A novel mathematical procedure to evaluate the effects of surface topography on the interfacial state of stress. Part I: Verification of the method for flat surfaces

A mathematical procedure is developed to utilize the complementary energy method, by minimization, in order to obtain an approximate analytical solution to the 3D stress distributions in bonded interfaces of dissimilar materials. The stress solutions obtained predict the stress jumps at the interfaces, which cannot be captured by current FEA methods. As a novel method, the penalty function is used to enforce the displacement boundary conditions at the interfaces. Furthermore, the mathematical procedure developed enables the integration of different interfacial topographies into the solution procedure. In order to incorporate the effects of surface topography, the interface is expressed as a general surface in Cartesian coordinates, i.e. F (x, y, z) = 0. In this paper, the flat interface problem, i.e. y = 0 surface is considered for verification of the method by comparison with the FEA method. A comparison of the results reveals our new mathematical procedure to be a promising and efficient method for optimizing interface topographies.     

Automated Nucleic Acid Purification for Large Samples

Advancements in the fields of genomic screening, molecular pathology and clinical research have resulted in a major increase in the demand for high quality DNA and RNA. This escalating demand has resulted in a sample preparation bottleneck and an emphasis on the development of new technologies to automate the purification process. Gentra has developed the AUTOPURE LS™ nucleic acid purification instrument, a platform capable of high-throughput sample purification from large samples, such as 10 mL whole blood. This article presents data showing the equivalency of DNA purified using manual and automated processing.     

Application of the Taguchi method for assessment of surface treatment procedures for Ti/n-type GaN contacts

Abstract

A Taguchi approach was used to appraise the relative importance of gallium nitride surface preparation on the processing of Ti/n-type GaN contacts. It is shown that a potassium hydroxide etch, silicon tetrachloride etch or hydrochloric acid clean prior to metal deposition can significantly improve the performance of the contact for the criteria of metal adhesion, contact resistance, electrical behaviour and across wafer uniformity.     

Bonding Optimization on Composite Surfaces using Atmospheric Plasma Treatment

The effect of atmospheric plasma treatment (APT) on the bonding performance of a cyanate ester and an epoxy carbon fiber reinforced composite fabricated with a polyester peel ply was evaluated. A room temperature (RT) cured epoxy, an elevated temperature cured epoxy, and a cyanate ester resin, were used as the bonding adhesives. Only small increases in the carboxyl species concentration were observed for both composite systems as a function of increasing plasma treatment. Lap shear (LS) tests of the bonded composites showed that the APT resulted in a 30% strength improvement for the RT cured epoxy bonded specimens while the cyanate ester composite exhibited negligible increases due to the formation of a highly oxidized, weakly bonded ash. Contact angle measurements indicated that the temperature exposure associated with the curing of the elevated temperature adhesives also reduced the efficacy of APT. Modifications of the bonding surface of these composites by the incorporation of a plasma responsive (PR) layer resulted in significant LS improvements. After incorporating the PR layer, the improvement in adhesive strength was over 225% that of an untreated specimen and approximately 190% that of the equivalently treated unmodified system. Bond strengths correlated with corresponding increases in carboxyl concentrations after APT.     

Comparative Study of the Results of Various Experimental Tests Used for the Analysis of the Mechanical Behaviour of an Adhesive in a Bonded Joint

The use of adhesively bonded joints is often limited by a lack of reliable models able to accurately predict their behaviour in industrial applications, in which the stress distribution is often complex. The mechanical behaviour of an adhesive in a bonded joint is often heavily dependent on its stress state (i.e., the tensile–shear combinations). Thus, a large experimental database is required to accurately represent the complex behaviour of an adhesive in a bonded joint. On the one hand, the initial yield surface (initial elastic limit) often has to be described taking into account the two stress invariants, hydrostatic stress and von Mises equivalent stress, and on the other hand the non-linear behaviour of the adhesive is also quite complex to model. However, the mechanical response of adhesively bonded joints often presents quite large stress concentrations; thus, the analysis of experimental tests is made particularly difficult. Obtaining reliable experimental results makes it possible to contribute to optimization of an adhesive in a bonded joint. This paper presents comparisons between results of different experimental tests (with bulk and bonded joints), some of them are designed to greatly limit the edge effects. Results are presented for two adhesives under proportional monotonic loadings. The two adhesives have very different behaviours (a ductile adhesive and a brittle adhesive) and two different surface preparations of aluminium substrates (a mechanical preparation and a chemical preparation recommended by the adhesive manufacturer) were studied.     

Surface preparation effect on performance of adhesively-bonded graphite/epoxy composites subjected to impact

The influence of pre-bond surface preparation on performance of adhesively-bonded composites subjected to impact was investigated in this study. Impact test was carried out on adhesively bonded graphite/epoxy composite specimens at different low impact energies ranging from 5 to 20 J using the drop-weight impact test. Post-impact ultrasonic evaluation was performed in order to determine the resulting internal damage due to impact on the adhesive bondline. The ultrasonic C-scan of the gated ultrasonic wave signal was acquired and the ensuing debond area in the adhesive bond was determined quantitatively for specimens made from substrates with different surface preparations such as paper peel ply, sandblasting, and sandpaper abrasion. In order to determine the flexural load bearing capacity and stiffness reduction after impact, a three-point bending test was conducted on unimpacted and impacted specimens. A comparative study was performed to evaluate the performance of adhesively-bonded composites with different surface preparations. The results revealed that paper-peel ply performed the best in terms of resistance to debond area formation in the adhesive layer, as well as in terms of retention of flexural load bearing capacity and stiffness after impact.     

Miniaturization of Sample Processing for Bioassays

LLNL has recently licensed its hand-held advanced nucleicacid analyzer, the HANAA, for commercial development. This (as is characteristic of other miniaturized, portable diagnostic instruments being developed around the world) typically still requires manual sample handling and preparation by skilled operators. Dielectrophoretic and acoustic manipulation of fluids and particles are techniques that have shown good promise to reduce the demands on manual sample preparation both for miniature and for table-top instrumentation.     

Origin of hard and soft zone formation during cladding of austenitic/duplex stainless steel on plain carbon steel

Abstract

Weld cladding of corrosion resistant materials is an attractive alternative to bulk usage. Such processes are generally associated with potential deterioration of mechanical properties at the clad/metal interface, often attributed to the formation of hard and soft zones which is usually minimised by control of dilution and use of a buffer layer. To ascertain the origin of the hard and soft zones, weld cladding of austenitic stainless steel (309L) was attempted on plain carbon (0.28 wt-%) steel base metal at various heat inputs, and correspondingly different dilution percentages for the weld metal were obtained. The formation of hard and soft zones was observed mainly on the austenitic stainless steel (i.e. weld metal) side. Typically close to the welding interface (i.e. visible fusion line) a hard zone was observed, followed, but only at higher dilution, by a soft zone. In general, with increasing dilution, the peak hardness and width of the hard zone increased, and the chromium and nickel concentrations of the soft zone significantly decreased. The hard zones were martensitic regions which, at higher dilution, contained chromium carbide precipitates. These precipitates may explain the chromium depletion in the adjacent soft zone. The nickel depletion may be a result of the positive interaction coefficient between nickel and carbon. Since hard/soft zone formation was least at lower dilution, cladding of duplex and superduplex stainless steel was conducted on the same plain carbon base with and without a 309L buffer at an optimum dilution of about 15%. Even at this dilution, direct cladding always resulted in hard and soft zone formation, which was significantly reduced or eliminated by indirect cladding. The hard zones were identified as regions with extensive carbide precipitation (no evidence of martensite formation was found), which occurred in duplex/superduplex grades even at 15% dilution.     

Diffusion annealing and laser surface alloying with aluminium to enhance oxidation resistance of carbon steels

Abstract

The oxidation behaviour of a carbon steel was investigated after modifying the surface in three different ways, namely: (a) physical vapour deposition of aluminium by a resistance heating evaporation technique; (b) diffusion annealing of predeposited aluminium; and (c) laser surface alloying with predeposited aluminium. While diffusion annealing at a temperature of 1023 K was found to result in the formation of Al₁₃Fe₄ and Al₅Fe₂ phases, laser surface alloying resulted in a case containing Al₁₃Fe₄ and Al₂Fe phases. The oxidation behaviour of all the above specimens was studied at 873 K for a total duration of up to 200 h. The mass change and morphological modifications to the surface were monitored at regular intervals of 25 h. It was observed that aluminising results in a substantial enhancement of the high temperature oxidation resistance in comparison with the untreated base material, by virtue of the superior oxidation resistance of the intermetallic phases formed on the surface. Furthermore, laser surface alloying was found to result in better oxidation resistance, in comparison with diffusion annealed and as deposited specimens.