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Offshore and Structural Mechanics

Characterization of the Compressive Behavior of Glass Fiber Reinforced Polyurethane Foam at Different Strain Rates

[+] Author and Article Information
Huiyang Luo, Yanli Zhang

School of Mechanical and Aerospace Engineering, Oklahoma State University, Stillwater, OK 74078

Bo Wang

 Technology, American Bureau of Shipping (ABS), Houston, TX 77060

Hongbing Lu

Department of Mechanical and Energy Engineering, University of North Texas, Denton, TX 76207hongbing.lu@unt.edu

J. Offshore Mech. Arct. Eng 132(2), 021301 (Mar 10, 2010) (12 pages) doi:10.1115/1.4000396 History: Received July 03, 2008; Revised June 20, 2009; Published March 10, 2010; Online March 10, 2010

A glass fiber reinforced polyurethane foam (R-PUF), used for thermal insulation of liquefied natural gas tanks, was characterized to determine its compressive strength, modulus, and relaxation behavior. Compressive tests were conducted at different strain rates, ranging from 103s1 to 10s1 using a servohydraulic material testing system, and from 40s1 to 103s1 using a long split Hopkinson pressure bar (SHPB) designed for materials with low mechanical impedance such as R-PUF. Results indicate that in general both Young’s modulus and collapse strength increase with the strain rate at both room and cryogenic (170°C) temperatures. The R-PUF shows a linearly viscoelastic behavior prior to collapse. Based on time-temperature superposition principle, relaxation curves at several temperatures were shifted horizontally to determine Young’s relaxation master curve. The results show that Young’s relaxation modulus decreases with time. The relaxation master curve obtained can be used to convert to Young’s modulus at strain rates up to 103s1 following linearly viscoelastic analysis after the specimen size effect has been considered.

Copyright © 2010 by American Society of Mechanical Engineers
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Figures

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Figure 2

Schematic of the split Hopkinson pressure bar for testing R-PUF at high-strain rates

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Figure 3

Optical micrographs of the PU foam surface: (a) x-y plane and (b) x-z plane

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Figure 4

SEM micrographs of the R-PUF: (a) x-y plane and (b) x-z plane

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Figure 8

Contours of displacements and strains in the z-direction measured using DIC. (a) Displacement field W(1 pixel=86.1 μm), (b) displacement field U(1 pixel=86.1 μm), (c) strain field εz, and (d) strain field εx.

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Figure 9

Axial strain as a function of subset size (1 pixel=86.1 μm)

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Figure 10

Compressive stress-strain curves of the R-PUF in the z-direction after scaling for the specimen size effect (23°C at a strain rate of 0.01 s−1). The curves represent the results for a large sample with diameter of 76.2 mm and length of 76.2 mm, and a small sample with diameter of 17.8 mm and length of 12.7 mm.

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Figure 12

Typical SHPB testing results of a R-PUF specimen. (a) Original oscilloscope recording and (b) stress equilibrium check and strain rate history.

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Figure 15

Compressive properties of the R-PUF at different strain rates (data at high-strain rates were scaled using the size factor in Fig. 1): (a) Young’s modulus and (b) yield strength. (Note that the sudden change in property at 20–40 s−1 is due to the dynamic buckling of glass fibers at these high-strain rates.)

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Figure 16

Typical deformed images of the R-PUF under compression at a high-strain rate obtained using high-speed photography. (a) εz=0%, (b) εz=2.73%, (c) εz=5.29%, (d) εz=10.8%, (e) εz=21.4%, and (f) εz=29.1%.

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Figure 17

Compressive stress-strain curves from bar signals and image analysis using DIC, and transverse strains corresponding to different axial strains. The dynamic Poisson’s ratio of the foam when compressed in the z-direction is determined as 0.054.

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Figure 18

Axial strain filed measured using DIC on an area of 250×50 pixels; axes are given in number of pixels, 1 pixel=37.74 μm in this case

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Figure 19

SEM micrographs of a failed R-PUF specimen compressed on a SHPB by 40% at a strain rate of 1100 s−1: (a) loading in the z-direction and (b) loading in the x-direction

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Figure 20

Relaxation modulus data of the R-PUF. (a) Young’s relaxation modulus curves at several temperatures. (b) Relaxation master curve referred to 23°C. The inset is the shift factors at several temperatures.

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Figure 21

Comparison of Young’s modulus between direct experimental measurement and calculated value using Eq. 9 from master curve in Fig. 2 for the foam in the z-direction

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Figure 1

R-PUF cylindrical specimens and the fiber orientations. (a) An undeformed specimen with axial direction in the z-direction; speckles are used for deformation measurements using DIC. (b) A compressed specimen after compression at 40% strain on a SHPB test at a strain rate of 1100 s−1.

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Figure 14

Compressive stress-strain curves of the R-PUF in the x-direction (high-strain rate data were corrected with the size factor shown in Fig. 1): (a) at 23°C and (b) at −170°C

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Figure 13

Compressive stress-strain curves of the R-PUF in the z-direction (high-strain rate data were corrected with the size factor shown in Fig. 1): (a) at 23°C and (b) at −170°C

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Figure 11

Size factors for Young’s modulus and yield strength of the R-PUF at low-strain rates

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Figure 7

Reference and deformed images of a foam in the z-direction compressed at a strain rate of 0.01 s−1. (a) t=0 s(εz=0), (b) t=10 s(εz=0.10), (c) t=20 s(εz=0.20), and (d) t=30 s(εz=0.30).

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Figure 6

DMA testing results for the R-PUF in the z-direction

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Figure 5

DMA schematic setup for the measurement of the flexural properties of the R-PUF in the z-direction

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