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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In applications requiring high velocity interactions of energetic materials, the shock response of the crystal-binder interface is of great importance. We demonstrate a technique for capturing the high localized deformation of the crystal-binder interface using time resolved Raman spectroscopy at nanosecond intervals. A bi-crystal interface of polydimethylsiloxane (PDMS) sandwiched between sucrose crystals is used in the method, with the sample as a whole put on a glass surface and impacted from the opposite end. Aluminum cylindrical flyers with thicknesses of 25-50 um and diameters of 1 mm were accelerated utilizing the Laser Induced Projectile Impact Test (LIPIT) to create high velocity shock compression loads. The velocity of the projectiles was determined using heterodyne photon doppler velocimetry (het-PDV) and ranged from 0.5 to 1.5 km/s. Full field measurements of the 532nm Raman spectroscopic response were acquired using an in-house designed laser array configuration with 27 discrete laser subsets. The pressure and temperature distributions over the interface were calculated using the pre-calibrated peak shifts of the sucrose CH and CH2 bonds. The highly localized deformation generated by pressure and temperature rise as the shock front travels across the interface were measured in-situ by the time resolved Raman spectroscopic response. The results showed
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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.
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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.
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