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Two types of additive manufacturing (AM) techniques, fused filament fabrication (FFF) and selective laser sintering (SLS), were employed to fabricate specimens from three widely used AM polymers: polylactic acid (PLA), acrylonitrile butadiene styrene (ABS), and polyamide (PA): the first two using FFF and PA using SLS. The single‐edge notch bending (SENB) test was employed to measure the mode‐I fracture toughnesses of bulk and AM specimens in three directions relative to the printing direction. Unlike their bulk counterparts, AM polymers exhibited anisotropic fracture toughness. The degree of anisotropy in FFF‐PLA was much larger than FFF‐ABS and SLS‐PA. The smallest fracture toughness value for each AM polymer was achieved along the transverse printing direction. Different AM techniques affect fracture toughness differently. For instance, FFF is shown to increase PLA's plastic deformation, while SLS reduces PA's ductility. The decrease in fracture toughness is consistently observed when transitioning from bulk polymers to AM ones, reaching as much as 75%. This study provides insights into the fracture toughness of AM parts made via popular plastics, which are crucial for designing structural parts using FFF and SLS processes. Highlights: Additively manufactured polymers always exhibit some anisotropic fracture toughnessDifferent additive manufacturing techniques affect the fracture toughness differentlyFused filament fabrication increases polylactic acid's plastic deformationBut selective laser sintering reduces polyamide's ductility [ABSTRACT FROM AUTHOR]

Because additively manufactured materials are increasingly being used in load-bearing structures, strength research has become critical. Surprisingly, numerous studies have reported the tensile strength measurements, but only a few studies have presented meaningful results for the shear strength measurements of additively manufactured polymers. Hence, this paper proposes a combined experimental and numerical investigation of a new interlayer shear strength measurement approach, and it targeted the applications of the same polyamide (PA12) specimens made with fused deposition modeling (FDM) and selective laser sintering (SLS). A necking-shaped shear specimen was developed to measure the pure shear strengths with the aid of a three-dimensional (3D) finite element analysis. The results showed that the specimens made with FDM and SLS exhibited totally different shear failure behaviors. The ultimate shear strength of the FDM-PA specimens had more than a 32% increase over that of the SLS-PA specimens. An interface mechanics assumption was employed to explore the different shear failure mechanisms with the support of a fractography analysis. [ABSTRACT FROM AUTHOR]

Unlike isotropic homogenous materials, 3D printing polymers are anisotropic in terms of their strength and fracture toughness properties, according to previous research. Therefore, the lowest strength must be accurately measured before the extensive application of any 3D printing materials. Certain 3D printing materials have been increasingly employed in large structures; therefore, the size effect of their strengths has become very important. In this work, we performed a systematic experimental investigation on the minimum strengths and size effects of four common 3D printing polymers made by fusion deposition modeling and injection molding. Three types of tensile specimens with very different cross-sectional areas were selected. For each material system, two types of specimens with different printing surface angles were employed. The results from 200 specimens demonstrated that the minimum tensile strengths in the build direction of some specimens were only 25% that of the other directions in the same specimens, and 59% of the injection-molded specimens composed of the same material. No general conclusion on the size effects of the tensile strengths could be made for most specimens. Only the strengths along the build direction decreased up to 72% in two types of specific specimens if their cross-sectional areas increased by 60 times. Although their strengths were anisotropic, the stiffness properties of the 3D printing polymers were isotropic. Digital image correlation measurements showed that the Young's moduli of all specimens composed of the same material were similar. A dual-notch mechanics model was employed to explain the anisotropic strengths, and a fractography analysis supported the size effect. [Display omitted] • Tensile strength along the build direction was the minimum strength. • No general conclusion on the size effects of the tensile strengths could be made. • Strengths decreased 72% in only PLA and PC specimens if their areas increased by 60 times. [ABSTRACT FROM AUTHOR]

The present work investigated the development of sand bed scour by submerged circular impinging vertical turbulent jets over a wide regime of jet diameters, exit velocities and impinging distances. The scour hole profiles were recorded with the optical method in order to present dynamic scour hole characteristics and their variation with any time. The results revealed that the scour profiles in the equilibrium state are similar and the geometries of the sour hole in the jet central region are only determined primarily by jet momentum flux. A semi-empirical equation has been developed for the maximum dimensionless scour depth in equilibrium state based on the obtained experimental data and theoretical analysis. The evolution of scour hole is found to be logarithmic, and three phases of temporal evolution were further distinguished. Finally, models for the prediction of variation of scour depth with time have been developed and validated, which could be used to assess the scour effects generated by impinging turbulent jets. The overall temporal evolution of the scour depth can be predicted by the erosion parameter E c and the dimensionless time to reach equilibrium τ ∞ , which is superior to existing models in predicting the scouring process. • The scour hole profiles were recorded with the optical method in order to present dynamic scour hole characteristics. • A semi-empirical equation has been developed for the maximum dimensionless scour depth in equilibrium state. • Models for the prediction of variation of scour depth with time have been developed and validated. [ABSTRACT FROM AUTHOR]

Nanofibers based on phenanthroimidazole derivatives PCC, PDC, and PSC were fabricated by organogelation processes, and their fluorescence sensory properties towards acid were investigated. It was found that the emission of PCC in the nanofiber-based film could be quenched significantly upon exposure to gaseous TFA due to the formation of protonated PCC, in which ICT (intramolecular charge transfer) would occur. On the other hand, TFA vapor led to the emitting colors of PDC and PSC in the nanofiber-based films to turn to yellow and green from sky blue and blue, respectively. Additionally, we found that the decay times of PCC were 0.1 s and 1.9 s in probing the saturated vapor of TFA in nanofiber-based film and in spin-coated film, respectively. The results suggested that the high surface-to-volume ratio and large interspace in the nanofiber-based networks favored the enhanced adsorption, accumulation, and diffusion of gaseous molecules, resulting in such a high performance. [ABSTRACT FROM AUTHOR]

β-Iminoenolate boron complexes bearing non-planar phenothiazine (P2B and P16B) and phenothiazine-S,S-dioxide (PO2B and PO16B) have been synthesized. It was found that P2B and P16B gave ICT emission due to the strong electron-donating ability of phenothiazine, and no ICT emission was detected for PO2B and PO16B because of the weak electron-donating ability of phenothiazine-S,S-dioxide. In particular, P2B and P16B exhibited high-contrast MFC behaviors compared with that of PO2B. On account of the formation of π-aggregates in the as-synthesized crystals of P2B, several channels for the changes in emitting colors in different solid states were provided. The as-synthesized crystals of P2B emitting weak brown light could be changed into ground powders emitting deep orange-red light after grinding. When the ground powders were fumed with DCM for 5 s or heated at 160 °C for 3 s, the samples emitted yellow and yellowish green light, respectively. Although the π-conjugated skeleton of P16B was the same as that of P2B, the introduction of a long carbon chain led to different MFC properties. The as-synthesized crystals of P16B emitting yellow light could be changed into ground powders emitting orange light upon grinding. Particularly, the amorphous state of P16B could be self-recovered to the crystalline state when placed at room temperature for 5 min because the π–π interactions might be restrained by the long carbon chain, leading to low stability of the amorphous state. In the case of PO2B, low-contrast and reversible MFC properties were observed under grinding/fuming or heating treatment. No mechanofluorochromism was observed for PO16B since a long carbon chain was involved and no ICT emission happened. Therefore, we suggested that the introduction of a non-planar conjugated unit and the construction of a D–π–A system with ICT emission were favorable for designing MFC dyes with high-contrast changes in emitting colors in different solid states, and the introduction of a long carbon chain could improve the MFC reversibility so as to gain self-recovering MFC materials. [ABSTRACT FROM AUTHOR]

D–π–A type phenothiazine modified triphenylacrylonitrile derivatives PVTPAN, P3TPAN and P10TPAN have been synthesized. The three compounds exhibited AIE behaviors, and the emission enhancement of PVTPAN and P3TPAN, in which 3-position in phenothiazine was functionalized, was more significant than P10TPAN (10-position of phenothiazine was modified) with more twisted conformation. The single crystal structures revealed that the multiple intermolecular interactions, including π–π interactions and hydrogen bonds of C–H…π and C–H…N, would lock the molecular conformation and reduce the energy loss via non-radiative channels, leading to enhanced emission. Interestingly, the as-synthesized crystals of PVTPAN, P3TPAN and P10TPAN emitting strong yellow, yellowish-orange and yellowish-green light under UV irradiation could be transferred into powders emitting red, orange-red and orange light, respectively, after grinding. Such mechanochromism was reversible upon the treatment of grinding and heating/fuming with DCM. The XRD patterns suggested that the mechanochromism originated from the transition between the crystalline and amorphous states. It should be noted that the recovery time of the emission of the ground powders to that of crystals for P10TPAN was much shorter than those for PVTPAN and P3TPAN at the same temperature. Extremely, the emission of the ground powders of P10TPAN could recover at room temperature, but that of PVTPAN and P3TPAN could not. We suggested that such different mechanochromic behaviors of P10TPAN from PVTPAN and P3TPAN were coming from its more twisted conformation, which could lead to the exothermic peak appearing at lower temperatures and the easier molecular rearrangement of the amorphous state into crystals. Therefore, we could prepare the dyes with stable or reversible mechanochromism by tuning the molecular conformation. [ABSTRACT FROM AUTHOR]

Carbazole and tert-butylcarbazole functionalized β-iminoenolate boron complexes CB and TCB with different mechanofluorochromic (MFC) properties have been synthesized. It was found that the as-synthesized crystal of CB emitted orange light under UV illumination because of the appearance of the emission from the monomers and the excimers. After grinding for a while, the ground powder 1 of CB emitted bright yellow light since the ratio of the emission intensity from excimers/monomers decreased. On further grinding for a long time, the excimers disappeared and the obtained amorphous ground powder 2 emitted dark green light. However, TCB emitted sky blue light in the as-synthesized crystal because no excimer was formed due to the steric hindrance of tert-butyl. After grinding, the amorphous ground powder of TCB emitted bright green light derived from excimers. It should be noted that the fluorescence quantum yield of TCB in an amorphous solid state reached 0.53, which was the highest one for the β-iminoenolate boron complexes that have ever been reported. In addition, the emission changes of CB and TCB in different solid states were reversible upon repeating the treatment of mechanical grinding and fuming with CH2Cl2. Therefore, the obtained β-iminoenolate boron complexes might be used as sensors and memory chips on the basis of the solid fluorescence response to external mechanical forces and organic solvents. [ABSTRACT FROM AUTHOR]

In the marine fire suppression system, continuous delivery of dry chemical powder to the fire source with long powder discharge range and high dispersion concentration is essential. The work is devoted to experimental and numerical studies of the flow characteristics of the dry chemical powder jet from a horizontal injector with a wide range of Stokes numbers between 6 to 30 and Reynolds numbers between 4792 to 23,960 by considering the effect of gravitational acceleration. A CFD-based Eulerian–Eulerian multiphase model combined with Standard k- ω turbulence model was used to predict flow characteristics of particle-laden jet using dimensionless numbers, including the solid volume fraction, the normalized velocity magnitude, and the turbulent viscosity ratio. Experimental studies have been carried out for three different inflow velocities (2.06, 2.45, and 2.81 m/s). The results indicate that the particle density plays a significant role in the dispersion of the particles in the radial and axial directions. The transition from U-shaped to V-shaped solid dispersion structure on the ground can be captured with the increase of particle density. Moreover, the higher level turbulence intensity enhances the solid dispersion concentration. Finally, it was found that the Portland cement powder exhibits better discharge performance in terms of solid discharge range and dispersion concentration in comparison with other dry powders. These results have implications in the design of powder-based fire suppression system. Further studies should aim to the in-depth research on the fire extinguishing mechanism of the Portland cement powder, especially the fire suppression effectiveness and thermal decomposition process. [ABSTRACT FROM AUTHOR]