Investigation of Murine Vaginal Creep Response to Altered Mechanical Loads

The vagina is a viscoelastic fibromuscular organ that provides support to the pelvic organs. The viscoelastic properties of the vagina are understudied but may be critical for pelvic stability. Most studies evaluate vaginal viscoelasticity under a single uniaxial load; however, the vagina is subjected to dynamic multiaxial loading in the body. It is unknown how varied multiaxial loading conditions affect vaginal viscoelastic behavior and which microstructural processes dictate this. Therefore, the primary objective was to develop methods using extension-inflation protocols to quantify vaginal viscoelastic creep under various circumferential and axial loads. The second objective was to quantify vaginal creep and collagen microstructure in the fibulin-5 wildtype and haploinsufficient vaginas. To evaluate pressure-dependent creep, the fibulin-5 wildtype and haploinsufficient vaginas (n=7/genotype) were subjected to various constant pressures at the physiologic length for 100 seconds. For axial length-dependent creep, the vaginas (n=7/genotype) were extended to various fixed axial lengths then subjected to the mean in vivo pressure for 100 seconds. Second harmonic generation imaging was performed to quantify collagen fiber organization and undulation (n=3/genotype). Increased pressure significantly increased creep strain in the wildtype, but not the haploinsufficient vagina. Axial length did not significantly affect the creep rate or strain in both genotypes. Collagen undulation varied through the depth of the subepithelium but not between genotypes. These findings suggest that the response to loading may vary with biological processes and pathologies, therefore, evaluating vaginal creep under various circumferential loads may be important.

Delayed buckling of spherical shells due to viscoelastic knockdown of the critical load

We performed dynamic pressure buckling experiments on defect-seeded spherical shells made of a common silicone elastomer. Unlike in quasi-static experiments, shells buckled at ostensibly subcritical pressures, i.e. below the experimentally determined critical load at which buckling occurs elastically, often following a significant delay period from the time of load application. While emphasizing the close connections to elastic shell buckling, we rely on viscoelasticity to explain our observations. In particular, we demonstrate that the lower critical load may be determined from the material properties, which is rationalized by a simple analogy to elastic spherical shell buckling. We then introduce a model centred on empirical quantities to show that viscoelastic creep deformation lowers the critical load in the same predictable, quantifiable way that a growing defect would in an elastic shell. This allows us to capture how both the deflection at instability and the time delay depend on the applied pressure, material properties and defect geometry. These quantities are straightforward to measure in experiments. Thus, our work not only provides intuition for viscoelastic behaviour from an elastic shell buckling perspective but also offers an accessible pathway to introduce tunable, time-controlled actuation to existing mechanical actuators, e.g. pneumatic grippers.

Brittle Creep and Viscoelastic Creep in Lower Palaeozoic Shales from the Baltic Basin, Poland

The paper analyses the mechanical properties of shales from the Baltic Basin, focusing on creep strain in conditions of variable stress and elevated temperature (85 °C). Rock samples were collected from drill cores from various depths between 3600–4000 m. A series of creep tests was performed using a triaxial apparatus in simulated pressure and temperature conditions in the reservoir. The creep tests were conducted at variable levels of differential stress in variable time intervals. The laboratory experiments were performed in order to study brittle and viscoelastic creep proceeding in time in shales rich in organic matter and clay minerals. Creep compliance of shale formations rich in organic matter influences the success of hydraulic fracturing procedures, as well as migration of natural gas during exploitation. Laboratory characteristics of geomechanical properties (compressive strength, strain and elastic moduli) is crucial for planning natural gas exploitation from unconventional resources. The results indicate that the level of constant differential stress and creep time significantly influence the mechanical properties of shales. The paper presents the differences between brittle and viscoelastic strain registered during creep tests at variable stress conditions and time intervals. In viscoelastic creep tests, creep strain is over two times larger in the second stage of creep in comparison to the magnitude of strain registered in the first stage. In brittle creep tests, axial strain in the first creep stage is two times larger than in viscoelastic creep tests in the second stage. Based on the experiments, elastic parameters, i.e., Young’s modulus and Poisson’s ratio, have been determined for each of the analysed samples. In brittle creep tests, Young’s modulus is smaller than in viscoelastic creep tests. In viscoelastic creep tests Young’s modulus increases in successive stages. Whereas Poisson’s ratio is larger for samples from brittle creep tests than for samples from viscoelastic creep tests and does not change with subsequent creep stages in viscoelastic creep tests.

Humidity Effect on Dynamic Electromechanical Properties of Polyacrylic Dielectric Elastomer: An Experimental Study

In this research, by utilizing the Very-High-Bond (VHB) 4905 elastomer, we carry out an experimental examination on the humidity effect on dynamic electromechanical performances of dielectric elastomers, including the dynamic response and viscoelastic creeping. Firstly, we experimentally analyze effects of the pre-stretch, peak voltage, waveform and frequency of the dynamic response of VHB 4905 elastomer under several ambient humidities. In general, the amplitude of dynamic deformation gradually adds up with the increasing humidity. Besides, it is found that the amplitude affected by different parameters shows diverse sensitivity to humidity. Subsequently, effect of humidity on the viscoelastic creeping of VHB 4905 is explored. The results demonstrate that, subject to different ambient humidities, the viscoelastic creeping under Alternating Current (AC) voltage is similar to that under Direct Current (DC) voltage. Furthermore, the equilibrium position of dynamic viscoelastic creep enlarges gradually with the humidity, regardless of voltage waveforms. For the dielectric elastomer with a pre-stretch ratio of 3, when the humidity increases from 20% to 80%, the increase of average equilibrium position of dynamic viscoelastic creep is larger than 1599%.