Stresses environmental attack affected

The last severe environment on our list is external stress. External stresses (14,33) especially affect the water uptake or saltwater penetration of adhesive bonds. However, some adhesives may appear insensitive to stresses. Presumably, the critical stress level (34) is so high that no environmental attacks can take place below that level. Thus, it is difficult to discuss the effect of external stresses without considering the fracture mechanics of adhesive joints. In general, external stresses accelerate bond degradation when the joint is exposed to severe environments. 

However, other approaches have also successfully been adopted. For example, Dickie and Ward [53] have studied single-lap joints exposed to a high humidity at moderately elevated temperatures but maintained a constant stress on the joints. Also, periodically the joints were removed from the high-humidity environment and exposed to a salt solution for a short time period. Using this accelerated-ageing test they were able to rank the durability performance of various adhesive systems in a comparatively short timescale. Further, they reported that not only were the kinetics of mechanisms of environmental attack accelerated, but also the exact details of the mechanisms were affected by the levels of the applied load. For example, for joints which consisted of bonded galvanised steel substrates, the effect of relatively high applied loads was to prevent the formation of an effective barrier of corrosion products, i.e. passivation of the substrate surface was prevented. This allowed the electrochemical corrosion process to proceed unimpeded, and hence at a faster rate than for similar, but unstressed, joints. Thus,... 

Host plant preferences of insect pests are often Influenced by environmental conditions. The effects of plant stress on susceptibility to Insect attack have been obseirved in many crop plants. Stress factors affecting the physiological state of the plant Include drought, disease, chemical pollutants, and high salt concentrations. Profound... 

Under psycho-physical, dietary, and environmental stress conditions, or after medication intake, there is an imbalance of microflora that makes the body susceptible to attack by pathogens. A proper diet is one of the main factors that influence the qualitative and quantitative composition of intestinal microflora. The most common approach involves the consumption of traditional foods, such as yogurt and fermented milk, which essentially contain probiotics, defined as live microorganisms, which can positively affect the host by improving its intestinal microbial balance. 

The first stage was considered to be a slow environmental stress crack (ESC), produced by slight chemical attack in the weld. ESC may be defined as a reduction in the tolerance of mechanical stress caused by special chemical environments, or, alternatively, as a reduction in the time to fracture under a fixed strain. Other examples have been the failure of blow-moulded polyethylene bottles when filled with detergent, and the weakening of crash helmets made of polycarbonate when affected by petrol or adhesive badges. 

Both stiffness and toughness can be affected by environmental effects snch as thermal and oxidative ageing, ultraviolet (UV) ageing, and chemical attack (including the special case of environmental stress corrosion). 

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