Virtual evaluation of energy exchange between two deformable body during low-energy impact. Application to the case of “Head on bonnet” impact according to WG17 and ACEA protocol

All over the world, and mainly in United States, since 1977 to 1991 the research centers of the main automotive companies had processed several statistical data on real accidents between vehicles and pedestrians taking care, obviously, to pedestrians' injury[7]. In latest years a research group of EEVC (European Enhanced Vehicle Safety Committee) had examined the statistic works of past twenty years and subsequent several scientific papers, and had realized some documents about "pedestrian test" procedures[2,6,7]. In reference papers of period 1977 - 1997 and in EEVC documents, the scientists describe a proposed homologative test for child-head impact; it is represented by an impact simulation of a standardized impactor on car bonnet, in order to evaluate the child-head injury as deceleration of its center of gravity. Injury evaluation criteria is an energy criterion and is quantified by the HPC index (Head Performance Criteria also called, without distinction, HIC - Head Injury Criteria) defined as follows: [GRAPHICS] in which t(1) and t(2) are respectively initial and final impact's instants (t(2)-t(1) is the integration range) and a is acceleration resultant vector, measured by an accelerometer mounted in the head-impactor. Impact duration generally doesn't exceed the 20 ms (milliseconds); in this range we can recognize two different phases: the first phase, dependent mainly on the local bonnet stiffness, with a length of time never greater than 4 ms and a second phase until the end of the phenomenon, dependent on boundary conditions and impacting bodywork parts, impacting engine parts and so on. In the first phase we can find the maximum acceleration's value, that heavily affects the final value of HPC [3]. In 1998, WG 17 has published a new testing protocol that was approved by EuroNCAP and European Community and fixed it as homologative criteria since 2010[10. For the period until 2010 the automotive companies had unanimously accepted a testing protocol less restrictive than the EuroNCAP one: the ACEA Protocol [11, 14] that will became a homologative criteria since 2005; this fact involves new compatibility problem for new car designer, because since 2005 all the vehicle will be homologated For determining the entity of damage on pedestrians we refer to the measurement scale called Abbreviated Injury only if they will respect the biomechanical values' range imposed by new testing protocol. Our paper pays attention on Child Head impact phenomena (the high priority impact both for WG17 and for ACEA protocol) and on the factors that affect the increase of HIC values. Other studies had demonstrated that in the middle area of the bonnet the acceleration peak, that heavily affects the value of HIC, strictly depends on "structural" parameters [3] of the bonnet; our paper is based on this studies and wants to reproduce, using numeric simulation, the Child impact dynamic (a very low-energy impact.... only 150 Joule) in order to demonstrate the importance of kinetic phenomena despite of the deformation's one. We can justify the validity of this hypothesis by the comparison of impact simulation of ACEA and WG17 test; these protocols are different for mass of impactor and speed of impact (for ACEA we have an impactor of 3.5 Kg that hits the bonnet with a 35 Km/h speed while for WG17 we have an impactor of 2.5 Kg that hits with a 40 Km/h speed). Child Head impact simulation in accordance with WG17 protocol was made and correlated on physical tests made on a FIAT Punto 60 by TNO (with an average correlation index of +/-10%)[5]. The test simulated, for the evaluation aim of our study, was made on a sub-system composed by the complete bonnet with its catch and hinges and was simulated in the explicit FEM code PamCrash.