Exploring the impact of cellulose nanocrystals and annealing treatment on the interlayer bond strength of polylactic acid 3D printed composites

The extrusion-based 3D printing process offers advantages, such as high precision, low cost, high speed, simplicity, and the ability to deposit multiple materials simultaneously. However, using 3D printing composite materials with orthogonal anisotropy can limit the interlayer bonding strength of printed parts. In this study, the interlayer tensile strength of 3D-printed polylactic acid (PLA) was affected by adding 0.5 wt% cellulose nanocrystals (CNC) and 1.2 wt% di-cumyl peroxide (DCP) to PLA, and annealing at low temperature (373 K, 1 h). The effects of annealing and CNC were determined by mechanical testing, scanning electron microscopy, differential scanning calorimetry, expansion testing, rheological testing, and x-ray diffraction analysis. After annealing, the interlayer gap increased due to crystal shrinkage, leading to a 25.9% decrease in tensile strength. However, the addition of CNC and DCP significantly improved the flow properties of the sample, resulting in better interlayer bonding and a 52.7% increase in tensile strength.     

Comparison of methods for evaluating bond strength between concrete substrate and repair materials

This investigation was aimed at studying the effect of test methods on bond strength between concrete substrate and repair material. Four test methods with cementitious or modified-cementitious repair materials, and two surface roughnesses were studied. The methods used were pull-off, slant shear, splitting prism and a new direct shear named Bi-Surface shear test. While the coefficient of variation (COV) for each type of test was acceptable, the bond strengths from some tests were up to eight times larger than those obtained from others. It is imperative that the bond tests be selected such that they represent the state of stress the structure is subjected to in the field. The new test method was easy to carry out and had reasonable results and can be developed by further investigations.     

Kissing bond detection in structural adhesive joints using nonlinear dynamic characteristics

In this paper, the effect of kissing bond on nonlinear dynamic behavior of structures with flexible adhesive joint is investigated. Bilinear characteristic due to opening and closing in kissing bond region results in nonlinear dynamic behavior of the structure such as harmonic distortion in response to harmonic excitation. So, the higher-order harmonics can be considered as Nonlinear Damage Indicator Functions (NDIF) for the purpose of damage identification. A two-dimensional (2D) finite element model of a beam connected to a rigid support via a flexible adhesive layer is used to investigate the efficiency of the proposed NDIFs in kissing bond detection. Kissing bond is introduced to the model via contact elements. NDIFs are extracted for the finite element model using single tone stepped-sine test simulation. Parameters such as amplitude of excitation, size and location of kissing bond region as well as friction between kissing surfaces, are studied. The results proved that the NDIFs are sensitive to the size and location of kissing bond. Consequently, in an experimental damage identification procedure, NDIFs can be used as an indicator of kissing bond type damages in adhesive joints.     

Investigations on the compressive strength of silica fume concrete using statistical methods

The present paper deals with a mathematical model developed using statistical methods to predict the 28-day compressive strength of silica fume concrete with water-to-cementitious material (w/cm) ratios ranging from 0.3 to 0.42 and silica fume replacement percentages from 5 to 30. Strength results of 26 concrete mixes, on more than 300 test specimens, have been analyzed for statistical modeling. The ratios of compressive strengths between silica fume and control concrete have been related to silica fume replacement percentage. The expression, being derived with strength ratios and not with absolute values of strength, is independent of the specimen parameters and is applicable to all types of specimens. On examining the validity of the model with the results of previous researchers, it was observed that for results on both cubes and cylinders, predictions were obtained within 7.5% of the experimentally obtained values.     

Environment-friendly deposition of SiCN interlayer for reinforcing carbon fibers in C/SiC composites

Several C/SiC composites with no interlayer, single pyrocarbon (PyC) interlayer and PyC/SiCN interlayer were fabricated by polymer infiltration and pyrolysis process. The microstructure and mechanical properties were investigated. The results verified that SiCN interlayer was formed on carbon fibers. Both bulk density and flexural stress of C/SiC composite with PyC/SiCN interlayer were slightly higher than composite fabricated with single PyC interlayer. When the weight fraction of SiCN interlayer in the composite was about 18 wt%, the flexural stress of the composite was enhanced to 416 MPa from 352 MPa for composite with single PyC interlayer. The observations of pulled-out fibers on fracture surfaces revealed non-catastrophic fracture features for PyC/SiCN deposited C/SiC composite.     

人工神经网络基团键贡献法预测烷烃闪点

建立了一个基于人工神经网络方法的基团键贡献模型,用于预测烷烃闪点。该模型既考虑了分子中基团的特性,又考虑了基团之间的连接性(化学键)。以16种烷烃基团键作为神经网络的输入参数,研究了44种烷烃的闪点与分子结构之间的相关性。结果表明,闪点预测值与实验值符合良好,绝对平均绝对误差6.0 K,绝对平均相对误差2.15%,优于传统基团贡献法所得结果。该方法的提出不仅揭示了烷烃闪点与分子结构之间的定量关系,而且为工程上提供了一种预测有机物闪点的新的有效方法。     

Prediction of the flash points of alkanes by group bond contribution method using artificial neural networks

A group bond contribution model using artificial neural networks, which had the high ability of nonlinear of prediction, was established to predict the flash points of alkanes. This model contained not only the information of group property but also connectivity in molecules. A set of 16 group bonds were used as input parameters of neural networks to study the correlation of molecular structures with flash points of 44 alkanes. The results showed that the predicted flash points were in good agreement with the experimental data that the absolute mean absolute error was 6.9 K and the absolute mean relative error was 2.29%, which were superior to those of traditional group contribution methods. The method can be used not only to reveal the quantitative correlation between flash points and molecular structures of alkanes but also to predict the flash points of organic compounds for chemical engineering. __________ Translated from Chemical Engineering (China), 2007, 35(4): 38–41 [译自: 化学工程]     

The use of USPV to anticipate failure in concrete under compression

The use of the ultrasonic pulse velocity tester is introduced as a tool to monitor basic initial cracking of concrete structures and hence to introduce a threshold limit for possible failure of the structures. Experiments using ultrasonic pulse velocity tester have been carried out, under laboratory conditions, on various concrete specimens loaded in compression up to failure. Special plots, showing the relation between the velocity through concrete and the stress during loading, have been introduced. Also, stress–strain measurements have been carried out in order to obtain the corresponding strains. Results showed that severe cracking occurred at a stress level of about 85% of the rupture load. The average velocity at this critical limit was about 94% of the initial velocity and the corresponding strain was in the range of 0.0015 to 0.0021. The sum of the crack widths has been estimated using special relations and measurements. This value that corresponds to the 94% relative velocity was between 5.2 and 6.8 mm.     

The role of interlayer grafting on the mechanical properties of magadiite/styrene‐butadiene rubber composites

This work examines the mechanisms by which magadiite (MGD), a synthetic layered silicate, acts as an active filler to provide high levels of mechanical reinforcement in styrene‐butadiene rubber (SBR) composites. Cetyltrimethylammonium (CTA⁺) expands the MGD layer spacing and promotes intercalation of SBR and silane coupling agent (Si69); the resulting CMGD/SBR composites have greater tensile moduli than comparable silica/SBR composites. CMGD was reacted in solution with Si69 (or MPTES) to prepare “pre‐grafted” MGD with varying levels of interlayer silane functionalization (SMGD). If the silane graft density is relatively low, the resulting SMGD/SBR composite has mechanical properties comparable to CMGD composites prepared with Si69 added during batch mixing. However, SMGD with high silane graft density does not permit SBR intercalation and produces composites with inferior mechanical properties, demonstrating the necessity of silane‐mediated interlayer grafting. Omitting Si69 from the formulation dramatically reduces the level of mechanical reinforcement as measured by DMA and tensile testing. Adding extra bulk sulfur (to replace sulfur omitted with Si69) does not produce composites with mechanical properties comparable to CMGD/SBR or SMGD/SBR prepared with Si69. This work demonstrates that silane‐mediated SBR‐MGD grafts within the MGD interlayer space are essential for achieving high levels of mechanical reinforcement in MGD/SBR composites. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45025.     

Effect of corrosion on bond in reinforced concrete under cyclic loading

Cyclic loading can result in severe deterioration in the bond between reinforcing steel bar and the surrounding concrete, especially when the reinforcement is corroded. In this study, tests were carried out for bond stress-slip response of corroded reinforcement with concrete under cyclic loading. Parameters investigated include: corrosion level, confinement, bar type, and loading history. The results revealed that bond behaviour was significantly reduced under cyclic loading. Degradation in bond was significantly less for deformed bars than for smooth bars at the initial loading cycle, but the difference was diminished with loading. The bond reduction was more substantial for unconfined steel bars than for confined bars. The relatively high level of corrosion caused degradation primarily in the initial five cycles, the effect of corrosion being decreased with loading. It was also demonstrated that the cyclic bond stress-slip curves depended on loading history.     

聚乙烯管道热熔接头可靠性评价方法研究

介绍了目前国内外聚乙烯管道熔接接头可靠性评价的短期力学性能和长期耐慢速裂纹拓展的主要检测方法及其适用性研究.近年来,超声波相控阵被广泛应用于熔接接头可靠性无损检测,其自动化检测和结果智能化判定是一个重要的研究方向.目前我国标准中对熔接接头可靠性检测的项目较少,为使聚乙烯承压管道向厚壁、大口径、高耐压、耐高温、长寿命和更...     

The relationship between substrate roughness parameters and bond strength of ultra high-performance fiber concrete

The bonding that exists between the old concrete and the new concrete depends largely on the quality of substrate surface preparation. The accurate representation of substrate surface roughness can help determine very precisely the correct bonding behavior. In this work, an experimental investigation was carried out to quantify the normal concrete (NC) substrate roughness parameters and evaluate their relationship with the bonding performance of ultra high-performance fiber concrete (UHPFC), used as a repair material. The bond strength was quantified based on the results of the pull-off test, splitting cylinder tensile test, and the slant shear test. Three types of NC substrate surface preparation were used: as-cast (without surface preparation) as reference, wire-brushed, and sand-blasted (SB); the roughness of which was determined using an optical three-dimensional (3D) surface metrology device (Alicona Infinite Focus). It was observed from the result of the pull-off test that failure occurred in the substrate, even though adequate substrate surface roughness was provided. Moreover, analysis of the splitting cylinder tensile and slant shear test results showed that the substrate surface preparation method had a significant influence in bonding strength between UHPFC and the NC substrate. The composite UHPFC/NC substrate having a SB surface behaved closely as a monolithic structure under splitting and slant shear tests. An excellent correlation (R ²?>?85%) was obtained between the substrate roughness parameters and the results of the splitting cylinder tensile and slant shear tests.     

Influence of different surface treatments on zirconia/resin shear bond strength using one-bottle universal adhesive

ABSTRACT

This study was designed to evaluate the effect of different treatments on the zirconia/resin shear bond strength (SBS) using commercial one-bottle universal adhesive. Zirconia discs with different surface treatments (blank control; airborne-particle-abrasion; glazing) were bonded to the bovine enamel surfaces using one-bottle universal adhesive. All specimens were tested for SBS (MPa) before and after 10000 thermocycles. Statistically analysis were conducted by using one-way analysis of variance and multiple-comparison least significant difference tests (α = 0.05). Airborne-particle-abrasion group showed higher SBS (36.19 ± 11.86) than control group (14.98 ± 5.90) and glazing group (10.63 ± 5.39) (p < 0.05). After thermocycling test, the SBS significantly decreased for control group (8.84 ± 2.55) and glazing group (6.18 ± 2.78) while not for airborne-particle-abrasion group (41.5 ± 7.95). One-bottle universal adhesives combined with airborne-particle-abrasion showed quite high SBS of zirconia/resin, which was appropriate for bonding of zirconia restoration.     

Improvement of flexural bond strength of zirconia-resin cement by surface patterning using sub-nanosecond UV laser

Zirconia is a dental material that shows excellent biocompatibility and high strength in clinical applications. This study aims to evaluate the effects of ultrafast laser applications. The surface nanostructures were classified into three groups. Group 1 was generated using the burst mode, with three different distances between dots: 52 µm (Group 1a), 104 µm (Group 1b), and 156 µm (Group 1c). Group 2 was processed using the scanning mode configuration, with a set of parallel lines. Group 3 was also processed using this scanning configuration creating a set of square-shaped patterning. Group 4 was the control group. After the surface treatments, a pair of zirconia specimens was bonded end to end with resin cement. Flexural bond strength (FBS) test was applied in a universal test machine. Multiple comparisons were performed using a one-way analysis of variance and the Tukey's HSD test. All the samples that were treated with the laser showed higher FBS values than the untreated surface. Using the burst mode, preformed circular-shaped surface on an angle of 90⁰ at 52 µm distance (Group 1a) showed the highest FBS values among all groups (p < .05). Groups 2 and 3 had significantly higher values than 1b and 1c.     

Microstructural analysis of the adhesion mechanism between old concrete substrate and UHPFC

The performance of any repaired concrete structure, and thus its service life, depends on the quality of the interfacial transition zone of the composite system formed by the repair material and the existing concrete substrate. In this work, the properties of the interfacial transition zone between normal concrete (NC) substrate as an old concrete and ultra-high performance fiber-reinforced concrete (UHPFC) as a repair material was investigated. Pull-off and splitting cylinder tensile tests were performed to quantify the bond strength in direct and indirect tensions, respectively. The microstructure of the interfacial transition zone was also studied using scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM/EDS). Different types of NC substrate surface preparation methods were used. An optical three-dimensional surface metrology device was used to estimate the substrate roughness parameters. Based on the results, high interfacial bond strength was achieved on the 3rd, 7th, and 28th days. The pull-off test results revealed that all failures occurred in the substrate, regardless of the substrate surface roughness. The majority of failures in the split tensile test also occurred in the substrate. SEM/EDS proved that the use of UHPFC as a repair material chemically, physically, and mechanically improved the repaired interfacial transition zone to become stronger and denser, as well as more uniform, and durable. Moreover, the use of UHPFC increased the service life of repaired structures and minimized the number and extent of interventions to the lowest possible level.     

The effect of lateral pressure on the bond of round reinforcing bars in concrete

Lateral pressure is found to significantly increase the bond strength that can be mobilized at the bar/concrete interface. For round bars the effect is mainly frictional and can result in an increase in pull-out load of as much as 200% for lateral pressures close to the cube strength of the concrete. Two different methods of bond testing, the standard cube pull-out and the more representative semi-beam test, were modified to investigate the influence of lateral stress on ultimate bond strength. The enhanced bond strengths obtained in the tests are compared with those predicted using a theoretical approach to estimate the frictional bond strength; reasonable agreement is shown.     

Analytic methods and theory of quantitative stereology for the determination of concrete proportioning in structural components

The analytic methods and the theory of the quantitative stereology combining with micro-analysis for concrete proportion on structural components were systemically described in this paper, specifically, the effective mix proportion of structural concrete was identified through three levels macroscopic (coarse aggregate), mesoscopic (fine aggregate) and microscopic (cement paste). Moreover, the porosity and hydration degree of structural concrete could be understood based on the compositional analysis of aggregate and cement stone. The results of the experimental analysis indicated that the mix proportion of concrete was similar to the effective mix proportion, the systemic error of the method could be amended, and the relative errors among configurations of materials were less than 4%, which validated the accuracy and the applicability of the methods.     

Experimental study on bond strength of fiber reinforced polymer rebars in normal strength concrete

The bond behavior of reinforcing bars is an important issue in the design of reinforced concrete structures and the use of fiber reinforced polymer (FRP) rebars is a promising solution to handle the problems of steel reinforcement corrosion. This study investigates the bond characteristics of carbon and aramid FRP (CFRP and AFRP) bars embedded in normal strength concrete. A pullout test was performed on 63 normal strength concrete specimens reinforced with FRP and steel rebars with different embedment lengths and bar diameters. The average bond stress versus slip curve is plotted for all specimens and their failure modes are identified. The effects of the embedment length and diameter of an FRP rebar on its bond strength are examined in this work. The bond strengths obtained from the test results are compared with the predictions by the bond strength equation proposed by Okelo and Yuan (2005), and its validity is evaluated.     

The Non-destructive Testing of Adhesively Bonded Structure: A Review

The types of defect encountered in adhesive joints and the non-destructive testing techniques available to detect them are reviewed. Three types of defect: complete voids or dis-bonds, poor cohesive strength of the adhesive layer and poor adhesion between the adhesive layer and adherend are commonly present. It is shown that a variety of techniques is available for dis-bond and void detection, ultrasonics and sonic vibration being the most commonly used. The detection of poor cohesive and adhesive properties, however, is much more difficult than void and dis-bond detection and is the subject of current research. At present there is only one commercially available instrument which claims to predict cohesive strength. There is no reliable non-destructive test to detect poor adhesion.     

The effect of the bond between the matrix and the aggregates on the cracking mechanism and fracture parameters of concrete

The aggregate–matrix interface plays a leading role in the fracture mechanisms and in the fracture response of concrete. In this work, the influence of the interface on the macroscopic fracture parameters of concrete is investigated. Eleven concrete batches were cast with the same matrix. Different—crushed or rounded—aggregates from the same quarry were used, and several surface treatments were applied to improve or degrade the bond between the matrix and the particles. Fracture tests (three-point bending tests and Brazilian splitting tests) were carried out to determine the fracture energy and other relevant fracture parameters of the concrete batches. The modulus of elasticity and the compressive strength were obtained from uniaxial compression tests. The macroscopic fracture behaviour was modeled by the cohesive crack model with a bilinear softening curve. The results show that concretes with the same matrix and aggregates, and similar behaviour under uniaxial compression, can give very different fracture responses. The work shows how fracture behaviour is governed by the interfacial properties that are also behind the cracking mechanism.