Time-dependent behavior of adhesively bonded composite–composite beams under flexural loading

Abstract

Time-dependent behavior is characteristic of adhesively bonded structureswhen put under constant load (creep). In this study, adhesively bonded beam specimens prepared by adhesively bonding two unidirectional carbon fiber laminated beams were subjected to accelerated three-point bending creep tests. A three-point bending test was selected because of its simplicity and the fact that bending stresses tend to develop in structures under load even if not subjected to direct flexural load. The aim of this study is to predict the long-term behavior and to investigate the long-term creep response of the adhesively bonded composite system. The long-term creep behavior was predicted by time–temperature superposition principle (TTSP) and construction of the master curve at a reference temperature.     

The change in reactivity of silicate anions during the hydration of calcium silicates and cement

The change in reactivity of the silicate anion complex of tricalcium silicate, β-dicalcium silicate and portland cement during hydration was studied by measuring the rate constant ($\text{k}$) of the silicomolybdic acid formation. The three anhydrous starting materials have identical $\text{k}$ values indicating their identical monosilicate anion structure. During hydration the value of $\text{k}$ decreases continuously; the rate of decrease of $\text{k}$ is highest in case of cement and lowest in case of β-dicalcium silicate. The decrease of $\text{k}$ can be attributed to the formation of bridging oxygen ions in the silicate complex. $\text{k}$ tends to reach a final, limiting value; beyond this $\text{k}$ can still be decreased by special techniques as e.g. by regrinding and rehydrating the hardened samples.     

Constitution of tars from flash pyrolysis of Australian coals: 3. A structural study of Millmerran asphaltene components by hydrogenolysis

Asphaltenes derived from tar from the flash pyrolysis of Millmerran coal have been separated into acid, base, polyfunctional and neutral fractions by ion-exchange chromatography. Each fraction was studied by high-pressure catalytic hydrogenolysis followed by g.c.-m.s. analysis of the volatile products. The high content of n-alkanes from C₉ to C₃₂ in the organic products highlights the unusual maceral composition of Millmerran coal and its high $\text{H}\text{C}$ ratio. The results show that most of the asphaltenes are made up of small 1 — or 2-ring aromatic units probably linked by methylene chains bonded through intermediate functional groups. In some cases, the asphaltene structures appear to be ‘simpler’ than the corresponding coal-tar resin structures in the maltenes. Because no amphoteric molecules were detected these results support the concept of an acid-base structure for coal-derived asphaltenes.     

Experimental investigation on the bending behaviour of hybrid and steel thin walled box beams—The role of adhesive joints

In the automotive design, nowadays there are two fundamental drivers. On one hand there are the environmental problems, on the other hand there are the safety matters. Within this contest, the weight reduction has become a key driver in the design of vehicles and it is necessary to consider and to study the use of nonconventional materials taking advantage from their high potential of weight reduction and energy absorption capability.In this perspective, the aim of this work is the study of the structural behaviour of box beams by means of a series of three points bending tests. The examined cross sections are those typically used in automotive construction. Different type of materials (steel, composite) and joining technologies (adhesive, spot weld) have been examined, considering different configurations. The work put in evidence the advantages coming from the use of adhesive, which allows structures with important weight reduction and better mechanical properties than traditional joining solutions.     

Analysis of a retort oil from an Israeli shale by gas chromatography-mass spectrometry-selected ion monitoring

The oil from an Israeli shale was fractionated by solvent extraction and silica gel column chromatography. The straight-chain pentane and benzene soluble materials were analysed by the gas chromatography-mass spectrometry-selected ion monitoring method. By monitoring specific fragment ions with representative $\text{m}\text{z}$ values, information was obtained of the chemical structures of the fractions.     

Dynamic mechanical response of adhesively bonded beams: Effect of environmental exposure and interfacial zone properties

The dynamic mechanical response of adhesively bonded metal beams has been examined over a broad temperature range using a dynamic mechanical thermal analyzer (DMTA). The measured dynamic mechanical response of the bonded beam contains rich information about the viscoelastic properties of the adhesive resin such as glass transition temperatures. The measured storage moduli and loss factors of the bonded beams are very sensitive to changes in the properties of adhesive induced by exposing the beam specimens to environmental attack. Dynamic mechanical responses associated with dry adhesive resin, water plasticized resin, and aggregated water were observed for an electro-galvanized steel/epoxy beam exposed to water. The effect of the presence of an interfacial accommodation zone (IAZ) between adhesive resin and metal substrate was also examined; measured properties were very sensitive to the presence of a low modulus IAZ. It was successfully demonstrated in this study that the DMTA testing of bonded beams is a potentially useful tool for studying dynamic performance and durability of automotive adhesive joints.     

Non-destructive evaluation of bonded structures with lock-in thermography

Lock-in thermography is employed for non-destructive evaluation of several types of bonded structures, which are commonly encountered in industrial applications. Specimens were fabricated to simulate: adhesively bonded aluminium joints, which are commonly used in aeronautical and automotive fields; bonds between pipes of poly(vinyl chloride) (PVC) employed in the transport of liquids (sewage systems); and bonds between plates of Plexiglas which are widely used in the manufacturing of aquaria. Amongst bonded structures, the composite materials are very important, which are generally made of carbon, glass, or Kevlar^? aramid fibers and epoxy resin, and which find application in many industrial fields, especially the aeronautical industry, because of their higher strength and lower weight as compared to metallic materials. It is known that surface plasma treatment of a material improves its adhesion, but it is also known that this treatment will degrade over time if the material is not bonded immediately. Thus, to assure quality, any bonded system should be monitored by the most effective non-destructive technique. To obtain information about the ability of lock-in thermography to assess the performance of such plasma treatment, several specimens were fabricated from either composites (carbon, or Kevlar^? fabric layers plus epoxy resin), or glass plates with and without surface plasma treatment before bonding. In addition, a sample was obtained from a piece of a typical insulated wall of refrigerator vehicles, which actually is a sandwich of polyurethane foam between two plates of fiberglass. The results obtained show that lock-in thermography is a useful tool for non-destructive evaluation of bonded structures.     

Numerical and experimental investigations of the dynamic response of bonded beams with a single-lap joint

The need to design lightweight structures and the increased use of lightweight materials in industrial fields, have led to wide use of adhesively bonding in recent years. In the design of mechanical systems, which consist of adhesively bonded joints, for minimum vibration response, a specific knowledge of the damping capacity of the component materials and joints is important. It is believed that adhesively bonded joints act to augment the system damping capacity in view of the increasing use of viscoelastic materials in their design. The aim of this paper is to provide an efficient numerical technique for the prediction of the dynamic response of bonded beams with a single-lap joint and to validate the predictions via experimental tests. The finite element method was used to predict the natural frequencies, mode shapes and frequency response functions of the beams. The dynamic test software and the data acquisition hardware were used in the experimental measurement of the dynamic response of the joints. The frequency response functions of the joints of different adherend widths and of different adhesive layer thickness were measured. The frequency response functions and mode shapes predicted using the finite element method were compared with those measured experimentally. The coordination of the numerical and experimental techniques makes it possible to find an efficient tool for studying the dynamic response of bonded beams with a single-lap joint.     

The external surface of microporous carbons,derived from adsorption and immersion studies

Three typical samples of active carbons have been investigated by using immersion calorimetry and adsorption techniques (t-plot, $\text{t}\text{F}$ method of Dubinin and direct analysis of the adsorption isotherm before and after prefilling), in order to obtain their external surface area. The different techniques lead to consistent results.     

Effect of the temperature on the strength of adhesively bonded single lap and T joints for the automotive industry

Adhesively bonded lap and T joints are used extensively in the manufacture of automotive structures. In order to determine the effect of using a structural adhesive instead of spot-welding, a detailed series of tests, supported by finite element analyses, was conducted using a range of loadings. The adhesive was a toughened epoxy and the adherend was a grade of mild steel typical of that used in the manufacture of car bodyshells. The lap joints were tested in tension (which creates shear across the bondline) and three point bending. Previous studies at room temperature have shown that joint failure is dictated by adherend yielding and adhesive strain to failure. In the present study, to asses the effect of temperature that an automotive joint might experience in service, tests were carried out at ?40 and +90 °C. It is shown that the failure criterion proposed at room temperature is still valid at low and high temperatures, the failure envelope moving up and down as the temperature increases or decreases, respectively.     

In situ laser raman spectroscopy of the sulfiding of Moγ-Al2O3 catalysts

Raman spectra of sulfided $\text{Mo}\text{γ-}\text{Al}{}_2\text{O}{}_3$ catalysts were obtained using in situ techniques for two sulfiding methods. For samples sulfided by 10% $\text{H}{}_2\text{S}\text{H}{}_2$ at 400 °C, MoS₂ structures were observed. A stepwise sulfiding using 10% $\text{H}{}_2\text{S}\text{H}{}_2$, with spectra recorded at 150, 250, and 350 °C, resulted in observation of molybdenum oxysulfide, reduced molybdate, and surface “MoS₂” phases. Reexposure of these samples to air led to radical modification of the oxysulfide structures as well as transformation of some sulfide phases. A model incorporating terminal and bridging MoS bonding and anion vacancies is proposed. This model is based on the conversion of isolated and aggregated molybdate and MoO₃ species to oxysulfide and reduced molybdenum phases. Conversion of reduced molybdenum phases to sulfides is observed to be slow.     

Influence of stress state and temperature on secondary relaxations in polymeric glasses

Using a range of dynamic techniques, the components of the complex tensile modulus $\text{E1}$, Poisson's ratio $\text{v1}$, shear modulus $\text{G1}$ and bulk modulus $\text{K1}$ have been determined at 21°C in the frequency range 10^?2 to 10⁷ Hz for PMMA and rigid PVC. Some $\text{E1}$ data obtained for PMMA over a range of both frequency and temperature are also presented. A frequency dependence of the components of both $\text{G1}$ and $\text{K1}$ is observed throughout the broad β relaxation regions in PMMA and PVC at room temperature. The β relaxation mechanisms are discussed in relation to the calculated relaxation magnitudes, relaxation time distributions and activation energy distribution. From an analysis of dynamic mechanical and strain-birefringence data, an assessment is made of the validity of the frequency-temperature superposition procedure as applied to secondary relaxation phenomena in polymeric glasses.     

Study of structural parameters on some petroleum aromatic fractions by 1H n.m.r./i.r. and 13C, 1H n.m.r. spectroscopy

¹H n.m.r. and i.r. spectroscopy were used to derive molecular parameters of petroleum fractions. Relative amounts are estimated of methylene and methyl groups in substituted alkyl side chains bonded to the aromatic ring system. The resolution of equation combinations leads to estimation of $\text{H}{}_{\mathrm{s}}\text{C}{}_{\mathrm{s}}\text{(=x)}$, which is an important parameter for structural analysis. Several petroleum fractions were characterized in terms of hypothetical average molecular structures using ¹H n.m.r./i.r. procedures, ¹³C coupled proton n.m.r. and Brown—Ladner methods. It is proposed that the ¹H n.m.r./i.r. method gives more precise average molecular parameters than the Brown-Ladner method with the most precise analytical procedure, up to date, being ¹³C coupled proton n.m.r. analysis. The Brown-Ladner method is especially suitable for structural analysis of low aromaticity molecular structures with long straight-chain alkyl substituents.     

Joining and mechanical testing of oxide/oxide (Nextel ™610/alumina-zirconia) ceramic composites

Efficient joining materials and techniques are of critical importance for the integration of CMCs in high performance structures. Continuous oxide fiber Nextel^? 610/alumina-zirconia composites were successfully joined to themselves by using a novel glass-ceramic based on the SiO₂-CaO-Al₂O₃-MgO-Y₂O₃-ZrO₂ system.Crystallization kinetic of the novel glass-ceramic was studied using Matusita, Sakka and Ozawa equations. Single lap off-set shear tests and four-point bending tests were performed at room temperature and at 850 °C to investigate the mechanical strength of the joints. Thermal ageing was performed at 850 °C for 100 h in air to check the thermal stability of the joined components. The results showed that the joints were oxidation resistant and the joined interfaces were well bonded. Single lap off-set shear tests on joined samples resulted in delamination of the composites. The average flexural strengths of the joined samples were 71 MPa and 81 MPa, at room temperature and at 850 °C, respectively.     

Dehydrogenation and hydrogenation activity of palladium-tin-silica and nickel-tin-silica

The dehydrogenation of cyclohexanone, cyclohexylamine, cyclohexane, and 2-propanol and the hydrogenation of ethylene on palladium-tin-silica and the hydrogenation of benzene on nickel-tin-silica were studied. With nickel-silica and nickel-tin-silica, the deposition of carbonaceous materials was studied kinetically. The X-ray photoelectron spectra of palladiumtin and nickel-tin were observed. The catalytic activities of both of the catalyst systems changed in similar ways with the change of the $\text{Pd}\text{Sn}$ and $\text{Ni}\text{Sn}$ ratios (atomic) of the catalysts. The dehydrogenation activity increased to a maximum and then decreased, with a decrease in the $\text{Pd}\text{Sn}$ and $\text{Ni}\text{Sn}$ ratios. The hydrogenation activity of the tin-containing catalysts was much lower than that of the tin-free catalysts. Carbonaceous materials were observed to be deposited more easily on the nickel-silica than on the nickel-tin-silica. The palladium-4d and nickel-3d band peaks of the alloy systems obtained by X-ray photoelectron spectroscopy were observed to shift to higher binding energies as the $\text{Pd}\text{Sn}$ and $\text{Ni}\text{Sn}$ ratios decreased. From the amount of carbon monoxide adsorbed, the surface concentration of palladium and that of nickel on the catalysts were suggested to be the minor determining factor of the catalytic activity studied here. It is concluded that tin is not just a diluent for the active metal but is an effective component to weaken the adsorption bond.     

Liquefaction of coal and related materials

Bituminous coal, lignite, peat and more recent biogenic materials were reacted as water slurries at 380 °C for short periods with hydrogen, directly and from the water-gas shift reaction, without organic solvents or catalysts. Conversion increases with $\text{H}\text{C}$ and $\text{O}\text{C}$ ratio of the input material and reaches 90%. For benzene-soluble products $\text{O}\text{C}$ is always lower and $\text{H}\text{C}$ usually higher than for the original material.     

Gel permeation chromatographic analysis of solubilized lignite asphaltenes and preasphaltenes

GPC data have been measured for a series of acetylated solvent refined lignite (SRL) asphaltenes and preasphaltenes and model compounds. Two structural parameters, the degree of aromatic condensation ($\text{H}{}_{\mathrm{aru}}\text{C}{}_{\mathrm{ar}}$) and the molar hydrogen to carbon ratio ($\text{H}\text{C}$), were employed to correct the molecular weight of the samples for linear molecular size. For the model compounds, $\text{H}{}_{\mathrm{aru}}\text{C}{}_{\mathrm{ar}}$ was more effective, whereas the SRL materials were better adjusted by $\text{H}\text{C}$. The calibration standards deemed most suitable for determination of molecular weight of SRL by GPC are the SRL samples themselves.     

Mercerization of cellulose: 1. Determination of the structure of Mercerized cotton

The structure of Mercerized cellulose II has been determined using the intensity data from Mercerized cotton fibres and rigid body least squares refinement techniques. The crystal structure consists of an array of antiparallel chain molecules essentially identical to that found for regenerated cellulose II. The unit cell is monoclinic with dimensions $\text{a = 8.02}\text{A}\text{?}$, $\text{b = 8.99}\text{A}\text{?}$, $\text{c = 10.36}\text{A}\text{?}$ (fibre axis), and γ = 116.6°; the space group is P2₁ and the cell contains sections of two cellulose chains which pass through the centre and corner of the ab projection. The final crystallographic R value was 0.263, based on intensity data for 30 observed and 11 unobserved non-meridional reflections. The CH₂OH groups of the corner chains are oriented near to the gt position while those of the centre chain are near to the tg position. The possibility of alternative side group orientations, most likely on the exterior of the cellulose II crystallites, has been demonstrated for both the Mercerized and regenerated structures. The percentage of these other orientations is small in the case of Fortisan (rayon) but substantial in the case of Mercerized cotton.     

Pyrolysis studies of organic oxygenates. 2. Benzyl phenyl ether pyrolysis under batch autoclave conditions

Benzyl phenyl ether (BPE) is a reactive organic oxygenate which contains the ether functionality believed to be present in subbituminous and bituminous coals. With an $\text{H}\text{C}$ of 0.92 it has a $\text{hydrogen}\text{carbon}$ ratio similar to that found in bituminous coals. Benzyl phenyl ether reacts readily at 375 °C either in the presence or absence of added donor hydrogen sources. The major products are toluene and phenol. Other, heavier products are also produced in significant quantities. In general, as available donor hydrogen is reduced, the products tend to have higher molecular weights. Conventional pyrolysis products become lighter (more desirable) materials when the pyrolysis is carried out in the presence of added hydrogen.     

Relationship between crystal structure and properties and irradiation behavior of reactor graphites

Nineteen graphites manufactured with a wide variety of raw materials and processes were characterized and correlations sought between structure and properties. The raw materials of the graphites ranged from highly isotropic cokes to anisotropic needle cokes and processing from conventional molding and extrusion to proprietary processes that produce high-strength isotropic materials. Strength, thermal expansivity, and the degree of anisotropy were found to correlate with crystallite size. The materials with relatively large mean apparent crystallite sizes $\text{L}\text{?}{}_{\mathrm{c}}$ were anisotropic and weak and had low thermal expansivity values, whereas those with smaller $\text{L}\text{?}{}_{\mathrm{c}}$ values were isotropic and strong and had large thermal expansivities. The graphites were irradiated at 625°–1625°C to fluences of up to 2 × 10²²n/cm². The only strong correlation found was between preirradiation strength and dimensional and volumetric changes. High-strength isotropic graphites with intermediate $\text{L}\text{?}{}_{\mathrm{c}}$ values were most stable with lowest initial contraction rates and lowest expansion rates after turnaround. Low-strength isotropic graphites were in the same range of stability with low-strength anisotropic graphites, except distortion was minimized due to their isotropic dimensional changes