Related Rule Study of Subcritical Crack Growth and Threshold Values in Transversely Isotropic Slates

Elastic parameters and the subcritical crack growth of different bedding angle slate specimens were studied using uniaxial compression testing and the double torsion constant displacement load relaxation method using SANS and MTS Insight machines. To study the relations of the mode-I stress intensity factor K I versus the subcritical crack growth velocity V , the fracture toughness K I C , the stagnation speed, and the threshold values, the double torsion constant displacement load relaxation method was carried out. The related rules between the bedding angles (β) and the uniaxial compressive strength, fracture toughness, and threshold values were investigated. Experimental results show that the uniaxial compression, the fracture toughness, and the threshold value curves move to the bottom then increase with the increase of the β angle. In addition, its fracture toughness is minimal when the β angle of the slates is 45°, and crack initiation and crack propagation are generated under load, which can lead to the failure of the slate. lg K I - lg V relations of transversely isotropic slates measured by this method are in accordance with linear rules, which is in good agreement with the Charles theory. The range of K 0 / K I C for these different bedding angle slates is from 0.511 to 0.789. The test results would provide the basis for studying seepage and time-dependency of rock engineering stability.

Comparative Study of Long- and Short-Stretch Woven Compression Bandages

Compression therapy using bandages or socks is the most common treatment for venous leg ulcers and edema. This article aims to compare the compression between long- and short-stretch bandages (LSB and SSB, respectively). Load-elongation curves, cyclic loading, and elastic recovery are investigated for both Cotton/Polyamide/Polyurethane and 100% bleached cotton bandages as LSB and SSB, respectively. Static (resting) and dynamic (working) pressures are measured on seven male legs, 31 ± 3.6 years old, using both two and three layers bandaging. Picopress pressure tests are performed on the ankle and mid-calf positions at gradual decreasing compression from the ankle to the knee. The deviation percentage between the experimental results by Picopress and theoretical calculations using Laplace's law and Al-Khaburi equations is compared. LSB recovered approximately 99% of its original length after stress-relaxation whereas SSB recovered only 93% of its original length after 5 days of cyclic load-relaxation. Moreover, SSB lost approximately 28.6% of its activity after wearing on the human leg for 5 days.

Glass Fiber-Reinforced PTFE Gasketed-Joint Under a Retorque

Joints gasketed with viscoelastic seals often receive an application of a secondary torque, i.e., retorque, in order to ensure joint tightness and proper sealing. The importance of understanding gasketed joint behavior under various loading conditions and test parameters is paramount to a successful design. The motivation of this study is to characterize and analytically model the initial and retorque load relaxation response of a single 25% glass-fiber reinforced polytetrafluorethylene (PTFE) gasket-bolted joint with serrated flange detail by a single set of experimentally determined modeling constants. The Burger-type viscoelastic modeling constants of the material are obtained through optimization from a baseline load relaxation data and compared to a variety of test cases for both initial and reloadings. Determination of a retorque parameter, α, allowing modeling constants identified from an initial loading to predict the retorque relaxation showed the retarded elasticity or K2 term to be most influential in predicting retorque response. Finally, the validity of the viscoelastic model with the retorque parameter is shown to reasonably predict retorque relaxation responses of all test cases investigated.

Flexure and pressure-loading effects on the performance of structure–battery composite beams

The effects of sustained three-point bend loading and hydrostatic pressure on the mechanical and energy-storage performance of three structure–battery beam prototypes were experimentally investigated. The SB beams, designed for unmanned underwater vehicle applications, were fabricated using marine-grade structural composite constituents and commercial rechargeable lithium-ion “pouch” cells. Low-temperature cure materials and multistep processing were used in fabrication to avoid exposing the cells to temperatures above 60℃. The results showed load relaxation (up to 6–18%) under constant displacement three-point bending within the elastic regime due to viscoelastic shear in adhesive bond layers between components and lamina. Concurrent cell charge–discharge during sustained load bending had a small effect on the load (∼1% change or less). Energy storage capacity under hydrostatic pressures up to 2 MPa, equivalent to 200 m ocean depth, showed a 6–8% decrease in capacity. The results highlighted the need for some design changes to improve structure–battery component performance including: exclusive use of high-temperature cure resins (epoxy or vinyl ester) to improve structural performance and enable single-step fabrication, and transverse (fiber) reinforcement to strengthen the interlayer bonds and embedded cell pockets to minimize load relaxation effects and maximize component bending strength.

Viscoelastic Properties of the Hindfoot Bones

Category: Hindfoot Introduction/Purpose: Obtaining and maintaining compression at an arthrodesis site is a key factor in achieving successful bony union. Bones, like other collagen containing tissues, are known to exhibit viscoelastic properties that may lead to stress relaxation at the arthrodesis site. The viscoelastic properties of the hindfoot bones when subjected to compression (as occurs during fusion surgery) are not known. The objective of this study was to quantify the viscoelastic properties of hindfoot bones under compression by measuring the time course of stress relaxation. Methods: 19 cadaveric human bone cubes 10 mm on each side consisting of trabecular and subchondral bone were cut from the hindfoot bones including the talus, calcaneus, and distal tibia. Each cube was scanned with micro computed tomography (µCT) to quantify bone volume/total volume ratio (BV/TV), trabecular thickness, trabecular separation, trabecular number, and connectivity density. Each specimen was then immersed in a saline bath and compressed 1 mm at a strain rate of 1 mm/s using an MTS machine (Fig 1). This compressed position was then held for 3 hours while the load was recorded. Following the compression test, each specimen was re-scanned with µCT. Results: Two distinct patterns of load relaxation were found. The first consisted of a uniform exponential decay. The second had a similar exponential decay but included a plateau occurring between 1-6 minutes. This second pattern was reflected in the average fractional load relaxation graph (Fig 2). The average peak load was 24.14 kg (SD ± 15.07 kg) and average end relaxation was 2.93 kg (SD ± 3.81 kg). The average time to achieve 95% decay in total load was 34.7 min (SD ± 29.1 min) although removing some outliers, it decreased to 24.9 min (SD ± 18.4 min) which is more representative of the overall data. Averages of BV/TV, trabecular thickness, and trabecular separation increased after stress relaxation while average connectivity density and trabecular number decreased. Conclusion: These data suggest that, due to the viscoelastic properties of bone, approximately 95% of an applied compressive load generated by a fixed displacement is lost within the first 30 minutes. Applied clinically, these findings may have a significant impact on the optimal surgical technique used for osteosynthesis and arthrodesis. Specifically, these data call into question whether the compression applied during surgery can be maintained throughout the healing phase without the application of continuous compression via an external fixator or internal continuous compression device. At minimum, these data suggest that lag or compression screws should be retightened prior to wound closure.

THE EFFECT OF DISPLACEMENT RATE ON VISCOELASTIC PROPERTIES OF RAT CERVIX

Determining the viscoelastic response of cervix at different displacement rates can provide sufficient information for the normal mechanical behavior of the tissue in assessment of cervical insufficiency. The objective of this study was to investigate the effect of displacement rate on viscoelastic properties of rat cervix. Different displacement rates were employed to measure the tensile and load-relaxation properties of cervices from virgin and 16 days post-conception pregnant rats. After preconditioning, the displacement of 2[Formula: see text]mm was applied to the distal halves of five pregnant rat’s cervices and 1 millimeter to the five virgin samples circumferentially. Uniaxial tension tests were employed at the displacement rates of 0.01, 0.1 and 1 mm/s randomly and were held for 10 min while the tissues were relaxed. Tensile and load-relaxation curves were well described by a quasi-linear viscoelastic model. Statistical analysis revealed significant correlation between the change in displacement rate and the elastic response, as well as the viscous response of the virgin samples. For pregnant samples, though, the correlation was found significant between the displacement rate and the elastic response of the tissue. Virgin tissue is strongly viscoelastic. Quantitative measurements of cervical mechanical properties will lead to a more accurate assessment of cervical insufficiency and prediction of preterm birth.