Small Mass Impact Research Articles

Bird strike simulation on a bonded Ti/CFRP leading edge of an engine fan blade

Bird strike represents a critical loading scenario for aeronautical structures, especially for engine fan blades. It is, therefore, of great importance both to protect the leading edge of the fan blade from impact damage and to be able to detect impact damage directly. In the present study, an explicit finite element (FE) model was developed using the LS-DYNA software to simulate bird strike on the leading edge of a CFRP fan blade protected by an adhesively bonded Ti layer. The model accounts for damage on the CFRP blade through a progressive damage modeling scheme and for debonding through a cohesive zone modeling scheme but not for damage on the Ti layer. For the modeling of the bird, the smooth particle hydrodynamics (SPH) method was used, due to the large deformations that were expected. Using the model, a parametric study on the effects of bird mass and impact energy was performed. The numerical results show that impact damage depends more on impact velocity than the bird mass. In all cases, debonding of the tip of the leading edge was predicted, while for combinations of small bird mass and large impact velocity a more extensive debonding of the protective layer was predicted. Regarding damage in the CFRP, only matrix cracking on the leading edge has been predicted. Aiming to assess the effectiveness of FBGs to detect debonding of the Ti layer due to bird strike, an FBG network has been modeled into the bondline and a study was performed on the correlation of the measured strains with impact damage.

A perturbation-based model for the prediction of responses involving delamination during small mass impacts on orthotropic composite plates

An analytical model was developed for the prediction of small-mass-impact response including delamination effects in orthotropic composite plates, which mainly focuses on the relatively weak resistance of fiber reinforced polymer (FRP) laminates to small mass impact. Mode parameters of the delaminated structure obtained through perturbation-based dynamic analysis were substituted into the dynamic impact model. The alterations of the structural response caused by the occurrence of delamination were predicted by applying the premeasured delamination threshold load (DTL) to the developed model. The occurrence of delamination was clearly shown by a descent following a sudden rise on the velocity curve of the impact point. Impact tests with different initial energy were conducted on FRP laminated plates of varying thickness. The experimental results were well consistent with the analytical prediction. The validity of the proposed model was further confirmed using exploratory experiments with the velocity measurement point located nearby the impact point. The proposed model was more practically applicable than finite element modelling due to its efficiency in terms of time.

Assessment of Impact Detection Techniques for Aeronautical Application: ANN vs. LSSVM

The impact localization in composite panels is assessed using two machine learning techniques: least square support vector machines (LSSVM) and artificial neural networks (ANN) with local strain signals from piezoelectric sensors. Sensor signals from impact experiments on a composite plate as well as signals simulated by a finite element model are used to train and test models. A comparative study shows that LSSVM achieves better accuracy than ANN on identifying location of impacts for a combination of large mass impact and small mass impact, in particular when less data is available for training which is more appropriate for real aeronautical application. Additionally, LSSVM is more capable of identifying new impact events which have not been considered in the training process.

Optimization of composite plates subjected to buckling and small mass impact using lamination parameters

The small mass impact, such as runway debris and dropped tools, is a major issue during the design of composite plates, as it may significantly reduce the plate strength and stiffness without any visible damage. This work presents a novel optimization procedure to maximize the critical buckling load of composite plates subjected to small mass impact considering impact response and delamination threshold load predicted by closed forms solutions. The optimization is based on lamination parameters combined with laminate databases. In the first phase of the optimization a simplified model is used to obtain an approximate optimum. A design optimization using refined finite element model starting from the previous phase optimum is performed. The proposed algorithm is efficient, robust and applicable to many optimization problem for composite plates.

Structural Optimization of a Composite Plate Subjected to a Small Mass Impact

This paper presents the optimization of a composite plate subjected to a small mass impact under a mode II delamination constraint, considering impact response and delamination threshold load predicted by closed form solutions. These solutions are inexpensive from a computational point of view and have good agreement with experimental data. Therefore, this approach can be very interesting when working with optimization methods that require a large number of iterations to converge. On other hand, the closed form solutions have implicit limitations. An example is the plate and impactor masses ratio that must be within a defined range, otherwise the impact model does not apply. Consequently, the plate optimization problem must be well stated and constrained in order to provide results comprised in the closed form solutions validity. In this work, the optimization problem and solution are stated and discussed. The optimization method employed is the simulated annealing, together with appropriate optimization heuristics. Final optimization results are validated by finite element analysis with the capability of predicting damage by mixed-mode delamination in composites under impact. The results obtained using the proposed optimization procedures agree remarkably well with the finite element predictions.

Small Mass Impact Testing of Sandwich Structures

Abstract Small mass impacts pose a threat to modern lightweight materials even at low velocities. In this study we describe a test method with a suitable setup to access the materials impact response to masses below 500 g and velocities up to 20 m/s. The setup uses a highspeed camera and adapted semi-automatic image processing algorithms to extract the time-force history of the indenter. The results show good agreement to analytical impact models for hollow core sandwich structures.

Analytical model for delamination growth during small mass impact on plates

An analytical model is presented for delamination initiation and growth and the resulting response during small mass impact on orthotropic laminated composite plates, which typically is caused by runway debris and other small objects. The solution is obtained by a fast stepwise numerical solution of a single integral equation. Delamination size, load and deflection history are predicted by extension of an earlier elastic impact model by the author. Good agreement is demonstrated in comparisons with finite element simulations and experiments.

Effect of permanent indentation on the delamination threshold for small mass impact on plates

Small mass impacts on composite structures are common cases caused by hailstones and runway debris. Small mass impactors usually result in a wave controlled local response, which is independent of boundary conditions. This response occurs before the reflection of waves from the boundaries and cannot be modeled by large mass drop weight tests. An elasto-plastic contact law, which accounts for permanent indentation and damage effects, was used here to study small mass impact on laminated composite plates. By comparing with results from the Hertzian contact law, it was found that damage can change the dynamic response of the structure significantly with increasing impact velocity. Due to smaller contact force generated for the case of using elasto-plastic contact, the central displacement of the plate is also less than the one using Hertzian contact law. The linearized version of the contact law was then used to derive the closed-form approximations of the contact force, indentation and plate central displacement for the impact loading of composite laminates. The threshold velocity for delamination onset under small mass impact was predicted analytically based on the obtained peak contact forces by combining with an existing quasi-static delamination threshold load criterion. A good agreement was found between the predicted threshold values and published experimental results.

An improved mixed numerical-experimental method for stress field calculation

In this work a numerical-experimental method is used to study the dynamic behavior of an aluminum plate subjected to a small mass impact. The out-of-plane displacements, due to transient bending wave propagation, were assessed for successive time instants, using double pulse TV-holography, also known as pulsed ESPI. The experimental setup and the image processing methods were improved to allow the calculation of the plate transient stress field. Integral transforms are used to obtain the strain fields from spatial derivatives of displacements noisy data. A numerical simulation of the plate transient response was carried out with FEM Ansys ®. For this purpose a PZT transducer was used to record the impact force history, which was inputted in the numerical model. Finally, the comparisons between numerical and experimental results are presented in order to validate the present methodology.

An investigation on the effect of a small mass impact on sandwich composite plates

This paper investigates the high strain rate response of polymer matrix sandwich composites utilizing foam-filled honeycomb cores by means of a small mass impact. The reinforcement fiber chosen for use in this paper is a simple weave carbon fiber. This paper also investigates the effects of utilizing different laminate configurations in conjunction with investigation of the effectiveness of such laminate during impact loading. In an effort to ascertain more information about the impact situation, compression after impact tests are performed to determine the extent of the damage by observing the degradation of strength associated with the impact event.

Closed form prediction of peak load and delamination onset under small mass impact

Small mass impact on plates is a common response type caused by hailstones and runway debris. The response in such cases is wave controlled and the load and deflection are out of phase and independent of plate size and boundary conditions. Previous solutions to the problem have been based on laborious numerical approaches not suitable for design. This paper presents a closed form solution based on three asymptotic cases with deformation purely by indentation, bending or shear. Solutions are presented for a Hertzian contact law, suitable for monolithic plates, and for a linear contact law, suitable for sandwich panels. A good agreement is shown with “exact” numerical solutions and experimental results. The solution is also combined with an existing quasi-static delamination threshold load criterion. Comparison with several published experiments indicates that the criterion also may be applicable to small mass impacts.

Engineering Method for Prediction of Impact Response and Damage in Sandwich Panels

An engineering method is suggested for prediction of impact response and damage of flat sandwich panels. The approach accounts for local core crushing, delamination and large face sheet deflections and does not rely on empirical indentation laws. Different models are suggested depending upon the impactor mass being either larger or significantly smaller than the mass of the impacted panel. The solution for large mass impact is based on closed form expressions. The solution for small mass impact is obtained from a dimensionless two-parameter integral equation. The validity of the approach is demonstrated on a number of static indentation experiments and impacts on sandwich panels.

Mass criterion for wave controlled impact response of composite plates

Abstract Impact duration strongly influences the impact response of plates. Long impacts cause a quasi-static response influenced by the plate size and boundary conditions. Short impacts cause a response governed by wave propagation unaffected of plate size and boundary conditions. This paper shows that the response type is governed by the impactor–plate mass ratio and not by impact velocity and derives a criterion for small-mass (wave controlled) impact response of orthotropic plates. Published criteria for large-mass (quasi-static) impact are discussed. Small-mass impacts on composite laminates are shown to be more critical than large-mass impacts of the same energy. Use of mass criteria for selecting analytical response models and considerations for sandwich plates are also discussed.

Measured charge generation by small mass impact at velocities between 1 and 45 km/s

The electrical charge that is generated by the impact of a small mass at velocities between 1 and 45 km/s was investigated using the Electrostatic Dust Accelerator of the Max-Planck-Institut in Heidelberg (MPI) and the Plasma Accelerator of the Lehrstuhl fu¨r Raumfahrttechnik (LRT) of the Technische Universita¨t Mu¨nchen (TUM). Glass beads were accelerated, and the targets were of different materials i.e., (Au, W, Fe, Al). The mass/velocity range of the accelerated small masses was: MPI: 10 −15g<m< 10 −10g/ 1km/s<v< 45km/s TUM/LRT: 10 −10g<m< 10 −5g/ 2km/s<v< 20km/s. The total yield of the impact charge Q ± total of both polarities can be described by the empirical formula: Q ± total =C ϱm αv β[Coulomb], Cϱ being a function of the density ratio of target/projectile, α is approximately unity and β between 2.92 and 3.77. The charge detector is described and the results that were obtained in test series at both facilities are discussed in relation to the empirical formula.