Deep Transfer Learning Approach for Localization of Damage Area in Composite Laminates Using Acoustic Emission Signal.

Intelligent composite structures with self-aware functions are preferable for future aircrafts. The real-time location of damaged areas of composites is a key step. In this study, deep transfer learning was used to achieve the real-time location of damaged areas. The sensor network obtained acoustic emission signals from different damaged areas of the aluminum alloy plate. The acoustic emission time-domain signal is transformed into the input image by continuous wavelet transform. The convolutional neural network-based model automatically localized the damaged area by extracting features from the input image. A small amount of composite acoustic emission data was used to fine-tune some network parameters of the basic model through transfer learning. This enabled the model to classify the damaged area of composites. The accuracy of the transfer learning model trained with 900 samples is 96.38%, which is comparable to the accuracy of the model trained directly with 1800 samples; the training time of the former is only 17.68% of that of the latter. The proposed method can be easily adapted to new composite structures using transfer learning and a small dataset, providing a new idea for structural health monitoring.

Microfluidic Chip with Fiber-Tip Sensors for Synchronously Monitoring Concentration and Temperature of Glucose Solutions.

Monitoring the properties of fluids in microfluidic chips often requires complex open-space optics technology and expensive equipment. In this work, we introduce dual-parameter optical sensors with fiber tips into the microfluidic chip. Multiple sensors were distributed in each channel of the chip, which enabled the real-time monitoring of the concentration and temperature of the microfluidics. The temperature sensitivity and glucose concentration sensitivity could reach 314 pm/°C and -0.678 dB/(g/L), respectively. The hemispherical probe hardly affected the microfluidic flow field. The integrated technology combined the optical fiber sensor with the microfluidic chip and was low cost with high performance. Therefore, we believe that the proposed microfluidic chip integrated with the optical sensor is beneficial for drug discovery, pathological research and material science investigation. The integrated technology has great application potential for micro total analysis systems (µ-TAS).

Identification of Multiple Mechanical Properties of Laminates from a Single Compressive Test.

In-plane elastic and interlaminar properties of composite laminates are commonly obtained through separate experiments. In this paper, a simultaneous identification method for both properties using a single experiment is proposed. The mechanical properties of laminates were treated as uncertainties and Bayesian inference was employed with measured strain-load curves in compression tests of laminates with embedded delamination. The strain-load curves were separated into two stages: the pre-delamination stage and the post-delamination stage. Sensitivity analysis was carried out to determine the critical properties at different stages, in order to alleviate the ill-posed problem in inference. Results showed that the in-plane Young's modulus and shear modulus in elastic properties are dominant in the pre-delamination stage, and the interlaminar strength and type I fracture toughness in interlaminar properties are dominant in the post-delamination stage. Five times of property identification were carried out; the maximum coefficient of variation of identified properties was less than 1.11%, and the maximum error between the mean values of the identified properties and the ones from standard experiments was less than 5.44%. The proposed method can reduce time and cost in obtaining multiple mechanical properties of laminates.

Analysis and simulation of fracture behavior in naturally occurring Bouligand structures.

Through natural selection processes, refined biological materials could be created that adapt to various environments and exhibit specific functions. Such materials include typical Bouligand structures that can be widely observed in marine creatures that have hard shells. Consisting of a helicoidal arrangement of aligned fibrils, layered single-twisted Bouligand-type structures (SBS) display exceptional fracture and damage resistance. A much more primitive and rarer type of this formation, the double-twisted Bouligand-type structures (DBS), has been discovered in ancient fish scales, and this architecture could provide added rigidity and significantly contribute to toughness when facing fracture risk. In this work, we describe a computational modeling approach to investigating fracture behaviors and toughening mechanisms in Bouligand structures. To achieve qualitative insights into the fracture behaviors of DBS and SBS, we applied these two configurations, which were identified from corresponding biological materials, to analyze load-displacement responses during single edge notched (SEN) tensile testing; the toughening mechanism is also discussed further. The results clearly show that the arrangement of helix fibrils and interlaminar properties play a major role in the resulting fracture behaviors of Bouligand architectures. This is of interest for the future design of engineering materials and structures that require composites with enhanced toughness, and could deepen our understanding of the structure-property relationship of Bouligand-type structures in bionic design. STATEMENT OF SIGNIFICANCE: In this work, a novel numerical modeling approach based on the extended finite element method (XFEM) has been established to evaluate the fracture behavior of a naturally-occurring Bouligand-type helicoidal structure subjected to the single edge notched (SEN) tensile loading. The roles of the biological features (i.e., layered arrangement of collagen fibrils and interbundle fibrils) on the fracture resistance and toughening mechanism of the Bouligand-type structures have been uncovered and analyzed quantitatively. This is of interest for future design of engineering materials and structures that require composites with enhanced toughness, and can deepen the understanding of the structure-property relationship of the Bouligand-type structure in bionic design.

ZrW2O8/ZrO2 Composites with Low/Near-Zero Coefficients of Thermal Expansion Fabricated at Ultralow Temperature: An Integration of Hydrothermal Assembly and a Cold Sintering Process.

ZrW2O8/ZrO2 composites with tunable low/near-zero coefficients of thermal expansion (CTE) are promising candidates in several fields including aerospace, precision manufacturing and measurement, electronic circuit, etc., for counteracting the thermal expansion effect. However, bottleneck issues (such as the unstable decomposition of ZrW2O8 phase, manufacturing size limitation, etc.) caused by conventional high-temperature sintering impede the development and application of ZrW2O8/ZrO2. To solve these scientific issues, a methodology integrating hydrothermal assembly with a cold sintering process (CSP) is exploited. The ZrW2O8/ZrO2 composite powders with a mace-like structure, in which the spherical ZrO2 nanoparticles peripherally embed on the rod-like ZrW2O8 matrix particles, are hydrothermally assembled. Then, the relatively dense ZrW2O8/ZrO2 composites with excellent low or even near-zero CTE are successfully achieved by CSP (as low as 190 °C) with a postannealing treatment (550 °C). The evolution of sintering densification, phase composition, and microstructure followed by the fundamental mechanism regarding the hydrothermal assembly of the mace-like structure and densification of CSP are investigated in detail. This research not only effectively overcomes the bottleneck issues of ZrW2O8/ZrO2 via integrating the hydrothermal assembly with the sintering technology at ultralow temperature but also develops a promising prospect for the fabrication of a broader range of metastable functional materials.

Plane stress measurement of orthotropic materials using critically refracted longitudinal waves.

Stress measurement in anisotropic materials attracted considerable interest from the field of engineering. This study presents an improved ultrasonic method for measuring plane stresses in orthotropic materials. This study considers the velocity variations of critically refracted longitudinal (LCR) waves induced by orthotropic anisotropy and internal stresses. The stress-acoustic relation in orthotropic materials is used as theoretical basis for the proposed ultrasonic method. To verify the accuracy and usability of these relations, measurements are performed on a 6061 aluminum alloy and a carbon fiber reinforced plastic (CFRP) composite are subjected to under tensile tests. As preconditions for testing, longitudinal wave velocity is measured and LCR wave wedges are designed for these two materials. Stress-acoustic coefficients are determined by a series of calibration tests. Finally, we confirm the accuracy of this method by comparing the acoustoelastic coefficients of the 6061 aluminum alloy with certain reference values. The acoustoelastic coefficients of CFRP composite are compared with the theoretically predicted curve to assess its usability for orthotropic materials.

Application of CCG Sensors to a High-Temperature Structure Subjected to Thermo-Mechanical Load.

This paper presents a simple methodology to perform a high temperature coupled thermo-mechanical test using ultra-high temperature ceramic material specimens (UHTCs), which are equipped with chemical composition gratings sensors (CCGs). The methodology also considers the presence of coupled loading within the response provided by the CCG sensors. The theoretical strain of the UHTCs specimens calculated with this technique shows a maximum relative error of 2.15% between the analytical and experimental data. To further verify the validity of the results from the tests, a Finite Element (FE) model has been developed to simulate the temperature, stress and strain fields within the UHTC structure equipped with the CCG. The results show that the compressive stress exceeds the material strength at the bonding area, and this originates a failure by fracture of the supporting structure in the hot environment. The results related to the strain fields show that the relative error with the experimental data decrease with an increase of temperature. The relative error is less than 15% when the temperature is higher than 200 °C, and only 6.71% at 695 °C.

Super flame-retardant lightweight rime-like carbon-phenolic nanofoam.

The desire for lightweight nanoporous materials with high-performance thermal insulation and efficient anti-ablation resistance for energy conservation and thermal protection/insulation has greatly motivated research and development recently. The main challenge to synthesize such lightweight materials is how to balance the relationship of low thermal conductivity and flame retardancy. Herein, we propose a new concept of lightweight "rime-like" structured carbon-phenolic nanocomposites to solve this problem, where the 3D chopped network-structured carbon fiber (NCF) monoliths are incorporated with nanoporous phenolic aerogel to retain structural and functional integrity. The nanometer-scaled porous phenolic (NP) was synthesized through polymerization-induced phase separation and ambient pressure drying using phenolic resin (PR) solution as reaction source, ethylene glycol (EG) as solvent and hexamethylenetetramine (HMTA) as catalyst. We demonstrate that the as-prepared NCF-NP nanocomposite exhibits with a low density of 0.25-0.35 g/cm(3), low thermal conductivity of 0.125 Wm(-1)K(-1) and outstanding flame retardancy exceeding 2000 °C under arc-jet wind tunnel simulation environment. Our results show that the synthesis strategy is a promising approach for producing nanocomposites with excellent high-temperature heat blocking property.

ZrB2-CNTs Nanocomposites Fabricated by Spark Plasma Sintering.

ZrB2-based nanocomposites with and without carbon nanotubes (CNTs) as reinforcement were prepared at 1600 °C by spark plasma sintering. The effects of CNTs on the microstructure and mechanical properties of nano-ZrB2 matrix composites were studied. The results indicated that adding CNTs can inhibit the abnormal grain growth of ZrB2 grains and improve the fracture toughness of the composites. The toughness mechanisms were crack deflection, crack bridging, debonding, and pull-out of CNTs. The experimental results of the nanograined ZrB2-CNTs composites were compared with those of the micro-grained ZrB2-CNTs composites. Due to the small size and surface effects, the nanograined ZrB2-CNTs composites exhibited stronger mechanical properties: the hardness, flexural strength and fracture toughness were 18.7 ± 0.2 GPa, 1016 ± 75 MPa, and 8.5 ± 0.4 MPa·m1/2, respectively.

Enhanced thermal shock resistance of ceramics through biomimetically inspired nanofins.

We propose here a new method to make ceramics insensitive to thermal shock up to their melting temperature. In this method the surface of ceramics was biomimetically roughened into nanofinned surface that creates a thin air layer enveloping the surface of the ceramics during quenching. This air layer increases the heat transfer resistance of the surface of the ceramics by about 10,000 times so that the strong thermal gradient and stresses produced by the steep temperature difference in thermal shock did not occur both on the actual surface and in the interior of the ceramics. This method effectively extends the applications of existing ceramics in the extreme thermal environments.

Facile route to nitrides: transformation from single element to binary and ternary nitrides at moderate temperature through a new modified solid-state metathesis.

In this article, we report a new modified solid-state metathesis pathway to synthesize nitrides using Li(3)N as a nitrification reagent to transform single element to nitrides. In this process, not only binary (including mono- and multinitrides) but also ternary nitrides can be approached by varying the molar ratio of Li(3)N to a single element. A possible two-step reaction mechanism for Li(3)N and single elements was proposed. This study provides a promising route to meet the increasing demand in energy savings and environmental protection for materials synthesis.