A Possibility for Quantitative Detection of Mechanically-Induced Invisible Damage by Thermal Property Measurement via Entropy Generation for a Polymer Material.

Entropy generation from a mechanical and thermal perspective are quantitatively compared via molecular dynamic (MD) simulations and mechanical and thermal experiments. The entropy generation values regarding mechanical tensile loading-which causes invisible damage-of the Polyamide 6 (PA6) material are discussed in this study. The entropy values measured mechanically and thermally in the MD simulation were similar. To verify this consistency, mechanical and thermal experiments for measuring entropy generation were conducted. The experimentally obtained mechanical entropy was slightly less than that calculated by MD simulation. The thermal capacity is estimated based on the specific heat capacity measured by differential scanning calorimetry (DSC), applying the assumed extrapolation methods. The estimated entropy generation was higher than the aforementioned values. There is a possibility that the entropy-estimating method used in this study was inappropriate, resulting in overestimations. In any case, it is verified that entropy increases with mechanical loading and material invisible damage can be qualitatively detected via thermal property measurements.

Molecular Dynamics Simulation for Evaluating Fracture Entropy of a Polymer Material under Various Combined Stress States.

Herein, the stress-state dependence of fracture entropy for a polyamide 6 material is investigated through molecular dynamics simulations. Although previous research suggests that a constant entropy increase can be universally applied for the definition of material fracture, the dependence of stress triaxiality has not yet been discussed. In this study, entropy values are evaluated by molecular dynamics simulations with varied combined stress states. The calculation is implemented using the 570,000 all-atom model. Similar entropy values are obtained independently of stress triaxiality. This study also reveals the relationship between material damage, which is correlated with void size, and the entropy value.

Formulation of non-linear viscoelastic-viscoplastic constitutive equation for polyamide 6 resin.

In this study, a non-linear viscoelastic-viscoplastic constitutive equation for polyamide 6 (PA6) is formulated and a new model is suggested for the viscoplastic part of the equation. The suggested model is empirical but can accurately predict the viscoplastic strain. In this study, creep and recovery tests are conducted to evaluate the viscoplastic strain. Using a non-linear dashpot, a viscoplastic strain is formulated and its parameters are identified for PA6. In addition, a stress relaxation test is conducted, and the relationship between the viscoelastic strain and stress is identified when considering the viscoplastic strain. In this study, the time-temperature superposition principle is thoroughly applied to include the effect of elevated temperature on the viscoelastic-viscoplastic behavior. All material constants in the non-linear viscoelastic-viscoplastic constitutive equation including the time-temperature superposition principle for PA6 are presented in this study.

Preliminary study of optimal measurement location on vibroarthrography for classification of patients with knee osteoarthritis.

[Purpose] The aims of the present study were to investigate the most suitable location for vibroarthrography measurements of the knee joint to distinguish a healthy knee from knee osteoarthritis using Wavelet transform analysis. [Subjects and Methods] Participants were 16 healthy females and 17 females with severe knee osteoarthritis. Vibroarthrography signals were measured on the medial and lateral epicondyles, mid-patella, and tibia using stethoscopes with a microphone while subjects stood up from a seated position. Frequency and knee flexion angles at the peak wavelet coefficient were obtained. [Results] Peak wavelet coefficients at the lateral condyle and tibia were significantly higher in patients with knee osteoarthritis than in the control group. Knee joint angles at the peak wavelet coefficient were smaller (more extension) in the osteoarthritis group compared to the control group. The area under the receiver operating characteristic curve on tibia assessment with the frequency and knee flexion angles was higher than at the other measurement locations (both area under the curve: 0.86). [Conclusion] The tibia is the most suitable location for classifying knee osteoarthritis based on vibroarthrography signals.

Evidence of Quantum Resonance in Periodically-Ordered Three-Dimensional Superlattice of CdTe Quantum Dots.

Semiconductor quantum dot (QD) superlattices, which are periodically ordered three-dimensional (3D) array structures of QDs, are expected to exhibit novel photo-optical properties arising from the resonant interactions between adjacent QDs. Since the resonant interactions such as long-range dipole-dipole Coulomb coupling and short-range quantum resonance strongly depend on inter-QD nano space, precise control of the nano space is essential for physical understanding of the superlattice, which includes both of nano and bulk scales. Here, we study the pure quantum resonance in the 3D CdTe QD superlattice deposited by a layer-by-layer assembly of positively charged polyelectrolytes and negatively charged CdTe QDs. From XRD measurements, existence of the periodical ordering of QDs both in the lamination and in-plane directions, that is, the formation of the 3D periodic QD superlattice, was confirmed. The lowest excitation energy decreases exponentially with decreasing the nano space between the CdTe QD layers and also with decreasing the QD size, which is apparently indicative of the quantum resonance between the QDs rather than a dipole-dipole Coulomb coupling. The quantum resonance was also computationally demonstrated and rationalized by the orbital delocalization to neighboring CdTe QDs in the superlattice.

Evaluation of damage progression and mechanical behavior under compression of bone cements containing core-shell nanoparticles by using acoustic emission technique.

In this work, the effect of the incorporation of core-shell particles on the fracture mechanisms of the acrylic bone cements by using acoustic emission (AE) technique during the quasi-static compression mechanical test was investigated. Core-shell particles were composed of a poly(butyl acrylate) (PBA) rubbery core and a methyl methacrylate/styrene copolymer (P(MMA-co-St)) outer glassy shell. Nanoparticles were prepared with different core-shell ratio (20/80, 30/70, 40/60 and 50/50) and were incorporated into the solid phase of bone cement at several percentages (5, 10 and 15 wt%). It was observed that the particles exhibited a spherical morphology averaging ca. 125 nm in diameter, and the dynamic mechanical analysis (DMA) thermograms revealed the desired structuring pattern of phases associated with core-shell structures. A fracture mechanism was proposed taking into account the detected AE signals and the scanning electron microscopy (SEM) micrographs. In this regard, core-shell nanoparticles can act as both additional nucleation sites for microcracks (and crazes) and to hinder the microcrack propagation acting as a barrier to its growth; this behavior was presented by all formulations. Cement samples containing 15 wt% of core-shell nanoparticles, either 40/60 or 50/50, were fractured at 40% deformation. This fact seems related to the coalescence of microcracks after they surround the agglomerates of core-shell nanoparticles to continue growing up. This work also demonstrated the potential of the AE technique to be used as an accurate and reliable detection tool for quasi-static compression test in acrylic bone cements.

Temperature-dependent water solubility of iodine-doped single-walled carbon nanotubes prepared using an electrochemical method.

We demonstrate that iodine-doping into single-walled carbon nanotubes (SWCNTs) can be effectively done using an electrochemical method. It is shown by in situ Raman measurements that the iodine-doping level can be easily and finely controlled because de-doping is also possible by changing the polarity. In situ synchrotron XRD measurements reveal that iodine molecules are mainly inserted into the hollow core of SWCNTs. The dispersion state of the iodine-doped SWCNTs in water as a function of temperature is also investigated. It is shown that the iodine-doped SWCNTs can be homogeneously dispersed in water at low temperature (ca. <15 °C).