Research progress of residual stress measurement methods.

Residual stress refers to self-equilibrating stress present within materials, with the potential to significantly affect manufacturing processes and performance. Therefore, accurately and quantitatively measuring residual stress is always of great importance. This study provides a comprehensive review of various characterization techniques for residual stress, including their principles, development history, applications, and limitations. Initially, several destructive techniques such as the hole-drilling method, ring-core method, deep hole drilling method, slitting method, and contour method are summarized. Subsequently, three nondestructive techniques based on X-ray/electron diffraction, magnetic signals, and ultrasonic signals are evaluated. In the final part of this overview, special attention is given to a newly-developed technique for measuring residual stress, which combines incremental focused ion beam (FIB) milling and digital image correlation (DIC). Our review aims to guide further investigations on residual stress and identify the future development of techniques for measuring residual stress.

Mechanisms of Heat-Treatment-Induced Cracking in Additively Manufactured IN738 Alloy.

The present study conducts a comprehensive study on heat-treatment-induced cracking of Inconel 738 (IN738) alloy fabricated by laser powder bed fusion (LPBF) using scanning electron microscopy (SEM), energy dispersion spectrum (EDS), and electron backscatter diffraction (EBSD). The results indicate that the macroscopic crack is dominantly triggered by the strain-age cracking mechanism and propagates along grain boundaries. The initiation of cracking is facilitated by the superimposition of residual stress induced by the LPBF process and contraction stress induced by precipitation, while the reopening of compress pores at grain boundaries weakens the grain boundaries and provides fast channels for cracking. These results revealed the coupling effects in triggering heat-treatment-induced cracking, offering a fundamental guideline for crack control during heat treatment of additively manufactured IN738 alloy.

Detection of Sudan Dyes Based on Inner-Filter Effect with Reusable Conjugated Polymer Fibrous Membranes.

Developing effective methods for detecting illegal additives in food or seasoning is of great significance. In this study, a sensing strategy for selective detection of Sudan dyes was designed based on the fluorescence inner-filter effect (IFE) by using poly(phenylenevinylene) (PPV) solid materials in combination with an optimized experimental protocol. Two types of fluorescent solid materials, electrospun fibrous membranes and drop-cast films, were fabricated with PPV as the fluorophore and poly(vinyl alcohol) as the matrix, respectively. Sudan dyes greatly quenched the fluorescence of the membrane and film, whereas other food colorings or possible food ingredients displayed a much smaller or negligible quenching effect. The sensing mechanism was studied, and the selectivity was ascribed to IFE, which requires the overlap between the absorption of the analyte and absorption/emission of the sensing material. The form of materials (membrane or film), the content of PPV, and the cross-linking process did not have much influence on the selectivity and sensitivity, which is consistent with the IFE mechanism and demonstrates the advantage of not requiring strict control of the preparative process. All the cross-linked materials were found to be stable against water/humidity and displayed good reversibility in sensing and can be reused at least for 10 cycles with negligible influence on the sensing performance. A cross-linked membrane was selected for detecting Sudan dyes in chili powder because folding did not affect the mechanical stability of the membrane. Two different protocols were used to pretreat the chili samples, which allowed the detection of Sudan dyes in chili powder as well as the discrimination of Sudan dyes from synthetic food coloring such as allura red. This study provides a facile and cost-effective method for preparing reusable sensing materials for detecting some dyes in commercial foods or food seasonings.

Fluorescence Quenching of a Conjugated Polymer by Synergistic Amine-Carboxylic Acid and π-π Interactions for Selective Detection of Aromatic Amines in Aqueous Solution.

Fluorescence sensing of amine in aqueous solution is challenging. The various basicity and chemical structures of amines may lead to poor selectivity in aqueous solution, and selective fluorescence detection of primary aromatic amine is rarely reported. This paper presents design and synthesis of a fluorescent conjugated polymer for rapid and selective sensing of aromatic amines in aqueous solution. The fluorescent conjugated polymer, poly[fluorenyl-alt-p-phenyleneethynylene] with pendant carboxylic acid groups and long alky chains, is synthesized via palladium-catalyzed Sonogashira coupling reaction. The fluorescence of the polymer is selectively quenched by the aromatic amines in aqueous solution, whereas the aliphatic amines enhance the fluorescence of the polymer. The high selectivity to the aromatic amines, particularly to the environmentally important p-phenylenediamine, likely originates from the amplified π-π fluorescence quenching synergized by amine and carboxylic acid interaction. Our results demonstrate an effective material design strategy that may be extended to fluorescence sensing of other aromatic compounds.

Preparation of fluorescent conjugated polymer fibrous membranes for rapid recognition of aromatic solvents.

Fluorescent poly(phenylenevinylene) (PPV)/poly(vinyl alcohol) (PVA) fibrous membrane was prepared via electrospinning of PPV precursor and PVA aqueous solution followed by thermal elimination. Further cross-linking produced the cross-linked membrane PPV/CPVA. Both PPV/PVA and PPV/CPVA membranes were found to have similar morphology and photophysics. These membranes showed a great fluorescence quenching response to aromatic solvents and a much smaller response to other organic solvents. Water also effectively quenched the fluorescence of PPV/PVA but not that of PPV/CPVA. This was attributed to un-cross-linked PVA being able to dissolve in water and the cross-linking improving the resistance of the membrane toward water. The sensing behavior was found to have good reversibility. The contact angle study showed that addition of only about 1% of PPV into the matrix reduced the hydrophilicity of the membrane significantly, suggesting that the PPV chains would be located at the surface of the fibers. X-ray photoelectron spectroscopy (XPS) investigation further confirmed such surface enrichment of PPV in the binary polymer blends. The PPV chain on the surface facilitated the π-π interaction between the polymer backbones and the aromatic molecules, thus leading to good selectivity and fast response of the two fibrous membranes toward aromatic solvents.

Receptor-free poly(phenylenevinylene) fibrous membranes for cation sensing: high sensitivity and good selectivity achieved by choosing the appropriate polymer matrix.

Poly(phenylenevinylene)/polyimide (PPV/PI) and poly(phenylenevinylene)/ polymethylmethacrylate (PPV/PMMA) fibrous membranes without any deliberately introduced receptors were prepared as fluorescence sensing materials through electrospinning, followed by thermal treatment. Both of these membranes displayed higher sensitivity toward most cations compared to the corresponding spin-coated films. PPV/PMMA membranes were more sensitive than PPV/PI membranes toward Cu(2+) and Fe(3+). About 4.5 fold of intensity enhancement upon 20 nM of Cu(2+), 80% of quenching upon 20 nM of Fe(3+) with fast response and simple regeneration were realized for PPV/PMMA membrane. The preliminary investigation into the mechanism revealed that the properties of the polymer matrix and thermal treatment of the membrane played important roles in the sensing performance.