In this study, in-situ synchrotron measurements were conducted on a thermal barrier coating system with a superalloy substrate. The use of high energy X-rays affords real-time monitoring of complex mechanical behavior; when coupled with an integrated high temperature and mechanical loading testing apparatus, variations in realistic representative service loading can be investigated for their influence on each constituent of the multi-layer system. This provides a method for identifying how changes in loading conditions across a turbine blade may play a role in the initiation of failure mechanisms and lifetime expectancy of the coating. Collecting diffraction measurements on a 2D area detector, elastic strain analysis is conducted to elucidate the influence of external thermal loading, applied mechanical loading, and internal cooling on the thermally grown oxide and ceramic zirconia topcoat. Analysis has identified the influence of these loadings on the system. Investigating the texturing and preferred orientation of the diffraction rings, further insight on the influence of plastic strain is discussed including how grains’ lattice plane rotation and subdivision during high temperature loading affect the residual strain state. Together the non-destructive methods of evaluating the multi-layer system shed light on the performance of thermal barrier coatings in operational conditions.
This research focuses on developing relationships between the mechanical properties and the optical spectra of carbon nanofiber (CNF) reinforced polymer composites. Stress distribution in the sample at discrete force increments is obtained and the effects on the G band of the Raman spectra of CNF are observed. A linear shift of carbon nanofiber Raman frequencies is created by strain applied to their molecular structure. Results of loading experiments combined with optical spectroscopy shows the stress dependency of these bands which can be used in the future for design, analysis and non-destructive structural stress testing of CNF/polymer composites. I. Introduction HE potential of polymer resins enhanced with carbon nanofibers for aerospace applications, where weight, stiffness and strength are critical, presents a need for developing a test methodology to assess the effect of CNF reinforcements with conventional laminates such as glass fiber and carbon fiber reinforced polymer composites. 1 Nanocomposites are a novel class of composite materials that have received special attention because of their improved properties at very low loading levels compared with conventional filler composites. 2 Conventional polymer composite materials reinforced by continuous fibers have excellent in-plane strength, but are usually weak against matrix-dominated failures. Single wall carbon nanotubes (SWNT) , multi-wall carbon nanotubes (MWNT), as well as carbon nanofibers (CNF) are being used for reinforcing polymer matrices for improved mechanical, thermal, and electrical properties. Carbon nanofibers, though not as perfect in structure compared to carbon nanotubes, are demonstrated to have positive impacts on properties of polymer composites and show potential to be reasonably used in metal matrix composites as well because of their mechanical and physical properties. The typical diameters of SWNTs are in the range of 0.7–1.5 nm, MWNTs in the 10–50 nm range, and that for CNF in the 60–200 nm range. In SWNT and MWNT, the graphitic planes are parallel to the tube axis, while in carbon nano fibers, they make a small angle to the CNF axis. 1 CNFs offer chemically facile sites that can be functionalized with additives thereby resulting in a strong interfacial bond with the matrix. Thermoplastics such as polypropylene, polyester, polycarbonate, nylon, poly (ether sulfone), poly (phenylene sulfide), acrylonitrile-butadiene-styrene, thermosets such as epoxy as well as thermoplastic elastomers such as butadiene-styrene diblock copolymer have been reinforced with carbon nanofibers. CNF composites have been studied basically for their potential to improve composite strength. However, unless the nanofiber-matrix interface has been modified, the poor load transfer through this interface, will lead to an interfacial debonding and subsequent premature failure. The structure and properties of the fiber-matrix interface play a huge role in the physical and mechanical properties of the composite. Particularly, for mechanical applications, the nature of the interface determines the load transfer efficiency from the matrix to the fibers, which in turn determines the stress resistance of the composite. The focus of CNF-reinforced composites has been the engineering applications that require superior strength, stiffness, electrical and thermal conductivities.
Chromium-doped α-alumina is naturally photo-luminescent with spectral properties that are characterized by R-lines with two distinct peaks known as R1 and R2. When the material is subjected to stress, shifts in the R-lines occur, which is known as the piezospectroscopic (PS) effect. Recent work has shown that improved sensitivity of the technique can be achieved through a configuration of nanoparticles within a polymer matrix, which can be applied to a structure as a stress-sensing coating. This study demonstrates the capability of PS coatings in mechanical tests and investigates the effect of nanoparticle volume fraction on sensing performance. Here, measurements of spectral shifts that capture variation in stress of the coating during mechanical testing and in the region of substrate damage showed that stress contours are more noticeable on a soft laminate than hard laminate. It was found that the 20 % volume fraction PS coating showed the most distinct features of all the coatings tested with the highest signal-to-noise ratio and volume fraction of α-alumina. Post failure assessment of the PS coatings verified that the coatings were intact and peak shifts observed during mechanical testing were due to the stress in the substrate. The results suggest the ability to design and tailor the “sensing” capability of these nanoparticles and correlate the measured stress variations with the presence of stress and damage in underlying structures. This study is relevant to nondestructive evaluation in the aerospace industry, where monitoring signs of damage is of significance for testing of new materials, quality control in manufacturing and inspections during maintenance.
Additive manufacturing (AM) is becoming prevalent in industry due to its cost-effectiveness, ability to produce parts with complex geometries, and reduced design-to-manufacture time. Although AM shows promising capabilities, inherent defects in printed parts affect their performance during service life, especially when it comes to the corrosion performance of AM metallic parts. AlSi10Mg is one type of metallic material that is printed using the powder bed fusion (PBF) process and commercially available for automotive and aerospace applications. It has been studied extensively for its mechanical behavior and characteristics. Multiple studies have been done to understand the corrosion performance of AlSi10Mg. However, conclusions on how the manufacturing process affects AlSi10Mg's corrosion performance have been contradictory, and there is very limited information on how this material performs under stress corrosion cracking (SCC). Here we show localized strain marking the eventual failure region of the AlSi10Mg sample subjected to SCC using in-situ micro- digital image correlation (DIC). The DIC maps revealed multiple regions of high strain around the electrochemically induced pit on the sample, which were associated with the pit-to-crack transition during the SCC process. The initial effort to apply the in-situ micro-DIC technique led to the capture of microstrains associated with early initiation of SCC accelerated by manufacturing defects in AM-prepared alloys. This study is a step forward towards better understanding of how the manufacturing process affects SCC behavior of AM-prepared alloys. The findings of this work can provide data that will aid in the development of improved manufacturing processes for creating AM alloy parts with better reliability and corrosion resistance.
