Publications

2020

• Tensile and ductile fracture properties of as-printed 316L stainless steel thin walls obtained by directed energy deposition
  
  • Margerit Pierre
  • Weisz-Patrault Daniel
  • Ravi-Chandar Krishnaswamy
  • Constantinescu Andrei
  Additive Manufacturing, Elsevier, 2020, 37, pp.101664. Mechanical properties of as-printed 316L stainless steel thin-walled structures obtained by directed energy deposition are investigated. In-situ tensile and fracture tests are performed on small samples obtained from a additively manufactured square section tube and extracted with three different orientations with respect to the part build direction. Despite a strongly oriented microstructure resulting from the process, as-printed specimens exhibit a reduced anisotropy in comparison with thick or polished samples commonly reported in the literature. Moreover, it is shown using a simple model that the reduced dentified anisotropy can be explained by considering the material thickness variation pattern only, resulting from the layer stacking process. Fracture tests are analyzed using an adapted digital image correlation procedure that evaluates the specimen fracture toughness from experimentally computed J-integrals. Using time reversal, strain fields in regions close to the crack path are identified. Stress fields are then computed from the constitutive behavior identified in tensile tests. A regularization procedure is proposed to enforce the stress equilibrium. Finally, the J-integral is computed using various integration contours in order to validate its path-independance. On this basis, a nearly isotropic fracture toughness is identified. Additional scanning electron microscope observations show that fracture surface features are independent from specimen orientation. This apparent isotropy is explained by the isotropic distribution of lack-of-fusion defects driving crack initiation and propagation. (10.1016/j.addma.2020.101664)
  DOI : 10.1016/j.addma.2020.101664
• Freezing a rivulet
  
  • Monier Antoine
  • Huerre Axel
  • Josserand Christophe
  • Séon Thomas
  Physical Review Fluids, American Physical Society, 2020, 5 (6). We investigate experimentally the formation of the particular ice structure obtained when a capillary trickle of water flows on a cold substrate. We show that after a few minutes the water ends up flowing on a tiny ice wall whose shape is permanent. We characterize and understand quantitatively the formation dynamics and the final thickness of this ice structure. In particular, we identify two growth regimes. First, a 1D solidification diffusive regime, where ice is building independently of the flowing water. And second, once the ice is thick enough, the heat flux in the water comes into play, breaking the 1D symmetry of the problem, and the ice ends up thickening linearly downward. This linear pattern is explained by considering the confinement of the thermal boundary layer in the water by the free surface. The partial freezing of Niagara falls and the cancellation of thousands of flights during the cold snap of winter 2019 are a few examples of the disturbances caused by extreme weather events. Indeed, the accretion of ice on superstructures such as planes [1-3], power-lines [4], bridge cables [5] or wind turbines [6] can have dramatic consequences. Nowadays, the main strategy to prevent most of these undesirable effects is to develop anti-icing surfaces [7, 8], but new paradigms could emerge from a better understanding of the freezing dynamics in complex configurations. When water flows on a cold surface for example, the resulting ice structure is reminiscent from the manifold interaction between the heat transport and the flow [9]. The presence of a free-surface is also determinant in these problems, resulting in the apparition of a tip on frozen sessile drops [10], or in the explosion of droplets cooled from the outside-in [11] in static conditions. Freezing of capillary flows, widely encountered in the previous examples, can consequently reveal a very rich behavior [12-15] as in the formation of icicles [16] or ice structures following drop impact on cold surfaces [17]. In this Letter, we investigate experimentally the freezing of a capillary water river, the so-called rivulet [18-20] (see Fig. 1), flowing over a cold substrate. We show for the first time that the growing ice structure reaches a static shape after few minutes. The water then flows on a tiny ice wall that thickens downward, an observation we quantitatively explain considering the confinement of a thermal boundary layer. These results bring new understanding of the ice crust formation in the presence of streaming water and improve the prediction of its shape. The experiment consists in flowing distilled water dyed with fluorescein at 0.5 g.L −1 along a cold aluminum block of 10 cm long, with an inclination of α = 30 • to the horizontal. The temperature of the injected water T in ranges from 8 to 35 • C, see Fig. 1(a). The water is injected through a needle (inner diameter 1.6 mm) at a flow-rate Q = 20 mL.min −1 , such that there is no meander at room temperature [19]. A straight water rivulet is then formed [18], with a typical width of 2 mm, a thickness of h w = 800 µm, and a characteristic velocity of the buoyant flow u 0 ≈ 10 cm.s −1. As the Reynolds number of the flow is sufficiently small (Re = u 0 h w /ν = 80), the flow is laminar and mass conservation implies that the liquid layer thickness h w is constant [18]. The temperature of the aluminum substrate T s is set by plunging the block in liquid nitrogen for a given amount of time so that it ranges from −9 to −44 • C. T s is measured during the experiment and remains constant (±1 • C). Experiments performed with substrate temperatures below −44 • C consistently lead to the fracture [21] or the self-peeling [22] of the ice and are not considered here. Upon contact with the cold substrate, the water freezes and an ice layer grows while the water continues to flow on top, as shown on the sequence of snapshots of inset in Fig. 1(a) and in the Sup. Mat. movie. During that process, the fluorescein concentrates between the ice dendrites, causing self-quenching and fluorescence dimming in the ice [23]. This allows us to clearly distinguish between the ice and the water layers under UV light. The ice layer thickness h i (x, t) is then measured using a Nikon D800 camera recording from the side at 30 fps. The setup is placed in a humidity control box to avoid frost formation (H r ≈ 5 − 10%). Figure 1(b) presents the ice layer profile along the direction of the flow (x = 0 at the needle) at different times for T in =10 • C and T s =-36 • C. The analysis is restricted to the middle of the plate (x ∈ [1,8] cm) to avoid input and output influences. At early times, the ice layer grows homogeneously along the plane and the successive profiles are parallel to the substrate. After that, the ice layer continues to grow but not uniformly: its thickness increases along the plane. Finally, the ice layer stops growing and the system reaches a permanent regime consisting of a static ice structure, of thickness h max , on top of which a water layer is flowing. The final shape of the ice can be well described by a line of slope β as illustrated by the dashed lines in Fig. 1(b): h max (x) = h i,0 + βx, with β varying in our experiments between 0 and 4 •. The (10.1103/PhysRevFluids.5.062301)
  DOI : 10.1103/PhysRevFluids.5.062301