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Research Papers

Tensile and Fracture Characterization of PETI-5 and IM7/PETI-5 Graphite/Epoxy Composites Under Quasi-Static and Dynamic Loading Conditions

[+] Author and Article Information
Dongyeon Lee

Department of Mechanical Engineering, Auburn University, Auburn, AL 36830

Hareesh V. Tippur1

Department of Mechanical Engineering, Auburn University, Auburn, AL 36830tippuhv@auburn.edu

Brian J. Jensen, Philip B. Bogert

 NASA Langley Research Center, Hampton, VA 44313

The failure of specimen was assessed based on the SHPB signals and not based on direct optical evidence. Hence, the failure strain value reported is only an estimate.

Although the second step yields accurate estimation of displacements, the resulting data could be noisy due to the choice of the subimage size, number of overlapping pixels of neighboring subimages, accuracy of the minimization scheme used, etc. Therefore, for a better interpretation of the resulting displacement fields and for obtaining strain components, if needed, additional smoothing step is desirable. However, the displacement data from either the second or third step yield negligible difference in fracture parameter estimates.

This observation also emphasizes the need for using full-field real-time deformation measurements in favor of conventional global measurements, such as crack initiation load, to assess critical fracture parameters. That is, if crack initiation toughness alone was evaluated for these two composites, say, using global crack initiation load measurement, one might come to the conclusion that the two material systems have similar fracture performance. This, however, is incorrect if post-initiation responses of the two materials, measured optically in this case, are taken into consideration, showing the superiority of PETI-5 based composite.

1

Corresponding author.

J. Eng. Mater. Technol 133(2), 021015 (Mar 21, 2011) (11 pages) doi:10.1115/1.4003487 History: Received August 30, 2010; Revised December 15, 2010; Published March 21, 2011; Online March 21, 2011

Tensile and fracture responses of the phenylethynyl terminated imide oligomer (PETI-5) are studied. Since this polymer is a candidate aerospace structural adhesive as well as a matrix material in composite systems, neat as well as fiber reinforced forms of PETI-5 are studied under static and dynamic loading conditions. A split-Hopkinson tension bar apparatus is used for performing tensile tests on dogbone specimens. The dynamic fracture tests are carried out using a drop tower in conjunction with 2D image correlation method and high-speed digital photography on edge cracked specimens in three-point bend configuration. A toughened neat epoxy system, Hexcel 3900, is also studied to provide a baseline comparison for neat PETI-5 system. The tensile stress-strain responses show PETI-5 to have excellent mechanical characteristics under quasi-static and dynamic loading conditions when compared with 3900. Fracture behavior of PETI-5 under quasi-static and impact loading conditions also shows superiority relative to 3900. The dynamic fracture behavior of a PETI-5 based graphite fiber reinforced composite, IM7/PETI-5, is also studied and the results are comparatively evaluated relative to the ones corresponding to a more common aerospace composite system, T800/3900-2 graphite/epoxy. Once again, the IM7/PETI-5 system shows excellent fracture performance in terms of dynamic crack initiation and growth behaviors.

Copyright © 2011 by American Society of Mechanical Engineers
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Figures

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Figure 1

Schematic of tensile SHPB apparatus used for testing dogbone specimens at high-strain rates

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Figure 2

Tensile stress-strain responses for four PETI-5 dynamic tensile tests carried out using split-Hopkinson bar apparatus (strain rate ∼2000/s). Inset shows geometry of a dogbone specimen used in the test (units in mm).

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Figure 3

Schematic of the experimental set-up for dynamic fracture: (1) high-speed digital camera, (2) impact tup of the drop tower, (3) delay generator, (4) lamp control unit, (5) pair of light sources, (6) specimen, and (7) copper tape

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Figure 4

Sliding (u) and opening (v) displacement fields assessed from image correlation for a dynamic case of neat PETI-5. The image represents the event occurred at t=250 μs (10 μs after crack initiation). Each contour represents a displacement increment of 10 μm and the scale is in mm.

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Figure 5

Dynamic stress intensity factor histories for two neat PETI-5 tests under impact loading. Inset indicates geometry of a specimen used in the test (units in mm). Impact velocity=4.8 m/s and t=0 corresponds to impact.

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Figure 6

Comparison of quasi-static (strain rate ∼4×10−4/s) stress-strain response of PETI-5 and 3900

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Figure 7

Tensile stress-strain responses for four Hexcel 3900 dynamic tensile tests carried out using split-Hopkinson bar apparatus (strain rate ∼1600/s) in comparison with averaged stress-strain (strain rate ∼2000/s) response of PETI-5

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Figure 8

Comparison of quasi-static stress intensity factors for PETI-5 and 3900 determined from three-point symmetric bend tests at 0.005 mm/s on single edge notch (SEN) samples using DIC. Beam dimensions are in mm and open symbols indicate value at sudden fracture.

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Figure 9

(a) Stress intensity factor history and (b) crack extension history of neat PETI-5 and 3900 under dynamic loading (impact velocity 4.8 m/s). Time zero in (b) indicates the time when crack initiation takes place.

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Figure 10

Fracture surface of PETI-5. Arrow indicates crack growth direction. Numerous parabolic surface features both on the midplane and near free surface in the SEM image are clearly evident.

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Figure 11

Representative speckle images for the β=45 deg case with full-field sliding displacement (u) and opening displacement (v) contours (crack initiates at t∼125 μs). Moving crack tip is indicated by an arrow. Units in the axes of displacement fields are in mm. Color-bar indicates displacement in μm. Contour interval is 5 μm.

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Figure 12

Crack growth histories and crack tip velocities for (a) IM7/PETI-5 and (b) T800/3900-2 composites with two different fiber orientations relative to the impact direction. Data for T800/3900-2 are after Ref. 22.

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Figure 13

Modes-I and -II stress intensity factor histories for two fiber orientation angles β=0 deg and 45 deg: (a) IM7/PETI-5 and (b) T800/3900-2 (25). Crack initiation values are marked by arrows.

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Figure 14

Energy release rate histories for T800/3900-2 and IM7/PETI-5 composite for 0 deg and 45 deg fiber orientations relative to the impact/initial notch orientation direction (crack initiation is indicated by arrows): delayed crack initiation and higher fracture toughness for IM7/PETI-5 are evident

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Figure 15

Fractographic examination of fractured T800/3900-2 and IM7/PETI-5 (β=45 deg) samples: substantially higher roughness due to fiber pull-out is evident in IM7/PETI-5 relative to the one for T800/3900-2

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Figure 16

Strain signals for PETI-5 and 3900 from tensile split-Hopkinson bar apparatus. (a) Typical incident, reflected, and transmitted waveforms and (b) overlay of εI and εT−εR signals on a shifted time axis.

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