A Testing Platform for Durability Studies of Polymers and Fiber-reinforced Polymer Composites under Concurrent Hygrothermo-mechanical Stimuli

1Department of Mechanical and Aerospace Engineering, University of California, Davis
Published 12/11/2014
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Summary

The durability of polymers and fiber-reinforced polymer composites in service is a critical aspect for their designs and condition-based maintenance. We present a novel low-cost laboratory testing platform for the investigation of the influence of concurrent mechanical and environmental loadings, and may help design more efficient yet safer composite structures.

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Gomez, A., Pires, R., Yambao, A., La Saponara, V. A Testing Platform for Durability Studies of Polymers and Fiber-reinforced Polymer Composites under Concurrent Hygrothermo-mechanical Stimuli. J. Vis. Exp. (94), e52464, doi:10.3791/52464 (2014).

Abstract

The durability of polymers and fiber-reinforced polymer composites under service condition is a critical aspect to be addressed for their robust designs and condition-based maintenance. These materials are adopted in a wide range of engineering applications, from aircraft and ship structures, to bridges, wind turbine blades, biomaterials and biomedical implants. Polymers are viscoelastic materials, and their response may be highly nonlinear and thus make it challenging to predict and monitor their in-service performance. The laboratory-scale testing platform presented herein assists the investigation of the influence of concurrent mechanical loadings and environmental conditions on these materials. The platform was designed to be low-cost and user-friendly. Its chemically resistant materials make the platform adaptable to studies of chemical degradation due to in-service exposure to fluids. An example of experiment was conducted at RT on closed-cell polyurethane foam samples loaded with a weight corresponding to ~50% of their ultimate static and dry load. Results show that the testing apparatus is appropriate for these studies. Results also highlight the larger vulnerability of the polymer under concurrent loading, based on the higher mid-point displacements and lower residual failure loads. Recommendations are made for additional improvements to the testing apparatus.

Introduction

Polymer and fiber-reinforced polymer (FRP) composites have been adopted in a variety of engineering structures, ranging from aircraft and spacecraft, naval vessels, civil infrastructure, (see for examples reviews of Katnam et al.1, Hollaway2, Mouritz et al.3), cars and trains, wind turbine blades, to prosthetics and biomaterials for sutures and implants. These materials’ durability is affected by complex service scenarios, which may include a combination of a) thermo-mechanical loading, e.g., freeze-thaw cycles in civil infrastructure4, subsonic/supersonic flight profiles5, wear in metal-backed polyethylene6); b) degradation due to environmental and chemical agents, e.g., sea water, de-icing, hydraulic fluid for aerospace and naval structures7-10, degradation of polymethylmethacrylate dental composites due to saliva11; c) complex interactions of materials in fastened or bonded joints, e.g., galvanic corrosion and debonding between dissimilar materials, whether in a carbon/fiber patch repair on an aircraft aluminum skin, or a carbon/PEEK bone plate fastened by stainless steel12.

There is unfortunately limited knowledge of the impact of concurrent in-service stimuli on the long-term durability of these materials. Most polymers may be categorized as viscoelastic materials. Mechanical loadings and environmental conditions significantly influence the viscoelastic response of polymers. Hence, reliable models for these materials’ long-term behavior should be able to incorporate time-dependent responses to coupled hygrothermal, mechanical, chemical stimuli This in turn will improve design predictions, safety and condition-based maintenance/replacement protocols.

There is a large literature body on experimental testing on hygrothermal effects, for example hygrothermal diffusion tests: if the scale of the samples allows it, the material samples may be positioned in a chamber at desired humidity and temperature levels. The samples are removed periodically to measure their mass and/or volume changes for a given amount of time, from weeks to years10,13-17. The hygrothermal test may be followed by mechanical testing, i.e., residual static/fatigue strength/fracture mechanics testing17-19, which only gives information on the effect of hygrothermal stimulus on the mechanical responses of materials. Test data may be fitted to diffusion models of varying complexity, from simple Fickian diffusion to models that include dependency on concentration, stress, temperature, reversible physical aging/plasticization and irreversible chemical reactions. This experimental output may be further incorporated in structural analyses.

Few authors have addressed the impact of simultaneous hygrothermal and mechanical stimuli. Among those researching FRP composites, Neumann and Garom20 immersed stressed and unstressed specimens in distilled water. Stress was applied by positioning the specimens inside compressed stainless steel springs, tuning the load by using different spring stiffnesses and compressive loads. A similar procedure is reported by Wan et al.21. Helbling and Karbhari22 employed a bending fixture inside an environmental chamber for different relative humidity percentages (RH%) and temperature levels. The pre-conditioned specimens were subjected to a given bending strain level, corresponding to a percentage of the static ultimate tensile strain for that composite. Kasturiarachchi and Pritchard23 prepared a stainless steel 4-point bending jig (one per specimen) that was positioned on a shelf in a large glass desiccator. The desiccator was partially filled with distilled water, had small leaks to prevent the buildup of pressure, and was placed in a humidity chamber at 95% RH. Gellert and Turley7 investigated marine-grade FRP composite specimens for their durability under combined creep loading and 100% RH. Their samples were loaded in 4-point bending at a constant load equal to 20% of the failure static flexure load, while fully immersed in sea water. The creep deflection was acquired periodically by using a thickness gauge between the outer surface of the beam in the central cross-section, and a glass plate (it is inferred that such measurement was performed outside the chamber). Abdel-Magid et al.24 placed samples of glass/epoxy in an Invar environmental fixture which was provided by NASA Langley, as the specimens were loaded in tension along the fiber direction, at 20% of the ultimate axial load. Ellyin and Rohrbarcher25 ran hygrothermal tests for up to 140 days, and then tested the specimens in fatigue on a hydraulic testing machine. The specimens were wrapped in a wet cheese cloth connected to a tube and a water supply. Earl et al.26 positioned their loading fixture and the specimens in a large environmental chamber (5.5 m3).

As discussed in many experimental studies, the environmental conditions affect the polymers’ mechanical properties and responses. Some limited experiments also show that the existence of mechanical stress/strain influences the diffusion process in the polymers. Hence, to enhance understanding on the overall performance of polymer-based materials under both mechanical and non-mechanical effects, there is a need for concurrent testing.

There were several objectives behind the design of the testing platform discussed in this paper. First, the platform is part of the experimental setup in a multi-year investigation on the hygrothermo-mechanical behavior of different types of FRP sandwich composites for wind turbine and naval engineering applications. The test data are used to calibrate the parameters in the viscoelastic constitutive equations for the polymeric composites. The constitutive models are based on the work developed over the years by Muliana and collaborators27-30. The second objective was to have a low-cost and user-friendly testing platform, for example one that could be easily relocated in a laboratory (e.g., to a scale for mass measurements, or to the source of the fluid, e.g., one coming from a faucet, a fumehood or a flammable cabinet). The third goal was to create a testing platform that is resistant to a number of chemicals commonly used in service (particularly hydraulic fluid, de-icing, cleaning solvents for aerospace applications8-10), thus specimens could be immersed in such chemicals, and their durability could be assessed.

The chamber (Figure 1) was constructed with high-density polyethylene, which has high chemical resistance. As mentioned above, it is expected that future work will include hygrothermo-mechanical investigation of composites immersed in hydraulic fluid, de-icing, cleaning solvents. Since thermal regulation is an integral aspect of testing, expanded polystyrene foam was fit around the sides of the tank and secured in place by tape and the steel frame itself, to prevent heat exchange with the environment.

The lid of the chamber (Figure 2) was manufactured from transparent, 9.525 mm-thick polycarbonate, allowing the users to observe the specimens during testing without disturbing the test. The lid is secured in place by aluminum T-bars, which were machined to slide under overhanging brackets on the sides of the tank.

Bending in the specimens is enacted by three aluminum blocks, which hang down from the lid, and are fastened through slots in the lid. The three blocks allow up to four specimens to be tested at one time, while the lid slots allow the block spacing to be adjusted depending on the length of the specimens. Each block is rounded at the contact edge to a 12.7 mm diameter, in adherence to ASTM standard D790-10. The specimens are positioned beneath two of the three blocks, with an upward force applied at its center to induce bending (Figures 1-2).

The apparatus was designed with maximum versatility and ease-of-use in mind. Casters with 41.275 mm diameter are fastened beneath the chamber for mobility purposes. Above them, the tank is supported by a welded steel frame with a wire mesh bottom and cross beams for support. Angle stock spacers for the outside tank corners were manufactured to keep the insulation from being crushed by the overhead weight and displacement gauges (string pot apparatus, discussed later). Around the top, angle stock was used again for framing. Pulley and string potentiometer systems to measure mid-span deflection are mounted on four steel, square-tubing arches (Figure 3). The center two arches out of these four carry the string potentiometers and are adjustable to account for specimen versatility. The string potentiometers were constructed using a torsional spring (as can be found in retractable key lanyards) and potentiometers with three-pronged electronic outputs. The pulleys are aligned and mounted for use with a steel cable running from a rigid connection by the specimen to a hanging rod over the side of the chamber for adjustable weight application.

The load is applied to the specimen using a series of cables, pulleys, linkages and bolts. First, the specimen is placed into the U-bolt so that the 10 mm cross bar is contacting the middle of the span. A 9.525 mm diameter steel rod with eye bolts at each end is then connected to the U-bolt. This steel connection passes through the lid of the chamber. A steel cable and Kevlar thread are attached to the eyebolt opposite the U-bolt. This allows the Kevlar thread from the string potentiometer to read data from a rigid point. The steel cable continues upwards and passes over two pulleys that allow the load to be applied at the periphery of the tank. The cable is then attached to a 9.525 mm diameter steel rod that serves as a slotted weight hanger. This hanger provides a place where the slotted weights can be set in order to apply the desired load.

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Protocol

1. Loading the Specimens

  1. Raise the lid of the tank and rest it upon the side supports (Figure 4).
  2. Place the specimen in the U-bolt, and ensure that the cross bar is making contact at the center of the specimen.
  3. Rest the ends of the specimen on the aluminum supports hanging from the lid. The ends of the specimens should have 5-10 mm of overhang.
  4. Repeat steps 1.2-1.4 for all of the specimens that will be tested.
  5. Remove the lid supports, lower lid, and make sure that the lid is seated on the lip of the tank.
  6. Apply the desired force by adding weights to the steel rod next to the outer pulley.

2. Measuring Displacement

  1. Ensure that the string potentiometer line is pulled taut.
  2. Using a digital multimeter, measure the resistance across the outer pins of the potentiometer (Figure 3), with black to Pin 1 and red to Pin 3, and record the reading.
  3. Convert the resistance reading into a displacement reading by computing the calibration factor (in this case, 1 kΩ corresponds to a 64.895 mm displacement).
  4. Repeat steps 2.1-2.3 for each specimen.

3. Weighing the Specimens

  1. Before beginning the weighing procedure, record the displacement data and prepare an interim holding chamber filled with the testing fluid at RT, as per ASTM D522931, or the appropriate testing standard.
  2. Remove the slotted weights from the ends of the steel cables.
  3. Raise the lid of the tank and rest it upon the side supports.
  4. Remove the specimen and place it into the prepared interim holding chamber. Repeat this step for all of the specimens.
  5. Remove the specimens and dry them individually using a microfiber cloth in order to remove excess fluid.
  6. Place the specimen on a high-precision scale and record the data reading.
  7. Repeat steps 3.5-3.6 for all specimens and then follow Protocol Step 1.

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Representative Results

The testing apparatus has successfully held specimens immersed in a fluid under three-point bending. With reasonable precisions, specimens can be loaded and tested with accurate readouts from the potentiometers for mid-point deflection changes. The change in electrical resistance can be recorded to 4 significant figures, resulting in a displacement resolution of the order of 0.1 µm.

Hygrothermo-mechanical tests were conducted at RT on two groups of four specimens of closed-cell polyurethane foam, with nominal dimensions 215 mm length x 24 mm width x 18 mm thickness. One group was tested in the chamber under dry conditions, intended as a) in air, inside the tank, and b) at ambient relative humidity of ~50% RH (the test took place at the end of June in a laboratory situated in hot and dry northern California Central Valley, in USA). This first group of samples is herein indicated as ‘dry specimens’. The second group of samples was tested in the tank while fully immersed in deionized water (100% RH, herein indicated as ‘wet specimens’). The specimens were loaded with hanging weights approximately equal to 50% of their ultimate load under static dry conditions, resulting in (1.780 ± 0.116) kg. The application of each hanging weight took few seconds, to achieve quasi-static loading conditions. It was expected that the foam would have a nonlinear viscoelastic behavior, but it was not known a priori how the concurrent stimuli would decrease the foam durability with respect to the dry specimens.

Resistance measurements on the digital multimeter were taken for each specimen, at approximately 15 min intervals for the first 6 hr of testing. Measurements were taken again after an additional 18 hr. From this, the change in mid-span deflection was tracked. Based on the data collected, the displacement after 24 hr for the dry specimens was (2.141 ± 0.371) mm, while the displacement for the wet specimens was significantly higher, and equal to (14.41 ± 3.62) mm (Figure 5, Table 1).

Following each trial run, the specimens were then tested for residual strength by loading them until failure. The wet specimens were found to have a residual failure load equal to (2.970 ± 0.246) kg, as compared to the residual failure load of (3.623 ± 0.0967) kg for the dry specimens, (Figure 6, Table 2). The resolution for the residual failure load measurements was ± 0.194 kg.

Figure 1
Figure 1. Overview of major components of testing apparatus. A. High density polyethylene tank. B. Expanded polystyrene foam insulation. C. Slotted polycarbonate lid. D. Aluminum T bar and overhang bracket. E. Three-point bending supports. F. Bottom frame. G. Angle spacers. H. Top frame. I. String potentiometer assemblies. J. Lower loading assembly. K. Slotted weights and hanger. Please click here to view a larger version of this figure.

Figure 2
Figure 2. Detailed view of lid. A. High density polyethylene tank. C. Slotted polycarbonate lid. D. Aluminum T-bar and overhang bracket. E. Three-point bending supports. J. Lower loading assembly. Please click here to view a larger version of this figure.

Figure 3
Figure 3. String potentiometer assembly of the testing apparatus. Please click here to view a larger version of this figure.

Figure 4
Figure 4. Lid supports of the testing apparatus. Please click here to view a larger version of this figure.

Figure 5
Figure 5. Mid-span displacement variation with time, for dry and wet specimens. Please click here to view a larger version of this figure.

Figure 6
Figure 6. Box plots of residual loads to failure, for dry and wet specimens, showing the larger vulnerability of the wet specimens. Please click here to view a larger version of this figure.

Figure 7
Figure 7. Pictures of foam specimens after residual bending strength tests: (A) and (B) dry specimens, (C) and (D) wet specimens. The nominal specimen width is 24 mm. Please click here to view a larger version of this figure.

Hours from
start
Change of displacement (mm), specimen 1 Change of displacement (mm), specimen 2 Change of displacement (mm), specimen 3 Change of displacement (mm), specimen 4
0.000 0.000 0.000 0.000 0.000
0.230 0.454 0.130 1.298 0.195
0.730 0.714 2.141 1.298 1.817
0.980 0.779 2.141 1.298 1.817
1.310 0.779 2.076 1.298 1.817
1.810 1.038 2.141 1.947 1.817
2.010 0.973 2.206 1.947 1.817
2.350 1.363 2.076 1.947 1.882
2.610 1.363 2.076 1.947 1.752
2.730 1.428 2.076 1.947 1.752
3.230 1.557 2.076 2.596 1.817
3.480 1.298 2.076 1.947 1.947
3.810 1.622 2.076 2.596 1.817
4.010 1.622 2.076 2.596 1.817
4.230 1.557 2.076 2.596 2.012
4.480 1.557 2.076 2.596 2.012
4.730 1.622 2.076 2.596 2.012
4.980 1.752 2.141 2.596 1.947
5.230 1.752 2.076 3.244 1.947
5.510 1.687 2.141 2.596 2.012
5.780 1.557 2.076 2.596 1.882
5.980 1.687 2.076 2.596 1.947
6.310 1.622 2.141 2.596 1.882
6.480 1.622 2.206 2.596 2.012
23.550 1.882 2.206 2.596 1.882
23.967 1.752 2.271 2.596 1.947

Table 1. Displacement vs. time of foam specimens at ambient relative humidity (dry specimens).

Hours from
start
Change of displacement (mm), specimen 1 Change of displacement (mm), specimen 2 Change of displacement (mm), specimen 3 Change of displacement (mm), specimen 4
0.000 0.000 0.000 0.000 0.000
0.303 3.245 0.000 1.298 0.000
0.653 3.439 0.195 2.596 0.000
0.903 4.932 1.168 3.894 1.168
1.163 4.932 1.168 3.245 1.233
1.433 6.295 2.206 4.543 2.012
1.703 6.360 2.466 4.543 2.142
2.013 7.074 2.855 5.192 2.077
2.253 7.203 2.790 5.192 2.077
2.763 7.917 3.310 5.841 3.180
3.013 7.917 3.634 5.841 3.180
3.283 8.047 4.413 5.841 3.180
3.513 7.917 4.153 5.841 3.180
3.753 7.917 3.699 6.489 3.245
4.013 9.734 5.192 7.787 4.478
4.253 10.448 4.802 8.436 4.608
4.513 10.448 4.802 8.436 4.478
4.783 10.448 4.802 8.436 4.478
5.013 10.448 5.127 8.436 4.737
5.313 10.383 4.737 8.436 4.608
5.513 11.421 5.711 9.085 5.581
5.753 11.421 5.646 9.085 5.711
6.033 11.551 5.776 9.085 5.516
6.333 11.486 6.035 9.085 5.581
6.503 11.551 6.360 9.734 6.035
23.300 16.937 10.383 14.277 9.734
23.650 17.067 10.318 15.575 9.734
23.983 17.002 10.253 14.277 10.383
24.250 17.262 10.253 14.926 9.994
24.983 18.625 11.486 16.224 11.292

Table 2. Displacement vs. time of foam specimens at 100% RH (wet specimens).

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Discussion

From the acquired data, it can be seen that the concurrent testing scenario did affect the durability of the closed-cell polyurethane foam specimens. This can be seen by comparing the significantly different displacements (Figure 5) and residual loads to failure (Figure 6) of dry and wet specimens. Figure 7 shows pictures of the specimens after the residual strength tests. It should also be observed that, while the displacement of the dry specimens reached steady state within the observation interval of 24 hr, those of the wet specimens did not. Hence, future tests will be conducted for a longer time interval, to either achieve a steady-state behavior of the conditioned specimens or establish that such steady-state may not be possible within a given testing time frame (for example, if the material experiences degradation that leads to failure).

The boxplots of Figure 6 show that the distribution of residual loads to failure for the wet specimens is statically different and lower with respect to the case for the dry specimens.

A direct comparison of this outcome cannot be made with the literature because of the relatively limited published data and the different materials and load profiles selected by various authors. However, the representative results obtained with this fixture concur with the trend observed of Gellert and Turley7 about “significantly higher” creep deflections experienced by their glass-fiber reinforced samples.

The testing apparatus may be improved in order to increase its robustness and ease of use. Sliding mounts will be added at the base of the top frame supports to hold the potentiometers in a more secure manner. This will reduce the possibility of movement and, therefore, increase the accuracy of the readings. Moreover, the potentiometers will be connected to small breadboards into three-pin screw terminals. This will also enhance the accuracy of the readings because it will eliminate the need to touch the potentiometer while taking measurements.

Additional improvements are planned to further increase the flexibility of the apparatus. For example, a new lid will be developed in order to create an airtight seal when testing potentially harmful chemicals. This change will likely lead to a modification of Protocol Step 1. An immersion heater may also be added in order to allow for testing at elevated temperatures. When testing a saline solution, a magnetic stir bar could be considered in place of an expensive stainless steel immersion heater. This would require a modification to the base of the apparatus for the incorporation of a magnetic source. The resulting testing apparatus will provide a broader picture of how the concurrent testing affects the durability of polymers and polymeric matrix composites under a variety of in-service conditions.

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Disclosures

The authors have nothing to disclose.

Acknowledgements

The authors thank Destiny Garcia, Serena Ferraro, Erik Quiroz and Steven Kern (Advanced Composites Research, Engineering and Science laboratory) for their help in designing and manufacturing the test setup. Shawn Malone, Michael Akahori, David Kehlet (Engineering Fabrication Lab) are acknowledged for their suggestions and assistance in the machining process. The support of the National Science Foundation (collaborative grant CMMI-1265691 and its REU supplement) and the Office of Naval Research (N00014-13-1-0604 to A. Muliana, Texas &M University (Principal Investigator), and V. La Saponara, managed by program director Yapa Rajapakse) are gratefully appreciated.

Materials

Name Company Catalog Number Comments
Aluminum 6061 rectangular bars McMaster-Carr, USA 8975K268, 1668T72, 7062T17,  Part of testing platform
Aluminum 6061 90° angles McMaster-Carr, USA 8982K91, 8982K14  Part of testing platform
440C stainless steel McMaster-Carr, USA 6253K52 Part of testing platform
High-density polyethylene sheets Tap Plastics, USA N/A (0.236 in. thick x 10.75 in. wide x 16.75 in. long) Part of testing platform
High-density polyethylene sheets Tap Plastics, USA N/A (0.354 in. thick x 6 in. wide x 10 in. long) Part of testing platform
High-density polyethylene sheets Tap Plastics, USA N/A (0.354 in. thick x 6 in. wide x 16.75 in. long) Part of testing platform
Polycarbonate sheets Tap Plastics, USA N/A (0.375 in thick, 11.5 in. wide, 17.5 in long) Part of testing platform
Expanded polystyrene foam Home Depot Model # 310880 Internet # 202532855 Part of testing platform
Galvanized steel rope McMaster-Carr, USA 3498T63 Part of testing platform
Steel eye bolt McMaster-Carr, USA 3013T341 Part of testing platform
Low-carbon steel 90° angle McMaster-Carr, USA 9017K444  Part of testing platform
Low-carbon steel rods McMaster-Carr, USA 8920K84, 8920K75, 8920K231, 8920K135, 8920K84    Part of testing platform
Low-carbon steel tubes McMaster-Carr, USA 6527K314, 8910K394, 8910K395, 8920K94   Part of testing platform
304 stainless steel U-bolt McMaster-Carr, USA 8896T104 Part of testing platform
Steel pulley McMaster-Carr, USA 3099T34 Part of testing platform
1008 carbon steel sheets McMaster-Carr, USA 9302T113 Part of testing platform
Light duty swivel casters Harbor Freight, USA 41519 Part of testing platform
100-lbf Vinyl Weight Set Overstock.com 11767059 Part of testing platform
Closed-cell polyurethane foam General Plastics, USA FR-3704 Testing samples
Deionized water Faucet, PurLab filtering system N/A Conditioning fluid of tank
Torsional spring Retractable Key Clip, Ebay, USA Lot 10 Used to build string potentiometer
Kevlar thread Cabela’s IK-321909 Used to build string potentiometer
10 kOhm potentiometer Ebay, USA 3590S-2-103L Used to build string potentiometer
Digital multimeter Harbor Freight, USA 98674 Used to take resistance measurements of string potentiometer

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