Damage In Sandwich Structures Research Articles

Assessment of Internal Damage in Sandwich Structures by Post-Processing of Mode Shapes Using Curvelet Transform.

Identification and quantification of structural damage is one of the crucial aspects of proper maintenance of mechanical and civil structures, which is directly related to their integrity and safety. The paper presents a novel approach for detecting various types of damage in sandwich structures by processing the mode shapes using a hybrid algorithm based on the curvelet transform and the standardized damage index concept. The proposed approach uses the properties of directional selectivity, absence of the boundary effect, typical of such a class of transforms, and excellent filtration capabilities of the curvelet transform as well as the classification hypothesis in the standardized damage index, which allows the exclusion of irrelevant information and emphasizes proper damage location and shape. The proposed hybrid algorithm allowed to successfully identify a subsurface core damage in sandwich structures, such as local lack of a core or its debonding from facings. The performed quantification study aimed to evaluate the correctness of identified damage shape confirmed the validity and accuracy of the proposed algorithm not only for the damage detection and localization but also for the estimation of the size of structural damage.

Experimental study and progressive failure analysis of stitched foam‐core sandwich composites subjected to low‐velocity impact

The effects of the impact energy and stitch density on low‐velocity impact (LVI) damage in sandwich structures composed of a stitched foam core and woven face sheets have been investigated both experimentally and numerically. In the LVI experiment, force–time curves are recorded and analyzed to study the impact response of stitched foam core sandwich structures. In the numerical investigation, a progressive damage model for the composite structures is developed based on the continuum damage mechanics (CDM) approach in which the damage forms of the composites are predicted by the strain‐based Hashin criteria. The damage evolution is simulated by the stiffness degradation procedure in exponential form. Interface elements are established between the panels and foam core to simulate the initiation and propagation of inter‐laminar damage. The stitching region is simplified as glass fiber reinforced resin columns, and its mechanical parameters are estimated by a mixed series‐parallel model. The numerical and experimental results are in good agreement, which demonstrates the validity of the progressive failure analysis method. The energy absorption is discussed using a numerical method, and the results help to characterize the damage mechanisms of LVI on stitch foam‐core sandwich composites. POLYM. COMPOS., 39:624–635, 2018. © 2016 Society of Plastics Engineers

Measurement of Sub-Surface Core Damage in Sandwich Structures Using In-situ Hertzian Indentation During X-ray Computed Tomography

Composite sandwich structures with honeycomb cores show varying properties in geometry and mechanical behavior depending on the studied scale. Herein a new test and evaluation method for sub-surface core damage in the indentation area of honeycomb sandwich structures using computed tomography is presented. The combination of X-ray micro-computed tomography (X-μCT) and an image analysis procedure adjusted to the detection of core deformation mechanisms allows the extraction and quantification of externally invisible, sub-surface damage in the sandwich composite. For this specific contact or indentation loading case on the sandwich face sheet an in-situ device is introduced, enabling a 3D analysis of the structural change during progressing indentation depth.

Mechanical behavior and acoustic emission technique for detecting damage in sandwich structures

The present study investigates the mechanical behavior under static and dynamic loadings and assesses damage by the acoustic emission method of two types of sandwich composite materials. The sandwich structures under study are both made of cross-ply laminates as skins and PVC closed-cell as foam with different densities: 60kgm−3 and 100kgm−3. The mechanical behavior tests were conducted in static and cyclic fatigue loadings under 4-point bending. The sandwich structures considered in fatigue tests were damaged by a various number of shear damages in the foam. Static tests were performed to determine the failure parameters and characteristics used in fatigue tests. The damage density effect on the stiffness, hysteresis loops, dissipated energy and damping of sandwich structures, were studied for various numbers of cycles during cyclic fatigue tests. The acoustic emission method was used to identify and characterize the local damage in both types of sandwich materials under static 4-point bending tests.

Guided wave based detection of damage in honeycomb core sandwich structures

We report on the Lamb wave type guided wave propagation in honeycomb core sandwich structures. An experimental study supported by theoretical evaluation of the guided wave characteristics is presented that proves the potential of Lamb wave type guided wave for detection of damage in sandwich structures. A sandwich panel is fabricated with planar dimension of 600mm×600mm, having a core thickness of 7mm, cell size of 5mm and 0.1mm thick aluminum face sheets. Thin piezoelectric patch actuators and sensors are used to excite and sense a frequency band limited guided wave with a central frequency. A linear phased array of piezoelectric patch actuators is used to achieve higher signal strength and directivity. Group velocity dispersion curves and corresponding frequency response of sensed signal are obtained experimentally. Linearity between the excitation signal amplitude and the corresponding sensed signal amplitude is found for certain range of parameters. The nature of damping present in the sandwich panel is monitored by measuring the sensor signal amplitude at various different distances measured from the center of the linear phased array. Indentation and low velocity impact induced damages of increasing diameter covering several honeycomb cells are created. Crushing of honeycomb core with rupture of face sheet is observed while introducing the damage. The damages are then detected experimentally by pitch-catch interrogation with guided waves and wavelet transform of the sensed signal. Signal amplitudes are analyzed for various different sizes of damages to differentiate the damage size/severity. Monotonic changes in the sensor signal amplitude due to increase in the damage size has been established successfully. With this approach it is possible to locate and monitor the damages with the help of phased array and by tracking the wave packets scattered from the damages.

Predicting Failure Loads of Impact Damaged Honeycomb Sandwich Panels

This article examines the effect of impact damage on minimum gauge sandwich structures. This type of damage can cause panels to fail at significantly lower loads than an equivalent undamaged panel. For this reason, impact damage in sandwich structures has been a focus for numerous studies, including those that examine damage tolerance of structures and also their post-impact residual strength. This study specifically examines thin-gauge Nomex honeycomb panels and is concerned with barely visible impact damage (BVID), the type of damage that can be found on aircraft components. An overview of BVID, including an in-depth examination of the failure modes and failure loads of damaged panels is presented in this article. The study uses nonlinear finite element models to accurately predict failure loads, and understand how this type of damage causes panels to fail at reduced loads. A simplified, closed-form analytical model is developed to predict failure stresses through a complete range of damage sizes and depths. This study finishes by comparing numerical and analytical models against experimental values. Good agreement is seen in all cases for a wide variety of damage sizes.