Digital cameras are rapidly supplanting film, even for very high speed and ultra high-speed applications. The benefits of these cameras, particularly CMOS versions, are well appreciated. This paper describes how a pair of synchronized digital high-speed cameras can provide full-field dynamic deformation, shape and strain information, through a process known as 3D image correlation photogrammetry. The data is equivalent to thousands of non-contact x-y-z extensometers and strain rosettes, as well as instant non-contact CMM shape measurement. A typical data acquisition rate is 27,000 frames per second, with displacement accuracy on the order of 25-50 microns, and strain accuracy of 250-500 microstrain.
High-speed 3D image correlation is being used extensively at the NASA Glenn Ballistic Impact Research Lab, in support of Return to Flight activities. This leading edge work is playing an important role in validating and iterating LS-DYNA models of foam impact on reinforced carbon-carbon, including orbiter wing panel tests. The technique has also been applied to air blast effect studies and Kevlar ballistic impact testing. In these cases, full-field and time history analysis revealed the complexity of the dynamic buckling, including multiple lobes of out-of-plane and in-plane displacements, strain maxima shifts, and damping over time.
Biomechanics place huge challenges on existing measurement technologies for determining the mechanical properties of these materials, as well as just measuring the full-field displacement and strain of these materials. 3D Image Correlation Photogrammetry is proving to be a powerful tool for these measurements, providing full-field 3D measurement of the specimens under normal loadings, even at high-speed. This optical technique is independent of the material that it is measuring, providing a non-contact measurement of any material or geometry type. The results are then directly comparable to finite element models for model verification, iteration and boundary condition determination. This paper discusses the theory of the technology, and its application in deformation and strain measurement of real biomechanic applications, from tissues and organs to ligaments and bones.
3D image correlation is a robust method for measuring full-field displacements and strains using a calibrated pair of video cameras. Underlying principles and benefits are reviewed, and the method is compared to both 3D ESPI and 2D image correlation. Several applications combining image correlation photogrammetry with stroboscopic illumination and/or high-speed video cameras are presented. Operational strains in ionic polymeric muscle samples and electro-restrictive actuators are determined. The use of short-duration white light pulses to study automobile tires on road wheels at speeds up to 150 miles per hour is demonstrated. Initial work measuring strains on an 18" flywheel in a spin pit at up to 35,000 rpm is described. A notched rubber dogbone sample is pulled to failure at 125% strain in 38 milliseconds, and hundreds of full-field strain maps are captured. This paper includes discussion of sample preparation methods and special lighting systems, including pulsed arc lamps and pulsed lasers. A matrix of capability using available high speed cameras is included.
Sheet metal manufacturers are under constant and increasing pressure to improve and document quality, while reducing cost. Furthermore, OEMs are shifting responsibility for quality inspections to suppliers, adding extra burdens. Exciting, shapely product designs are placing greater demands on both quality assurance and development departments. Deep drawing and other new advanced forming methods push materials to their limits. There is a new stamping quality control tool available for easy, effective and reliable determination of shape, strains and thinning. Full-field optical vision systems, based on the well-known principles of circle grid analysis and photogrammetry, provide automated analysis and quantitative color maps for every square inch of complex parts. Quality results are displayed on a 3D computer model, using the actual measured dimensions of the real part, allowing it to be viewed from any angle. One of the key features of this system is a dynamic link between the forming limit diagram and the strain/thinning color map. When a point is clicked on either display, a second crosshair automatically highlights that same point on the other display, and a detail box presents all measured and calculated quantities. Critical points can be identified at a glance so that corrective action can be taken. Examples shown include before and after die optimization, and analysis of a 1.2 meter long B-pillar stamping.