Environmental attack mechanisms

The above methodology successfully predicted the loss of Joint strength as a function of the time that the epoxy/steel Joints were exposed to water at various temperatures [6,43,65]. However, several problems do occur in applying this model in practice. For example, the critical concentration of water necessary for the environmental attack mechanism to initiate has to be known, or has to be fixed empirically by an initial calculation using known strength-loss data. Also, the detailed kinetics of the mechanism have to be known. 

Type of attack Environmental Cause of attack Mechanical factors Corrosion product... 

The primary environmental fate mechanism followed by stored or buried HD is hydrolysis. Although HD is rapidly hydrolyzed (a half-life of 4 to 8 min at 25° C in distilled water has been reported [Bartlett and Swain, 1949]), the overall process of hydrolytic destruction is limited by the very low water solubility of HD. Intermediate hydrolysis products and/or water-insoluble thickeners that can coat or encapsulate droplets of mustard retard hydrolysis. Because of low water solubility and formation of intermediate products, bulk amounts of HD may persist undispersed under water for some time. However, HD dispersed as droplets or mist, as in the case of an aerial attack, is expected to hydrolyze rapidly in humid air. 

The SMP based syntactic foam may be used in various lightweight composite stmctures. Most of the time, the structures may be used outdoors with uncontrolled environmental attacks. The most obvious environmental attacks include ultraviolet (UV) radiation, moisture, temperature, and combinations of these single factors, such as hydrothermal attacks. Polymers are extremely sensitive to these environmental attacks due to the photophysical and photochemical effects. Often, the combination of UV radiation with oxidative and hydrolytic factors leads to more severe degradation than that from a single factor. The damage may range from mere surface discoloration to extensive loss of mechanical properties. 

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. 

J. Comyn, "Kinetics and Mechanisms of Environmental Attack," in Durability of Structural Adhesives, Ed., A.J. Kinloch, Applied Science Publishers, London and New York (1983). 

The first group can be described as absolute requirements , i.e. the requirements that have to be fulfilled without any compromises. Typical examples of these are temperature resistance or resistance to chemical or other types of environmental attack. Also certain mechanical properties may become absolute requirements if, for example, a minimum value is set for the property in the specification. Any adhesive failing to meet any of the absolute requirements leads to that particular adhesive being dropped from of the selection process. If none of the adhesives satisfies all the absolute requirements, it is obvious that either the requirements that have been set are unrealistic and have to be reconsidered or adhesive bonding is not the correct method for the case studied. 

Therefore, the first and the most important problem is the fibre-matrix adhesion. The role of the matrix in a fibre-reinforced composite is to transfer the load to the stiff fibres through shear stresses at the interface. This process requires a good bond between the polymeric matrix and the fibres. Poor adhesion at the interface means that the full capabilities of the composite cannot be exploited and leaves it vulnerable to environmental attacks that may weaken it, thus reducing its life span. Insufficient adhesion between hydrophobic polymers and hydrophilic fibres result in poor mechanical properties of the natural fibre-reinforced polymer composites. Pre-treatments of the natural fibre can clean the fibre surface. 

Long-term durability of adhesively bonded joints may require resistance to a number of individual or combined degradation modes, including environmental attack, fatigue and time-dependent failures. Time-dependent failure mechanisms are often characterized nsing either a strength approach, involving creep and creep-rupture tests, or a fracture approach, in which debond rate is determined. In creep-rupture tests, adhesive joints are subjected to... 

To prepare the FRP composite, the respective fiber is embedded in a polymer matrix mostly thermoset or thermoplastic resins. The role of the matrix is (i) to bind the fibers together, (ii) to transfer stresses between fibers, and (iii) to protect them against environmental attack and damage due to mechanical abrasion. The matrix also controls the processability, the maximum service temperatures, as well as the flammability and corrosion resistance of FRP. Most FRPs are made in order to improve mechanical performances such as elastic properties (modulus of elasticity) and ultimate properties (strength, toughness). To some extent and based on the choice of constituents, preparation of composites makes it also possible to tailor other physical properties, such as electrical conductivity, mass transport properties, heat conduction, etc. [49]. 

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