Periodic Reporting for period 4 - PURPOSE (Opening a new route in solid mechanics: Printed protective structures)
Periodo di rendicontazione: 2022-09-01 al 2024-08-31
This project investigates dynamic fragmentation in metals, traditionally analyzed within a statistical framework that attributes limited energy absorption in protective structures to material and geometric flaws. Here, we propose an alternative approach, integrating a deterministic component into fragmentation mechanisms. To validate this new theory, we have devised a comprehensive experimental, analytical, and numerical methodology to address several canonical fragmentation problems, each defined by distinct geometric and loading conditions that make them readily identifiable from a mechanical standpoint. The research examines 3D-printed specimens of four engineering metals, designed with controlled porous microstructures and commonly utilized in aerospace and civilian security applications. The objective is to determine whether, at sufficiently high strain rates, fragmentation mechanisms transition from being defect-controlled to inertia-controlled. Validating this new statistical-deterministic framework has revealed that defects play a reduced role in high-rate fragmentation, which would lower the entry barriers for 3D-printed materials in energy-absorbing applications. This shift has the potential to reduce production, transportation, repair, and energy costs of protective structures without compromising their energy absorption capabilities. Preliminary outcomes of the project have demonstrated through a tripartite approach —experiments, computational models, and analytical formulations— that multiscale inertia in porous materials serves as a robust regularization mechanism which delays plastic localization and fracture. At high strain rates, inertia diminishes the influence of defects in the dynamic fracture of ductile porous materials.
Mechanical characterization within wide ranges of strain rate and temperature: We have conducted comprehensive micro- and macro-mechanical characterization of 3D-printed metals. The experiments included tension, compression, and shear tests across a range of strain rates and temperatures. We utilized X-ray tomography and electron backscatter diffraction analysis to investigate the initial distribution of porosity and the crystallographic structure of the 3D-printed specimens. The surface roughness of the samples was characterized using a 3D optical profilometer.
Dynamic fragmentation experiments:
• Dynamic expansion of rings.
• Dynamic expansion of thin-walled cylinders.
• Dynamic axial penetration of thin-walled cylinders.
• Dynamic collapse of thick-walled cylinders.
• Dynamic torsion of thin-walled cylinders.
• Dynamic compression of shear-compression samples.
• Plate-impact tests.
Selected samples have been subjected to X-ray tomography and scanning electron microscopy analysis both before and after testing to establish correlations between the porous microstructure and the fracture pattern, as well as the effect of the crystallographic structure on the fracture characteristics.
Constitutive modelling: We have modeled the mechanical behavior of four additively manufactured materials. We employed macroscopic elastic-plastic models, with yielding characterized by criteria that account for anisotropy, tension-compression asymmetry, and porosity (including microinertia effects), as well as crystal plasticity models for both BCC and FCC crystallographic structures. The parameters for these models were identified through mechanical characterization experiments. The constitutive models have been implemented in ABAQUS subroutines, which have been made available in public repositories.
Analytical approach: We have developed specific 1D and 2D linear stability analyses, along with 2D nonlinear two-zone models, to predict the formation of necking and shear band instabilities in additively manufactured materials. The theoretical solutions have been validated through finite element simulations and experimental studies. The codes with the analytical models have been made available in public repositories.
Computational approach: We have developed a novel methodology to incorporate the actual distribution of porosity, crystallographic structure, and surface roughness of 3D-printed materials into finite element models. We have also developed an innovative strategy for topology optimization of the porous and crystallographic microstructure, aimed at designing synthetic microstructures that enhance the energy absorption capacity of additively manufactured protective structures.
As of now, we have published 23 scientific articles presenting the results of the project in the most prestigious journals in Solid Mechanics. Two additional articles have just been accepted for publication, and another is currently under review. We anticipate publishing at least 10 to 15 more scientific articles based on the project's findings.
Dynamic fragmentation experiments:
• Dynamic expansion of rings.
• Dynamic expansion of thin-walled cylinders.
• Dynamic axial penetration of thin-walled cylinders.
• Dynamic collapse of thick-walled cylinders.
• Dynamic torsion of thin-walled cylinders.
• Dynamic compression of shear-compression samples.
• Plate-impact tests.
Selected samples have been subjected to X-ray tomography and scanning electron microscopy analysis both before and after testing to establish correlations between the porous microstructure and the fracture pattern, as well as the effect of the crystallographic structure on the fracture characteristics.
Constitutive modelling: We have modeled the mechanical behavior of four additively manufactured materials. We employed macroscopic elastic-plastic models, with yielding characterized by criteria that account for anisotropy, tension-compression asymmetry, and porosity (including microinertia effects), as well as crystal plasticity models for both BCC and FCC crystallographic structures. The parameters for these models were identified through mechanical characterization experiments. The constitutive models have been implemented in ABAQUS subroutines, which have been made available in public repositories.
Analytical approach: We have developed specific 1D and 2D linear stability analyses, along with 2D nonlinear two-zone models, to predict the formation of necking and shear band instabilities in additively manufactured materials. The theoretical solutions have been validated through finite element simulations and experimental studies. The codes with the analytical models have been made available in public repositories.
Computational approach: We have developed a novel methodology to incorporate the actual distribution of porosity, crystallographic structure, and surface roughness of 3D-printed materials into finite element models. We have also developed an innovative strategy for topology optimization of the porous and crystallographic microstructure, aimed at designing synthetic microstructures that enhance the energy absorption capacity of additively manufactured protective structures.
As of now, we have published 23 scientific articles presenting the results of the project in the most prestigious journals in Solid Mechanics. Two additional articles have just been accepted for publication, and another is currently under review. We anticipate publishing at least 10 to 15 more scientific articles based on the project's findings.
Mechanical characterization within wide ranges of strain rate and temperature: this is likely the most exhaustive micro and macro-mechanical characterization of printed metals ever performed.
Dynamic fragmentation experiments: We have designed two new experimental configurations: dynamic expansion of rings and dynamic axial penetration of thin-walled cylinders. We conducted seven different dynamic fracture and fragmentation experiments, along with in-situ real-time tests using X-ray imaging. This work has yielded the first images of the evolution of the porous microstructure within the material under impact loading. We established the first correlation between the topology and morphology of the porous and crystallographic microstructures before and after testing, tracking changes in the shape and size of individual voids and grains.
Constitutive modelling: We have utilized macroscopic elastic-plastic models and crystal plasticity models to characterize the behavior of printed metals across a wide range of strain rates and temperatures. The constitutive models have been validated through in-house characterization tests and implemented into ABAQUS using user subroutines, resulting in the most comprehensive database to date for modeling additively manufactured metals.
Analytical approach: We have developed the first theoretical models based on linear stability analysis and nonlinear localization analysis to predict necking and shear banding in additively manufactured materials subjected to dynamic loading. These analytical models enabled identification of loading rates, specimen dimensions, and material parameters where inertia, rather than defects, governs localization and fracture in additively manufactured samples at high strain rates.
Computational approach: We have developed the first computational framework which incorporates the actual distribution of porosity, crystallographic structure, and surface roughness of 3D-printed materials into finite element models, providing a multi-scale approach to the effect that material and geometric defects have on plastic localization and fragmentation of printed metals. Through microstructurally-informed finite element simulations of fragmentation problems, validated by our own experimental results, we have drawn definitive conclusions regarding the effects of microstructural and surface defects on plastic flow localization and fracture dynamics of additively manufactured materials under various dynamic loading conditions.
Dynamic fragmentation experiments: We have designed two new experimental configurations: dynamic expansion of rings and dynamic axial penetration of thin-walled cylinders. We conducted seven different dynamic fracture and fragmentation experiments, along with in-situ real-time tests using X-ray imaging. This work has yielded the first images of the evolution of the porous microstructure within the material under impact loading. We established the first correlation between the topology and morphology of the porous and crystallographic microstructures before and after testing, tracking changes in the shape and size of individual voids and grains.
Constitutive modelling: We have utilized macroscopic elastic-plastic models and crystal plasticity models to characterize the behavior of printed metals across a wide range of strain rates and temperatures. The constitutive models have been validated through in-house characterization tests and implemented into ABAQUS using user subroutines, resulting in the most comprehensive database to date for modeling additively manufactured metals.
Analytical approach: We have developed the first theoretical models based on linear stability analysis and nonlinear localization analysis to predict necking and shear banding in additively manufactured materials subjected to dynamic loading. These analytical models enabled identification of loading rates, specimen dimensions, and material parameters where inertia, rather than defects, governs localization and fracture in additively manufactured samples at high strain rates.
Computational approach: We have developed the first computational framework which incorporates the actual distribution of porosity, crystallographic structure, and surface roughness of 3D-printed materials into finite element models, providing a multi-scale approach to the effect that material and geometric defects have on plastic localization and fragmentation of printed metals. Through microstructurally-informed finite element simulations of fragmentation problems, validated by our own experimental results, we have drawn definitive conclusions regarding the effects of microstructural and surface defects on plastic flow localization and fracture dynamics of additively manufactured materials under various dynamic loading conditions.