Current Mechanics and Advanced Materials - Current Issue

Background: Bistable composite laminates are emerging as smart structures in automotive and aerospace applications. However, the behavior of the wave propagation within such laminates has not been investigated, which hinders their implementation in Structural Health Monitoring (SHM) and Non-Destructive Evaluation (NDE).

Objective: As a result, this manuscript examines the propagation behavior of guided waves in bistable composite structures. By understanding the effect of pre-stressing in bistable composite laminates on the characteristics of propagating waves, such as velocity and amplitude, a more knowledgeable decision about their applications in flaw detection and assessment can be made.

Methods: The fundamental symmetric (S0) and anti-symmetric (A0) Lamb wave modes were investigated during propagation in two bistable composite laminates, [0/90]T and [02/902]T, and were assessed experimentally and numerically using ABAQUS. For the tested frequencies, which ranged from 60 kHz to 250 kHz, the behavior of the propagating wave was evaluated for both stable configurations and across two different actuators that were lined up with the fiber directions. Signal processing techniques were thus extensively used to enhance the measured signals and identify both the group velocities and the amplitudes’ trend of the S0 and A0 wave modes.

Results: Our results showed that there is a minimal variation (typically below 1%) in the amplitude and velocity of the A0 and S0 modes when the composite plates switch between the first stable configuration and the second stable configuration in both composite plates. These results were numerically validated by replicating the bi-stability of the composites. The numerical data were in relatively close agreement (10% average error) with the experimental values and trends. Furthermore, the bistable effect was examined in detail relative to a reference numerical flat (monostable) plate. Although the bistable effect induced a notable amount of internal residual stress, this did not significantly impact the propagating wave modes, with a maximum difference of about 2% when comparing wave velocities.

Conclusion: The effect on the wave propagation behavior along different directions of both stable configurations was shown to be minimal. These results, which were validated numerically, clear the ambiguity on the usage of these laminates in experimental health monitoring.

Background: Ultrafine-grained (UFG) titanium, which is with high yield strength, biocompatibility and corrosion resistance, is widely used in biomedical and industrial applications. However, adiabatic shear localization (ASL) is often observed in UFG materials due to their worse deformation stability under impact loading. This instability will easily result in the formation of adiabatic shear bands (ASB), a narrow band located in the ASL zoom, and finally cause the fracture of the material. The main objective of this work is to study the adiabatic shear behavior of UFG titanium under impact loading, including macro- and micro-properties, temperature rise, ASB failure, etc.

Methods: A synchronization apparatus, which consisted of Kolsky bar system, high-speed camera system and high-speed infrared temperature measuring system, was set up to carry out the in-situ study of the mechanical properties, temperature rise, and adiabatic shear failure process of UFG pure titanium. Microstructure of the material was also analyzed in this work.

Results: The critical strain of UFG pure titanium for adiabatic shear localization is about 0.37 and 0.69 for UFG Ti and CG Ti, respectively. The peak shear stress of UFG Ti is 500MPa. The propagation velocity of ASB in UFG titanium is 533~800m/s, and 160~320m/s for CG Ti. The temperature rsie within ASB of UFG titanium is 307~732°C, and 212-556°C for CG Ti. The intense temperature rise is after the peak stress and the initiation of ASB most of the time.

Conclusion: UFG Ti has good mechanical properties, however, it is easier to form ASB and cause adiabatic shear failure under impact loading when compared with CG Ti. Temperature rise may not play a major role in the formation of ASB in UFG Ti, but maybe the consequence of ASB. Results of this work will help researchers better understand the failure of UFG metals under impact loading.

Nanocomposites comprised of a polymer matrix and various types of nanosized fillers have remained one of the most important engineering materials and continue to draw great interest in the research community and industry. In particular, graphene in nanocomposites that possess high thermal conductivity and excellent mechanical, electrical, and optical properties have turned out to be promising fillers for making the next generation of advanced high-performance materials.

Though large-scale production of graphene-based nanocomposites is a bit challenging due to the mechanical, functional, and interfacial properties of the graphene and polymer matrix under severe loading conditions, the automotive and off-highway machinery industries are expected to utilize the most modern composite materials, such as graphene-based nanocomposites, to create lighter, stronger, safer, and more energy-efficient cars in the future. Graphene-based material strategies have been investigated and demonstrated to be effective for structural applications in various industries, including electronics, electromechanical, and energy systems. However, currently, there is only limited research highlighting the specific knowledge available for design engineers and researchers involved in providing lightweight but strong solutions using graphene-based materials for automotive and off-highway vehicle applications.

The present review presents an overview of the latest studies that utilize graphene-based nanomaterials and their composites in automotive and off-highway machinery applications. First, the paper describes the concept of traditional composites used presently in the engineering industries by considering its advantages and limitations. Then, it highlights the key benefits of using nanostructured carbon materials, such as graphene, through some recent studies available in the literature. Subsequently, it depicts the various mechanisms of integrating graphene as polymer reinforcements within the composite materials based on the survey and their related modelling, designing, and manufacturing capabilities suitable for the automotive and off-highway machinery industry. Finally, it outlines the available experimental evidence for graphene-based composites. To lay the groundwork for future work in this exciting area, the paper discusses the current challenges as well as future prospects in the field.

Aims: This work is to give a further insight into the residual stress distribution within APS TBC system within TBC system by coupled thermo-mechanic finite element analysis.

Background: Thermal barrier coatings are a typical example of ceramic-metal system. Thermal stress could be produced in the thermal barrier coatings on account of the difference in coefficient of thermal expansion between the ceramic and metal and poses a serious threat to the performance and lifetime of the thermal barrier coatings.

Objective: Many previous studies used out-of-plane stresses to estimate the delamination of the bond-coat/TGO and TGO/top-coat interfaces. However, in the present work, normal and tangential stresses under thermal-mechanic coupling analysis are adopted to investigate the interface delamination.

Methods: A thermo-mechanic coupled model comprised of a top-coat, thermally grown oxide, bond-coat, and substrate, is built for the stress evolution within the thermal barrier coatings using finite element method. The thermal conduction is carried out inside the thermal barrier coating model according to Fourier's law, with the temperature gradient forming in the model.

Results: The top-coat is subjected to tensile stresses around the peak, with the stress magnitudes decreasing with the oxide thickness. Normal tension acts across the oxide/bond-coat interface around the peak while normal compression acts around the valley. Their magnitudes are dependent on the oxide thickness but are not so sensitive to the substrate surface heat transfer coefficient.

Conclusion: The TC/TGO interface might be subjected to tensile or compressive stresses around the peak, which depends on the TGO thickness. The TGO/BC interface is subjected to tensile stresses around the peak and compressive stresses around the valley, the magnitudes of which increase with the TGO thickness. The influence of substrate film coefficient on the stress across the TGO/BC interface is not so significant.

Discussion: The thermo-mechanic coupled model is employed to analyze the top-coat stress, bond-coat plastic strain and interface stress. Simultaneously, a parametric study is performed on the effect of thermally grown oxide thickness and substrate surface heat transfer coefficient on the above mechanical responses.