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The Development of Constitutive Equations of Polycarbonate and Modeling the Impact Behavior
(01. Izmir Institute of Technology, 2023) Sarıkaya, Mustafa Kemal; Güden, Mustafa; Taşdemirci, Alper
The Johnson and Cook (JC) flow stress and damage parameters of a polycarbonate were determined by the mechanical tests and numerical simulations. The experimental tests included quasi-static and high strain rate tension and compression, quasi-static notched-specimen tension, quasi-static indentation (QSI), low velocity impact (LVI) and projectile impact (PI). The flow stress equation determined from the experimental average true stress-true strain curve well agreed with the effective stress-strain obtained from the quasi-static numerical tension test. The numerical QSI force-displacement curve based on the experimental average true stress-true strain equation was further shown to be very similar to that of the experiment. The LVI and PI test simulations were then continued with the experimental average true stress-true strain equation using five different flow stress-strain rate relations: JC, Huh and Kang, Allen-Rule and Jones, Cowper-Symonds and the nonlinear rate approach. No strain rate sensitivity in the LVI tests was ascribed to low strain rate dependency of the flow stress at intermediate strain rates and large strains. On the other side, all the stress-strain rate relations investigated nearly predicted the experimental damage types in the PI tests, except the Cowper-Symonds relation which predicted the fracture of the polycarbonate plate at 140 m s-1. The absorbed energy at 160 m s-1 test was determined 1.6 times that of the QSI test, proving an increased energy absorption of the tested polycarbonate at the investigated impact velocities. The verified parameters were finally used to model the damages formed on a canopy against bird strike.
Experimental and Numerical Investigation of the Quasi-Static and High Strain Rate Crushing Behavior of Single and Multi-Layer Zig-Zag 1050 H14 Al Trapezoidal Corrugated Core Sandwich Structures
(Izmir Institute of Technology, 2014) Kılıçaslan, Cenk; Güden, Mustafa; Taşdemirci, Alper; Güden, Mustafa; Taşdemirci, Alper
The quasi-static and dynamic crushing behavior of single, double and multi-layer zig-zag 1050 H14 Al trapezoidal corrugated core sandwich structures in 0°/0° and 0°/90° core orientations and with and without interlayer sheets were investigated both experimentally and numerically at varying impact velocities. The numerical simulations were conducted using the finite element code of LS-DYNA. The effect of fin wall imperfection was assessed through the fin wall bending and bulging. The numerical homogenization of the single layer corrugated structure was performed using MAT26 honeycomb material model. The buckling stress of single- and double-layer corrugated sandwich structures increased when the strain rate increased. The increased buckling stresses were ascribed to the micro inertial effects. The initial buckling stress at quasi-static and high strain rate was numerically shown to be imperfection sensitive. Increasing the number of core layers decreased the buckling stress and increased the densification strain. The panels tested with spherical and flat striker tips were not penetrated and experienced slightly higher deformation forces and energy absorptions in 0°/90° corrugated layer orientation than in 0°/0° orientation. However, the panels tested using a conical striker tip were penetrated/perforated and showed comparably smaller deformation forces and energy absorptions, especially in 0°/90° layer orientation. The homogenized models predicted the low velocity compression /indentation and projectile impact tests of the multi-layer corrugated sandwich with an acceptable accuracy with reduced computational time.

Phd Degree / Doktora

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