Humidity effects on the fatigue of fiber reinforced polymers in micro/nano functional systems
Micro and nano functional systems like MEMS and smart systems are exposed to complex and very challenging service conditions (e.g., within airframe structures or in automotive applications under the hood or within battery cells). Therefore, long-term reliability and structural integrity of the chassis and the printed wiring boards is of rising concern. These parts are often made of fiber reinforced polymers (FRP). Recently, a new methodology for accelerated fatigue testing of FRP has been presented. It allows predictions of the structural fatigue lifetime of FRP for arbitrary temperatures and loading ratios based on three simple tests: visco-elastic DMA, constant strain rate tests, and high cycle fatigue tests at a limited number of temperatures and strain rates. However, this methodology considers dry samples only, which does not suffice for describing the behavior of micro/nano systems as mentioned before. The paper reports the approach to the expansion of this testing methodology to the effects of humidity. Choosing PCB material as typical example for FRP in micro/nano systems, the time / temperature shift functions have been determined for wet and dry samples. Subsequently, constant strain rate and high cycle fatigue tests have been performed. They show a substantial loss in strength and fatigue lifetime for FRP structures that were exposed humid environment. Determining the quantities measuring this reduction in structural reliability paves the way to the expansion of the fatigue testing methodology towards accounting for the humidity effect as well.
Toolbox for visco-elastic material modeling of smart lightweight structures
More and more functionalities will soon be integrated into lightweight structures made of fiber reinforced polymers (FRP). This combines very different materials in minimum space. Consequently, thermo-mechanical risks are of vital concern. The complex stress situation requires a systematic and scalable toolbox of methodologies capable of keeping pace with the growing number and diversity in possible failure risks and sites. The paper reports the status of ongoing work on three expansions to an accelerated test methodology that allows determining the lifetime of FRP structures based on three types of tests: visco-elastic dynamic mechanical analysis, constant strain rate and fatigue tests. The expansions attempt to cover in addition i) various levels of moisture content within the polymer, ii) several laminate stack configurations, and iii) different loading situations including the mix of tension, bending, and shear.
A quantitative measure of nitrifying bacterial growth.
Nitrifying bacteria convert ammonia (NH3) to nitrate (NO3-) in a nitrification reaction. Methods to quantitatively separate the growth rate of these important bacterial populations from that of the dominant heterotrophic bacteria are important to our understanding of the nitrification process. The changing concentration of ammonia is often used as an indirect measure of nitrification but ammonification processes generate ammonia and confound this approach while heterotrophs remove nitrate via denitrification. Molecular probe methods can tell us what proportion of the microbial community is nitrifying bacteria but not their growth rate. The technique proposed here was able to quantify the growth rate of the nitrifying bacterial populations amidst complex ecological processes. The method incubates [methyl-3H] thymidine with water samples in the presence and absence of an inhibitor of nitrification-thiourea. The radioactively labeled DNA in the growing bacteria was extracted. The rate of incorporation of the label into the dividing bacterial DNA was used to determine bacterial growth rate. Total bacterial community growth rates in full-scale and pilot-scale fixed-film nitrifying reactors and an activated sludge reactor were 2.1 x 10(8), 4.1 x 10(8) and 0.4 x 10(8)cell ml(-1)d(-1), respectively; the growth rate of autotrophic-nitrifying bacteria was 0.7 x 10(8), 2.6 x 10(8) and 0.01 x 10(8)cell ml(-1)d(-1), respectively. Autotrophic-nitrifying bacteria contributed 30% and 60% of the total bacterial community growth rate in the nitrifying reactors whereas only 2% was observed in the activated sludge reactor that was not designed to nitrify. The rates of ammonia loss from the nitrifying reactors corresponded to the rate of growth of the nitrifying bacteria. This method has the potential to more often identify factors that enhance or limit nitrifying processes in both engineered and natural aquatic environments.