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.
Accelerated testing for long-term fatigue strength of various FRP laminates for marine use
Abstract The prediction of long-term fatigue life of various FRP laminates combined with resins, fibers and fabrics for marine use under temperature and water environments were performed by our developed accelerated testing methodology based on the time–temperature superposition principle (TTSP). The five kinds of FRP laminates were prepared under three water absorption conditions of Dry, Wet and Wet + Dry after molding. The three-point bending constant strain rate (CSR) and fatigue tests for these FRP laminates at three conditions of water absorption were carried out at various temperatures and loading rates. As results, the mater curves of fatigue strength as well as CSR strength for these FRP laminates at three water absorption conditions are constructed by using the test data based on TTSP. It is possible to predict the long-term fatigue life for these FRP laminates under an arbitrary temperature and water absorption conditions by using the master curves. The characteristics of time, temperature and water absorption dependencies of flexural CSR and fatigue strengths of these FRP laminates are clarified.
Experimental study on bond behavior between high-strength grout and deformed steel bars
Abstract Engineers use high-strength grout to connect precast concrete members for its low shrinkage and good fluidity. This paper designed 87 central pull-out specimens using the control variable method to investigate the influence of factors on the rebar-grout bond property such as compressive strength, steel grade, bar diameter, transverse stirrups, and bond length. The test result showed that improving grout compressive strength (85.8 MPa–122.7 MPa) cannot improve the bond strength, which was different from the previous conclusions of concrete pull-out tests. Simplified formulas were presented for calculating the characteristic values of bond strength, residual strength, peak slip, and relative slip of load-holding platform based on parameter analysis. Then, a constitutive model was proposed to predict the full-range bond stress-slip curve of high-strength grout and steel bar specimens from bearing to failure. Finally, FEA models were established to evaluate the applicability and accuracy of the constitutive model.