On relevance of time-dependent Poisson’s ratio for determination of relaxation function parameters

The current study concerns the determination of material constants of a three-dimensional linear viscoelastic model. It is assumed that the constitutive equation utilizes a Prony series as a memory function. A method for the evaluation of relaxation function parameters is presented which can be used for arbitrary loading histories. The proposed methodology is applied to the identification of the viscoelastic constants of acrylonitrile butadiene styrene (ABS). For that purpose, a number of rheological tests in tension have been performed on ABS standard dogbone specimens. The significance of the time-dependent Poisson’s ratio for the determination of material parameters is investigated. It is found that taking into account the measurements of specimen’s lateral contraction over time has a particularly strong influence on the identified values of parameters responsible for the bulk behavior. Several boundary value problems have been analyzed in order to assess the influence of the material parameter values on the obtained solutions. It is demonstrated that some oversimplifications assumed during the determination of viscoelastic constants can lead to a loss of precision or even wrong results.

Cyclic behavior of self-tapping screwed laminated bamboo lumber connections subjected to cycle loadings

Self-tapping screws are commonly used to connect critical structural components, such as legs to rails in chair construction, using laminated bamboo lumber (LBL) materials. The loosening of a connection is commonly seen in self-tapping screwed LBL connections before actual breakage of connections happens. The loosening of connections, especially those associated with chair legs, can significantly affect chair stability. Current furniture performance test standards have not address this issue, i.e., the minor loosening of a connection is not treated as a failure in the current standard because of the lack of better understanding the load-rotation-time behavior of various connections subjected to the cyclical loads. The effects of cyclic loading magnitude and orientation on the load-rotation-time behavior of L-shaped, end-to-side, single self-tapping screwed LBL connections were investigated. Results indicated that the Burger and Kelvin models could be used to describe the cyclic and recovery behavior of studied connections. Increasing the cyclic loading magnitude resulted in a decreasing trend for all viscoelastic constants. The most significant decrease in all viscoelastic constants occurred when the cyclic loading magnitude applied to connections increased from 50 to 60% of its corresponding ultimate static resistance loads.

Biomechanical Properties of the Human Soft Palate

Objective: To measure biomechanical properties of the human soft palate and the variation across anatomic regions. Design: Ex vivo analysis of human tissue. Patients/participants: Ten palates harvested from 10 normal adult human cadavers (age range, 37 to 90 years). Interventions: Computer-controlled uniaxial stress-relaxation mechanical properties tested in physiological saline at 37°C. Main Outcome Measures: Measurement of Young modulus, Poisson ratio, and determination of viscoelastic constants c, τ1, and τ2 by curve-fitting of the reduced relaxation function to the data. Results: One hundred sections were tested from the 10 palates, representative of 10 anatomic zones. The mean Young modulus range was 585 Pa at the posterior free edge to 1409 Pa at regions of attachment. The mean Poisson ratio in the inferior-superior direction was 0.45 (SD 0.26) and in the lateral direction, was 0.30 (SD 0.21). The mean viscoelastic constants for 1-mm extensions were C = −0.1056 (±0.1303), τ1 = 11.0369 (±9.1865) seconds, and τ2 = 0.2128 (±0.0792) seconds, and for 2-mm extensions were C = −0.1111 (±0.1466), τ1 = 14.3725 (±5.2701) seconds, and τ2 = 0.2094 (±0.0544) seconds. Conclusions: The results show agreement with values of the Young modulus estimated by authors (Ettema and Kuehn, 1994; Berry et al., 1999) undertaking finite element modeling of the palate. However, other modulus measurements based on closing pressure are considerably different. The spatial distribution of viscoelastic parameters across the palate shows good consistency.