Masoud Yekani Fard

Damage assessment of CFRP composites using a time–frequency approach

A damage assessment methodology using a time–frequency signal processing technique is presented in this article. Delaminations are detected in composite structures with multiple stiffeners. Because Lamb waves are complex in nature, due to wave dispersion and scattering, a robust signal processing technique is required to extract features from Lamb wave signals. In this article, the matching pursuit decomposition algorithm is used for extracting wavelets from the Lamb wave signals in the time–frequency domain. A small time–frequency atom dictionary is defined to avoid the exhaustive search over the time–frequency domain and to reduce the computation costs. The propagation characteristics of Lamb waves in stiffened composite panels are investigated. The delaminations are detected by identifying the converted Lamb wave modes introduced by the structural imperfection. A two-step damage detection approach, which uses both pulse-echo and pitch-catch active sensing schemes, is developed for the identification of delaminations. The delamination is quantified using a signal energy-based damage index. The matching pursuit decomposition algorithm is further used to localize the delamination position by solving a set of nonlinear equations. The results show that the matching pursuit decomposition algorithm can be used to identify and localize the seeded delaminations in composite structures with complex geometries and material properties.

Characterization of Epoxy Resin Including Strain Rate Effects Using Digital Image Correlation System

AbstractThe mechanical response of epoxy resin Epon E 863 has been studied in tension, compression, and flexure. The epoxy resins have been tested at different strain rates ranging from 5.9×10-5 to 0.03  s-1. Two types of dog-bone geometries have been used in the tension tests. Small sized cubic, prismatic, and cylindrical samples were used in compression tests. Beams with quarter deep notches or grooves were tested at their midpoints in flexural tests. Strains were measured by using a digital image correlation technique, extensometer, strain gages, and actuator. Observation of sample geometry during tension tests at constant elongation rate shows necking and crazing in Epon E 863. Cubic, prismatic, and cylindrical compression samples undergo a stress drop at yield, but only cubic samples experience strain hardening before failure. Characteristic points of tensile and compressive stress strain relation and load deflection curve in flexure, such as proportional elastic limit stress (PEL), ultimate tensile ...

Analytical Solution for Flexural Response of Epoxy Resin Materials

AbstractA piecewise linear parametric uniaxial stress-strain approach has been used to obtain the closed form nonlinear moment curvature response on the basis of strain compatibility in bending for epoxy resin materials. The stress-strain curves, consisting of a bilinear ascending curve followed by strain softening and constant plastic flow in tension and compression, are described by two main parameters, with an additional five nondimensional tensile and seven nondimensional compressive parameters. The main parameters are the modulus of elasticity and strain at the proportional elastic limit point in tension. Parametric studies show that ultimate tensile stress and compressive yield stresses and tension and compression flow stresses have the highest effects on flexural load carrying capacity. Moment curvature equations, in conjunction with softening localization and static equilibrium conditions, were used to simulate the flexural load-deflection response of a beam under three-point bending (3PB) conditi...

Nonlinear Flexural Behavior and Moment Curvature Response of Epoxy Resin Using Digital Image Correlation Technique

The effect of local strain and stress distribution in the polymeric material under out of plane loading is of interest in many applications. Flexural testing of the epoxy resins is necessary to evaluate their mechanical behavior. In this paper, the flexural response of epoxy resin Epon E 863 and hardener EPI-CURE 3290 has been investigated in three point bending (3PB) at different strain rates. Beam specimens with rectangular cross sections, different lengths and span to depth ratios were tested at their mid point. Samples had either a quarter deep notch or a groove at their mid points. Strain fields within the beam structure were determined using digital image correlation technique. Load deflection curves at different load speeds were obtained. Limit of proportionality (LOP), modulus of rupture (MOR), initial slope of the load-deflection curve, and slope of load deflection curve after the LOP point were extracted. The nonlinear moment curvature behavior has been obtained. The results show that a digital image correlation can accurately categorize the material response of an epoxy resin for flexural loading. Results clearly display the strain rate effects on the material response.

A simplified approach for flexural behavior of epoxy resin materials

A piecewise-linear parametric uniaxial tension and compression stress–strain model with a simplified post-peak response is developed to obtain the nonlinear load deflection response of epoxy resin materials which are considerably stronger in compression than tension. This model could be used to obtain flexural strength when the complete post-peak behavior of the material in tension and compression is not available. The tension and compression stress–strain curves are bilinear for pre-peak response followed by a constant flow stress in tension and a constant yield stress in compression in the post-peak response. The simulations and experiments reveal the suitability of this model for predicting the three-point bending and four-point bending response. This model gives an upper bound estimate for flexural over-strength factor.

Influence of load type and stress gradient on flexural strength of epoxy resin polymeric material

AbstractA piecewise-linear parametric uniaxial stress-strain approach has been used to obtain the nonlinear moment curvature response based on strain compatibility in bending for epoxy resin materials. It has been shown that the direct use of tension and compression stress-strain models underestimates the flexural strength of epoxy resin materials in a three-point bending (3PB) setup. An analytical and experimental investigation is conducted to better evaluate the degree of flexural overstrength for epoxy resin material. Four-point bending (4PB) and a round plate (RP) supported on three symmetrically arranged pivot points on a circle are chosen. An algorithm is developed to obtain the load-deflection response of the 4PB and RP samples from the nonlinear moment curvature curve. Small-sized 3PB tests are conducted to examine the size effects on the flexural response. The experimental nonlinear load-deflection responses obtained in the various load arrangements are satisfactorily simulated through the develo...

Prediction of scour depth around bridge piers using Gaussian process

A reliable prognostics framework is essential to prevent catastrophic failure of bridges due to scour. In the U.S., scour accounts for almost 60% of bridge failures. Currently available techniques in the literature for predicting scour are mostly based on empirical equations and deterministic regression models, like Neural Networks and Support Vector Machines, and do not predict the evolution of scour over time. In this paper, we will discuss a Gaussian process model, which includes Bayesian uncertainty for prediction of time-dependent scour evolution. We will validate the model on the experimental data conducted in four different flumes in different conditions. The robustness of the algorithm will also be demonstrated under different scenarios, like lack of training data and equilibrium scour conditions. The results indicate that the algorithm is able to predict the scour evolution with an error of less than 20% for most of the time, and 5% or less given enough training data.

Damage detection in composite structures using Lamb wave analysis and time-frequency approach

A methodology based on Lamb wave analysis and time-frequency signal processing has been developed for damage detection and structural health monitoring of composite structures. Because the Lamb wave signals are complex in nature, robust signal processing techniques are required to extract damage features. In this paper, Lamb wave mode conversion is used to detect the damage in composite structures. Matching pursuit decomposition algorithm is used to represent each Lamb wave mode in the time-frequency domain. Results from numerical Lamb wave propagation simulations and experiments using orthotropic composite plate structures are presented. The capability of the proposed algorithm is demonstrated by detecting seeded delaminations in the composite plate samples. The advantages of the methodology include accurate time-frequency resolution, robustness to noise, high computational efficiency and ease of post-processing.

Structural health monitoring and damage detection in composite panels with multiple stiffeners

The propagation characteristics of Lamb waves in stiffened carbon fiber polymer matrix composite panels are investigated. In particular, the group velocities of S0 and A0 modes are calculated, the attenuation of Lamb waves induced by stiffeners and delaminations are evaluated, and the angular dependence of group velocity is subsequently examined. A two step damage detection and characterization approach, which uses both pulse-echo and pitchcatch active sensing schemes, is presented for the identification of delaminations in the stiffened composite panels. The matching pursuit decomposition algorithm is used for the representation of the sensor signal in the time-frequency domain. The delaminations are detected by identifying the converted Lamb wave modes in the time-frequency domain. The results confirm that the proposed approach is able to effectively identify the presence of seeded delaminations in complex composite structures.

Inverse analysis of constitutive relation for epoxy resin materials

A multilinear parametric uniaxial stress strain approach has been used to obtain the closed form nonlinear load deflection response based on strain compatibility in bending for epoxy resin materials. A constitutive model for tension and compression of resin polymeric material is proposed. The model consists of a bilinear ascending curve followed by strain softening and constant plastic flow in tension and compression. The model is described by two main parameters; modulus of elasticity in tension and strain at the proportional elastic limit point in tension, in addition to twelve non-dimensional tension and compression parameters. Moment curvature equations, deformation localization, and static equilibrium conditions were used to simulate the flexural load deflection response of a beam under threepoint bending and four-point bending conditions. Results show a considerable underestimation in flexural load carrying capacity if the uniaxial tension and compression stress strain curves are directly used.