Influencing Factors in Acoustic Emission Detection: A Literature Review Focusing on Grain Angle and High/Low Tree Ring Density of Scots Pine

Abstract

Among non-destructive testing (NDT) techniques applied to structural health monitoring in existing timber structures, ranging from visual inspection to more sophisticated analysis, acoustic emission (AE) is currently seldomly used to detect mechanical stresses in wooden building assets. This paper presents the results from a systematic literature review on AE NDT applied to monitor micro and macro fracture events in softwood, specifically Scots pine. This survey particularly investigates its application with respect to the tree rings density and grain angle inspection, as influencing factors well correlated with physical and mechanical characteristics of wood. The literature review was performed in a three-step process defined by the PRISMA (Preferred Reporting Items for Systematic reviews and Meta-Analyses) flow diagram, leading to the selection of 31 documents from different abstract and citation databases (Scopus, Web of Science and Google Scholar). The outcomes have highlighted how laboratory experiments, including several types of tests (tensile, cutting, compressive, etc.), were conducted in most cases, while a very limited number of studies investigated on in situ monitoring. In addition, theoretical approaches were often explored in parallel with the experimental one. It emerges that—for tree ring density studies—a multi-technique approach, which may include microscopic observations, could be more informative. Indeed, although not widely investigated, high/low tree ring density and grain angle were found as influencing factors on the AE parameters detected by the sensors, during condition and structural health monitoring experiments.

Differential Energy Criterion of Brittle Fracture as a Criterion for Wood’s Transition to the Plastic Deformation Stage

“…We consider the loading process as a step (discrete) process, in which each step represents the destruction of the next meso-element or connection of elements in the structure [32] and is accompanied by the pulse of the aforementioned acoustic emission. The adequacy of this assumption is confirmed by the experiments known in the literature [10,14,19,30,33]. The load and displacement variations at each stage of such a process are assumed to be sufficiently small and are therefore bound by Hooke's law.…”

“…A number of other destruction criteria are known, which are based on the analysis of the conversion of input energy into the potential energy of elastic deformation, as well as into the energy of micro-, meso-and macro-destruction, acoustic emission [12][13][14], heat transfer [15] and other physical effects [16][17][18][19][20][21][22][23]. In any case, it is important to know the load-displacement (stress-strain) relationship since energy is determined by integrating the above dependence [24], and the graphical representation of these dependencies in the form of load-displacement (stress-strain) curves improves the understanding of the behavior of materials under load [4,25].…”

Exploring the Capability of Acoustic Emission Technique in Early Detecting Mode I and Mixed Mode Fractures

“…These techniques are fundamental to support conservation activities but also for understanding how the degradation processes are triggered by surrounding environmental conditions. In the case of structural health monitoring of objects made of wood, the most widely used nondestructive testing (NDT) techniques for the assessment of timber structures are visual, acoustic, vibration, strength or moisture grading probing techniques [1].…”

“…It is currently used to study the mechanism of damage occurring at certain stress levels in wood, as extensively reported by Baensch [13]. The AE applied to monitor wooden samples, is mainly used in laboratory experiments, as highlighted in Boccacci et al [1], while a very limited number of studies are dealing with the monitor of movable wooden objects in situ, specifically wooden furniture [14]. During tests, the AE instrumentation is often found coupled with the Universal Testing Machine, mainly to perform fracture tests, but also tensile, cutting, compressive and torsional tests on the samples, and with the Digital Image Correlation often used to record images during the experiments.…”

“…Looking in detail at the AE activity recorded in softwood samples as one of the widely used building construction materials in Scandinavian Countries [1], it emerged that although not thoroughly investigated, high/low tree ring density and grain angle can be influencing factors to be analysed in depth [20,23,6,10].…”