Keywords: Creep, Recovery, Viscoelasticity, Polymers
RH148
Abstract
Creep-Recovery measurements aid in process design, comparing formulations, and understanding part failure. To assess creep and recovery, accurately and precisely controlled stress is applied and removed from samples. The Discovery™ ARES-G3™ Strain-Controlled Rheometer can perform stress-controlled creep-recovery tests on samples ranging from oils to polymers to stiff plastics.
Creep measurements assess the performance of materials subjected to small stress values, relative to the yield stress of the material, for extended periods of time. Below the yield stress of a material, the material may deform and fail during the end use application because of microstructural rearrangements. Creep testing may be used to determine the linear viscoelastic region [1] and yield stress of a material [2]. Creep testing is typically followed by a zero-stress step to monitor the material’s recovery. Creep and creep-recovery measurements are used to assess processing impacts due to polymer elasticity [3], sagging in paints [4], and failures in plastics [5]. Creep-Recovery testing is available on the ARES-G3 Rheometer for a wide range of materials using both the Advanced Peltier System (APS) and Forced Convection Oven (FCO). This application note discusses the range of materials that may be tested in creep-recovery on the ARES-G3 Rheometer.
During a creep-recovery test, a sudden stress value is applied to a material for a specified period and then it is suddenly removed. The strain response is monitored over time and depends on a material’s ability to store or dissipate energy, as shown in Figure 1. Purely elastic materials respond immediately to the applied stress with constant strain until the stress is removed, resulting in the strain returning immediately to zero. The energy is stored until the stress is removed. Purely viscous materials respond to the applied stress with a linear increase in the strain over time. When the stress is removed, a purely viscous material maintains the final strain achieved with no recovery. Purely viscous materials dissipate energy and are unable to recoil when the stress is removed. Viscoelastic materials – having both viscous and elastic character – respond to the applied stress somewhere in between the 2 boundaries. Initially, the elastic component of the material responds to the stress, resulting in a sharp increase in the strain. The viscous component responds next, and the linear strain region can be used to calculate the low shear viscosity. The slope in the linear region of the strain versus time is equal to the stress σ0 divided by the viscosity 𝜂𝜂, as shown in Figure 2. When the stress is removed, a viscoelastic material shows some initial recovery related to the elastic component. The recovery gradually increases over time to an equilibrium value. The more elastic a sample is, the more strain is recovered and vice versa. In the recovery zone of Figure 2, the black line achieves a lower strain value compared to the blue line, which indicates a larger recovery response.
Additional parameters obtained from creep-recovery tests include the time dependent creep compliance J(t) [3, 6], and the time dependent recoverable compliance Jr(t) with applied stress σ0, time dependent strain γ(t), and recoverable strain γR(𝑡).
Materials with higher creep compliance values deform more under the applied stress – the materials are more compliant. Materials with higher recoverable compliance are more elastic and recover more when the stress is removed.
S600 standard oil (1.054 Pa.s at 25 °C) was measured with a 50 mm upper stainless steel parallel plate, lower APS plate fixture, and the APS providing temperature control at 25 °C. PDMS (polydimethylsiloxane) was measured with 25 mm upper and lower stainless steel parallel plates with the FCO providing temperature control at 30 °C. ABS (acrylonitrile-butadiene-styrene) was tested with the rectangular torsion fixture and the FCO providing temperature control at 30 °C.
During stress-controlled testing on the ARES-G3 Rheometer, the motor position control loop is affected by an external stress loop as depicted in Figure 3, where torque 𝑀𝑀 is the control variable and Ѳ is the displacement. The transducer is used as a sensor, which requires that the sample be part of the loop [7]. To optimize performance, a Conditioning – Stress Control step is used to determine PID coefficients for each sample. This step is expanded in Figure 4. Once the PID coefficients have been optimized for a certain material’s test conditions, a precomputed stress control file may be loaded instead of running the stress control step each time.
To ensure optimal performance during creep tests, Conditioning – Transducer steps are added to the procedure as shown in Figure 4. These steps allow for sample relaxation and prepare the transducer for the measurement. Additional information is available in TRIOS™ Software Help.
Creep-Recovery on a Standard Oil
Standard oils are purely viscous fluids and results for S600 are shown in Figure 5. In the creep zone, 10 Pa of stress is applied to the S600 sample for 30 s. The oil responds with a linear increase in strain. The viscosity calculated from the slope is 1.073 Pa.s, which is within 2% of the certified value. When the stress is removed, the oil remains strained and does not recover.
Creep-Recovery on PDMS
PDMS is a viscoelastic silicone putty with polymer melt properties at ambient conditions. The results are shown in Figure 6. In the creep zone, 30 Pa of stress is applied. The PDMS shows an initial sharp increase in strain due to the elasticity of the sample. The slope levels off to a constant value as the viscous component responds. When the stress is removed, the strain decreases as the sample begins to recoil due to the elastic properties. The strain does not recover all the way to zero because of energy dissipation due to the viscous properties. The compliance and recoverable compliance results are shown in Figure 7. The compliance increases over time as the sample moves more under the applied stress. When the stress is removed, the elastic component dominates the response and there is a sharp increase in the recoverable compliance. As the viscous component responds, the value increases more gradually.
Creep-Recovery on a Stiff Material
ABS is comprised of 3 different polymer blocks with different glass transitions. Around ambient conditions, ABS is above the butadiene transition but below the next transition around 115 °C. At 30 °C, the sample is quite stiff, allowing it to be tested in the rectangular torsion fixture. The sample exhibits predominately elastic behavior as shown in Figure 8. In the creep zone, 3000 Pa of stress is applied for 30 seconds. The sample responds immediately to the stress and remains strained until the stress is removed. When the stress is removed, the sample’s elasticity causes it to recoil fully.
Purely viscous S600 oil, viscoelastic PDMS, and stiff ABS were tested on the ARES-G3 Rheometer. These examples demonstrate the range of materials that may be evaluated with creep-recovery tests. Each sample was subjected to a stress value and the strain response was monitored. S600 oil demonstrates a purely viscous response to the stress application. Stiff ABS demonstrates a predominately elastic response to the stress application at 30 °C. PDMS exhibits viscoelastic properties falling on the spectrum between purely viscous and purely elastic samples.
References
- K. Whitcomb, “RH111: Determining the Linear Viscoelastic Region in Creep and Stress Relaxation Tests,” TA Instruments Application Note.
- A. Franck, “AAN016: Understanding Rheology of Structured Fluids,” TA Instruments Application Note.
- A. Franck, “AAN022: Creep Recovery Measurements of Polymers,” TA Instruments Application Note.
- P. Whittingstall, “RH059: Paint Evaluation Using Rheology,” TA Instruments Application Note.
- J. Jansen, “Understanding Creep Failure of Plastics,” Plastics Engineering, pp. 32-36, July/August 2015.
- C. Macosko, Rheology Principles, Measurements, and Applications, Wiley-VCH, 1994.
- A. Franck, “ARES-G2: A New Generation of Separate Motor and Transducer Rheometers,” Applied Rheology, vol. 18, pp. 44- 47, 2008.
Acknowledgement
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This paper was written by Kimberly A. Dennis, PhD, Senior Applications Scientist.
TA Instruments, ARES-G3, and TRIOS are trademarks of Waters Technologies Corporation.
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