Deformation of Poly-l-lactid acid (PLLA) under Uniaxial Tension and Plane-Strain Compression

The ability of PLLA, either amorphous or semicrystalline, to plastic deformation to large strain was investigated in a wide temperature range (Td = 70–140 °C). Active deformation mechanisms have been identified and compared for two different deformation modes—uniaxial drawing and plane-strain compression. The initially amorphous PLLA was capable of significant deformation in both tension and plane-strain compression. In contrast, the samples of crystallized PLLA were found brittle in tensile, whereas they proved to be ductile and capable of high-strain deformation when deformed in plane-strain compression. The main deformation mechanism identified in amorphous PLLA was the orientation of chains due to plastic flow, followed by strain-induced crystallization occurring at the true strain above e = 0.5. The oriented chains in amorphous phase were then transformed into oriented mesophase and/or oriented crystals. An upper temperature limit for mesophase formation was found below Td = 90 °C. The amount of mesophase formed in this process did not exceed 5 wt.%. An additional mesophase fraction was generated at high strains from crystals damaged by severe deformation. After the formation of the crystalline phase, further deformation followed the mechanisms characteristic for the semicrystalline polymer. Interlamellar slip supported by crystallographic chain slip has been identified as the major deformation mechanism in semicrystalline PLLA. It was found that the contribution of crystallographic slip increased notably with the increase in the deformation temperature. The most probable active crystallographic slip systems were (010)[001], (100)[001] or (110)[001] slip systems operating along the chain direction. At high temperatures (Td = 115–140 °C), the α→β crystal transformation was additionally observed, leading to the formation of a small fraction of β crystals.

Crystal Visco-Plastic Model for Directionally Solidified Ni-Base Superalloys Under Monotonic and Low Cycle Fatigue

Nickel-base superalloys (NBSAs) are extensively utilized as the design materials to develop turbine blades in gas turbines due to their excellent high-temperature properties. Gas turbine blades are exposed to extreme loading histories that combine high mechanical and thermal stresses. Both directionally solidified (DS) and single crystal NBSAs are used throughout the industry because of their superior tensile and creep strength, excellent low cycle fatigue (LCF), high cycle fatigue (HCF), and thermomechanical fatigue (TMF) capabilities. Directional solidification techniques facilitated the solidification structure of the materials to be composed of columnar grains in parallel to the <001> direction. Due to grains being the sites of failure initiation the elimination of grain boundaries compared to polycrystals and the alignment of grain boundaries in the normal to stress axis increases the strength of the material at high temperatures. To develop components with superior service capabilities while reducing the development cost, simulating the material’s performance at various loading conditions is extremely advantageous. To support the mechanical design process, a framework consisting of theoretical mechanics, numerical simulations, and experimental analysis is required. The absence of grain boundaries transverse to the loading direction and crystallographic special orientation cause the material to exhibit anisotropic behavior. A framework that can simulate the physical attributes of the material microstructure is crucial in developing an accurate constitutive model. The plastic flow acting on the crystallographic slip planes essentially controls the plastic deformation of the material. Crystal Visco-Plasticity (CVP) theory integrates this phenomenon to describe the effects of plasticity more accurately. CVP constitutive models can capture the orientation, temperature, and rate dependence of these materials under a variety of conditions. The CVP model is initially developed for SX material and then extended to DS material to account for the columnar grain structure. The formulation consists of a flow rule combined with an internal state variable to describe the shearing rate for each slip system. The model presented includes the inelastic mechanisms of kinematic and isotropic hardening, orientation, and temperature dependence. The crystallographic slip is accounted for by including the required octahedral, cubic, and cross slip systems. The CVP model is implemented through a general-purpose finite element analysis software (i.e., ANSYS) as a User-Defined Material (USERMAT). Uniaxial experiments were conducted in key orientations to evaluate the degree of elastic and inelastic anisotropy. The temperature-dependent modeling parameter is developed to perform non-isothermal simulations. A numerical optimization scheme is utilized to develop the modeling constant to improve the calibration of the model. The CVP model can simulate material behavior for DS and SX NBSAs for monotonic and cyclic loading for a range of material orientations and temperatures.

Heat Treatment of the Surface of the ChS57 Alloy with Powerful Nanosecond Ultraviolet Laser Pulses

The threshold of optical breakdown of the nickel alloy ChS57 (Inconel) was measured at a wavelength of 0.355 μm with a laser pulse duration of 10 ns. Heat treatment of ChS57 above pulse energy density threshold (1 - 2.5 J/cm2) occurred mainly in the ablative mode with almost no melting. The elemental composition of the surface layer did not change at an irradiation in a fixed spot. When a laser beam moves along the surface of the sample at a speed of 1 mm / s and at pulse energy density of about 0.02 J/cm2, oxygen was detected in the elemental composition (3 – 4 wt. %). However, the proportions of the elemental composition of the alloy remained virtually unchanged. Heat treatment under threshold at pulse energy density ≥ 0.25 J/cm2 revealed a rise of the surface layer with traces of high-temperature plastic deformation in the form of slippage on grain boundaries and crystallographic slip.

Magnetic, electrical and mechanical properties of Fe40Mn40Co10Cr10 high entropy alloy

A prototypical, single-phase, and non-equiatomic high entropy alloy Fe40Mn40Co10Cr10 has been mechanically deformed at room and cryogenic temperatures. Plastic deformation was accommodated via crystallographic slip at room temperature while transformation induced plasticity (TRIP) has been observed in samples deformed at 77 K. The stress-induced martensitic transformation occurred from face-centered cubic (FCC) to hexagonal close-packed (HCP) structures. A detailed electron backscatter diffraction analysis was utilized to detect phase change and evaluate the evolution of the HCP phase volume fraction as a function of plastic strain. Physical properties of undeformed and deformed samples were measured to elucidate the effect of deformation-induced phase transitions on the magnetic and electrical properties of Fe40Mn40Co10Cr10 alloy. Relatively small magnetic moments along with non-saturating magnetic field dependencies suggest that the ground state in the considered material is ferrimagnetic ordering with coexisting antiferromagnetic phase. The temperature evolution of the coercive fields has been revealed for all samples. The magnitudes of the coercive fields place the considered system into the semi-hard magnetic alloys category. The temperature dependence of the zero-field cooled (ZFC) and field cooled (FC) magnetization was measured for all samples in the low field regime and the origin of irreversibility in ZFC/FC curves was discussed. Besides, the temperature dependence of the resistivity in all samples was measured and the possible conduction mechanisms were discussed.

Continuum damage model for low-cycle fatigue of metals: An overview

This work aims to give a comprehensive and accurate view of those micromechanical models that have been developed over the past two decades by our research team. We hereby refer to the distinct capabilities and theoretical difficulties of such models published in various journals by discussing them in a more integrated manner. We also believe that our community can find a significant benefit from this work through below mentioned discussions, which answer many of the questions asked by researchers and those interested in this field. Returning to this work topic, the low-cycle fatigue (LCF) life is highly affected by microstructural instabilities, inhomogeneity and shear bands formation. The first type of damage is related to the nuclei of fatigue microcracks, for several FCC and BCC metals generally governed by continuous irreversible slips within the intensive slip bands. The second type of damage concerns ductile damage caused by cavitation induced by plastic strain and hydrostatic stress. Thereby, fatigue-failure is a relevant topic that requires further explanations to better understand the plausible damaging mechanisms of microcracks and/or microvoids at lower levels of observation. To model the microcracks initiation, some attempts were conducted via micromechanical models assuming, for example, that the microcracks initiate at the crystallographic slip system level describing the LCF response of metals under simple and complex loading paths. In this overview, the latest development of mixed approach with double character called Micromechanical Determinist-Probabilistic Model (MDPM) coupled with damage will be presented with its generalized structure.

Crystal Visco-Plastic Model for Ni-Base Superalloys Under Thermomechanical Fatigue

Gas turbine blades are subjected to complex mechanical loading coupled with extreme thermal loading conditions which range from room temperature to over 1000°C. Nickel-base superalloys exhibit high strength, good resistance to corrosion and oxidation, long fatigue life and is capable of withstanding high temperatures for extended periods of time. Consequently, Ni-base superalloys (NBSAs) are highly suitable as blading materials. The cyclic strains due to mechanical as well as thermal cycling leads to Thermomechanical fatigue (TMF). Damage from TMF takes the form of microstructural material cracking which consequently lead to the failure of the component. In order to increase the service life and reliability and reduce operating costs, development of simulations that accurately predict the material behavior for TMF is highly desirable. To support the mechanical design process, a framework consisting of theoretical mechanics, experimental analysis and numerical simulations must be used. Capturing the effects of thermomechanical fatigue is extremely important in the prediction of the material behavior and life expectation. Single crystal (SX) Ni-base superalloys exhibit anisotropic behavior. A modeling framework which is capable of simulating the physical attributes of the material microstructure is essential. Crystallographic slip along the slip planes controls the microstructural evolution of the material Crystal Visco-Plasticity (CVP) theory captures anisotropic behavior as well as the slip along the slip planes. CVP constitutive models can capture rate-, temperature, and history-dependence of these materials under a variety of conditions. Typical CVP formulations consist of a flow rule, internal state variables, and parameters. The model presented in the current study includes the inelastic mechanism of kinematic hardening and isotropic hardening which are captured by the back stress and drag stress, respectively. Crystallographic slip is accounted for by the incorporation of twelve octahedral six cubic slip systems. An implicit integration scheme which uses Newton-Raphson iteration method is used to solve the required solutions. The CVP model is implemented through a general-purpose finite element analysis software (i.e., ANSYS) as a User-Defined Material (USERMAT). A small batch of uniaxial experiments were conducted in key orientations (i.e., [001], [011], and [111] to assess the level of elastic and inelastic anisotropy. Modeling parameters are expressed as temperature-dependent to allow for simulation under non-isothermal conditions. An optimization scheme based in MATLAB utilizes this experimental data to calibrate the CVP modeling constants. The CVP model has the capability to simulate material behavior for monotonic and cyclic loading coupled with in phase and out phase temperature cycling for a variety of material orientations, strain rates, strain and temperature ranges. A CVP model that predicts SX behavior across various rates, orientations, temperatures and load levels have not been presented before now.