Analysis of impact-induced damage and delamination in the composite shell of a helmet Praveen K. Pinnoji, Puneet Mahajan * Department of Applied Mechanics, Indian Institute of Technology Delhi, New Delhi, India article info Article history: Received 27 November 2009 Accepted 7 March 2010 Available online 12 March 2010 Keywords: Composite shell Helmet Impact Damage Delamination Cohesive zone model abstract The impact energy absorption by helmets is of vital importance to the safety of motorcyclists during acci- dents. The paper is concerned with the study of damage and delamination, which are the principal modes of failure and energy absorption, in a composite shell and their influence on the impact performance of a helmet. Numerical simulations were conducted with different composite shells made of cross-ply lami- nate, woven fabric, and glass mat. The effectiveness of the numerical model is established using available experimental results from the literature. Hashin failure criteria and cohesive zone model (CZM) were used for predicting the in-plane damage and delamination in composite plies, respectively. An interface layer having a bilinear relationship between traction and relative displacement was placed between the plies of the composite shell to predict the delamination. The influence of damage and delamination in shells made of composite materials on impact-induced forces is evaluated and their performance is com- pared with helmet shells made of Acrylonitrile Butadiene Styrene (ABS). Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction It is established that crash helmets offer protection to the hu- man head during accidents, by cushioning the head so that the time of actual impact is extended thereby reducing the level of acceleration and force on the head. A helmet consists of five main elements, namely outer shell, liner foam, padding foam, strap, and visor. A typical motorcycle helmet is shown in Fig. 1. The outer shell provides the defence against the initial impact and holds all the components together. It spreads the impact over a large area of helmet, prevents the liner foam fracture and its penetration by sharp objects, and protects the face [1]. The liner foam undergoes plastic deformation and absorbs major part of the energy during accidents. Its low yield stress and flat nature of post yield stress– strain curve also ensures that impact forces experienced by the head are low. The padding foam does not absorb energy and is used for comfort and fitting purpose of different sizes of head. The strap is used for the retention system and keeps the head and helmet to- gether. The visor protects the eyes of the rider from rough weather conditions, dust and insects. But a helmet cannot completely pro- tect the wearer against all foreseeable impacts because materials have their own definite limit to energy absorption capability. When the liner foam, which is the dominant material in the hel- met, crushed completely the unabsorbed energy will be transferred to the head and the impact forces developed will be very high. These impact forces will be reduced if the outer shell absorbs some additional energy during impact. In helmet, the market dominant outer shell is made of thermoplastics (either Acrylonitrile Butadi- ene Styrene or Polycarbonate) or composites of epoxy with fibers of glass, carbon or Kevlar. The outer shell is typically 3–5 mm thick. The helmet shells made from composite materials are generally more expensive than the conventional thermoplastic material. When subjected to impact loading, the dominant failure mecha- nisms taking place in composite laminates are a complex combina- tion of various interlaminar and intralaminar damage mechanisms such as fiber breakage, matrix cracking and delamination. It has been conjectured that besides absorbing energy by deformation the composite shell also absorbs energy through these damage mechanisms. It has been suggested that these damage modes in a helmet with a composite shell help to reduce peak accelerations experienced by the head [2]. This suggestion however needs further investigation to justify the high cost of helmets with composite shell in comparison with helmets having thermoplastic shell. Kostopou- los et al. [2] studied the effect of helmet shells made up of woven fab- ric (carbon, glass, Kevlar) and glass mat reinforced composites on impact performance. Chang–Chang failure criterion and a tie break interface algorithm based on interlaminar shear stresses were used to examine damage initiation and delamination in shells, respec- tively. The acceleration of the head and regions of delamination in these helmet shells during top impact were shown in the paper, although no comparisons with ABS or Polycarbonate (PC) helmet shell were made. The woven Kevlar fabric shell responded better 0261-3069/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2010.03.011 * Corresponding author. Tel.: +91 1126591229; fax: +91 1126581119. E-mail addresses: mahajan@am.iitd.ernet.in, mahajan@am.iitd.ac.in (P. Mahajan). Materials and Design 31 (2010) 3716–3723 Contents lists available at ScienceDirect Materials and Design journal homepage: www.elsevier.com/locate/matdes