Qualitative Investigation of Damage Initiation at Meso-Scale in Spheroidized C45EC Steels by Using Crystal Plasticity-Based Numerical Simulations

This research uses EBSD data of two thermo-mechanically processed medium carbon (C45EC) steel samples to simulate micromechanical deformation and damage behavior. Two samples with 83% and 97% spheroidization degrees are subjected to virtual monotonic quasi-static tensile loading. The ferrite phase is assigned already reported elastic and plastic parameters, while the cementite particles are assigned elastic properties. A phenomenological constitutive material model with critical plastic strain-based ductile damage criterion is implemented in the DAMASK framework for the ferrite matrix. At the global level, the calibrated material model response matches well with experimental results, with up to ~97% accuracy. The simulation results provide essential insight into damage initiation and propagation based on the stress and strain localization due to cementite particle size, distribution, and ferrite grain orientations. In general, it is observed that the ferrite–cementite interface is prone to damage initiation at earlier stages triggered by the cementite particle clustering. Furthermore, it is observed that the crystallographic orientation strongly affects the stress and stress localization and consequently nucleating initial damage.

Α numerical evaluation of various damage models for thermoplastic composite materials subjected to quasi-static tensile loading

In the present work, damage propagation in thermoplastic composite laminates subjected to quasi-static tensile loading was numerically simulated using different damage models. The simulation was performed using the commercial finite element suite LS-Dyna, where various material models were evaluated based on their ability to simulate the thermoplastic composite behavior. More specifically, material models based on Tsai-Wu failure criteria (MAT_055), progressive damage modeling (MAT_162), micromechanical material modeling (MAT_215) and continuum damage modeling (MAT_261) were implemented, comparing the output failure strength and load – displacement curve with experimental data available in the literature. The approach of damage evolution and failure type for each model was taken under consideration as well. Overall, the progressive damage model (MAT_162) presented the most accurate prediction of the material’s failure behavior.

Contribution of lignin to the stress transfer in compression wood viewed by tensile FTIR loading

To achieve efficient utilization of compression wood (CW), a deeper insight into the molecular interactions is necessary. In particular, the role of lignin in the wood needs to be better understood, especially concerning how lignin contributes to its mechanical properties. For this reason, the properties of CW and normal wood (NW) from Chinese fir (Cunninghamia lanceolata) have been studied on a molecular scale by means of polarized Fourier transform infrared (FTIR) spectroscopy, under both static and dynamic loading conditions. Under static tensile loading, only molecular deformations of cellulose were observed in both CW and NW. No participation of lignin could be detected. In relation to the macroscopic strain, the molecular deformation of the cellulose C-O-C bond was greater in NW than in CW as a reflection of the higher microfibril angle and the lower load taken up by CW. Under dynamic deformation, a larger contribution of the lignin to stress transfer was detected in CW; the molecular deformation of the lignin being highly related to the amplitude of the applied stress. Correlation analysis indicated that there was a direct coupling between lignin and cellulose in CW, but there was no evidence of such a direct coupling in NW.

Electro-mechanical studies of multi-functional glass fiber/epoxy reinforced composites

An experimental study is performed to investigate the electro-mechanical response of three-dimensionally conductive multi-functional glass fiber/epoxy laminated composites under quasi-static tensile loading. To generate a three-dimensional conductive network within the composites, multi-wall carbon nanotubes are embedded within the epoxy matrix and carbon fibers are reinforced between the glass fiber laminates using an electro-flocking technique. A combination of shear mixing and ultrasonication is employed to disperse carbon nanotubes inside the epoxy matrix. A vacuum infusion process is used to fabricate the laminated composites of two different carbon fiber lengths (150 µm and 350 µm) and four different carbon fiber densities (500, 1000, 1500, 2000 fibers/mm2). A four circumferential probe technique is employed to measure the in-situ electrical resistance of composites under tensile load. Although composites of both carbon fiber lengths showed significant decrease of sheet resistance under no mechanical load conditions, composites of 350 µm long carbon fibers showed the lowest resistivity of 10 Ω/sq. Unlike the resistance values, composites of 350 µm carbon fibers showed a significant decrease in Young’s modulus compared to 150 µm counterparts. For the electro-mechanical response, composites containing carbon fibers of 150 µm long demonstrated a maximum value of percentage change in resistance. These results were then compared to both 350 µm and no added carbon fibers under quasi-static tensile loading.