Лако-красочные материалы — производство

The stresses that a joint experiences during environmental exposure also influence its durability, i. e., it exhibits either decreased lifetime or decreased residual strength [1,7,10,107]. As with moisture, there may be a critical stress level below which failure does not occur [108,109] or is not accelerated (depending on the moisture level). The type of stress is also important. For example, cyclic stresses degrade the bond more rapidly than constant stresses [5,7].

The stresses on a joint make primary and secondary chemical bonds, both within the polymer itself and across the polymer-oxide interface, more susceptible to environmental attack by lowering the activation energy for bond breaking [1,14,92]. The stresses can also increase the rate of transport of moisture in the adhesive, possibly via crazing or the formation of microcracks [1,14,92,110] or increasing the free volume of the polymer to allow more moisture ingress. Joints subjected to thermal ‘‘spikes’’ or cycling, such as those present in high-speed military aircraft, are particularly vulnerable to this type of aging. Thus, weight gains in bonded composite systems that encountered one to four spikes (0°C-150°C-0°C) per day were proportional to the total number of spikes [111,112], suggesting that water was entering the microcracks formed during thermal cycling. Additionally, studies of the chemical hydrolysis of epoxides by water (80°C) indicated that although unstressed samples were unaffected for up to 3 months, stressed systems induced hydrolysis of ester groups within days [113,114].

The stress at the crack tip of a wedge test specimen, together with the presence of moisture at the tip, serve to make this test specimen more severe than soaked lap shear specimens or similar types and a better evaluation of relative durability. In fact, Boeing has correlated the results of wedge tests from actual aircraft components with their in­service durability [5,115]. Wedge test specimens fabricated from components that had exhibited service disbonds showed significant crack growth during the first hour of expo­sure whereas those fabricated from good components showed no crack growth during this time period. In contrast, lap shear specimens and porta shear specimens all demonstrated high bond strengths regardless of the service conditions.