A collaborative testing and analysis effort investigating the effects of threaded fastener size on load-displacement behavior and failure was conducted to inform the modeling of threaded connections. A series of quasistatic tension tests were performed on #00, #02, #04, #06 and #4 (1/4”) A286 stainless steel fasteners (NAS1351N00-4, NAS1352N02-6, NAS1352N04-8, NAS1352N06-10, and NAS1352N4-24, respectively) to provide calibration and validation data for the analysis portion of the study. The data obtained from the testing series reveals that the size of the fastener may influence the characteristic stress-strain response, as the failure strains and ultimate loads varied between the smaller (#00 and #02) and larger (#04, #06, and #4) fasteners. These results motivated the construction of high-fidelity finite element models to investigate the underlying mechanics of these responses. Two threaded fastener models, one with axisymmetric threads and the other with full 3D helical threads, were calibrated to subsets of the data to compare modeling approaches, analyze fastener material properties, and assess how well these calibrated properties extend to fasteners of varying sizes and if trends exist that can inform future best modeling practices. The modeling results are complemented with a microstructural analysis to further investigate the root cause of size effects observed in the experimentally obtained load-displacement curves. These analyses are intended to inform and guide reduced-order modeling approaches that can be incorporated in system level analyses of abnormal environments where modeling fidelity is limited and each component is not always testable, but models must still capture fastener behavior up to and including failure. This complimentary testing and analysis study identifies differences in the characteristic stress-strain response of varying sized fasteners, provides microstructural evidence to support these variations, evaluates our ability to extrapolate calibrated properties to different sized fasteners, and ultimately further educates the analysis community on the robustness of fastener modeling.
In Part I, equal channel angular extrusion (ECAE) was demonstrated as a novel, simple-shear deformation process for producing bulk forms of the low ductility Fe-Co-2V (Hiperco 50A®) soft ferromagnetic alloy with refined grain sizes. Microstructures and mechanical properties were discussed. In this Part II contribution, the crystallographic textures and quasi-static magnetic properties of ECAE-processed Hiperco were characterized. The textures were of a simple-shear character defined by partial {110} and (111) fibers inclined relative to the extrusion direction, in agreement with the expectations for simple-shear deformation textures of BCC metals. These textures were observed throughout all processing conditions and only slightly reduced in intensity by subsequent recrystallization heat treatments. Characterization of the magnetic properties revealed a lower coercivity and higher permeability for ECAE-processed Hiperco specimens relative to the conventionally processed and annealed Hiperco bar. The effects of the resultant microstructure and texture on the coercivity and permeability magnetic properties are discussed.
Processing of the low workability Fe-Co-1.5V (Hiperco® equivalent) alloy is demonstrated using the Laser Engineered Net Shaping (LENS) metals additive manufacturing technique. As an innovative and highly localized solidification process, LENS is shown to overcome workability issues that arise during conventional thermomechanical processing, enabling the production of bulk, near net-shape forms of the Fe-Co alloy. Bulk LENS structures appeared to be ductile with no significant macroscopic defects. Atomic ordering was evaluated and significantly reduced in as-built LENS specimens relative to an annealed condition, tailorable through selection of processing parameters. Fine equiaxed grain structures were observed in as-built specimens following solidification, which then evolved toward a highly heterogeneous bimodal grain structure after annealing. The microstructure evolution in Fe-Co is discussed in the context of classical solidification theory and selective grain boundary pinning processes. Magnetic properties were also assessed and shown to fall within the extremes of conventionally processed Hiperco® alloys. Hiperco® is a registered trademark of Carpenter Technologies, Readings, PA.
