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Creep Response (creep + response)
Selected AbstractsSome aspects of the mechanical response of BMI 5250-4 neat resin at 191°C: Experiment and modeling,JOURNAL OF APPLIED POLYMER SCIENCE, Issue 3 2008M. B. Ruggles-Wrenn Abstract The inelastic deformation behavior of BMI-5250-4 neat resin, a high-temperature polymer, was investigated at 191°C. The effects of loading rate on monotonic stress,strain behavior as well as the effect of prior stress rate on creep behavior were explored. Positive nonlinear rate sensitivity was observed in monotonic loading. Creep response was found to be significantly influenced by prior stress rate. Effect of loading history on creep was studied in stepwise creep tests, where specimens were subjected to a constant stress rate loading followed by unloading to zero stress with intermittent creep periods during both loading and unloading. The strain-time behavior was strongly influenced by prior deformation history. Negative creep was observed on the unloading path. In addition, the behavior of the material was characterized in terms of a nonlinear viscoelastic model by means of creep and recovery tests at 191°C. The model was employed to predict the response of the material under monotonic loading/unloading and multi-step load histories. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008 [source] The compressive creep properties of normal and degenerated murine intervertebral discsJOURNAL OF ORTHOPAEDIC RESEARCH, Issue 1 2004Erika I. Palmer Abstract Identifying mechanisms by which degeneration alters intervertebral disc material properties and biomechanical behavior is important for clarifying back pain risk factors as well as for evaluating the efficacy of novel interventions. Our goal was to quantify and characterize degeneration-dependent changes in the disc's response to compression using a previously established murine model of disc degeneration. We performed compressive creep tests on normal and degenerated murine intervertebral discs and parameterized the biomechanical response using a previously established fluid-transport model. Using a series of biochemical and histological assays, we sought to determine how biomechanical alterations were attributable to degeneration-related changes in tissue morphology. We observed that with moderate degeneration, discs lost height (mean ± std. dev. of 0.44 ± 0.01 vs. 0.36 ± 0.01 mm, p < 0.0001), increased in proteoglycan content (31 ± 4 vs. 43 ± 2 ,g/ml of extract, p < 0.0002), became less stiff (2.17 ± 0.66 vs. 1.56 ± 0.44 MPa, p < 0.053), and crept more. Model results suggested that the increased creep response was mainly due to a diminished strain-dependent nuclear swelling pressure. We also noted that the model-derived tissue properties varied with the applied load magnitude for both normal and degenerated discs. Overall, our data demonstrate that architectural remodeling stimulated by excessive loading diminishes the disc's ability to resist compression. These results are similar to degeneration-dependent changes reported for human discs. © 2003 Orthopaedic Research Society. Published by Elsevier Ltd. All rights reserved. [source] Medial collateral ligament autografts have increased creep response for at least two years and early immobilization makes this worseJOURNAL OF ORTHOPAEDIC RESEARCH, Issue 2 2002G. M. Thornton Recent evidence has shown that 10,40% of knee joints reconstructed with soft-tissue autografts have a recurrence of abnormal joint laxity over time. One possible explanation is the "stretching out" (or unrecovered creep) of the graft tissue. To test in vitro creep and creep recovery of fresh anatomic ligament autografts in an extra-articular environment, 16 rabbits underwent an orthotopic medial collateral ligament (MCL) autograft procedure to one hindlimb. Three subgroups of animals had either unrestricted cage activity for 1 year (n = 5) or 2 years (n = 5) or pin-immobilization for the first 6 weeks followed by cage activity for the remainder of 1 year (n = 6). Following laxity measurements, to test their creep response, isolated MCL grafts were cyclically and then statically creep tested in vitro at 4.1 MPa, allowed to recover at zero load for 20 min, and finally elongated to failure. Due to differences in cross-sectional area between the grafts and normal MCLs, two normal control groups were tested: stress-matched tested at 4.1 MPa (16.2 N; n = 7) and force-matched tested at 29.1 N (7.1 MPa; n = 6). Ligament grafts had normal laxity but significantly increased creep and decreased creep recovery compared to normal MCLs after 1 and 2 years of healing (p < 0.0004). Graft failure stress was also significantly less than normal (p < 0.0001). Immobilized grafts had significantly greater creep compared to non-immobilized grafts at 1 year of healing (p < 0.05). These results support previous observations concerning material inferiority of fresh anatomic rabbit MCL autografts, but add the concept that such grafts also have increased potential to creep with either slower or incomplete recovery when subjected to low stresses in vitro. Joint and ligament laxities in situ were normal in this model, however, suggesting either that in vivo MCL graft stresses are lower than those used here in vitro or that these tissues have other mechanisms by which they can recover their functional length in vivo. © 2002 Orthopaedic Research Society. Published by Elsevier Science Ltd. All rights reserved. [source] Maleated amorphous ethylene propylene compatibilized polyethylene nanocomposites: Room temperature nonlinear creep responsePOLYMER ENGINEERING & SCIENCE, Issue 8 2010Ali Shaito Nonlinear creep of polyethylene and its nanocomposites remains an area of significant interest. Maleated polyethylene is often used as a compatibilizer to ensure enhanced dispersion. This article investigates blown films of linear low-density polyethylene and its nanocomposites with montmorillonite-layered silicate (MLS). An amorphous ethylene propylene copolymer grafted maleic anhydride (amEP) was added to enhance the interaction between the PE and the MLS. Tensile results indicate that the addition of amEP and MLS separately and together produces a synergistic effect on the mechanical properties of the neat PE. Nonlinear creep was analyzed by examining creep and recovery of the films with a Burger model and the Kohlrausch-Williams-Watts relation. A consistent decrease in unrecoverable plastic strain was obtained in the nanocomposite samples. A decreased retardation time associated with MLS presence was determined. POLYM. ENG. SCI., 50:1620,1632, 2010. © 2010 Society of Plastics Engineers [source] | ||||||||||