
was to not only lighten the braid but to reduce the formation of resin pockets
that may develop at the crossings of large-sized (12k) tows. A cursory exami-
nation of the resulting data as shown appears to indicate that the nominal
tensile properties were not affected except for the transverse strain. Based on
density and fiber volume ratio measurements to be performed shortly, the eval-
147
! II
_--d
IL__. _.'__:
TAB ADHESIVE
PLAN
a
_c
e
II
t
SIDE VIEW
GLASS / EPOXY
COMPOSITE TAB
TYPICAL BRAIDED SPECIMEN GEOMETRY
TYPICAL DIMENSIONS OF FOUR TYPES OF SPECIMENS*
Plain braids 0,750 0.352 0.102 2.75 4.5 0.25 2.5
Braids with
microballoons 0.753 0,354 O. 104 2.75 4.5 0.25 2.5
0.877 0.354 O. 104 2.75 4.5 0.25 2.5
0.878 0.362 O. 112 2.75 4.5 0.25 2.5
Braids with
axial fibers
Braids with
axial fibers &
microballoons
148
All dimensions in inches. Braids with axials have 37% (26 tows out of 70)
axial fibers.
Figure 1. Tensile Specimen Geometry and Dimensions
TABLE-I • TENSILE PROPERTIES OF BRAIDS*
Braid Load Stress Strain Modulus Trans.Strain Poisson's
angle Pmax (_max El I m,'L'¢ E 1 1 _ 12 ratio
(deg) (kip) (ksi) (%) (Msi) (%) V 12
Plain braids 23/30 13.22 +1.7 173.5+_23 1.47_+o.o9 11.8 2.31_+o.36 1.57
Braids with 2 I/3O 13.44+2.3 172.1_+29 1.52_+0.11 11.3 2.69-+0.40 1.77
microballoons
Braids with - 15.81+I.0 179.0-k 14 1.34__+0.07 13.4 I . 19-k-0.27 0.89
_Lxial fibers
Braids with
axial fibers &
mlcroballoons
13.14+__2.3 135.3_+27 1.26-_+0. lO 10.7 1.39-+0.33 I. I0
The experimental data shown here in tabular form is for convenience only. Comparison of
data among the various cases shown here is not recommended. Extensive data reduction and
detailed analyses are planned for the purpose of comparing the properties on a rational basis.
Factors such as braid angle, fiber volume ratio, failure zones, size effect, density, specimen
imperfections, if any, etc., are expected to influence the preliminary data shown above.
149
150
uation of specific mechanical properties will be conducted. If the succeeding
analyses prove to be encouraging, a corollary that needs to be addressed is the
percentage of microballoons that could be introduced in a unit volume of the
braid resulting in one or more than one designated optimum property or proper-
ties. Further, what is the effect of introducing other grades of microballoons
as to size and specimen thickness on these properties ?
A nonuniform drift in the braid angle was noticed which may be at-
tributed to the compression molding process. In order to stabilize the braid
angle as well as to further improve the mechanical properties of the braids si-
multaneously, ,axial fibers(tows) were introduced during the preform fabrica-
tion. As expected, preliminary data reveal an improvement in the nominal
tensile strength and modulus, and a decrease in the strains. The percentage of
axial fibers selected in this study was random. Optimization techniques may
be applied to determine the percent of axial fibers that can be embedded in
braided specimens.
The experimental results with a matrix containing microballoons and
braids containing axials were obtained very recently and are being analyzed
critically to understand their significance.
,
.
.
.
Conclusions
Four different types of braided specimens were studied to assess their
tensile mechanical properties. Factors that affect the properties are
identified for further data reduction and analyses.
A preliminary evaluation on the effect of microballoons in the plain
braided composites on the mechanical properties is presented. Many
of the properties appear to remain unchanged. Further analyses based
on density measurements may show ,'m improvement in some of the
specific properties.
Axial fibers were introduced in the braids to improve the mechanical
properties further, and the preliminary results are encouraging.
Through these studies, it has become clear that many factors affect the
mechanical properties ranging from preform and composite fabrication
through testing and ration_ data reduction.
Acknowledgements
This work is supported by the National Aeronautics and Space
Administration through a Grant Number NAGW-1331 to the Mars Mission
Research Center at North Carolina State and North Carolina Agricultural and
Technical State Universities. Prof. EI-Shiekh, a member of the MMRC at
NCSU, contributed the braided prefonns.
References
i°
,
,
.
.
Simonds, R.A., Stinchcomb, W., and Jones, R.M., "Mechanical Behavior
of Braided Composite Materials", Composite Materials: Testing and
Design (Eighth Conference), ASTM STP 972, J.D. Whitcomb, Ed.,
American Society for Testing and Materials, Philadelphia, 1988.
Ko, F. K., '_Fensile Strength and Modulus of a Three-Dimensional Braid
Composite", Composite Materials: Testing and Design (Seventh
Conference), ASTM STP 893, J.M. Whitney, Ed., American Society for
Testing and Materials, Philadelphia, 1986.
Macander, A.B., Crane, R.M., and Camponeschi, E.T., Jr.,"Fabrication
and Mechanical Properties of Multidimensionally (X-D) Braided
Composite Materials ", Composite Materials:Testing and Design (Seventh
Conference), ASTM STP 893, J.M. Whitney, Ed., American Society for
Testing and Materials, Philadelphia, 1986.
Mohamed, M.H., Zhang, Z., and Dickinson, L., "Manufacture of Multi-
Layer Woven Preforms", Advanced Composites & Processing Technology ;
MD-Vol.5, Editors: T.H. Tsiang and R.A.Taylor, The American Society of
Mechanical Engineers.
E1-Shiekh, A.H.M., "The Effect of Processes and Processing Parameters
on the Properties of 3-D Composites", 33rd International SAMPE
Symposium & Exhibition, Anaheim, CA., March 7-10, 1988.
151



Tensile properties of textile composites

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server