Periodic Report Summary 1 - IMPACTSMART SANDWICH (Active Control of Impact Response of Smart Sandwich Structures)
1. Project Context and Objectives
Among other structural configurations, smart sandwich structures with composite faces, foam core and embedded piezoelectric actuators and sensors, combine the superior mechanical properties of sandwich structures, such as, high flexural stiffness to mass ratio, with the capability to monitor the structural response on-site in real-time and to adapt their response according to selected control criteria. At high loading rates, such as in the case of impact loading, damage may occur in the form of matrix cracks, core crushing and interfacial delaminations, which in cases of low-velocity impact may be invisible. Thus, prediction and monitoring of both global dynamic response and local through-thickness stress field are essential in order to design within appropriate safety limits. Inversely, accurate prediction of the piezoelectric sensory signals during impact enables estimation of a damage incident during function and facilitate identification of impact parameters. Moreover, the formulation of computationally efficient methodologies for predicting the structure-impactor structural system’s response is crucial in order to develop realistic real-time active control algorithms and relevant applications by using the piezoelectric actuators.
In this context, the objectives of the project include the development of a computationally efficient impact mechanics methodology for prediction of the global and local through-thickness impact response of sandwich composite structures with piezoelectric transducers, identification of impact parameters by means of the sensory signals and active control for achieving minimization of impact force and maximum energy harvesting. A further objective is the experimental validation of the developed methodology by conducting impact testing measurements using a configuration designed and constructed within the project.
2. Work Performed
2.1 Modelling of Impact Response of Smart Sandwich Composite Structures
Novel laminate mechanics methodologies have been formulated for predicting the low-velocity impact response of composite and sandwich composite structures with piezoelectric transducers. They rest on higher-order layerwise kinematic assumptions developed for simulating the through-thickness impact response and enable computationally efficient prediction of stress at the interface between different materials by using a low number of discrete layers to model the laminate through-thickness. Linearized contact laws have been implemented for simulating the impactor-target interaction, yielding the coupled electromechanical plate-impactor system. Ritz-type and finite element solutions have been developed for beams, plates and shells in order to achieve calculation of their modal matrices. A Guyan reduction technique or a multiplication with the eigenvectors have been applied on the modal matrices to yield maximum two laminate variables per vibration mode and form an impactor-target linear system of minimum size. Predictions include temporal variations of impact force, displacements, electric potential, velocities, strains and stresses. As far as computational efficiency is concerned, the developed methodologies save around 99% of computational effort compared to the solution of the full impactor-target systems. Their high accuracy is validated by comparisons with published numerical and experimental results, and with measurements conducted in a low-cost impact testing configuration developed in the context of the project.
2.2 Impact Identification, Active Control and Energy Allocation
The impact identification technique developed within the project is based on the aforementioned impact mechanics methodologies and constrained optimization algorithms. It enables identification of impactor material, mass and velocity, in addition to impact location and leads to full reconstruction of the global and local through-thickness impact response, including impact force, displacements and stresses at the interface between different material layers of the sandwich structure. Thus, a conclusion can be drawn on whether damage has occurred in the form of delamination or core crushing.
As far as active impact control is concerned, the impactor-target system is formulated in state-space. A current density proportional gain (P) output feedback control algorithm has been developed for minimizing the impact force by activating a surface attached piezoceramic layer. The feasibility of shape-control towards minimization of impact force in composite and sandwich composite plates with piezoceramic active patches has been extensively studied and a morphing mechanism has been designed.
The impact mechanics methodology has been extended to take into account structural damping provided by the viscoelastic nature of the composite materials. In order to quantify the electric energy stored in the piezoelectric transducers during the impact event, the energy allocation/transformation during impact has been predicted. Energy harvesting in circuits connected to the piezoelectric transducers is an ongoing research topic.
2.3 Experimental Validation
A low-cost portable impact testing configuration for small to medium-sized beams, plates and shells has been designed and set-up. The configuration is equipped for measurement of impact force and adjustment of the impact velocity. A real-time data acquisition and control system enables high-speed simultaneous sampling of up to 16 sensory signals and driving of up to 8 actuators having independent voltage. Sandwich plates with monolithic or composite faces, foam core and piezoelectric patches have been fabricated and tested. Predictions for impact force and sensory signals acquired using the aforementioned impact mechanics methodologies have been validated with measured data, yielding a very good correlation.
3. Significant Research Results
The most significant research results achieved during the fellowship are listed below:
• Prediction of the global and local through-thickness low-velocity impact response of smart composite and sandwich composite structures, including prediction of interfacial stress and incorporation to failure criteria.
• Development of a novel twofold computational efficient impact analysis methodology,
o by implementing condensation techniques on the modal matrices
o by using a smaller number of discrete layers compared to linear layerwise laminate theories while capturing piecewise higher-order through thickness distributions of displacement, strain and stress variables.
• Development of an impact identification technique enabling posteriori extraction of impact location, impactor material, mass and velocity, and reconstruction of the impact event, including impact force time history and estimation of whether damage has occurred.
• Formulation of output feedback control algorithms for reduction of impact force in composite and sandwich composite plates with piezoelectric layers.
• Prediction of the energy allocation and transformation during impact.
• Design and construction of a low-cost portable experimental configuration for impact testing.
Most of the above research results have been presented in four high-ranked peer-reviewed journal and three conference publications.
4. Potential Impact and Use
Directions towards potential use of the project results are summarized below:
• The impact mechanics methodology developed in the project could be implemented in the design phase of industrial or commercial products encompassing sandwich configurations and fully elastic low-velocity impacts.
• The capability of interfacial stress prediction is a very useful design tool, which enables lighter structural parts and lower fuel consumption in aerospace, automotive and railway applications.
• Real-time monitoring of the response of sandwich structures by means of the piezoelectric transducers and comparison with the predicted response provides the opportunity to design engineers to tend to even lighter structural configurations by fulfilling the requirements of the relevant norms about regular inspection and thus to lower fuel consumption.
• The impact identification methodology developed during the project enables estimation of a probable damage incident due to impact and can be used for locating the areas requiring local inspection during service or repair of the structure.
The potential scientific impact of the project’s results is expected to be quantified in terms of citation indexes of the relevant publications in a long-term horizon. So far, the review comments received denote the novelty and practical value of studying impacts on sandwich structures with piezoelectric transducers. As far as the socio-economical impact is concerned, on the basis of the directions towards usage listed above, there can be potential industrial interest in the project’s results. Moreover, the dissemination activities of the project encompassing a wider audience from the civil society revealed high public interest for the experimental configuration and the project’s results in general.
5. Public Website of the Project
http://impact-smart.mech.ntua.gr/(öffnet in neuem Fenster)
Among other structural configurations, smart sandwich structures with composite faces, foam core and embedded piezoelectric actuators and sensors, combine the superior mechanical properties of sandwich structures, such as, high flexural stiffness to mass ratio, with the capability to monitor the structural response on-site in real-time and to adapt their response according to selected control criteria. At high loading rates, such as in the case of impact loading, damage may occur in the form of matrix cracks, core crushing and interfacial delaminations, which in cases of low-velocity impact may be invisible. Thus, prediction and monitoring of both global dynamic response and local through-thickness stress field are essential in order to design within appropriate safety limits. Inversely, accurate prediction of the piezoelectric sensory signals during impact enables estimation of a damage incident during function and facilitate identification of impact parameters. Moreover, the formulation of computationally efficient methodologies for predicting the structure-impactor structural system’s response is crucial in order to develop realistic real-time active control algorithms and relevant applications by using the piezoelectric actuators.
In this context, the objectives of the project include the development of a computationally efficient impact mechanics methodology for prediction of the global and local through-thickness impact response of sandwich composite structures with piezoelectric transducers, identification of impact parameters by means of the sensory signals and active control for achieving minimization of impact force and maximum energy harvesting. A further objective is the experimental validation of the developed methodology by conducting impact testing measurements using a configuration designed and constructed within the project.
2. Work Performed
2.1 Modelling of Impact Response of Smart Sandwich Composite Structures
Novel laminate mechanics methodologies have been formulated for predicting the low-velocity impact response of composite and sandwich composite structures with piezoelectric transducers. They rest on higher-order layerwise kinematic assumptions developed for simulating the through-thickness impact response and enable computationally efficient prediction of stress at the interface between different materials by using a low number of discrete layers to model the laminate through-thickness. Linearized contact laws have been implemented for simulating the impactor-target interaction, yielding the coupled electromechanical plate-impactor system. Ritz-type and finite element solutions have been developed for beams, plates and shells in order to achieve calculation of their modal matrices. A Guyan reduction technique or a multiplication with the eigenvectors have been applied on the modal matrices to yield maximum two laminate variables per vibration mode and form an impactor-target linear system of minimum size. Predictions include temporal variations of impact force, displacements, electric potential, velocities, strains and stresses. As far as computational efficiency is concerned, the developed methodologies save around 99% of computational effort compared to the solution of the full impactor-target systems. Their high accuracy is validated by comparisons with published numerical and experimental results, and with measurements conducted in a low-cost impact testing configuration developed in the context of the project.
2.2 Impact Identification, Active Control and Energy Allocation
The impact identification technique developed within the project is based on the aforementioned impact mechanics methodologies and constrained optimization algorithms. It enables identification of impactor material, mass and velocity, in addition to impact location and leads to full reconstruction of the global and local through-thickness impact response, including impact force, displacements and stresses at the interface between different material layers of the sandwich structure. Thus, a conclusion can be drawn on whether damage has occurred in the form of delamination or core crushing.
As far as active impact control is concerned, the impactor-target system is formulated in state-space. A current density proportional gain (P) output feedback control algorithm has been developed for minimizing the impact force by activating a surface attached piezoceramic layer. The feasibility of shape-control towards minimization of impact force in composite and sandwich composite plates with piezoceramic active patches has been extensively studied and a morphing mechanism has been designed.
The impact mechanics methodology has been extended to take into account structural damping provided by the viscoelastic nature of the composite materials. In order to quantify the electric energy stored in the piezoelectric transducers during the impact event, the energy allocation/transformation during impact has been predicted. Energy harvesting in circuits connected to the piezoelectric transducers is an ongoing research topic.
2.3 Experimental Validation
A low-cost portable impact testing configuration for small to medium-sized beams, plates and shells has been designed and set-up. The configuration is equipped for measurement of impact force and adjustment of the impact velocity. A real-time data acquisition and control system enables high-speed simultaneous sampling of up to 16 sensory signals and driving of up to 8 actuators having independent voltage. Sandwich plates with monolithic or composite faces, foam core and piezoelectric patches have been fabricated and tested. Predictions for impact force and sensory signals acquired using the aforementioned impact mechanics methodologies have been validated with measured data, yielding a very good correlation.
3. Significant Research Results
The most significant research results achieved during the fellowship are listed below:
• Prediction of the global and local through-thickness low-velocity impact response of smart composite and sandwich composite structures, including prediction of interfacial stress and incorporation to failure criteria.
• Development of a novel twofold computational efficient impact analysis methodology,
o by implementing condensation techniques on the modal matrices
o by using a smaller number of discrete layers compared to linear layerwise laminate theories while capturing piecewise higher-order through thickness distributions of displacement, strain and stress variables.
• Development of an impact identification technique enabling posteriori extraction of impact location, impactor material, mass and velocity, and reconstruction of the impact event, including impact force time history and estimation of whether damage has occurred.
• Formulation of output feedback control algorithms for reduction of impact force in composite and sandwich composite plates with piezoelectric layers.
• Prediction of the energy allocation and transformation during impact.
• Design and construction of a low-cost portable experimental configuration for impact testing.
Most of the above research results have been presented in four high-ranked peer-reviewed journal and three conference publications.
4. Potential Impact and Use
Directions towards potential use of the project results are summarized below:
• The impact mechanics methodology developed in the project could be implemented in the design phase of industrial or commercial products encompassing sandwich configurations and fully elastic low-velocity impacts.
• The capability of interfacial stress prediction is a very useful design tool, which enables lighter structural parts and lower fuel consumption in aerospace, automotive and railway applications.
• Real-time monitoring of the response of sandwich structures by means of the piezoelectric transducers and comparison with the predicted response provides the opportunity to design engineers to tend to even lighter structural configurations by fulfilling the requirements of the relevant norms about regular inspection and thus to lower fuel consumption.
• The impact identification methodology developed during the project enables estimation of a probable damage incident due to impact and can be used for locating the areas requiring local inspection during service or repair of the structure.
The potential scientific impact of the project’s results is expected to be quantified in terms of citation indexes of the relevant publications in a long-term horizon. So far, the review comments received denote the novelty and practical value of studying impacts on sandwich structures with piezoelectric transducers. As far as the socio-economical impact is concerned, on the basis of the directions towards usage listed above, there can be potential industrial interest in the project’s results. Moreover, the dissemination activities of the project encompassing a wider audience from the civil society revealed high public interest for the experimental configuration and the project’s results in general.
5. Public Website of the Project
http://impact-smart.mech.ntua.gr/(öffnet in neuem Fenster)