CRACK PROPAGATION BEHAVIOR OF LATERALLY CONSTRAINED POLYMERS USED AS DIELECTRIC ELASTOMERS

ABSTRACT 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.

Changes in the mechanical properties of snow relevant to crack propagation in the hours and minutes following loading

<p>Since most dry slab avalanches occur during or immediately following loading by snowfall or wind deposition, it is important to understand changes in the mechanical properties of the snowpack in the minutes and hours following loading. To investigate these temporal changes we conducted a series of 15 Propagation Saw Test (PST) experiments on a flat, uniform site. The existing snowpack at our site contained a layer of surface hoar buried 2 cm below the snow surface. We used a 5 mm sieve to add 10 cm of snow into a 120 cm by 30 cm cardboard frame and completely isolated our blocks. We then conducted PSTs on the buried surface hoar layer from 4 – 453 minutes after adding the sieved snow. We sprayed dye on the side of our tests and filmed them with a high speed camera at 3000 frames per second. Immediately following our tests we measured the density of the sieved snow, and we collected three SnowMicroPen (SMP) profiles along the length of each PST. In one case we collected SMP data at 10 cm increments along our beam prior to conducing our PST to better assess vertical and lateral variations in slab properties induced by sieving. We utilize Digital Image Correlation analyses of the high speed videos to assess the slab elastic modulus (E), the weak layer specific fracture energy (wf), and the crack propagation speed (c) of each test. All our tests fully propagated to the end of the PST columns. Critical cut lengths (rc) ranged between 1.5 and 9 cm, with rc generally increasing over time, in line with the gradual stiffening of the slab observed in the SMP measurements. Our results provide additional information about the temporal changes of mechanical properties immediately following loading, and will better inform modeling efforts attempting to assess these changes.</p>

Comparison of peridynamic and phase-field models for dynamic brittle fracture in glassy materials

We report computational results obtained with three different models for dynamic brittle fracture. The results are compared against recent experimental tests on dynamic fracture/crack branching in glass induced by impact. Two peridynamic models (one using the meshfree discretization, the other being the LS-DYNA’s discontinuous-Galerkin implementation) and a phase-field model lead to interesting and important differences in terms of reproducing the experimentally observed fracture behavior and crack paths. We monitor the crack branching location, the angle of crack branching, the crack propagation speed, and some particular features seen in the experimental crack paths: small twists/kinks near the far edge of the sample. We discuss the models’ performance and provide possible reasons behind the failure of some of the models to correctly predict the observed behavior.

Study on the Fracture Behavior of Cracks Emanating from Tunnel Spandrel under Blasting Loads by Using TMCSC Specimens

Radial cracks may exist around tunnel edge, and these cracks may propagate and weaken tunnel stability under nearby blasting operations. In order to study the blast-induced fracture behavior of radial cracks emanating from a tunnel spandrel, a tunnel model containing a spandrel crack (TMCSC) with different inclination angles was proposed in this paper. Crack propagation gauges (CPGs) and strain gauges were used in the experiments to measure crack initiation moment and propagation time. Finite difference models were established by using AUTODYN code to simulate crack propagation behavior and propagation path. ABAQUS code was used to calculate dynamic stress intensity factors (SIFs). The results show that (1) crack inclination angles affect crack initiation angles and crack propagation lengths significantly; (2) critical SIFs of both mode I and mode II decrease gradually with the increase of the crack propagation speed; (3) the dynamic energy release rates vary during crack propagation; and (4) there are “crack arrest points” on the crack propagation paths in which the crack propagation speed is very small.

Investigation of Crack Propagation Characteristics Using Instrumented Charpy and DWT Tests for Full-Scale Burst Tested 1219 mm OD Grade 550 (X80) Linepipe

TransCanada, on behalf of the Coastal GasLink (CGL) project, has carried out two full-scale burst tests [1, 2] at the Spadeadam test site of DNV GL, to validate the effectiveness of crack arrestors and refine the propagation control design for the large-diameter, X80 linepipe required for this project. The tests were supported by LNG Canada and TransCanada Technology Management Program. For these full-scale burst tests, Grade 550linepipe having Charpy energies from 125 to over 450 J were produced using thermomechanical controlled processing (TMCP) technology. This paper describes propagation and arrest properties of the X80 linepipe materials having various Charpy energy values from the aspect of crack propagation energy and crack propagation speed relationships from instrumented Charpy and press-notched (PN) and static pre-cracked drop-weight tear (SPC-DWT) tests, together with in-situ observation of crack propagation by high-speed video camera. It was found that crack propagation speed is greatly affected by crack propagation energy measured by both Charpy and instrumented DWT tests. The crack propagation energy is lower in DWTT specimens with a higher separation index. It is not clear whether the crack propagation energy is only affected by the separations. However, the crack velocity is higher in DWTT specimens with a higher separation index. It is assumed that the crack propagation speed might be not only affected by separation but also low propagation energy. The testing data obtained from Charpy and instrumented DWT tests are compared with the fracture speed data measured from the full-scale burst test. The correlation between Charpy energy and crack propagation energy in DWTT is also compared with the predictions of an empirical equation.

Crack Branching Characteristics at Different Propagation Speeds: From Quasi-Static to Supersonic Regime

Classical dynamic fracture mechanics predicts that the crack branching occurs when crack propagation speed exceeds a subsonic critical velocity. In this paper, we performed simulations on the dynamic fracture behaviors of idealized discrete mass–spring systems. It is interesting to note that a crack does not branch when traveling at supersonic speed, which is consistent with others' experimental observations. The mechanism for the characteristics of crack branching at different propagation speeds is studied by numerical and theoretical analysis. It is found that for all different speed regimes, the maximum circumferential stress near the crack tip determines the crack branching behaviors.

Effect of Contact between the Crack Faces on Crack Propagation

In mixed-mode crack propagation the crack faces frequently touch each other. The ensuing friction is expected to decrease the crack propagation speed. This effect is usually not taken into account, however, a realistic prediction of this effect may increase the calculated life and consequently increase the length of the inspection intervals. In this paper, penalty contact conditions are introduced in between the crack faces of the automatically generated mesh in a cyclic crack propagation. Special attention is given to the contact formulation and the area in which contact is defined. It is shown that the resulting crack propagation rate is significantly reduced by the introduction of friction provided that positive Mode-I is not significantly involved.