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Research Papers

Stress Whitening Quantification of Thermoformed Mineral Filled Acrylics

[+] Author and Article Information
E. M. Gunel1

Department of Civil, Structural and Environmental Engineering, State University of New York at Buffalo, Buffalo, NY 14260emgunel@buffalo.edu

C. Basaran

Department of Civil, Structural and Environmental Engineering, State University of New York at Buffalo, Buffalo, NY 14260cjb@buffalo.edu

1

Corresponding author.

J. Eng. Mater. Technol 132(3), 031002 (Jun 15, 2010) (11 pages) doi:10.1115/1.4001262 History: Received April 23, 2009; Revised December 10, 2009; Published June 15, 2010; Online June 15, 2010

Stress whitening problem in thermoformed alumina trihydrate (ATH) reinforced poly(methyl methacrylate) (PMMA) was studied. In situ heavy-gage thermoforming of acrylics was entirely replicated under laboratory controlled conditions at different operation parameters. Samples were monitored with optical microscope after the completion of the thermoforming operation. For stress whitening quantification, a new index was proposed from image histograms of processed optical micrographs. Results indicated that stress whitening in PMMA/ATH samples increases with level of plastic deformation at all thermoforming conditions. The influence of the forming rate and forming temperature on the degree of stress whitening was explained in terms of change in material behavior and microdeformation mechanisms around two characteristic temperatures of PMMA/ATH. Developed method for stress whitening quantification characterizes different levels of stress whitening with single numeric values. It is shown that stress whitening index and density of microdeformation features display a strong correlation. Higher density of particle cracks at low forming temperatures results in higher stress whitening levels. Increased surface irregularity and large size voids at high forming temperatures produce lower stress whitening.

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Copyright © 2010 by American Society of Mechanical Engineers
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Figures

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Figure 1

Engineering stress-strain curves of PMMA/ATH samples in forming step at 0.9 mm/s forming rate, 75°C forming temperature

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Figure 2

Loading-displacement history of PMMA/ATH sample thermoformed at 0.9 mm/s forming rate, 75°C forming temperature (deformation limit 13.5 mm)

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Figure 3

SEM images of undeformed PMMA/ATH sample

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Figure 4

SEM images of thermoformed PMMA/ATH samples at 0.9 mm/s forming rate and 90°C forming temperature (fifth thermoforming cycle)

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Figure 5

SEM images of thermoformed PMMA/ATH samples at 0.09 mm/sec forming rate and 125°C forming temperature (fifth thermoforming cycle)

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Figure 6

SEM images of thermoformed PMMA/ATH samples at 0.009 mm/s forming rate and 100°C forming temperature (fifth thermoforming cycle)

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Figure 7

Processed optical images of thermoformed PMMA/ATH sample at 0.009 mm/s forming rate, 100°C forming temperature (a) at undeformed state and at the end of (b) first, (c) second, (d) third, (e) fourth, and (f) fifth thermoforming cycle

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Figure 8

Image histograms of thermoformed PMMA/ATH sample (a) at 0.009 mm/s forming rate, 100°C forming temperature and (b) at 0.9 mm/s forming rate, 75°C forming temperature

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Figure 9

Normal probability plots of image histograms of thermoformed PMMA/ATH sample at 0.009 mm/s forming rate, 100°C forming temperature

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Figure 10

Image histograms of thermoformed PMMA/ATH sample (a) at 0.009 mm/sec forming rate, 100°C forming temperature and (b) at 0.9 mm/s forming rate, 75°C forming temperature (gray level in natural logarithmic scale)

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Figure 11

Normal probability plots of image histograms of thermoformed PMMA/ATH sample at 0.009 mm/s forming rate, 100°C forming temperature (gray level in natural logarithmic scale)

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Figure 12

Change in stress whitening ΔG in thermoformed PMMA/ATH sample at (a) 0.9 mm/s, (b) 0.09 mm/s, and (c) 0.009 mm/s forming rate

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Figure 13

Stress whitening (ΔG) versus areal crack density for various thermoforming conditions

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