<p>Aeronautical products additively manufactured by Selective Laser Melting (SLM), are known to have fatigue properties which are negatively impacted by porosity defects, microstructural features and residual stresses. Little research is available studying these phenomena with respect to the short fatigue crack growth (FCG) inconsistency problem, the large focus being on the long FCG. This thesis seeks to add useful knowledge to the understanding of the mechanisms for short crack growth variability in SLM manufactured Ti-6Al-4V, with the two variables for the process conditions and build directions investigated. An in-situ FCG investigation using x-ray synchrotron computed micro-tomography (μXSCT) was used to visually observe and quantify the short crack path evolution. Crack growth, deflections and porosity interactions were noted and discussed in relation to microstructure, build layer thickness and build layer orientation. A novel use of in-situ energy dispersive x-ray diffraction (EDD) was able to show the lattice strains evolving as a propagating crack moved through a small region of interest. The results presented show the ability to reliably obtain all six elastic strain tensor components, and interpret useful knowledge from a small region of interest. </p> <p> </p> <p>There are conflicting views in literature with respect to the damage tolerance behavior of as built SLM manufactured Ti-6Al-4V. In the 2018 review by Agius et al., the more prominent studies were considered with Leuders et al. showing the highest long FCG rates for cracks parallel to the build layer and Cain et al. showing cracks propagating through successive build layers as highest [1]–[3]. Cain et al. and Vilaro et al. report significant anisotropy in long FCG for different build orientations whereas Edwards and Ramulu present similar FCG behavior for three different build directions [2]–[5]. Kruth et al. concluded that for optimized build parameters without any (detectable) pores, the building direction does not play a significant role in the fracture toughness results [6]. All of the mentioned literature reported martensitic microstructures and the presence of prior grain structures for as built SLM Ti-6Al-4V.</p> <p> </p> <p>No studies to the authors knowledge have considered the short FCG of SLM manufactured Ti‑6Al‑4V and its implications to the conflicting damage tolerance behaviors reported in literature [1]. In this work small cross-sectional area (1.5 x 1.5 ) samples in two different build conditions of as built SLM Ti-6Al‑4V are studied. The short FCG rate of three different build directions was considered with cracks parallel to the build layers shown to be the most damaging. The microstructure and build layer are shown to be the likely dominant factors in the short FCG rate of as built Ti-6Al-4V. In terms of porosity, little impact to the propagating short crack was seen although there is local elastoplastic behavior around these defects which could cause toughening in the non-optimized build parameter samples tested. The fracture surfaces were examined using a Scanning Electron Microscope (SEM) with the results showing significant differences in the behavior of the two build conditions. From the microindentation hardness testing undertaken, the smooth fracture surface of the optimized sample correlated with a higher Vickers Hardness (VH) result and therefore higher strength. The non-optimized samples had a ‘rough’ fracture surface, a lower VH result and therefore strength. Furthering the knowledge of short FCG in SLM manufactured Ti-6Al-4V will have positive implications to accurately life and therefore certify additive manufactured aeronautical products.</p>

Micro-mechanical Raman spectroscopy is an excellent tool for direct stress measurements in the structure. The presence of mechanical stress changes the Raman frequency of each Raman modes compared to the Raman frequencies in absence of stress. This difference in Raman frequency is linearly related to stress induced and can be calibrated to stress by uniaxial or biaxial tension/compression experiments. This relationship is not generally linear for non-linear behavior of the materials which limits its use to experimentally study flow stress and plastic deformation behavior of the material. In this work strontium titanate ceramic particles dispersed inside epoxy resin matrix were used to measure stress in epoxy resin matrix with non-linear material behavior around it. The stress concentration factor between stress induced inside ceramic particles and epoxy resin matrix was obtained by non-linear constitutive finite element model. The results of finite element model were used for training a machine learning model to predict stress in epoxy resin matrix based on stress inside ceramic particles. By measuring stress inside ceramic particles using micro-mechanical Raman spectroscopy, the stress inside epoxy matrix was obtained by pre-determined stress concentration factor.

This thesis investigates failure in sandwich panels due to bending, specifically localised buckling or wrinkling, a predominant failure mechanism for thin gauge honeycomb sandwich panels loaded in bending or compression. Over the past 60 years, considerable work has been devoted to understanding wrinkling and trying to predict failure loads in damaged and undamaged panels accurately. Existing wrinkling expressions were shown to over-estimate failure loads by over 100%. Discrepancies between wrinkling expressions and experimental failure loads were previously attributed to imperfections and irregularities in the structure. The aim of this thesis is to investigate this problem and try to accurately predict failure loads and understand the underlying failure mechanisms in damaged and undamaged panels, using a combination of numerical and analytical techniques. Classical wrinkling models use a continuum core to model complex cellular honeycomb cores. This type of model reduces complex cellular geometry to a series of effective properties that provide constant support to the face sheet. In reality, honeycomb cores provide support around the periphery of the cell walls and not across the entire surface of the face sheet. Due to the nature of wrinkling and the size of the wavelength, incorrect representation of the core could affect the failure loads and model. This study made direct comparisons between linear buckling loads of a discrete-cored sandwich panel and a continuum-cored sandwich panel. Discrete properties were converted to continuum properties within a Finite Element package. The result conclusively showed that both models predict the same linear failure loads, disproving the theory that the core representations contribute to the difference between experimental and analytical models. It was also shown that existing wrinkling models can accurately predict linear wrinkling loads. These linear model loads do not necessarily match the collapse strength of the physical panel and in most cases predict a significantly higher value. The research then moves on to developing expressions to convert cellular geometry into continuum properties accurately. Expressions are developed for honeycomb structures with fillets in their junctions. Both out-of-plane and in-plane modulus properties are reviewed and the models are verified against Finite Elements models and experimental results. Studies showed that the restrained in-plane modulus can be up to ten times stiffer than the commonly used free modulus value. This has a significant effect on the wrinkling stress. By using the correct value, the discrete model and continuum models predict the same loads. The classical wrinkling expressions also predict the same wrinkling stress as the Finite Element models. After establishing that the core representation is not the cause of the prediction error, the thesis turns to non-linear Finite Element models to predict failure loads and failure mechanism of thin-gauge sandwich honeycomb structures loaded in bending. A continuum three-dimensional non-linear Finite Element model, with bilinear plasticity, is compared with a set of experiments that use different types of Nomex cores and face sheets. The models show that the panels fail prematurely due to core crushing because of wrinkles forming in the face sheets. Experimental results indicate similar trends. The final section examines the affect of impact damage in honeycomb sandwich structures. Due to the thin face sheets and thick cores used on many aircraft and marine components, sandwich panels offer little resistance to impact events. Resulting damage usually consists of a layer of crushed core and a shallow dent in the face sheet. This type of damage often leads to a significant reduction in the load-carrying capacity of the panel through a full range of damage sizes. Finite element and analytical models were developed to accurately predict and capture the localised wrinkling failure mechanism which occurs in the impacted area. Models were directly compared to experimental results, with a high degree of correlation. The numerical and analytical models showed that impact damaged panels were failing due to wrinkling instability and not due to premature core crushing, which is the case with undamaged panels. They showed that two factors influence the wrinkling failure load: damage depth and damage diameter.

This thesis investigates failure in sandwich panels due to bending, specifically localised buckling or wrinkling, a predominant failure mechanism for thin gauge honeycomb sandwich panels loaded in bending or compression. Over the past 60 years, considerable work has been devoted to understanding wrinkling and trying to predict failure loads in damaged and undamaged panels accurately. Existing wrinkling expressions were shown to over-estimate failure loads by over 100%. Discrepancies between wrinkling expressions and experimental failure loads were previously attributed to imperfections and irregularities in the structure. The aim of this thesis is to investigate this problem and try to accurately predict failure loads and understand the underlying failure mechanisms in damaged and undamaged panels, using a combination of numerical and analytical techniques. Classical wrinkling models use a continuum core to model complex cellular honeycomb cores. This type of model reduces complex cellular geometry to a series of effective properties that provide constant support to the face sheet. In reality, honeycomb cores provide support around the periphery of the cell walls and not across the entire surface of the face sheet. Due to the nature of wrinkling and the size of the wavelength, incorrect representation of the core could affect the failure loads and model. This study made direct comparisons between linear buckling loads of a discrete-cored sandwich panel and a continuum-cored sandwich panel. Discrete properties were converted to continuum properties within a Finite Element package. The result conclusively showed that both models predict the same linear failure loads, disproving the theory that the core representations contribute to the difference between experimental and analytical models. It was also shown that existing wrinkling models can accurately predict linear wrinkling loads. These linear model loads do not necessarily match the collapse strength of the physical panel and in most cases predict a significantly higher value. The research then moves on to developing expressions to convert cellular geometry into continuum properties accurately. Expressions are developed for honeycomb structures with fillets in their junctions. Both out-of-plane and in-plane modulus properties are reviewed and the models are verified against Finite Elements models and experimental results. Studies showed that the restrained in-plane modulus can be up to ten times stiffer than the commonly used free modulus value. This has a significant effect on the wrinkling stress. By using the correct value, the discrete model and continuum models predict the same loads. The classical wrinkling expressions also predict the same wrinkling stress as the Finite Element models. After establishing that the core representation is not the cause of the prediction error, the thesis turns to non-linear Finite Element models to predict failure loads and failure mechanism of thin-gauge sandwich honeycomb structures loaded in bending. A continuum three-dimensional non-linear Finite Element model, with bilinear plasticity, is compared with a set of experiments that use different types of Nomex cores and face sheets. The models show that the panels fail prematurely due to core crushing because of wrinkles forming in the face sheets. Experimental results indicate similar trends. The final section examines the affect of impact damage in honeycomb sandwich structures. Due to the thin face sheets and thick cores used on many aircraft and marine components, sandwich panels offer little resistance to impact events. Resulting damage usually consists of a layer of crushed core and a shallow dent in the face sheet. This type of damage often leads to a significant reduction in the load-carrying capacity of the panel through a full range of damage sizes. Finite element and analytical models were developed to accurately predict and capture the localised wrinkling failure mechanism which occurs in the impacted area. Models were directly compared to experimental results, with a high degree of correlation. The numerical and analytical models showed that impact damaged panels were failing due to wrinkling instability and not due to premature core crushing, which is the case with undamaged panels. They showed that two factors influence the wrinkling failure load: damage depth and damage diameter.

This thesis investigates failure in sandwich panels due to bending, specifically localised buckling or wrinkling, a predominant failure mechanism for thin gauge honeycomb sandwich panels loaded in bending or compression. Over the past 60 years, considerable work has been devoted to understanding wrinkling and trying to predict failure loads in damaged and undamaged panels accurately. Existing wrinkling expressions were shown to over-estimate failure loads by over 100%. Discrepancies between wrinkling expressions and experimental failure loads were previously attributed to imperfections and irregularities in the structure. The aim of this thesis is to investigate this problem and try to accurately predict failure loads and understand the underlying failure mechanisms in damaged and undamaged panels, using a combination of numerical and analytical techniques. Classical wrinkling models use a continuum core to model complex cellular honeycomb cores. This type of model reduces complex cellular geometry to a series of effective properties that provide constant support to the face sheet. In reality, honeycomb cores provide support around the periphery of the cell walls and not across the entire surface of the face sheet. Due to the nature of wrinkling and the size of the wavelength, incorrect representation of the core could affect the failure loads and model. This study made direct comparisons between linear buckling loads of a discrete-cored sandwich panel and a continuum-cored sandwich panel. Discrete properties were converted to continuum properties within a Finite Element package. The result conclusively showed that both models predict the same linear failure loads, disproving the theory that the core representations contribute to the difference between experimental and analytical models. It was also shown that existing wrinkling models can accurately predict linear wrinkling loads. These linear model loads do not necessarily match the collapse strength of the physical panel and in most cases predict a significantly higher value. The research then moves on to developing expressions to convert cellular geometry into continuum properties accurately. Expressions are developed for honeycomb structures with fillets in their junctions. Both out-of-plane and in-plane modulus properties are reviewed and the models are verified against Finite Elements models and experimental results. Studies showed that the restrained in-plane modulus can be up to ten times stiffer than the commonly used free modulus value. This has a significant effect on the wrinkling stress. By using the correct value, the discrete model and continuum models predict the same loads. The classical wrinkling expressions also predict the same wrinkling stress as the Finite Element models. After establishing that the core representation is not the cause of the prediction error, the thesis turns to non-linear Finite Element models to predict failure loads and failure mechanism of thin-gauge sandwich honeycomb structures loaded in bending. A continuum three-dimensional non-linear Finite Element model, with bilinear plasticity, is compared with a set of experiments that use different types of Nomex cores and face sheets. The models show that the panels fail prematurely due to core crushing because of wrinkles forming in the face sheets. Experimental results indicate similar trends. The final section examines the affect of impact damage in honeycomb sandwich structures. Due to the thin face sheets and thick cores used on many aircraft and marine components, sandwich panels offer little resistance to impact events. Resulting damage usually consists of a layer of crushed core and a shallow dent in the face sheet. This type of damage often leads to a significant reduction in the load-carrying capacity of the panel through a full range of damage sizes. Finite element and analytical models were developed to accurately predict and capture the localised wrinkling failure mechanism which occurs in the impacted area. Models were directly compared to experimental results, with a high degree of correlation. The numerical and analytical models showed that impact damaged panels were failing due to wrinkling instability and not due to premature core crushing, which is the case with undamaged panels. They showed that two factors influence the wrinkling failure load: damage depth and damage diameter.

This thesis investigates failure in sandwich panels due to bending, specifically localised buckling or wrinkling, a predominant failure mechanism for thin gauge honeycomb sandwich panels loaded in bending or compression. Over the past 60 years, considerable work has been devoted to understanding wrinkling and trying to predict failure loads in damaged and undamaged panels accurately. Existing wrinkling expressions were shown to over-estimate failure loads by over 100%. Discrepancies between wrinkling expressions and experimental failure loads were previously attributed to imperfections and irregularities in the structure. The aim of this thesis is to investigate this problem and try to accurately predict failure loads and understand the underlying failure mechanisms in damaged and undamaged panels, using a combination of numerical and analytical techniques. Classical wrinkling models use a continuum core to model complex cellular honeycomb cores. This type of model reduces complex cellular geometry to a series of effective properties that provide constant support to the face sheet. In reality, honeycomb cores provide support around the periphery of the cell walls and not across the entire surface of the face sheet. Due to the nature of wrinkling and the size of the wavelength, incorrect representation of the core could affect the failure loads and model. This study made direct comparisons between linear buckling loads of a discrete-cored sandwich panel and a continuum-cored sandwich panel. Discrete properties were converted to continuum properties within a Finite Element package. The result conclusively showed that both models predict the same linear failure loads, disproving the theory that the core representations contribute to the difference between experimental and analytical models. It was also shown that existing wrinkling models can accurately predict linear wrinkling loads. These linear model loads do not necessarily match the collapse strength of the physical panel and in most cases predict a significantly higher value. The research then moves on to developing expressions to convert cellular geometry into continuum properties accurately. Expressions are developed for honeycomb structures with fillets in their junctions. Both out-of-plane and in-plane modulus properties are reviewed and the models are verified against Finite Elements models and experimental results. Studies showed that the restrained in-plane modulus can be up to ten times stiffer than the commonly used free modulus value. This has a significant effect on the wrinkling stress. By using the correct value, the discrete model and continuum models predict the same loads. The classical wrinkling expressions also predict the same wrinkling stress as the Finite Element models. After establishing that the core representation is not the cause of the prediction error, the thesis turns to non-linear Finite Element models to predict failure loads and failure mechanism of thin-gauge sandwich honeycomb structures loaded in bending. A continuum three-dimensional non-linear Finite Element model, with bilinear plasticity, is compared with a set of experiments that use different types of Nomex cores and face sheets. The models show that the panels fail prematurely due to core crushing because of wrinkles forming in the face sheets. Experimental results indicate similar trends. The final section examines the affect of impact damage in honeycomb sandwich structures. Due to the thin face sheets and thick cores used on many aircraft and marine components, sandwich panels offer little resistance to impact events. Resulting damage usually consists of a layer of crushed core and a shallow dent in the face sheet. This type of damage often leads to a significant reduction in the load-carrying capacity of the panel through a full range of damage sizes. Finite element and analytical models were developed to accurately predict and capture the localised wrinkling failure mechanism which occurs in the impacted area. Models were directly compared to experimental results, with a high degree of correlation. The numerical and analytical models showed that impact damaged panels were failing due to wrinkling instability and not due to premature core crushing, which is the case with undamaged panels. They showed that two factors influence the wrinkling failure load: damage depth and damage diameter.

Being a widely used structure, composite shells have been studied for a long time. The features of small thickness, heterogeneity, and anisotropy of composite shells have created many challenges for analyzing them. A number of theories have been developed for modeling composite shells, while they are either not practical for engineering use, or rely on assumptions that do not always hold. Consequently, a better theory is needed, especially for the application on challenging problems such as shells involving thermoelasticity, viscoelasticity, or viscoplasticity.<br><br>In this dissertation, a shell theory based on mechanics of structure genome (MSG), a unified theory for multiscale constitutive modeling, is developed. This theory is capable of handling fully anisotropy and complex heterogeneity, and because the derivation follows principle of minimum information loss (PMIL) and using the variational asymptotic method (VAM), high accuracy can be achieved. Both a linear version and a nonlinear version using Euler method combined with Newton-Raphson method are presented. This MSG-based shell theory is used for analyzing the curing process of composites, deployable structures made with thin-ply high strain composite (TP-HSC), and material nonlinear shell behaviors.<br><br>When using the MSG-based shell theory to simulate the curing process of composites, the formulation is written in an analytical form, with the effect of temperature change and degree of cure (DOC) included. In addition to an equivalent classical shell theory, a higher order model with the correction from initial geometry and transverse shear deformation is presented in the form of the Reissner-Mindlin model. Examples show that MSG-based shell theory can accurately capture the deformation caused by temperature change and cure shrinkage, while errors exist when recovering three-dimensional (3D) strain field. Besides, the influence of varying transverse shear stiffness needs to be further studied.<br><br>In order to analyze TP-HSC deployable structures, linear viscoelasticity behavior of composite shells is modeled. Then, column bending test (CBT), an experiment for testing the bending stiffness of thin panels under large bending deformation, is simulated with both quasi-elastic (QE) and direct integration (DI) implementation of viscoelastic shell properties. Comparisons of the test and analysis results show that the model is capable of predicting most of the measured trends. Residual curvature measured in the tests, but not predicted by the present model, suggests that viscoplasticity should be considered. A demonstrative study also shows the potential of material model calibration using the virtual CBT developed in this work. A deployable boom structure is also analyzed. The complete process of flattening, coiling, stowage, deployment and recovery is simulated with the viscoelastic shell model. Results show that major residual deformation happens in the hoop direction.<br><br>A nonlinear version of the MSG-based general purpose constitutive modeling code SwiftComp is developed. The nonlinear solving algorithm based on the combined Euler-Newton method is implemented into SwiftComp. For the convenience of implementing a nonlinear material model, the capability of using user material is also added. A viscoelastic material model and a continuum damage model is tested and shows excellent match when compared with Abaqus results with solid elements and UMAT. Further validation of the nonlinear SwiftComp is done with a nonlinear viscoelastic-viscoplastic model. The high computational cost is emphasized with a preliminary study with surrogate model.

The initiation of high explosives (HEs) under shock loading lacks a comprehensive understanding: particularly at the particle scale. One common explanation is hot spot theory, which suggests that energy in the material resulting from the impact event is localized in a small area causing an increase in temperature that can lead to ignition. This study focuses on the response of HMX particles (a common HE) within a polymer matrix (Sylgard-184^®), a simplified example of a polymer bonded explosive (PBX). A light gas gun was used to load the samples at impact velocities ranging from 370 to 520 m/s. The impact events were visualized using X-ray phase contrast imaging (PCI) allowing real-time observation of the impact event. The experiments used three subsets of PBX samples: multiple particle (production grade and single crystal), drilled hole, and milled slot. Evidence of damage and deformation occurred in all of the sample types. While the necessary impact velocity for consistent hot spot formation leading to reactions was not reached, the damage (particularly cracking) that occurred provides a useful indication of where hot spots may occur when higher velocities are reached. With the multiple particle samples, evidence of cracking and debonding occurred throughout. One sample showed significant volume expansion due to possible reaction. The samples containing drilled holes demonstrated the expected pore collapse behavior at these velocities, as well as damage downstream from the holes under various two-hole arrangements. Milled slot samples were tested to simulate existing cracks in the HMX. These samples showed increased damage at the site of the milled slot, as well as unique cracking behavior in one of the samples.

<div>Sensing of mechanical state is critical in diverse fields including biomedical implants, intelligent robotics, consumer technology interfaces, and integrated structural health monitoring among many others. Recently, materials that are self-sensing via the piezoresistive effect (i.e. having deformation-dependent electrical conductivity) have received much attention due to their potential to enable intrinsic, material-level strain sensing with lesser dependence on external/ad hoc sensor arrays. In order to effectively use piezoresistive materials for strain-sensing, however, it is necessary to understand the deformation-resistivity change relationship. To that end, many studies have been conducted to model the piezoresistive effect, particularly in nanocomposites which have been modified with high aspect-ratio carbonaceous fillers such as carbon nanotubes or carbon nanofibers. However, prevailing piezoresistivity models have important limitations such as being limited to microscales and therefore being computationally prohibitive for macroscale analyses, considering only simple deformations, and having limited accuracy. These are important issues because small errors or delays due to these challenges can substantially mitigate the effectiveness of strain-sensing via piezoresistivity. Therefore, the first objective of this thesis is to develop a conceptual framework for a piezoresistive tensorial relation that is amenable to arbitrary deformation, macroscale analyses, and a wide range of piezoresistive material systems. This was achieved by postulating a general higher-order resistivity-strain relation and fitting the general model to experimental data for carbon nanofiber-modified epoxy (as a representative piezoresistive material with non-linear resistivity-strain relations) through the determination of piezoresistive constants. Lastly, the proposed relation was validated experimentally against discrete resistance changes collected over a complex shape and spatially distributed resistivity changes imaged via electrical impedance tomography (EIT) with very good correspondence. Because of the generality of the proposed higher-order tensorial relation, it can be applied to a wide variety of material systems (e.g. piezoresistive polymers, cementitious, and ceramic composites) thereby lending significant potential for broader impacts to this work. </div><div><br></div><div>Despite the expansive body of work on direct current (DC) transport, DC-based methods have important limitations which can be overcome via alternating current (AC)-based self-sensing. Unfortunately, comparatively little work has been done on AC transport-deformation modeling in self-sensing materials. Therefore, the second objective of this thesis is to establish a conceptual framework for the macroscale modeling of AC conductivity-strain coupling in piezoresistive materials. For this, the universal dielectric response (UDR) as described by Joncsher's power law for AC conductivity was fit to AC conductivity versus strain data for CNF/epoxy (again serving as a representative self-sensing material). It was found that this power law does indeed accurately describe deformation-dependent AC conductivity and power-law fitting constants are non-linear in both normal and shear strain. Curiously, a piezoresistive switching behavior was also observed during this testing. That is, positive piezoresistivity (i.e. decreasing AC conductivity with increasing tensile strain) was observed at low frequencies and negative piezoresistivity (i.e. increasing AC conductivity with increasing tensile strain) was observed at high frequencies. Consequently, there exists a point of zero piezoresistivity (i.e. frequency at which AC conductivity does not change with deformation) between these behaviors. Via microscale computational modeling, it was discovered that changing inter-filler tunneling resistance acting in parallel with inter-filler capacitance is the physical mechanism of this switching behavior.</div>

This work presents findings in the combined experimental and computational study of the effects of anisotropy and microstructure on the behavior of HMX-based energetic materials. Large single crystal samples of β-HMX were meticulously created by solvent evaporation for experimental purposes, and respective orientations were identified via x-ray diffraction. Indentation modulus and hardness values were obtained for different orientations of β-HMX via nanoindentation experiments. Small-scale dynamic impact experiments were performed, and a viscoplastic power law model fit, to describe the anisotropic viscoplastic properties of the crystal. The anisotropic fracture toughness and surface energy of β-HMX were calculated by studying indentation-nucleated crack system formations and fitting the corresponding data to two different models, developed by Lawn and Laugier. It was found that the {011} and {110} planes had the highest and lowest fracture toughnesses, respectively. Drop hammer impact tests were performed to investigate effects of morphology on the impact-induced thermal response of HMX. Finally, the anisotropic properties obtained in this work were applied in a cohesive finite element simulation involving the impact of a sample of PBX containing HMX crystals with varying orientations. Cohesive finite element models were generated of separate microstructure containing either anisotropic (locally isotropic) or global isotropic properties of HMX particle. In comparison, the isotropic model appeared to be more deformation resistant.