Nondestructive Testing (NDT), Non Linearity, Leakage

Chapter 7

Abstract

responses.

1. Introduction

boundaries, dislocations, etc.

139

Time Domain Analysis of Elastic

Nonlinearity in Concrete Using

Mourad Bentahar, Charfeddine Mechri, Paola Antonaci,

Concrete and consolidated granular media in general exhibit a strong nonlinear hysteretic elastic behavior when excited by ultrasonic wave perturbations. Due to the sensitivity of their elastic properties to the small changes that can appear in their microstructure, the dynamic stress-strain relationship considered at low strains is affected by the presence of microcracks and hence the progression of damage. Tracking the nonlinear behavior can be made through the dependence on the excitation amplitude of the amplitude of higher order harmonics or of the resonance frequency of the sample. The present chapter shows a time domain analysis of elastic nonlinearity based on the break of the superposition principle when

ultrasonic continuous waves are propagating in concrete samples. The latter, which can be of different microstructures (grain sizes, mortar, or polymer matrix), helps to understand the physical mechanisms involved in the different nonlinear elastic

Numerous experimental observations have shown that microinhomogeneous media manifest a "new" nonlinear elastic behavior different from the one explained by the classical theory of nonlinear elasticity proposed by Landau and applicable to homogeneous and crystalline media. The "new" properties, described in the framework of the nonlinear mesoscopic elasticity (NME) formalism, include most types of rocks, concrete, bones, damaged metals, and composites, which manifest the same macroscopic observations despite the existing differences in their microstructures and chemical constituents [1–8]. Indeed, all these materials share the characteristic of being complex with contacts, or microdefects (i.e., cracks), grain

Observations on these materials concern quasi-static and dynamic acoustic experiments. In quasi-static tests, the stress-strain relationship is governed by an unusual multivalued function where the observed hysteretic loop contains small inner loops showing the presence of a memory effect [9]. In dynamic experiments with a propagating acoustic wave, the proportionality between input and output

Keywords: concrete, damage progression, nonlinear ultrasound, nonlinear elasticity, microcracking, nondestructive testing

Continuous Waves

Antonio Gliozzi and Marco Scalerandi
