Mechanical Interlocking Effect Research Articles

Bondline Thickness Effects on Damage Tolerance of Adhesive Joints Subjected to Localized Impact Damages: Application to Leading Edge of Wind Turbine Blades.

The leading edges of wind turbine blades are adhesively bonded composite sections that are susceptible to impact loads during offshore installation. The impact loads can cause localized damages at the leading edges that necessitate damage tolerance assessment. However, owing to the complex material combinations together with varying bondline thicknesses along the leading edges, damage tolerance investigation of blades at full scale is challenging and costly. In the current paper, we design a coupon scale test procedure for investigating bondline thickness effects on damage tolerance of joints after being subjected to localized impact damages. Joints with bondline thicknesses (0.6 mm, 1.6 mm, and 2.6 mm) are subjected to varying level of impact energies (5 J, 10 J, and 15 J), and the dominant failure modes are identified together with analysis of impact kinematics. The damaged joints are further tested under tensile lap shear and their failure loads are compared to the intact values. The results show that for a given impact energy, the largest damage area was obtained for the thickest joint. In addition, the joints with the thinnest bondline thicknesses displayed the highest failure loads post impact, and therefore the greatest damage tolerance. For some of the thin joints, mechanical interlocking effects at the bondline interface increased the failure load of the joints by 20%. All in all, the coupon scale tests indicate no significant reduction in failure loads due to impact, hence contributing to the question of acceptable localized damage, i.e., damage tolerance with respect to static strength of the whole blade.

PRODUCTION OF INDIVIDUALISED MULTI-MATERIAL COMPONENTS USING A ROBOT-BASED PROCESS CHAIN

The production of individual work pieces in small to medium batch sizes requires an adaptation of the manufacturing strategy. Particularly, the manufacturing of multi-material components out of metal and plastic is characterized by high production costs as well as high production times. To resolve this challenge, a new, modular process chain for the production of these structures in a single manufacturing cell was developed. In this cell, a robot manufactures multi-material components with multiple end-effectors in several successive process steps. In the first step, interlocking structures are manufactured on a metal part by a surface structuring tool. Afterwards, an extruder is used to add a thermoplastic onto the structured metal part. Due to mechanical interlocking effects, the applied thermoplastic shows improved adhesion behavior. A study was conducted to analyze the achievable joint strength of the additive material application onto the structured metal samples. Investigations to determine the achievable manufacturing quality of a robot guided milling process for multi-material parts have been carried out. A recently developed suction hood is used to capture the metal and plastic chips. In this paper, the results regarding the efficiency of the individual end-effectors, including the extraction hood, are presented and it is demonstrated how they interact within the robot-based process chain.

Enhanced interfacial adhesion of carbon fiber/epoxy composites by synergistic reinforcement with multiscale “rigid-flexible” structure at interphase

Carbon fiber (CF) reinforced matrix composites have wide application prospect, however, the interfacial adhesion of composites is weak due to smooth and chemically inert of CF surface. To settle this problem, two kinds of synergistic hierarchical reinforcements with multiscale “rigid-flexible” structures [CF-graphene oxide (GO)-carbon nanotubes (CNTs)- polyamide (PA) and CF-CNTs-GO-PA] were fabricated through simple esterification and anionic polymerization reactions. The effects of the interface thickness and grafting sequence on the interfacial properties of the composites were explored. The interfacial shear strength (IFSS) of the CF-GO-CNTs-PA/resin composites was 88.3 MPa, which exceeded those of CF, CF-GO-CNTs, CF-GO-PA, and CF-CNTs-GO-PA by 80.9%, 11.5%, 2.3%, and 1.4%, respectively. This was mainly ascribed to the thicker gradient interface layer , the synergistic effects of GO and CNTs, the rigid and flexible structure, as well as the interfacial interactions and mechanical interlocking effect in the CF-GO-CNTs-PA/resin composite. This work provides a novel concept for the construction of interfaces suitable for advanced CF composites with different structures.

Role of dry ozonization of basalt fibers on interfacial properties and fracture toughness of epoxy matrix composites

Abstract The mechanical properties of basalt fiber-reinforced epoxy composites (BFRPs) are significantly dependent on the interfacial adhesion between basalt fibers (BFs) and the epoxy matrix. In this study, we proposed a simple and efficient method for deep and stable penetration of BFs into the epoxy matrix through dry-ozone treatments. To confirm the efficiency of the proposed method, BFRPs were fabricated using two types of composites: untreated BFs and dry-ozonized BFs in varying amounts, and the optimum amount of BFs for all the composites fabricated in this work was 60 wt%. With the addition of this amount of dry-ozonized BFs, the interlaminar shear strength and fracture toughness of the composites were enhanced by 21.2 and 23.2%, respectively, as compared with untreated BFs. The related reinforcing mechanisms were also analyzed, and the enhanced interfacial adhesion was mainly attributed to the mechanical interlocking effect. This approach shows that the dry-ozone treatment of BFs is a simple and efficient method for the preparation of BFRPs with excellent interfacial adhesion, which can be a potential application in the auto parts industry.

Fabrication, mechanical and thermal properties of copper coated graphite films reinforced copper matrix laminated composites via ultrasonic-assisted electroless plating and vacuum hot-pressing sintering

In the trend of integration of structure and function, novel thermal management materials are urgent to be developed owning good mechanical properties as well as excellent thermal properties. In this work, a complete and dense copper (Cu) coating with a thickness of 400 nm was deposited on the surfaces of graphite films (GFs) by ultrasonic-assisted electroless plating, which can improve the surface roughness of GFs and interfacial bonding between GFs and Cu foils. Cu-coated and uncoated GFs reinforced copper matrix laminated composites (noted as GFs(Cu)/Cu and GFs/Cu) were fabricated by optimized vacuum hot-pressing sintering. Based on the scratch test and microstructures of GFs(Cu) and GFs(Cu)/Cu composites, addition of Cu coating can improve the interfacial adhesion and element diffusion behavior, and intuitively explains the mechanical interlocking effect at the C–Cu interface. On basis of the nanoindentation mechanical properties across GFs(Cu)/Cu and GFs/Cu interfaces, the interfacial transition region with Cu coating of ~3 μm is three times of that without Cu coating. Flexural strength of 27.6 vol% GFs(Cu)/Cu laminated composite is 45.9 MPa, which is 56% higher than that of the corresponding GFs/Cu laminated composite. In-plane thermal conductivity (TC) and out-of-plane coefficient of thermal expansion (CTE) of 69.4 vol % GFs(Cu)/Cu laminated composite are 1177.8 W/m·K and −2.5 ppm/K respectively, which could be explained on the theoretical calculations and interfacial structures. In conclusion, the Cu coating on surfaces of GFs can generally improve the interfacial bonding, mechanical properties and thermal properties of GFs(Cu)/Cu laminated composites. It provides an effective way to develop novel structural and functional materials for thermal management.

Effect of IR‐laser treatment parameters on surface structure, roughness, wettability and bonding properties of fused deposition modeling‐printed PEEK/CF

AbstractIn this study, laser surface treatment was applied to alter the surface texturing and chemical compositions of fused deposition modeling (FDM)‐printed PEEK/CF samples to improve the deficiency of inert surface of PEEK as adherend substrate. The influence of IR‐laser parameters including treatment gaps, single pulse energy and pulse widths on surface properties and shear bond strength were discussed. The results indicated that surface roughness was enhanced with decreasing treatment gap or increasing pulse energy, which reached the highest value of Ra = 32.44 μm at 0.4*0.4 mm2 treatment gap and 300 mJ single pulse energy. By adjusting laser pulse width, surface wettability changed from hydrophobicity to hydrophilicity. After micro‐second laser ablation, the texturing structure was changed and acted as mechanical interlocking effect, and therefore make the shear bond strengths improve from 3.28 to 6.42 MPa compared with the untreated groups. On the other hand, functional groups on substrate surface were activated after nano‐second laser ablation, which contributes to an enhancement of shear bond strength through chemical interaction between adhesives and substrates. Therefore, our work highlights an efficient method of laser surface treatment on the adhesion property of FDM‐printed substrates.

Effects of steel fiber grout on the mechanical performance and failure characteristics of fully grouted bolts

In real engineering, with the gradual deterioration of mining geological conditions, the improvement of the bearing performance of fully grouted bolts is beneficial to better control of the roadway surrounding rock. For this purpose, based on the idea of steel fiber reinforced concrete, a systematic laboratory study was conducted to investigate the effect of steel fiber grout on the bearing performance of anchored specimens. Three steel fibers with different diameters (0.5, 1.0 and 1.5 mm) were selected. Fiber specimens and fiber-free specimens were prepared and a series of pull-out tests were conducted. The experimental results show that the steel fiber grout can effectively improve the bearing performance of the anchored specimens and enhance the anti-destructive ability of the anchored specimens. The steel fibers in the grout can enhance the mechanical interlocking effect, the dilatancy effect and the friction effect of the anchoring interface, thereby improving the ability of the anchored specimen to withstand additional loads. The reinforcement effect of steel fiber on the bearing performance of the anchored specimens presents an obvious diameter dependence. With the increase of the steel fiber diameter, the bearing properties of the anchored specimens tend to increase and the cumulative AE counts, cumulative AE hits, cumulative AE energy and AE events also increase. Moreover, the steel fiber grout can more fully mobilize the bearing performance of the anchoring interface, and the number of microcracks in the fiber specimens increases during the bearing process. This research results may provide a useful reference for the support design of fully grouted bolts.

Tribological and Thermo-Mechanical Properties of TiO2 Nanodot-Decorated Ti3C2/Epoxy Nanocomposites.

The micromorphology of fillers plays an important role in tribological and mechanical properties of polymer matrices. In this work, a TiO2-decorated Ti2C3 (TiO2/Ti3C2) composite particle with unique micro-nano morphology was engineered to improve the tribological and thermo-mechanical properties of epoxy resin. The TiO2/Ti3C2 were synthesized by hydrothermal growth of TiO2 nanodots onto the surface of accordion-like Ti3C2 microparticles, and three different decoration degrees (low, medium, high density) of TiO2/Ti3C2 were prepared by regulating the concentration of TiO2 precursor solution. Tribological test results indicated that the incorporation of TiO2/Ti3C2 can effectively improve the wear rate of epoxy resin. Among them, the medium density TiO2/Ti3C2/epoxy nanocomposites gained a minimum wear rate. This may be ascribed by the moderate TiO2 nanodot protuberances on the Ti3C2 surface induced a strong mechanical interlock effect between medium-density TiO2/Ti3C2 and the epoxy matrix, which can bear a higher normal shear stress during sliding friction. The morphologies of worn surfaces and wear debris revealed that the wear form was gradually transformed from fatigue wear in neat epoxy to abrasive wear in TiO2/Ti3C2/epoxy nanocomposites. Moreover, the results of thermo-mechanical property indicated that incorporation of TiO2/Ti3C2 also effectively improved the storage modulus and glass transition temperature of epoxy resin.

Aspects on bond strength in sheet structures from TMP and CTMP – a review

AbstractHigh yield pulps (HYP), manufactured in mechanical and chemimechanical pulping processes, are mainly used in graphic papers and paper grades where a high bulk is preferable, like in paperboards. Moreover, packaging papers with very high demands on both dry and wet strength could be manufactured from HYP in a near future. Preferred bonds between fibre components (long fibres, shortened fibres and fines) in the various paper grades are quite different. In the review, plausible effects of mechanical interlocking, intermolecular interactions (“physical bonding”), hydrogen bonds, intermixing of polymers, additives and possible specific interactions in the formation of strong bonds in sheet structures from HYP are discussed. A required condition for high bond strength in sheets from HYP furnishes is that fibre components are forced into sufficiently close contact. This is to a great extent impeded if the fibre walls are too stiff. Consequently, the current review focuses on both how fibre fractions should preferably be developed for different end uses and how suitable bonds might be achieved in different paper grades. The ideal type of bonds is certainly different depending on the demands on the final paper quality.

Ni/AlN Composite Coating for Corrosion and Elements Interdiffusion Resistance in Molten Fluoride Salts System

The Ni/AlN composite coating was prepared for increasing corrosion and elements interdiffusion resistance of GH3535 alloy in molten fluoride salts, and the effect of mechanical interlocking on adhesion strength between AlN layer and nickel coating was also studied. Results indicated that the adhesion strength between AlN layer and nickel coating could be significantly enhanced through mechanical interlocking effect, which effectively prevented the nickel coating from flaking off at elevated temperature. Through an etching pre-treatment of AlN layer, the corrosion resistance of the Ni/AlN coated GH3535 alloy in molten FLiNaK salt was further improved, and elements interdiffusion between the substrate and nickel coating was completely suppressed. AlN layer as a diffusion barrier remained compact and continuous in Ni/GH3535 system after high-temperature molten salt corrosion. Moreover, a Ni–P layer consisting of Ni3P and Ni phases formed in the Ni coating after corrosion.

Laser-induced groove optimization for Al/CFRP adhesive joint strength

The ability to achieve sufficient values of adhesive strength depends on many parameters such as type and thickness of the adhesive, and especially appropriate surface preparation. While it is possible to increase the adhesive strength of a surface prepared for bonding by making it chemically suitable, forming of the structures that will provide mechanical interlocking can also increase the adhesive shear strength. Mechanical interlocking groove parameters such as depth, width, and number per unit area formed on the AA2024-T3 aluminum surface by using fiber laser were optimized for adhesive bonding strength with carbon fiber reinforced polymer (CFRP). The top surface and cross-sectional images were used to determine the geometrical change of the grooves produced on the aluminum surface. As the depth of the grooves increased, the adhesion surface area of the samples increased by approximately 308%. According to the single-lap shear strength test results, the increase in the depth of the grooves, hence the adhesion surface area and the entrance width of the grooves have a common effect on the shear strength. While increase in the adhesive surface area and entrance width of the grooves increases the shear strength, decreasing in the entrance width causes the decrease in the shear strength even if the surface area increases. In this study, with the effect of mechanical interlocking caused by laser-induced grooves, approximately 180% increase in adhesive shear strength (26.48 MPa) was achieved compared to the abraded sample. After mechanical tests, failure modes were also observed by using the a photo camera. It was determined that the effect of created grooves with optimized geometrical sizes on the AA2024-T3 aluminum surfaces changed the failure mode that occurred at the CFRP surfaces from adhesive failure to fiber tear failure.

Microstructure and Improved Thermal Shock Behaviour of an In Situ Formed Metal-Enamel Interlocking Coating

A novel metal-enamel interlocking coating was designed and prepared in situ by co-deposition of Ni-enamel composite layer and subsequent air spray of enamel with 10 wt% nanoscale Ni. During the firing process, the external enamel layer was melted and jointed with the enamel particles at the upper part of the Ni-plating layer to form the enamel pegs. Thermal shock tests of pure enamel, enamel with 10 wt% Ni composite and metal-enamel interlocking coatings were conducted at 600 °C in water and static air. The results indicated that the metal-enamel interlocking showed superior thermal shock resistance to both pure enamel and enamel with 10 wt% Ni composite coatings. The enhanced performance was mainly attributed to the advantageous effects of mechanical interlocking of the enamel pegs formed at the enamel/Ni-plating interface. Meanwhile, during thermal shock test, big clusters formed by nanoscale Ni agglomerations were oxidised to be a Ni/NiO core–shell structure while small single nanoscale Ni grains were oxidised completely, which both improved the thermal shock resistance of enamel coating significantly.

Selective Defect Passivation and Topographical Control of 4‐Dimethylaminopyridine at Grain Boundary for Efficient and Stable Planar Perovskite Solar Cells

AbstractRecent progress in highly efficient perovskite solar cells (PSCs) has been made by virtue of interfacial engineering on 3D perovskite surfaces for their defect control, however, the structural stability of the modified interface against external stimuli still remains unresolved. Herein, 4‐dimethylaminopyridine (DMAP) is introduced to develop a facile technique for selectively passivating the grain boundary (GB) and controlling the topographical boundary of the perovskite surface near the GB. Through the surface treatment of DMAP, strongly bound DMAP crystals are selectively formed at the GB, which serves two functions: nonradiative recombination at GB is effectively reduced by healing the uncoordinated Pb2+ while adhesion strength between the perovskite and the poly(triaryl amine) (PTAA) polymer is significantly enhanced by a mechanical interlock effect. A planar PSC with DMAP treatment exhibits a champion power conversion efficiency of 22.4%, which is not only much higher than the 20.04% observed for a nontreated control device, but also the highest among the planar PSCs using PTAA polymers as a hole transport material. Furthermore, the use of DMAP leads to a substantial improvement in the device stability under damp‐heat test and light irradiation.

New insights into interface interactions of CNT-reinforced epoxy nanocomposites

Interfacial shear strength between carbon nanotubes (CNTs) and polymer matrices is a fundamental but unresolved issue. Here, through molecular dynamics simulation (MD), we revealed that there are two completely different interfaces between CNTs and epoxy matrix, namely the weak interface (τemb) at the embedded CNT region, and the strong pull-out interface (τpull−out) occurred at the entry position of CNT. Strikingly, the load transfer capability at the CNT-embedded interface is very poor, yielding a low τemb of ~6.8 MPa, which is one order of magnitude lower than that of the pull-out interface (τpull−out~132 MPa). Moreover, both of τemb and τpull−out depend on the diameters of CNT, but are not affected by CNT length. We also found that CNT defects, especially the topological defects such as CNT turns or kinks, are able to significantly increase the interfacial shear strength because of strong mechanical interlocking effect. Our results provide valuable insights into the CNT-polymer interface interactions, which are of great importance for exploring the reinforcing mechanisms for CNTs reinforced polymer nanocomposites.

Atomistic simulations of functionalization of aramid fiber‐epoxy nanocomposite

AbstractOwing to its high degree of crystallinity and orientation, the surface of aramid fiber is smooth, causing its low bonding strength with polymer matrix. This has restricted the application of aramid fiber in reinforced polymer materials. Effective methods are by introducing functional groups through surface modification and by increasing its surface roughness thereby greatly improving its bonding strength with the polymer. In this work, molecular dynamics (MD) simulation study fiber functionalized with hydroxyl (OH), carboxyl (COOH), and the silane coupling agent as nanofillers for polymer nanocomposites. The interfacial characteristics and the mechanical behavior of polymer nanocomposites are investigated. The results show that the functionalization can enhance the interfacial shear stress and tensile strength. The functional group not only provides a stronger interface, but also provides additional mechanical interlocking effect, which effectively improves load‐bearing transmission capacity. The analysis of the micro‐mechanism from the energy level also provides new insights for the functionalized design of nanocomposites.

Improvement of hydroxyapatite formation ability of titanium-based alloys by combination of acid etching and apatite nuclei precipitation.

The authors aimed to improve hydroxyapatite formation ability of Ti6Al4V, Ti-15Mo-5Zr-3Al alloy, Ti-12Ta-9Nb-6Zr-3V-O alloy (Gummetal®) and commercially pure Ti (cpTi) mesh by a combination of acid etching and apatite nuclei precipitation. Surfaces of specimens were etched with H2SO4 for pores formation on the specimens. Thus-etched specimens were soaked in an alkalinised simulated body fluid (SBF), which was adjusted at higher pH than that of conventional SBF and this solution was subsequently heated. By this treatment, apatite nuclei were precipitated in the pores of the specimens. By a soak in the conventional SBF to check hydroxyapatite formation ability, hydroxyapatite was covered the entire surfaces of the specimens within 1 day and high hydroxyapatite formation ability was successfully shown. The adhesion strength of the hydroxyapatite film formed in the above SBF test showed larger value as increasing the surface roughness of the specimens by adjusting the above acid etching condition depending on the kinds of Ti-based alloys. This is because the adhesion of the hydroxyapatite film occurred by the mechanical interlocking effect. In addition, this method showed shape selectivity of the materials because similar hydroxyapatite formation ability could be introduced to the cpTi mesh.

Improving the interfacial shear strength of carbon fibre and epoxy via mechanical interlocking effect

In this study, an effective and environmentally friendly method is proposed to enhance the interfacial shear strength between the carbon fibre and resin by the mechanical interlocking effect. The Fenton chemical reaction was used to oxidize and construct uneven structure on the carbon fibre surface, followed by functionalization with amine (-NH2) groups. The carbon fibre surface morphology was first designed, and the mechanical properties of the interface were investigated through molecular dynamics simulation. It was found that the interface shear strength increased by 277.9% and 133.6%, in comparison with the untreated CF and –NH2@CF, respectively. The microbond test showed that the interface shear strength improved by 251.1% and 159.4%, respectively, proving the effectiveness of our design within a certain error range. Moreover, the tensile strength increased by 8.39%. The proposed method opens up a new way to enhance the interface mechanical properties of fibre reinforced composite materials.

Functionalization enhancement interfacial bonding strength between graphene sheets and calcium silicate hydrate: Insights from molecular dynamics simulation

The macroscopic mechanical performance of functionalized graphene sheets (FGS) reinforced cement composite is strongly associated with the interfacial bonding performance between FGS and calcium silicate hydrate (CSH). Herein, the interfacial bonding performance of CSH gel incorporated FGS with different functional groups (-H, –NH2, –CH3, –OH and –COOH) at various coverage degrees was studied using molecular dynamics. The simulated results indicated that the interfacial bonding performance between FGS and CSH can be optimized through tuning the functional group. Our simulations also indicated that the interfacial adhesive strength increase with the coverage degrees of the functionalized group. The GCOOH sheet, the GOH sheet and the GNH2 sheet can effectively increase the interfacial bonding strength of the composite due to these strong interactions with the CSH gel, but the addition of GCH3 sheet, GH sheet and pristine graphene sheet cannot. Subsequently, the static molecular structure, the dynamic properties and the mechanical interlocking effect were analyzed to elucidate the underlying mechanism of the discrepancy of interfacial bonding properties for different FGS/CSH composites. Our study explains the enhanced mechanism for the CSH gel incorporated various FGS at molecular level, providing a basis for the optimal design of cement-based materials using nanotechnology.

Atomistic Investigation on the Wetting Behavior and Interfacial Joining of Polymer-Metal Interface.

Polymer-metal hybrid structures can reduce the weight of components while ensuring the structural strength, which in turn save cost and subsequently fuel consumption. The interface strength of polymer-metal hybrid structure is mainly determined by the synergistic effects of interfacial interaction and mechanical interlocking. In this study, the wetting behavior of polypropylene (PP) melt on metal surface was studied by molecular dynamics simulation. Atomistic models with smooth surface and nano-column arrays on Al substrate were constructed. Influences of melt temperature, surface roughness and metal material on the wetting behavior and interfacial joining were analyzed. Afterwards the separation process of injection-molded PP-metal hybrid structure was simulated to analyze joining strength. Results show that the initially sphere-like PP model gradually collapses in the wetting simulation. With a higher temperature, it is easier for molecule chains to spread along the surface. For substrate with rough surface, high density is observed at the bottom or on the upper surface of the column. The contact state is transitioning from Wenzel state to Cassie–Baxter state with the decrease of void fraction. The inner force of injection-molded PP-Fe hybrid structure during the separation process is obviously higher, demonstrating a greater joining strength.

Low Water Absorption, High-Strength Polyamide 6 Composites Blended with Sustainable Bamboo-Based Biochar.

To promote the application of polyamide 6 (PA6) in wood–plastic composites, the negative effects associated with the thermal degradation of plant fibers must be overcome. In this study, waste bamboo fibers were subjected to pyrolysis and ball milling to afford nano bamboo-based biochar (BC), which was subsequently used as reinforcement to prepare PA6/BC nano composites by injection molding. In addition, the processing fluidity, water absorption, mechanical properties, and interface compatibility of PA6/BC composites were discussed. Results revealed that a BC content of less than 30 wt% is beneficial to improve the processing fluidity of the composites. With the increase in the BC content, the density of the PA6/BC composites gradually increased, while the water absorption of the PA6/BC composites gradually decreased, and the maximum decrease was 46%. Compared to that of pure PA6, the mechanical strength of PA6/BC composites was improved by the addition of BC, and the maximum tensile/flexural strength and modulus of PA6/BC composites increased by 41%/72% and 195%/244%, respectively. However, the impact strength decreased by 27%. After immersion treatment, the dimensional stability and mechanical strength of the composites decreased, while toughness improved. At a BC content of less than 40 wt%, BC particles exhibited good dispersibility and wettability in the PA6 matrix, and the rough surface and rich pore structure of BC rendered strong mechanical interlocking effects and good interface compatibility, thereby enhancing the mechanical properties of the composites.