PSI - Issue 25

Hassan Mansour Raheem et al. / Procedia Structural Integrity 25 (2020) 3–7 Author name / Structural Integrity Procedia 00 (2019) 000–000

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1. Introduction Hydrogels are 3D cross-linked networks that highly utilized as a tissue scaffold. Because they exhibited viscoelastic behavior and their mechanical behavior resembles the native tissue such as the nucleus pulposus (Raheem, Bay, and Rochefort 2019). For instance, Agarose, low acylgellan gum and alginate were used as a promising material for NP replacements (Pereira et al. 2011). Since hydrogels show a non-linear behavior under mechanical compression test, it is important to understand their mechanical behavior out of the test range by adapting the hyperelastic models such as Ogden, Neo-Hookean, Mooney-Rivlin, and Yeoh models to be implied in the future in the biomedical applications such as NP replacement. Moreover, understanding the viscoelastic properties of the hydrogel is crucial to develop a material for replacing the degeneration NP (Sasson et al. 2012). It is also vital for modeling to understand the compression behavior of natural tissue (Gefen et al. 2005). Hyperelastic models have been used in literature to characterize a natural tissue such as whale blubber (Zenier and Parmigiani 2011), or hydrogel like agar, where the compression data was modeled using Ogden model (Yu, Santos, and Campanella 2012). Hydrogels are viscoelastic material, soft and fragile materials that contain high water percent (80-90 %) (Buckwalter 1995). This makes them difficult to measure their mechanical properties specifically in tension (Sasson et al. 2012). Because the slipping in the grip during the using of the common tensile test. There are no abundance standards available to guide the testing of the hydrogels. In addition, few studies have been addressed hyperelastic models for precursory materials that potentially would be used as NP replacement (Sasson et al. 2012). Therefore, the results of the simulation by performing ABAQUS, and the candidate hyperplastic models must fit the experimental data. Hence, predict the realistic behavior of the material out of the test range. Thus, the goal of this study is to utilize the strain energy density functions (SEDFs) to determine a suitable constitutive model that represents the nonlinear behavior of the agarose hydrogels under unconfined compression test. In addition, it is to predict the behavior of these materials out of range of the test (i.e. in tension) and validate the results using FE. 2. Materials and methods 2.1. Preparing of the hydrogel Agarose (2 wt. %, A2) powder were dissolved in a heated deionized water at 80 ⁰C. Then poured into a premade mold as cylindrical shape (25 mm diameter, 4 mm height). Then, let them cool down at room temperature and take them out from the mold. Number of the specimens are 5. 2.2. Mechanical testing An unconfined uniaxial compression test was performed on the samples under loading rate 1 mm/min by using INSTRON at room temperature. A preloading condition 1.5 N load was applied to guarantee a full contact between the sample and the plantain. Then, the samples were compressed until 25% of the sample height. The nominal stress and nominal strain were recorded from the machine test. 2.3. Constitutive hyperelastic models Hyperelastic models namely Ogden, Neo-Hookean, Mooney-Rivlin, and Yeoh (Ali, Mosseini, and Sahari 2010)are used in this study to characterize the response of the Aagrose hydrogel (2 wt.% Aagrose) under unconfined compression. These models are well described in literature(Previati, Gobbi, and Mastinu 2017; Zenier and Parmigiani 2011; Ali, Mosseini, and Sahari 2010). Briefly, they depend on the strain energy density functions (W) (Ali, Mosseini, and Sahari 2010). Hydrogels are considered as incompressible materials due to their high water content and exhibit large strain under low loading condition. Therefore, W is postulated applicable to describe the non-linear behavior of the hydrogel.