Creep in future long-term space technology materials is a critical concern due to the duration of potential missions to Mars and beyond. Structural and skin components in long-term mission spacecraft will undergo creep deformation and eventual failure if not designed to be sufficiently creep resistant. The microstructural deformation mechanisms that control the creep behavior must be understood to intelligently inform the design of new creep resistant alloys and enhance those already in service. Using lightweight single phase β Ti alloys, an analysis tool was developed to measure grain boundary sliding (GBS) and intragranular slip in-situ via a Heaviside function-based algorithm. The data needed for the analysis tool includes an electron backscattered diffraction generated microstructural map and high-resolution digital image correlation (HRDIC) strain fields. This testing technique advances the state of the art by facilitating in-situ measurement of these microstructural deformation mechanisms without the need to interrupt creep testing and introduce unwanted thermic cyclic effects. Proof-of-concept experiments utilizing this analysis tool on a single phase β Ti alloy in room temperature creep rapidly identified the dominant deformation mechanism to be intragranular slip and glide creep without the need for destructive and expensive post-mortem testing.
EXTENDED ABSTRACT

Pre-Northridge moment frames with PJP Welded Column Splices (WCS) are highly vulnerable to brittle fracture much before the connection develops the strength of the upper connected column due to the inherent crack-like flaw (unfused region of the weld) and the low toughness of the weld material. Given that the consequences of fracture are catastrophic and that retrofitting these splices can be highly disruptive to building operations, accurately estimating their fracture risk is of great importance. To achieve this, a probabilistic quantification of splice fracture is necessary, along with tools that simulate splice fracture and post-fracture response in a global frame assessment framework.

A framework to probabilistically assess the fracture strength of these splices is presented which addresses shortcomings of previous research and performance assessment guidance that do not consider key mechanistic or statistical effects. A new element model (in OpenSees), which is informed by the fracture mechanics-based estimates of splice strength and existing material models in OpenSees, is developed to simulate the splice fracture and post-fracture response. Application of the new splice element in assessment of a 20-story building to scaled ground motions is demonstrated.
EXTENDED ABSTRACT

Ductile fracture is affected by the state of stress, which is commonly described by two parameters, stress triaxiality and Lode parameter. While the effects of triaxiality are well known, the effect of the Lode parameter are uncertain. This uncertainty results in particular from the difficulty to vary the Lode parameter at controlled triaxiality. Recent experiments by the authors suggest that the Lode parameter does indeed affect ductile fracture to some extent. The aim of this work is to analyze the mechanisms behind these apparent effects of the Lode parameter.To accomplish this, an advanced multi-surface porous-plasticity model that accounts for both homogeneous and inhomogeneous yielding is used in an Abaqus Umat to simulate proportional loading of a single integration point. Within this modeling framework, the effect of Lode parameter is inherently captured through the competition between the two main modes of inhomogeneous yielding: internal necking and internal shearing of the intervoid ligament. The ability of this constitutive formulation to capture the effects of the Lode parameter that were observed in experiments is examined.
EXTENDED ABSTRACT

High cycle fatigue (HCF) in composite structures leads to damage accumulation and associated stiffness
degradation, which are challenging to quantify. This work uses a medium wave infrared to monitor selfheating in chopped carbon fiber/acrylonitrile butadiene styrene specimens subjected to tension-tension
fatigue loading. An innovative rapid testing protocol that correlates the generated full-field temperature
maps and stiffness degradation data has been developed providing a comprehensive understanding of
material behavior under cyclic loading. Results contribute to the fundamental understanding of HCF in
composite materials and develop more accurate predictive models for fatigue life. Rapid testing has allowed
correlating process parameters with the microstructure and structural integrity of additively manufactured
(AM) composites.
EXTENDED ABSTRACT

This study investigated the accumulation of damage in periodic, engineered disbond arrays and its effect on the shear strength and failure mode of single lap joints. The impact of surface contamination on shear strength was also analyzed. Experimental results showed that surface contamination had a significant negative impact on shear strength, with a reduction of up to 98% in specimens with 100% contamination. The use of a disbond stripe resulted in a slight reduction of only 3.89% in shear strength. However, no progressive accumulation of damage in bonds was observed in the current set of experiments. Further investigation is required to examine the relationship between crack mode and design configuration. This study highlights the importance of addressing these factors in the design and analysis of bonded structures to ensure their lifetime and durability.
EXTENDED ABSTRACT

Maraging steels are a class of precipitation hardened steels wherein different micro-mechanisms of deformation such as planar slip, interaction with coherent/incoherent precipitates, and reverted austenite controls the overall mechanical behavior of the material. High Pressure Torsion (HPT) processing adds to this complexity by pumping in a large density of dislocations that form sub-grain boundaries and cellular structures, leading to changes in precipitate morphology and stability upon ageing. This results in a drastic change in the deformation accommodation mechanism. While these steels are known to display high fracture toughness in the hot rolled condition (hereafter referred to as: as-received), this study reports for the first time their KIC values after deformation processing, including the effect of grain size refinement, dislocation density and texture induced anisotropy. To accomplish these measurements in the small volume discs that are produced by HPT, small-scale clamped beam bend geometries were utilized for the first time. KIC measurements were carried out for both cases in the unaged, peak-aged and over-aged conditions. DIC strain mapping has been made use of to quantify the crack tip opening displacement and process zone evolution ahead of the crack tip.
EXTENDED ABSTRACT

In this study, static fracture experiments under mode-I and mixed mode loading, and fatigue testing under mode-I loading were carried out on double cantilever beam (DCB) specimens, and subsequent healing of the delamination was investigated. Thermoplastic healants dispersed in a thermoset CFRP composite were used to perform the healing, triggered through brief heating in an oven. It was observed from the test results that delaminations can be healed efficiently and the healing was found to be repeatable. As a result of healing, significant crack closure was observed and the fatigue crack growth rate was considerably reduced. These findings can be helpful in extending the service life of laminated composites.
EXTENDED ABSTRACT

A wavelet-enriched adaptive hierarchical, coupled crystal plasticity – phase-field finite element model is developed in this work to simulate crack propagation in complex polycrystalline microstructures. The model accommodates initial material anisotropy and crack tension-compression asymmetry through orthogonal decomposition of stored elastic strain energy into tensile and compressive counterparts. The crack evolution is driven by stored elastic and defect energies, resulting from slip and hardening of crystallographic slips systems. A FE model is used to simulate the fracture process in a statistically equivalent representative volume element reconstructed from electron backscattered diffraction scans of experimental microstructures. Multiple numerical simulations with the model exhibits microstructurally sensitive crack propagation characteristics.
EXTENDED ABSTRACT

Most studies on the fracture of bulk or ribbon superconductors are based on superconducting critical state models that do not consider temperature changes. Most of the research objects of the thermal-mechanical-electric-magnetic model only focus on the distribution of magnetic field current and stress, while the thermal-mechanical-electric-magnetic model with cracks is rarely involved. The research in this paper will be based on a generalized critical state model that considers both temperature and magnetic field effects to investigate the effects of thermal and magnetic effects on cracks in superconducting structures.
EXTENDED ABSTRACT

A complete constitutive theory is presented to enable ductile fracture simulations under complex loadings that may involve shear-dominated stress states or even negative triaxialities. The yield criteria accounting for various forms of anisotropy is supplemented with evolution equations to complete the constitutive theory formulation. State-of-the-art ductile fracture theory can only be fully exploited when a robust implementation enabling structural computations is available. This work set out to address the latter within a multisurface framework. A complete constitutive theory of plastic porous materials incorporating homogeneous (HY) and multiple (n) unhomogeneous yieldings (UY), named HUNnY is developed. The capabilities of the new formulation and its implementation are demonstrated by simulating fracture in tension, fracture in shear of top hat specimen and fracture by shear banding. The predictive theory promises to completely change our understanding of some of these most challenging problems that remained elusive for decades.
EXTENDED ABSTRACT