Micromechanical Testing Research Articles

Implementation of bio-inspired organic/inorganic layer structures as interphase in carbon fiber reinforced concrete

Biological materials found in nacre or glass sponges reveal specific layered organic and inorganic structures known for their fracture toughness caused by crack deflection and bridging. This work aims to bio-mimic the brick-and-mortar (BnM) structure from nacre and the layer-by-layer (LbL) structure from the glass sponge filaments to incorporate these effects into the contact zone of carbon fiber-reinforced concrete (CFRC) in order to prevent brittle composite failure. To build up BnM- and LbL-structure materials such as nanoclays, sodium water glass, and polymer dispersions were selected since they are well-established low-cost materials in building. Nanoclays were analyzed regarding their size, dispersibility and exfoliation. Montmorillonite (MMT) was used to be mixed with polymers to produce self-assembled BnM-structured films and coatings. Also, LbL-structures were formed by alternating layers of sodium water glass and polymer. Scanning electron microscopy and energy-dispersive X-ray spectroscopy were used to verify the morphology. The MMT-containing coatings demonstrated enhanced nucleation potential when exposed to cementitious eluate. Micromechanical pull-out tests on single carbon fibers with BnM- and LbL-coatings embedded in concrete demonstrate the potential to increase composite toughness. The successful implementation of the bio-inspired structures using affordable materials lays the groundwork for their scalability and integration into composite structures for building.

Toughness enhancement in TiN/Zr0.37Al0.63N1.09 multilayer films

The hardness and fracture toughness of high-temperature wear-resistant transition metal aluminum nitride multilayer films depend largely on the constituting layer's structure, compositional modulation, morphology, and interface coherency. We present a study on 1-micron thick multilayered films consisting of stacked layers of TiN and Zr0.37Al0.63N1.09, each layer being 10 nm thick. The films were grown using ion-assisted reactive magnetron sputtering on MgO(001) and Si(001) at substrate temperatures ranging from ambient to 900°C. By increasing growth temperature, we found that the ZrAlN layers transition from near amorphous to nanocrystalline wurtzite to decomposed c-ZrN and w-AlN domains. Concurrently, the TiN layers exhibit strong fiber texture, polycrystallinity, and epitaxial growth carried by the ZrN domains. Both hardness and fracture stress, evaluated by nanoindentation and micromechanical tests, increase with temperature from H=24 GPaMgO, 23 GPaSi to 36 GPaMgO, 30 GPaSi, and σFSi= 6.1-7.7 GPa, respectively. An improved fracture toughness of KIC=2.4-2.8 MPa√m is related to different toughening mechanisms for the various microstructures. The difference in hardness between the substrates is related to compressive stress due to the deposition conditions and thermal contraction. The superior fracture stress is attributed to dense multilayers, free from macroscopic defects due to ion-assisted growth. After being deposited at 200°C, the multilayers remained thermally stable when vacuum annealed for 15 hours at 900°C, with no significant change in phase composition or hardness. The improved hardness, toughness, and temperature stability of the otherwise brittle nitrides are promising for industrial applications.

TRIBOLOGICAL TESTS OF ACRYLIC TEETH FOR REMOVABLE DENTURES

The functionality of removable dentures is ensured by proper diagnostics and clinical modeling. Noless important are the structural conditions and the biotribological and biomechanical context, whichdetermine the wear resistance in the contact of opposing teeth and the contact of the denture plate with thestomatognathic system. The aim of the study is to evaluate acrylic teeth used in prosthetic reconstructionsbased on microstructural, micromechanical and tribological tests. Samples for testing were taken from teethfor removable dentures made by various manufacturers. Microstructural analyses were performed using anoptical microscope and a scanning microscope. Microhardness and elasticity coefficient measurements wereperformed on the NHT device. Tribological tests were performed on a Roxana Machine Works tester using afriction node: ball – 3 discs made of the tested material. The conducted research allowed for the evaluation ofthe structural quality of acrylic teeth and the determination of the tribological interaction resulting from thecontact of synthetic and natural teeth in the presence of artificial saliva.

Insights into the Mechanical Characterization of Mouse Brain Tissue Using Microindentation Testing

AbstractIndentation testing is the most common approach to quantify mechanical brain tissue properties. Despite a myriad of studies conducted already, reported stiffness values vary extensively and continue to be subject of study. Moreover, the growing interest in the relationship between the brain's spatially heterogeneous microstructure and local tissue stiffness warrants the development of standardized measurement protocols to enable comparability between studies and assess repeatability of reported data. Here, we present three individual protocols that outline (1) sample preparation of a 1000‐µm thick coronal slice, (2) a comprehensive list of experimental parameters associated with the FemtoTools FT‐MTA03 Micromechanical Testing System for spherical indentation, and (3) two different approaches to derive the elastic modulus from raw force‐displacement data. Lastly, we demonstrate that our protocols deliver a robust experimental framework that enables us to determine the spatially heterogeneous microstructural properties of (mouse) brain tissue. © 2024 Wiley Periodicals LLC.Basic Protocol 1: Mouse brain sample preparationBasic Protocol 2: Indentation testing of mouse brain tissue using the FemtoTools FT‐MTA03 Micromechanical Testing and Assembly SystemBasic Protocol 3: Tissue stiffness identification from force‐displacement data

Optimization of cellulose nanocrystal (CNC) concentration in polycaprolactone bio-composites for bio-plotting: a robust interpretation of the reinforcement mechanisms

Bioabsorbable and biodegradable composites have experienced rapid growth, owing to their high demand in the biomedical sector. Polymer-cellulose nanocrystal (CNC) compounds were developed using a medical-grade poly (ε-caprolactone) (PCL) matrix to improve the stiffness and load-bearing capacity of pure PCL. Five PCL/CNCs filament grades were melt-extruded, pelletized, and fed into an industrial bioplotter to fabricate specimens. To assess the effects of CNCs on pure PCL, 14 tests were conducted, including rheological, thermomechanical, and in situ micro-mechanical testing, among others. The porosity and dimensional accuracy of the samples were also documented using micro-computed tomography while scanning electron microscopy was employed for morphological characterization. Overall, the 4.0 wt % CNCs loading accomplished the optimum mechanical response, with an increase in its tensile (19.1%) and flexural strength (12.6%) compared to pure PCL. Concurrently, this grade exhibited the highest MFR, minimum porosity, and highest nominal-to-actual geometric accuracy, thereby convincingly interpreting the reinforcement mechanisms.

Strength and stiffness characterization of ultra-high performance concrete (UHPC) cement paste phases through in-situ micro-mechanical testing

Compressive strength and stiffness of concrete materials involve the complex interaction of aggregates and hardened cement, which can involve even more complex phases formed through hydration processes. Typically, the strength and stiffness of concrete materials are determined through material-scale testing, focusing on the bulk behavior while ignoring individual constitutive material interactions. This study aims to identify individual cement phase contributions to bulk concrete properties through the characterization of individual hardened cement-paste phases using micro-scale compressive testing. In this study, in-situ micro-compression measurements are used to generate statistical distributions of compressive strength and stiffness for five hardened cement phases and one phase composite. Results from the in-situ micro-compression testing indicate that AFt/AFm and CH/C–S–H micropillars exhibit higher stiffness and strength when compared to hardened C–S–H and C-A-S-H phases. Comparing measured micropillar strength and stiffness values with traditional nano-indentation methods indicates that nano-indentation approaches may provide a higher estimation of individual phase stiffness due to material confinement boundary effects.

Mitigating the corrosion of AISI 301LN steel in molten carbonate salts by doping with alumina nanoparticles for thermal energy storage applications

Molten carbonate salt nanofluids have become an excellent strategy to enhance the power generation efficiency of the next-generation concentrated solar power (CSP) technology due to their excellent thermophysical properties at high temperatures. The corrosion behaviour in contact with salt nanofluids is one of the main issues to address for implementing cost-effective steels as construction materials for CSP. In the present work, the effects of molten salt nanofluids with Al2O3 nanoparticles (<50 nm and 1.0 wt%) on the corrosion behaviour of AISI 301LN stainless steel have been investigated. Corrosion tests were carried out in a static molten salt mixture of Li2CO3–Na2CO3–K2CO3 at 600 °C for 1000 h. Complementary electron microscopy, microanalysis and optical characterization techniques and micromechanical tests were carried out to study the oxide scales formed after corrosion tests in salt nanofluids and base salt of reference. Results evidenced that doping nanoparticles in molten salts significantly decreased the corrosion rate by ∼50%, compared with a base salt. Incorporating nanoparticles into the oxide scale, forming a nanostructure of reticulated Al2O3 nanoparticles generated a more homogeneous and dense oxide scale, increasing its hardness about 30% and enhancing its effectiveness as a protective barrier.

Hardness and fracture toughness enhancement in transition metal diboride multilayer films with structural variations

The simultaneous enhancement of hardness (H) and fracture toughness (KIC) through the formation of superlattice structures challenges the conventional belief that these quantities are mutually exclusive. Here, this approach has been applied to the transition metal diborides, whose inherent brittleness severely restricts their application potential. The mechanical properties of TiB2/TaB2 systems as a function of bi-layer period Λ are investigated, combining theoretical and experimental approaches. Density Functional Theory is used to investigate the structural stability and mechanical properties of stoichiometric hexagonal TiB2/TaB2 superlattices for Λ = 3.9 – 11.9 nm. The calculations predict the highest H = 38 GPa and KIC (100) of 3.3 MPa.m1/2 at the value of Λ = 5.2 nm. Motivated by the theoretical results, multilayer films with Λ = 4–40 nm were prepared by direct current magnetron sputtering. Due to the sputtering effects, the deposited diboride films differ significantly from the view of stoichiometry and structure. A detailed structure investigation reveals TiB2/TaB2 in form of superlattices exhibiting coherent interfaces for Λ = 4 nm. For higher Λ, parts of TaB2 layers transform from the crystalline to the disordered phase. These transformations are reflected in the mechanical properties as measured by nanoindentation and micromechanical bending tests. The evolution of hardness follows Hall-Petch behavior, reaching a maximum of 42 GPa at Λ = 6 nm. Enhancing fracture toughness involves more complex mechanisms resulting in two KIC maxima: 3.8 MPa.m1/2 at Λ = 6 nm and 3.7 MPa.m1/2 at Λ = 40 nm.

Micromechanical properties of micro- and nanocrystalline CVD diamond thin films with gradient microstructures and stresses

Chemical vapor deposition produced diamonds attract considerable scientific and industrial interest due to their exceptional mechanical and functional properties. Here, hot filament (HF) chemical vapor deposition was used to synthesize two diamond thin films with different cross-sectional microstructure and residual stresses. Structural characterization by scanning electron microscopy, Raman spectroscopy, and cross-sectional synchrotron x-ray nanodiffraction revealed different diamond morphologies. While the microcrystalline diamond film exhibits pronounced microstructural gradients expressed by gradually increasing (i) intensities of the 111 Debye-Scherrer rings, (ii) ⟨110⟩ fiber texture sharpness, (iii) grain size, and (iv) slightly oscillating residual stress at the level of −0.5 GPa, the nanocrystalline diamond film showed no pronounced cross-sectional variation of microstructure above the nucleation zone of ∼0.5 μm and a steady stress level of 0.25 GPa. In situ micromechanical cantilever bending tests revealed highly different mechanical properties of the two films. In detail, Young's modulus of 830 ± 53 and 459 ± 53 GPa, fracture stress of 12.4 ± 0.8 and 7.8 ± 1.0 GPa, and fracture toughness values of 6.9 ± 0.4 and 3.6 ± 0.3 MPa⋅m½ were evaluated for the micro- and nanocrystalline diamond films, respectively. In summary, this study provides valuable insights into the microstructure-residual stress correlation in micro- and nanocrystalline diamond films, especially illuminating their influence on micromechanical properties.

Analysis of micromechanical and tribotechnical properties of fluoropolymers during friction on 40Kh steel

The paper presents the results of micromechanical and tribotechnical tests of samples of polytetrafluoroethylene, radiation-modified (PTFE – RM according to modes A and B), coked in comparison with the PTFE standard (Fluoroplast -F4). The influence of the surface roughness of a 40Kh steel counter-tile on the coefficients of friction against time at a constant sliding speed is considered. Micromechanical tests were carried out using the method of instrumental indentation with a triangular Berkovich diamond pyramid. Tribotechnical tests were performed on a friction machine with a vertical axis of rotation of the spindle according to the scheme “the end of the sample of the fluoropolymer disk is the end of the counter-flange of the annular sleeve (steel 40Kh)” without lubrication and when the friction pair is lubricated with MGE-10A hydraulic oil. It was found that during friction without lubrication, PTFE samples had the maximum friction coefficients, and F4K20 samples had the minimum friction coefficients when working with 40X steel counter-tiles treated with P180 sandpaper. PTFE-RM (A) samples had the best wear resistance, followed by decreasing F4K20, PTFE – RM (B) and PTFE under friction without lubrication. In the presence of lubricant, the samples of the PTFE standard were inferior in wear resistance to the F4K20 samples, but were 5-7% better than the samples of PTFE-RM (B) and (A), respectively.

Friction welding joint analysis of dissimilar nanocomposites: PEEK/aluminum to PEEK/titanium

AbstractWelding of dissimilar materials is extremely challenging to the researchers. In present work, for the first time, poly(ether ether ketone) (PEEK) based aluminum (Al), titanium (Ti) reinforced nanocomposites with varying reinforcements of 10, 20, 30, and 40 vol% were developed using powder metallurgy technology. Additionally, for the first time, the two dissimilar nanocomposites of PEEK/Al and PEEK/Ti were successfully welded by friction welding (FW) process to produce FW PEEK/Al‐PEEK/Ti nanocomposites with equal metallic reinforcements (viz., 10–40 vol% Ti or Al). The nanocomposites were characterized precisely and correlated by physical, microstructural, structural, thermal, and micromechanical tests. Crystallinity being a factor of melting temperature affects significantly the micromechanical characteristics influenced by reinforced particle‐concentrations. The FW PEEK/30Al‐PEEK/30Ti was found as best material since it showed highest nanohardness (0.652 GPa) and elastic modulus (15.902 GPa) in FW Joint at 40 mN compared to other FW nanocomposites. At the FW joint section, it was discovered that the reinforced particles were being transferred through an interdiffusion mechanism. Mobility of the nanoparticles was influenced by the concentration of the reinforced particles, which further modified the matrix's crystallization behavior and consequently influenced the FW nanocomposites' micromechanical properties. Therefore, the present work has suggested a feasible route for applying thermoplastic nanocomposites in the biomedical and aerospace industries.

Elemental distribution and fracture properties of magnetron sputtered carbon supersaturated tungsten films

The combination of strength and toughness is a major driving force for alloy design of protective coatings, and nanocrystalline tungsten (W)-alloys have shown to be promising candidates for combining strength and toughness. Here we investigate the elemental distribution and the fracture toughness of carbon (C) alloyed W thin films prepared by non-reactive magnetron sputtering. W:C films with up to ~4 at.% C crystallize in a body-centered-cubic structure with a strong 〈hh0〉texture, and no additional carbide phases are observed in the diffraction pattern. Atom probe tomography and X-ray photoelectron spectroscopy confirmed the formation of such a supersaturated solid solution. The pure W film has a hardness ~13 GPa and the W:C films exhibit a peak hardness of ~24 GPa. In-situ micromechanical cantilever bending tests show that the fracture toughness decreases from ~4.5 MPa·m1/2 for the W film to ~3.1 MPa·m1/2 for W:C films. The results show that C can significantly enhance the hardness of W thin films while retaining a high fracture toughness.

Comprehensive 3-D homogenization approach for predicting mechanical properties and creep behavior of polymer nanocomposites reinforced with graphene nanoplatelets

The aim of this study is to provide a comprehensive 3-D homogenization approach based on the high-fidelity generalized method of cells method to predict the mechanical properties and creep behavior of polymer nanocomposite reinforced with graphene nanoplatelets. Several fundamental experimental aspects including the volume fraction, size, and random orientation of graphene nanoplatelets, the interphase region, and the aggregation state of nanofillers of graphene are taken into account. The present study introduces two novel perspectives. At first, the aggregation of graphene nanoplatelets is formulated as dependent on volume fraction. Then the size of graphene is formulated as an important factor affecting the interphase region and the mechanical properties of polymer nanocomposite. Micromechanical tests conducted using the proposed method illustrate the good agreement with experimental data. The results of the parametric study on different experimental aspects revealed important findings. (i) For a specific value of graphene nanoplatelet volume fraction, the mechanical properties of polymer nanocomposite are at the best state. By increasing the graphene content beyond the specified graphene content value, the effective properties of graphene are decreased. (ii) The size of graphene nanoplatelets and the characteristics of the graphene/polymer interphase region have a direct effect on the properties of the nanocomposite.

Studies on the microhardness, dielectric properties, and luminescence of L-threonine and Nd: L-threonine crystals

Neodymium-doped L-threonine and pure L-threonine single crystals were grown by slow evaporation technique and characterized by X-ray diffraction (XRD), micromechanical hardness testing, dielectric and photoluminescence studies. Single crystal XRD studies reveals the lattice parameters of the grown crystals. Vickers and Knoop hardness testing methods are employed to evaluate the hardness properties of the crystals. Directional anisotropy is analysed through Knoop hardness measurements. Dielectric studies are conducted to examine the electrical properties of the crystals. Photoluminescence studies reveals that Neodymium-doped L-threonine and pure L-threonine crystals exhibit blue fluorescence. The key novelty of this study is the exploration of the physical and chemical properties of neodymium-doped L-threonine crystals in comparison with undoped L-threonine.

Composition and micromechanical properties of the femoral neck compact bone in relation to patient age, sex and hip fracture occurrence.

Current clinical methods of bone health assessment depend to a great extent on bone mineral density (BMD) measurements. However, these methods only act as a proxy for bone strength and are often only carried out after the fracture occurs. Besides BMD, composition and tissue-level mechanical properties are expected to affect the whole bone's strength and toughness. While the elastic properties of the bone extracellular matrix (ECM) have been extensively investigated over the past two decades, there is still limited knowledge of the yield properties and their relationship to composition and architecture. In the present study, morphological, compositional and micropillar compression bone data was collected from patients who underwent hip arthroplasty. Femoral neck samples from 42 patients were collected together with anonymous clinical information about age, sex and primary diagnosis (coxarthrosis or hip fracture). The femoral neck cortex from the inferomedial region was analyzed in a site-matched manner using a combination of micromechanical testing (nanoindentation, micropillar compression) together with micro-CT and quantitative polarized Raman spectroscopy for both morphological and compositional characterization. Mechanical properties, as well as the sample-level mineral density, were constant over age. Only compositional properties demonstrate weak dependence on patient age: decreasing mineral to matrix ratio (p = 0.02, R2 = 0.13, 2.6 % per decade) and increasing amide I sub-peak ratio I~1660/I~1683 (p = 0.04, R2 = 0.11, 1.5 % per decade). The patient's sex and diagnosis did not seem to influence investigated bone properties. A clear zonal dependence between interstitial and osteonal cortical zones was observed for compositional and elastic bone properties (p < 0.0001). Site-matched microscale analysis confirmed that all investigated mechanical properties except yield strain demonstrate a positive correlation with the mineral fraction of bone. The output database is the first to integrate the experimentally assessed microscale yield properties, local tissue composition and morphology with the available patient clinical information. The final dataset was used for bone fracture risk prediction in-silico through the principal component analysis and the Naïve Bayes classification algorithm. The analysis showed that the mineral to matrix ratio, indentation hardness and micropillar yield stress are the most relevant parameters for bone fracture risk prediction at 70 % model accuracy (0.71 AUC). Due to the low sample number, further studies to build a universal fracture prediction algorithm are anticipated with the higher number of patients (N > 200). The proposed classification algorithm together with the output database of bone tissue properties can be used for the future comparison of existing methods to evaluate bone quality as well as to form a better understanding of the mechanisms through which bone tissue is affected by aging or disease.

Enhancing performance of hybrid carbon/glass textile-reinforced composites: Theoretical prediction and experimental characterization

Multi-scale analysis is of great interest to calculate the final response of hybrid textile-reinforced carbon/glass epoxy (TRCG) composites based on micromechanical tests and geometrical properties. A novel multi-scale model and experiments are utilized to enhance the performance of TRCG composites. Experiments are conducted on TR glass/epoxy, carbon/epoxy, and 7 hybrid TRCGs. The model integrated viscoplastic-material and nonlinear-geometry properties and damage initiation-propagation mechanisms to address stress–strain curves that incline with experimental data with less than 11 % deviation. The microscopic scanning is utilized to study the fracture mechanisms and delaminated areas of specimens. The tensile curves and delaminated-area of TRCG composites are affected by the local glass/carbon mixed ratio and stacking arrangement. Alternative arrangement, H3:[G/C/G/C]S, provides better mechanical properties and a lower isotropy-mismatch effect and delaminated area compared to H1:[2G/2C]S, and H2:[2C/2G]S, and H4:[C/G/C/G]S. H5:[G/2C/G/C]S with the highest possible properties provides the lowest delaminated-area, because of bridging-glass layers to mitigate brittleness and primary failures of carbon, yet is low enough to restrict premature delamination. H6:[4G/2C/4G] and H7:[2G/2C/G]s with high isotropy-mismatch hustle premature failure and fail to deliver expected performance. The fracture mechanisms of TRCG composites are utilized to propose two 10-layer composites with alternative arrangements, which they are experimented afterwards to study the damage modes. It is concluded that H8:[G/C/G/C/G] laminate has the highest mechanical properties and energy absorption amongst TRCG composites. In which with highest integrity, the delaminated area is limited to the vicinity of fracture zone.

Recycled carbon fiber potential for reuse in carbon fiber/PA6 composite parts

Pyrolysis reclaiming is the most promising process to treat high volumes of composite waste with an advantageous carbon footprint. This paper aims to compare pyrolysis reclaimed carbon fibers (RCF) to virgin sized fibers (VF) and de-sized fibers (VFT) in their capability to bond to a polyamide 6 matrix. Micromechanical tensile testing of single fiber samples of the three fiber types was conducted. A minor reduction in tensile strength and an unchanged elastic modulus of the RCF compared to VF was observed. Scanning electron microscopy and atomic force microscopy scans were used to evaluate the morphology of the fibers. To evaluate the surface energy of the fibers, tensiometric testing was conducted. RCF showed a better adhesion capability compared to VFT through higher total surface energy. Moreover, X-ray spectrophotometry scans highlighted a higher proportion of functional groups at the RCF surface compared to VFT. Finally, pull-out tests underlined a decrease of the interfacial shear strength of RCF and VFT by 35% compared to VF. Overall, this study’s results further the understanding of the impact of the pyrolysis reclaiming process on RCF mechanical and adhesion properties.

Real-time evaluating temperature-dependent interfacial shear strength of thermoplastic composites based on stress impedance effect of magnetic fibers

Thermoplastic composites are susceptible to service environments. Precise evaluation of the temperature-dependent interfacial shear strength (IFSS) is essential for determining and predicting mechanical properties and service life. However, a great challenge exists in the measurement of temperature-dependent IFSS by micromechanical tests, due to polymer deformation at elevated temperatures. Here, we report a novel prediction methodology based on the interfacial stress impedance effect of magnetic fibers (MFs) for non-destructive and real-time evaluation of temperature-dependent IFSS. The MFs are optimized by annealing and surface modification to obtain a linear relationship between the impedance property of MF and the radial compressive stress. Through the impedance measurement and corresponding IFSS micromechanical test, a correlation model between the IFSS of composite and the impedance of MF is established. The IFSS at elevated temperatures calculated by this model are consistent with the results of the micromechanical test by more than 95 %, and allows for evaluating the IFSS of composites beyond the glass transition point. The prediction methodology utilizing MF features the merits of convenient operation from the impedance measurement as well as high reliability and wide operating temperature from the nonmechanical and nondestructive test, paving a novel avenue to real-time evaluate temperature-dependent IFSS of advanced thermoplastic composites.

Viability of recycled fibres extracted from EoL composites

Recycling thermoset-based composites is a technological and economic challenge. The predominating recycling methods (mechanical recycling and pyrolysis) downgrade the fibre properties (length, strength) and are energy-intensive. To avoid downcycling and to ensure economic feasibility, technological solutions need to be developed to enable recycling of long reinforcing fibres that can also be used in demanding applications. Thermochemical recycling, where reinforcing fibres are extracted from the depolymerized matrix, can be considered as an advanced option for recovery of higher quality fibres from EoL composites. Additional steps are required to use these fibres in new composite structures. Thermochemical recycling removes the sizing from the fibre surface making it difficult to handle and resulting in poor fibre-matrix compatibility especially in the case of glass fibres. In this study, we discuss the re-sizing of recycled fibres. We have focused on fibres extracted from glass fibre reinforced composites (GFRPs) from EoL wind turbine blades and carbon fibre reinforced composites (CFRPs) collected from aeronautics industry. We demonstrate the steps for a batch re-sizing process and study the effects of the re-sizing on the properties of composites made of recycled fibres. The interfacial properties are analysed with micro-mechanical testing using the microbond method – both for thermoset and thermoplastic composites – and with tensile tests for thermoplastic composites. Re-sizing significantly eases the handling of the fibres, but the interfacial compatibility is improved only in the case of GFRPs as thermochemically recycled carbon fibres (rCF) exhibit very good interfacial performance even without sizing. This study highlights that with proper recovery and resizing processes, recycled fibres can be viable feedstocks for various applications and – with future improvements on the fibre recovery methods – even approach the applicability of virgin fibres.

Development of modular cryogenic indentation apparatus for investigating micro-region mechanical properties of materials

High-precision cryogenic micromechanical testing apparatuses are of significant importance in investigating the micro-region mechanical properties and deformation behaviors of materials at cryogenic temperatures. In this study, a modular cryogenic indentation apparatus was designed and developed utilizing a contact-atmosphere hybrid refrigeration technique with a minimum temperature of −150 °C. The temperature of both the indenter tip and the specimen was independently controlled through a dual-channel proportional–integral–derivative (PID) feedback loop to eliminate contact thermal drift to within 0.1 nm/s. The application of cryogenic nitrogen (N2) atmosphere refrigeration eliminated additional stiffness at the indenter tip completely. The feasibility of the apparatus was demonstrated through calibrations and tests conducted on standard fused silica and single crystal nickel. Highly accurate indentation curves and micro-region mechanical properties of single crystal nickel were obtained at variable cryogenic temperatures. With decreasing temperature, both slip bands and pile-up around residual indentation imprints were weakened. Notably, at −150 °C, the slip bands disappeared, and the phenomenon of pile-up was replaced by sink-in. This developed cryogenic indentation apparatus will be a powerful tool to reveal the effect of cryogenic temperatures on the evolution mechanisms of materials’ micro-region mechanical properties.