Showing papers on "Digital image correlation published in 2017"

Influence of processing and orientation print effects on the mechanical and thermal behavior of 3D-Printed ULTEM® 9085 Material

Abstract: In this paper, we investigate the print orientation effects on the macrostructure, the mechanical and thermal properties, and the strain field behavior of ULTEM® 9085 using a Stratasys Fused deposition modeling (FDM) 400 Printer. The tensile strength, failure strain, Poisson’s ratio, coefficient of thermal expansion and modulus were all shown to vary significantly depending on the build orientation of identical dogbones. FDM parts ranged in strength from 46 to 85% of strengths attainable from comparable injection-molded parts. The coefficient of variation (CV) increased from 2 to 13% as the primary layer orientation deviated from the primary load direction. CAT scan and SEM were employed to relate the corresponding macrostructure to the mechanical response of the material along the parts’ 3-primary directions, using digital image correlation (DIC). The fracture surfaces of these parts further suggest that 3D FDM materials behave more like laminated composite structures than isotropic cast resins and therefore design allowables should reflect actual part build configurations.

High-Speed Photography and Digital Optical Measurement Techniques for Geomaterials: Fundamentals and Applications

Abstract: Geomaterials (i.e. rock, sand, soil and concrete) are increasingly being encountered and used in extreme environments, in terms of the pressure magnitude and the loading rate. Advancing the understanding of the mechanical response of materials to impact loading relies heavily on having suitable high-speed diagnostics. One such diagnostic is high-speed photography, which combined with a variety of digital optical measurement techniques can provide detailed insights into phenomena including fracture, impact, fragmentation and penetration in geological materials. This review begins with a brief history of high-speed imaging. Section 2 discusses of the current state of the art of high-speed cameras, which includes a comparison between charge-coupled device and complementary metal-oxide semiconductor sensors. The application of high-speed photography to geomechanical experiments is summarized in Sect. 3. Section 4 is concerned with digital optical measurement techniques including photoelastic coating, Moire, caustics, holographic interferometry, particle image velocimetry, digital image correlation and infrared thermography, in combination with high-speed photography to capture transient phenomena. The last section provides a brief summary and discussion of future directions in the field.

Experimental and computational study of the damage process in CFRP composite beams under low-velocity impact

Abstract: Damage process in composites subjected to low-velocity impact is investigated both experimentally and numerically. Drop-weight impact experiments are carried out, in which a model unidirectional [ 0 5 / 90 3 ] s CFRP laminate beam is impacted by a cylindrical head creating an almost uniform two-dimensional loading condition. Initiation and progression of damage, consisting of matrix cracks and delamination, are visualized in real-time via ultra-high-speed camera at rates up to 60,000 fps and the sequence of failure events are clearly captured. Evolution of dynamic strain fields in the laminate is then quantified by a Digital Image Correlation (DIC) analysis and the resulting final failure patterns are characterized by a digital microscope. In the computational part, a three-dimensional finite element analysis is performed using ABAQUS/Explicit to simulate the experiments. In these simulations, the intraply matrix damage in the middle 90° layers is modeled using a Continuum Damage Mechanics (CDM) based composite failure theory with LaRC04 initiation criterion and implemented via a user-written VUMAT subroutine. Delamination is modeled using cohesive interface elements that are introduced between the 0°/90° interfaces. Damage initiation time, location and the interaction of failure modes are compared with the experimental data. Real-time observations of the sequentially occurring diagonal matrix cracking followed by dynamic delaminations are made. In addition to the major diagonal matrix cracks, existence of multiple diagonal micro-matrix cracks near the upper interface are shown which are also predicted by the simulations. Finally, experimentally obtained real-time strain field values, failure mechanisms and the failure sequence are shown to be in good agreement with the simulations. We believe that the elaborate experimental results presented here for an idealized composite layup can serve as a benchmark test case to validate composite and interface damage modeling methods.

Highly curved image sensors: a practical approach for improved optical performance

Abstract: The significant optical and size benefits of using a curved focal surface for imaging systems have been well studied yet never brought to market for lack of a high-quality, mass-producible, curved image sensor In this work we demonstrate that commercial silicon CMOS image sensors can be thinned and formed into accurate, highly curved optical surfaces with undiminished functionality Our key development is a pneumatic forming process that avoids rigid mechanical constraints and suppresses wrinkling instabilities A combination of forming-mold design, pressure membrane elastic properties, and controlled friction forces enables us to gradually contact the die at the corners and smoothly press the sensor into a spherical shape Allowing the die to slide into the concave target shape enables a threefold increase in the spherical curvature over prior approaches having mechanical constraints that resist deformation, and create a high-stress, stretch-dominated state Our process creates a bridge between the high precision and low-cost but planar CMOS process, and ideal non-planar component shapes such as spherical imagers for improved optical systems We demonstrate these curved sensors in prototype cameras with custom lenses, measuring exceptional resolution of 3220 line-widths per picture height at an aperture of f/12 and nearly 100% relative illumination across the field Though we use a 1/23” format image sensor in this report, we also show this process is generally compatible with many state of the art imaging sensor formats By example, we report photogrammetry test data for an APS-C sized silicon die formed to a 30° subtended spherical angle These gains in sharpness and relative illumination enable a new generation of ultra-high performance, manufacturable, digital imaging systems for scientific, industrial, and artistic use

Active Slip System Identification in Polycrystalline Metals by Digital Image Correlation (DIC)

Abstract: In this paper, a novel approach was proposed to increase the confidence of active slip system identification in polycrystalline metals. The approach takes advantage of microscale deformation tracking via Digital Image Correlation (DIC) combined with scanning electron microscopy (SEM). The experimentally-obtained high-resolution deformation fields were mapped to an undeformed configuration, which gives slip traces suitable for comparison with undeformed crystal orientation data. A metric, named herein as the ‘relative displacement ratio’ (RDR), is calculated from the displacement fields near slip traces to characterize the localized deformation due to slip. In validation cases, the experimentally-measured RDRs matched well with RDRs theoretically-calculated from active slip systems. In test cases, active slip system identification by incorporating RDR as an additional constraint was demonstrated to be preferable to using Schmid factor alone as a constraint. The proposed approach supplements existing techniques for slip system identification with increased confidence.

Characterisation of strain localisation processes during fatigue crack initiation and early crack propagation by SEM-DIC in an advanced disc alloy

Abstract: Fatigue failure processes in metallic materials are closely related to the evolution of strain localisation under cyclic loading. Characterisation of this strain localisation is important in understanding the mechanisms of fatigue crack initiation and propagation, and provides critical validation data to develop appropriate crystal plasticity models for prediction of these processes. In this study, strain localisation during fatigue crack initiation and early crack propagation in an advanced Ni-based superalloy for turbine disc application has been characterised at the grain level with a sub-micron resolution by digital image correlation on SEM images using secondary γ′ themselves as the speckle pattern. The obtained full-field strains have been analysed in global coordinates associated with the applied loading direction and in terms of the local coordinates associated with individual slip bands. Deformation arising from in-plane and out-of-plane dislocation slip can be identified by a combination of shear strain ɛxy and transverse strain ɛyy in the local slip band coordinates in combination with EBSD analysis. Cracks preferentially initiate from slip/strain bands adjacent and parallel to twin boundaries and then propagate along the slip/strain bands, leading to the onset of significant transverse strain ɛyy in the local band coordinates as a consequence of crack opening. Crack propagation is closely related to strain accumulation at the crack tip which is determined by the grain orientation and grain size. Transverse strain ɛyy in local slip band coordinates together with the inclination angle between dislocation slip direction on an activated {111} plane and the slip trace of this {111} plane at the specimen surface is proposed to be a cracking indicator/fracture criterion.