Energy dissipation via acoustic emission in ductile crack initiation

The final publication is available at Springer via http://dx.doi.org/10.1007/s10704-016-0096-8.This article presents a modeling approach to estimate the energy release due to ductile crack initiation in conjunction to the energy dissipation associated with the formation and propagation of transient stress waves typically referred to as acoustic emission. To achieve this goal, a ductile fracture problem is investigated computationally using the finite element method based on a compact tension geometry under Mode I loading conditions. To quantify the energy dissipation associated with acoustic emission, a crack increment is produced given a pre-determined notch size in a 3D cohesive-based extended finite element model. The computational modeling methodology consists of defining a damage initiation state from static simulations and linking such state to a dynamic formulation used to evaluate wave propagation and related energy redistribution effects. The model relies on a custom traction separation law constructed using full field deformation measurements obtained experimentally using the digital image correlation method. The amount of energy release due to the investigated first crack increment is evaluated through three different approaches both for verification purposes and to produce an estimate of the portion of the energy that radiates away from the crack source in the form of transient waves. The results presented herein propose an upper bound for the energy dissipation associated to acoustic emission, which could assist the interpretation and implementation of relevant nondestructive evaluation methods and the further enrichment of the understanding of effects associated with fracture

Digital Image Correlation Techniques for NDE and SHM

Monitoring and analyzing the integrity of structures, infrastructure, and machines is essential for economic, operational, and safety reasons. The assessment of structural integrity and dynamic conditions of those systems is important to ensure safe operation and achieve or even extend the design service life. Recent advancements in camera technology, optical sensors, and image processing algorithms have made optically based and noncontact measurement techniques such as photogrammetry and digital image correlation (DIC) appealing methods for nondestructive evaluation (NDE) and structural health monitoring (SHM). Conventional sensors (e.g., accelerometers, strain gages, string potentiometers, LVDTs) provide results only at a discrete number of points. Moreover, these sensors need wiring, can be time-consuming to install, may require additional instrumentations (e.g., power amplifiers, data acquisition), and are difficult to implement on large-sized structures without interfering with their functionality or may require instrumentation having a large number of data channels. On the contrary, optical techniques can provide accurate quantitative information about full-field displacement, strain, geometry, and the dynamics of a structure without contact or interfering with the structure’s functionality. This chapter presents a summary review of the efforts made in both academia and industry to leverage the use of DIC systems for NDE and SHM applications in the fields of civil, aerospace, and energy engineering systems. The chapter also summarizes the feasibility of the approaches and presents possible future directions of the measurement approach