Mechanoluminescent (ML) sensor consisting of polymer and functional ceramic powder (SrAl₂O₄:Eu²⁺) emits intensive green light repeatedly accompanied by mechanical stimulation even in elastic defamation region. The ML intensity is proportional to Mises strain energy. Thus, when dispersedly coated onto a structure, each particle acts as a sensitive mechanical sensor, while the two-dimensional (2D) emission pattern of the whole assembly reflects the dynamical strain/stress distribution. Actually, we have successfully visualized (1) a crack existing, generation and propagation in structural health monitoring (SHM), (2) strain distribution in 3D designed industrial parts such as car body and various joints for light weighting. In addition, we introduce recent mechanoluminescent studies on evaluating functional degradation of structural materials, such as (3) detection of plastic defamation area, (4) CFRP (carbon fiber reinforced plastic) (5) delamination of dissimilar material adhesive joint and functional recovering for circular economic industries.

Adhesive bonding is an interfacial phenomenon that is critical for assembling polymer-matrix composite materials, enabling them to maintain their lightweight and high-stiffness properties. However, the lack of understanding of the adhesion mechanisms at the molecular level has limited the reliability of adhesive bonding for industrial uses. In this article, using soft X-ray microscopy, we demonstrate the visualization of one of the important factors of adhesion; that is, physical and chemical states at the adhesive interface. For this purpose, we prepared a model adhesive interface composed of a thermosetting epoxy resin adhered to a plasma-pretreated thermoplastic resin. We succeeded in observing multiscale phenomena in the adhesion mechanisms, including sub-mm complex interface structure, sub-μm distribution of the functional groups, and molecular-level covalent-bond formation. These results provide a benchmark for further research to examine how physical and chemical states correlate with adhesion, and demonstrate that soft X-ray imaging is a promising approach for visualizing the physical and chemical states at adhesive interfaces from the sub-mm level to the molecular level.

Polymer composites with inorganic fillers such as glass fiber, talc, clay, etc., are industrially important due to their improved properties over the original polymer materials. Interfacial adhesion between matrix and filler is a key factor that influences the properties of composites. Despite numerous studies related to this issue, the details of the mechanisms of interfacial adhesion between matrix and filler are not yet fully understood due to a lack of techniques for characterizing interfacial structure at the molecular level that occur in small areas of the polymer system. Fourier transform infrared (FTIR) microscopic techniques combined with two-dimensional (2D) correlation spectroscopy has been developed as a technique that can probe the chemical structures (formation of interactions and chemical bonds) related with the interfacial adhesion. This paper intends to provide a comprehensive overview of recent progress for visualization techniques of matrix-filler structures of polymer composites using FTIR microscopy.

Strain sensors are a key for the development of next-generation soft robots. For such applications, the electronic sensing using flexible electronic materials are continuing to be at the forefront of the sensing technology. Recently, optical-sensing technology have attracted much attention because of high spatiotemporal resolution, high sensitivity and noninvasively. Especially, chiral-nematic liquid-crystalline elastomers have much attention to be applied to optical strain sensors because of the change of selective reflection wavelength in response to the strain. Here, we introduce our recent research on optical strain sensors using chiral-nematic liquid-crystalline elastomers. There is remarkable maturity in the development of numerous stimuli-responsive liquid-crystalline elastomers. However, the control of the recovery process after the removal of the stimuli has remained difficult. Interestingly, our simple materials design concept of “layering the elastomers with other materials” allows us to arbitrary control the recovery process in both macroscopic deformation and microscopic molecular orientation. Based on the concept, we have succeeded to fabricate a mechano-optical sensor with high sensitivity and high spatiotemporal resolution. This concept would open a pathway to control the recovery process in various stimuli-responsive liquid-crystalline elastomers.

X-ray computed tomography (CT) is a non-destructive three-dimensional visualization method that uses the high penetrating power of hard X-rays with energy of 10 keV or higher to visualize the inside of a sample. We realized 4D (3D＋time) X-ray CT with a temporal resolution of 10 ms and a pixel size of 4.2 μm using a white synchrotron beam from a bending magnet source of the third-generation synchrotron facility. Real-time observation of the tensile fracture process of a tire rubber sample was successfully achieved for the first time. It is expected that our approach has various applications, such as mechanical fracture of soft materials, adhesive interface fracture, friction, and wear mechanisms.