Assessment of interfacial bonding between polymer threads and epoxy resin by transverse fibre bundle (TFB) tests

Two experimental approaches were employed to assess the fibre/matrix adhesion between polymer threads and epoxy resin by transverse fibre bundle (TFB) tests. The first approach was to measure interfacial bonding strength of the fibre/matrix interface in dog-bone-shaped tensile specimens by applying normal stress until failure, simulating the Mode I failure mode. The second approach was to determine the fibre/epoxy interfacial bonding strength in shear (simulating the Mode II failure mode) by means of a V-notched beam shear testing method, i.e. a modified Iosipescu test. In both methods, polymer threads were transversely incorporated in the middle section of the specimens. It was found that both methods were simple, reliable, and sensitive to changes in the fibre/matrix adhesion conditions, though interpretation of the test results was somewhat complex. The two experimental approaches were able to produce consistent results and can thus be adopted as alternative methods for determining the interfacial bonding properties between fibres and matrix in composite systems where conventional micro-mechanical or macro-mechanical testing methods cannot be used.     

Estimation of the strength of adhesion between a thermoresponsive polymer coating and nitinol wire

As polymer coatings become more widely used in the biomedical device industry, both to improve biocompatibility and as coatings for localised drug delivery, quantitative methods to measure the adhesive strength between coatings and substrates become a very important consideration. The aim of this study was to take a method for estimating the interfacial fracture toughness of a film to a flat substrate and apply it to Nitinol wires used in the production of medical devices. An investigation into the affect of surface roughness on the fracture toughness was also conducted. For the present study, a thermoresponsive based Poly (N-isopropylacrylamide) polymer was coated onto nitinol wire substrates and the adhesion strength between the polymer and wire was measured using a nanoindentation technique. Different surface treated nitinol wires, with different surface topography and roughness were used, and the affect of these surface properties on adhesion strength was investigated. Results showed that it was possible to apply the delamination technique to wire samples and obtain fracture toughness values. Results also showed that the surface roughness is an important parameter that can affect the adhesion between a coating and the substrate. It was found that, as the average surface roughness increased so also did the adhesive strength between the coating and wire sample.     

On the evaluation of the adhesion of electroplated Ni coatings upon steel substrate with extended microbridge technique

The interfacial adhesion of electroplated Ni coatings on steel substrate was assessed using an extended microbridge technique (eMBT), and the fracture path of interfacial cracks was examined through cross-sectional high-resolution SEM observation in loading condition. The results indicated that the magnitude of interfacial toughness increased ten to hundred times as the cleaning time of substrate surface was prolonged in a limited duration. The cross-sectional SEM examination revealed that the weak interface fracture was related to brittle mechanism, whereas the strong interface was a reflection of ductile one. In case of the duration time of substrate pre-cleaning, which was commonly adopted in applications of electroplate engineering, the over-erosion of substrate surface appeared and the steel surface became rough. In this case, the mechanical stick force, in addition to the physical bonding force across the interface, was observed to play a crucial role in the coatings integrity and bonding reliability.     

Measurement of the fracture toughness of polymer-non-polymer interfaces

One of the primary factors limiting the development of a better understanding of polymer-non-polymer adhesion is the lack of a good testing method for the measurement of the strength of the interface. In this paper polymer-non-polymer adhesion is evaluated in terms of the fracture toughness of the interface using an asymmetric double cantilever beam testing geometry. The test is applied to the measurement of polystyrene (PS)-glass and PS-silicon (native oxide) interfaces modified by PS-poly(2 vinylpyridine) (PVP) and PS-poly methyl methacrylate (PMMA) diblock copolymers. The importance of mixed mode crack propagation is demonstrated and it is shown that through an appropriate choice of sample geometry, the crack-tip trajectory can be controlled so that the crack is forced to propagate along the interface. The PS-glass test, in particular, is shown to overcome many of the traditional problems of adhesion measurements, such as failure, away from the interface and effects of far-field deformation in the polymer. The interfacial fracture toughness of the PS-glass and PS-silicon interfaces without copolymer modification are approximately the same and weak with values of 1 J m^–2. The addition of the block copolymers results in significant (>40-fold) increases in the interfacial fracture energies. The increase in fracture toughness is dependent on the quantity and degree of organization of the block copolymer at the interface.     

Shear strength and fracture toughness of carbon fibre/epoxy interface: effect of surface treatment

Textile-reinforced composites have become increasingly attractive as protection materials for various applications, including sports. In such applications it is crucial to maintain both strong adhesion at fibre–matrix interface and high interfacial fracture toughness, which influence mechanical performance of composites as well as their energy-absorption capacity. Surface treatment of reinforcing fibres has been widely used to achieve satisfactory fibre–matrix adhesion. However, most studies till date focused on the overall composite performance rather than on the interface properties of a single fibre/epoxy system. In this study, carbon fibres were treated by mixed acids for different durations, and resulting adhesion strength at the interface between them and epoxy resin as well as their tensile strength were measured in a microbond and microtensile tests, respectively. The interfacial fracture toughness was also analysed. The results show that after an optimum 15–30 min surface treatment, both interfacial shear strength and fracture toughness of the interface were improved alongside with an increased tensile strength of single fibre. However, a prolonged surface treatment resulted in a reduction of both fibre tensile strength and fracture toughness of the interface due to induced surface damage.     

Fracture mechanisms of epoxy filled with ozone functionalized multi-wall carbon nanotubes

This work focused on the fracture mechanisms and reinforcing effects of ozone-treated multi-walled carbon nanotubes (MWCNTs) in epoxy matrix. Ozone functionalization of MWCNTs was found to be of help for a better dispersion and stronger interfacial bonding with epoxy matrix, which in turn improve the strength and fracture toughness of the resin. The MWCNT/epoxy composites showed complicated failure modes than the conventional fibrous composites, which have been quantitatively investigated and correlated with the fracture toughness of the nanocomposites studied.     

An Investigation on Vibration Welding of Amorphous and Semicrystalline Polymers

Vibration welding technique has been used to study the weld zone of thermoplastic polymers using ABS (amorphous), PC (amorphous), PMMA (amorphous), and PBT (semicrystalline). Polymers were welded using alike components and combinations of semicrystalline polymer with different amorphous polymers. Mechanical testing of welded polymers has proved that the tensile strength, elongation at break, and deformation was highest for PC–PC weld and least for ABS–ABS weld, when alike polymers were welded. However, welding of semicrystalline and amorphous polymer shows enormous reduction in its tensile strength as well as other tensile properties. Also, the tensile fracture of PBT with other amorphous polymers always occurred at weld zone which was not always in case of alike polymer welds. The weld strength of these polymers was observed to be dependent on the mechanical interlocking among the layers and not on interfacial bonding. This phenomenon may be due to the difference in glass transition temperatures of semicrystalline and amorphous polymers. XRD, FESEM, and AFM have been used in this study to observe the morphology of welded surfaces.     

Failure processes in particulate filled polypropylene

In this paper the common degradation effect of silicon oxide filler on fracture strain and fracture toughness of isotactic polypropylene is investigated by analysing the failure processes in the composite material by microscopic methods. Experiments demonstrate that, although fracture of the polymer regions absorbs considerable energy by plastic deformation, void formation and cracking of the interface between the polymer and the filler usually requires very little energy. These weak interfaces do not resist cracking and are the cause of brittleness in particulate filled systems. The crucial parameters influencing the fracture data of the composite were found to be the volume fraction of the filler and the interfacial adhesion between polymer matrix and particles. As the interfacial fracture energy is usually much smaller than the polymer fracture energy, the composite toughness drops when filler is added. Using a model which describes the individual steps of crack formation and final fracture, an attempt is made to explain the decrease of crack resistance of the polymer matrix with increasing filler fraction and to calculate the fracture energy of the composite by introducing partial values of crack resistance of the matrix and the interface, respectively. In addition, it is discussed how a coarse spherulitic morphology of the matrix, as produced by isothermal crystallization from the melt, can modify this behaviour.     

电热作用对碳纤维树脂基复合材料力学性能的影响

采用复合材料电热实验平台,测试碳纤维树脂基复合材料（Carbon Fiber Reinforced Polymer,CFRP）电热作用下温度场变化规律,同时从单丝拉伸断裂界面剪切强度、短梁剪切性能变化和剪切断口等多方面揭示电热作用对CFRP力学性能的影响机制。结果表明:电热作用会使CFRP整体温度迅速升高,在约4 min时达到稳态温度,随着电流强度的增大,CFRP层板表面温度越高,当电流强度为8 A（0.44 A/mm2）时,CFRP的表面温度达到151℃;单丝拉伸和短梁剪切界面强度都随着电流强度增加呈现先增加后降低的趋势;小电流时,电热作用产生较少的焦耳热,优化界面性能,提高界面剪切强度,大电流时,电热作用产生的焦耳热过大,对界面产生烧蚀等不可逆损伤,降低了界面结合性能。      

PA6/ABS共混物的脆-韧转变研究

采用熔融共混方法制备了苯乙烯-马来酸酐共聚物(SMA)增容的PA6/ARS共混物,结合吴守恒的临界基体层厚度(IDc)理论,考察了基体层厚度与界面粘接对PA6/ABS共混物脆一韧转变的影响.结果表明,温度低于8℃,当ID减小时,冲击强度先缓慢增加,当ID＜ID.时,共混物缺口冲击强度急剧增加;测试温度处于13℃～23℃...     

Adhesion of polymer thin-films and patterned lines

The adhesion of interfaces in thin-film structures containing ductile polymer blanket films and patterned lines is reported. The intent of the study was to demonstrate that both the film thickness and the aspect ratio of patterned lines have a significant effect on the interfacial fracture energy of interfaces adjacent to the ductile polymer. In particular, there is currently limited understanding of the effect of dimensional constraint in the plane of the film on local plasticity and associated interfacial fracture energies. Accordingly, the interfacial adhesion of patterned structures containing arrays of polymer/SiO₂ lines with varying aspect ratios was investigated. Macroscopic adhesion values were determined by measuring the critical strain energy release rate, G _c, for debonding of the patterned interface. The yield properties of the polymer films as a function of film thickness was also investigated. Decreasing aspect ratio of the polymer lines was found to significantly increase interface fracture energy and is rationalized in terms of the effect of stress state on the extent of plastic deformation in the polymer line.     

Process and mechanical properties of carbon/carbon–silicon carbide composite reinforced with carbon nanotubes grown in situ

A hierarchical C_f/C–SiC composite was fabricated via in situ growth of carbon nanotubes (CNTs) on fiber cloths following polymer impregnation and pyrolysis process. The effects of CNTs grown in situ on mechanical properties of the composite, such as flexural strength, fracture toughness, crack propagation behavior and interfacial bonding strength, were evaluated. Fiber push-out test showed that the interfacial bonding strength between fiber and matrix was enhanced by CNTs grown in situ. The propagation of cracks into and in fiber bundles was impeded, which results in decreased crack density and a “pull-out of fiber bundle” failure mode. The flexural strength was increased while the fracture toughness was not improved significantly due to the decreased crack density and few interfacial debonding between fiber and matrix, although the local toughness can be improved by the pull-out of CNTs.     

Surface treatment of coir (Cocos nucifera) fibers and its influence on the fibers’ physico-mechanical properties

Coir, an important lignocellulosic fiber, can be incorporated in polymers like polyacrylate in different ways for achieving desired properties and texture. But its high level of moisture absorption, poor wettability and insufficient adhesion between untreated fiber and the polymer matrix lead to debonding with age. In order to improve the above qualities, adequate surface modification is required. In our present work, fiber surface modification by ethylene dimethylacrylate (EMA) and cured under UV radiation. Pretreatment with UV radiation and mercerization were done before grafting with a view to improve the physico-mechanical performance of coir fibers’. The effects of mercerization on shrinkage and fiber weight losses were monitored at different temperature and alkali concentration. We observed that, fiber shrinkage is higher at low temperature and 20% alkali treated coir fibers yielded maximum shrinkage and weight losses. It was found that higher shrinkage of the polymer grafted fiber showed enhanced physico-mechanical properties. The grafting of alkali treated fiber shows an increase of polymer loading (about 56% higher) and tensile strength (about 27%) than 50% EMA grafted fiber. The fiber surface topology and the tensile fracture surfaces were characterized by scanning electron microscopy and were found improved interfacial bonding to the modified fiber–matrix interface.     

Effects of titanium surface modifications on bonding behavior of hydroxyapatite ceramics and titanium by hydrothermal hot-pressing

In order to improve the interface strength in the bonded body of hydroxyapatite (HA) ceramics and Ti disks prepared by a hydrothermal hot-pressing (HHP) method, the effects of Ti surface modification on the bonding behavior were investigated. The reaction layer composed of titanium dioxide and sodium titanate was formed on the Ti surface using a 5 M NaOH solution with the objective of increasing the interface strength between the Ti substrate and HA ceramics to be formed by the HHP method. Three conditions with different temperature and treatment times were tested to modify the Ti surface. A mixture of calcium hydrogen phosphate dihydrate and calcium hydroxide was used as a starting powder material for solidifying HA. Solidification of HA and its bonding with Ti were achieved simultaneously by using the HHP method at the low temperature as low as 323 K. 3-point bending tests were conducted to obtain an estimate of the interface fracture toughness of HA/Ti. The Ti surface modification conducted at 323 K for 2 h using the hydrothermal NaOH solution was shown to be most effective among the three conditions tested. The hydrothermal Ti surface modification enabled us to increase significantly the interface fracture toughness. The enhancement of the interface fracture toughness was possibly due to the presence of anatase formed on the Ti surface and the good adhesion in the bioactive layer.     

Toughening of carbon fiber-reinforced epoxy polymer composites utilizing fiber surface treatment and sizing

Toughening of fiber-reinforced epoxy composites while maintaining other mechanical properties represents a significant challenge. This paper presents an approach of enhancing the toughness of a DGEBA/mPDA-based carbon fiber-reinforced epoxy composite, without significantly reducing the static-mechanical properties such as flexural properties and glass transition temperature. The impact of combining an UV-ozone fiber surface treatment with an aromatic and aliphatic epoxy fiber sizing on composite toughness is investigated. Carbon fiber-epoxy adhesion was increased as measured by the single fiber interfacial shear test. The Mode I composite fracture toughness was enhanced by 23% for the UV-ozone fiber surface treatment alone. With the addition of an aromatic and aliphatic fiber sizing, the composite fracture toughness was further increased to 50% and 84% respectively over the as-received, unsized fiber. The increased fiber/matrix adhesion also improved the transverse flexural strength.     

Interactions of a liquid crystalline polymer with polycarbonate and poly(ethylene terephthalate)

Thermal behaviour of blends of a liquid crystalline copoly(ester amide) (Vectra B950) with two isotropic polymers has been studied by differential scanning calorimetry. One of the isotropic polymers is an amorphous polymer – polycarbonate, the other is a semi-crystalline polymer – poly(ethylene terephthalate). It was found that the glass transition temperature of polycarbonate decreases with increasing Vectra concentration in the blend, suggesting a partial miscibility between the Vectra liquid crystalline polymer (LCP) and polycarbonate. The miscibility is enhanced through heat treatment at elevated temperatures presumably due to a transesterification reaction. Moreover, the presence of the amorphous poly- carbonate hinders the crystallization of the liquid crystalline polymer in the blends. It was also observed that heat treatment of the Vectra LCP and poly(ethylene terephthalate) blends causes a loss in crystallinity and shifts in transition temperatures of poly(ethylene terephthalate), indicating that exchange reactions occur between Vectra B950 and poly(ethylene terephthalate). Based on these results, a new strategy, in situ compatibilization, is proposed to improve the interfacial adhesion between an LCP and an isotropic polymer. This revised version was published online in November 2006 with corrections to the Cover Date.     

Quantitative evaluation of the adhesion of metallic coatings using cylindrical substrate

This paper introduces an effective interfacial fracture toughness test based on interface fracture mechanics theory. This testing method uses a circumferentially notched tensile (CNT) specimen, which is ideally suited for determining the interfacial fracture resistance of coatings. Unlike other interfacial fracture tests, this test is simple to prepare, requires minimum test setup and is easy to model. An interfacial pre-crack was generated between a nickel coating and mild steel cylindrical substrate to evaluate adhesion strength. In situ acoustic and SEM analyses were used to determine the crack initiation or the critical load of failure. The critical energy release rate, critical stress intensity factors and phase angle were determined using the J integral which was determined by applying the critical load to the finite element model. A detailed finite element analysis was carried out to study the effect of different interface pre-crack positions and mode mixity on energy release rate for different notch angles and elastic modulus ratios. The cracking resistance of the interface was characterised by the notch angle of CNT specimens. The analysis showed an increase in interfacial fracture toughness as phase angle increases and was significant when the phase angle was large. The combined results of computational and experimental analysis showed that any defect or stress concentration at the interface could significantly weaken the adhesion of coating.     

纳米SiO2质量分数对胶粘碳纤维增强树脂复合材料板-钢搭接界面黏结性能的影响

胶黏剂力学性能对碳纤维增强树脂复合材料(CFRP)加固钢结构的界面黏结性能影响显著。基于研制的胶黏剂配比,分析了不同纳米SiO₂质量分数对胶黏剂常温固化后基本力学性能及微观结构的影响,制作了31个CFRP板-钢板双搭接试件,对其进行了常温固化后的承载能力、有效黏结长度、传力模式、黏结-滑移本构等试验研究,得出了纳米SiO₂质量分数对CFRP板-钢板搭接试件界面黏结性能的影响规律,并与常用商品胶黏剂进行了比较。研究结果表明:随纳米SiO₂质量分数的增加,胶黏剂应力-应变关系由线性转变为非线性,应变能、断裂伸长率及剪切强度分别最高提升了292.10%、202.88%和133.12%。微观结构分析表明纳米SiO₂的添加使断面粗糙度显著增加,形成了密集的塑性空穴,产生了更多的微裂纹,使胶黏剂的韧性大幅度提高。当纳米SiO₂质量分数从0增至1wt%,搭接试件破坏模式由界面破坏逐渐变为CFRP板层离破坏。掺入纳米SiO₂能显著增加搭接试件的极限承载力(提升256.96%)及界面有效黏结长度(提升3倍),提高CFRP表面的应变及界面剪应力峰值。纳米SiO₂质量分数为0与0.5wt%的搭接试件的黏结-滑移曲线为双线性三角形模型,纳米SiO₂质量分数为1wt%的搭接试件的黏结-滑移曲线为三线性梯形模型,黏结界面韧性大幅提升。CFRP-钢界面承载能力受胶黏剂拉伸强度与断裂伸长率的双重影响,非线性高强度(即具有较高应变能)胶黏剂对应的CFRP-钢搭接接头具有更好的界面性能。      

Prediction of interfacial fracture between concrete and fiber reinforced polymer (FRP) by using cohesive zone modeling

Interfacial debonding between concrete and fiber reinforced polymer (FRP) is investigated through integrating experiments and computations. An experimental program is designed to evaluate interfacial fracture parameters of mode-I through cutting and bonding specimens with an FRP sheet. The evaluated fracture parameters, i.e. the fracture energy and the bonding strength, are confirmed by predicting FRP debonding failure with the cohesive zone modeling approach. In the cohesive zone model, a traction-separation relation for FRP debonding is proposed with a shape index while providing various initial descending slopes. Computational results of the cohesive zone model agree well with three-point bending test results for both FRP debonding and plain concrete fracture. Furthermore, both experimental and computational results demonstrate that the fracture energy and the cohesive strength are essential fracture parameters for the prediction of FRP debonding behavior.     

纤维表面涂层对SiC(f)/SiC复相陶瓷力学性能和界面结构的影响

报道了化学气相浸渍（CVI）工艺制备的SiC（f）/SiC复相陶瓷中纤维表面涂层对复合材料力学性能和显微结构的影响。SEM观察表明：C或B N表面涂层改变了SiC（f）/SiC复相陶瓷中纤维与基体间的强界面结合,使断裂过程中的界面解离和纤维拔出大大增加,与此同时材料的断裂韧性和断裂功明显提高。说明C或BN纤维表面涂层能够大大地改善SiC（f）/SiC复相陶瓷的脆性断裂行为模式。高分辨电镜的观察证实在CVI过程初期,纤维表面首先发生石墨界面相的沉积,该界面相具有明显的层状晶格条纹,而纤维表面C涂层为无定型态。