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Compressive behavior of pultruded composite plates with circular holes
Composite Structures 65 (2004) 29–36 www.elsevier.com/locate/compstruct
Compressive behavior of pultruded composite plates with circular holes M. Saha b
a,*
, R. Prabhakaran b, W.A. Waters Jr.
c
a Department of Mechanical Engineering, Tuskegee University, Tuskegee, AL 36088, USA Department of Mechanical Engineering, Old Dominion University, Norfolk, VA 23529, USA c Lockheed Martin Engineering, Hampton, VA 23681, USA
Available online 7 November 2003
Abstract Compressive behavior of E-glass fiber/isophthalic polyester resin matrix pultruded composite sheet material is reported for two thicknesses. The effect of circular holes on compressive strength and failure strain was also investigated. In the first series of tests, compression tests were performed with plate specimens without any hole. These specimens were instrumented with several pairs of back-to-back axial strain gages at different locations on the surface of the specimen to capture global buckling (if any) during compression. In the second series of tests, plate specimens with circular holes were prepared. A wide range of diameter to width ratio of D=W ¼ 0:075–0.75 was chosen. The open-hole specimens were also instrumented with several strain gages (combination of single and strip gages) in and around the surface of the hole to determine the strain distribution, strain concentration factor, transverse and through-the-thickness normal strain, etc. A relationship between the compressive strength and hole-diameter was established, and compared with the base line material properties. Finally, an attempt was made to determine the compressive failure initiation mechanisms and failure progression mechanisms in the presence of holes through sectioning and examining under a stereo microscope. 2003 Elsevier Ltd. All rights reserved. Keywords: Pultruded composites; Circular holes; Compressive strength; Plate specimen
1. Introduction The pultrusion process has been very successful in producing low-cost, high-quality structural products. Various cross-sectional shapes with continuous length can be produced in a single step process. Pultruded products, based on relatively inexpensive constituents such as E-glass fibers and polyester resins, were originally used for commercial applications where lifetime cost benefits, lightweight and corrosion resistance made them attractive over steel or aluminum. In recent years, significant progress has taken place in pultrusion technology. New reinforcements, such as S-glass, carbon, and Kevlar, as well as new matrix resins, such as epoxy, phenolic, and even thermoplastics, have opened new applications in aerospace, sporting and transportation industries. Incorporation of composites into structural *
Corresponding author. Tel.: +1-334-727-8950; fax.: +1-334-7244224. E-mail address: [email protected] (M. Saha). 0263-8223/$ - see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.compstruct.2003.10.002
applications requires drilling holes to facilitate bolting or riveting to other structural elements. These holes introduce stress concentrations which significantly reduce the failure stress of the composite panel. Experimental studies have shown that severe strength reduction of advanced composites can occur due to the presence of defects such as hole, slit, and damage [1–4]. This is due to the fact that hole, slit and damage can cause strain concentration in the vicinity of the defect. However, the strength reduction can be even more in the case of compression as compared to tension. It is known that the composite material is not totally brittle and some stress relief occurs around the defects. The high stresses at the defect boundary initiate local failure which result in a redistribution of stresses. Compressive behavior of laminated composites with holes have been studied by a number of investigators. Waas and Babcock [5] studied the compressive failure in graphite-epoxy laminates containing a single hole with the aid of real time holographic interferometry and photomicrography. They observed that damage initiates by a combination of fiber
30
M. Saha et al. / Composite Structures 65 (2004) 29–36
microbuckling and delamination. The 0 ply microbuckling originates at the hole edges at 80% of the ultimate compressive strength and propagates into the interior of the specimen. The length of the buckled zone increases with increasing applied load, propagating stably until it reaches a critical length. Then unstable growth begins and the microbuckle traverses the specimen completely. The inplane compressive fracture behavior of carbon fiber-epoxy multidirectional laminates containing a single hole was studied by Soutis and Fleck [6]. They have reported up to 40% reduction in the compressive strength, and the dominant failure was fiber microbuckling in the 0 plies. They also developed a model to predict the compressive strength and found good agreement. However, these studies focused on graphite-epoxy composite laminates with small holes typical of those used for aircraft fasteners. Recently, Rhodes et al. [7] extended this study to evaluate the effect of panel width on compressive strength of graphiteepoxy laminates with various hole sizes ranging from D=W ¼ 0:02 to 0.75. The point stress failure criterion was evaluated to predict the compressive strength and good agreement was found between experiment and prediction. The test results included panels with machined cracks, punched holes and drilled holes. The parameters were shown to have an upper limit given by the reduction in specimen cross-sectional area and lower limit given by the effect of stress concentration. Such studies related to pultruded composites are scarce. The present experimental investigation was conducted to gather a data base for pultruded composites. The compressive behavior of pultruded composites, in the form of sheets of two different thicknesses, is reported in this paper. A wide range of hole sizes is considered to evaluate the compressive strength, and the failure modes.
2. Experimental procedure 2.1. Test material The material used in this investigation is E-glass fiber-reinforced polymer matrix (GFRP) pultruded composite. The material was manufactured by Creative Pultrusions of Alum Bank, PA [8]. The trade name is Pultrex-1500. The matrix material is isophthalic polyester. The pultruded composite consists of layers of unidirectional roving fibers in the pultrusion direction sandwiched between layers of continuous strand mats (CSM). The lay-up configuration and percent glass content in each layer are shown in Fig. 1 [9]. It is seen from the figure that the 6.3-mm thick sheet has two roving layers and five CSM layers of which one is fine and four are coarse, while the 12.7-mm thick sheet has four roving layers and nine CSM layers of which three are fine and six are coarse. The compositions of the
Fig. 1. Lay-up configuration of two pultruded composite sheets.
constituent layers of various pultruded composite sheets are summarized in Table 1. It is interesting to note that the 6.3-mm thick sheet has a higher amount of total glass as well as unidirectional roving but a lower amount of CSM as compared to the 12.7-mm thick sheet. 2.2. Test specimen preparation All the test specimens were cut from the pultruded composite sheets of 6.3-mm and 12.7-mm thicknesses, using a water-cooled diamond saw. The width and length of the plate specimens were determined so that a strength failure was more likely to occur than a buckling failure. Buckling analyses were performed on various plate specimens to determine the lowest critical load. The predicted buckling load was used to determine the specimen dimensions. The material properties used in the buckling analysis were taken from Ref. [10]. Two different sets of plate specimens were made. The specimens of 152-mm length and 102-mm width were cut from the 6.3-mm thick sheet, while the specimens of 254mm length and 178-mm width were cut from the 12.7mm thick sheet. The length direction of all the specimens Table 1 Summary of the compositions of various constituent layers of pultruded composite Glass fiber reinforcement Total glass in composite, percent Roving as a percent of total glass CSM as a percent of total glass Coarse CSM as a percent of total glass Fine CSM as a percent of total glass
Sheet thickness 6.3-mm
12.7-mm
60
58
42
36
58
64
45
46
13
18
M. Saha et al. / Composite Structures 65 (2004) 29–36
was chosen parallel to the pultrusion direction (parallel to the roving fibers). After cutting, the ends and sides of each specimen were machined flat and parallel, using a surface grinding machine (Brown and Sharpe 510) within 0.0025-mm, to permit uniform compression loading. Circular holes were machined in the center of some specimens of each thickness using a diamond impregnated core bit. A wide range of diameter to width ratios (D=W ) of 0.075–0.75 was chosen. Two plexi-glass plates were placed on top and bottom of the specimens during the drilling operation to avoid any machining induced damage, especially on the exit side. The drilling operation was performed in three steps, starting with a small drill to the final hole size. For the case of bigger holes, a reamer tool was used with a very small radial feed until the required hole size was achieved. At least three specimens for each value of D=W were tested. 2.3. Instrumentation One specimen of each thickness without a hole was instrumented with several pairs of back-to-back axial strain gages at various important locations. The 152mm · 102-mm plate specimen was instrumented with a total of three pairs of back-to-back strain gages; two pairs at 25.4-mm from the loading end and specimen edges, and one pair at the specimen center. The 254mm · 178-mm plate specimen was instrumented with a total of five pairs of back-to-back strain gages; two pairs at 38-mm from the loading end and 25.4-mm from specimen edges, one pair at specimen center, and one pair at one-third location along the vertical axis of symmetry. All the strain gages were oriented parallel to the loading direction. The purpose of these back-to-back strain gages was to capture any possible buckling or bending, and also to verify uniform introduction of compression loading. At least one specimen of each hole size was instrumented with several strain gages. One pair of back-toback strain gages was placed at 25-mm from the loading edge along the vertical axis of symmetry, and oriented parallel to the loading direction. These gages were used to measure the far field surface strain. One strip gage, consisting of ten gages, along with several single gages were mounted on the section between the hole edge and the specimen edge along the horizontal axis of symmetry. The strip gage was placed very close to the hole edge. All the strain gages were oriented parallel to the direction of the applied load to capture the strain gradient due to the presence of hole. Three additional strain gages were placed inside the hole surface. At the intersection of the hole edge and the horizontal axis of symmetry, one strain gage was placed with orientation parallel to the applied load to measure the strain concentration, and one strain gage was placed with orientation through the thickness to monitor failure––in
31
particular delamination that might initiate at the hole edge. At the intersection of the hole edge and the vertical axis of symmetry, one strain gage was placed with orientation perpendicular to the applied load to monitor the transverse strain. 2.4. Test fixture and testing procedure The compression fixture used in this study is similar to the fixture described in Ref. [11]. This fixture has two pairs of adjustable grips to accommodate specimens of different thicknesses. The specimen was placed within the adjustable end grips and the screws were tightened while pressing the specimen-fixture together using two C-clamps, so that the clamped boundary conditions can be applied at the loaded ends. Two side fixtures with knife-edge restraints attached were placed along the side of the specimen so that simply supported boundary conditions can be applied to prevent wide column buckling. A gap of approximately 6.3-mm was left between the side supports and the end grips to allow for compression of the panel without loading the fixture. A typical instrumented specimen in the test fixture ready for compression testing is shown in Fig. 2. All the test specimens were loaded quasi-statically in axial compression using a Tinius Olsen 1.78-MN capacity hydraulic testing machine. During the testing, two direct current differential transformer (DCDT) displacement sensors were used: (i) at the specimen center and oriented transverse to the specimen to measure the out-of-plane displacements (specimens with hole had the DCDT offset) and (ii) alongside the specimen oriented parallel to the direction of load to measure the end shortening. All the electrical signals (strain gages, load cell and DCDT’s) were recorded continuously using a WIN5000e data acquisition system. The
Fig. 2. Photograph of the test fixture and the instrumented specimen.
M. Saha et al. / Composite Structures 65 (2004) 29–36
loading platen was adjusted by applying 20% of the expected ultimate load while monitoring the back-to-ack axial strain gages before final loading. The purpose was to place the specimen-fixture assembly so that it was aligned with the testing machine. A constant cross-head rate of 1.2-mm/min was applied during the test. 2.5. Strain concentration The removal of potential load carrying material from the center of a panel causes internal load transfer and strain concentration which not only depends on the shape of the cutout, but on the amount of material removed. For circular holes in infinite isotropic plates, the stress concentration factor on the hole centerline normal to load application is 3.0 [12]. The orthotropic extension of this theory, developed by Tan [13], expresses the stress concentration factor of an infinite, homogeneous orthotropic plate as a function of laminate material properties as follows: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi rffiffiffiffiffiffiffi E E 11 11 KT1 ¼ 1 þ 2 m12 þ ð1Þ E22 G12 where E11 , E22 , m12 and G12 are the Young’s moduli in the pultrusion direction and perpendicular to the pultrusion direction, the Poisson’s ratio and the shear modulus respectively. The values are taken from Ref. [10]. This relationship yields a value of 3.68 for the 6.3-mm thick specimen and a value of 3.60 for the 12.7-mm thick specimen. Any deviation from these values for different hole diameters must be due to the finite size of the specimen since hole size does not appear in the stress concentration factor. Thus, the finite width correction factor (FWC) must be included to determine the stress concentration factor for finite size specimens (KTg ). The approximate orthotropic FWC is given by following expression [13]: h
d 2 d 4 i d 6 1 1 2 w þ w KT 3 1 wd w KT ¼ 2 KTg ð2Þ where KTg is the stress concentration factor of the finite plate based on gross section area. The improved theory for FWC factor is given by following expression [14]: 6 3 1 wd KT1 1 d M ¼ þ KTg 2 þ 1 d 3 2 w w " 2 # 1 d M KT 3 1 ð3Þ w
3. Results and discussion 3.1. Response of specimens without holes As mentioned before, one specimen of the 6.3-mm thickness was instrumented with three-pairs of backto-back axial strain gages and one specimen of the 12.7-mm thickness was instrumented with five-pairs of back-to-back axial strain gages to capture any possible buckling and also to verify the uniformity of the load introduction. Typical stress–strain responses of the two back-to-back strain gages located at the plate center are shown in Fig. 3 for the 6.3-mm thick specimen and in Fig. 4 for the 12.7-mm thick specimen. As seen in the figures, the two back-to-back strain gage data are very consistent up to 91% of the failure load for the 6.3-mm thick specimen and up to 96% of the failure load for the 12.7-mm thick specimen. Beyond those load levels, the two back-to-back strain gage readings show a little divergence until complete failure. The stress–strain behavior is linear until failure for both the thicknesses. The stress–strain responses for other pairs of back-to-back strain gages are found to be very similar. The out of plane displacement behavior also shows no evidence of global buckling. A summary of the test results is shown in Table 2. The table shows the individual test results and the average values. The failure strain values shown in the parentheses were calculated from the failure stress and the compressive modulus [10]. It can be seen from Table 2 that the average failure stress and strain are higher for the 6.3-mm thick specimens as compared to 12.7-mm thick specimens. The average compressive failure stress of the 6.3-mm plate specimens is very close to the compressive strength of the material reported in Ref. [10], while the average compressive failure stress of
300
250
Compressive Stress (MPa)
32
200
150
3,6
100
50
where sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi 3ð1wd Þ 18 3 1 1 2þð1wd Þ 2 M ¼ 2 2 wd
0 0
ð4Þ
0.5
1.0 1.5 Axial Compressive Strain (%)
2.0
Fig. 3. Compressive stress–strain response for 6.3-mm thick plate specimen with back-to-back strain gages near plate center (3,6).
M. Saha et al. / Composite Structures 65 (2004) 29–36
Table 2 Summary of the compression test results for undamaged plate specimens
250
200 Compressive Stress (MPa)
33
150
Specimen thickness (mm)
Specimen ID
Failure load (kN)
Failure stress (MPa)
Failure Straina (%)
6.3
1 2 3 Average
183.6 183.8 165.5 177.6
266.4 261.9 256.3 261.5
1.52(1.49) (1.49) (1.44) 1.52
12.7
1 2 3 Average
491.1 486.3 508.7 495.4
226.6 220.7 231.0 226.1
1.33(1.27) (1.23) (1.29) 1.33
100 4,9
50
0 0
0.5 1.0 Axial Compressive Strain (%)
1.5
Fig. 4. Compressive stress–strain response for 12.7-mm thick plate specimen with back-to-back strain gages near plate center (4,9).
the 12.7-mm plate specimens is approximately 8% lower then the compressive strength of the material. 3.2. Response of specimens with holes The compression response of specimens with holes discussed here includes the stress vs. strain behavior, residual compressive strength, strain concentration, strain distribution in the vicinity of the hole, and the compressive failure modes. The strain gage data include the axial surface strain and the strains on the curved surface of the hole. As mentioned before, the surface strain gage was located 25-mm below the loading end along the vertical axis of symmetry and oriented parallel
a Failure strain in parenthesis is calculated from compressive failure stress and modulus of the bar specimen. Failure stress and strain are 267 MPa and 1.7% for the 6.3-mm thickness and 245 MPa and 1.45% for the 12.7-mm thickness [10].
to the applied load. Strain values on the curved hole surface were measured using strain gages orientated parallel to the loading direction, transverse to the loading direction and in the thickness direction. Typical stress vs. strain responses of the 6.3-mm thick specimens for two hole sizes (D=W ¼ 0:25 and 0.75) are shown in Figs. 5 and 6. The behavior of axial surface strain and axial strain inside the hole is found to be fairly linear up to failure. The surface strain is compressive for D=W ¼ 0:25, and as the D=W increases the surface strain changes from compressive to tensile (Fig. 6). This is due to the fact that the surface strain gage was located 25-mm from the specimen edge, and as the hole diameter increases the distance between the gage and the hole edge decreases, which may change the state of stress from compressive to tensile stress. The magnitude of the surface strain decreases as the hole diameter increases.
160
140
120 Compressive Stress (MPa)
Hole l Edge (Transverse)
Surface (Axial)
Surface (axial) Hole edge (transverse)
HoleEdge (Thickness)
100
Hole edge (thickness)
80
Hole edge (axial)
60
40
e Edge Hole (Axial) i
20
0 -2.0
-1.5
-1.0
-0.5
0
0.5
1.0
1.5
2.0
Strain (%)
Fig. 5. Compressive stress–strain plots for 6.3-mm thick plate specimen with D=W ¼ 0:25.
34
M. Saha et al. / Composite Structures 65 (2004) 29–36 100
Compressive Stress (MPa)
80 Hole e Edge (Transverse)
Surface (axial) Hole edge (transverse)
60
e Surface (Axial) Hole Ed dge (Axial)
40
d Hole Edge (Thickness)
Hole edge (thickness)
Hole edge (axial)
20
0 -2.0
-1.5
-1.0
-0.5
0 0.5 Strain (%)
1.0
1.5
2.0
Fig. 6. Compressive stress–strain plots for 6.3-mm thick plate specimen with D=W ¼ 0:75.
All the axial strain gages located inside the hole failed before the specimen failure. The strain gages were out of range at approximately 1.625% strain. However, the response of the transverse strain and the thickness strain gages are found to be non-linear for the smaller hole diameter, and as the hole diameter increases the behavior shows linear tendency. There is a load drop before the final failure of approximately 80 MPa for the plate with D=W ¼ 0:25, and 65 MPa for plate with D=W ¼ 0:75 which is believed due to matrix cracking. In fact, matrix cracking was heard during the compression testing when the axial compressive stress level reached approximately those values, and the cracking noise continued with increasing load until complete failure. The load drops observed in the thickness gage indicate that delamination may occur at those stress levels. The axial strain distributions in the vicinity of the hole for a particular applied load are shown in Fig. 7 for
Ratio of Measured Strain to Applied Strain
10 D/W = 0.25 D/W = 0.50 D/W =0.75 D/W = 0.075 (Strain at Hole Edge NOT Measured)
8
6
the 6.3-mm thick specimen. The remote strain corresponding to the applied load at which strain distribution was plotted was equal to 0.0009. This strain value was well below the failure strain. The measured strain values were normalized by the applied remote strain. It can be mentioned that the axial strain values on the surface of the hole were measured using special type strain gages except for hole of D=W ¼ 0:075. For this specimen, the hole was too small to mount the strain gage on the surface of the hole and only the surface strains near the hole were measured. The horizontal axis represents the distance measured from the plate center normalized by the half-width of the specimen. The strain values are found to be highest on the surface of the hole due to stress concentration, and these values are found to be higher for the larger hole size as shown in Fig. 7. The strain values decay as the distance from the edge of the hole increases. The decay rate increases as the size of the hole decreases. The variation of strain concentration (KT ) factor as a function of hole diameter for the 6.3-mm thick specimens is shown in Fig. 8. The theoretical strain concentration factors for various hole diameters were calculated from Eqs. (2) and (3) and are superimposed in Fig. 8. It can be seen that the experimental values agreed well with improved theory as compared to approximate theory for all the hole diameters.
4
3.3. Failure analysis 2
0
0
0.25
0.50
0.75
1.00
Ratio of Distance Measured from Center to Half Width Along the Transverse Axis
Fig. 7. Axial strain distribution in the vicinity of hole (6.3-mm thick).
An attempt was made to determine the failure initiation mechanisms due to the presence of the hole. Some specimens were loaded until the first cracking noise and then unloaded immediately. The specimens were removed from the test fixture for microstructural examination. The specimens were sectioned in the vicinity of
Strain Concentration Factors based on Gross Area
M. Saha et al. / Composite Structures 65 (2004) 29–36
35
10 Experimental Data Improved Theory Approximate Theory
8
6
4
2
0 0
0.25
0.50 D/W
0.75
1.00
Fig. 8. Variation of strain concentration factor as function of hole diameter (6.3-mm thick).
the hole using a diamond-impregnated saw, and the cross-sections were examined under a stereo microscope. A typical photomicrograph showing failure initiation mechanisms for the 6.3-mm thick specimen is given in Fig. 9. It can be seen from this figure that the delamination initiated at the edge of the hole due to the stress concentration. Possibly, the delamination triggered microbuckling of the roving layer. All the specimens with holes fractured due to failure initiated at the edge of the hole in the form of delamination between the roving and CSM layers, which triggered fiber breakage as well as shearing of the CSM layers. The failure surface extends toward the edges of the specimen. The post-failure mechanisms show multiple delaminations, fiber microbuckling and shearing of CSM layers as shown in Fig. 10. The delamination was found to be more pronounced as the hole diameter increased. A summary of the compression test results for various hole diameters is shown in Table 3 for both the
Fig. 9. Photomicrograph of the cross-section in the vicinity of the hole showing failure initiation in the form of delamination.
Fig. 10. Photograph of the cross-section in the vicinity of the hole showing post-failure mechanisms in the form of delamination, fiber breakage, and shear failure.
thicknesses. As mentioned before, at least three specimens were tested for each case and the average values are shown in Table 3. The failure stress was calculated by dividing the failure load by the net area of crosssection. The failure strains were determined by dividing the failure stress by the compressive modulus given in Ref. [10].
Table 3 Summary of the compression test results for plate specimens with holes Specimen thickness
Hole diameter (mm)
Hole dia to width ratio (D=W )
Average failure load (kN)
Average failure stress (MPa)
Average failure straina (%)
6.3-mm
7.6 15.4 22.3 33.0 44.5 56 66.7
0.07 0.15 0.22 0.32 0.43 0.54 0.65
132 106 95 71 63 56 43
193 154 140 104 96 82 65
1.08 0.86 0.78 0.58 0.54 0.46 0.37
12.7-mm
13.5 26.7 41.2 62.3 82.5 101.5 123.6
0.07 0.15 0.23 0.35 0.46 0.57 0.69
343 279 265 222 199 131 106
159 129 121 102 90 61 48
0.89 0.72 0.67 0.57 0.50 0.34 0.27
a
Failure strain was calculated based on the failure stress and the compressive modulus from Ref. [10].
36
M. Saha et al. / Composite Structures 65 (2004) 29–36
concentration factor based on gross area agreed well with the improved theory. The compressive strength was found to be higher for 6.3-mm thick material as compared to 12.7-mm thick material. However, as the ratio of hole diameter to specimen width increased, the differences between the two thicknesses become less significant. It was found that delamination occurred first, which might have triggered fiber failure. The post failure analysis suggested that the compressive failure mechanisms consist of delamination, fiber microbuckling and shearing of CSM layers. References
Fig. 11. Compressive strength as a function of hole diameter to specimen width.
The variation of the compressive strength as a function of hole diameter to width ratio for both the thicknesses is shown in Fig. 11. The compressive strengths of the specimens without holes are also included. The 6.3mm thick specimens exhibit higher compressive strengths as compared to the 12.7-mm thick specimens, probably because of more internal flaws in the thicker material. As the ratio of hole diameter to specimen width increases, the differences between the two thicknesses become less significant.
4. Conclusions The compressive behavior of the pultruded composite plate specimens with and without holes was investigated for two different thicknesses. Several pairs of back-toback strain gages and DCDT devices were used to monitor global buckling and bending during compression testing. Strain gages were used both inside the hole as well as in the vicinity of the hole to capture strain distribution, strain concentration and through-thethickness normal strain, etc. A wide range of hole diameter to width ratios (D=W ¼ 0:25–0.75) was used to determine the compressive strength as a function of hole size. An attempt was made to determine the compressive failure initiation as well as post failure mechanisms. The strain at the hole edge was found to be higher as the diameter of the hole increased. The measured strain
[1] Garbo SP. Compression strength of laminates with unloaded fastener holes. AIAA Paper No. 80-0709, 1980. [2] Starnes JH, Rhodes MD, Williams JG. Effects of impact damage and holes on the compressive strength of graphite/epoxy laminates. Nondestructive evaluation and flaw critically of composite materials, ASTM STP 696. 1979. p. 147–71. [3] Lessard LB, Chang FK. Damage tolerance of laminated composites containing an open hole and subjected to compressive loadings: part I-analysis. J Compos Mater 1991;25:2–43. [4] Lessard LB, Chang FK. Damage tolerance of laminated composites containing an open hole and subjected to compressive loadings: part II-experiments. J Compos Mater 1991;25:44–64. [5] Waas A, Babcock Jr CD. Observation of the initiation and progression of damage in compressively loaded composite plates containing a cutout. NASA Progress Report, Grant NSG-1483. 1986. [6] Soutis C, Fleck NA. Static compression failure of carbon fibre T800/924C composite plate with a single hole. J Compos Mater 1990;24(5):536–58. [7] Rhodes MD, Mikulas Jr MM, McGowan PE. Effects of orthotropy and width on the compression strength of graphite-epoxy panels with holes. AIAA J 1987;22(9):1283–92. [8] Shaping the future. Creative Pultrusion Inc. Alum Bank. Pennsylvania, USA. [9] Devara S. An investigation of the influence of tightening torque and sea water on pultruded FRP omposite bolted joints. MS Thesis. Old Dominion University, VA, 1994. [10] Saha M, Prabhakaran R, Waters WA. Compressive properties of pultruded composites. Mech Compos Mater 2000;36(6):781–90. [11] NASA/aircraft industry standard specification for graphite fiber/ toughened thermoset resin composite material. NASA Reference Publication 1142. Complied by the ACEE Composites Project Office. Langley Research Center. Hampton. VA, 1985. [12] Peterson RE. Stress concentration factors. New York: Wiley; 1974. p. 110–1. [13] Tan SC. Laminated composites containing an elliptical opening. I. Approximate stress analysis and fracture model. J Compos Mater 1987;2(10):925–48. [14] Tan SG. Stress concentration in laminated composites. Pennsylvania: Technomic Publishing Company; 1994.
Compressive behavior of pultruded composite plates with circular holes M. Saha b
a,*
, R. Prabhakaran b, W.A. Waters Jr.
c
a Department of Mechanical Engineering, Tuskegee University, Tuskegee, AL 36088, USA Department of Mechanical Engineering, Old Dominion University, Norfolk, VA 23529, USA c Lockheed Martin Engineering, Hampton, VA 23681, USA
Available online 7 November 2003
Abstract Compressive behavior of E-glass fiber/isophthalic polyester resin matrix pultruded composite sheet material is reported for two thicknesses. The effect of circular holes on compressive strength and failure strain was also investigated. In the first series of tests, compression tests were performed with plate specimens without any hole. These specimens were instrumented with several pairs of back-to-back axial strain gages at different locations on the surface of the specimen to capture global buckling (if any) during compression. In the second series of tests, plate specimens with circular holes were prepared. A wide range of diameter to width ratio of D=W ¼ 0:075–0.75 was chosen. The open-hole specimens were also instrumented with several strain gages (combination of single and strip gages) in and around the surface of the hole to determine the strain distribution, strain concentration factor, transverse and through-the-thickness normal strain, etc. A relationship between the compressive strength and hole-diameter was established, and compared with the base line material properties. Finally, an attempt was made to determine the compressive failure initiation mechanisms and failure progression mechanisms in the presence of holes through sectioning and examining under a stereo microscope. 2003 Elsevier Ltd. All rights reserved. Keywords: Pultruded composites; Circular holes; Compressive strength; Plate specimen
1. Introduction The pultrusion process has been very successful in producing low-cost, high-quality structural products. Various cross-sectional shapes with continuous length can be produced in a single step process. Pultruded products, based on relatively inexpensive constituents such as E-glass fibers and polyester resins, were originally used for commercial applications where lifetime cost benefits, lightweight and corrosion resistance made them attractive over steel or aluminum. In recent years, significant progress has taken place in pultrusion technology. New reinforcements, such as S-glass, carbon, and Kevlar, as well as new matrix resins, such as epoxy, phenolic, and even thermoplastics, have opened new applications in aerospace, sporting and transportation industries. Incorporation of composites into structural *
Corresponding author. Tel.: +1-334-727-8950; fax.: +1-334-7244224. E-mail address: [email protected] (M. Saha). 0263-8223/$ - see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.compstruct.2003.10.002
applications requires drilling holes to facilitate bolting or riveting to other structural elements. These holes introduce stress concentrations which significantly reduce the failure stress of the composite panel. Experimental studies have shown that severe strength reduction of advanced composites can occur due to the presence of defects such as hole, slit, and damage [1–4]. This is due to the fact that hole, slit and damage can cause strain concentration in the vicinity of the defect. However, the strength reduction can be even more in the case of compression as compared to tension. It is known that the composite material is not totally brittle and some stress relief occurs around the defects. The high stresses at the defect boundary initiate local failure which result in a redistribution of stresses. Compressive behavior of laminated composites with holes have been studied by a number of investigators. Waas and Babcock [5] studied the compressive failure in graphite-epoxy laminates containing a single hole with the aid of real time holographic interferometry and photomicrography. They observed that damage initiates by a combination of fiber
30
M. Saha et al. / Composite Structures 65 (2004) 29–36
microbuckling and delamination. The 0 ply microbuckling originates at the hole edges at 80% of the ultimate compressive strength and propagates into the interior of the specimen. The length of the buckled zone increases with increasing applied load, propagating stably until it reaches a critical length. Then unstable growth begins and the microbuckle traverses the specimen completely. The inplane compressive fracture behavior of carbon fiber-epoxy multidirectional laminates containing a single hole was studied by Soutis and Fleck [6]. They have reported up to 40% reduction in the compressive strength, and the dominant failure was fiber microbuckling in the 0 plies. They also developed a model to predict the compressive strength and found good agreement. However, these studies focused on graphite-epoxy composite laminates with small holes typical of those used for aircraft fasteners. Recently, Rhodes et al. [7] extended this study to evaluate the effect of panel width on compressive strength of graphiteepoxy laminates with various hole sizes ranging from D=W ¼ 0:02 to 0.75. The point stress failure criterion was evaluated to predict the compressive strength and good agreement was found between experiment and prediction. The test results included panels with machined cracks, punched holes and drilled holes. The parameters were shown to have an upper limit given by the reduction in specimen cross-sectional area and lower limit given by the effect of stress concentration. Such studies related to pultruded composites are scarce. The present experimental investigation was conducted to gather a data base for pultruded composites. The compressive behavior of pultruded composites, in the form of sheets of two different thicknesses, is reported in this paper. A wide range of hole sizes is considered to evaluate the compressive strength, and the failure modes.
2. Experimental procedure 2.1. Test material The material used in this investigation is E-glass fiber-reinforced polymer matrix (GFRP) pultruded composite. The material was manufactured by Creative Pultrusions of Alum Bank, PA [8]. The trade name is Pultrex-1500. The matrix material is isophthalic polyester. The pultruded composite consists of layers of unidirectional roving fibers in the pultrusion direction sandwiched between layers of continuous strand mats (CSM). The lay-up configuration and percent glass content in each layer are shown in Fig. 1 [9]. It is seen from the figure that the 6.3-mm thick sheet has two roving layers and five CSM layers of which one is fine and four are coarse, while the 12.7-mm thick sheet has four roving layers and nine CSM layers of which three are fine and six are coarse. The compositions of the
Fig. 1. Lay-up configuration of two pultruded composite sheets.
constituent layers of various pultruded composite sheets are summarized in Table 1. It is interesting to note that the 6.3-mm thick sheet has a higher amount of total glass as well as unidirectional roving but a lower amount of CSM as compared to the 12.7-mm thick sheet. 2.2. Test specimen preparation All the test specimens were cut from the pultruded composite sheets of 6.3-mm and 12.7-mm thicknesses, using a water-cooled diamond saw. The width and length of the plate specimens were determined so that a strength failure was more likely to occur than a buckling failure. Buckling analyses were performed on various plate specimens to determine the lowest critical load. The predicted buckling load was used to determine the specimen dimensions. The material properties used in the buckling analysis were taken from Ref. [10]. Two different sets of plate specimens were made. The specimens of 152-mm length and 102-mm width were cut from the 6.3-mm thick sheet, while the specimens of 254mm length and 178-mm width were cut from the 12.7mm thick sheet. The length direction of all the specimens Table 1 Summary of the compositions of various constituent layers of pultruded composite Glass fiber reinforcement Total glass in composite, percent Roving as a percent of total glass CSM as a percent of total glass Coarse CSM as a percent of total glass Fine CSM as a percent of total glass
Sheet thickness 6.3-mm
12.7-mm
60
58
42
36
58
64
45
46
13
18
M. Saha et al. / Composite Structures 65 (2004) 29–36
was chosen parallel to the pultrusion direction (parallel to the roving fibers). After cutting, the ends and sides of each specimen were machined flat and parallel, using a surface grinding machine (Brown and Sharpe 510) within 0.0025-mm, to permit uniform compression loading. Circular holes were machined in the center of some specimens of each thickness using a diamond impregnated core bit. A wide range of diameter to width ratios (D=W ) of 0.075–0.75 was chosen. Two plexi-glass plates were placed on top and bottom of the specimens during the drilling operation to avoid any machining induced damage, especially on the exit side. The drilling operation was performed in three steps, starting with a small drill to the final hole size. For the case of bigger holes, a reamer tool was used with a very small radial feed until the required hole size was achieved. At least three specimens for each value of D=W were tested. 2.3. Instrumentation One specimen of each thickness without a hole was instrumented with several pairs of back-to-back axial strain gages at various important locations. The 152mm · 102-mm plate specimen was instrumented with a total of three pairs of back-to-back strain gages; two pairs at 25.4-mm from the loading end and specimen edges, and one pair at the specimen center. The 254mm · 178-mm plate specimen was instrumented with a total of five pairs of back-to-back strain gages; two pairs at 38-mm from the loading end and 25.4-mm from specimen edges, one pair at specimen center, and one pair at one-third location along the vertical axis of symmetry. All the strain gages were oriented parallel to the loading direction. The purpose of these back-to-back strain gages was to capture any possible buckling or bending, and also to verify uniform introduction of compression loading. At least one specimen of each hole size was instrumented with several strain gages. One pair of back-toback strain gages was placed at 25-mm from the loading edge along the vertical axis of symmetry, and oriented parallel to the loading direction. These gages were used to measure the far field surface strain. One strip gage, consisting of ten gages, along with several single gages were mounted on the section between the hole edge and the specimen edge along the horizontal axis of symmetry. The strip gage was placed very close to the hole edge. All the strain gages were oriented parallel to the direction of the applied load to capture the strain gradient due to the presence of hole. Three additional strain gages were placed inside the hole surface. At the intersection of the hole edge and the horizontal axis of symmetry, one strain gage was placed with orientation parallel to the applied load to measure the strain concentration, and one strain gage was placed with orientation through the thickness to monitor failure––in
31
particular delamination that might initiate at the hole edge. At the intersection of the hole edge and the vertical axis of symmetry, one strain gage was placed with orientation perpendicular to the applied load to monitor the transverse strain. 2.4. Test fixture and testing procedure The compression fixture used in this study is similar to the fixture described in Ref. [11]. This fixture has two pairs of adjustable grips to accommodate specimens of different thicknesses. The specimen was placed within the adjustable end grips and the screws were tightened while pressing the specimen-fixture together using two C-clamps, so that the clamped boundary conditions can be applied at the loaded ends. Two side fixtures with knife-edge restraints attached were placed along the side of the specimen so that simply supported boundary conditions can be applied to prevent wide column buckling. A gap of approximately 6.3-mm was left between the side supports and the end grips to allow for compression of the panel without loading the fixture. A typical instrumented specimen in the test fixture ready for compression testing is shown in Fig. 2. All the test specimens were loaded quasi-statically in axial compression using a Tinius Olsen 1.78-MN capacity hydraulic testing machine. During the testing, two direct current differential transformer (DCDT) displacement sensors were used: (i) at the specimen center and oriented transverse to the specimen to measure the out-of-plane displacements (specimens with hole had the DCDT offset) and (ii) alongside the specimen oriented parallel to the direction of load to measure the end shortening. All the electrical signals (strain gages, load cell and DCDT’s) were recorded continuously using a WIN5000e data acquisition system. The
Fig. 2. Photograph of the test fixture and the instrumented specimen.
M. Saha et al. / Composite Structures 65 (2004) 29–36
loading platen was adjusted by applying 20% of the expected ultimate load while monitoring the back-to-ack axial strain gages before final loading. The purpose was to place the specimen-fixture assembly so that it was aligned with the testing machine. A constant cross-head rate of 1.2-mm/min was applied during the test. 2.5. Strain concentration The removal of potential load carrying material from the center of a panel causes internal load transfer and strain concentration which not only depends on the shape of the cutout, but on the amount of material removed. For circular holes in infinite isotropic plates, the stress concentration factor on the hole centerline normal to load application is 3.0 [12]. The orthotropic extension of this theory, developed by Tan [13], expresses the stress concentration factor of an infinite, homogeneous orthotropic plate as a function of laminate material properties as follows: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi rffiffiffiffiffiffiffi E E 11 11 KT1 ¼ 1 þ 2 m12 þ ð1Þ E22 G12 where E11 , E22 , m12 and G12 are the Young’s moduli in the pultrusion direction and perpendicular to the pultrusion direction, the Poisson’s ratio and the shear modulus respectively. The values are taken from Ref. [10]. This relationship yields a value of 3.68 for the 6.3-mm thick specimen and a value of 3.60 for the 12.7-mm thick specimen. Any deviation from these values for different hole diameters must be due to the finite size of the specimen since hole size does not appear in the stress concentration factor. Thus, the finite width correction factor (FWC) must be included to determine the stress concentration factor for finite size specimens (KTg ). The approximate orthotropic FWC is given by following expression [13]: h
d 2 d 4 i d 6 1 1 2 w þ w KT 3 1 wd w KT ¼ 2 KTg ð2Þ where KTg is the stress concentration factor of the finite plate based on gross section area. The improved theory for FWC factor is given by following expression [14]: 6 3 1 wd KT1 1 d M ¼ þ KTg 2 þ 1 d 3 2 w w " 2 # 1 d M KT 3 1 ð3Þ w
3. Results and discussion 3.1. Response of specimens without holes As mentioned before, one specimen of the 6.3-mm thickness was instrumented with three-pairs of backto-back axial strain gages and one specimen of the 12.7-mm thickness was instrumented with five-pairs of back-to-back axial strain gages to capture any possible buckling and also to verify the uniformity of the load introduction. Typical stress–strain responses of the two back-to-back strain gages located at the plate center are shown in Fig. 3 for the 6.3-mm thick specimen and in Fig. 4 for the 12.7-mm thick specimen. As seen in the figures, the two back-to-back strain gage data are very consistent up to 91% of the failure load for the 6.3-mm thick specimen and up to 96% of the failure load for the 12.7-mm thick specimen. Beyond those load levels, the two back-to-back strain gage readings show a little divergence until complete failure. The stress–strain behavior is linear until failure for both the thicknesses. The stress–strain responses for other pairs of back-to-back strain gages are found to be very similar. The out of plane displacement behavior also shows no evidence of global buckling. A summary of the test results is shown in Table 2. The table shows the individual test results and the average values. The failure strain values shown in the parentheses were calculated from the failure stress and the compressive modulus [10]. It can be seen from Table 2 that the average failure stress and strain are higher for the 6.3-mm thick specimens as compared to 12.7-mm thick specimens. The average compressive failure stress of the 6.3-mm plate specimens is very close to the compressive strength of the material reported in Ref. [10], while the average compressive failure stress of
300
250
Compressive Stress (MPa)
32
200
150
3,6
100
50
where sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi 3ð1wd Þ 18 3 1 1 2þð1wd Þ 2 M ¼ 2 2 wd
0 0
ð4Þ
0.5
1.0 1.5 Axial Compressive Strain (%)
2.0
Fig. 3. Compressive stress–strain response for 6.3-mm thick plate specimen with back-to-back strain gages near plate center (3,6).
M. Saha et al. / Composite Structures 65 (2004) 29–36
Table 2 Summary of the compression test results for undamaged plate specimens
250
200 Compressive Stress (MPa)
33
150
Specimen thickness (mm)
Specimen ID
Failure load (kN)
Failure stress (MPa)
Failure Straina (%)
6.3
1 2 3 Average
183.6 183.8 165.5 177.6
266.4 261.9 256.3 261.5
1.52(1.49) (1.49) (1.44) 1.52
12.7
1 2 3 Average
491.1 486.3 508.7 495.4
226.6 220.7 231.0 226.1
1.33(1.27) (1.23) (1.29) 1.33
100 4,9
50
0 0
0.5 1.0 Axial Compressive Strain (%)
1.5
Fig. 4. Compressive stress–strain response for 12.7-mm thick plate specimen with back-to-back strain gages near plate center (4,9).
the 12.7-mm plate specimens is approximately 8% lower then the compressive strength of the material. 3.2. Response of specimens with holes The compression response of specimens with holes discussed here includes the stress vs. strain behavior, residual compressive strength, strain concentration, strain distribution in the vicinity of the hole, and the compressive failure modes. The strain gage data include the axial surface strain and the strains on the curved surface of the hole. As mentioned before, the surface strain gage was located 25-mm below the loading end along the vertical axis of symmetry and oriented parallel
a Failure strain in parenthesis is calculated from compressive failure stress and modulus of the bar specimen. Failure stress and strain are 267 MPa and 1.7% for the 6.3-mm thickness and 245 MPa and 1.45% for the 12.7-mm thickness [10].
to the applied load. Strain values on the curved hole surface were measured using strain gages orientated parallel to the loading direction, transverse to the loading direction and in the thickness direction. Typical stress vs. strain responses of the 6.3-mm thick specimens for two hole sizes (D=W ¼ 0:25 and 0.75) are shown in Figs. 5 and 6. The behavior of axial surface strain and axial strain inside the hole is found to be fairly linear up to failure. The surface strain is compressive for D=W ¼ 0:25, and as the D=W increases the surface strain changes from compressive to tensile (Fig. 6). This is due to the fact that the surface strain gage was located 25-mm from the specimen edge, and as the hole diameter increases the distance between the gage and the hole edge decreases, which may change the state of stress from compressive to tensile stress. The magnitude of the surface strain decreases as the hole diameter increases.
160
140
120 Compressive Stress (MPa)
Hole l Edge (Transverse)
Surface (Axial)
Surface (axial) Hole edge (transverse)
HoleEdge (Thickness)
100
Hole edge (thickness)
80
Hole edge (axial)
60
40
e Edge Hole (Axial) i
20
0 -2.0
-1.5
-1.0
-0.5
0
0.5
1.0
1.5
2.0
Strain (%)
Fig. 5. Compressive stress–strain plots for 6.3-mm thick plate specimen with D=W ¼ 0:25.
34
M. Saha et al. / Composite Structures 65 (2004) 29–36 100
Compressive Stress (MPa)
80 Hole e Edge (Transverse)
Surface (axial) Hole edge (transverse)
60
e Surface (Axial) Hole Ed dge (Axial)
40
d Hole Edge (Thickness)
Hole edge (thickness)
Hole edge (axial)
20
0 -2.0
-1.5
-1.0
-0.5
0 0.5 Strain (%)
1.0
1.5
2.0
Fig. 6. Compressive stress–strain plots for 6.3-mm thick plate specimen with D=W ¼ 0:75.
All the axial strain gages located inside the hole failed before the specimen failure. The strain gages were out of range at approximately 1.625% strain. However, the response of the transverse strain and the thickness strain gages are found to be non-linear for the smaller hole diameter, and as the hole diameter increases the behavior shows linear tendency. There is a load drop before the final failure of approximately 80 MPa for the plate with D=W ¼ 0:25, and 65 MPa for plate with D=W ¼ 0:75 which is believed due to matrix cracking. In fact, matrix cracking was heard during the compression testing when the axial compressive stress level reached approximately those values, and the cracking noise continued with increasing load until complete failure. The load drops observed in the thickness gage indicate that delamination may occur at those stress levels. The axial strain distributions in the vicinity of the hole for a particular applied load are shown in Fig. 7 for
Ratio of Measured Strain to Applied Strain
10 D/W = 0.25 D/W = 0.50 D/W =0.75 D/W = 0.075 (Strain at Hole Edge NOT Measured)
8
6
the 6.3-mm thick specimen. The remote strain corresponding to the applied load at which strain distribution was plotted was equal to 0.0009. This strain value was well below the failure strain. The measured strain values were normalized by the applied remote strain. It can be mentioned that the axial strain values on the surface of the hole were measured using special type strain gages except for hole of D=W ¼ 0:075. For this specimen, the hole was too small to mount the strain gage on the surface of the hole and only the surface strains near the hole were measured. The horizontal axis represents the distance measured from the plate center normalized by the half-width of the specimen. The strain values are found to be highest on the surface of the hole due to stress concentration, and these values are found to be higher for the larger hole size as shown in Fig. 7. The strain values decay as the distance from the edge of the hole increases. The decay rate increases as the size of the hole decreases. The variation of strain concentration (KT ) factor as a function of hole diameter for the 6.3-mm thick specimens is shown in Fig. 8. The theoretical strain concentration factors for various hole diameters were calculated from Eqs. (2) and (3) and are superimposed in Fig. 8. It can be seen that the experimental values agreed well with improved theory as compared to approximate theory for all the hole diameters.
4
3.3. Failure analysis 2
0
0
0.25
0.50
0.75
1.00
Ratio of Distance Measured from Center to Half Width Along the Transverse Axis
Fig. 7. Axial strain distribution in the vicinity of hole (6.3-mm thick).
An attempt was made to determine the failure initiation mechanisms due to the presence of the hole. Some specimens were loaded until the first cracking noise and then unloaded immediately. The specimens were removed from the test fixture for microstructural examination. The specimens were sectioned in the vicinity of
Strain Concentration Factors based on Gross Area
M. Saha et al. / Composite Structures 65 (2004) 29–36
35
10 Experimental Data Improved Theory Approximate Theory
8
6
4
2
0 0
0.25
0.50 D/W
0.75
1.00
Fig. 8. Variation of strain concentration factor as function of hole diameter (6.3-mm thick).
the hole using a diamond-impregnated saw, and the cross-sections were examined under a stereo microscope. A typical photomicrograph showing failure initiation mechanisms for the 6.3-mm thick specimen is given in Fig. 9. It can be seen from this figure that the delamination initiated at the edge of the hole due to the stress concentration. Possibly, the delamination triggered microbuckling of the roving layer. All the specimens with holes fractured due to failure initiated at the edge of the hole in the form of delamination between the roving and CSM layers, which triggered fiber breakage as well as shearing of the CSM layers. The failure surface extends toward the edges of the specimen. The post-failure mechanisms show multiple delaminations, fiber microbuckling and shearing of CSM layers as shown in Fig. 10. The delamination was found to be more pronounced as the hole diameter increased. A summary of the compression test results for various hole diameters is shown in Table 3 for both the
Fig. 9. Photomicrograph of the cross-section in the vicinity of the hole showing failure initiation in the form of delamination.
Fig. 10. Photograph of the cross-section in the vicinity of the hole showing post-failure mechanisms in the form of delamination, fiber breakage, and shear failure.
thicknesses. As mentioned before, at least three specimens were tested for each case and the average values are shown in Table 3. The failure stress was calculated by dividing the failure load by the net area of crosssection. The failure strains were determined by dividing the failure stress by the compressive modulus given in Ref. [10].
Table 3 Summary of the compression test results for plate specimens with holes Specimen thickness
Hole diameter (mm)
Hole dia to width ratio (D=W )
Average failure load (kN)
Average failure stress (MPa)
Average failure straina (%)
6.3-mm
7.6 15.4 22.3 33.0 44.5 56 66.7
0.07 0.15 0.22 0.32 0.43 0.54 0.65
132 106 95 71 63 56 43
193 154 140 104 96 82 65
1.08 0.86 0.78 0.58 0.54 0.46 0.37
12.7-mm
13.5 26.7 41.2 62.3 82.5 101.5 123.6
0.07 0.15 0.23 0.35 0.46 0.57 0.69
343 279 265 222 199 131 106
159 129 121 102 90 61 48
0.89 0.72 0.67 0.57 0.50 0.34 0.27
a
Failure strain was calculated based on the failure stress and the compressive modulus from Ref. [10].
36
M. Saha et al. / Composite Structures 65 (2004) 29–36
concentration factor based on gross area agreed well with the improved theory. The compressive strength was found to be higher for 6.3-mm thick material as compared to 12.7-mm thick material. However, as the ratio of hole diameter to specimen width increased, the differences between the two thicknesses become less significant. It was found that delamination occurred first, which might have triggered fiber failure. The post failure analysis suggested that the compressive failure mechanisms consist of delamination, fiber microbuckling and shearing of CSM layers. References
Fig. 11. Compressive strength as a function of hole diameter to specimen width.
The variation of the compressive strength as a function of hole diameter to width ratio for both the thicknesses is shown in Fig. 11. The compressive strengths of the specimens without holes are also included. The 6.3mm thick specimens exhibit higher compressive strengths as compared to the 12.7-mm thick specimens, probably because of more internal flaws in the thicker material. As the ratio of hole diameter to specimen width increases, the differences between the two thicknesses become less significant.
4. Conclusions The compressive behavior of the pultruded composite plate specimens with and without holes was investigated for two different thicknesses. Several pairs of back-toback strain gages and DCDT devices were used to monitor global buckling and bending during compression testing. Strain gages were used both inside the hole as well as in the vicinity of the hole to capture strain distribution, strain concentration and through-thethickness normal strain, etc. A wide range of hole diameter to width ratios (D=W ¼ 0:25–0.75) was used to determine the compressive strength as a function of hole size. An attempt was made to determine the compressive failure initiation as well as post failure mechanisms. The strain at the hole edge was found to be higher as the diameter of the hole increased. The measured strain
[1] Garbo SP. Compression strength of laminates with unloaded fastener holes. AIAA Paper No. 80-0709, 1980. [2] Starnes JH, Rhodes MD, Williams JG. Effects of impact damage and holes on the compressive strength of graphite/epoxy laminates. Nondestructive evaluation and flaw critically of composite materials, ASTM STP 696. 1979. p. 147–71. [3] Lessard LB, Chang FK. Damage tolerance of laminated composites containing an open hole and subjected to compressive loadings: part I-analysis. J Compos Mater 1991;25:2–43. [4] Lessard LB, Chang FK. Damage tolerance of laminated composites containing an open hole and subjected to compressive loadings: part II-experiments. J Compos Mater 1991;25:44–64. [5] Waas A, Babcock Jr CD. Observation of the initiation and progression of damage in compressively loaded composite plates containing a cutout. NASA Progress Report, Grant NSG-1483. 1986. [6] Soutis C, Fleck NA. Static compression failure of carbon fibre T800/924C composite plate with a single hole. J Compos Mater 1990;24(5):536–58. [7] Rhodes MD, Mikulas Jr MM, McGowan PE. Effects of orthotropy and width on the compression strength of graphite-epoxy panels with holes. AIAA J 1987;22(9):1283–92. [8] Shaping the future. Creative Pultrusion Inc. Alum Bank. Pennsylvania, USA. [9] Devara S. An investigation of the influence of tightening torque and sea water on pultruded FRP omposite bolted joints. MS Thesis. Old Dominion University, VA, 1994. [10] Saha M, Prabhakaran R, Waters WA. Compressive properties of pultruded composites. Mech Compos Mater 2000;36(6):781–90. [11] NASA/aircraft industry standard specification for graphite fiber/ toughened thermoset resin composite material. NASA Reference Publication 1142. Complied by the ACEE Composites Project Office. Langley Research Center. Hampton. VA, 1985. [12] Peterson RE. Stress concentration factors. New York: Wiley; 1974. p. 110–1. [13] Tan SC. Laminated composites containing an elliptical opening. I. Approximate stress analysis and fracture model. J Compos Mater 1987;2(10):925–48. [14] Tan SG. Stress concentration in laminated composites. Pennsylvania: Technomic Publishing Company; 1994.