Juan C. Suárez

Grain Coarsening and Boundary Migration during Welding of Invar Fe-36Ni Alloy

To assess the metallurgical reasons for the susceptibility of Invar Fe-36Ni alloy to reheat cracking, a number of specimens have been prepared and welded in a weld simulator, with a gas tungsten arc welding procedure and heat inputs ranging from 1.3 to 7.5MJ/m. Grain coarsening in the heat affected zone was predicted with fair accuracy by using a diffusive model. Samples with two beads-on-plate were used to study the effect of reheating of the first bead by the weld thermal cycle of the second pass. Grain-boundary migration was found to be sensitive to boundary orientation and morphology. Filler metal containing Ti-Mn was responsible for the further impeding of boundary migration owing to the presence of precipitates. A clean surface and the use of oxygen-free shielding gas were compulsory requirements to obtain sound weldments; otherwise preferential diffusion of oxygen along grain boundaries and subsurface inclusions were observed in the joint.

Optical fibre sensors for monitoring of welding residual stresses

Abstract Transient and residual strains in the heat affected zone (HAZ) of a TIG arc welded aluminium–magnesium alloy test plate have been measured on line using fibre Bragg grating sensors. The optical fibre sensor operates as a reflection filter: when a light beam travelling along the fibre optic reaches the Bragg grating, only a certain wavelength will be reflected. The reflected wavelength will change if any strain is applied to the fibre sensor. An optical fibre sensor was bonded on the bottom surface of the welding plate; during the weld thermal cycle, the strains experienced by the sensor were correlated to those by the plate. Thermal strains were measured on line during the welding procedure. Subtracting from the total strain the contribution of the fibre thermal expansion, it was possible to determine the pattern and rate of both dynamic and residual strains developed in the HAZ of the welded specimens. The final goal is to act actively during welding in order to minimise the residual stresses and distortions.

Case studies in adhesives selection

Abstract The selection of an adhesive for a particular application is not as easy as endeavour as it might originally appear. To achieve optimum performance when bonding two materials, one must carefully plan every stage of the bonding process. The selection of an adhesive is a critical factor that will influence each step. The adhesive selection will be dependent primarily on: • The type and nature of substrates to be bonded. • The method of curing that are available and practical. • The expected environments and stresses that the joint will undergo in service. The adhesive selection process is difficult because many factors must be considered, and there is no universal adhesive that will fulfil every application. It is usually necessary to compromise when selecting a practical adhesive system. Adhesive properties limit performance. We need a way of surveying properties, to get a feel for the values design-limiting properties can have. One property can be displayed as a ranked list or bar chart. But it is seldom that the performance of a component depends on just one property. Almost always it is a combination of properties, for instance, of the strength-to-shear modulus ratio. This suggests the idea of plotting one property against another, mapping out the fields in property-space occupied by each adhesive type. The resulting charts are helpful in many ways. They condense a large body of information into a compact but accessible form; they reveal correlations between adhesive properties, which aid in checking and estimating data. In this paper, we have also used the methodology from Professor Michael F. Ashby of Cambridge University in order to solve general practical cases where we had chosen adhesive bonding as a joining process.

Degradation in Seawater of Structural Adhesives for Hybrid Fibre-Metal Laminated Materials

The adhesives used for applications in marine environments are subject to particular chemical conditions, which are mainly characterised by an elevated chlorine ion content and intermittent wetting/drying cycles, among others. These conditions can limit the use of adhesives due to the degradation processes that they experience. In this work, the chemical degradation of two different polymers, polyurethane and vinylester, was studied in natural seawater under immersion for different periods of time. The diffusion coefficients and concentration profiles of water throughout the thickness of the adhesives were obtained. Microstructural changes in the polymer due to the action of water were observed by SEM, and the chemical degradation of the polymer was monitored with the Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC). The degradation of the mechanical properties of the adhesive was determined by creep tests with Mixed Cantilever Beam (MCB) specimens at different temperatures. After 180 days of immersion of the specimens, it was concluded that the J-integral value (depending on the strain) implies a loss of stiffness of 51% and a decrease in the failure load of 59% for the adhesive tested.

Welding of invar Fe-36Ni alloy for tooling of composite materials

Summary Invar alloy has traditionally been used almost exclusively in the manufacture of high-precision measuring instruments. Recently, however, new structural uses, such as the manufacture of bulky composite tooling for the aerospace industry, have forced us to reconsider all the welding procedures previously used. The aim of this research is to investigate some of the problems of welding Invar, proposing practical solutions based on experience. Several issues were considered: i.e. precautions regarding purity of inert shielding gas and the absence of surface residue, the role of titanium added to the filler metal, sensitivity of the alloy to reheat cracking and the existence of precipitates and inclusions, etc. The presence of characteristic banding in the weld pool, its origin and effects on joint properties, were also investigated.

Influence of post-curing temperature on the structure, properties, and adhesion of vinyl ester adhesive

Vinyl ester (VE) resins are widely used as thermoset adhesives in structural joints and composites, but complete curing under environmental conditions is not produced. The existing literature dealing with the effect of post-curing on the structure, viscoelastic, mechanical, and adhesion properties of VE resin is scarce. Therefore, in this study, VE resin was subjected to different post-curing temperatures (50–140 °C) for one hour, and the changes in structure and properties were assessed. The degree of cross-linking of the VE resin depended on the post-curing temperature and cure started to be completed above 100 °C, a temperature close to the glass transition temperature (115 °C) of the completely cross-linked polymer. Furthermore, gel formation in VE resin was evidenced for post-curing temperature below 100 °C. In order to fully cross-link the VE resin, post-curing at 140 °C for one hour was necessary, and it was evidenced by an increase in the glass transition temperature and in the mechanical properties; an increase in adhesion to cold rolled steel was obtained although the shear strength was lower than in the joint produced with the non-post-cured VE resin.

Mode II fracture energy in the adhesive bonding of dissimilar substrates: carbon fibre composite to aluminium joints

The end-notched flexure (ENF) test calculates the value of mode II fracture energy in adhesive bonding between the substrates of same nature. Traditional methods of calculating fracture energy in the ENF test are not suitable in cases where the thickness of the adhesive is non-negligible compared with adherent thicknesses. To address this issue, a specific methodology for calculating mode II fracture energy has been proposed in this paper. To illustrate the applicability of the proposed method, the fracture energy was calculated by the ENF test for adhesive bonds between aluminium and a composite material, which considered two different types of adhesive (epoxy and polyurethane) and various surface treatments. The proposed calculation model provides higher values of fracture energy than those obtained from the simplified models that consider the adhesive thickness to be zero, supporting the conclusion that the calculation of mode II fracture energy for adhesives with non-negligible thickness relative to their adherents should be based on mathematical models, such as the method proposed in this paper, that incorporate the influence of this thickness.

CFD Simulations on the Vortex-Induced Vibrations of a Flexible Cylinder With Wake Interference

In this work, CFD computations showing the dynamic response of a long flexible cylinder subject to a stepped current immersed in the wake of another cylinder are presented. These two cylinders are placed upstream in tandem configuration, where the flexible cylinder is excited by vortex shedding mechanisms. This work completes from the computational point of view, the research started 2 years ago with experiments conducted at the E.T.S.I. Navales towing tank of the Technical University of Madrid. The flexible cylinder studied is 3 m long having an external diameter of 16 mm. A combination of two codes that simulate the fluid-structure interaction phenomenon was used to obtain the velocity and pressure fields and also to measure the deformation of the cylinder at the same points where the strain gauges where placed during the experiment. This code communicates a finite volume (FV) software that solves the Navier-Stokes equations and reports the shear and pressure fields on the flexible cylinder to a second finite element (FEM) code that is able to compute stresses and deformations. Deformations are reported back to the first fluid solver in order to compute the next time step. In the experiments, only the 65% length of the cylinders were under the water surface, consequently a VOF technique was used to simulate the free surface separation between air and water. The numerical stability of these two combined codes is one of the most delicate aspects of the simulation. Taking into account that the upstream cylinder was orders of magnitude more rigid than the downstream one, we considered the upstream cylinder as stationary and consequently having no role during the FEM calculation. Boundary conditions for the flexible cylinder where such that they should imitate the universal joints used in the experiments. The fundamental natural frequencies of oscillation were monitored and compared to the towing tank experiments.Copyright

ENF test in the adhesive bonding of aluminium–composite joints and evaluation of its reliability with Weibull distribution

The composite materials of the polymeric matrix reinforced with carbon fibre have an extensive industrial application as they provide light and resistant structures. However, in many products (automobiles, aircraft, etc.), the composite materials must be joined to other components manufactured with aluminium alloys. The use of structural adhesive to bond these materials may be a good alternative if a specific design of the adhesive joint is carried out by maximising its performance and reducing its limitations. In the current work, the end-notched flexure (ENF) fracture test is used to assess the mechanical behaviour of the adhesive joint to facilitate the choice of the best adhesive and surface treatment of the adherends. However, in industrial applications of a great technical requirement (where the safety of staff or property may be at risk), the former experimental results are not enough and must be complemented with methods that can provide additional guarantee for a suitable reliability. For this purpose, a statistic analysis of the obtained experimental data has been carried out by means of the application of a Weibull distribution, in order to propose the adhesive and surface treatment that best combines the mechanical performance and high reliability. At the end, we can conclude that the epoxy adhesive with sand-blasting treatment for the aluminium and the peel ply for the carbon fibre has the greatest reliability (more than 90% for loads until 900 N).

Finite Element Modeling of the Dynamic Response of a Steel Pipe During Water Hammer

Water hammer imposes a steep rise in pipe pressure due to the rapid closure of a valve or a pump shutdown. Transversal strain waves propagate along the pipe wall at sonic velocities, and dynamic stresses are developed in the material, which can interact with discontinuities and contribute to an unexpected failure. Pressure increase has been modeled as a simple step front in a finite element model of a short section of a steel pipe. Boundary conditions have been considered to closely resemble the conditions of longer pipe behavior. The shock traveling along the length of the fluid-filled pipe causes a vibration response in the pipe wall. Dynamic strains and stresses follow the water hammer event with a certain delay, as is shown from the results of the FEA. Response of the material is strain rate dependent and dynamic peak stresses are substantially larger than the expected from the static pressure loads. Damping of the waves, neither by the material of the pipe nor by the interaction fluid-pipe, has not been considered in this simple model. Hoop, axial, radial, and Von Mises equivalent stresses have been evaluated both for the overshooting and the following phase of decompression of a pipe without discontinuities. However, dynamic stresses can be enhanced in the presence of discontinuities such as laminations, thickness losses in the pipe wall due to corrosion, changes in the wall thickness in neighboring pipe sections, dents, etc. These dynamic effects are able to explain how certain discontinuities that were reported as passing an Engineering Critical Assessment can eventually cause failure to the integrity of the structure. Deflections in the pipe wall can be altered by the welded transition from a pipe with a certain thickness to another with a smaller thickness, and wavelength changes of one order of magnitude can be expected. This can result in different approaches towards the risk assessment for discontinuities in the proximity of changes in wall thickness.Copyright