Roberto A. Osegueda

Nondestructive construction error detection in large space structures

Continuum modeling of large space structures is extended to the problem of detecting construction errors in large space structures such as the proposed space station. First-order dynamic sensitivity equations for structures involving eigenfrequencies, modal masses, modal stiffnesses, and modal damping are presented. Matrix equations relating changes in element parameters to dynamic sensitivities are summarized. The sensitivity equations for the entire dynamical system are rearranged as a system of algebraic equations with unknowns of stiffness losses at selected locations. The feasibility of the formulation is numerically demonstrated on a simply-supported Euler-Bernouilli beam with simulated construction defects. The method is next extended to large space structures modelled as equivalent continua with simulated construction defects.

Non-destructive testing of aerospace structures: granularity and data mining approach

For large aerospace structures, it is extremely important to detect faults, and nondestructive testing is the only practical way to do it. Based on measurements of ultrasonic waves, Eddy currents, magnetic resonance, etc., we reconstruct the locations of the faults. The best (most efficient) known statistical methods for fault reconstruction are not perfect. We show that the use of expert knowledge-based granulation improves the quality of fault reconstruction.

A constitutive-microdamage model to simulate hypervelocity projectile-target impact, material damage and fracture ☆

Abstract A set of constitutive-microdamage equations are presented that can model shock compression and the microdamage and fracture that can evolve following hypervelocity impact. The equations are appropriate for polycrystalline metals. For impact at a projectile velocity of 6.0 km/s, numerical simulations are preformed that describe the impact of spherical soda-lime glass projectiles with aluminum 1100 rectangular target plates. Three ratios of the projectile diameter to the target thickness are chosen for the simulations, providing a wide range of damage features. The simulated impact damage is compared with experimental damage of corresponding test specimens, illustrating the capability of the model.

Localization and quantification of damage in a space truss model using modal strain energy

This paper addresses the issue of localizing and quantifying damage using changes in the vibrational characteristics of structures. The method considers the mode shapes of the structure pre- and post-damage measured via modal analysis. Values of the modal shapes are used to compute the strain energy distribution in the structural elements. Using the assumption that the element modal strain energy is the same pre- and post-damage, and characterizing the damage as a scalar quantity of the undamaged stiffness matrix, an expression is obtained for element damage factors that quantify the magnitude of the damage for each mode shape. Due to numerical instabilities in the computation of this expression, filters are applied that overcome some of the instabilities but reduce the true amplitude of damage. The modified-filtered expression was very effective in localizing the actual damage. After localization, the magnitudes of damage are computed using the original unfiltered expression. The method is tested using experimental data from a 3D scale model of a space structure, subjected to 18 different damage scenarios. The damage forms consist of a 180 degree cut (Type I), a 50 percent reduction of the area over one-third the element length (Type II), and a complete cut through the element section (Type III). These types of damages correspond to magnitudes of (alpha) equal to -0.17, -0.5, and -1.0 respectively. The method is ale to detect Type I damage for only one of four cases, Type II for all the three cases and Type III damage for all single and double-location cases, excluding the cases that involves a damage insensitive element.

Artificial neural networks for structural damage detection and classification

An analysis of artificial neural networks on damage assessment of an aluminum cantilever beam was conducted. The neural networks were trained and tested with deterministic data of resonant frequency information to test their ability in determining the magnitude, location and type of damage on the beam. Being a preliminary study, no experimental data has been included, since no information was found in the literature where neural networks were used in determining the type of damage on a structure. This paper includes a discussion on the theory of neural network and the process involved in developing the architecture for three layer backpropagation neural networks for damage assessment. The neural networks were tested for three types of damage using four damage magnitudes.

Detection of cracks at rivet holes in thin plates using Lamb-wave scanning

This paper describes a Lamb-wave scanning method for the detection of notches simulating cracks at rivet holes in thin plates. The approach requires the generation of an ultrasonic So-Mode Lamb wave using an incident transmitter excited with a tone burst centered at a near non-dispersive frequency. Area scans are performed on a plate with a hole with a notch to generate times series information which is used to create animations illustrating the wave propagation characteristics. The time series are subject to a sifting process to obtain intrinsic mode functions which contain narrow frequency banded information of the signals. The Hilbert-Huang transform is applied to the intrinsic mode functions which permit the computation of the signal energy as a function of time, proportional to the square of the amplitude of the analytical signal. Animations of the propagation of the Lamb-wave energy illustrate that a potential scanning approach is to acquire time series along a line between the transmitter and the hole, capturing wave scattering from the hole and reflections from the notches. The times of flight and amplitudes of the notch-reflected energy are used to calculate coordinates of the source of the reflections by a geometric approach. The identified coordinates of the reflections outline the extent of the notch at the rivet hole. Results of experiments conducted on thin square plates with a single hole with notches of various sizes compare favorably with the actual notches.

Simulations of hypervelocity impact damage and fracture of aluminum targets using a constitutive-microdamage material model

The material damage and fracture of Aluminum 1100 target plates that experience hypervelocity impact by glass projectiles traveling at 6 km/s are simulated using a proposed constitutive-microdamage material model. The model is best suited for polycrystalline metals that are subject to hypervelocity impact at the lower range of velocities. Simulations are performed for three projectile diameter-target thickness ratios that produce a wide range of damage features. The predicted damage is compared with that of the corresponding test laboratory specimens, illustrating the capability of the constitutive-microdamage model.

Fuzzy logic in nondestructive testing of aerospace structures

In nondestructive testing, to locate the faults, we send an ultrasonic signal and measure the resulting vibration at different points. To describe and combine the uncertainty corresponding to different measurements and fuzzy estimates, we used fuzzy logic. As a result, we get reasonably simple computational models which lead to as good fault detection as the known more complicated models.

Fusion of Modal Strain Energy Differences for Localization of Damage

The fusion of multiple modal strain energy differences is proposed for the detection of damage at single and multiple locations. The approaches assume the existence of several modal shapes of the structure in the undamaged and damaged states. Modal curvatures obtained through iterative high- order spline fits of the shapes permit the determination of the modal strain energy content of the structure in both states. Locations with increases in the modal strain energy between the undamaged and damaged structure are indicative of possible damage. The damage indications resulting from multiple modes are fused according to three rules: (1) an Average Standard Norm, (2) a union of probability mass functions and (3) a weighted intersection of the probabilities that the strain energy are greater than zero. the third rule requires knowledge of the statistics of the modal shape measurements. While not strictly adhering to the classical data fusion methodologies such as, Dempster-Shafer or Bayesian, the combination of information at the probabilistic level yields result that are consistent with those expected in a formal data fusion formulation. The fusion algorithms for the location of damage are tested using five modal shapes obtained from aluminum beams in undamaged and damaged states. One and two locations at different damage magnitudes are considered. The results indicate that the last two approaches are superior to the standard norm method.

Statistical and dempster-shafer techniques in testing structural integrity of aerospace structures

We describe the existing statistics-related methods of testing structural integrity for aerospace structures, describe their drawbacks, how they can be overcome, and compare the resulting techniques.