Guided Lamb waves for identification of damage in composite structures: A review

This paper is a review of low-velocity impact responses of composite materials. First the term ‘low-velocity impact’ is defined and major impact-induced damage modes are described from onset of damage through to final failure. Then, the effects of the composite's constituents on impact properties are discussed and post-impact performance is assessed in terms of residual strength.

Review of low-velocity impact properties of composite materials

Mechanical metamaterials exhibit properties and functionalities that cannot be realized in conventional materials. Originally, the field focused on achieving unusual (zero or negative) values for familiar mechanical parameters, such as density, Poisson's ratio or compressibility, but more recently, new classes of metamaterials — including shape-morphing, topological and nonlinear metamaterials — have emerged. These materials exhibit exotic functionalities, such as pattern and shape transformations in response to mechanical forces, unidirectional guiding of motion and waves, and reprogrammable stiffness or dissipation. In this Review, we identify the design principles leading to these properties and discuss, in particular, linear and mechanism-based metamaterials (such as origami-based and kirigami-based metamaterials), metamaterials harnessing instabilities and frustration, and topological metamaterials. We conclude by outlining future challenges for the design, creation and conceptualization of advanced mechanical metamaterials.

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Flexible mechanical metamaterials

Localized fluid flow measurements with an He-Ne laser spectrometer

This paper reviews the common shapes of collapsible energy absorbers and the different modes of deformation of the most common ones. Common shapes include circular tubes, square tubes, frusta, struts, honeycombs, and sandwich plates. Common modes of deformation for circular tubes include axial crushing, lateral indentation, lateral flattening, inversion and splitting. Non-collapsible systems, such as lead extrusions or tube expansions, are considered to be beyond the scope of this review.

Collapsible impact energy absorbers: an overview

Axial crush of metallic honeycombs

Experimental results are presented for contact and low-velocity-impact re sponses of composite laminates by rigid spheres. The contact phenomenon is different in small and large indentation stages, which is due to occurrence of laminate damage. The contact force corresponding to the onset of laminate damage is found to be independent of the indentor size. In the small indentation stage where the plate is intact, the change of laminate stacking sequence has an insignificant effect on the force-indentation relation ship. However, the force-indentor size relationship does not follow the modified contact law for thin laminates indented by large indentors. Beyond the small indentation stage, damage occurs. The size of the delamination area is proportional to the applied loads and the number of repeated loading cycles. The passage of the delamination crack front can be detected using strain gauges. Also, the indentation spring becomes stiffer under repeated loading. Finally, the delamination damage and the stra...

Response of Composite Laminates to Contact Loads and Relationship to Low-Velocity Impact

A method to identify both the impact location and the transverse impact force history from the strain responses at certain points on a rectangular plate is presented. The governing equations of the plate were obtained by applying the Reissner-Mindlin plate theory and the Rayleigh-Ritz method. The strain response was related to the impact force by solving the above equations using the eigenmode expansion method. A mutuality relationship among any pairs of strain responses was used to find the impact location without knowing in advance the impact force history. The force history was subsequently determined after the impact location was identified. The conjugate gradient method was adopted to search for the optimal impact location as well as the force history. Numerical verification was performed using randomly generated impact locations and force histories to simulate impact events. The excellent agreement showed the effectiveness and the validity of the proposed method.

On the Inverse Problem of Rectangular Plates Subjected to Elastic Impact, Part I: Method Development and Numerical Verification

Experimental results for in-plane tensile and transverse impact responses of stitched composite laminates were presented in this paper. (02/902). E-glass/epoxy laminates of 2.8 mm nominal thickness fabricated by resin transfer molding were used as the specimens. The through-the-thickness reinforcement was provided by untwisted Kevlar-29 ravings of 1,000 and 3,000 denier. In the in-plane tensile test, although the damage mechanism of the stitched laminates was much affected by the loading directions, the stiffness was not significantly affected by the addition of the stitch threads. In the impact test using a hemispherically tipped impactor, stitching was found to significantly reduce the delamination crack area, and the 3,000 denier threads provided a better resistance to the propagation of these cracks. It was also found that, although the stitch step, stitch spacing, as well as the impact location relative to the stitch threads affected the size and the shape of the delamination area during impact, the ...

Behavior of Stitched Laminates under In-Plane Tensile and Transverse Impact Loading

With a blue light-emitting diode and a phosphor layer to downconvert blue light to a second light, such as yellow, white light can be produced. The authors developed a one-dimensional model to describe the light propagating in the phosphor layer in terms of light absorption, conversion, and reflection. The parameters required for the model were determined from the data obtained by using multiple-layer phosphor films. The model predicts that, with a reflector between the diode and the phosphor layer that is blue-light transparent but reflects other visible light, the normalized white light intensity is above 0.9, higher than that of conventional packages (0.6–0.8).

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Enboa Wu

Papers

Axial crush of metallic honeycombs

Response of Composite Laminates to Contact Loads and Relationship to Low-Velocity Impact

On the Inverse Problem of Rectangular Plates Subjected to Elastic Impact, Part I: Method Development and Numerical Verification

Behavior of Stitched Laminates under In-Plane Tensile and Transverse Impact Loading

Modeling white light-emitting diodes with phosphor layers