The Biomechanics of Cartilage—An Overview

Articular cartilage (AC) sheathes joint surfaces and minimizes friction in diarthrosis. The resident cell population, chondrocytes, are surrounded by an extracellular matrix and a multitude of proteins, which bestow their unique characteristics. AC is characterized by a zonal composition (superficial (tangential) zone, middle (transitional) zone, deep zone, calcified zone) with different mechanical properties. An overview is given about different testing (load tests) methods as well as different modeling approaches. The widely accepted biomechanical test methods, e.g., the indentation analysis, are summarized and discussed. A description of the biphasic theory is also shown. This is required to understand how interstitial water contributes toward the viscoelastic behavior of AC. Furthermore, a short introduction to a more complex model is given.

Comparison of the effect of argon, hydrogen, and nitrogen gases on the reduced graphene oxide-hydroxyapatite nanocomposites characteristics

In this study, the effect of the argon, nitrogen, and hydrogen gases on the final properties of the reduced graphene oxide- hydroxyapatite nanocomposites synthesized by gas injected hydrothermal method was investigated. Four samples were synthesized, which in the first sample the pressure was controlled by volume change at a constant concentration. In subsequent samples, the pressure inside the autoclave was adjusted by the injecting gases. The initial pressure of the injected gases was 10 bar and the final pressure considered was 25 bar. The synthesized powders were consolidated at 950 °C and 2 MPa by spark plasma sintering method. The final samples were subjected to Vickers indentation analysis. The findings of this study indicate that the injection of argon, hydrogen, and nitrogen gases improved the mechanical properties of the nanocomposites. Injection of gases increased the crystallinity and particle size of hydroxyapatite, and this increase was greater for nitrogen gas than for others. Injection of these gases increased the rate of graphene oxide reduction and in this case the effect of nitrogen gas was greater than the others.

An Expanding Cavity Model for Indentation Analysis of Shape Memory Alloys

The mechanical and functional responses of shape memory alloys (SMAs), which are often used in small volume applications, can be evaluated using instrumented indentation tests. However, deciphering the indentation test results in SMAs can be complicated due to the combined effects of the non-uniform state of stress underneath the indenter and stress-induced phase transformation. To address this issue, an expanding cavity model (ECM) applicable to spherical indentation of SMAs is developed in this work based on an analytical solution for an internally pressurized hollow sphere. Analytical expressions for key indentation parameters such as the mean contact pressure and size of the transforming zone are obtained, whose validity is evaluated by recourse to finite element simulations and published experimental data for a Ni–Ti alloy. It is shown that the ECM predicts the above parameters reasonably well for indentation strains varying from 0.01 to 0.04. Also, a method is proposed to determine the critical stress required to initiate phase transformation under uniaxial compression based on the application of the ECM to interpret the indentation stress–strain response.

Nano indentation Analysis of Multi Stage Spark Plasma Sintered Hydroxyapatite-Calcium Titanate Biocomposite

Despite being highly bioactive and biocompatible, some of the limitations like poor fracture toughness, lack of electrical conductivity and antimicrobial properties restrict the use of monolith hydroxyapatite (HA) as bone replacement material. In this paper, we address one such issue and will demonstrate how CaTiO3 (CT) addition enhances physical properties like strength, fracture toughness etc. Therefore, the strategy in the current research is to develop dense HA-CT biocomposites using innovative multi-stage spark plasma sintering (MSSPS) technique (at 50 MPa, 1200oC, 5 min), that can mimic the function and properties close to that of natural bone. Fine scale microstructural characterization using TEM reveals the presence of twins in CaTiO3 grains and the grain size of HA is around 1-2 µm. Phase analysis using x-ray diffraction analysis revealed an absence of α and β-tricalcium phosphate (Ca3(PO4)2) or CaO phase which is also supported by Fourier transformed infra-red spectroscopy. Elastic modulus of 46-135 GPA is obtained using nanoindentation. Based on the available empirical models, it has been observed that the experimentally measured density hardness, and elastic modulus match reasonably well with that of the natural cortical bone.

A 2-D Axisymmetric Indentation Model for Peening Residual Stress in Metallic Material and its Experimental Verification

Various impact analysis models have been used for analytical prediction of peeningresidual stress. In this paper, a new approach based on finite element (FE) analysis was proposed topredict the peening residual stress through single indentation analysis using the dent profilegenerated on a shot-peened surface. Three analysis models (rigid, elastic, and plastic shots) werecompared each other, and the dent obtained in the plastic shot impact analysis model showed a dentprofile almost identical to that of the experimentally obtained dent. A rigid indenter modelconstructed using the dent profile obtained by dynamic impact analysis, and it integrated into thesingle indentation analysis model. The FE surface residual stress obtained in the center of the dentof the indentation analysis model was found to be almost identical to the surface residual stressmeasured by X-ray diffraction (XRD), thus verifying the validity of the proposed single basicindentation FE model based on impact analysis.