Cohesive stress

Where, xxy, c

shear stress, cohesion and internal friction angle in interface. 

No detailed data. 70 % clay/30 % sand. Clay of medium plasticity shear test at 34 % water content gave a cohesion c = 15 kN/m and internal shear angle (jj = 22.2°. " ) High plasticity clay shear test at 34 % water content gave c = 13 kN/m ( ) = 14.0°. Clay, shear test c = 45 kN/m = 24.5°. ) Silt, shear test at 16 % water content c = 27 kN/m, = 38°. ) Clay, shear test at 27 % water content, c = 55 kN/m, (fi = 17.5° c effective-stress cohesion, effective-stress internal friction angle. 

The science of adhesion recognizes two types of failure. As already mentioned in Section 12.3.2, adhesive failure occurs when the bond between the adhesive and the adherend breaks on stressing. Cohesive failure is when the failure takes place within either the substrate or the adherend. Many cases are known where the adhesive bond between two substances is stronger than the substances themselves. 

Most solid surfaces are marred by small cracks, and it appears clear that it is often because of the presence of such surface imperfections that observed tensile strengths fall below the theoretical ones. For sodium chloride, the theoretical tensile strength is about 200 kg/mm [136], while that calculated from the work of cohesion would be 40 kg/mm [137], and actual breaking stresses are a hundreth or a thousandth of this, depending on the surface condition and crystal size. Coating the salt crystals with a saturated solution, causing surface deposition of small crystals to occur, resulted in a much lower tensile strength but not if the solution contained some urea. 

A material s flow function is usually measured on the same tester as the wall friction angle, although the cell arrangement is somewhat different (Fig. 6). ConsoHdation values are easily controUed, and the cohesive strength of the bulk soHd is determined by measuring interparticle shear stresses while some predeterrnined normal pressure is being appHed. 

The modulus of elasticity can also influence the adhesion lifetime. Some sealants may harden with age as a result of plasticizer loss or continued cross-linking. As a sealant hardens, the modulus increases and more stress is placed on the substrate—sealant adhesive bond. If modulus forces become too high, the bond may faH adhesively or the substrate may faH cohesively, such as in concrete or asphalt. In either case the result is a faHed joint that wHl leak. 

Overland water flow appHes shear forces to sod surfaces. When shear forces exceed the stress required to overcome cohesive forces between sod particles, the particles are detached and suspended in the flow. Suspended particles are carried into surface sod with infiltrating water where they block pores and initiate seal formation (47). Thus, erosion results in reduced water infiltration as well as loss of sod from the field and consequent downstream water pollution. If erosion is controlled, good water infiltration is maintained. 

Fracture mechanics (qv) affect adhesion. Fractures can result from imperfections in a coating film which act to concentrate stresses. In some cases, stress concentration results in the propagation of a crack through the film, leading to cohesive failure with less total stress appHcation. Propagating cracks can proceed to the coating/substrate interface, then the coating may peel off the interface, which may require much less force than a normal force pull would require. 

The reason for the activity of the above named classes of liquids is not fully understood but it has been noted that the most active liquids are those which reduce the molecular cohesion to the greatest extent. It is also noticed that the effect is far more serious where biaxial stresses are involved (a condition which invariably causes a greater tendency to brittleness). Such stresses may be frozen in as a result of molecular orientation during processing or may be due to distortion during use. 

Another distinction to be made is illustrated with the peel test shown in Fig. 1. Application of stress may cause the joint to fail either adhesively or cohesively . Adhesive failure, shown in Fig. la, is thought ideally to correspond to a perfect... 

The aim of this chapter is to describe the micro-mechanical processes that occur close to an interface during adhesive or cohesive failure of polymers. Emphasis will be placed on both the nature of the processes that occur and the micromechanical models that have been proposed to describe these processes. The main concern will be processes that occur at size scales ranging from nanometres (molecular dimensions) to a few micrometres. Failure is most commonly controlled by mechanical process that occur within this size range as it is these small scale processes that apply stress on the chain and cause the chain scission or pull-out that is often the basic process of fracture. The situation for elastomeric adhesives on substrates such as skin, glassy polymers or steel is different and will not be considered here but is described in a chapter on tack . Multiphase materials, such as rubber-toughened or semi-crystalline polymers, will not be considered much here as they show a whole range of different micro-mechanical processes initiated by the modulus mismatch between the phases. 

Pure PDMS networks are mechanically weak and do not satisfy the adhesive and cohesive requirements needed for most applications in which the silicone adhesive joint is subjected to various stresses. For crosslinked silicones to become high performing adhesives, they need to be strengthened. 

APAOs has limited their utility in a number of applications. The broad MWD produces poor machining and spraying, and the low cohesive strength causes bond failures at temperatures well below the softening point when minimal stress is applied. To address these deficiencies, metallocene-polymerized materials have been developed [17,18]. These materials have much narrower MWDs than Ziegler-Natta polymerized materials and a more uniform comonomer distribution (see Table 3). Materials available commercially to date are better suited to compete with conventional EVA and EnBA polymers, against which their potential benefits have yet to be realized in practice. 

The high cohesive strength developed during the curing of these materials tends to place stress on their adhesive properties. 

K, stress intensity factor ctc cohesive stress G, energy release rate. 

The cohesive stress ac is assumed to be constant (Dugdale model) as in Eq. (7.5). Chan, Donald and Kramer [87] found a good agreement between the critical energy release rate GIC, as estimated by the Dugdale model and G)C as computed from the actual stress and displacement profiles in their experiments. 

The deformation zones were calculated for the polymers of Table 5.1 and Table 6.1 according to the Dugdale-Barenblatt-model. Yield stress ay from tensile tests was used instead of the cohesive stress ctc since a reasonable agreement of ay and ctc... 

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