Interfacial Bond Strength Research Articles

Effect of Cavity Cleanser With Long-Term Antibacterial and Anti-Proteolytic Activities on Resin-Dentin Bond Stability.

ObjectiveSecondary caries caused by oral microbiome dysbiosis and hybrid layer degradation are two important contributors to the poor resin–dentin bond durability. Cavity cleansers with long-term antimicrobial and anti-proteolytic activities are in demand for eliminating bacteria-induced secondary caries and preventing hybrid layers from degradation. The objectives of the present study were to examine the long-term antimicrobial effect and anti-proteolytic potential of poly(amidoamine) dendrimers with amino terminal groups (PAMAM-NH2) cavity cleanser.MethodsAdsorption tests by attenuated total reflectance–infrared (ATR-IR) spectroscopy and confocal laser scanning microscopy (CLSM) were first performed to evaluate whether the PAMAM-NH2 cavity cleanser had binding capacity to dentin surface to fulfill its relatively long-term antimicrobial and anti-proteolytic effects. For antibacterial testing, Streptococcus mutans, Actinomyces naeslundii, and Enterococcus faecalis were grown on dentin surfaces, prior to the application of cavity cleanser. Colony-forming unit (CFU) counts and live/dead bacterial staining were performed to assess antibacterial effects. Gelatinolytic activity within the hybrid layers was directly detected by in situ zymography. Adhesive permeability of bonded interface and microtensile bond strength were employed to assess whether the PAMAM-NH2 cavity cleanser adversely affected resin–dentin bonding. Finally, the cytotoxicity of PAMAM-NH2 was evaluated by the Cell Counting Kit-8 (CCK-8) assay.ResultsAdsorption tests demonstrated that the binding capacity of PAMAM-NH2 on dentin surface was much stronger than that of 2% chlorhexidine (CHX) because its binding was strong enough to resist phosphate-buffered saline (PBS) washing. Antibacterial testing indicated that PAMAM-NH2 significantly inhibited bacteria grown on the dentin discs as compared with the control group (p < 0.05), which was comparable with the antibacterial activity of 2% CHX (p > 0.05). Hybrid layers conditioned with PAMAM-NH2 showed significant decrease in gelatin activity as compared with the control group. Furthermore, PAMAM-NH2 pretreatment did not adversely affect resin–dentin bonding because it did not decrease adhesive permeability and microtensile strength. CCK-8 assay showed that PAMAM-NH2 had low cytotoxicity on human dental pulp cells (HDPCs) and L929.ConclusionsPAMAM-NH2 cavity cleanser developed in this study could provide simultaneous long-term antimicrobial and anti-proteolytic activities for eliminating secondary caries that result from a dysbiosis in the oral microbiome and for preventing hybrid layers from degradation due to its good binding capacity to dentin collagen matrix, which are crucial for the maintenance of resin–dentin bond durability.

Interfacial bond strength of epoxy and PUR adhesively bonded timber-concrete composite joints manufactured in dry and wet processes

This paper investigated the shear bond strength of adhesively bonded timber-concrete composite (TCC) joints between spruce wood and cast-in-situ concrete (i.e., TCC joint produced in wet bond process), and between spruce wood and prefabricated concrete (i.e., produced in dry bond process). Two types of adhesive including two-component polyurethane (PUR) and two-component epoxy were used for bonding wood and the concrete blocks. In fabrication of the wet bond TCC joints, three waiting times defined as the interval times between the starting time of adhesive cross-linking process and starting time for pouring fresh concrete were considered for both PUR and epoxy adhesives. These three waiting times were 0, 0.5tgel and tgel in which tgel represents the gelation time of the corresponding PUR and epoxy measured by oscillatory test. The wet TCC joints manufactured at 0, 0.5tgel and tgel waiting times and the dry TCC joints were tested under shear load to obtain their shear strength. The values of the shear strength of the wet and dry bond TCC joints were discussed, compared and correlated with respect to the failure modes. The experimental results showed that the average shear strength of wet bond TCC joints bonded with PUR and epoxy both reduced by around 47% when increasing the waiting time from 0 to tgel. It should be attributed to that the adhesives tended to have a weaker bond with cast-in-situ concrete at a longer waiting time, based on optical microscopy observations. Both PUR and epoxy bonded dry TCC joints possessed similar average shear strength and identical failure modes, i.e., failure in adhesive area and small portion of concrete fracture. Moreover, the use of epoxy in wet bond TCC joints led to higher shear strength than that in the dry bond TCC joints, while this trend was conversely observed in PUR bonded wet and dry TCC joints.

Estimation of bond strengths and design parameters for precast concrete-UHPC shear key interface in box beam bridge

Adjacent precast prestressed concrete box beam bridge is often the choice for short-to-medium span bridges. However, it has been reported that box beam bridges experienced reoccurring longitudinal cracks at beam-shear key interfaces. The interface experiences a complex stress state represented by tensile/compressive stress perpendicular to the interface and longitudinal and vertical shear stress parallel to it. In this study, small-scaled specimens were cast together with the box beams and UHPC shear keys of a real bridge and were tested using four different test methods that try to replicate the actual stress conditions. The interface experienced satisfactory bond strength under different stress scenarios even at early age. The cohesion and friction coefficients were determined based on analysis of the experimental data and can be used to estimate the interfacial bond capacity at early age of UHPC. The interfacial bond capacity of a typical bridge was also determined and compared with the stress due to design truck load. The results indicate that the early age shrinkage and thermal load on the interface is more critical compared to the effects of the truck load. When subjected to the design truck load specified by current code, only 4.9% of the bond capacity was used, while the interfacial bond strength during the early age was exceeded. This indicates that the load is not the critical factor that causes bond failure at beam-shear key interface in box beam bridges. Furthermore, the cohesion value is sufficient to resist the applied load and the shear reinforcement can be optimized in the future application of UHPC in box beam bridges. Meanwhile, the early age tensile bond strength was exceeded and therefore the use of shear reinforcement to carry the early age tensile stress is necessary to prevent cracking.

Bending behaviour of steel cable reinforced 3D printed concrete in the direction perpendicular to the interfaces

The performance of 3D printed concrete (3DPC) specimens subjected to bending perpendicular to the interfaces has not been effectively investigated. In this study, steel cables were placed in the direction perpendicular to the printing interfaces, and the bending behaviour of reinforced 3D printed concrete was experimentally studied in this direction using the four-point bending test. The effects of the reinforcement ratio, hook length at the ends of the steel cables and the loading direction on the failure mode, as well as the flexural capacity, post-peak behaviour, and anisotropic behaviour of the specimens were analysed in detail. The results showed that with an appropriate reinforcement ratio and hook length, the reinforcement can greatly improve the post-peak bending behaviour in the direction perpendicular to the interfaces for 3DPC specimens. Moreover, since the flexural capacity depends on the compressive strength of concrete and the tensile strength of steel cables, rather than the interfacial bond strength, no obvious anisotropy of the 3DPC specimens can be found. This research will contribute to the structural design of 3DPC and its practical application in building structures.

Design of a bi-layer coating configuration on ultra-high molecular weight polyethylene (UHMWPE) fibre surface to derive synergistic response on interfacial bond strength

In this study, we demonstrate a design of a stable bi-layer coating configuration on UHMWPE fibres surface. UHMWPE fibres show abundant promise as load bearing reinforcement in fibre-based composites; however, its inert chemical nature has been a consistent hurdle for developing high performance composites. An efficient and environmentally friendly two-step coating technique has been performed on UHMWPE fibre surface; viz. Physisorption of polydopamine (PDA) followed by the construction of Polyamide 6,6 coating over PDA layer. The coated fibres revealed improved tensile strength in comparison to the pure UHMWPE fibres. Transverse fibre bundle tension (TFBT) test was used to assess the interfacial bonding strength of the coated fibre with an epoxy matrix. The bi-layer coated fibre/epoxy demonstrated an improvement of ∼ 40% in bonding strength compared to the pure UHMWPE/epoxy system. This bi-layer coated UHMWPE fibres could be of immense potential for several applications, including biomedical engineering.

Effect of casting and curing temperature on the interfacial bond strength of epoxy bonded concretes

The effect of casting and curing temperature on concrete-epoxy-concrete interfacial bond strength was evaluated. To cover a wide range of climatic conditions, six different temperature values from 5 to 55 °C were considered. The interfacial bond strength of the composites was experimentally assessed using pull-off, wedge splitting and bi-surface shear tests. The individual materials, epoxy and cement paste, were also characterized by means of viscosity, shore D hardness and setting time measurements. Results show that the casting and curing temperature dramatically impacts the mechanical properties of the epoxy bonded concretes. Indeed, increasing temperature from 5 to 55 °C leads to a decrease in bond strength up to 65%. The significant drop in setting time of epoxy compared to cement at high temperature, the inferior epoxy mechanical properties, as well as the reduction in epoxy thickness due to material loss in pores and microcracks can explain this decrease.

Mechanical performance of concrete strengthened by modified epoxy resin bonded CFRP

Mechanical performance of concrete strengthened by modified epoxy resin bonded CFRP (carbon fiber reinforced polymer) is present in this paper, with particular focus on the interface bond strength and flexural strength. A type of modified epoxy resin adhesive which bonds CFRP sheets to strengthen plain concrete was developed. The interface bond strength between CFRP and concrete and the flexural strength of CFRP reinforced concrete were comprehensively tested. The influence of the CFRP size, number of CFRP bonding layers, and concrete strength was further analyzed. The results showed that CFRP significantly improved the maximum bearing capacity, displacement and failure energy of plain concrete. The bearing capacity of plain concrete improved almost linearly with an increasing number of CFRP layers. CFRP-reinforced low-density concrete has a better strengthening effect than high-density concrete. The final strength and failure energy of the CFRP-reinforced concrete increased significantly, compared to that of plain concrete. A model for calculating the maximum flexural strength of reinforced concrete structures is proposed.

On the fatigue properties of a third generation aluminium-steel butt weld made by Hybrid Metal Extrusion & Bonding (HYB)

The present investigation is concerned with the high-cycle axial fatigue behaviour of a third generation Al-steel butt weld made by Hybrid Metal Extrusion & Bonding (HYB). In this particular weld, metallurgical bonding is achieved by a combination of microscale mechanical interlocking and intermetallic compound (IMC) formation, where the IMC layer is in the sub-micrometre range (<1µm). During high-cycle axial fatigue testing this microstructure provides a high intrinsic resistance against interfacial cracking. In the as-welded condition, fatigue fracture typically initiates at the weld toe on the aluminium side of the joint due to the unfavourable effect of having a geometrical stress riser localised inside the soft heat-affected zone. Since the interfacial bond strength is not a limiting factor, the fatigue properties of the Al-steel HYB butt weld are seen to fully match those of corresponding Al-Al weldments produced by gas metal arc welding, laser beam welding and friction stir welding.

Mechanical properties and drying shrinkage of alkali-activated seawater coral aggregate concrete

Alkali-activated materials (AAMs) are identified as environmental-friendly substitutes for ordinary Portland cement (OPC) for lowing greenhouse-gas emissions and achieving efficient waste recycling. In this paper, well-durable AAMs were utilized to prepare the slag-based alkali-activated seawater coral aggregate concrete (AACAC) and three alkaline dosages (Na2O-to-binder ratios of 3, 4, and 6% by weight) were designed to determine their mechanical characteristics and drying shrinkage. The tested results indicated that the failure modes for all coral aggregate concrete (CAC) were characterized by the broken coral aggregates originated from their low strength and high brittleness. As the alkaline concentration increased, the compressive strength (f cu), splitting tensile strength (f ts), axial compressive strength (f c), elastic modulus (E c), and interfacial bond strength (f b) between the new concrete and the old concrete, were gradually enhanced. Additionally, the (f b/f ts) ratio can be stabilized at about 0.70. Compared with the cement-based CAC, the AACAC achieved higher f ts, E c, and f c because of the improved interfacial microstructures between the aggregate and the paste matrix after using AAMs. Moreover, the utilization of coral aggregates can lower the drying shrinkage (approximately 14.7%) of conventional alkali-activated concrete (AAC) at the same alkaline content owing to the internal self-curing effect of porous aggregates.

Heat Transfer-Based Non-isothermal Healing Model for the Interfacial Bonding Strength of Fused Filament Fabricated Polyetheretherketone.

Fused Filament Fabrication (FFF) as an Additive Manufacturing (AM) method for Polyetheretherketone (PEEK) has established a promising future for medical applications so far, however interlayer delamination as a failure mechanism for FFF implants has raised critical concerns. A one-dimensional (1D) heat transfer model (HTM) was developed to compute the layer and interlayer temperatures by considering the nature of 3D printing for FFF PEEK builds. The HTM was then coupled with a non-isothermal healing model to predict the interlayer strength through thickness of a FFF PEEK part. We then conducted a parametric study of the primary temperature effects of the FFF system, including the print bed, nozzle, and chamber temperatures, on layer healing. The heat transfer component of the model for the FFF PEEK layer healing assessment was validated separately. An idealized PEEK cube design (10x10x10 mm3) was used for model development and 3D printed in commercially available industrial and medical FFF machines. During the printing and cooling processes of FFF, thermal videos were recorded in both printers using a calibrated infrared camera. Thermal images were then processed to obtain time-dependent layer temperature profiles of FFF PEEK prints. Both the theoretical model and experiments confirmed that the upper layers in reference to the print bed exhibited higher temperatures, thus higher healing degrees than the lower layers. Increasing the print bed temperature increased the healing of the layers allowing more layers to heal 100%. The nozzle temperature showed the most significant effect on the layer healing, and under certain nozzle temperature, none of the layers healed adequately. Although environment temperature had less impact on the lower layers closer to the print bed, 100% healed layer number increased when the chamber temperature increased. The model predictions were in good agreement with the experimental data, particularly for the mid-part of FFF PEEK cubes printed in both FFF machines.

Experimental evaluation of carbon/epoxy laminates with concentrated carbon nanotube interlayers for high damping

Carbon nanotubes (CNTs) embedded in carbon/epoxy (c/ep) composites offer a lightweight, stiff solution with high damping for structural components. To manufacture CNT/c/ep hybrid composites with high concentrations of CNTs, a CNT yarn interlayer concept is used. Through a mechanism known as stick-slip, the interface between CNTs and c/ep laminates dissipate energy during dynamic cycling. It is predicted that altering the interfacial bond strength can enhance dynamic properties: loss factor, loss modulus, and storage modulus. In this research, the effect of surface treatments on CNT yarns prior to inserting the CNTs into the laminate was explored to determine if the dynamic properties of the composite were enhanced. It was determined that 2,3-dibromo-1,4-butanediol (23D14B), Triton X-100 (TX) and a solution of sulfuric and nitric (S/N Acid) were the best treatments of 12 tested based on the increased dynamic properties. For a composite with 5 vol% CNT yarn interlayers, 23D14B, TX, and S/N Acid increased Loss Factor by 230%, 130%, and 160%, respectively. Elevated temperature, moisture-saturated/elevated temperature, and cyclic loading frequency were investigated on 5vol. % CNT interlayers to determine their effects on dynamic properties of the CNT hybrid composites.

Adhesion-cohesion balance of prepreg tack in thermoset automated fiber placement. Part 1: Adhesion and surface wetting

The constitution of prepreg tack in automated fiber placement (AFP) is affected by a sensitive balance between adhesive interfacial bond strength and cohesive strength of the prepreg resin. In an effort to explore the role of interfacial liquid-solid interaction on the tack of commercial aerospace-grade epoxy prepreg, a surface wetting analysis was performed on AFP-related substrates. The standard test liquid combination water/diiodmethane and extracted neat epoxy resin were used for contact angle measurement employing the sessile drop method and the Owens-Wendt-Rabel-Kaelble (OWRK) model. Additional rheological and topographical analyses were carried out to account for viscous resin flow on surfaces of different roughness. The results from the material characterization are discussed against the background of tack measurement by probe tack testing utilizing a rheometer. Significant differences between the investigated surfaces in terms of both the maximum tack level and the onset temperatures of adhesion were found as a function of test parameters relevant for contact formation. General agreement with the experimental tack results was observed employing a topographically extended version of the Dahlquist criterion. For each substrate, a temperature-dependent critical storage modulus could be determined that conforms to the onset temperature of tackiness. Contact angle measurements revealed a correlation between the thermodynamic work of adhesion and maximum tack and, moreover, the tack onset in the adhesive regime when additionally incorporating surface topography. Matching ratios of polar and dispersive surface free energy and surface tension components were found to favor the molecular interaction at the interface between prepreg resin and substrate.

Behavior of novel hybrid lightweight concrete composites under drop-weight impact loading

This investigation evaluated the feasibility of combining lightweight concrete (LWC) and engineered cementitious composite (ECC) in specific configurations to develop a lightweight hybrid composite with improved impact resistance. In this study, three mixtures were investigated: one LWC mixture fully produced with lightweight fine and coarse expanded slate aggregates; one ECC mixture developed with polyvinyl alcohol fibers (ECCP); and one ECC mixture developed with steel fibers (ECCS). For testing, cylindrical specimens and small-scale beams were constructed in a two-layer composite system having different configurations (i.e., different arrangements and depths). Additional specimens fully cast with LWC, ECCP, and ECCS were tested for comparison. The interface bond strength between LWC and either ECCP or ECCS was also evaluated. The performance of the developed composites was assessed under static flexural loading and drop-weight impact. The results showed that both ECCP and ECCS could achieve a good bonding with LWC substrate, i.e., above 90% of that measured for the monolithic LWC specimens, even with no surface preparations (i.e., roughening or using shear keys). Combining LWC with either ECCP or ECCS generated novel lightweight hybrid composites that can offer superior performance for different structural lightweight members exposed to high impact loading. For given arrangement and depth, the LWC-ECCS composite exhibited higher mechanical and impact performance whereas the LWC-ECCP was distinguished by achieving an excellent impact resistance at a lighter self-weight.

Improving bond strength and mass transport properties of spacer-concrete interface with textured spacers

Spacers are important devices used in all concrete structures to support reinforcing steel and achieve the required cover. However, spacers induce a weak, porous and microcracked interface that facilitates ingress of aggressive agents. This paper aims to address the problem by developing a method to produce cementitious spacers with a range of small-scale surface textures including grooves, squares, rectangles, hemispheres and pyramids. The textured spacers were cast in Portland cement mortar or concrete, cured up to 28 days, and tested for tensile bond strength, accessible porosity and mass transport properties. The properties were correlated to surface characteristics to establish the effects of texture on spacer-concrete interface. Results show that textured spacers promote mechanical interlocking with concrete, thereby increasing bond strength, resistance against microcracking and pressure-induced flow. The use of certain textures can compensate for the negative effects of spacers, achieving similar bond strength and permeability to monolithic concrete without the interface.

Development of data-driven prediction model for CFRP-steel bond strength by implementing ensemble learning algorithms

Bonding carbon fiber reinforced polymer (CFRP) laminates has been broadly utilized in steel structure rehabilitation. As for the final strengthened capacity, the bond strength between CFRP and steel usually plays a dominant role instead of the CFRP’s mechanical property. However, the bond behavior of the CFRP-steel (CS) interface is very complicated with various failure modes and consequently the bond strength is hard to estimate leading to the CFRP strengthened steel structure insecure. Under this circumstance, in order to accurately predict the bond strength of CS, efficient data-driven models were developed through implementing ensemble learning (EL) algorithms named by “gradient boosting decision tree (GBDT)” and “random forest (RF)” as two representative ones on a collected CS single-shear test database. These models’ performances on bond strength prediction were compared and also three representative machine learning algorithms “artificial neural network (ANN)”, “support vector machine (SVM)” and “classification and regression tree (CART)” are utilized for validating the necessity. The comparison results indicate that the model generated by the GBDT algorithm attains the best accuracy for CS interfacial bond strength prediction (R2 = 0.98) among the ensemble and machine learning algorithms. Through model explaning analysis, the mechanism behind the GBDT based prediction model was also carefully verified. After these tests and analyses, the GBDT based model was proved to have the potential to facilitate the design and evaluation of CFRP strengthened steel structures.

Development of Intertwined Infills to Improve Multi-Material Interfacial Bond Strength

Abstract Recently, the availability of various materials and ongoing research in developing advanced systems for multi-material additive manufacturing (MMAM) have opened doors for innovation in functional products. One major concern of MMAM is the strength at the interface between materials. This article hypothesizes overlapping and interlacing materials to enhance the bonding strength. To test this hypothesis, we need a computer-aided manufacturing (CAM) tool that can process the overlapped material regions. However, existing computational tools lack key multi-material design processing features and have certain limitations in making full use of the material information, which restricts the testing of our hypothesis. Therefore, this research also develops a new MMAM slicing framework that efficiently identifies the boundaries for materials to develop different advanced features. By modifying a ray tracing technology, we develop layered-depth material images (LDMI) to process the material information from computer-aided design (CAD) models for slicing and process planning. Each sample point in the LDMI has associated material and geometric properties that are used to identify the multi-material regions. Based on the material information in each slice, interlocking joint (T-Joint) and interlacing infill are generated in the regions with multiple materials. Tensile tests have been performed to verify the enhancement of mechanical properties by the use of overlapping and interlacing materials.

Experimental investigation of the bond-slip behaviour between corrugated steel plates and concrete in CSRC structures

Corrugated steel and reinforced concrete (CSRC) composite structures refer to composite structures that contain reinforced concrete and corrugated steel plates as load-bearing components. To study the interfacial bond-slip behaviour between the corrugated steel plate and concrete, a total of sixteen CSRC specimens were designed, and push-out tests were performed. The influences of the concrete strength, the thickness of the concrete protective layer, the stirrup ratio and the embedment length of the corrugated steel plate on the bond-slip behaviour of the CSRC composite structure were analysed. In addition, the contribution of corrugation ridges, corrugation valleys, and the inside and outside of the corrugated steel plate to the bond strength was studied. The results demonstrated that the final failure modes of CSRC specimens could be divided into two types, namely, bond-splitting failure and bond-anchoring failure; the strain of the corrugated steel plate in the CSRC specimen at different positions exhibited an approximately exponential distribution. The four main influencing parameters all had a positive impact on the interfacial bond strength of CSRC. From the perspective of the contribution of different parts of the corrugated steel plate to the ultimate bond strength of the CSRC specimen, the corrugation valley is weaker than the corrugation ridge, and the inside and outside of the corrugated steel plate are approximately the same. Through the method of linear fitting, the calculation formulas for the bond strength of CSRC were obtained. Based on the concept of interface failure energy (Gf), a more accurate bond-slip constitutive model of the CSRC was proposed, which could provide a certain basis for establishing the relevant theoretical calculation methods.

Investigating the compatibility of nickel coated carbon nanotubes and cementitious composites through experimental evidence and theoretical calculations

Nickel coated multi-walled carbon nanotubes (NiMCNTs) are favorable reinforcing nanofillers for modifying cementitious composites due to their preeminent mechanical properties, electrical conductivity, thermal properties and dispersibility. This paper investigates the mechanical properties and compatibility of NiMCNTs filled cementitious composites, having two different types of cement, two water to cement ratios, and two dosages of five types of NiMCNTs. The results show that 0.06 vol% NiMCNTs with small aspect ratios can significantly enhance the mechanical properties of cementitious composites, while NiMCNTs with large aspect ratios play a better strengthening effect at 0.03 vol%. The flexural strength/toughness of cementitious composites containing 0.06 vol% NiMCNTs with an aspect ratio of 200 can be increased by 19.65%/116.78%. Adding 0.03 vol% NiMCNTs with an aspect ratio of 1000 enhances the compressive strength/toughness of composites by 18.61%/47.44%. Besides, NiMCNTs have preferable compatibility to cementitious composites prepared by P·O 42.5R cement with a water to cement ratio of 0.3. The enhancement mechanism is related to the denser microstructure and effective suppression of microcracks in the cementitious matrix by NiMCNTs with filling, bridging and pull-out effects, as well as the high interface bond strength between NiMCNTs and matrix. A strength prediction model for NiMCNTs reinforced cementitious composites is also established to estimate the mechanical strength of cementitious composites containing NiMCNTs with different aspect ratios/contents, showing a small relative error within ±6%/±13% for predicted flexural/compressive strength values in comparison with the experimental results.

Effects of heat treatment on mechanical properties of 3D Si3N4f/BN/Si3N4 composites by PIP

The fabrication of three-dimensional silicon nitride (Si3N4) fiber-reinforced silicon nitride matrix (3D Si3N4f/BN/Si3N4) composites with a boron nitride (BN) interphase through precursor infiltration and pyrolysis (PIP) process was reported. Heat treatment at 1000–1200 °C was used to analyze the thermal stability of the Si3N4f/BN/Si3N4 composites. It was found after heat treatment the flexural strength and fracture toughness change with a pattern that decrease first and then increase, which are 191 ± 13 MPa and 5.8 ± 0.5 MPa·m1/2 respectively for as-fabricated composites, and reach the minimum values of 138 ± 6 MPa and 3.9 ± 0.4 MPa·m1/2 respectively for composites annealed at 1100 °C. The influence mechanisms of the heat treatment on the Si3N4f/BN/Si3N4 composites include: (Ⅰ) matrix shrinkage by further ceramization that causes defects such as pores and cracks in composites, and (Ⅱ) prestress relaxation, thermal residual stress (TRS) redistribution and a better wetting at the fiber/matrix (F/M) surface that increase the interfacial bonding strength (IBS). Thus, heat treatment affects the mechanical properties of composites by changing the properties of the matrix and IBS, where the load transfer efficiency onto the fibers is fluctuating by the microstructural evolution of matrix and gradually increasing IBS.

UHPC grout-filled pipe sleeve with bolts for large-diameter deformed bars: Analytical model on the tensile resistance and design method

A novel UHPC grout-filled pipe sleeve with bolts was put forward for connecting large-diameter deformed bars and its feasibility was experimentally verified in a previous study. But it is not easy to rapidly broaden its application partly due to the lack of relevant calculation and design knowledge. This study aims to establish an analytical model on the tensile resistance that is dependent on the deformed bar-UHPC grout and UHPC grout-steel pipe interfaces, and develop a design method for the proposed pipe sleeve. In the model, the former interface resistance was analytically calculated considering the friction resistance, bearing of ribs against UHPC grout, and steel pipe's confinement. A dimensionless index ηb-g was defined to measure the relationship of the bond stress and pressure at the interface, and calibrated based on the experimental results. The latter interface resistance was analytically calculated considering the interface bond strength and resistance of bolts. Experimental results were adopted to verify the analytical model. A design method was developed for the pipe sleeve based on the verified analytical model, and a design case was given to illustrate how to use it. Results show that the analytical model can accurately predict the two interface resistances. ηb-g is respectively recommended for three categories of pipe sleeves, that are distinguished by the existence of bolts and grout type. The design method is convenient to provide a feasible design scheme of the pipe sleeve for a specified large-diameter deformed bar, of which the embedded length can be reduced to be 6 ~ 7 times bar diameter.