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Abstract: This paper presents an experimental and numerical analysis of the mechanical behaviour of orthopaedic implants with crack-type defects, considering the principles and advantages of the modern X-FEM method, which was used due to limitations of traditional FEM in terms of crack growth simulation, especially for complex geometries. In X-FEM, the finite element space is enriched with discontinuity functions and asymptotic functions at the crack tip, which are integrated into the standard finite element approximation using the unity division property. Though rare, femoral component failures are well-documented complications that can occur after hip prosthetic implantation. Most stem fractures happen in the first third of the implant due to the loosening of the proximal stem and fixation of the distal stem, leading to bending and eventual fatigue failure. The main goal of this paper was to obtain accurate and representative models of such failures. Experimental analyses of the mechanical behaviour of implants subjected to physiological loads, according to relevant standards, using a new combined approach, including both experiments and numerical simulations was presented. The goal was to verify the numerical results and obtain a novel, effective methodology for assessing the remaining fatigue life of hip implants. For this purpose, the analysis of the influence of Paris coefficients on the total number of cycles was also considered. Hence, this simulation involved defining loads to closely mimic real-life scenarios, including a combination of activities such as ascending stairs, stumbling, and descending stairs. The tensile properties of the titanium alloy were experimentally determined, along with the Paris law coefficients C and m. The finite element software ANSYS 2022R2 version was used to develop and calculate the three-dimensional model with a crack, and the resulting stresses, stress intensity factors, and the number of cycles presented in the figures, tables, and diagrams. The results for the fatigue life of a partial hip implant subjected to various load cases indicated significant differences in behaviour, and this underscores the importance of analysing each case individually, as these loads are heavily influenced by each patient’s specific activities. It was concluded that the use of numerical methods enabled the preliminary analyses of the mechanical behaviour of implants under fatigue loading for several different load cases, and these findings can be effectively used to predict the possibility of Ti-6Al-4V implant failure under variable cyclic loads.
Keywords: structural integrity, fatigue fracture, extended finite element method (XFEM), experimental testing, DIC, numerical simulations, stress intensity factor, orthopaedic implants, crack-type defect
Published in DKUM: 21.03.2025; Views: 0; Downloads: 7
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Abstract: A discussion on the conventional creep crack growth parameters, e.g. the experimental C*-integral, C*exp, or the experimental Ct-integral, Ct,ssc, shows that the physical meaning of these parameters for growing cracks in elastic–plastic, creeping materials is not fully clear. Therefore, a comparison is presented in this paper between the conventional creep crack growth parameters, several J-integral related parameters and the crack driving force (CDF), which has been used in linear elastic and elastic–plastic fracture mechanics. The CDF for elastic–plastic, creeping materials is derived from basic thermodynamic principles and by applying the concept of configurational forces (CFs). A comprehensive numerical study is performed where crack propagation is modelled by alternating creep and crack extension steps at constant loads in a compact tension specimen made of the nickel-base superalloy Waspaloy at a temperature of 700 °C. The CDF is evaluated by a CF-based post-processing procedure after a conventional finite element computation. This procedure is applicable for small-scale creep (ssc-), transition creep (tc-) and “moderate” extensive creep (ec-) conditions. For more pronounced ec-conditions, the procedure might have to be adapted. It is shown that C*exp and Ct,ssc reflect the time derivative of the CDF during the creep stages. In contrast, the variations of the CDF coincide well with that of J-values estimated from the crack-tip opening displacement.
Keywords: fracture mechanics, creep crack growth, crack driving force, C*-integral, J-integral, configurational force concept, finite element method
Published in DKUM: 20.03.2025; Views: 0; Downloads: 5
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Abstract: This work presents the development of a J-integral estimation procedure for deep and shallow cracked bend specimens based upon plastic ηpl factors for a butt weld made in an S690 QL high strength low alloyed steel. Experimental procedures include the characterization of average material properties by tensile testing and evaluation of base and weld metal resistance to stable tearing by fracture testing of square SE(B) specimens containing a weld centerline notch. J-integral has been estimated from plastic work using a single specimen approach and the normalization data reduction technique. A comprehensive parametric finite element study has been conducted to calibrate plastic factor ηpl and geometry factor λ for various fixture and weld configurations, while a corresponding plastic factor γpl was computed on the basis of the former two. The modified ηpl and γpl factors were then incorporated in the J computation procedure given by the ASTM E1820 standard, for evaluation of the plastic component of J and its corresponding correction due to crack growth, respectively. Two kinds of J-R curves were computed on the basis of modified and standard ηpl and γpl factors, where the latter are given by ASTM E1820. A comparison of produced J-R curves for the base material revealed that variations in specimen fixtures can lead to ≈10% overestimation of computed fracture toughness JIc. Furthermore, a comparison of J-R curves for overmatched single-material idealized welds revealed that the application of standard ηpl and γpl factors can lead to the overestimation of computed fracture toughness JIc by more than 10%. Similar observations are made for undermatched single material idealized welds, where fracture toughness JIc is overestimated by ≈5%.
Keywords: metal weld, strength mismatch, fracture, plastic correction factors, fixture rollers, J-R resistance curve
Published in DKUM: 20.03.2025; Views: 0; Downloads: 3
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Abstract: Knowledge of defect formation mechanisms in the manufacturing process helps improve product quality. In this study, defect formation due to re-melting of each layer in selective laser melting of Ti-6Al-4V demonstrated the physical behavior in the manufacture of metallic parts. The re-melting strategy was based on scanning with low energy density (ED) and increased ED with various combinations of laser processing parameters. The increased EDs and their parameters, namely laser power, scanning speed, and hatch distance, were selected based on the previous research experience by the authors. The concept of selecting a low ED followed by a high ED was to reduce the spattering of the powder material during the process. The low ED caused partial sintering of the powder, while the high ED caused the melting of the material, resulting in different metallurgical properties of the manufactured parts. Densities, pore properties, porosity in the initial layers, surface morphologies, and microstructures in the defective areas of the samples were studied to determine the effects of re-melting. Advantages and disadvantages were found with respect to the range of applications of the products
Keywords: re-melting, pore properties, defect, surface morphology, Ti-6Al-4V, selective laser melting
Published in DKUM: 14.03.2025; Views: 0; Downloads: 2
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Abstract: In our study, we explored the complexities of overhead transmission line (OTL) engineering, specifically focusing on their responses to varying atmospheric conditions (ambient temperature, ambient humidity, solar irradiance, ambient pressure, wind speed, wind direction), and electric current usage. Our goal was to comprehend how these independent variables impact critical responses (dependent variables) such as conductor temperature, conductor sag, tower leg stress, and vibrations – parameters crucial for electric distribution. We modelled the target output variable as a polynomial of a certain degree of the input variables. The precise forms of the polynomial were determined using the genetic algorithms (GA). Developed models are essential for quantifying the influence of each input parameter, enriching our understanding of essential system elements. They provide long-term predictions for assessing transmission line lifespan and structural stability, with particularly high precision in forecasting temperature and sag angle. It is important to note that certain engineering parameters, such as material properties and load considerations, were not included in our research, potentially influencing accuracy.
Keywords: Overhead Transmission Lines (OTL), machine learning, modelling, optimization, genetic algorithms (GA)
Published in DKUM: 10.03.2025; Views: 0; Downloads: 3
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