Abstract

The nonlinear spring model combined with dislocation dipole theory was applied to describe the acoustic nonlinearity during the fatigue process in metals. The spring stiffness changes with fatigue degree. For the early stage, spring stiffness approaches infinity, and the heavier nonlinearity mainly results from the increase of dislocation density. Further fatigue leads to the occurrence of micro-cracks, during which spring stiffness begins to decrease. Abundant micro-crack sprouting accelerates the crack’s expansion, and spring stiffness drops quickly, which causes the obvious decline in the transmitted harmonic amplitudes. Solutions obtained from the nonlinear wave equation with dislocation terms were added into the spring model. Varying spring stiffness was chosen for simulating the fatigue process. Then, nonlinear harmonic variation during this process was observed, which was classified into three stages: (I) the early dislocation fatigue stage; (II) the micro-crack sprouting stage; (III) the crack expansion stage. Nonlinear acoustic measurements were carried out on an aluminum alloy specimen during its fatigue process until cracks could be seen clearly. Harmonic variations in experiments can also be classified into the same three stages as the numerical results, which provides a theoretical and experimental reference for fatigue evaluation in metals using the nonlinear acoustic method.

Highlights

  • Metal materials are widely used in practical engineering applications, but long-term in-service work makes them prone to failure due to fatigue damages

  • The linear acoustic method is invalid for detecting fatigue damages of materials, while the finite amplitude sound wave can produce obvious waveform distortion when it propagates through a medium with fatigue damages, which has been proven to be an effective method for evaluating fatigue damages in metal materials [1,2,3,4,5]

  • Nonlinear acoustic theory in solids established by researchers such as Landau, Murnaghan, and Goldberg provides the basis for nonlinear ultrasonic testing [6,7]

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Summary

Introduction

Metal materials are widely used in practical engineering applications, but long-term in-service work makes them prone to failure due to fatigue damages. Theoretical models were proposed to explain the mechanism of the nonlinear acoustic effect caused by micro-damages. Some experiments [9,10,18,19] were carried out to observe the interface were established to study the contact nonlinearity of acoustics based on the contact interface nonlinear properties of material delamination and microstructure changes caused by fatigue model [16,17]. Material delamination and microstructure changes caused by fatigue damages, which established the Nonlinear Lamb waves have been commonly applied to evaluate the fatigue damages or detect the connection between micro-damages and acoustical nonlinearity. For the late stage damages, varying stiffness was chosen for modeling the different fatigue degree with the sprouting of micro-cracks. Whole fatigue process of the sample, which certified the numerical models

Nonlinearity
Nonlinearity Due to Micro-Cracks at Late Stage Fatigue
Computation Results
I: Early dislocation fatigue stage
Experiment Results and Discussions
Conclusions
Full Text
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