The goal is to determine how microstructural factors, especially the architecture of microstructural factors, control fatigue damage in 3D reinforced polymer composites. Test materials were fabricated from various preforms, including stitched quasi-isotropic laminates, and through-the-thickness angle interlock, layer-to-layer angle interlock, and through-the-thickness stitching effect weaves. Preforms were impregnated with a tough resin by a special vacuum infiltration method. Most tests are being performed in uniaxial compression/compression loading. In all cases to date, failure has occurred not by delamination, but by shear failure, which occurs suddenly rather than by gradual macroscopic crack growth. Some theoretical aspects of bridging are also examined

Cox, B. N.

Dadkhah, M. S.

Inman, R. V.

Mitchell, M. R.

Morris, W. L.

Schroeder, S.

English

NASA Technical Reports Server

N93-30850
MICROMECHANICS OF FATIGUE IN WOVEN AND STITCHED COMPOSITES
B.N. Cox, M.S. Dadkhah, R.V. Inman,
M.R. Mitchell, W.L. Morris, and S. Schroeder
Rockwell International Science Center
1049 Camino Dos Rios
Thousand Oaks, CA
Summary
The goal of this basic research program is to determine how microstructural
factors, especially the architecture of reinforcing fibers, control fatigue damage in 3D
reinforced polymer composites.
Test materials are being fabricated from various preforms, including stitched
quasi-isotropic laminates; and through-the-thickness angle interlock, layer-to-layer angle
interlock, and through-the-thickness stitching effect weaves. Preforms are impregnated
with a tough resin by a special vacuum infiltration method. Careful control of this
process and the cure cycle is necessary to obtain specimens free of bubbles and cracks.
Most tests are being performed in uniaxial compression/compression loading.
In all cases to date, failure has occurred not by delamination (as in a 2D composite) but
by shear failure, which occurs suddenly rather than by gradual macroscopic crack growth.
Special apparatus has been set up to enable optical images of the gauge section
to be captured, digitized, and stored at various epochs in the fatigue life. These images
are subsequently analyzed by our unique high accuracy strain mapping instrument,
HASMAP, to reveal the evolution of strain fields-during fat-igue. The strain fields are
measured with sufficient resolution to illuminate micromechanical events on the scale of
individual fiber tows. They reveal the subtle damage that foreshadows eventual failure.
Some theoretical aspects of bridging effects have also been examined,
especially the role of the bridging length scale, which is the increment of crack growth
over which bridging by 3D tows becomes effective. Among other things, these considera-
tions reveal why impact damage can be such a strong function of specimen size and
shape.
579
https://ntrs.nasa.gov/search.jsp?R=19930021661 2020-03-17T04:30:51+00:00Z
MATERIAL MATRIX
Preforms
Most preforms in the material matrix are either woven fiber mats supplied by
Textile Technologies Inc. (TTI - Hatboro, Pennsylvania) or stitched quasi-isotropic
laminates supplied by HiTech/Hexcel Inc. (Reno, Nevada). They are described in the
following table. The TTI preforms vary in thickness from 0.35" to 0.5", while each
Hexcel panel is approximately 5/16" thick.
Woven preforms from TTh
Table 1
Preform Matrix
Warp Stitching Approximate
Panel Weave Pattern and Weft Fiber Dimensions
A layer-to-layer AS4 AS4 38". 4" ____
angle interlock _ o_ _x" o/.:_ _ • o
B layer-to-layer AS4 S-glass 30" Q" _<'°-_ _ _ °_ °
angle interlock ,)y(. "_-_ !i _ ,, _-_, _ _ ,,
C through-the-thickness AS4 S-glass 38" _ 4" _ . >__>.__ _-" , _- __.-_-_....
angle interlock LAYER TO LAYER
ANGLE INTERLOCK
D through-the-thickneSSangleinterlock AS4 AS4 38" _ 4" o
E through-the-thickness ASQ ASQ 12" _ 9"
stitching effect
F through-the-thickness ASg E-glass 12" _ 9H
stitching effect
THROUGH-THE-THICKNESS
ANGLE INTERLOCK
/ " , . ,,
'o o:o o_o olo o_o o_o
'o o!o o_o o_o o:o o;o
Warp Stitching Approximate o oo o:o o_o o_o o_o
and Weft Fiber Dimensions o o:o o,o o!o o!o o:o
o o:o o!o o:o o:o oEo
Stitched preforms from Hexceh
Panel Stitching Pattern
G 6 rows/in. T300 S-glass [2" _ 12"
H 12 rows/in. T300 S-glass 12" _ 12"
[ Two half-thickness panels T300 S-glass 12" _ 12"
each stitched at
6 rows/in.;
then joined at
6 rows/in.
"'STITCHING" EFFECT
Resin
Most composites have been filled with Tactix 138 resin and H41 hardener,
supplied by Dow Chemical (Freeport, Texas). A few specimens were filled early in the
program with Epon 828 (Shell Chemical Co.). Since the Tactix 138 is tougher, it gives a
much superior product (see "Specimen Fabrication" below).
Odd Specimens
The basic material matrix is supplemented from time to time by small
specimens received from other research groups.
580
SPECIMEN FABRICATION
All the preforms in Table I are being processed in-house into composite speci-
mens within this program. The processing route is as follows.
Impregnation and cure using Tactix 138 resin and H41 hardener (Dow
Chemical) are effected in a reusable aluminum mold. The preform is placed in the mold,
heated to 65°C, and degassed in a vacuum of ~I Torr. The resin is then mixed with the
hardener and also heated to 6_°C and degassed. The resin mixture is then poured over
the preform (still at 65°C) and the whole assembly is degassed twice again. The first
degassing results in bubbling out of some volatiles followed by apparent boiling of the
resin mixture as the pressure falls further. The second degassing pass produces very
little emission of volatiles down to the pressure at which boiling began on the first
degassing, where the degassing is now stopped. The mold is then closed and the specimen
cured, the curing cycle (which is chosen to maximize resin toughness) comprising 2 hrs at
120°C and 2 hrs at 177°C. All fabrication runs to date have produced 10"×4" panels,
from which 12-16 rectangular fatigue specimens can be machined.
In contrast to specimens cured with the more brittle resin Epon 828, in which
extensive microcracking was found emanating from interior sites where glass interlock
tows abutted AS4 warp or weft tows, the specimens cured with Tactix 13g are almost
entirely free of microcracks. The only microcracks found are in surface resin layers. If
excess resin is allowed to cure outside the preform, large, regularly spaced cracks will
form in it. If care is taken to avoid excess resin (which is not especially easy for the TTI
preforms, whose thickness varies), microcracks are still occasionally found in surface
pockets of resin between tows (Fig. 1). Such microcracks extend into the composite until
they encounter the internal microstructure. Thus their average size is about the
diameter of one fiber tow. The large openings of these cracks attest to the substantial
residual stresses in the epoxy that evidently drive the microcracking.
(a) (b)
Fig. I Two surface microcracks found in a composite of Tactix 138/H_1 and preform
from panel D of Table 1.
581
ASSESSMENT OF PREFORM INFILTRATION
Fabricated composites have been sectioned and polished perpendicular and
parallel to the warp fibers to allow porosity and fiber volume fractions to be estimated.
Figure 2(a) shows an optical micrograph of a typical section through a through-the-
thickness angle interlock composite (panel D of Table 1). There is no visible porosity.
Figure 2(b) shows an SEM micrograph of a section of a single tow, in which it is clear
that complete wetting of each individual fiber has occurred.
From Fig. 2(a), the volume fraction constituted by warp, weft, and interlock
tows can be estimated at - 0.7; while from Fig. 2(b), the volume fraction of fibers within
each tow is also - 0.7. Consequently, the net fiber volume fraction of the composite is
- 0.5. More accurate values will be obtained from resin extraction measurements.
(a) (b)
Fig. 2 (a) Optical micrograph of section parallel to warp fibers through composite of
panel D of Table I with Tactix 13g/H41 matrix. (b) SEM micrograph of section
through an individual warp tow in (a).
582
TEST MATRIX
Macroscopic Tests
The basic test matrix consists of tests on specimens made from the preforms
of Table 1. Panels fabricated as described in "Specimen Fabrication" are cut into
rectangular specimens of dimensions 4"× 1/2"× panel thickness. These are being
subjected to fully reversed (R : -1), tension-tension (R : 0), and compression-compression
(R :-co) uniaxial fatigue loading. For each R ratio, five load amplitudes are being
covered, makinga matrix of 15 tests for each material in the material matrix.
The material matrix includes the six weave types supplied by TTI and the three
stitching patterns in the Hexcel panels (Table 1). Because of weave or stitching asym-
metry, neither the TTI nor the Hexcel specimens possess in-plane isotropy, being instead
orthotropic. Specimens are being tested with the load axis lying along both in-plane
directions of reflective symmetry. The material matrix therefore comprises lg cases.
The total number of tests in the basic matrix is accordingly 270
(15 loads × Ig materials).
Micromechanical Tests
When test results look especially significant or interesting, the micro-
mechanics of the failure are investigated using either the actual specimens in the matrix
or nominally identical ones in repeat tests. The micromechanical investigations are
based primarily on HASMAP, as described on the following pages.
583
IN SITUDIGITAL IMAGECOLLECTIONFORHASMAP
HASMAP (for high accuracy strain field mapper) determines fields of
differential surface deformation by comparing digital images of deformed and
undeformed material. In its usual mode, HASMAP achieves very high spatial resolution
by analyzing optical or SEM micrographs. The micrograph images are digitized by a CCD
camera, which provides the maximum possible spatial stability. The pixel density in the
digitization is increased by digitizing small areas on each micrograph at a time so that
the entire micrograph is represented by arrays of between 1000 × 1000 and 4000 × 4000
pixels.
In this program, we have developed special apparatus to streamline this
process by obviating the use of micrographs, opening the way to substantial decreases in
labor costs and increasing the volume of data that can be analyzed. The CCD camera is
placed in situ in the test apparatus (Fig. 3). It is mounted on a precise x-y-z stage, which
can be scanned to obtain an array of overlapping (typically by 1096) digital images
directly from the specimen surface. At 60X, the entire specimen gauge section can be
covered by a 2 × 2 array of digitized images, each of which is represented by a 4g0 × 512
array of pixels (the intrinsic array size of the CCD camera).
While the rigid structure of the x-y=z stage ensures the absence o[ relative
rotations between digitized images in the 2 × 2 array, relative translations are known
only to within a few microns. These translational errors are computed and eliminated by
calculating cross-correlations within the overlapping strip between each pair of images
and adjusting their assigned positions within the computer accordingly.
Fig. 3 Loading frame and CCD camera mounted on x-y-z stage.
584 ORIGINAL pAGL
BLACK AND WHITE pHOTOGRAPH
DISPLACEMENT VECTORS CALCULATED BY HASMAP
The net result of the procedure described on the previous page is a digitized
1000 _ [000 pixel image of the entire specimen width over a good portion of the gauge
length. Relative displacements between pairs of such images (usually recorded at
constant load before and after the application of fatigue cycles) are found by high speed
cross-correlation calculations. The calculations are performed over a 16 × 16 array of
points, with the analysis for each point carried out over a 32 × 32 pixel image segment.
The displacement accuracy of this analysis is _+0.2 pixels, corresponding to ~ + 0.2 um on
the specimen. (While HASMAP can achieve displacement accuracy of *_ 100k when high
magnification optical micrographs are used, 1 such precision would be superfluous here.
Furthermore, data must be recorded over as much of the gauge section as possible when
testing smooth specimens because deformation and eventual failure can be found
anywhere on it. Limitations of data storage capacity dictate that this be done at
moderate magnification.)
A typical array of displacement vectors is shown in Fig. 4.
(a) (b)
Stress ' " _ ' ' ' " _ "
axis
°
Fig. 4 Deformation in a stitched composite (supplied by B. Dexter, NASA Langley;
not listed in Table 1) resulting from g000 compression-compression fatigue
cycles to a peak load of -260 MPa. Images were recorded at -130 MPa.
(a) Digitized image of gauge section, which has been decorated with SiC
particles to enhance contrast. (b) Differential displacement vector field
computed for the area of (a) showing the irreversible displacements associated
with fatigue damage.
_M.R. 3ames, W.L. Morris, and B.N. Cox, "A High Accuracy Automated Strain Field
Mapper," Exptl. Mech. 30, 60-67 (1990).
585
DISPLACEMENT AND STRAIN FIELDS FROM HASMAP
Continuous full field deformation maps are deduced from vectors such as those
of Fig. 4 as follows. Contours of constant u and v (the displacements perpendicular and
parallel to the sample axis) are formed by linear interpolation between values of the
appropriate component of the vector array. These are then smoothed by an algorithm
which dilates and then erodes each contour level, removing a prescribed amount of high
frequency noise. There results a set of contours (Fig. 5(a)) which, for the present
application, have a resolution comparable to high resolution moir6 fringes, with the
advantage over the moir6 method that the absolute sense of the local deformation (i.e.,
whether it is compressive or tensile) is always known. The derivatives of these
displacements are then taken to generate a complete set of strain fields relative either
to the specimen or to the local principal stress axes as desired. Figure 5(b) shows the
strain component Eyy derived from Fig. 5(a), where the y-axis is the stress axis.
The zone of heavy deformation (strains ~ 10 TM) striking across Fig. 5(b) from
center=left to center coincides with a single stitching tow, the only one failing in the
field of view. Longer range deformation shows that the gauge section has also sheared
during fatigue. This is clearest in Fig. 4(b): the directions of the displacement vectors
at the top left and bottom right indicate tensile strain normal to the diagonal that runs
from bottom left to top right. The specimen ultimately failed along this diagonal.
(a)
®
(b)
!iiiiiiiiiiiil
I Stress axis
®
\
Fig. 5
® ®
(a) Contours of the displacement v (parallel to the stress axis) derived from the
vectors of Fig. 4. Symbols + and - show local maxima and minima. Contours
are separated by 0.6 _m in displacement. (b) The strain field e derived from
(a). In this black and white rendition of a color image, imp_rYtant zones of
maximum tension and compression have been labeled. Each gray level
indicates a strain increment of 1.1 × I0 TM.
586
EXTRINSIC FACTORS IN CRACK BRIDGING
Crack bridging by 3D fiber tows is the primary source of macroscopic crack
resistance following delamination in impact tests, and it is probably one of the key
[actors in preventing microscopic cracks from propagating into macroscopic cracks
during fatigue of pristine material. Extensive theoretical studies of bridged cracks have
recently been conducted, mainly under other funding. 2'3 This work has shown very
clearly that the mechanics of bridged cracks depend strongly on extrinsic factors. In
particular, the stress required to propagate a matrix crack and the transition from intact
bridging ligaments (ductile failure) to ruptured ligaments (brittle failure) depend strongly
on load distribution and the size of the specimen and any notch.
Within this program, the significance of these results for impact testing in 3D
composites, and possibly for fatigue resistance too, has also been considered. A central
role is played by the 'bridging length scale," the characteristic length of initial crack
growth over which the bridging zone matures. 3 This is a critical engineering property of
any material containing bridged cracks. It can vary from the order of the fiber diameter
to several meters, depending on the nature of the bridging ligaments. The length to
which a microcrack might propagate before arresting is determined by the bridging
length scale; the size of a delamination crack caused by impact will depend on how large
the stressed area is relative to the bridging length scale; and whether such a bridged
delamination will subsequently propagate to failure will depend on the specimen size in
terms of the bridging length scale.
While no predictive theory for bridged cracks that is accurate for all load
configurations and specimen sizes has yet been demonstrated, it is clear from
simulations3 that the critical information is the relation between the opening displace-
ment u of a bridged crack and the tractions p imposed by the bridging ligaments. If p(u)
is not known and conventional engineering methods of fracture mechanics are applied,
severely nonconservative predictions are possible. The effects discussed in Ref. 3 may
account for many of the discrepancies reported for different kinds of impact tests
involving specimens of different sizes and shapes. 4
ACKNOWLEDGMENTS
Work supported by NASA Contract No. NASI-18840 under the technical moni-
toring of Dr. Charles Harris. Dr. Mac Puckett of Dow Chemical kindly supplied the
Tactix 138 resin and H41 hardener, along with greatly appreciated advice on their use.
Dr. Benson Dexter of NASA Langley supplied several specimens of stitched composites.
Dr. Gary Farley of NASA Langley provided valuable assistance in procuring woven
preforms.
2B.N. Cox and D.B. Marshall, "Stable and Unstable Solutions for Bridged Cracks in
Various Specimens," Acta Metall., in press.
BB.N. Cox) "Extrinsic Factors in the Mechanics of Bridged Cracks)" submitted to Acta
Metal].
4P.E. Reed, "Impact Testing of Plastics for Design, ') in ATA-MAT 89, Turin, Italy, June
1989 (Associazione Tecnica Dell'automobile, Italy, 1989).
587



Micromechanics of fatigue in woven and stitched composites

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