This paper considers a number of NP-complete problems, and provides faster algorithms for solving them. The solutions are based on a recursive partitioning of the problem domain, and careful elimination of some of the branches along the search without actually checking them. The time complexity of the proposed algorithm is of the form O(2en) for constant 0 e 1, where n is the output size of the problem. In particular, such algorithms are presented for the Exact SAT and Exact Hitting Set problems with e = 0.2985. Both algorithms improve on previous ones proposed in the literature.

The insulation resistance of polyurethane and poly (para dichloroxylelene) conformal coating was characterised following exposure to hygrothermal environment. Three types of test specimens were used—the standard Y type pattern, the ‘comb’ pattern and the ‘twin’ conductor type. The PCB with the comb pattern has been found to be the best testing specimen for the evaluation and comparison of conformal coating materials and processes. Using this type of PCB, the effects of predrying and thickness in polyurethane coated PCBs have been studied. While predrying has proved to be beneficial for preventing degradation of insulation resistance upon exposure to hygrothermal conditions, only marginal improvement of resistance is obtained when the polyurethane coating thickness has been doubled from 50 to 100 µm. Tin‐plated copper exhibited enhanced insulation resistance compared with bare copper for both polyurethane and polyxylelene coatings. Finally, a thin polyxylelene coating has demonstrated improved moisture barrier properties compared with a thicker polyurethane coating as determined from the resistance degradation with exposure duration to heat and humidity.

A study was made to characterize the surface of oxides formed on Al 2024 in comparison to those formed on Al 1100, using Auger, ESCA, SIMS, FTIR and SEM/EDAX techniques. The pretreatments studied were chromate conversion coating, chromic acid anodizing and sulfuric acid anodizing. The specimens were evaluated as treated and after exposure to laboratory conditions (T=25≠3°C and relative humidity of 60≠10%) for 6 months. In addition, a new surface pretreatment was studied, which was based on McDonnell Douglas P.S. 13201 specification. Adhesive bond strength of silicone rubbers and epoxy adhesives to pretreated Al 2024 was determined, using single lap joint specimens. It was shown that the main difference between the anodic coatings on Al 1100 and Al 2024 was the presence of copper on the surface in the latter. The presence of copper led us to postulate the mechanism of polymerization inhibition of 2 parts polysiloxane adhesive. The most suitable treatments for adhesive joining were found to be chromic acid sealed or unsealed anodizing for both Al 2024 and Al 1100. Our standard chromic acid anodizing gave higher bond strength for aluminum-adhesive joints than the “Douglas” process. as determined with several epoxy adhesives. Correlation between microscopic and macroscopic phenomena was established.

In the present work chemical interactions between sealed chromic-acid-anodized or conversion-coated Al1100, two proprietary primers and PR-420, a polyurethane adhesive, were studied. The chemical compositions of the primers and the adhesive were determined. All the relevant combinations of substrate-primer-adhesive were evaluated, using FTIR, Auger and SIMS spectroscopies. Single lap joint shear strength of the various combinations served as an indicator to the preferred surface treatment for a sound adhesive bonding. Chromic acid anodizing with a commercial AD-6 primer gave the best results. Correlation between chemical interaction and bond strength was observed.

The benefits of adhesively bonded structures are well known. However, the most significant factor limiting the extensive application of metal bonding in primary aerospace structures is the prevailing lack of confidence in its long term durability under hostile environmental conditions.1,2

The effect of moisture content in four commercial epoxy film adhesives on their performance was studied. The investigation included conditioning of the adhesives under humidity and drying cycles, determination of moisture level following each conditioning step, and evaluation of the shear strength of the conditioned adhesive. Experimental results have shown that absorbed moisture caused reduction in adhesive shear strength. Predrying of the uncured adhesive under vacuum (3–5 mm Hg), for 3–4 hours at room temperature, was very effective in removing the absorbed humidity and consequently bond strength was regained, provided that the absorbed moisture was below the 0.3% level. Above this threshold value, irreversible deterioration occurred and drying resulted in only partial recovery of adhesive bond strength.

Single lap joint specimens bonded with two commercial epoxy-based film adhesives, FM-73 and FM-300K, were exposed to combinations of different types of ‘load’, ie mechanical load, temperature and moisture/water. The applied mechanical load was high—90% of the nominal strength of the specimen (as determined in tensile experiments). The initial strength of the joint and the residual strength at various time intervals were then determined. The specimens were not influenced by a single type of ‘load’ or even by a combination of two types of ‘load’. However, combining all three ‘loads’ caused spontaneous failure of the specimen. When applying a constant load of up to 60% of the nominal strength, no spontaneous failures were observed within 30 days (even after exposure to hot-wet environment).

The moisture level of two commercial overaged epoxy film adhesives has been controlled by drying under vacuum and/or exposure to humid atmosphere. Shear and peel bond strengths of the conditioned adhesives were evaluated. Predrying of the uncured adhesive under vacuum (3–5 mm Hg) at room temperature is shown to be very effective for bond strength enhancement. Additional humidifying/drying circles show the same effect but some irreversible degradation occurs and only partial improvement of adhesive bond strength is achieved.