Abstract
This paper presents results from numerical and experimental investigation on Charpy tests in order to point out failure mechanisms and to evaluate new polymeric blends PP + PA6 + EPDM. Charpy tests were done for initial velocity of the impactor of 0.96 m/s and its mass of 3.219 kg and these data were also introduced in the finite element model. The proposed model takes into account the system of four balls, including support and the ring of fixing the three balls and it has a finer discretization of the impact area to highlight the mechanisms of failure and their development in time. The constitutive models for four materials (polypropylene with 1% Kritilen, two blends PP + PA6 + EPDM and a blend PA6 + EPDM) were derived from tensile tests. Running simulations for each constitutive model of material makes possible to differentiate the destruction mechanisms according to the material introduced in the simulation, including the initiation and the development of the crack(s), based on equivalent plastic strain at break (EPS) for each material. The validation of the model and the simulation results were done qualitatively, analyzing the shape of broken surfaces and comparing them to SEM images and quantitatively by comparing the impact duration, energy absorbed by the sample, the value of maximum force during impact. The duration of the destruction of the specimen is longer than the actual one, explainable by the fact that the material model does not take into account the influence of the material deformation speed in Charpy test, the model being designed with the help of tests done at 0.016 m/s (1000 mm/min) (maximum strain rate for the tensile tests). Experimental results are encouraging for recommending the blends 20% PP + 42% PA6 + 28% EPDM and 60% PA6 + 40% EPDM as materials for impact protection at low velocity (1 m/s). Simulation results are closer to the experimental ones for the more brittle tested materials (with less content of PA6 and EPDM) and more distanced for the more ductile materials (with higher content of PA6 and EPDM).
Highlights
Simulation results are closer to the experimental ones for the more brittle tested materials and more distanced for the more ductile materials
The results reported in the literature show that polymers have a stress–strain curve dependent on the strain rate [24,47]
The material model was based on experimental data obtained from traction, for a speed of v = 1000 mm/min [27] because it was estimated that, for these materials, the influence on the shape of the stress–strain curve of the test rate, except for very small test strain rate (v = 10 mm/min), is low and the characteristic values obtained from the tensile tests are in a narrow range
Summary
Charpy tests have been characterized materials for impact resistance till the XIX century [1,2,3,4,5], but relevance had been initiated by Charpy [6] the test keeping his name till nowadays. A model of Charpy test characterizing the behavior of an armor eel under high strains and strain rates, at elevated temperatures. The constants of constitutive and failure model were deduced from tensile tests This simulation used explicit time integration and was run for a total time of 2 × 10−3 s. Recent tests provided complex data on the behavior of polymeric materials, with increasingly complex constitutive models, which take into account several factors: Temperature, deformation rate, structure, and stress. Experimental and obtained by simulation, have emphasized the importance of modeling the material at high deformation rate, [22,37,38,39,40]. Simulation of Charpy tests are few, but the interest in using them as shock absorbers and protective materials makes this subject to be the treated in this study
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