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.
Evaluation of buckling load and dynamic performance of steel shear wall retrofitted with strips made of shape memory alloy
One of the most imperfections of the steel shear wall is out of plane displacements that cause severe damage in both structural and non-structural elements. In this paper, the effects of shape-memory alloys on steel shear walls are investigated. First, a numerical analysis using the finite element method in ABAQUS software has been carried out according to an experimental test. The results of the numerical analysis have been verified with experimental results. Next, shape-memory alloy fibers have been added vertically and horizontally to various parts of the steel shear wall. The results show that retrofitting with the shape-memory alloy reduces the out-of-plane displacement of the steel shear wall under both cyclic and seismic loadings. Besides, the buckling load in the steel shear wall increases when it is retrofitted with the shape-memory alloy. Also, the total out-of-plane movement (accumulated absolute displacements) of the steel shear wall and non-structural damage are controlled by the characteristic of a shape-memory alloy material called “super-elastic”.