Characterization of fatigue behavior of polydimethylsiloxane

Polydimethylsiloxane (PDMS) is an elastomeric material with myriad applications, particularly in the field of biomedicine. Many devices in various applications utilize PDMS, particularly in those that experience cyclic loads, such as micropumps, soft robotics, wearable devices, and tissue scaffolds. All such applications that experience repeated loading require knowledge of the fatigue behavior of PDMS for a reliable design and to predict the device's useful life. While the fatigue behavior of several materials is well-characterized, the fatigue life of PDMS remains poorly understood, especially under constant-force loading. In this thesis, we conduct force-controlled fatigue testing of PDMS to address this gap in the literature. To achieve this purpose, we custom-built a uniaxial stretcher and integrated it with a force control feedback system, equipped with a digital camera to capture images at the appropriate moments. We employed a hybrid method, where crosshead displacement was used for the bulk of the fatigue test, while at specific cycles, we took DIC measurements for high-accuracy strain measurements. We conducted experiments on 77 PDMS samples with maximum stress amplitudes ranging from 0.85 to 2.8 MPa. The outcome of this mechanical characterization of the fatigue behavior of PDMS research yielded some quantitative results. Firstly, the equation of the S-N curve is found to be $S = 2.994 \times N^{-0.007586}$. All Samples tested at or below a stress level of 2.25 MPa survived 1,000,000 cycles without failing, and the samples tested at the stress level of 2.5 MPa experienced at least one failure in our tests. We fitted the Weibull distribution to the data of stress levels where failures were observed. The shape parameter decreased from 12.79 at 2.5 MPa to 6.37 at 2.8 MPa, indicating a higher failure probability at higher stress amplitudes. We quantified the ratcheting behavior of PDMS using DIC analysis and discovered that ratcheting strain increases logarithmically with fatigue cycles. We also found that whenever the samples reach a ratcheting strain of 0.17, failure becomes very likely, suggesting that this ratcheting strain is a precursor to failure. At one cycle, our S-N curve predicts a failure stress value that is remarkably close to the ultimate tensile strength of PDMS ($\sim 3$ MPa) that we determined from tensile tests separately, further strengthening confidence in our results.