5 research outputs found
Determination of the Constants of GTN Damage Model Using Experiment, Polynomial Regression and Kriging Methods
No full textDamage models, particularly the Gurson–Tvergaard–Needleman (GTN) model, are widely used in numerical simulation of material deformations. Each damage model has some constants which must be identified for each material. The direct identification methods are costly and time consuming. In the current work, a combination of experimental, numerical simulation and optimization were used to determine the constants. Quasi-static and dynamic tests were carried out on notched specimens. The experimental profiles of the specimens were used to determine the constants. The constants of GTN damage model were identified through the proposed method and using the results of quasi-static tests. Numerical simulation of the dynamic test was performed utilizing the constants obtained from quasi-static experiments. The results showed a high precision in predicting the specimen’s profile in the dynamic testing. The sensitivity analysis was performed on the constants of GTN model to validate the proposed method. Finally, the experiments were simulated using the Johnson–Cook (J–C) damage model and the results were compared to those obtained from GTN damage model
Dynamic material characterization of polymeric foam by means of experimental, analytical, phenomenological and numerical methods
No full textThe main objective of this study was to advance the experimental, numerical, phenomenological and analytical methods of assessing the dynamic compressive response of polymeric foams, especially at an intermediate strain rate range of 50 s-1 to 600 s-1. Different experimental apparatuses and techniques, including the universal compression/tension testing machine (strain rates up to 0.5 s-1), custom-build droptower (a strain rate range of 50 s-1 to 200 s-1) and pneumatic testing apparatus (a strain rate range of 200 s-1 to 600 s-1) were utilized in the macro-mechanical characterization. The microstructure of the investigated polymeric foams was studied by means of Scanning Electron Microscopy (SEM) and Energy Dispersive x-ray Spectroscope (EDS). Numerical, analytical and phenomenological models were modified and calibrated to characterize and predict polymeric foams’ anisotropic rate-sensitive mechanical response. In the first phase of the study, the compressive response of polyether sulfone (PES) foam was investigated under both quasi-static and elevated strain rates. Anisotropic behavior was assessed by testing three orthogonal loading directions, revealing distinct deformation mechanisms. Elevated strain rates, ranging from 50 s-1 to 200 s-1, showcased a substantial rate dependency. The compressive response was simulated using Finite Element Analysis (FEA), achieving an impressive average validation metric of 97%. Localized deformation in the foam rise direction was identified, with specialized equations developed to quantify this phenomenon. The observed variation in the deformation mechanism and rate sensitivity of PES foams when loading in different material directions granted the need for further investigation on the influence of the specimen shape and profile on the mechanical characterization of polymeric foams. In the second phase, rigid Polyvinyl Chloride (PVC) foams were investigated for the effects of the specimen size and density variation on deformation mechanisms and mechanical properties. Different through-thickness direction density variation patterns, varying from 2.6% to 26.3%, governed localized deformation and post-yield stress drop-off behavior. Plateau stress exhibited sensitivity to foam density, while specimen thickness influenced loading elastic modulus. Intermittent unloading-reloading cyclic testing revealed that the thickness effect on the apparent unloading elastic modulus was negligible despite the thickness’s significant effect on the loading modulus. The limitations of Split Hopkinson Pressure Bar (SHPB) apparatuses in dynamic testing limited the exploration of specimen size influence in the dynamic mechanical response of polymeric foams. Consequently, most research on the influence of specimen size on the mechanical response was limited to the quasi-static loading regime. The specimen size effect was reported to be negligible in the quasi-static regime. A droptower testing machine equipped with a 45.45 kg dropping entity and a novel energy dissipation system was utilized to test specimens with different profile sizes and shapes dynamically. The findings from this work, for the first time, revealed that, unlike the quasi-static regime, dynamic behavior was sensitive to specimen profile, prompting the use of impact velocity rather than engineering strain rate in reporting rate sensitivity. Equations were developed to quantify the localized deformation of polymeric foams based on the specimen’s instantaneous dimensions during dynamic compression testing, revealing the significant effect of the specimen size on the correlation between the engineering and localized strain rates. Thus, the specimen thickness and profile size should be consistent in the quasi-static and dynamic experiments to achieve an accurate mechanical characterization, which has rarely been achieved in previous studies. In the study’s third phase, a pneumatic apparatus was utilized to characterize PVC foams under various strain rates, up to 600 s-1, corresponding to 15 m/s impacting velocity, and loading directions to address prior technical limitations. PVC foams with six different nominal densities were subjected to strain rates ranging from 0.005 s-1 to 600 s-1. Specimens possessing a consistent profile size and shape were utilized in the quasi-static and dynamic tests. A modified Nagy model coupled with a nonlinear Avalle relationship accurately represented stress/strain responses. It was found that conducting tests at a single dynamic strain rate (e.g. 400 s-1) effectively captures the rate sensitivity of PVC foam within the 200 s-1 to 600 s-1 strain rate range. Additionally, the influence of relative density on the foam’s rate sensitivity was quantified. In the final phase of the study, the characteristics of PVC foam obtained from the previous phases of this study were utilized to design, test and validate the foam section of a novel energy dissipation system involving AA6061 extrusions and a PVC foam. This hybrid energy dissipation system demonstrates enhanced mechanical performance, surpassing traditional axial crushing modes regarding energy absorption effectiveness
High-capacity, idealized energy absorbing mechanisms achieved via lightweight AA6061-T4/PVC foam composite structures
No full textAbstract: There is accelerating demand for energy absorbing structures fabricated from lightweight materials with idealized, near-constant force responses to simultaneously resolve the engineering challenges of vehicle mass reduction and improved occupant safety. A novel compounded energy dissipation system comprised of AA6061-T4 extrusions subjected to hybrid cutting/clamping and compressed H-series PVC foam, which benefited from the steady-state force response of each individual technology, was investigated utilizing quasi-static experiments and finite element modelling. Identical structures were also subjected to axial crushing to compare the newly proposed system to the current state-of-the-art in practical implementation. The cutting/foam crushing system exhibited highly stable collapse mechanisms, aided by the presence of the foam cores, and exceeded the energy absorbing capacity of the traditional crushing mode by approximately 13.4 % while simultaneously eliminating the excessive peak forces and structural instabilities. The simulated deformation profiles were consistent with the experiments and the force responses were predicted with an average error of 12.4 %.Communication présentée lors du congrès international tenu conjointement par Canadian Society for Mechanical Engineering (CSME) et Computational Fluid Dynamics Society of Canada (CFD Canada), à l’Université de Sherbrooke (Québec), du 28 au 31 mai 2023
A Novel Testing Apparatus For Investigating Dynamic Compression Response Of Polymeric Foam At Intermediate Strain Rates
No full textConference abstract. Part of the Proceedings of the Canadian Society for Mechanical Engineering International Congress 2022
Influence of Extruded Tubing and Foam-Filler Material Pairing on the Energy Absorption of Composite AA6061/PVC Structures
No full textThere is accelerating demand for energy-absorbing structures fabricated from lightweight materials with idealized, near-constant force responses to simultaneously resolve the engineering challenges of vehicle mass reduction and improved occupant safety. A novel compounded energy dissipation system composed of AA6061-T6 and AA6061-T4 tubing subjected to hybrid cutting/clamping and H130, H200 and H250 PVC foam compression was investigated utilizing quasi-static experiments, finite element simulations and theoretical modeling. Identical structures were also subjected to axial crushing to compare with the current state of the art. The novel cutting/foam crushing system exhibited highly stable collapse mechanisms that were uniquely insensitive to the tube/foam material configuration, despite the disparate material properties, and exceeded the energy-absorbing capacity and compressive force efficiency of the axial crushing mode by 14% and 44%, respectively. The simulated deformation profiles and force responses were consistent with the experiments and were predicted with an average error of 12.4%. The validated analytical models identified numerous geometric/material configurations with superior performance for the compounded AA6061/PVC foam cutting/foam crushing system compared to axial crushing. An Ashby plot comparing the newly obtained results to several findings from the open literature highlighted the potential for the compounded cutting/foam crushing system to significantly outperform several alternative lightweight safety systems
