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.
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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.
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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.
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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.
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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.
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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.
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Understanding the remaining life of a component is critical to maintaining safe operation and is necessary for budgeting repairs. This paper uses Finite Element Analysis to predict the performance of a steam header under realistic loading scenarios, comparing the difference between life expectancies of service-aged material to that of virgin material.
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This poster presentation summarizes the results from several projects in this field conducted at the TU Darmstadt to identify and describe the various influences on crack growth under thermo-mechanical fatigue (TMF) loading. The activation of damage mechanisms under TMF loading and interactions between them are dependent of the temperature cycle and the respective load phasing. Depending on the type of loading (force- vs. strain-control), contrary influences of the phase shift on the TMF crack growth rates are found. To describe crack growth under creep-fatigue and TMF conditions, the linear accumulation model ‘O.C.F.’ was developed – based on the contributions of fatigue, creep and oxidation to crack growth per load cycle. This model is capable to reproduce the effects of time-dependent damage, different load ratios and TMF phase shifts, as well as component geometries. The model’s linear formulation allows assessing the dominant driver of crack growth at each stage of an experiment. These predictions are compared with fractographic investigations and in-situ observations of crack paths to identify the mechanisms of crack growth under different TMF load cycle forms.
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Existing ductile failure models such as the Gurson-Tvergaard-Needleman (GTN) model as well as more recent physics-based models (for instance, the Benzerga-Leblond coalescence model from 2014) were all derived for perfectly plastic porous materials using classical limit analysis, with plastic flow in the matrix being described by J2 flow theory. When extended heuristically to hardenable materials, these models do not account for the heterogeneity of plastic strain in the matrix, and are unable to capture the effect of hardening on the evolution of porosity, the primary damage variable.

This work uses “sequential limit analysis” (SLA) to first derive a hardening-sensitive void coalescence criterion for a cylindrical cell containing a coaxial cylindrical void of finite height, by discretizing the intervoid ligament into a finite number of shells in each of which the quantities characterizing isotropic hardening are considered to be homogeneous. Next, this new criterion is combined with a recently formulated hardening-sensitive void growth criterion (also derived using SLA) to obtain a hybrid model of ductile failure. The new constitutive formulation’s ability to remedy the two aforementioned shortcomings of existing models is examined, and a set of finite-element micromechanical unit cell calculations is used to further assess the model’s predictive capabilities.
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An analysis of the fatigue performance of 316L stainless steel bar drilled and tapped is performed. The effect of flow drilling and flow tapping on the material microstructure, microhardness and fatigue life is compared to the characteristics of conventional cutting processes.
The hardness recorded at the surface of flow formed holes is 62% higher than that of the raw material. In addition, grains are refined and plastically deformed by the flow processes.
Four-points bending fatigue tests were performed at 3 stress amplitudes and with a stress ratio of 0.1. The results revealed no significant differences in fatigue life for tests performed bending moment is equal to 75% and 60% of the yield bending moment. Nevertheless, when the maximum bending moment applied was limited to 50% of the yield bending moment, the specimens containing holes manufactured by the cutting endure more cycles. Fractographic observations revealed, for both specimens, that the failure initiated from the thread beneath the surface of maximum tensile stress. On the fracture surfaces of flow processed specimens, cracks initiated from the discontinuities observed at the peak of threads. In addition, secondary cracks are observed at the thread roots where to material is hard and the grains are refined.
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