This paper investigates the relationship between open hole tensile strength and distance between a hole and a stitch in a textile composite material. Tension tests were completed on various specimens with widths of 1 in., 2 in., 3 in. and a constant width to hole diameter ratio of 4. The composites tested were warp knits with AS4 fibers and 3501-6 resin. Test results show a small percent change of net strength with stitch location. However, due to the large scatter in data, the small 6% change in net strength is considered negligible

Pierucci, Kathleen A.

English

NASA Technical Reports Server

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A Study of How Stitch Placement Affects the Open Hole
Tension Strength of Stitched Textile Composite Materials
Kathleen A. Piemcci
Clarence C. Poe
Keith Furrow
Research And Technology Group
Materials Division
Mechanics And Materials Branch
Langley Research Center
Hampton, Virginia
August 8, 1995
599
https://ntrs.nasa.gov/search.jsp?R=19970003107 2020-06-18T00:11:30+00:00Z
Abstract
This paper investigates the relationship between open hole tensile
strength and distance between a hole and a stitch in a textile composite
material. Tension tests were completed on various specimens with
widths of 1 in., 2 in. or 3 in. and a constant width to hole diameter ratio
of 4. The composites tested were warp knits with AS4 fibers and 3501-6
resin. Test results show a small percent change of net strength with
stitch location. However, due to the large scatter in data, the small 6%
change in net strength is considered negligible.
Introduction and Background
There is strong interest in using composite materials for primary airplane components. The
chief advantage of composites over presently used metals is the low density of composites
compared to the high density of metals. The overall weight of an airplane would be reduced
thereby reducing fuel consumption. Another advantage of using composites is the stiffness of the
material. By using composites, future airplane wings will have a higher aspect ratio of length to
width because of the increased stiffness of the wings. This results in a more economical plane
because high aspect ratio wings reduce the drag on an aircraft. The third advantage of composites
over metals is that composite strength and stiffness can be tailored. Components of an aircraft will
need extra strength in certain directions depending on the design of the component. Anisotropic
composites can have increased strength and stiffness in different directions by laying extra fibers in
the directions of the needed extra strength.
Although there axe many benefits in using composite materials, the present cost of
manufacturing composites is high compared to the costs of manufacturing metals. As a result, an
effort has been made to develop cheaper methods of fabricating composites without a large
decrease in strength or stiffness. One of these new methods being researched is mechanically
stitching dry textile preforms. Fibers in the dry fabric axe oriented in different directions,
depending on where the most strength is needed, and then sewn together by machine. After being
stitched together, the material is cut to the desired size and shape. Resin is used to harden the
material, and bind the layers together. This was accomplished by resin transfer molding (RTM), in
which the material is impregnated with resin and cured. This new method has been proven to be
more cost effective than present manufacturing techniques. Stitching has also been found to reduce
delamination after impact.
Before stitched composites can be used regularly in commercial aircraft, the mechanical
properties of composites must be tested. Further tests are needed to determine the ability to
machine and fabricate final products from stitched composite parts. One concern about the
fabrication of the final components was the effect of hole placement near a stitch. The area
surrounding a stitch tends to have a pocket for resin which may make that area weaker than the rest
of the material. Since higher stress concentrations exist around a hole, there was concern over
whether the material would be weaker in this area if a hole was drilled in a resin enriched area near
a stitch. Therefore, the combination of high stresses between a hole and a stitch needed to be
examined. Open-hole tension tests determined if the distance between the edge of a hole and a
stitch affected the ultimate strength of a specimen.
6O0
Materials
All of the specimens tested were warp knit with 44% AS4 fibers in the +/- 45 o and 13,
directions and 12% AS4 fibers in the 90 ° direction. The A specimens had all seven layers stitched
at once, whereas the HI specimens had the +/-45* and 0* layers stitched together as a group, and
then a layer of 90 ° was stitched between two groups of +/-45 ° and 0 °.
Each row of stitches was sewn at 0.25 in. from the previous row in all three specimen
widths. Different stitch diameters were considered for testing. The diameter of the stitches for the
HI, A1, and A2 specimens was 0.0313 in. (1/32). This type of stitch was tested for the three
width sizes of 3 in., 2 in., and 1 in. However, for the A3 specimens a smaller stitch diameter of
0.156 in. (1/64) was tested. The reason for testing a small diameter stitch was a small diameter
might leave a smaller area or pocket than a large diameter stitch. Since resin tends to collect in
pockets near a stitch, a small area around a stitch was believed to collect less strength-reducing
resin than a large area around a stitch. However, this small diameter stitch was tested only in the 1
in. wide composites.
Different diameter holes were core drilled into the specimens. This type of drilling involves
a cylindrical-shaped drill-bit grinding a hole through the material. The ratio of the width to the hole
diameter of each specimen was kept constant at 4. The hole diameters were 0.25 in., 0.5 in., and
0.75 in for the width sizes of 1 in., 2 in., and 3 in. respectively. The edge of the holes were
located either touching a stitch, close to a stitch, or far from a stitch. For each specimen width, at
least 3 specimens were tested with each hole location. A sample specimen can be seen in Figure 1.
Procedures
Each specimen was measured, with a caliper, for width, thickness, diameter, and distance
of the longitudinal tangent of the hole from the nearest stitch. The width and thickness were
needed to calculate the cross sectional area of the specimen. The thickness of the specimen was
measured on both sides of a hole, and the average of the two measurements were recorded as the
thickness. The diameter of the drilled hole was measured to check for consistency. The distance
of the longitudinal tangent of the hole from the stitch was measured in order to compare the hole
placements of each sample.
After each specimen was measured, the ends of the specimens were taped to protect the
grips of the tension machine. The specimen was placed squarely in the lower grip of the tension
machine and the upper grip was maneuvered around the upper portion of the specimen and closed.
The machine applied a load to the specimen at a rate of 0.05 in./min, until fracture. The ultimate
tensile load was recorded from each experiment for later comparison. Specimens were tested in
groups according the width, and then hole placement with the largest specimens tested first.
The tension testing machine for the largest specimens of 3 in. width was a Materials
Testing Machine (MTS) Hydraulic Fatigue Test System, with a 100 KIP capacity. The machine
was controled with a MTS 458.20 microconsole. For the two smaller widths of 2 in. and I in., a
MTS Hydraulic Fatigue Test System, with a 50 KIP capacity and MTS 458.20 microconsole was
used for testing. A computer was connected to the microconsoles to read the output signals. The
data acquisition software program plotted the load vs. the grip stroke.
Results
The net open-hole tensile strength of the each specimen versus the distance of the edge of
the hole from the stitch was graphed. The net strength allowed for specimens of different cross
sectional areas to be compared. Calculations of the net strength were made by the following
formula:
(l) Net Strength = Load / ((Width-Diameter) • Thickness)
The average net strength of all the specimens by type, by size, and by the location of the edge of
the hole can be seen in Table 1. The net strength was used to calculate the percent of scatter in the
data. The difference between the highest and lowest net strengths of specimens with the same size,
stitch diameter, and stitch technique were divided by the net strength of the highest specimen as in
the following equation:
(2) (Highest net strength - Lowest net strength) / Highest net strength
601
For the largestwidth sizeof 3 in., boththe HI and the A2 specimens were graphed
together for comparison between materials. As can be seen in Figure 2, the distance between the
edge of the hole and the stitch did not affect the ultimate strength of either material. The average
strength of the H 1 material with the edge of the hole close to the stitch (HC), was compared with
the average of the H1 with the edge far from the stitch (HF). Although there was a slight 2.3%
decrease in the average strength of the H 1 specimens from HC to HF, Figure 2 shows the scatter
of the data points was as high as 7.4%. With this high percentage of scatter, it can not be
concluded that the strength changed significantly when the distance between the edge of the hole
and the stitch decreased. The average net strength of the A2 specimens with the edge close to the
stitch (A2C) was 79.33 ksi, and the average net strength of the specimens with the hole far from
the stitch (A2F) was 73.82 ksi. The A2 specimens similarly showed a decrease in average net
strength of 6.9%, but with as much as a 10.5% scatter of data. Since the scatter of data was higher
than the change from A2C to A2F, it was concluded that the change was negligible. Figure 2 also
shows that the open-hole tension strength of the H 1 and A2 Specimens are similar even though the
H 1 has a different stitching technique
Results from the tests with the 2 in. wide H1 and A1 specimens are shown in Figure 3 and
Figure 4. In Figure 3, the average net strength of the H1 with the edge close to the stitch (HC)
was 78.24 ksi. This strength is 1.8% less then the average net strength of 79.65 ksi for the H1
with the edge far from the stitch (HF). The percent scatter of the points was 11.4%, which was
higher than the decrease in net strength of 1.8%. As a result, the change in strength was negligible
when compared to the higher percent scatter of data. As can be seen in Figure 4, the average net
strength of the A1 specimens that had the edge close to the stitch (AIC) was 86.04 ksi. The
average net strength of the A 1 specimens that had the edge far from the stitch (A1F) was
86.28 ksi, which was less than 1% higher than the A1C specimens. This change in average net
strength was especially low when compared to the other tests. Since the 21.7% scatter of data was
significantly higher than the change of net strength, the change in net strength was considered to be
negligible. The average strengths of the H 1 for the 2 in. wide specimens was lower than the A 1
specimens even though the H 1 had a different stitching technique.
The results of the 1 in. wide specimens can be seen in Figure 5, Figure 6, and Figure 7.
In Figure 5, the H 1 specimen with the tangent close to the stitch (HC) had an average net strength
of 90.65 ksi. This net strength was 5.7% lower than the net strength of 96.16 ksi for the H 1 with
the edge far from the stitch (HF). However, the scatter of data was 15.5% which was higher than
the percent change in the net strengths of the specimens. The change in strength from HC to HF
was considered to be negligible since the percent scatter of data was higher than the percent change
in strength. In Figure 6 it can be seen that the AI specimen with the edge close to the stitch (A1C)
had an average net strength of 93.97 ksi. The AIC net strength was 8.0% lower than the average
net strength of 102.17 ksi for the A1 specimen with the edge far from the stitch (A1F). However,
since the scatter of data was 13.5%, the 8.0% change on net strength was considered negligible.
A third set of tensile tests was conducted with 1 in. wide A3 specimens These specimens had a
smaller stitch diameter of 0.0156 in. as compared to the stitch diameter of 0.0313 in. used in the
A 1 and H 1 specimens. As can be seen in Figure 7, the average net strength of the A3 specimens
with the edge close to the stitch (A3C) was 94.25 ksi. This net strength was 7.8% lower than the
average net strength of 102.17 ksi for the A3 specimen with the edge far from the hole (A3F).
The scatter in data was 12.4%, which was more than the change in net strength.
Conclusions
In conclusion, all the specimens had a percentage of scatter higher than the percent change
in net strength. As a result, there was no significant change in strength due to hole location near a
stitch. Future tests should focus more on why the percent scatter of data was so high, and more
tests with hybrid material. Hybrid material uses the same stitching technique as the H 1 specimens
but has stronger IM7 fibers in the 0 o direction.
602
References
Hawley, Arthur V., "Development of Stitched / RTM Primary Structures for Transport
Aircraft," NASA Contractor Report 191441, NASA Contract NS 1-18862, July 1993.
603
Table 1
Material
Hybrid Close
(HC)
Hybrid Far
(HF)
A2 Close
(A2C)
A2 Far
(A2F)
Hybrid Close
(HC)
Hybrid Far
(HF)
A1 Close
(AlC)
A1 Far
(AIF)
Width
(in.)
3
3
3
3
2
2
2
2
Ave. Net
Strenqth (ksi)
77.27
75.45
79.33
73.82
78.24
79.65
86.04
86.28
Material Width Ave. Net
(in.) Strenqth (ksi)
Hybrid Close 1 90.65
(HC)
Hybrid Far 1 90.16
(HF)
A1 Close 1 93.97
(A1C)
A1 Far 1 102.17
(A 1 F)
A3 Close 1 94.25
(A3C)
A3 Far 1 102.17
(A3F)
I
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I
I
I
I I
I I
I I
I I
I I
I I
I I
I I
I
I I
I I
I I
I I
I I
I
I
I
I
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I
'1 I
I I
I I
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Figure 1 I
Sample Specimen
Ratio of Width to Diameter = 4
,. Distance from the hole
Diameter
to the stitch
--_L Ira=
w
Width
604
J_
L
Net Strength Vs. Distance of
the Hole from the Stitch for H-75 and A-75
120
II0
100
90 "--
80
70
6O
5O I
0 0.02
Figure 2
o ......._o ........ o .... o -
I I I I
004 006 008 O.1
Distance from Stitch (in.)
! I
0.12 0.14 0.16
--e. -H-75 [
I----e--- A-75 .
IAverage Net StrengthJ
HC 77.27 (Ksl) I
HF 75.45 IA2C 79.33A2F 73.82
L
LI3
z
Net Strength Vs. Distance or
the Hole from the Stitch for All H150
120
I10
I00
9O
0
80 -----------"--
70
6O
50 I
0 0.02
Figure 3
o
o Oo
I I I 1 I !
0.04 t) 06 O.Otl [I.I 0.12 0 14
Dlst,_nce Irom the Stitch (In)
Average Net Strength I
HC 7824 (ksl) IHF 79.65
0.16
vP
u_
Z
Net Strength Vs. Distance of
the Hole from the Stltch for All A150
120
110
100
90
8O
70
60
50 I I I I I I I
0.02 0.04 0.06 0.08 O. I O. 12 O. 14
Distance From the Stitch (In.)
Figure 4
Average Net Strengm I
AIC 86.04 (kst) I
AIF 86.28 J
0.16
(/I
[p)
Z
20
10
00
o
90 o
8O
70
60
50
0.02
Flgure 5
Net Strength Vs. Distance of
the Hole from the Stitch for All H125
o o o _
o
o
o
o
o o
I
0.04
I 1 I I I
0.06 0.08 0.1 0.12 0.14
Distance from the Stitch (In.)
Average Net Strength I
HC 90.65 (ksl) IHF 96.16
0.16
6O6
vc
¢i.
Z
,2o I
Net Strength Vs. Distance of
the Hole from the Stitch for All A125
110
I00
90
80
70
60
50 ' '
0
Flgure 6
@
@ @
, I , , , I , , , I , , , I
0.04 0.08 O. 12 O. 16
Distance from the Stitch (in.)
I Average Net Strength
AIC 93.97 (ksi)
AIF 102.17
v
c-
c
.h
U3
W
Z
120
110
100
Net Strength Vs. Distance of
the Hole from the Stitch for All A325
@
@
@
._--------i
9O
80
70
60
50 I I I
0
Figure 7
X •
I I I I I I I ' I I I I I
0,04 0.08 O. I 2 O. 16
Distance from the Stitch (in.)
Average Net Strengt_
A3C 94.25 (ksl) /
A3F 102,17 I
607



A Study of How Stitch Placement Affects the Open Hole Tension Strength of Stitched Textile Composite Materials

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