Stressed Adhesive

Keywords Adhesive modulus Adhesys expert system Co-axial joints Compression Concealed joints Creep Elastic limit Epoxy Epoxy composite Einite element analysis Glue line thickness Goland and Reissner Hart-Smith Heat exchanger Hooke s Law Joint designs Joint thickness Lap shear strength Peel Plastic behaviour Polyurethane Pipe bonding Shear stresses Shear modulus Stress distribution Thick adherend shear test Tubular joints Volkersen equation Young s modulus 

When two materials are required to be fastened together, we expect some stress to be associated with the assembly in normal use. At the design stage, we have the opportunity to consider all practical options. If we wish to accommodate maximum loads, the adhesive would perform to the highest level if the stress were to put the adhesive into compression (Fig. 1). 

Redesign of the joint to avoid cutting the hole or slot, means that the applied load generates a tensile rather than a compressive load (Fig. 2). 

If the load is applied in an asymmetric maimer, the stress concentration will have implications with respect to the adhesive performance. In its extreme, this is considered to be a cleaving or peel stress (Fig. 3). 

The supplier of adhesive will normally provide strength data which is relevant to a specific use. Compressive strength would be needed for a grout to support a railway line, shear strength to assemble a bicycle frame, tensile strength in the manufacture of bonded rubber O rings and peel strength for the assembly of multilayer laminates. 

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Film Adhesion. The adhesion of an inorganic thin film to a surface depends on the deformation and fracture modes associated with the failure (4). The strength of the adhesion depends on the mechanical properties of the substrate surface, fracture toughness of the interfacial material, and the appHed stress. Adhesion failure can occur owiag to mechanical stressing, corrosion, or diffusion of interfacial species away from the interface. The failure can be exacerbated by residual stresses in the film, a low fracture toughness of the interfacial material, or the chemical and thermal environment or species in the substrate, such as gases, that can diffuse to the interface. 

Carter, G. F., Outdoor Durability of Adhesive Joints under Stress, Adhesives Age, October 1967. 

The improved mechanical properties (such as stress, adhesion and oxidation stability) with increasing silicon content have been noticed in other studies. Brors et al.217, have reported that the Si/W ratio should stay above 2.0 in order to have stable films. Shishino et al.245, in a study of open failures in WSix interconnects, come to the conclusion that by increasing the Si/W ratio from 2.44 to 2.58, the failure rate is decreased from 0.350% to 0.007%. Shioya et al.246 have shown that the room temperature stress reduces sharply with increasing silicon content for silane based films. For a SiH2Cl2 based film the dependence of the film stress on process parameters is less clear cut [Selbrede238]. 

Stress—adhesive film must be able to withstand stresses due to moisture (loss/gain), deformation from creep or cold flow, differential expansion and contraction, etc. 

Adhesives that reduce and dissipate stresses consist of low-modulus elastomeric types. Low-stress adhesives are required in attaching large die, large substrates, and flex circuitry. Low-modulus elastomeric adhesives are also used to attach and fillet large components to dampen vibration and shock. 

First generation of low-stress adhesives (paste and film)... 

Lastly, adhesives are used to dissipate stresses that may be generated from thermal excursions, mechanical shock, vibration, or moisture. Specially formulated adhesives are effectively used as underfills for flip-chip devices and ball-grid-array packages to compensate for mismatches of expansion coefficients among the solder, the silicon chip, and the ceramic or plastic-laminate substrate. Low-stress adhesives are also used to attach fragile devices such glass diodes and to dampen stresses due to vibration. 

A function of adhesives that is becoming increasingly important is to absorb and dissipate stresses produced in electronic assemblies due to changes in environmental conditions or accelerated testing. The need for stress-absorbing and dissipating adhesives has increased with advancements in microelectronics. Some advancements requiring low-stress adhesives are ... 

Adhesives formulated from flexible, elastomeric resins are now commercially available and effectively used to dissipate stresses. They are low-stress or no-stress adhesives that have a low modulus of elasticity. Several guidelines that can be followed to reduce stresses are the following ... 

Silver glass High-temperature stability. Low outgassing. Close CTE match to silicon. High-firing temperatures. High-stress adhesive. Voiding due to solvent. 

High-modulus adhesives are likely to have high bending stresses, high deflections, and low radii of curvature. A comparison of typical and ultra-low-stress conductive adhesives was reported using 250-mil square silicon die (20 mils thick) bonded to 10-mil-thick copper leadframes." A conventional silver-filled adhesive had a deflection of 3 mm and an ROC of 1,070 mm while a low-stress adhesive showed a deflection of 0.01 mm and an ROC >125,000 mm." ... 

Yuhua G, Gang L, Yin X, Yangchao T (2007) Study of the demolding process-implications for thermal stress, adhesion and friction control. J Micromech Microeng 17 9-19... 

Goland and Reissner(16) was limited because the peel and shear stresses were assumed constant across the adhesive thickness, the shear was assumed a maximum at the overlap end (and not zero as it must be at a free surface), and the shear deformation of the adherends was neglected. Later, several analysts including Renton and Vinson(17) and Allman) 18) produced solutions where the adherends were modelled to account for bending, shear and normal stresses. Adhesive shear stress was set to zero at the overlap ends. Allman additionally allowed for a linear variation of the peel stress across the adhesive thickness, although his adhesive shear stress was constant. 

See also adhesive bonding adhesive stress adhesion theory joining (of plastic parts). 

Bernstein, E. F., Marzec, U., dayman, M.D., Swanson, S., and Johnston, G. G. Platelet function following siarface injury and shear stress Adhesion, Aggregation, Release and Factor 3 activity. Ann. N. Y. Acad. Sci. 283 138,197T-... 

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