Jeffrey Abell

Geometric deviations remain a major barrier to the widespread industrial adoption of incremental sheet forming (ISF). Compared with conventional toolpath compensation that rely on extensive data generation and trial-and-error procedures, variation of toolpath styles offers a more direct and efficient strategy for mitigating geometric defects. In this study, multiple curvilinear toolpath strategies were investigated for a standard closed-contour ISF part to evaluate their effectiveness in reducing geometric deviations. Six toolpaths were examined, including three established types – convex, concave, and wavy – and three novel toolpaths proposed in this work: adaptive, cusp, and sine. The convex toolpath achieved the largest side-wall springback reduction relative to the linear baseline but introduced a significant bottom pillow effect and reduced formability. While the cusp toolpath effectively suppressed both springback and pillow formation, it resulted in local thickening and degraded surface finish. Overall, the sine toolpath provided the most balanced performance, achieving effective reduction of all major geometric defects. Numerical simulations reveal an inherent tradeoff between side-wall springback reduction and bottom pillow formation, as positive residual bending moments formed in the pillow region contribute to springback mitigation by promoting outward bending of the side walls.

This study aimed to quantify microplastic (MP) concentration and analyze the spatial and temporal variabilities of the concentrations during the tidal cycle in Humboldt Bay, California. To get an approximation of MP concentration, both water and sediment samples were taken at five different stations, twice during one tidal cycle. Sampling was conducted during two different cruises, on the 19th and 21st of September 2020. The samples were processed in the lab using a density separation procedure and filtration. MP concentrations in the different samples were determined using an average optical microscopy count. Comparison of the water column MP concentrations during ebb and flood tides shows higher concentrations during flood tide, 49.0 particles/L ± 32.37 (flood) vs 34.4 particles/L ± 16.32 (ebb), indicating that MPs are brought into Humboldt Bay from the ocean. The comparison of the MP concentrations during lower energy and higher energy conditions indicates that concentrations in the water column were elevated when there was greater tidal kinetic energy, approximated by the covariance of the measured velocity in North Bay Channel. This result was assumed to be caused by the strong tidal currents stirring up both sediments and the settled MPs into the water column. Due to lower tidal kinetic energy on the sediment sampling cruise day, we could not confirm that assumption. Water samples indicated that MPs are heterogeneously distributed in the bay, with higher concentrations found near the Entrance Channel and lower concentrations found further north in the bay. Sediment samples also indicate a heterogeneous distribution of MPs in the bay, with the lowest concentrations near the Entrance Channel, 15 particles/kg, where high tidal currents inhibit settling of particles.

This research examines factors that control pH in Humboldt Bay – a shallow, tidally-driven estuary in northern California (USA) that supports shellfisheries which are economically important to the state. Time-series data from hydrographic sensors at two Central and Northern California Ocean Observing System (CenCOOS) stations, as well as multiple 2021 undergraduate cruises, were used to understand the role of tides, biological productivity and carbonate dissolution in controlling pH on various timescales. Differences in pH, dissolved oxygen, chlorophyll, and temperature between an in-bay sensor and a coastal sensor indicate that the tidal flux exerts a long term, seasonal control on pH, but biological productivity substantially modifies carbon and oxygen thereby controlling pH on daily and weekly timescales. Sediment samples were also collected from the bay in 2021 to study carbonate dissolution. Sediments were incubated for three days in both stirred and unstirred conditions (to mimic tidal mixing and no tidal mixing respectively) and DO, pH and alkalinity were monitored. For all stirred incubations, large increases in pH and alkalinity suggested considerable carbonate sediment dissolution. When scaled to the bay’s in-situ suspended sediment concentrations, carbonate dissolution may exert a supplementary control on pH at similar time scales as biological productivity, but the magnitude of its effect is less.