Experimental investigation of impact on composite laminates with protective layers

This paper presents an experimental study of low energy impacts on composite plates covered with a protective layer. In service, composite materials are subjected to low energy impacts. Such impacts can generate damage in the material that results in significant reduction in material strength. In order to reduce the damage severity, one solution is to add a mechanical protection on composite structures. The protection layer is made up of a low density energy absorbent material (hollow spheres) of a certain thickness and a thin layer of composite laminate (Kevlar). Energy absorption ability of these protective layers can be deduced from the load/displacement impact curves. First, two configurations of protection are tested on an aluminium plate in order to identify their performance against impact, then the same are tested on composite plates. Test results from force–displacement curves and C-scan control are compared and discussed and finally a comparison of impact on composite plates with and without protection is made for different configurations

Significance of low energy impact damage on modal parameters of composite beams by design of experiments

This paper presents an experimental study on the effects of multi-site damage on the vibration response of composite beams damaged by low energy impacts around the barely visible impact damage limit (BVID). The variation of the modal parameters with different levels of impact energy and density of damage is studied. Vibration tests have been carried out with both burst random and classical sine dwell excitations in order to compare that which of the methods among Polymax and Half Bandwidth Method is more suitable for damping estimation in the presence of damage. Design of experiments (DOE) performed on the experimental data show that natural frequency is a more sensitive parameter for damage detection than the damping ratio. It also highlighted energy of impact as the factor having a more significant effect on the modal parameters. Half Bandwidth Method is found to be unsuitable for damping estimation in the presence of damage

Optical measurements and experimental investigations in repeated low-energy impacts in powerboat sandwich composites

In the world of powerboats competition, the high-performance sandwich-structured composites have completely replaced traditional materials. During the competition, the structure of this kind of ships is subjected to repeated impacts. It is then fundamental to understand the damage evolution in order to select the most appropriate materials and increase safety issues. The present study is aimed at analysing the behaviour of sandwich-structured composites undergoing repeated low-energy impacts. Three different materials have been analysed. Two are sandwich-structured composites used for the cockpit of offshore powerboats and differing only by the core cell thickness. The third material is composed only by the skin of the same sandwich structures, without the core. Impacts were made at three different energy levels: 15, 17.5 and 20 J. In addition to the parameters typically used for the assessment of the impact damage, a new damage assessment has been carried out by means of three-dimensional optical measurements of the imprinted volumes resulting from the impact events. This approach has allowed the definition of a correlation between the imprinted volumes and the number of impacts, until the complete perforation, for each single specimen. Finally, thanks to usual indexes and the imprinted volumes, some considerations are developed about the influence of the core cell thickness in powerboats design

Mechanical protection for composite structures submitted to low energy impact

Composite materials are widely used in aeronautical structures. These materials can be submitted to low energy impacts like tool drop, routine operations… Such impacts can generate damages in the material that significantly reduce the structure strength. A solution to reduce the severity of damages due to impact is to add a mechanical protection on composite structures (patent n° 2 930 478). In this paper, an experimental study on different concepts of protective layers is presented. This protection is made of a certain thickness of low density energy absorbent material (foam, honeycomb or stacking of hollow spheres) and a thin layer of composite laminate (Kevlar). Experimental impact tests with a spherical impactor of 20 mm diameter at low velocity and low energy are made on aluminum plates, with different protections, and for different levels of energy. Analyses of Load/Displacement curves enable to study the capability of each mechanical protection to absorb energy. Resistance of these protections is then compared and discussed, taking into account the thickness and the surface density of the protections

Damage localization using experimental modal parameters and topology optimization

This work focuses on the developement of a damage detection and localization tool using the Topology Optimization feature of MSC.Nastran. This approach is based on the correlation of a local stiness loss and the change in modal parameters due to damages in structures. The loss in stiness is accounted by the Topology Optimization approach for updating undamaged numerical models towards similar models with embedded damages. Hereby, only a mass penalization and the changes in experimentally obtained modal parameters are used as objectives. The theoretical background for the implementation of this method is derived and programmed in a Nastran input file and the general feasibility of the approach is validated numerically, as well as experimentally by updating a model of an experimentally tested composite laminate specimen. The damages have been introduced to the specimen by controlled low energy impacts and high quality vibration tests have been conducted on the specimen for dierent levels of damage. These supervised experiments allow to test the numerical diagnosis tool by comparing the result with both NDT technics and results of previous works (concerning shifts in modal parameters due to damage). Good results have finally been archieved for the localization of the damages by the Topology Optimization

Consequences of large impacts on Enceladus' core shape

International audienceThe intense activity on Enceladus suggests a differentiated interior consisting of a rocky core, an internal ocean and an icy mantle. However, topography and gravity data suggests large heterogeneity in the interior, possibly including significant core topography. In the present study, we investigated the consequences of collisions with large impactors on the core shape. We performed impact simulations using the code iSALE2D considering large differentiated impactors with radius ranging between 25 and 100 km and impact velocities ranging between 0.24 and 2.4 km/s. Our simulations showed that the main controlling parameters for the post-impact shape of Enceladus’ rock core are the impactor radius and velocity and to a lesser extent the presence of an internal water ocean and the porosity and strength of the rock core. For low energy impacts, the impactors do not pass completely through the icy mantle. Subsequent sinking and spreading of the impactor rock core lead to a positive core topographic anomaly. For moderately energetic impacts, the impactors completely penetrate through the icy mantle, inducing a negative core topography surrounded by a positive anomaly of smaller amplitude. The depth and lateral extent of the excavated area is mostly determined by the impactor radius and velocity. For highly energetic impacts, the rocky core is strongly deformed, and the full body is likely to be disrupted. Explaining the long-wavelength irregular shape of Enceladus’ core by impacts would imply multiple low velocity (<2.4 km/s) collisions with deca-kilometric differentiated impactors, which is possible only after the LHB period

Experimental investigation of impact behavior of wood-based sandwich structures

Low carbon emission and sustainable development are shared goals throughout the transportation industry. One way to meet such expectations is to introduce lightweight materials based on renewable sources. Sandwich panels with plywood core and fiber reinforced composite skins appear to be good candidates. Additional properties of wood such as fire resistance or thermal and acoustic insulation are also essential for many ap- plications and could lead to a new interest for this old material. In this paper, Sandwich panels with two different types of plywood and four different skins (aluminum and glass, CFRP, or flax reinforced polymer) are tested under low-velocity/low energy impacts and their behavior is discussed

Low intrusive coupling of implicit and explicit integration schemes for structural dynamics: application to low energy impacts on composite structures

Simulation of low energy impacts on composite structures is a key feature in aeronautics. Unfortunately they are very expensive: on the one side, the structures of interest have large dimensions and need fine volumic meshes (at least locally) in order to capture damages. On the other side small time steps are required to ensure the explicit algorithms stability which are commonly used in these kind of simulations [4]. Implicit algorithms are in fact rarely used in this situation because of the roughness of the solutions that leads to prohibitive expensive time steps or even to non convergence of Newtonlike iterative processes. It is also observed that rough phenomenons are localized in space and time (near the impacted zone). It may therefore be advantageous to adopt a multiscale space/time approach by splitting the structure into several substructures owning there own space/time discretization and their own integration schemes. The purpose of this decomposition is to take advantage of the specificities of both algorithms families: explicit scheme focuses on rough areas while smoother (actually linear) parts of the solutions are computed with larger time steps with an implicit scheme. We propose here an implementation of the Gravouil-Combescure method (GC) [1] by the mean of low intrusive coupling between the implicit finite element analysis (FEA) code Z-set and the explicit FEA code Europlexus. Simulations of low energy impacts on composite stiffened panels are presented. It is shown on this application that time step ratios up to 5000 can be reached. However, computations related to the explicit domain still remain a bottleneck in terms of cpu time

Modeshapes recognition using Fourier descriptors: a simple SHM example

The main objective of this study is to develop an alternative criterion for modeshape classification, as the currently available one, MAC (Modal Assurance Criteria), is only a vector correlation representing modeshape similarities. This new method is developed to provide a set of features (Fourier Descriptors) for comparing modeshapes with “local” similarities of higher interest than “global” similarities using nodal lines. These lines are able to characterize modeshapes very easily. So when damage occurs, we are able to track the few descriptors changes to localise the damage. We validated our method on a CFCF plate demonstrating the quality of the damage localisation and possible use in a “mode tracking” application (space structure)