Numerical modelling of composite structures impact damage at high strain rates

Ispitivanje i numeričko modeliranje udarnih opterećenja i oštećenja kompozitnih laminata perspektivno je područje znanstvenog istraživanja u kojem postoji značajan prostor za napredak. Uz druge probleme i poteškoće, zbog anizotropnosti, višeslojnosti konstrukcija te problematičnog spoja između pojedinih slojeva (eng. Interface), nesavršenosti i zaostalih naprezanja u procesu proizvodnje, njihovu nosivost ponekad je i pri statičkim uvjetima opterećenja teško procijeniti. Uz činjenicu da su im svojstva, u smislu krutosti i čvrstoća, ovisna o brzini deformacije, procjena njihovog integriteta pri udarnim opterećenjima još je znatno kompleksnija. U radu su, nakon iscrpnog pretraživanja literature i detaljnog opisivanja problematike, provedene numeričke analize jednostavnog problema balističkog udara kuglice u kvadratnu kompozitnu ploču koristeći komercijalni programski paket za numeričku analizu metodom konačnih elemenata Abaqus/Explicit. Za numeričko modeliranje korištena su tri pristupa modeliranju ploče: konvencionalnom ljuskom (bez definirane debljine), ljuskom s debljinom (3D ljuska) i klasičnim kontinuumom. Uz to, razmatran je i utjecaj određenih parametara na rezultate simulacije te različiti pristupi dobivanju udarne kontaktne sile. Odabrani razmatrani problem već je obrađen u literaturi eksperimentalno i numeričkim simulacijama, što omogućuje usporedbu rezultata ovdje provedenih simulacija s vjerodostojnim, odnosno provjerenim podacima. Pritom su u simulacijama korištena tri kriterija inicijacije oštećenja: Hashinov (kao jedini kriterij na temelju kojeg je u korištenom programskom paketu dostupan model oštećivanja), Danielov (NU teorija) i Raimondov. Posljednja dva kriterija u obzir uzimaju utjecaj brzine deformiranja na svojstva kompozitnog materijala, a njihovo korištenje moguće je jedino putem korisnički definiranih potprograma VUMAT (Fortran rutina). U radu su detaljno opisani svi navedeni korišteni kriteriji oštećenja, mogućnosti i karakteristike korištenog programskog paketa te korisnički definirani potprogrami VUMAT. Opisan je i čitav postupak kreiranja numeričkog modela. Nakon detaljnog prikaza i usporedbe rezultata, izvedeni su određeni zaključci o provedenom radu.Experimental investigation and numerical modelling of impact loads and damage of the composite materials is a perspective field of science with a great potential for improvements. Along other problems and difficulties, because of their anisotropy, laminated structure and problematic layer interfaces, imperfections and residual stresses from production processes, composites' load bearing capabilities are sometimes, even in the cases of static loading, hard to determine. Bearing in mind that their mechanical properties are strain rate dependent, analyzing their structural integrity in the cases of impact loading is an even greater challenge. In this Thesis, after a detailed literature survey and a brief problem description, numerical simulations of a simple ballistic impact problem of a ball hitting the quadratic composite panel were conducted using a commercially available software for Finite Element Method analysis Abaqus/Explicit. For the numerical modelling process, three different approaches for modelling of the composite panel were used: conventional shell (without defined thickness), shell with a defined thickness (3D shell), and a classic continuum approach. Along that, effects of some of the simulation parameters on the simulation results were analyzed, and different methods of determining the impact force were evaluated. The selected analyzed problem has previously been investigated experimentally and numerically in the available literature, which allows for the here obtained results to be validated against thrustworthy, proved data. In the simulations conducted, three different failure criteria were employed: Hashin (as the criterion that the only available damage model in the used software is based on), Daniel (the NU theory) and Raimondo. The last two criteria take the strain rate effects in consideration when calculating composite properties, and their usage is only possible by creating the user defined subroutines VUMAT (Fortran routine). In the Thesis, all named failure criteria have been briefly described, the possibilities and characteristics of the used software were presented, and the VUMAT subroutines were explained. Also, the process of numerical modelling has been explained. After displaying and comparison of the obtained results, the conclusions on the performed analyses have been made