A novel method for impact force estimation in composite plates under simulated environmental and operational conditions

Abstract

During its lifetime, an aircraft structure is subjected to various impacts from various sources such as tool drops, hail, ground service equipment, etc. In modern composite structures, these impacts have a significant chance of generating barely visible damage (BVID) which may lead to catastrophic failure of a structure if left undetected to grow. However, BVID is difficult to detect during routine visual inspection without specialised non-destructive inspection and thus there is large interest in developing monitoring systems for estimating the location and severity of impact events. Currently, most systems and methods have been developed for controlled lab conditions and do not consider the wide range of impact parameters in real life operation (environmental conditions, vibration, impactor stiffness, angle, etc) which may severely compromise the accuracy of these methods. In this study we have explored two methods for maximum impact force estimation, deconvolution and a novel gradient method, for the purpose of reliable severity assessment in composite aircraft structures under simulated environmental and operational conditions. It is shown that both methods allow accurate and robust estimation of the maximum impact force from various cases of impacts (variation of impact energy, mass, stiffness, angle, temperature, source) using minimum initial data from a single impact case. From further testing it is demonstrated that the gradient method is robust towards the effects of impact localisation errors and noise. The gradient method also has much less computational and storage requirements and is thus more feasible to integrate with current data acquisition systems being developed for structural health monitoring. Thus, we conclude that the proposed gradient method is suitable for impact force monitoring and severity assessment in composite aircraft structures in the simulated environmental and operational conditions.

Uncertainty quantification for impact location and force estimation in composite structures

“…A novel data-driven stochastic kriging-based method for impact location and maximum force estimation was developed. Comparison with a deterministic benchmark method developed in prior studies 5,6,13 indicates that the proposed method gives more reliable estimates for experimental impacts under various simulated environmental and operational conditions by estimating the uncertainty. The developed method highlights the suitability of data-driven methods for uncertainty quantification by taking advantage of the relationship between data points in the reference database that is a mandatory component of these methods (and is often seen as a disadvantage).…”

“…As the maximum impact force is random (ranging from 50 to 250 N), the error was taken as percentage towards the actual value as shown in equation (7). To summarise the error for a whole impact case, the mean (F max, mean ) and standard deviation (F max,std ) of the maximum force error for all impacts in the said case were calculated 13 F max, error = (F max, est À F max, act )…”

“…1 In thin-walled structures (as is usually found in composite airframe), impact events generate Lamb waves which propagate across the structure and can be recorded using optimally positioned (to provide best coverage and information with minimum sensors 2,3 ) strain sensors or accelerometers. 1,[4][5][6][7] Features of these recorded Lamb wave signals (such as time of arrival (ToA) and amplitude) contain information that can be used to estimate the location 1,5,6,[8][9][10][11][12] and severity of impacts, 7,[13][14][15] and many methods have been developed for this purpose. However, these features are subject to variation arising from measurement uncertainty 10,12,14,15 and changes in environmental and operational conditions.…”

“…However, these features are subject to variation arising from measurement uncertainty 10,12,14,15 and changes in environmental and operational conditions. 5,6,13 The uncertainty of extracted features, coupled with the inherent limit of accuracy in impact location and force estimation algorithms (as no algorithm is ever exact), [5][6][7][8]13,14,16 results in uncertainty in the final estimations.…”

“…1 Thus, there is interest in developing impact damage monitoring systems which can reliably inform operators of the occurrence and location of impact damage, allowing a condition-based maintenance and thus reduction in costs. 1 In thin-walled structures (as is usually found in composite airframe), impact events generate Lamb waves which propagate across the structure and can be recorded using optimally positioned (to provide best coverage and information with minimum sensors 2,3 ) algorithms (as no algorithm is ever exact), [5][6][7][8]13,14,16 results in uncertainty in the final estimations.…”