Damage assessment of NCF, 2D and 3D woven composites under compression after multiple-impact using acoustic emission

This study is devoted to the damage characterization of Non-Crimp Fabric (NCF), 2D plain-woven (2D-PW) and 3D orthogonal plain-woven (ORT-PW) carbon/epoxy laminates, subjected to compression after multiple-impact loading, using Acoustic Emission (AE). The ultrasonic C-scan images showed that the interlaminar damage area induced by the single and 3-impact in ORT-PW architecture is 3 and 2 times smaller than NCF and 2D-PW architectures respectively. The impacted specimens were then subjected to the in-plane compression load. Two indices, one based on the mechanical response and another one based on the AE behavior of the laminates, were proposed to compare the performance of different architectures. These indices showed that the ORT-PW had the best performance among all the architectures. Finally, AE was used to distinguish the different damage mechanisms including: matrix cracking, intra and inter-yarn debonding, defected-fiber breakage, intact-fiber breakage and z-binder fiber breakage in the CAI tests of the architectures

Compression after multiple low velocity impacts of NCF, 2D and 3D woven composites

This paper investigates the effect of the fabric architecture and the z-binding yarns on the compression after multiple impacts behavior of composites. Four fiber architectures are investigated: non-crimp fabric (NCF), 2D plain weave (2D-PW), 3D orthogonal plain (ORT-PW) and twill (ORT-TW) weave. The specimens were subjected to single and multiple low-velocity impacts at different locations with the same energy level (15 J). Non-destructive techniques including ultrasonic C-scanning, X-ray CT and Digital Image Correlation (DIC) are employed to quantitatively analyze and capture the Barely Visible Impact Damage (BVID) induced in the specimens. Although the absorbed energy was approximately the same, damage was the least in 3D woven architectures. In the case of compression after impact, 3D woven composites demonstrated a progressive damage behavior with the highest residual strength (~92%) while 2D plain weave and NCF specimens showed suddenly catastrophic damage and the residual strength of ~65% and ~55% respectively

A Model-Assisted Probability of Detection Framework for Optical Fiber Sensors

Optical fiber sensors (OFSs) represent an efficient sensing solution in various structural health monitoring (SHM) applications. However, a well-defined methodology is still missing to quantify their damage detection performance, preventing their certification and full deployment in SHM. In a recent study, the authors proposed an experimental methodology to qualify distributed OFSs using the concept of probability of detection (POD). Nevertheless, POD curves require considerable testing, which is often not feasible. This study takes a step forward, presenting a model-assisted POD (MAPOD) approach for the first time applied to distributed OFSs (DOFSs). The new MAPOD framework applied to DOFSs is validated through previous experimental results, considering the mode I delamination monitoring of a double-cantilever beam (DCB) specimen under quasi-static loading conditions. The results show how strain transfer, loading conditions, human factors, interrogator resolution, and noise can alter the damage detection capabilities of DOFSs. This MAPOD approach represents a tool to study the effects of varying environmental and operational conditions on SHM systems based on DOFSs and for the design optimization of the monitoring system

Adapted anisomorphic model for fatigue life prediction of CFRP laminates under constant amplitude loading

A new constant life diagram (CLD) model is proposed to predict the fatigue life of carbon fibre-reinforced epoxy laminates under constant amplitude (CA) loading. The CLD is asymmetric and non-linear, and it is built upon the anisomorphic CLD model. It consists of two sub-models; one sub-model is applicable to laminates with lay-ups characterised by a larger ultimate tensile strength (UTS) than absolute ultimate compressive strength (UCS): UTS ⩾ ∣UCS∣, while the second sub-model can be applied to those exhibiting the opposite tendency: ∣UCS∣ > UTS. Combined, the sub-models can predict the fatigue life of any carbon-epoxy laminate. The CLD can be constructed using only static strength data and fatigue life data related to one stress ratio (R), defined as either R=0.1 or R=-1.0. An experimental campaign was conducted on a carbon-epoxy laminate with a lay-up of [90/0/90] 2S to validate the first CLD sub-model. Additionally, a second case study from literature with a lay-up of [45/90/-45/0] 2S was employed for validation. The second CLD sub-model was evaluated using two coupon case studies from literature with lay-ups of [±60] 3S and [45] 16 . The predicted and experimentally obtained fatigue lives showed agreements for different R-ratios, and the observed prediction errors were in ranges similar to those of the original anisomorphic CLD model. Hence, the presented CLD model allows for fatigue life predictions in scales similar to experimental results while reducing the required experimental efforts with respect to the anisomorphic CLD model. Structural Integrity & Composite

Analysis of Stochastic Matrix Crack Evolution in CFRP Cross-Ply Laminates under Fatigue Loading

The present work aims at understanding the stochastic matrix crack evolution in CFRP cross-ply laminates under tension–tension fatigue loading. An experimental campaign was carried out on twenty-three specimens at different stress levels, while two optical techniques were used for the in-situ monitoring of the accumulation of transverse matrix cracks. The results showed a significant scatter in crack evolution among specimens. This stochastic behaviour was further investigated using image analysis and numerical modelling. It was found that transverse matrix cracks can be classified into the independent and dependent cracks based on a critical crack spacing. Furthermore, the severity of interaction among cracks was quantified by introducing a dependent crack ratio. Finally, a strength-based probabilistic model was proposed to describe the scattering regime of the crack evolution. The agreement between model and test results indicates that local strength variations of 90 plies are the dominant scattering source governing the initial fatigue resistance to cracking and determining the accumulation of transverse matrix cracks among specimens. These results may provide a new insight into the stochastic nature of matrix cracking in composite laminates and aid in the design of fatigue resistance properties.</p

Structural state awareness of composite structures by blending passive and active acoustic-based health monitoring methods

This study aims to demonstrate the effectiveness of blending passive and active acoustic-based health monitoring methods to impact damage diagnostics of composite structures. The structural state awareness is introduced as a term to characterize the health condition that a structure is and how this condition can be quantified by blending health monitoring techniques. To this aim, a Carbon Fiber Reinforces Polymer (CFRP) composite plate was fabricated and subjected to a simulated low-velocity impact by performing repeated quasi-static indentation tests where a loading-unloading-reloading test profile was adopted. Two Acoustic Emission (AE) broadband sensors and a network of eight piezoelectric (PZT) sensors were attached on the composite plate surface. AE (passive method) was employed during the loading and reloading phases of the indentation tests to in-situ monitor the damage initiation and progression, while scanning of the plate with Lamb waves (active method) was done to localize the damage when the structure was unloaded. The obtained results showed that the proposed blended passive and active acoustic-based method has the potential to provide useful information about the impact-induced damage in composite structures.Structural Integrity & Composite

Damage characterization of laminated composites using acoustic emission: A review

Damage characterization of laminated composites has been thoroughly studied the last decades where researchers developed several damage models, and in combination with experimental evidence, contributed to better understanding of the structural behavior of these structures. Experimental techniques played an essential role on this progress and among the techniques that were utilized, acoustic emission (AE) was extensively used due to its advantages for in-situ damage monitoring with high sensitivity and its capability to inspect continuously a relatively large area. This paper presents a comprehensive review on the use of AE for damage characterization in laminated composites. The review is divided into two sections; the first section discusses the literature for damage diagnostics and it is presented in three subsections: damage initiation detection, damage type identification and damage localization, while the second section is devoted to damage prognostics and it focuses on the remaining useful life (RUL) and residual strength prediction of composite structures using AE data. In every section, efforts have been made to analyze the most relevant literature, discuss in a critical manner the results and conclusions, and identify possibilities for future work.The paper has been published open accessStructural Integrity & Composite

A cycle-by-cycle fatigue damage analysis of biaxial composite structures utilizing acoustic emission

In the present work, a fatigue damage analysis of biaxial Carbon Fibre Reinforced Polymer (CFRP) specimens loaded at two different stress levels and a fatigue ratio, R=0.1, was performed. A cycle-by-cycle approach was used, utilizing results from Acoustic Emission (AE) measurements. The aim was to investigate the influence of the applied maximum stress level on the fatigue damage accumulation process and to examine the hypothesis that damage growth occurs in a portion of a load cycle. It was found that the damage process, for the specimens loaded in lower applied stress, was gradually increased where different damage mechanisms evolved slowly towards the end of life. On the other hand, for the specimens loaded in higher applied stress, the damage process accumulated evenly and several failure mechanisms occurred in parallel at a very early stage of the fatigue life. Finally, for both loading cases, a load threshold was found, below which, there was no damage growth, supporting the hypothesis of this work.</p

Experimental investigation on the effect of creep on the damage evolution of CFRP structures during fatigue loading

This paper presents an experimental investigation on the effect of creep on the damage evolution of Carbon Fiber Reinforced Polymer structures during fatigue loading. A new experimental campaign is proposed where unidirectional CFRP specimens are tested under the combination of fatigue and constant compressive load. The tests represent the loading that the lower part of an air-wing faces during the flight and parking process. Acoustic Emission technique is employed in order to monitor the damage progression and accumulation. The results of the acoustic emission are compared with reference tests where only fatigue loading is used and it is found that the acoustic emission patterns in terms of number and distribution of events over the duration of tests and energy accumulation is different for these two types of tests. The results indicate that the damage process on CFRP structures is different when creep is present.Structural Integrity & Composite