Analysis of humidity-dependent adhesion between a probe tip and a surface

Adhesive forces commonly exhibit a monotonic increase or a maximum with increasing relative humidity. However, anomalous behavior has been reported. Here, a numerical model of adhesive forces, comprised mainly of capillary and van der Waals forces, between a tip and a surface is established. It is described by a power law that considers the geometry, the liquid bridge wetting radius, the contact angle, and the separation distance. Capillary forces (sum of surface tension and Laplace pressure) and van der Waals forces are calculated. The latter cannot be neglected in the adhesion even at high humidity. Decrease in adhesion with increasing relative humidity can be attributed to a blunt tip shape, which is validated by experimental data. Specifically, the decrease in adhesion is attributed primarily to a transition from a rounded to a blunt tip shape. Structuring objects at the micro- or nanoscale can either increase or decrease adhesion as a function of relative humidity. This has a wide range of applications in robotic manipulation and can provide a better understanding of adhesion mechanisms in atomic force microscopy in ambient air.     

Identification of Dynamic (Young’s and Shear) Moduli of a Structural Adhesive Using Modal Based Direct Model Updating Method

In this paper, mechanical characteristics (Young’s modulus and shear modulus) of an adhesive are identified using modal based direct model updating method and experimental modal data. The results show that both Young’s and shear moduli of adhesive are frequency dependent. Also, it is demonstrated that the thickness and length of the adhesive-line have influence on these properties. All experiments and subsequent identifications are conducted both in bending and shear modes, and it has been shown that the shear modulus of adhesive is more sensitive to length and thickness variations. The repeatability and consistency of method is proved through repeating the process several times and with different adherends.     

Determination of Water Diffusion Coefficients and Dynamics in Adhesive/ Carbon Fiber Reinforced Epoxy Resin Composite Joints

To determinate the water diffusion coefficients and dynamics in adhesive/carbon fiber reinforced epoxy resin composite joints,energy dispersive X-ray spectroscopy analysis(EDX)is used to establish the content change of oxygen in the adhesive in adhesive/carbon fiber reinforced epoxy resin composite joints.As water is made up of oxygen and hydrogen,the water diffusion coefficients and dynamics in adhesive/carbon fiber reinforced epoxy resin composite joints can be obtained from the change in the content of oxygen in the adhesive during humidity aging,via EDX analysis.The authors have calculated the water diffusion coefficients and dynamics in the adhesive/carbon fiber reinforced epoxy resin composite joints with the aid of both energy dispersive X-ray spectroscopy and elemental analysis.The determined results with EDX analysis are almost the same as those determined with elemental analysis and the results also show that the durability of the adhesive/carbon fiber reinforced epoxy resin composite joints subjected to silane coupling agent treatment is better than those subjected to sand paper burnishing treatment and chemical oxidation treatment.     

Influence of Backbone Structure on Properties of Directly Polymerized Phenoxy Resins from Epichlorohydrin and Various Aromatic Dihydric Phenols Monomers

A series of phenoxy resins was directly prepared through the polymerization of each of the various aromatic dihydric phenols and epichlorohydrin.FTIR and 1H NMR spectra were recorded to characterize the structures of the re-sins.The GPC curves were used to determine the molecular weight distribution.In addition,the thermal properties of the resins were studied with differential scanning calorimetry(DSC)and thermal gravimetric analysis(TGA).The thermal stabilities of the polymers increased with the content of the benzene ring,pendant group increasing or biphenyl groups emerging.The adhesive properties of the polymers were evaluated in terms of the lap shear strength with Fe-Fe adherends.The fracture mechanisms were determined by SEM observation and it was found that there was an important participation of cohesive fracture mechanisms.Also,it has been demonstrated that the extension of these micro-cohesive mechanisms is directly correlated with the adhesive strength.According to these results,the phenoxy resin containing biphenyl groups presented a higher adhesive strength and could improve the adhesive property of the epoxy/phenoxy system to a certain extent.     

On the Impact of Elastic Bodies with Adhesive Forces

We analyze the problem of an elastic sphere impacting against another elastic body for the case when adhesive forces act between the bodies. Depending on the parameter describing the (relative) influence of the adhesive forces, the bodies will either separate or continue to be in contact and perform an oscillatory motion after the rebound. The value of the adhesive forces parameter defining separation and capture is calculated. This revised version was published online in July 2006 with corrections to the Cover Date.     

Progressive fracture of adhesively bonded composite structures

Progressive damage and fracture of adhesively bonded graphite/epoxy composite structures are evaluated via computational simulation. Load induced damage in both the adhesive bond and the adjoining laminate is considered. An integrated computer code is used for the simulation of structural degradation under loading. Damage initiation, growth, accumulation, and propagation to fracture are included in the simulations. Results show in detail the damage progression sequence and structural fracture resistance during different degradation stages. Design implications with regard to damage tolerance of adhesively bonded joints are examined. Influence of the type of loading as well as adhesive thickness on damage initiation and progression for an adhesively bonded composite structure are investigated.     

On the wall slip phenomenon of elastomers in oscillatory shear measurements using parallel-plate rotational rheometry: II. Influence of experimental conditions

The use of parallel-plate rotational rheometry to characterize ex situ pre-prepared samples of rubber-like polymers is motivated by, for example, the investigation of magneto-rheological elastomers. When exceeding a critical excitation amplitude in oscillatory shear experiments, these elastomeric samples are prone to slip at the sample-plate contact interface. This phenomenon, known as wall slip, starts to occur at the sample's outer rim and leads to an imperfect force transfer onto the sample. This results in a systematic error of measured rheological material quantities.A thorough investigation is presented to reveal how this phenomenon is affected by selected experimental conditions, namely the static axial preload and measuring frequency. For this purpose disc-shaped samples composed of an unfilled silicone rubber are prepared by casting and examined by means of a controlled stress rotational rheometer equipped with a serrated rotor configuration.The oscillatory strain sweep experiments suggest that wall slip, exclusively present at the serrated rotor surface, is significantly influenced by the static preload. In contrast, only a slight frequency dependence is observed within the examined experimental conditions.Further insights into the wall slip mechanism were attained by two novel methodologies. It is shown that it is possible to produce a master curve for the various applied preloads. This demonstrates that the physical mechanism behind wall slip is independent of the axial force. Furthermore, we derive an empirical model for the criterion governing the onset of wall slip. This links the critical stress at which wall slip is initiated to the static friction condition and geometrical aspects of the rotor configuration. From this it is anticipated that the conditions for reliable experiments involving ex situ pre-prepared samples composed of low damping elastomers can, in the future, be estimated a priori.     

Nondestructive examination of polymethacrylimide composite structures with terahertz time-domain spectroscopy

THz reflective time domain spectroscopy (THz-RTDS) has been considered as an effective method to detect hidden objects with potential for supplementing other NDE technologies for foam composite adhesive structure debonding defects. PMI (Polymethacrylimide) is a heat-resistant foam material, with the highest strength and stiffness to weight ratio. It is widely used in various parts of airplanes, especially the wing leading edge and rudder, landing gear doors, wing-body/wingtip fairings and so on. We analyzed the features of adhesive debonding defect based mainly on the variation of the time-domain wave form and compared with the inclusion defect. The quantification of degrees of adhesive debonding can be readily achieved with THz-RTDS images based on the delay of the wave front and the main reflection time-domain waveforms. Typical accuracy of about 100 μm was achieved.     

Preparation of chlorinated poly(propylene carbonate) and its distinguished properties

Poly(propylene carbonate) (PPC),the copolymerization product of carbon dioxide and propylene oxide,was chlorinated for the first time in our laboratory.Nuclear magnetic resonance (NMR) spectroscopy and ion chromatography test showed that chlorine atoms were successfully introduced onto the polymer chains of PPC.We named this newborn polymer material as chlorinated poly(propylene carbonate) (CPPC).It is worth noting that the reaction conditions of the chlorination of PPC were quite mild,which could be easily and simply realized at industrial level.What is more important is that CPPC possessed many more distinguished properties in solubility,wettability,adhesiveness,and gas barrier compared with PPC.For example,the bonding strength of CPPC as thermal adhesive is nearly four times higher than that of PPC for wood,stainless steel and glass.The oxygen permeability coefficient of CPPC exhibits a decrease of 33％ compared with that of PPC.Moreover,CPPC is quite stable in air,whereas it could be well biodegraded in soil compared with PPC.These results indicated that CPPC could be widely used in the fields of coating,adhesive,barrier materials and so on,which could greatly promote the development of PPC industry.     

Dust distribution of solid and adhesive mixed dust in a granular bed filter

Granular bed filters can effectively filter adhesive dust in high-temperature flue gas. In this study, polyvinyl chloride (PVC) powder was used as adhesive dust, and the mixture of PVC and ash powder was used to simulate solid and adhesive mixed dust. The effects of gas temperature, velocity, and inlet adhesive dust mass content on dust distribution in granular bed (GBF) were discussed. Results show that the mixed dust mainly accumulates on the upper part of the granular bed, and the mass of the collected dust decreases exponentially from the upper layer to the bottom layer in the GBF. The adhesive dust content collected in each layer differs from that of the incoming dust, and their deviation varies approximately linearly along with the depth of the bed. The total dust distribution and adhesive dust content deviation are influenced by gas temperature and inlet adhesive dust content but independent of gas velocity. The correlations of dust distribution of solid and adhesive mixed dust are presented based on the experimental results.