Browsing by Author "Espinosa-Leal, Leonardo"
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- Characterising the notch root radii and analyses of stress concentration factors near the dominant valleys of rough surface profiles
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2023) Gebrehiwot, Silas Z.; Espinosa-Leal, Leonardo; Remes, Heikki; Vermunt, MarinusSurface roughness is one of the key surface integrity factors affecting the strength and fatigue life of components. Stress concentrations occur due to the randomness of the surface profiles. The presence of a dominant valley, a complex geometry and interacting effects exasperate the severity of the stress concentrations. To estimate the theoretical stress concentration factor (SCF) at the valley, the notch root radius should be estimated carefully. We propose an effective method for estimating the root radius of the deepest valley using numerical derivative techniques. The surface roughness of a carefully sanded Alumec 89 block was measured using SJ-400 tester. The 1-D roughness data was used first to evaluate the root radius of the deepest valleys and then, estimate the SCF using analytical and computational methods. We used 2-D finite element (FE) models under uniaxial tension for the computational analyses. The validity of our method is based on determining the SCF using different theoretical methods and comparing the results to the FE calculations. The theoeritical estimations are made using the Neuber, Inglis and Arola-Ramulu approaches, whereas COMSOL Multiphysics is used for the FE analyses. Comparing the theoeritical methods with the FE calculations, the Arola-Ramulu approach was better, with a maximum of 16.3 % error. The minimum deviations can be explained by the model containing parameters such as Ry, Rz and Ra which are inherent to the roughness profile of the material. - Experimental investigation and modelling of the nonlinear creep behaviour of additive-manufactured carbon fibre-reinforced polyethylene terephthalate (CF-PET).
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2024-10) Gebrehiwot, Silas Z.; Espinosa-Leal, Leonardo; Linderbäck, Paula; Remes, HeikkiIn this paper, the nonlinear creep behaviour of additive-manufactured carbon fibre-reinforced polyethylene terephthalate (CF-PET) is characterised using experimental, theoretical and computational methods. The experimental approach investigates the influence of infill orientations on the creep deformation of the material. For the study, samples at 0°, 45○, and 90° infill orientations are produced with 90% infill density using fused filament fabrication (FFF). The infill orientation parameter highly influences the creep behaviour. Increasing the infill orientation from 0° to 90° monotonically improves the creep resistance of the material, which can be explained by orientation of the fibre-matrix reinforcement towards the uniaxial stresses. Surface examinations of creep-ruptured samples via scanning electron microscopy (SEM) reveal that a combination of matrix failure, fibre pull-out, fibre-matrix debonding, inter-layer debonding, and the presence of voids cause the fractures. Based on the experimental data, the primary and secondary creep responses are modelled theoretically and computationally. The theoretical model is based on the dependence of the material's creep on stress and time parameters at the transient and steady state stages. Combined stress and time functions are used to model the creep of the material. Parallelly, two-dimensional (2D) finite element (FE) analyses are made on COMSOL Multiphysics to model the creep computationally. The approach is based on the superposition of Norton's and Garofalo's creep models with predefined time hardening property. The results of the modelling are in good agreement with the experimental findings, showing a maximum of 1.04 % for the theoretical, and 2.9 % for the computational approaches. - On the Short-Term Creep and Recovery Behaviors of Injection Molded and Additive-Manufactured Tough Polylactic Acid Polymer
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2023-11) Gebrehiwot, Silas Z.; Espinosa-Leal, Leonardo; Andersson, Mirja; Remes, HeikkiThe creep and recovery behaviors of a tough polylactic acid polymer are investigated experimentally and theoretically. We studied the influence of manufacturing methods and parameters on the viscoelastic responses. Experimental comparisons were carried out on 13 different samples manufactured using fused deposition modeling (FDM) and injection molding methods. The sample variations in the FDM were based on four infill densities (70-100%) and 3 infill directions (0 ∘, 45 ∘, 90 ∘) . Theoretically, the Burgers and Weibull’s models are used to predict the creep and recovery responses of the samples. Our experimental findings suggest that the injection-molded samples perform better in creep for most of the cases. However, at higher stress loadings, the 90 and 100% infill density samples showed excellent creep resistance behaviors at the 90 ∘ infill direction. On the other hand, the theoretical creep and recovery predictions were based on the nonlinear least-squares regression method. The Burgers model predicted the creep responses with reasonable accuracies. A maximum of 5.83 % mean absolute percentage error (MAPE) was found for the 0° infill direction and 80% infill density sample. On the contrary, the model lacks accuracy in recovery strain predictions, showing an average of 173.15% MAPE for all studied samples. Introducing Weibull’s distribution improved the accuracies showing a 3.44% average MAPE for all samples. - Optimising the mechanical properties of additive-manufactured recycled polylactic acid (rPLA) using single and multi-response analyses methods
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2023-12) Gebrehiwot, Silas Z.; Espinosa-Leal, Leonardo; Linderbäck, Paula; Remes, HeikkiTaguchi’s design of experiment (DoE) and the grey relational analysis are used to optimise fused filament fabrication (FFF) parameters for the tensile strength and modulus of toughness (MoT) responses of a recycled polylactic acid (Reform-rPLA). The paper investigates the influences of the infill geometry, infill density, infill orientation, nozzle temperature and infill speed on the mechanical properties using the L18 orthogonal array that is based on the 2 1× 4 3 factor levels and 3 experimental repetitions. The output responses are first studied individually and combined as a multi-response optimisation using the grey relational analysis method. In the strength optimisation, the infill orientation and infill density are statistically significant with P-values α less than the 0.05 criterion. Similarly, the analysis of variance (ANOVA) for the MoT showed that infill orientation and infill geometry are statistically significant. For the multi-response optimisation, only the infill orientation is statistically significant. The mean response analyses identified factor levels that led to optimum strength and MoT responses. The confirmation tests are in good agreement with the response predictions. Using the first three influential factors, multiple variable linear regression models were developed. The predictive models showed average errors of 7.91 % for the tensile strength and 8.6 % for the MoT.