We are pleased to introduce the collection, 2022 Retrospective: Mechanics of Materials. This collection aims to showcase original research and review-based papers on the most exciting developments of the past couple of years. The work presented here highlights the broad diversity of research performed across the section, and aims to put a spotlight on the main areas of interest. All research presented here seeks to display strong advances in theory, experiment, and methodology with applications to compelling problems. Authors are encouraged to showcase their most recent discoveries and to kickstart scientific discourse around them.

University of Trento

University of Modena and Reggio Emilia

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Frontiers in Materials

Mechanics of Materials

Frontiers in Mechanical Engineering

2022 Retrospective: Mechanics of Materials

Corrigendum: Fabrication and surface characterization of titanium dioxide nanotubes on titanium implants

Titanium has been widely used in orthopedics and dental implants due to its excellent biocompatibility and mechanical properties. However, the surface of titanium is biologically inert and lacks biological activity, resulting in poor integration between titanium-based implants and surrounding natural bone tissue, which is a common challenge in its clinical application. Surface modification is currently an effective means to improve the biocompatibility and bioactivity of titanium implants. The natural tissues of the human body are assembled from nanomodules, so from a biomimetic perspective, nanostructures should have better biological activity. TiO2 nanotubes have unique physical and chemical properties due to their elastic modulus, large specific surface area, and regular hollow structure similar to those of bone tissue. This study used anodic oxidation technology to prepare TiO2 nanotubes on the surface of titanium. The surface properties of the nanotubes were evaluated using field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), atomic force microscopy profilometry, contact angle measurements, etc. The corrosion resistance was tested using an electrochemical workstation. The results indicate that anodic oxidation can be used to successfully prepare titanium dioxide nanotube arrays on the surface of titanium. The nanotubes not only exhibit a good structure but also improve the surface hydrophilicity and corrosion resistance of titanium, thereby demonstrating potential for clinical application.

Titanium has been widely used in orthopedics and dental implants due to its excellent biocompatibility and mechanical properties. However, the surface of titanium is biologically inert and lacks biological activity, resulting in poor integration between titanium-based implants and surrounding natural bone tissue, which is a common challenge in its clinical application. Surface modification is currently an effective means to improve the biocompatibility and bioactivity of titanium implants. The natural tissues of the human body are assembled from nanomodules, so from a biomimetic perspective, nanostructures should have better biological activity. TiO2 nanotubes have unique physical and chemical properties due to their elastic modulus, large specific surface area, and regular hollow structure similar to those of bone tissue. This study used anodic oxidation technology to prepare TiO2 nanotubes on the surface of titanium. The surface properties of the nanotubes were evaluated using field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), atomic force microscopy profilometry, contact angle measurements, etc. The corrosion resistance was tested using an electrochemical workstation. The results indicate that anodic oxidation can be used to successfully prepare titanium dioxide nanotube arrays on the surface of titanium. The nanotubes not only exhibit a good structure but also improve the surface hydrophilicity and corrosion resistance of titanium, thereby demonstrating potential for clinical application.

Fabrication and surface characterization of titanium dioxide nanotubes on titanium implants

Advancements in additive manufacturing technology (3D printing) have enabled us to fabricate reasonably good parts using continuous fiber-reinforced matrix composites. Unfortunately, most of these 3D-printed composite parts inherently possess a large number of voids originating from the trapped air within and between molten composite beads during the deposition stage. Removing the voids has thus become a key challenge in attempts to apply 3D printed composite parts for fabricating stiff/strong load-bearing structures. Here, we employed a classical process, viz. compression molding, to post-consolidate 3D-printed continuous carbon fiber-reinforced polyamide (CFPA), and to investigate the implications in terms of microscale parameters (void content) and mesoscale parameters (mechanical properties, plasticity, damage) using matrix-dominated lay-up of [±45]2s. We found that the proposed post-consolidation process could reduce the void of 3D-printed CFPA from 12.2% to 1.8%, enhancing the shear modulus and shear strength by 135% and 116%, respectively. The mesoscale analysis shows that, albeit with less ductility, the post-consolidated CFPA laminate was more resistant to damage than the 3D-printed CFPA. Classical compression molding is thus a promising technique for improving the physical and mechanical performances of 3D-printed composites by reducing inherent void built-ups.

Advancements in additive manufacturing technology (3D printing) have enabled us to fabricate reasonably good parts using continuous fiber-reinforced matrix composites. Unfortunately, most of these 3D-printed composite parts inherently possess a large number of voids originating from the trapped air within and between molten composite beads during the deposition stage. Removing the voids has thus become a key challenge in attempts to apply 3D printed composite parts for fabricating stiff/strong load-bearing structures. Here, we employed a classical process, viz. compression molding, to post-consolidate 3D-printed continuous carbon fiber-reinforced polyamide (CFPA), and to investigate the implications in terms of microscale parameters (void content) and mesoscale parameters (mechanical properties, plasticity, damage) using matrix-dominated lay-up of [±45]2s. We found that the proposed post-consolidation process could reduce the void of 3D-printed CFPA from 12.2% to 1.8%, enhancing the shear modulus and shear strength by 135% and 116%, respectively. The mesoscale analysis shows that, albeit with less ductility, the post-consolidated CFPA laminate was more resistant to damage than the 3D-printed CFPA. Classical compression molding is thus a promising technique for improving the physical and mechanical performances of 3D-printed composites by reducing inherent void built-ups.

Post-consolidation process for modifying microscale and mesoscale parameters of 3D printed composite materials

Significant attention has been paid to developing highly flexible and highly stretchable strain sensors due to the increasing demand for wearable devices such as motion-capturing devices and health-monitoring devices. Especially, carbon nanotube (CNT) network-based elastomeric sensors have been studied extensively for their unique strong piezoresistive response under large deformation. Despite its importance for the facile design of sensors, the effect of length and volume fraction of CNT on the piezoresistivity over a large strain range has not been fully uncovered. In this study, by combining coarse-grained molecular statics (CGMS) simulations and efficient percolation network analysis, we investigate the piezoresistive response of the CNT network for a wide range of the length and volume fraction and visualized the CNT network topology to understand the mechanism behind the piezoresistivity response. Based on the set of calculations, we obtain the design map of stretchability and sensitivity for the CNT-elastomer nanocomposite sensors over a wide range of design parameters of CNT, which can be used to fabricate the strain sensor with a desired performance.

Significant attention has been paid to developing highly flexible and highly stretchable strain sensors due to the increasing demand for wearable devices such as motion-capturing devices and health-monitoring devices. Especially, carbon nanotube (CNT) network-based elastomeric sensors have been studied extensively for their unique strong piezoresistive response under large deformation. Despite its importance for the facile design of sensors, the effect of length and volume fraction of CNT on the piezoresistivity over a large strain range has not been fully uncovered. In this study, by combining coarse-grained molecular statics (CGMS) simulations and efficient percolation network analysis, we investigate the piezoresistive response of the CNT network for a wide range of the length and volume fraction and visualized the CNT network topology to understand the mechanism behind the piezoresistivity response. Based on the set of calculations, we obtain the design map of stretchability and sensitivity for the CNT-elastomer nanocomposite sensors over a wide range of design parameters of CNT, which can be used to fabricate the strain sensor with a desired performance.

Coarse-grained modeling for predicting the piezoresistive response of CNT-elastomer nanocomposite

The present article investigates the microstructure and mechanical properties of A356 matrix composite reinforced with TiB2 particles synthesized by the salt metal reaction of as-cast and T6 state. Microscopic observations of the prepared composites reveal that in-situ grown TiB2 particles are characterized with regular shapes and nearly uniform distributed in the A356 matrix. A clear interface between the A356 matrix and TiB2 particles was observed. The detailed analysis of mechanical properties of synthesized composites of as-cast and T6 state show that the ultimate tensile strength and Young’s modulus of the synthesized composites increased with the increasing weight percentage (wt%) of in-situ TiB2 particles in the A356 matrix, but the Poisson’s ratio of the synthesized composites decreased with the increase of TiB2 particles wt%. The Young’s modulus of the composites increased by up to 10.8% and the Poisson’s ratio decreased by up to 3.2% with the increase of TiB2 particles wt%, compared to A356 alloy. With the increase of TiB2 particle wt%, the yield strength of the composites decreased at first (when the TiB2 particle wt% is less than 1%) then increased, while elongation and percent reduction in area increased at first and then decreased. Furthermore, T6 heat treatment can refine grain and effectively improve the mechanical properties of composites.

The present article investigates the microstructure and mechanical properties of A356 matrix composite reinforced with TiB2 particles synthesized by the salt metal reaction of as-cast and T6 state. Microscopic observations of the prepared composites reveal that in-situ grown TiB2 particles are characterized with regular shapes and nearly uniform distributed in the A356 matrix. A clear interface between the A356 matrix and TiB2 particles was observed. The detailed analysis of mechanical properties of synthesized composites of as-cast and T6 state show that the ultimate tensile strength and Young’s modulus of the synthesized composites increased with the increasing weight percentage (wt%) of in-situ TiB2 particles in the A356 matrix, but the Poisson’s ratio of the synthesized composites decreased with the increase of TiB2 particles wt%. The Young’s modulus of the composites increased by up to 10.8% and the Poisson’s ratio decreased by up to 3.2% with the increase of TiB2 particles wt%, compared to A356 alloy. With the increase of TiB2 particle wt%, the yield strength of the composites decreased at first (when the TiB2 particle wt% is less than 1%) then increased, while elongation and percent reduction in area increased at first and then decreased. Furthermore, T6 heat treatment can refine grain and effectively improve the mechanical properties of composites.

Microstructure and Mechanical Properties of Aluminum Matrix Composites Reinforced With In-Situ TiB2 Particles

Copper–graphite sandwich composites are new functional composites with excellent tribological and mechanical properties. More and more researchers prepare the sandwich composites by accumulative roll-bonding processes due to severe plasticity deformation. The microstructure and properties of copper–graphite sandwich composites prepared by continual annealing and accumulative roll-bonding processes for eight rolling cycles were investigated. The results showed that with the increase of rolling cycles, the dispersion of graphite particles between two copper layers along a rolling direction became more uniform, the pores at the interface between graphite particles and the Cu matrix gradually disappeared, and the hardness of copper–graphite composites gradually increased. After eight rolling cycles, the Vickers hardness of the composites increased by 140.8% as compared with that of annealed pure copper. During friction, the steel balls can meet some graphite particles in the composites rolled by four cycles. The friction coefficient and wear loss of copper–graphite composites gradually decreased with the increase of rolling cycles. Compared with the composites rolled by one cycle, the friction coefficient and wear loss of the composites rolled by eight cycles were reduced by 32.5 and 49.0%, respectively. The main wear mechanism gradually evolved from fatigue wear and abrasive wear to adhesive wear, and then to fatigue wear.

Copper&#x2013;graphite sandwich composites are new functional composites with excellent tribological and mechanical properties. More and more researchers prepare the sandwich composites by accumulative roll-bonding processes due to severe plasticity deformation. The microstructure and properties of copper&#x2013;graphite sandwich composites prepared by continual annealing and accumulative roll-bonding processes for eight rolling cycles were investigated. The results showed that with the increase of rolling cycles, the dispersion of graphite particles between two copper layers along a rolling direction became more uniform, the pores at the interface between graphite particles and the Cu matrix gradually disappeared, and the hardness of copper&#x2013;graphite composites gradually increased. After eight rolling cycles, the Vickers hardness of the composites increased by 140.8% as compared with that of annealed pure copper. During friction, the steel balls can meet some graphite particles in the composites rolled by four cycles. The friction coefficient and wear loss of copper&#x2013;graphite composites gradually decreased with the increase of rolling cycles. Compared with the composites rolled by one cycle, the friction coefficient and wear loss of the composites rolled by eight cycles were reduced by 32.5 and 49.0%, respectively. The main wear mechanism gradually evolved from fatigue wear and abrasive wear to adhesive wear, and then to fatigue wear.

The Bonding Interface and Tribological Properties of Cu–Graphite Sandwich Composites Fabricated via Accumulative Roll-Bonding Processes

The present study examines the processability of Fe-C alloys, with carbon contents up to 1.1 wt%, when using laser based powder bed fusion (LB-PBF). Analysis of specimen cross-sections revealed that lack of fusion porosity was prominent in specimens produced at low volumetric energy density (VED), while keyhole porosity was prominent in specimens produced at high VED. The formation of porosity was also influenced by the carbon content, where increasing the carbon content reduced lack of fusion porosity, while simultaneously increasing the susceptibility to form keyhole porosity. These trends were related to an improved wettability, viscosity, and flow of the melt pool as well an increased melt pool depth as the carbon content increased. Cold cracking defects were also observed in Fe-C alloys that had an as-built hardness ≥425 HV. Reducing the carbon content below 0.75 wt% and increasing the VED, which improved the intrinsic heat treatment during LB-PBF, were found to be effective mitigation strategies to avoid cold cracking defects. Based upon these results, a process window for the Fe-C system was established that produces high density (>99.8%), defect-free specimens via LB-PBF without the requirement of build plate preheating.

The present study examines the processability of Fe-C alloys, with carbon contents up to 1.1&nbsp;wt%, when using laser based powder bed fusion (LB-PBF). Analysis of specimen cross-sections revealed that lack of fusion porosity was prominent in specimens produced at low volumetric energy density (VED), while keyhole porosity was prominent in specimens produced at high VED. The formation of porosity was also influenced by the carbon content, where increasing the carbon content reduced lack of fusion porosity, while simultaneously increasing the susceptibility to form keyhole porosity. These trends were related to an improved wettability, viscosity, and flow of the melt pool as well an increased melt pool depth as the carbon content increased. Cold cracking defects were also observed in Fe-C alloys that had an as-built hardness ≥425 HV. Reducing the carbon content below 0.75&nbsp;wt% and increasing the VED, which improved the intrinsic heat treatment during LB-PBF, were found to be effective mitigation strategies to avoid cold cracking defects. Based upon these results, a process window for the Fe-C system was established that produces high density (&gt;99.8%), defect-free specimens via LB-PBF without the requirement of build plate preheating.