Contents lists available at ScienceDirect Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech Failure mode maps of bio-inspired sandwich beams under repeated low- velocity impact S.H. Abo Sabah a , A.B.H. Kueh b, ⁎ , N. Muhamad Bunnori c a School of Civil Engineering, Universiti Sains Malaysia, Engineering Campus, 14300, Nibong Tebal, Penang, Malaysia b Department of Civil Engineering, Faculty of Engineering, Universiti Malaysia Sarawak, 94300, Kota Samarahan, Sarawak, Malaysia c Department of Civil Engineering, Faculty of Engineering, University of Malaya, 50603, Kuala Lumpur, Malaysia ARTICLE INFO Keywords: A. Layered structures Impact behavior Failure map Modeling Sandwich structures ABSTRACT Existing sandwich structures failure maps are confined only to data from the quasi-static bending tests even for describing failure modes due to the impact event. Strengths of the constituent layers, which are not well-de- scribed by these maps, can reasonably change especially under the repeated impact load case. Hence, a new series of more realistic failure mode maps have been developed from the experimentally and numerically ob- tained observations on recently proposed bio-inspired dual-core sandwich beams in the presence of repeated low-velocity impacts of different energy levels. The beams consist of top and bottom carbon fiber reinforced polymer skins sandwiching the rubber and aluminum honeycomb cores. Departing from the modified Gibson model, an actual presentation of skin and core behaviors has been modeled following the trend of strengths variations for the construction of the failure mode maps when subjected to numerous impact numbers and energies. The produced maps offer the flexibility to accommodate the changes in strengths due to deterioration or densification of constituent layers after impact, and hence following more favorably the physical failure description of the sandwich beams. Accompanying these maps, a general set of mathematical expressions have also been produced for practical convenience. It is found that the failures from observations are within the proposed map boundaries with accuracies ranging from 85.7% to 100%. 1. Introduction As a classical innovation, the sandwich composite structure concept has continually sustained vitality and usefulness in numerous con- structional practices due to an ever-increasing demand for high stiffness and lightweight characteristics in applications [1]. In catering to a vast variety of applications; from marine, aerospace, and civil engineering to sporting appliances, there exist several different loading conditions that could impose a direct threat to the survivability of sandwich structures. More severe aftermath can be anticipated both structurally and in terms of human safety especially when the loading environment is of dynamic nature, such as in the presence of impact and blasting. In an impact event, sandwich structures can exhibit numerous modes of failures. Various different failure modes have been explored in detail in existing literature through comprehensive stress analyses using a properly de- fined set of criteria [2–5]. However, the prediction of failure in sand- wich structures remains challenging due to the nonlinear behavior of all integrating materials, making the task laborious and complex. In sum- marizing outcomes from a global-scale failure determination exercise, Hinton et al. [2] highlighted that the existing failure criteria are in- sufficient even for describing the failure modes of unidirectional lami- nates. It is well-known that numerous stress concentration mechanisms may be yielded at each constituent layer of a composite sandwich structure. These effects are considerably amplified especially in multi- layered structures, which are a formation of various different materials. In our most recent work, we have developed a bio-inspired sand- wich beam consisting of two carbon fiber reinforced plastic (CFRP) laminated skins (top and bottom) and a dual-core system (rubber and aluminum honeycomb) [3] for low-velocity impact resistance im- provement. We have also conducted experimental and numerical in- vestigations with a promising outcome showing that the new bio-in- spired sandwich beam is superior to the conventional design by up to five folds in terms of the overall impact resistance behavior. An overall comparative impact performance, conveying contribution from various responses, becomes possible by employing our newly defined index, in addition to single behavior assessment widely used in the current practice. It is granted that the relevant literature on sandwich structures damage under low-velocity impact offers some useful information on all https://doi.org/10.1016/j.compscitech.2019.107785 Received 4 February 2019; Received in revised form 16 July 2019; Accepted 17 August 2019 ⁎ Corresponding author. E-mail addresses: kbhahmad@unimas.my, ak_sibu@yahoo.com (A.B.H. Kueh). Composites Science and Technology 182 (2019) 107785 Available online 19 August 2019 0266-3538/ © 2019 Elsevier Ltd. All rights reserved. T