Pawan Pingle

Prediction of full field dynamic strain from limited sets of measured data

Dynamic response is an important consideration for design of structures due to operating or occasional loadings. The resulting dynamic stress strain is also of concern for fatigue and structural health monitoring. Typically, the actual loading and structural condition (boundary conditions, environmental condition, geometry, mechanical properties, etc.) are not necessarily known. Much effort is expended in attempting to identify the loads and appropriate model for prediction of these types of events. At best, the forces and actual boundary conditions are approximate and have an effect on the overall predicted response and resulting stress-strain that is identified for subsequent evaluation. Experimental data can only be obtained from limited sets of points, such as those typically collected with accelerometers. These are normally used in the evaluation the state of a structure in service condition. More recently, Digital Image Correlation (DIC) and Dynamic Photogrammetry (DP) have become very important techniques to measure the surface response. These are non-contact and full-field techniques, which allow that much more simultaneous data to be measure. The sets of limited surface data that are collected can be used in conjunction with an expansion algorithm to obtain full field information. The finite element model mass and stiffness matrices are used to obtain the normal constitutive relations as well as the modal characteristics. This information is used to develop the expansion algorithm and for the stress recovery during the back substitution process typically employed.

Comparison of Image Based, Laser, and Accelerometer Measurements

Non-contacting measurements are extremely useful for many applications in which contacting measurements are not appropriate or impossible. Within this work, digital image correlation, dynamic photogrammetry, three dimensional (3D) laser vibrometry and accelerometer measurements are all used to measure the dynamics of a structure to compare each of the techniques. Each approach has its benefits and drawbacks, so comparative measurements are made using these devices to show some of the strengths and weaknesses of each technique, especially when measuring in a 3D environment. Comparisons are made in all cases to a well-studied finite element model as well as to each other.

Full-Field Dynamic Stress/Strain from Limited Sets of Measured Data

Often times occasional events may cause significant displacement and corresponding stress strain damage to a structure. Using limited sets of measured data, expansion of real time data has been shown to provide accurate full field displacement results. This displacement data can be used in conjunction with the finite element model to identify full field dynamic stress-strain results. This approach is demonstrated for an analytical model to show the methodology proposed. Examples illustrating different configurations of measured data sets along with simulated noise are presented to illustrate the technique.

Limited Experimental Displacement Data used for obtaining Full-Field Dynamic Stress Strain Information

Full field displacement and full-field dynamic stress-strain information has been shown to be estimated accurately from limited sets of data using an expansion algorithm through analytical simulations. Experimental data is collected and used for the dynamic expansion process to predict dynamic stress-strain for a candidate structure. Experimental measurements are obtained in a controlled study using an impact excitation technique. Displacement data at limited locations is collected using dynamic photogrammetry and digital image correlation techniques. The full-field expanded displacement data is compared and validated with a reference solution. This full-field displacement data is then used to identify the dynamic stress-strain to show the usefulness of the technique.

A Review of Digital Image Correlation Applied to Structura Dynamics

A significant amount of interest exists in performing non‐contacting, full‐field surface velocity measurement. For many years traditional non‐contacting surface velocity measurements have been made by using scanning Doppler laser vibrometry, shearography, pulsed laser interferometry, pulsed holography, or an electronic speckle pattern interferometer (ESPI). Three dimensional (3D) digital image correlation (DIC) methods utilize the alignment of a stereo pair of images to obtain full‐field geometry data, in three dimensions. Information about the change in geometry of an object over time can be found by comparing a sequence of images and virtual strain gages (or position sensors) can be created over the entire visible surface of the object of interest. Digital imaging techniques were first developed in the 1980s but the technology has only recently been exploited in industry and research due to the advances of digital cameras and personal computers. The use of DIC for structural dynamic measurement has only...

On incompressibility of a matrix in naturally occurring composites

The work illustrates that a soft matrix, which has the Poisson ratio close to 0.5 and is reinforced with a rigid-line inclusion, possesses an interesting behavior at the inclusion/matrix interface. It experiences a hydrostatic stress state and behaves as an incompressible fluid under longitudinal and transverse loads. The stress singularities are eliminated ahead of the inclusion tips, and when interface defects are formed, their effect on the composite compliance is minimal. These observations have far reaching applications when one is interested in mechanisms of multifunctional property improvement of composites (such as toughness and stiffness) learned from naturally occurring composites.

Analysis of Multiple Rigid-Line Inclusions for Application to Bio-Materials

Hard biological materials such as nacre and enamel employ strong interactions between building blocks (mineral crystals) to achieve superior mechanical properties. The interactions are especially profound if building blocks have high aspect ratios and their bulk properties differ from properties of the matrix by several orders of magnitude. In the present work, a method is proposed to study interactions between multiple rigid-line inclusions with the goal to predict stress intensity factors. Rigid-line inclusions provide a good approximation of building blocks in hard biomaterials as they possess the above properties. The approach is based on the analytical method of analysis of multiple interacting cracks (Kachanov, 1987) and the duality existing between solutions for cracks and rigid-line inclusions (Ni and Nasser, 1996). Kachanov’s method is an approximate method that focuses on physical effects produced by crack interactions on stress intensity factors and material effective elastic properties. It is based on the superposition technique and the assumption that only average tractions on individual cracks contribute to the interaction effect. The duality principle states that displacement vector field for cracks and stress vector-potential field for anticracks are each other’s dual, in the sense that solution to the crack problem with prescribed tractions provides solution to the corresponding dual inclusion problem with prescribed displacement gradients. The latter allows us to modify the method for multiple cracks (that is based on approximation of tractions) into the method for multiple rigid-line inclusions (that is based on approximation of displacement gradients). This paper presents an analytical derivation of the proposed method and is applied to the special case of two collinear inclusions.Copyright

Analytical and Experimental Learning in a Vibrations Course at the University of Massachusetts Lowell

At the University of Massachusetts Lowell, Vibrations 22.550 is taught in a slightly different manner than traditional courses in other universities. The traditional single and multiple Degree of Freedom (DOF) systems are addressed along with transmissibility, isolation and related topics in the first half of the course. However, in the second half of the course, rather than explore continuous solutions, the course is focused on development of simplistic beam finite element discrete models along with experimental modal testing coupled with tuned absorber applications to detune critical modes of the structure. The structures studied have ranged from simplistic beams, tennis rackets, snowboards and similar equipment to wind turbine blades.

Alternate Techniques/New Approaches for Identification of Full Field Dynamic Stress Strain from Limited Sets of Measured Data for Wind Turbine Applications

Current design analysis for wind turbine blade applications is based on an estimation of the forces that drive the system during operation. Much effort has been expended in the estimation of these forces from fluid structure interaction models as well as from measured operating responses. However, the existing approaches can only provide an approximate estimate of the forces for the structural modeling of the blades, primarily because of a lack of distributed sensing capability and because the assumed boundary conditions are not always accurate. Two relatively new techniques are currently under investigation that enable a much better overall prediction of the response of the system. Full-field optical measuring techniques and real time data expansion approaches have the potential to greatly improve these required predictions. Three-dimensional digital image correlation (DIC) and dynamic point tracking (3DPT) have recently been explored as possible approaches to collect data on various types of structures using noncontacting measurement while the system is operating (rotating). A basic discussion on these imaging techniques is provided to show their usefulness in making structural dynamic measurements. The sets of limited surface data that are collected can be used in conjunction with an expansion algorithm to obtain fullfield information. The finite element model mass and stiffness matrices are used to obtain the normal constitutive relations as well as the modal characteristics. This information is used to develop the expansion algorithm and for the stress recovery during the back substitution process typically employed in the finite element approach. A basic discussion of the process used is presented along with several different test cases to demonstrate the optical measurement process and the ability to achieve full-field results. Some thoughts on the exploitation of these new tools and the vision forward are also presented.

Comparison of surface velocity measurement using a scanning laser vibrometer and acoustic holography.

A significant amount of interest exists in performing noncontacting full‐field surface velocity measurement. For many years traditional surface velocity measurements have been made by using a scanning Doppler laser vibrometer (SDLV). Nearfield acoustical holography is another approach that enables reconstruction of quantities such as the acoustic pressure, surface velocity, intensity, and power radiated from a structure into three‐dimensional space, based on the sound pressure measured at a two‐dimensional surface. Within this work the surface velocity of a clothes dryer panel is computed based on the acoustic field pressure measurements by using the Helmholtz equation least square (HELS) method, in which the reconstructed sound field is optimized by using spherical wave functions. The dryer panel is measured using an SDLV as well as a 64‐channel microphone array. The reconstructed full‐field surface velocity using the HELS method is compared to the measurement from the laser vibrometer during operation a...