An innovative technique for measuring the high strain rate respon of sandwich composites Hassan Mahfuz * , Wahid Al Mamun, Anwarul Haque, Sherida Turner, Hisham Mohamed, Shaik Jeelani Tuskegee University's Center for Advanced Materials, Tuskegee, AL 36088, USA Abstract Foam core sandwich composites have been tested under high strain rate (HSR) loading in the thickness direction. An in experimental con®guration has been designed for this purpose. Conventional split Hopkinson pressure bar (SHPB) techniq been modi®ed by replacing the steel transmitter bar by a polycarbonate bar. This modi®cation resulted in stronger signa transmitter bar, which would otherwise be insigni®cant especially when testing soft materials. New sets of mathematical formu- lations have been derived to account for the impedance mismatch between the incidence and transmitter bars. The modi®ed equations are ®rst veri®ed with a known material and then used for sandwich composites. Three types of cores with vario have been tested under compression at strain rates ranging from quasi-static to 1000 S ÿ1 . The compressive failure stress has been observed to be directly proportional to the core density, as well as to the strain rate. The strain rate sensitivity was moder sandwich composites mostly failed by the collapse of the foam-cell. Delamination did not play a major role in the failure pr Details of the mathematical derivations and the analysis of the HSR behavior are presented in this paper. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: Sandwich composites; High strain rate response 1. Introduction Our continuing quest for lighter and stronger mate- rials has almost forced us to lean towards composites and sandwich constructions for marine, automotive and aerospace structures. The currentthrustof the three major DoD agenciesincluding Navy,Air Force and Army, have identi®ed aordable light structures as their focalpoints of interests as we move to the next millen- nium [1±4]. To Navy, Sandwich constructions become especially signi®cant because of three reasons; (1) very low hull weight, (2) low signatures, and (3) high energy absorption during mine blast or ballistic impact. Sand- wich constructionsprovideuniqueadvantagesover conventional metalsand alloys on all of the three counts.A sandwich construction stiens a structure without materially increasing its weight. The deformable viscoelastic core also plays an importantrole in en- hancing the energy absorption capability during an im- pact.It internally dampens the impactenergy asthe stress waves propagate through the core. It is the intent of this study,to understand the response of the sand- wich composites under such stress wave loading. High strain rate (HSR) characterization of compos- ites has been a subjectof greatinterestamong re- searchers primarily because of the need to understand their behaviorin loading situationswherethey are subjected to a wide range of strain rates [5,6]. The strain rate sensitivity ofthe materialis something thathe designers have to know before designing a structure for dynamic loading situation. In generalthe subjectof HSR loading on compositesis complex.When this composite is bonded to viscoelastic core to form into a sandwich construction, the analysis becomes even more challenging. Typically,a split Hopkinson pressure bar (SHPB) is used to acquire the response of materials under HSR loading [7,8].The techniquehas been extended to composite materials without much of a change except that one has to be careful with the impedance match between thebars and the specimen-material [9±11]. While there are issues still to be resolved, as to how the constituent matrix and ®ber will aect the overall com- posite response [12], SHPB is a popular technique to www.elsevier.com/locate/compstruct Composite Structures 50 (2000) 279±285 * Corresponding author. Fax: +1-334-724-4399. E-mail address: ememah@acd.tusk.edu (H. Mahfuz). 0263-8223/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 3 - 8 2 2 3 ( 0 0 ) 0 0 1 1 8 - 5