Ala Hijazi

A novel ultra-high speed camera for digital image processing applications

Multi-channel gated-intensified cameras are commonly used for capturing images at ultra-high frame rates. The use of image intensifiers reduces the image resolution and increases the error in applications requiring high-quality images, such as digital image correlation. We report the development of a new type of non-intensified multi-channel camera system that permits recording of image sequences at ultra-high frame rates at the native resolution afforded by the imaging optics and the cameras used. This camera system is based upon the concept of using a sequence of short-duration light pulses of different wavelengths for illumination and using wavelength selective elements in the imaging system to route each particular wavelength of light to a particular camera. As such, the duration of the light pulses controls the exposure time and the timing of the light pulses controls the interframe time. A prototype camera system built according to this concept comprises four dual-frame cameras synchronized with four dual-cavity pulsed lasers producing 5 ns pulses in four different wavelengths. The prototype is capable of recording four-frame full-resolution image sequences at frame rates up to 200 MHz and eight-frame image sequences at frame rates up to 8 MHz. This system is built around a stereo microscope to capture stereoscopic image sequences usable for 3D digital image correlation. The camera system is used for imaging the chip‐workpiece interface area during high speed machining, and the images are used to map the strain rate in the primary shear zone.

A calibrated dual-wavelength infrared thermometry approach with non-greybody compensation for machining temperature measurements

We report the development of a new approach for determining temperatures using the dual-wavelength infrared thermometry technique, which does not presume greybody behaviour and compensates for the spectral dependence of emissivity. This approach is based on Plancks radiation equation and explicitly accounts for the wavelength-dependent response of the IR detector and the losses occurring due to each of the elements of the IR imaging system that affect the total radiant energy sensed in different spectral bands. A thorough calibration procedure is utilized to determine a compensation factor for the spectral dependence of emissivity, which is referred to as the non-greybody compensation factor (NGCF). Calibration and validation experiments are carried out on Aluminum 6061-T6 targets with two different surface roughnesses. Results show that this alloy does not exhibit greybody behaviour, even though the two spectral bands used were relatively close to each other, and that the spectral dependence of emissivity is influenced by the surface finish. It is found that non-greybody behaviour of low emissivity surfaces can lead to significant systematic error in dual-wavelength IR thermometry. The inclusion of the NGCF eliminates the systematic error caused by the invalidity of greybody assumption and thus improves the accuracy of the measurements. Non-greybody-compensated dual-wavelength thermography is used to measure the chip temperature along the tool–chip interface during orthogonal cutting of Al 6061-T6 and sample results at three different cutting speeds are presented.

Strength of Stiffened 2024-T3 Aluminum Panels with Multiple Site Damage

Two modie edlinkup modelsweredevelopedfordetermining thecritical stressbased on linkup (ligamentfailure ) of2024-T3aluminumpanelswithmultiplesitedamage.ThesemodelsweredevelopedforusewithstandardMilitary Handbook for Metallic Materials and Elements for Aerospace Vehicle Structures (MIL-HDBK-5G )yield strength values. For this investigation, ligament failure stresses predicted by these models are compared with test stresses determined from a variety of stiffened panels including single-bay panels with the lead crack centered between stiffeners and two-bay panels with the lead crack centered beneath a severed stiffener. The stresses predicted by the modie ed linkup models correlate well with the test data. The results of this investigation should add to the understanding of the extent to which nonlinear behavior can be modeled with simplie ed engineering models. Nomenclature a = lead crack half-length an = nominal lead crack half-length c = multiple site damage (MSD) crack length D = hole diameter Fcol = collapse stress ` = half-length for MSD crack and hole, cC D/2 L = ligament length t = panel thickness tS = stiffener thickness W = panel width WS = stiffener width

Linkup Strength of 2024-T3 Bolted Lap Joint Panels with Multiple Site Damage

A modified linkup model has been developed for determining the residual strength of 2024-T3 aluminum panels with multiple-site damage. This model was developed by semi-empirical analysis of test data from flat unstiffened open-hole panels. The model was later validated with test data from 36 open-hole stiffened panels. During the investigation, 36 bolted lap joint panels with different crack configurations were tested to further validate the previously developed modified linkup model. Stress intensity factors for the modified linkup model for the different crack configurations were determined from finite element analysis with the FRANC2D/L code. The residual strengths predicted by the modified linkup model correlate well with the test data, as was the case with the earlier studies of the stiffened panels. The results for the bolted lap joint panels show a sudden reduction in the residual strength at the early stages of multiple-site damage followed by a gradual linear decrease with increasing multiple-site damage crack size. The results also demonstrate that a stress-intensity-factor-based solution can be formulated with empirical analysis of test data from a simple configuration and then used to analyze more complex configurations.

Contribution of the Imaging System Components in the Overall Error of the Two-Dimensional Digital Image Correlation Technique

Digital image correlation (DIC) is one of the most widely used non-invasive methods for measuring full-field surface strains in a wide variety of applications. The DIC method has been used by numerous researchers for measuring strains during the plastic range of deformation where the strains are relatively large. The estimation of the amount of background strain error in the measurements is of prime importance for determining the applicability of this method for measuring small strains (such as the elastic strains in metals, ceramics, bone samples, etc.). In this study, the strain errors in 2D-DIC measurements associated with different types of imaging systems were investigated. In-plane rigid-body-translation, experiments were used to estimate the overall amount of error in DIC displacement and strain measurements. Different types of cameras having different types of sensors and different spatial resolutions were used in the study. Also, for the same type of camera, different types of lenses were used. Results show that the DIC measurement accuracy depends on the magnitude of image displacement and that different error estimation parameters can be used for quantifying the accuracy of the measurements. Also, the effect of the lens on measurement accuracy is more pronounced than that of the camera. Furthermore, imaging conditions such as image sharpness and camera gain also affect the accuracy. Further still, the measurement accuracy was found to be influenced by the direction of translation. The results indicate that measurement error can be reduced by orienting the camera such that the major displacement direction is parallel to the width direction of the image. The experimental approach used in this study can be used for quantitatively assessing the quality of the different types of cameras and lenses and to determine their suitability for use in experimental techniques that depend on image analysis such as DIC and particle image velocimetry (PIV).

Link-Up Strength of 2524-T3 and 2024-T3 Aluminum Panels with Multiple Site Damage

The aluminum alloy 2524-T3 is replacing 2024-T3 for many applications because 2524-T3 has a greater fracture toughness while retaining the same strength. Much attention has been given to the effect of multiple site damage on 2024-T3 aluminum; however, very little has been reported about 2524-T3. Twenty-two panels of 2524-T3, each with a different crack configuration, were tested for critical (linkup) strength, and the results were compared with an identical set of previously tested 2024-T3 panels with MSD. The panels were 24 in. wide with a midspan row of 0.25-in.-diam holes at 1-in. pitch. Each panel had a central lead crack with collinear MSD cracks emerging from both sides of the adjacent holes. A comparison of the results showed the 2524-T3 panels to average approximately 27% greater strength than the 2024-T3 panels. The linkup or plastic-zone-touch model used to predict the critical (link-up) strength of the panels was found to be highly conservative. Consequently the test data were used for a semi-empirical analysis to develop a modified link-up model for 2524-T3, similar to the one previously developed for 2024-T3. The average error between the critical strengths from testing and those predicted by the link-up model was approximately 20%, whereas that for the modified link-up model was approximately 3%.

Strength of 7075-T6 and 2024-T3 Aluminum Panels with Multiple-Site Damage

Much attention has been given to the development of technology for the purpose of determining the strength of aluminum panels that have multiple-site damage. The linkup model has been investigated because of its simplicity, and a number of modified linkup models have been presented. However, most of the attention has been given to 2024-T3 aluminum. Little attention has been given to 7075-T6 because it is a more brittle material with lower fracture toughness, making it more suitable to be analyzed by conventional linear elastic (brittle) fracture mechanics technology. The work presented here involves the study of 7075-T6 panels with multiple-site damage. Both the classical linear elastic fracture model and the linkup model are shown to be highly inaccurate. A modified linkup model and a modified brittle fracture model have both been developed by empirical analysis. Both of these models appear to have a high degree of accuracy over a wide range of crack geometry. The modified linkup model for 7075-T6 is then compared with a previously developed modified linkup model for 2024-T3. This comparison shows that 2024-T3 panels with multiple-site damage have greater strength than 7075-T6 panels, especially for panels with small ligament lengths.

An image-splitting-optic for dual-wavelength imaging

Dual-wavelength imaging is used in several scientific and practical applications. One of the most common applications is dual-wavelength thermography which has many advantages over single wavelength thermal imaging. Optical imagesplitters can be used to turn any imaging equipment into a dual-wavelength imaging system. In this paper, a new design of an image-splitting optic, for use in dual-wavelength imaging, is presented. The new design evades the limitations encountered with the basic image-splitter design where images can be captured at higher resolutions and frame rates. The new design also facilitates the adjustment of the image magnification. With very minor changes in the optical components, the image-splitter can be used in different thermal imaging techniques such as Infrared (IR) imaging and Laser Induced Fluorescence (LIF) imaging or any other technique that utilizes dual-wavelength imaging. Furthermore, with some modifications in the optical path, the image splitter can be used for imaging

A novel multichannel nonintensified ultra-high-speed camera using multiwavelength illumination

Multi-channel gated-intensified cameras are commonly used for capturing images at ultra-high frame rates. However, the image intensifier reduces the image resolution to such an extent that the images are often unsuitable for applications requiring high quality images, such as digital image correlation. We report on the development of a new type of non-intensified multi-channel camera system that permits recording of image sequences at ultra-high frame rates at the native resolution afforded by the imaging optics and the cameras used. This camera system is based upon the use of short duration light pulses of different wavelengths for illumination of the target and the use of wavelength selective elements in the imaging system to route each particular wavelength of light to a particular camera. A prototype of this camera system comprising four dual-frame cameras synchronized with four dual-cavity lasers producing laser pulses of four different wavelengths is described. The camera is built around a stereo microscope such that it can capture image sequences usable for 2D or 3D digital image correlation. The camera described herein is capable of capturing images at frame rates exceeding 100 MHz. The camera was used for capturing microscopic images of the chip-workpiece interface area during high speed machining. Digital image correlation was performed on the obtained images to map the shear strain rate in the primary-shear-zone during high speed machining.

Comparison of Residual Strength Estimates for Bolted Lap-Joint Panels

Fracture analysis of cracks along mechanically fastened joints is a central issue in the damage tolerance assessment of semimonoque aircraft structures. Nonlinear elastic-plastic finite element analysis, employing the cracktip-opening-angle criterion, was used to evaluate residual strength of bolted lap-joint 2024-T3 aluminum panels with multiple site damage subjected to Mode-I loading. A total of 36 different crack configurations were analyzed and compared to the experimental results. The effect of different aspects of the finite element modeling procedure on the accuracy of the residual strength predictions was investigated. Modeling of the geometric details of the test panels, such as fasteners and antibuckling restraints, was found to have a considerable effect on the predicted residual strengths. The nonlinear analyses provided reasonable estimates of the residual strength. The numerical results were compared with residual strength predictions obtained from a semi-empirical link-up model. Both approaches were comparable in terms of accuracy when compared to the test data.