Stress internal

Adhesives often operate with some additional stress in the joint arising from shrinkage of the adhesive relative to the substrates. The main reason for the shrinkage comes from equilibrium contact between the adhesive and substrates being established at temperatures above the subsequent operating temperature of the joint. Thus, since the adhesive and substrates usually have different coefficients of thermal expansion, thermal strains are introduced upon cooling. Other events such as loss of solvent, polymerization reaction, crosslinking, etc. may also be accompanied by volume contractions but are usually considered to be of secondary importance. 

Experimental work [118-125], especially that using photoelastic techniques, has established the presence of residual stresses in joints but the results have often not been quantitative. For the simpler case of polymeric films coated onto metallic substrates, photoelastic techniques and a method based upon the bimetallic strip principle have often been employed [126-131]. Using the latter method, Danneberg [130] showed that for a wide range of epoxy-based coatings on an aluminium substrate thermal contraction was a major cause of internal stress and that the stresses generated were of the order of 0.08 

MPa/°C. More recently Hahn [125] has studied residual stresses using a very sensitive electrical strain measuring technique and, although the method was not quantitative, he clearly showed that residual stresses were present in the interfacial regions of the adhesive layer. 

The theoretical analysis of the internal stresses in an adhesive joint is beset with great difficulties, even if it assumed that the adhesive is perfectly elastic. Bikerman [132], Wake [133], Harrison and Harrison [134], Carlson and Sapetta [135] and Inoue and Kobatake [123] have made this assumption and attempted theoretical predictions. Some of the main points to emerge are that the in-plane normal stresses, 0-22, are tensile in nature and are greatest and 

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Theory of the fictitious temperature field allows us to analyze the problems of residual stresses in glass using the mathematical apparatus of thermoelasticity. In this part we formulate the boundary-value problem for determining the internal stresses. We will Lheretore start from the Duhamel-Neuinan relations... 

A fan blade is continuously vibrating millions of cycles up and down ia operatioa over a short period of time. Each time a blade tip moves past an obstmction it is loaded and then unloaded. If forced by virtue of tip speed and number of blades to vibrate at its natural frequency, the ampHtude is greatly iacreased and internal stresses result. It is very important when selecting or rating a fan to avoid operation near the natural frequency. The most common method of checking for a resonance problem is by usiag the relatioa ... 

E. Orowan, ia Symposium on Internal Stresses in Metals, Institute of Metals, London, 1948, pp. 451—453. 

Corrosion attack on the polymer is influenced by permeation rate, as weU as internal stresses or fatigue, that distorts or fractures the resin glass fiber... 

Size reduction causes particle breakage by subjecting the material to contact forces or stresses. The apphed forces cause deformation that generates internal stress in the particles and when this stress reaches a certain level, particle breakage occurs. 

Measurements of stress relaxation on tempering indicate that, in a plain carbon steel, residual stresses are significantly lowered by heating to temperatures as low as 150°C, but that temperatures of 480°C and above are required to reduce these stresses to adequately low values. The times and temperatures required for stress reUef depend on the high temperature yield strength of the steel, because stress reUef results from the localized plastic flow that occurs when the steel is heated to a temperature where its yield strength is less than the internal stress. This phenomenon may be affected markedly by composition, and particularly by alloy additions. 

Cast iron is not a recommended constmction material for oleum. It is weU-documented that cast iron fails catastrophically by cracking in oleum service. The mechanism or the cause of cracking is not well understood, but failures result from a buildup of internal stresses within the cast iron. One notable exception is process iron, a proprietary cast iron of Chas. S. Lewis Co. (St. Louis, Missouri). Process iron has been used successfiiUy in oleum service. 

Air-Entrainment Agents. Materials that are used to improve the abiUty of concrete to resist damage from freezing are generally known as air-entrainment agents. These surfactant admixtures (see Surfactants) produce a foam which persists in the mixed concrete, and serves to entrain many small spherical air voids that measure from 10 to 250 p.m in diameter. The air voids alleviate internal stresses in the concrete that may occur when the pore solution freezes. In practice, up to 10% air by volume may be entrained in concrete placed in severe environments. 

Both Watts and sulfamate baths are used for engineering appHcation. The principal difference in the deposits is in the much lower internal stress obtained, without additives, from the sulfamate solution. Tensile stress can be reduced through zero to a high compressive stress with the addition of proprietary sulfur-bearing organic chemicals which may also contain saccharin or the sodium salt of naphthalene-1,3,6-trisulfonic acid. These materials can be very effective in small amounts, and difficult to remove if overadded, eg, about 100 mg/L of saccharin reduced stress of a Watts bath from 240 MPa (34,800 psi) tensile to about 10 MPa (1450 psi) compressive. Internal stress value vary with many factors (22,71) and numbers should only be compared when derived under the same conditions. 

A.STM B636, Std. Methodfor Measurement of Internal Stress of Plated Metallic Coatings with the Spiral Contractometer, American Society for Testing and... 

K. Parker, Internal Stress Measurements of Electroless Nickel Coatings by the Rigid Strip Method, ASTM STP 947, American Society for Testing and Materials, Philadelphia, Pa., 1987. 

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