A deformation mechanism map for a Ni-based superalloy is presented during cyclic loading at low (300°C), intermediate (550°C), and high (700°C) temperatures for low (0.7%) and high (1.0%) applied strain amplitudes. Strain mapping is performed via digital image correlation (DIC) during interrupted fatigue experiments at elevated temperatures at 1, 10, 100 and 1000 cycles, for each specified loading and temperature condition. The DIC measurements are performed in a scanning electron microscope, which allows high-resolution measurements of heterogeneous slip events and a vacuum environment to ensure stability of the speckle pattern for DIC at high temperatures. The cumulative fatigue experiments show that the slip bands are present in the first cycle and intensify with number of cycles; resulting in highly localized strain accumulation. The strain mapping results are combined with microstructure characterization via electron backscatter diffraction. The combination of crystal orientations and high-resolution strain measurements was used to determine the active slip planes. At low temperatures, slip bands follow the {111} octahedral planes. However, as temperature increases, both the {111} octahedral and {100} cubic slip planes accommodate strain. The activation of cubic slip via cross-slip within the ordered intermetallic γ' phase has been well documented in Ni-based superalloys and is generally accepted as the mechanism responsible for the anomalous yield phenomenon. The results in this paper represent an important quantifiable study of cubic slip system activity at the mesoscale in polycrystalline γ-γ' Ni-based superalloys, which is a key advancement to calibrate the thermal activation components of polycrystalline deformation models.
This paper describes a series of experiments and analyses that were used to examine crack growth near sapphire/epoxy interfaces. Adhesion of the epoxy to the sapphire was enhanced by coating the sapphire with mixtures of two silane coupling agents that form self-assembled monolayers. A new biaxial loading device was used to conduct a series of mixed-mode fracture experiments. Crack opening interferometry, atomic force microscopy, and angle-resolved X-ray photoelectron spectroscopy allowed cohesive zone sizes, fracture surface topographies, and loci of fracture to be established. The experiments were complemented by finite element analyses that accounted for the rate- and pressure-dependent yielding of the epoxy. The analyses also made use of traction-separation laws to represent the various interphases that were produced by the mixed monolayers. The intrinsic toughness (defined as the area underneath the traction-separation curve) of the bare sapphire interfaces was independent of mode-mix and lower than values from previous experiments with glass/epoxy and quartz/epoxy specimens. The increase in overall toughness with mode-mix was completely accounted for by viscoplastic dissipation in the epoxy outside the cohesive zone. The minimum toughness of the coated sapphire interfaces was about five times higher than the mode-mix independent intrinsic toughness of the uncoated specimens. The increase in overall toughness with mode-mix was almost completely accounted for by increases in the intrinsic toughness as the traction-separation law varied with mode-mix. As a result, viscoplastic dissipation outside the cohesive zone was minimal. Atomic force fractography and X-ray photoelectron spectroscopy indicated that the crack growth mechanisms and the loci of fracture in the coated and uncoated specimens were quite different.
Microstructure attributes are responsible for heterogeneous deformation and strain localization. In this study, the relation between residual strain fields and microstructure is examined and assessed by means of experiments and crystal plasticity modeling. The microstructure of rolled aluminum alloys (AA) in the 7050-T7451 condition was experimentally obtained with electron backscatter diffraction (EBSD) analysis along the rolling direction (L-T orientation), across the rolling direction (T-L orientation), and transverse to the rolling direction (T-S orientation). Each of these sections was also patterned using a novel microstamping procedure, to allow for strain mapping by digital image correlation (DIC). The measured microstructures were in turn used as input of an elasto-viscoplastic crystal plasticity formulation based on fast Fourier transforms (EVP-FFT). Comparisons between the strain maps obtained experimentally by the concurrent DIC-EBSD method and the EVP-FFT simulations were made for the three sections, corresponding to the initial textures. The comparisons showed that the predicted levels of strain concentration were reasonable for all three specimens from a statistical perspective, which is important to properly describe and predict the strains within an ensemble of components; however the spatial match with the actual strain fields needs improvement.
Surface strain measurements using image correlation require a pattern to be applied to the surface of the object being measured. Lithography, the most widely used method for repeatable patterning is expensive, requiring dedicated technical staff and significant infrastructure. Lithography is time consuming, often requiring several days for each patterning application, which limits throughput. An innovative method has been developed and tested whereby repeatable patterns for image correlation are applied without dedicated technical staff or special infrastructure and can be completed in a few minutes rather than days. This new method is more amenable to application of patterns to complex surface geometries and larger surface areas. The new micro stamping method allows for higher contrast patterning materials, which improves the accuracy of strain measurements using image correlation.Accurate surface strain measurements using image correlation are dependent on the application of a high-contrast pattern to the surface of the object being measured. Error in the strain measurement is dependent on the particular pattern applied, and repeatability of the pattern on various surfaces is ideal. Micro texture stamping is a repeatable, high throughput, high-resolution, low cost, parallel patterning method in which a stamp surface pattern is replicated into a material by mechanical contact. Details of the flexible micro textured stamps produced by 1900 Engineering have been published [1][2]. The stamps were fabricated with a 10 µm base-element size. The electron-beam lithography (EBL) process for generating the stamp master took 57 hours to complete a 12.7 mm x 12.7 mm area using an e-beam resist [3]. Without the stamping procedure, EBL would need to be repeated for each subsequent specimen to be patterned; however, after fabricating the stamp, the pattern application took approximately 10 minutes per subsequent specimen.A diagram for the stamp usage is in Figure 1. The procedure for application of the stamp to create a speckle pattern is: (1) Sonicate the specimen in acetone, then methanol, and dry; (2) With a fine liner, apply MCC primer on clean specimen; (3) Let stand for 15 s and gently apply compressed air from the top; (4) Bake for 3 min at 115 ºC on a hot plate, then remove the specimen. (5) Let the hot plate cool to 60 ºC, (6) Place two specimens side by side (to allow level stamping -the procedure can be applied to one specimen); (7) Apply Shipley 1805 photo resist on one specimen; (8) Wait 20 s; (9) Apply the Shipley in the same specimen; (10) Align the stamp by touching first the dummy specimen and then let the stamp lay down over the specimen to be stamped. (11) Adjust the hot plate for 115 ºC; (12) Place a piece of cook paper over the stamp, to allow a non-stick surface for the weight; (13) Apply weight (~4 psi); (14) Bake for 8 minutes; (15) Remove the weight and the specimen from the hot plate; (16) Carefully peel the stamp off the specimen; (17) Check the gauge section on the microscope for the patterns. The...
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