PSI - Issue 28

Bahador Bahrami et al. / Procedia Structural Integrity 28 (2020) 829–835 Bahrami et al./ Structural Integrity Procedia 00 (2020) 000–000

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1. Introduction Polymethyl-methacrylate (PMMA) has been a frequently-used material in biomedical applications in recent years owing to its favorable properties such as biocompatibility, low toxicity, aesthetic quality, etc.. While PMMA has wide applications in different medical fields, it is primarily used in two areas, namely dentistry and orthopedics. For example, it can be used as a dental material for the fabrication of a denture base or in the restoration and relining of dental prostheses (Frazer et al. (2005)). Besides, in orthopedics, it can be used as bone cement to fixate the joint between the metallic implant and the patient’s bone as well as filling prospective bone cavities. Despite to the wide applications of PMMA in biomechanics, still, two problems exist regarding the utilization of this material. The first one is the brittle behavior of PMMA which makes it prone to sudden fracture without any prior indications (Deb and Vazquez, 2001). Moreover, this material has poor mechanical properties in comparison with its adjacent components like tooth or bone (May-Pat et al. (2012)). The second problem of using PMMA is the possible existence of various kinds of defects in bone cement or dental prosthesis components. These defects have been reported to range from small pores to large cracks and voids (Taylor et al. (2004)). The combination of these defects as the stress raiser elements with the brittle nature of PMMA has caused several failures in recent years (Baleani et al. (2003)). A brief literature survey shows that most of the research articles in this area belong to the bone cement application of PMMA while the dental form of this material, despite its wide implementation, has been less investigated. Regarding the fracture studies of bone cement type of PMMA, Kotha et al. (2006) improved the tensile strength and fracture toughness of bone cement by adding the short titanium fibers to it. Ries et al. (2006) investigated the in vivo degradation of the fracture toughness in the components made of bone cements. There are also other research studies that have utilized different methods including small punch technique, acoustic emission, etc. to characterize various mechanical properties of bone cements such as fracture toughness and fatigue life (Giddings et al. (2001); Guandalini et al. (2004)). Considering the fracture investigation of PMMA with dental applications, there exist much fewer works. As an instance, Ayatollahi et al. (2020) used a strain-based fracture model to predict the mixed-mode fracture loads of shortened semi-circular bend (SSCB) specimens made of PMMA dental biomaterial. Considering the research gap in the analysis of PMMA dental materials, the current manuscript has two aims: first is to provide some experimental findings on the fracture load of PMMA dental material, and second is to assess the suitability of the generalized strain energy density (GSED) criterion for fracture prediction of polymeric components. The paper begins with experiments in which a number of mixed-mode fracture tests are conducted on the modified single edge notched bend (SENB) specimens and the respective fracture loads are recorded. Then, the generalized strain energy density criterion is introduced and the experimentally measured fracture loads are estimated by means of this criterion. It is shown that two types of critical distances can be used in combination with the GSED criterion. While both of the considered critical distances lead to satisfactory results, it is demonstrated that the estimations via the one specifically derived for the GSED criterion is more accurate. Nomenclature � - � Coefficients of the strain energy density criterion Crack length Tensile and shear modulus � �� Modes I and II stress intensity factors � Experimental fracture load Polar coordinate components � Critical distance Half of the support span T-stress term Width and thickness of the SENB specimen � Critical value of strain energy density � , �� and ∗ Normalized form of K I , K II and T