CRACK PROPAGATION BEHAVIOR OF LATERALLY CONSTRAINED POLYMERS USED AS DIELECTRIC ELASTOMERS
Dielectric elastomer-based transducers are rapidly gaining importance with the syntheses of new polymers that can potentially be used as dielectric materials. However, these materials are always prone to fracture in the presence of cracks and flaws. Failures originate from flaws (or notches), and a complete fracture may take place due to the propagation of cracks. The present work investigates the crack propagation behavior of two popular polymers, VHB 4910 and Ecoflex, that are widely used as dielectric elastomers. In this case, tensile loadings in laterally constrained boundary conditions are considered. The average crack propagation speed for Ecoflex is higher than that for VHB, implying that Ecoflex will fail earlier than that of VHB under similar conditions. Moreover, with increasing notch lengths at a fixed strain rate, the average crack propagation speed decreases appreciably but becomes constant for comparatively larger notches. The results also conclude that the average crack propagation speed and normalized crack tip diameter remain higher for VHB than for Ecoflex for larger normalized notch lengths. It is observed that the average crack propagation speed increases with strain rates, whereas the normalized crack tip diameter is independent of strain rates. Experimental results obtained here will provide a useful comparative insight to understand the failure behavior of two polymers widely used as dielectric elastomers.ABSTRACT
Experimental setup for determining crack propagation under a pure shear loading. (1) Specially designed fixture for the pure shear loading. (2) Notched pure shear specimen. (3) Crosshead of universal testing machine (UTM). (4) High-speed camera. (5) Computer to obtain data from the UTM. (6) Computer to obtain videos with the high-speed camera. (7) Tripod for the camera. (8) UTM controller. (9) Lamp to focus light on the specimen.
Geometry of a pure shear specimen with a notch.
Image analysis technique in PCC software.
Crack tip distance versus time plots at different strain rates for (a) VHB and (b) Ecoflex at a fixed notch length. Note that the logarithmic x axis plot is used.
Crack tip distance versus time plot at 0.1/min for (a) VHB and (b) Ecoflex. A straight line is fitted, and its equation with gradient is shown.
Crack tip distances covered after 25 ms of blunting of cracks in (i) VHB and (ii) Ecoflex at a strain rate of 10/min. Initial notches/cracks created in the specimen are shown in (a), blunting of cracks are shown in (b), while the crack tip distances covered after 25 ms of crack initiation are shown in (c). Normalized crack tip distances covered are 0.015 and 0.05 for (i, c) VHB and (ii, c) Ecoflex, respectively.
Variations of (a) average crack propagation speed, (b) fracture toughness, (c) failure stretch ratio, and (d) failure stress with different strain rates at a normalized notch length of 0.2 for both VHB and Ecoflex.
Blunting of VHB in a circular shape just before the crack propagation starts and the measurement of the normalized crack tip diameter at a fixed normalized notch length of 0.2 for varying strain rates of (a) 0.1/min, (b) 10/min, (c) 30/min, (d) 60/min, and (e) 80/min.
Blunting of Ecoflex in a circular shape just before the crack propagation starts and the measurement of the normalized crack tip diameter at a fixed normalized notch length of 0.2 for varying strain rates of (a) 0.1/min, (b) 10/min, (c) 30/min, (d) 60/min, and (e) 80/min.
Variation of normalized crack tip diameter with strain rates at a normalized notch length of 0.2 for both VHB and Ecoflex.
Crack speeds versus normalized notch lengths for (a) VHB and (b) Ecoflex at a fixed strain rate of 10/min.
Variations of normalized crack tip diameters of VHB at a fixed strain rate of 10/min for different normalized notch lengths: (a) 3.13E-7, (b) 0.003, (c) 0.006, (d) 0.015, (e) 0.025, (f) 0.031, (g) 0.046, (h) 0.062, (i) 0.078, (j) 0.100, (k) 0.200, (l) 0.330, and (m) 0.500.
Variation of diameters of Ecoflex at a fixed strain rate of 10/min for different normalized notch lengths: (a) 7.14E-7, (b) 0.001, (c) 0.002, (d) 0.007, (e) 0.014, (f) 0.035, (g) 0.071, (h) 0.014, (i) 0.210, (j) 0.350, and (k) 0.500.
Variation in diameters for different normalized notch lengths.
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