Abstract: This work presents an automated optimization strategy for steel truss structures capable of undergoing elasto-plastic response, targeting material-efficient designs without compromising structural reliability. The mechanical behavior is evaluated through a nonlinear finite element formulation accounting for both large-deformation effects and material yielding, where complementary strain energy associated with residual forces is adopted as a measure for governing inelastic evolution. Shape and size are optimized concurrently by treating nodal coordinates and cross-sectional areas as design variables, allowing exploration of a broader solution space and the emergence of improved structural layouts. To enhance search performance, a neural-network-based surrogate is embedded into the genetic algorithm, enabling adaptive learning from evolving candidate solutions and guiding the optimizer toward regions with higher potential for near-global optimality. A benchmark application demonstrates that the proposed hybrid framework achieves more favorable configurations than a conventional genetic algorithm, characterized by reduced material consumption and limited plastic engagement within predefined acceptability criteria, highlighting the promise of data-driven evolutionary procedures for advanced truss design under nonlinear conditions.

Keywords: Optimization, elasto-plastic design, steel trusses, genetic algorithm, neural network.

Abstract: Various adhesive strengths are governed by the intensity of the singular stress field generated at the interface edge, referred to as the Intensity of Singular Stress Field (ISSF). When the singular stress fields in a test specimen and an actual structure are geometrically similar, strength evaluation based on ISSF has been shown to be highly effective. However, since the singular stress field varies depending on the local geometry at the adhesive edge, the ISSF method cannot be directly applied to joints with different edge geometries.

In this study, a unified approach for evaluating adhesive strength under different singular stress fields is proposed by introducing a fictitious edge interface crack at the adhesive interface edge.

(1) The proposed edge interface crack, whose length is less than 1% of the adhesive layer thickness, establishes a one-to-one correspondence between the stress intensity factor (SIF) of the crack and the ISSF of an interface edge without a crack. For example, the adhesive strength of butt joints varying with adhesive layer thickness can be represented by a constant critical value K_1c of the edge interface crack.
(2) In the proposed edge interface crack method, direct comparison becomes possible even when the singularity indices at interface edges differ, because the SIF has a common physical unit. Furthermore, the dominant fracture mode—debonding or shear failure—can be readily identified from the relative magnitudes of K₁and K₂.
(3) To evaluate adhesive strength for different local geometries, an exact general solution for the stress intensity factors of an edge interface crack applicable to butt joints is derived, and its usefulness is demonstrated as follows.
(4) For lap joints with different edge geometries, the nominal average fracture stress may differ by more than a factor of two. It is shown that these differences can be consistently explained by a constant critical value K_1c using the edge interface crack method.
(5) Based on numerous experimental results, the nominal average fracture strength of butt joints is approximately twice that of lap joints. By introducing a fictitious edge interface crack with a crack length a=10^-4 mm at the interface edge, the adhesive strengths of butt joints (BJ) and lap joints (LJ) can be directly compared. The obtained critical values are $K_{1c}^{(B,ave)} = (12.2 \pm 4.70) \times 10^{-2}$ $\text{MPa}\sqrt{\text{m}}$ for butt joints and $K_{1c}^{(L,ave)} = (67.9 \pm 24.8) \times 10^{-2}$$\text{MPa}\sqrt{\text{m}}$ for lap joints, giving $(K_{1c}^{(L,ave)}/K_{1c}^{(B,ave)}) \approx 5.5$, indicating that lap joints exhibit higher strength than butt joints, consistent with common engineering understanding.
(6) Since the strengths of butt joints and lap joints can be expressed by constant values of K_1c or K_2c, and considering the ratio $(K_{1c}^{(L,ave)} / K_{1c}^{(B,ave)}) \approx 5.5$, the strengths specified for JIS standard specimens with adhesive thickness h=0.1mm were predicted. The predicted ratio $\sigma_c^B(\text{JIS}) / \tau_c^L(\text{JIS}) = 1.65$ agrees well with the experimental ratio $\sigma_c^{(B,ave)} / \tau_c^{(L,ave)} \approx 1.7$.

These results demonstrate that the edge interface crack method provides a practical and unified framework for evaluating adhesive strength across different geometries and loading conditions.

Keywords : Adhesive strength, butt joint, lap joint, singularity index, stress intensity factor, edge interface crack.

Abstract: The scatter of the fatigue life of additive manufacturing specimens is quite large because of the defects dispersed in material. Due to the various defects and the large life scatter of additive manufacturing material, the relationship between life and defect characterization parameters such as defect type, defect location, and defect size are complex. This paper analyses the effect of defects on fatigue life from the perspective of statistical sense. The statistical results based on both the minimum life of the samples under the same stress level and the half of the samples with the fatigue life less than the median show that the defect size has a great influence on the fatigue life. However, the "effective area", i.e., the projection area of a defect on the plane vertical the loading direction, is not an ideal parameter to characterize the defect. In other word, the sample with the maximum defect in term of the effective area is not necessarily the shortest life sample. Fatigue life is more sensitive to the defects on surface or near-surface than those inside a specimen. There is little difference in the hazards of the two types of defects to fatigue performance, porosity defect induced failure is slightly higher than non-fusion defect induced failure.

Keywords: Additive manufacturing material, defects, defect size, defect location, fatigue life.

Abstract: This study presents a novel method to determine the stress tensor for grain groups with preferred texture orientations and the Critical Resolved Shear Stresses (CRSSs) required to activate slip systems, applied to investigate the elastic-plastic behavior of textured duplex steel [1] and two-phase brass. The approach relies on in situ neutron diffraction measurements of lattice strains in ferritic and austenitic grains for steel, and in α- and β-phase grains for brass, during tensile testing. These measurements enabled direct experimental determination of the evolution of both the stress tensor and the Resolved Shear Stress (RSS) for groups of grains with similar orientations.
For the first time, CRSS values for slip systems in both phases of duplex steel and two-phase brass were directly obtained from experimental data. A key advantage of this methodology is that both grain stress tensors and CRSSs are determined for representative polycrystalline volumes without relying on elastic-plastic models.
Results show that heat treatment significantly hardens the ferritic phase in duplex steel, resulting in a markedly higher CRSS compared to the austenitic phase, strongly influencing the overall yield stress. In contrast, the CRSS values of α- and β-phase grains in two-phase brass are approximately equal, leading to more uniform mechanical behaviour.
Finally, the experimental data were compared with predictions from a multi-scale Elastic-Plastic Self-Consistent (EPSC) model using the experimentally obtained CRSSs as input. Direct determination of CRSS values reduces input parameters in multiscale models and allows verification of theoretical calculations.

Reference: [1] A. Baczmański, et al., International Journal of Mechanical Sciences 283 (2024) 109745.

Keywords: Elastic-plastic deformation, neutron diffraction, crystallographic slip, multiscale model
Acknowledgements: This work was financed by a grant from the National Science Centre, Poland (NCN), No. UMO-2023/49/B/ST11/00774.

Abstract: Crystallographic texture evolution during symmetric and asymmetric rolling of aluminum and copper was investigated experimentally and through modelling using the Finite Element Method (FEM) combined with crystal plasticity (CP) approaches. Rolling asymmetry was introduced by varying the ratio of the roll diameters, with the asymmetry coefficient defined as A=R₂ / R₁. A second type of asymmetry was generated by adjusting the material insertion angle α, defined as the angle between the incoming strip and the horizontal plane. Consequently, both flat and tilted entry configurations were examined. The variation of crystallographic texture across the thickness of the rolled bars was measured using X-ray diffraction and predicted using FEM–CP simulations. In parallel, the microstructure was characterized by Electron Backscatter Diffraction (EBSD). The dominant effect observed during asymmetric rolling is the homogenization of texture across the sample thickness, arising from the presence of strong shear stresses and strain components within the material. The extent of this homogenization depends on the selected rolling geometry parameters, namely A and α. Changes in texture distribution significantly influence the mechanical response of the material, particularly its plastic ductility and drawability. These effects manifest as an increased maximum strain at fracture and a reduction in planar plastic anisotropy. The developed computational software enables optimization of the rolling geometry parameters. Based on the modelling results, the most favorable rolling configurations have been identified and recommended for technological implementation.

Keywords: Rolling geometry, crystallographic texture, aluminium, X-ray diffraction, mechanical properties, numerical modeling

Abstract: The effect of long-term ageing on degradation of tensile properties and mechanism of a directionally-solidification superalloy DZ409 were studied, to simulate servicing of heavy industrial gas turbine blades during the long-term aged at 900℃ and 980℃. The results show that the spherical secondary γ' phase dissolves rapidly, the cubic γ' phase becomes smooth, and some γ' phases merge at 900℃ and 980℃. γ' phase coarsened continuously during long-term ageing, and the coarsening rate at 980℃ was much higher than that at 900℃. The MC carbides gradually transform into M₂₃C₆ and γ' phases. At 900℃ to 1000h and 980℃ to 100h, M₂₃C₆ began to precipitate at the MC/γ' interface. When (γ+γ') eutectic structure coexists with MC, M₂₃C₆ particles are gradually precipitated. No TCP phase was found after aged at 900℃/20000h and 980℃/3000h. Both the yield and tensile strength of the alloy at RT, 650℃ and 900℃ can be divided into three stages after aged at 900℃. The first stage (0~100/200h) has a rapid decline in strength, mainly due to the rapid dissolution of the spherical secondary γ' phase. In the second stage (100/200~2000h), the decline rate of the strength continued to slow down, mainly due to the coarsening of the γ' phase and the decrease of the cubic degree of the γ' phase, which led to the decrease of the mismatch of γ/γ', and the decomposition of MC carbide began. In the third stage (2500~8000h), the strength tends to be stable, which is because of the negative effect of continuous coarsening and merging of γ' phase on the deformation resistance of the alloy. The positive effect of M₂₃C₆ carbide formation on the pinning of dislocation and the inhibition of grain boundary slip makes the tensile properties of the alloy stable. Compared with the long-term aged at 900℃, the damage regularity of microstructure evolution and tensile properties of the alloy after 980℃ ageing is similar, but the change rate is faster.

Keywords: Long-term ageing, property degradation, directionally solidified superalloy DZ409, microstructure evolution.

Abstract: A large amount of primary energy is consumed in power plants, automobiles, and industrial facilities. However, only about one‑third of this energy is effectively utilized, while most of the remainder is dissipated as waste heat. To recover this unused thermal energy, thermoelectric conversion systems have attracted increasing attention from the viewpoint of the Sustainable Development Goals (SDGs). Inorganic thermoelectric materials have been extensively studied and are already employed in practical applications such as space probes (e.g., Voyager and Cassini). In recent years, organic thermoelectric materials based on conducting polymers, carbon nanotubes, and their composites have attracted growing interest because of their advantages, including low‑cost fabrication, low toxicity, abundant raw materials, mechanical flexibility, and solution processability. For practical applications, the fabrication of free‑standing and flexible thermoelectric films that can be handled without substrates is particularly important. In this invited presentation, simple methods for fabricating free‑standing organic thermoelectric films are introduced, based on two representative systems. First, free‑standing conducting polymer films are demonstrated, which can be obtained simply by washing ionic‑liquid‑modified PEDOT:PSS films in water, enabling easy peeling from the substrate. Second, free‑standing single‑walled carbon nanotube films prepared by vacuum filtration of their dispersions are presented, without the use of polymer binders or complicated post‑treatments. These examples highlight that free‑standing and flexible thermoelectric films can be fabricated through remarkably simple processes, providing a practical platform for organic thermoelectric devices.

Keywords: Organic thermoelectrics, free-standing film, conducting polymers, carbon-nanotubes.

Abstract: Ultrathin flexible organic optoelectronics have attracted growing interest for wearable and bio-integrated applications because their reduced thickness enables superior mechanical compliance and conformal contact compared with conventional flexible devices. These advantages are particularly important for human health monitoring, where intimate and stable contact with soft, curvilinear skin is essential for accurate signal acquisition. However, traditional flexible organic optoelectronic devices, typically thicker than 100 μm, often suffer from interfacial delamination, stress concentration, and degraded electrical stability under repeated deformation. Here, we present our previous efforts on the fabrication and structural engineering of printable ultrathin organic optoelectronic devices with total thicknesses below 10 μm. By combining interfacial material design with mechanical architecture optimization, we developed ultrathin device platforms that improve energy-level alignment, enhance interlayer adhesion, and reduce strain localization in multilayer structures. Representative devices based on ~1.5 μm polymer substrates and ultrathin encapsulation layers exhibited excellent flexibility, maintaining stable operation under bending radii down to 0.5 mm and after up to 10,000 bending cycles. Compared with conventional thick flexible electronics, these ultrathin architectures show greatly improved deformability, interfacial stability, and tolerance to cyclic mechanical loading. Our studies demonstrate that interface engineering, together with rational structural design, is essential for constructing mechanically robust and electrically stable ultrathin organic optoelectronics, providing useful guidelines for future wearable health monitoring, soft bio-interfaces, and highly compliant organic electronic skins.

Keywords: Ultrathin organic optoelectronics, interface engineering, flexible electronics, printable electronics, interfacial adhesion, cyclic deformation.

Abstract: Currently, most meta-structures with tailorable Poisson’s ratio were designed using isotropic materials such as polymers and metals. However, continuous fibre reinforced composite (CFRC) has demonstrated significant advantages when compared to these isotropic materials, but hitherto few relevant studies for CFRC meta-structures are reported. This study innovatively developed a concurrent level set-stream (L-S) topology optimization theory to acquire the CFRC meta-structures with customed Poisson’s ratios. First, the theory was proposed based on a parametric model that combines the level-set and stream functions to design the topological layout and fibre path of CFRC structures. Then, engineering structures with maximized stiffness or strength were designed to validate the method. Last, the theory was applied to high-precision design of CFRC meta-structures with customized Poisson’s ratio.

Keywords: Topology optimisation, continuous fibre-reinforced composite, negative Poisson's ratio, additive manufacturing, computational mechanics.

Acknowledgements: This work was supported by the National Natural Science Foundation of China (12302177), the Shenzhen Science and Technology Program, China (JCYJ20230807093602005), the Guangdong Basic and Applied Basic Research Foundation, China (2024A1515010203) and the Shenzhen Key Laboratory of Intelligent Manufacturing for Continuous Carbon Fibre Reinforced Composites (ZDSYS20220527171404011).

Abstract: Recently, MEMS accelerometers employing gold (Au) components prepared by electrodeposition have been reported to be small in size while retaining high sensitivity due to Au’s high mass density, enabling the detection of body tremors and muscular sounds. However, the low mechanical strength of Au compared to the Si-based materials used in conventional MEMS accelerometers leads to insufficient structural stability for long-term use of the device. To enhance the mechanical strength of Au-based materials, multi-layered metal technology has been proposed. Cantilever-like structures are commonly used in the movable components of MEMS devices. An effective strategy to enhance structural stability is to repeatedly perform electrodeposition of gold, followed by deposition of a material with high mechanical strength, to create a multi-layered structure. According to the Euler–Bernoulli beam theory, the structural stability of a cantilever-like structure can be improved by using materials with a high Young's modulus (E). However, although E is an intrinsic property of materials that should remain constant as the specimen size changes, the E of small specimens is reported to differ as the size changes. The E of a small specimen with a specific geometry is called the effective Young's modulus (E_eff). The E_eff of a micro-cantilever can be determined from its resonance frequency, which can be measured using a laser Doppler vibrometer. This report describes the preparation of various Ti/Au multilayered structures. Firstly, the effective permittivity of complex three-dimensional (3D) Ti/Au multilayered structures was determined from the resonance frequency obtained using a laser Doppler vibrometer. Next, the effects of these structures on long-term stability were studied using vibration tests. Finally, we discuss the relationship between Ti/Au multilayered structures with characteristic E_eff values and long-term structural stability. This could contribute to the design of highly sensitive and stable MEMS accelerometers.

Keywords: Electrodeposition, material evaluation, MEMS, inertial sensor, Young's modulus

Acknowledgements: We appreciate T. Sakai, S. Iida, and T. Konishi at NTT-AT Corp. for their technical support. This work was supported by JST, CREST Grant No. JPMJCR21C5, Japan

Abstract: We have been exploring how the exceptional properties of carbon nanotubes (CNTs) can be extended to macro-scale CNT-spun yarn to hasten their practical application. I will introduce our research on high-strength few-walled CNT (FWCNT) spun yarn as an alternative to commercially available carbon fibers.

We first established the key structural requirements of vertically aligned CNT arrays, including CNT height, density, and the number of CNTs, for reproducible and continuous yarn formation by direct drawing and twisting from CNT substrates. The untreated CNT spun yarns exhibited a tensile strength of approximately 1.5 GPa and a Young’s modulus of approximately 100 GPa. Therefore, a major challenge was to enhance their mechanical properties to those of high-performance aerospace-grade carbon fibers, which have tensile strengths of approximately 7 GPa and Young’s moduli of approximately 320 GPa.

To address this challenge, we developed a high-speed Joule annealing (JA) process in which an electric current is applied to CNT-spun yarns as they are continuously treated at 1 cm s-1. When the treatment temperature exceeded 3000 K, the intensity ratio of a graphitic G band (~1590 cm^-1) and a defect-induced D band (~1350 cm^-1) of the Raman spectra of the CNT-spun yarns increased markedly, indicating structural improvement in the CNTs, which contributed to enhanced tensile strength.

Furthermore, to suppress slippage between individual CNTs and CNT bundles, we developed a technique for introducing fine graphene oxide or graphene sheets into the nanoscale spaces within the CNT yarns. By combining this nano-space reinforcement strategy with high-temperature JA treatment, we achieved a tensile strength of 6.4 GPa and a Young’s modulus of 320 GPa. These values are close to those of the highest-strength commercial carbon fibers, demonstrating the potential of CNT spun yarns as next-generation lightweight, high-strength structural fibers．

Keywords: Carbon nanotubes (CNT), high-strength carbon nanotube spun yarn, Joule annealing, nano-space reinforcement

Acknowledgements: This work was partially supported by JSPS KAKENHI Grant Number JP24K00928.