Semi-crystalline polymer (SCP) has a very complex hierarchical structure, consisting of laminated crystalline and amorphous phases. Therefore, the macroscopic mechanical behavior of SCP relates to several different deformation mechanisms. Double yielding (DY) is one of the important mechanical behaviors of SCP, different from that of glassy polymer. The DY phenomenon can be accounted for by two yield criteria related to interactions between the crystalline and amorphous phases. Several mechanisms have been proposed to explain the DY phenomenon. The important point is that the thermal history of the SCP significantly affects DY. This indicates that DY is characterized by a molecular structure change in the amorphous and crystalline phases. In this paper, several kinds of proposed mechanisms for the DY phenomenon and the mechanical modeling of SCP characterized by the thermal history and strain rate are introduced.

This study investigated the effects of strain rate and loading mode on the strength properties and fracture morphology of epoxy adhesive joints by quasi-static tests and dynamic tests using the Split Hopkinson Bar (SHB) method. It was found that the bonding strength showed strain rate dependence for both tensile and shear loading modes. In addition, the loading mode dependence appeared, i.e. the strength under tensile loading higher than that under shear loading in both quasi-static and SHB tests. Furthermore, the fracture morphology differs depending on the loading mode, and in particular, voids are generated on the fracture surface during tensile deformation in the high strain rate range. Molecular Dynamics simulations were employed to evaluate the deformation modes of molecular chains in order to investigate the loading mode. It is found that the effect of bond stretching is larger in tensile deformation, while the effect of bending angle was larger in shear deformation. This difference in molecular deformation is thought to be responsible for the difference in loading mode. Furthermore, this comparison under tensile and shear loadings suggested the mechanism of void generation, which was experimentally observed during tensile deformation in the high strain-rate.

The mechanical model using fractional derivative viscous elements has been studied to represent mechanical behavior of viscoelastic materials. The present study explains the theoretical representation on the mechanical model using fractional derivative viscous elements which provides series expressions using Mittag-Leffler function for relaxation modulus, creep compliance, and complex modulus. Then, it is verified the practical aspects of this mechanical model with respect to the identification of parameters of this mechanical model. As the result, the series expressions using Mittag-Leffler function can be represent relaxation modulus less parameters than the Prony series based on conventional viscoelastic mechanical model. Furthermore, the time-temperature superposition principle can be applied to the mathematical representation of the series by Mittag-Leffler function. Also, it is shown that the five-parameter model based on the fractional derivative viscous elements can approximate the complex modulus even if the shape of the loss modulus is asymmetric with respect to its peak, and the relationship between complex modulus and relaxation modulus based on the five-parameter model is represented.

Polymeric materials exhibit strong temperature- and frequency-dependent viscoelastic behavior, and understanding their creep and relaxation mechanisms is essential for predicting long-term mechanical reliability. Conventional dynamic mechanical analysis (DMA) is limited to low-frequency measurements, making it difficult to evaluate the viscoelastic response in the GHz range relevant to modern electronic and structural applications. In this study, molecular dynamics (MD) simulations were performed to investigate the high-frequency viscoelastic behavior of amorphous polypropylene (PP) at 1 GHz. Periodic tensile deformation was applied, and the storage modulus E’and loss tangent tanδwere determined from the phase relationship between stress and strain. The results showed that E’decreased with increasing temperature, while tanδexhibited a pronounced peak near 300 K, corresponding to the glass transition temperature identified from a change in slope of the volume–temperature curve. Furthermore, the hysteresis loop area and potential energy variation per cycle increased proportionally with tanδ, demonstrating that molecular-scale energy dissipation governs macroscopic loss behavior. This research confirms the effectiveness of MD-based vibrational analysis for quantitatively evaluating viscoelastic mechanisms in polymers.

The application of Japanese traditional Kirigami techniques in engineering has great potential for expanding material functionality. In this study, we investigated the mechanical behavior of Kirigami-inspired honeycomb-like structures formed by applying tensile force to slit-patterned sheets, with the aim of evaluating their potential as cushioning materials. Three types of mechanical tests were conducted on specimens with different slit dimensions: tensile tests, out-of-plane displacement measurements, and compression tests. These tests evaluated the influence of slit dimensions on stretching properties, the formability of the honeycomb-like structures, and energy absorption characteristics during compression. The tensile tests revealed that structures with a lower ratio of honeycomb cell wall height to slit length exhibited greater elongation under smaller loads indicating better conformability to the shape of protected objects. Additionally, the results of the out-of-plane displacement measurement showed that a shorter slit length increases the force required for buckling, which could potentially affect the formability of the honeycomb-like structure by preventing the cell walls from rising properly. Finally, structures with taller cell walls exhibited higher energy absorption during compression tests. This indicates that the energy absorption capacity of the structures was significantly influenced by the amount of out-of-plane deformation.

Deflection iso ne ofth e mostim portantin dicators ofbr idgein tegrity inb ridgem aintenance managemet. nAmong deflections, dead load deflection is subject to regular bridge inspections. However, measuring dead load deflection involves long-term intermittentm easurements, necessitatingc onsideration ofre positioningm easurementeq uipmenta nd changes in the surrounding environment. Therefore, we propose a method for measuring bridge dead load deflection using a Laser level. This study formulates deflection equations for cases requiring reinstallation of measurement equipment and conducted verification experiments using model girders, confirming the feasibility of high-precision measurements. ltal so examined the impacto fa mbientli ghtco lor changes on laser detection.