Impact response of high density flexible polyurethane foam

Impact response of high density flexible polyurethane foam

E.Zaretsky a ,*,Z.Asaf b ,E.Ran b ,

F.Aizik b

a Department of Mechanical Engineering,Ben Gurion University of the Negev,P.O.Box 653,Beer Sheva 84105,Israel b

Plasan Sasa,Kibbutz Sasa,M.P.Merom Hagalil,13870,Israel

a r t i c l e i n f o

Article history:

Received 13June 2011

Accepted 9September 2011

Available online 16September 2011Keywords:Flexible foam

Shock compression Crush up stress

a b s t r a c t

The impact response of high density flexible polyurethane-based foam was studied in a series of symmetric (both the impactor and the sample made of the same foam)planar impact experiments,with continuous VISAR monitoring of the velocity of the rear sample surface.The impact velocities in these experiments varied from 43to 605m/s providing a sample compression over the 0.36e 51-MPa pressure range,with the strain rates changing,respectively,from 4Â103to 6Â105s À1.The linear shock velocity-particle velocity Hugomiot of the foam,U S ¼U S 0þsu ¼14.8þ1.318u ,was determined on the basis of the recorded velocity histories.The rise times of the velocity histories allows one to conclude that under shock compression above 3.2MPa,the initial structure of the foam is completely crushed and the foam resistance to the propagation of the shock is determined by the void-free foam material.The dynamic tensile (spall)strength of the foam,determined in a separate impact experiment with 1-mm thick foam impactor was found equal to 0.3MPa.Such unexpectedly low spall strength is possibly the result of substantial damage having taken place in the foam during compression.

Ó2011Elsevier Ltd.All rights reserved.

1.Introduction

The ability of polymeric foams to absorb energy of impact stands behind a wide variety applications in automotive industry,civil engineering,packaging and transportation of fragile goods.The quasi-static mechanical properties of the foams,which belong to a large group of materials with cellular structure,were studied intensely over last fifty years.The results of these multiple studies were carefully analyzed by Gibson and Ashby [1],who suggested a series of useful phenomenological relations between the densities,moduli,Poisson ’s ratios,and collapse stresses of the foams and the properties of the bulk material the foam was made of.Similar rela-tions for foams ’moduli,yield or collapse stress,etc.,were obtained as the result of the micromechanical study of the struts of the foam skeleton [2].The dynamic response of the foams,essential for their applications,was not addressed in these studies.An important feature of the dynamic response of the polymeric foams is their strong strain rate sensitivity [3].The in fluence of the strain rate on the mechanical response of foams was revealed in drop-weight [4e 6]and impact sleds [7]experiments and was found to be crucial for their energy absorbing ability.Unfortunately the results of these experiments do not provide data for a constitutive description

of the studied material.This problem was partly solved by use of the SHPB (Split Hopkinson Pressure Bar)[8e 11]study of the dynamic response of the foams.This method allows obtaining the stress-strain relations of the foams at the relevant (102s À1-103s À1)strain rates providing valuable information for the constitutive modeling of the foams and for energy absorption estimates.

Gas gun driven planar impact experiments are widely used for studying the dynamic response of metals,ceramics and polymers up to strain rates of about 106s À1and higher [12].The adjustment technique used in such experiments allows one to create exact uniaxial strain boundary conditions in the impacted sample.As a result,the initial parameters of the shock-induced stress pulse traveling through the studied sample are precisely de fined.The response of the impact-loaded sample being monitored by a Velocity Interferometer System for Any Re flector (VISAR)[13]provides accurate constitutive information about the studied sample.One motivation for the present work was to explore the possibility of using the VISAR-instrumented gun-driven planar impact experiment for obtaining such constitutive information about flexible polymeric foam.Another reason was dictated by the need to close the gap between the highest loading rates of SHPB technique,w 103s À1,and the lowest of the planar impact,w 104s À1.Addressing these two issues prompted us to perform a series of planar impact experiments with high density flexible polyurethane foam,accompanied by VISAR monitoring the velocity w of the free sample surface.

*Corresponding author.Tel.:þ97286477102;fax:þ97286477100.E-mail address:zheka@bgu.ac.il (E.

Zaretsky).Contents lists available at SciVerse ScienceDirect

International Journal of Impact Engineering

journal homep age:www.elsevie

/locate/ijimpeng

0734-743X/$e see front matter Ó2011Elsevier Ltd.All rights reserved.doi:10.1016/j.ijimpeng.2011.09.004

International Journal of Impact Engineering 39(2012)1e 7