Full-field monitoring methods for damage analysis on aeronautical CFRP specimens under fatigue loads

Abstract

The present paper is focused on full-field experimental monitoring procedures to be employed during HCF fatigue testing on two series of CFRP open hole samples. Two different experimental methodologies based on thermographic techniques and displacement measurements with Digital Image Correlation (DIC) analysis are employed for damage settlement and evolution to be detected up to failure, together with correspondent compliance analysis. Combined monitoring approaches, based on thermo-elastic and dissipative phenomena, together with stiffness properties variations, are claimed to offer precise damage state localization during tests in real time; in addition, DIC analysis is performed during low-frequency fatigue cycle is studied for better failure prediction and damage location. The thermal parameters and experimental compliance correlation seem to indicate similar signal variation during damage progress and after proper data elaboration; contemporaneous raw thermal measurements in critical zones of specimens under fatigue life offer a kind of delamination recognition at specific layer interface and location, as well as propagation before final failure. Sample under tensile load on the other hand reveal delaminations indirectly on the surface. In addition, non-destructive thermographic and ultrasound tests are performed at regular intervals during fatigue life.

Fatigue health monitoring of AISI 304 notched specimens by means of thermographic analysis and UT-based measurements

“…As indicated in Table 3, the AISI specimens were subjected to the fatigue tests under the load-control mode with sine wave shape employing a constant frequency of 9 Hz and a stress ratio of 0.1. As proposed in a previous work [12], a cumulative damage DK(N) parameter related to stiffness variation of specific specimen is evaluated through experimental data recorded during fatigue test as defined by the following equation:…”

“…Additionally, the experimental results include the compliance analysis where the stiffness behaviour is evaluated during all fatigue life until final failure [12]. In the next paragraph, the monitoring strategy consists of PZT sensors and thermographic setup and relative processing approaches are presented.…”

“…During fatigue tests, thermal acquisitions are recorded to evaluate crack initiation and propagation. Metallic samples are monitored at regular intervals with a micro-bolometric infrared thermal camera, as shown in the thermographic setup of Figure 4 (a) [12,18].…”

“…In previous work [12], the authors proposed two thermal contrast parameters based on reversible and irreversible thermoelastic sources for the cumulative fatigue damage identification and quantification in selected zones. Assuming initially undamaged conditions, the first damage index is denoted as Thermoelastic Contrast parameter Drev related to reversible heating sources that occur during the fatigue life (Ntot) and calculated as:…”

“…Therefore, a proper strategy through non-destructive and low expensive monitoring techniques represents a continuous challenge of Structural control and Health monitoring for the prevention and detection of crack initiation on industrial metallic components under fatigue loading to ensure stable and reliable operating condition during service life [9 -11]. In literature, different and alternative methods are proposed as advanced possibilities of several SHM indicators, such as smart materials [15], wireless sensors [14], infrared thermography [12,20], piezo-electric (PZT) sensors [13], radiographic techniques [16], 3D digital image correlation [17], and so on. These solutions and clearly proper combination of different approaches should enhance the intervention time or maintenance cost, which would prevent major collapses of monitored structure under fatigue loads [6].…”