Tensile and fracture behavior of polymer foams
Introduction
Over the last few years, rigid polymer foams are gaining interests as core materials for sandwich structures in aerospace, marine, automobiles and other commercial applications due to their higher energy absorption capabilities, especially in the event of impact loading [1], [2], [3]. Among polymer foams, polyvinyl chloride (PVC) and polyurethane (PUR) foams are the most widely used core materials in sandwich constructions. Among the three regions (skin, core and skin–core interface) of a sandwich construction, the core plays a very important role in increasing the stiffness and also in controlling the failure mechanisms, especially in flexure. The structural response of a polymer foam strongly depends on the foam density, cell microstructure such as cell size, shape, type (open or closed) etc., and solid polymer properties [4]. Due to the viscoelastic nature of the solid polymer materials, foams exhibit rate dependent behavior. When using a closed-cell polymer foam, strain rate effects can further complicate the structural response due to the presence of the membranes enclosing the cells, trapped gas and gas flow. Due to the complex structure of the sandwich construction, poor or missing bonding during fabrication can lead to cracking of the core. Furthermore, cracks in the core can be generated in-service, which eventually can lead to premature failure of the sandwich structure. For example, it was reported in the literature [5], [6], [7] that the fatigue loading on a sandwich panel can cause cracking at the skin–core interface located at the loading point, which propagated towards the support span and kinked to the other side of the skin. Thus, it is obvious that the fracture behavior of sandwich composites is very important to investigate. Although the fracture behavior of skin materials is relatively well known, very little information is available about core materials, especially for polymer foams. Therefore, a thorough understanding of the fracture behavior of polymer foams and the effect of various parameters influencing the fracture behavior are essential for efficient use of these materials in such applications.
Gibson and Ashby [4] provided an extensive investigation of different types of cellular materials for various applications ranging from packaging to lightweight structures. Ashby, Gibson and Maiti [8] were the first few researchers who investigated the fracture behavior of linear elastic foams using a discrete cell-by-cell extension model for crack growth. They also developed a theoretical model relating the fracture toughness with relative density of the foam and verified experimentally. Fowlkes [9] investigated the fracture behavior of polyurethane (PUR) foams with different geometry using linear elastic fracture mechanics (LEFM) in determining fracture toughness of PUR foam. It was concluded that the fracture toughness and critical energy release rate are fundamental properties that can characterize the tensile properties of PUR foam. Zenkert and Backlund [10] investigated the mode-I crack propagation of polyvinyl chloride (PVC) foams and suggested that LEFM model could be used to describe this class of material. They also performed mode-II and mixed-mode fracture tests on the same material [11] and found that fracture stress and crack propagation angle could accurately be predicted using strain energy density criterion. Despite their potential applications in a wide range of dynamic loading environment, investigation onto the dynamic fracture behavior of polymer foams is limited. Recently, El-Hadeck and Tippur [12] investigated the dynamic fracture behavior of epoxy syntactic foam using single edge notch bend (SENB) specimen under three-point loading. The dynamic load was applied using an instrumented drop tower with a hemispherical tup and the dynamic crack initiation and growth were measured using an optical method. They found that the crack speed increases with the increase of volume fraction of microballoons and there is no significant dependence of dynamic fracture toughness on crack speed in any of the volume fractions observed.
In this paper, a systematic study including tensile, quasi-static fracture and dynamic fracture behavior of different polymer foams is performed. Effects of loading rate, foam density, level of cross-linking, solid polymer material, and cell orientation on fracture toughness are addressed. Also, empirical relationships of tensile strength, modulus, and fracture toughness with relative foam density are evaluated for PVC foam. Finally, the fracture surfaces are examined using a scanning electron microscope (SEM) and digital camera.
Section snippets
Test materials
Two different types of polymer foams, namely, cross-linked polyvinyl chloride (PVC) and rigid polyurethane (PUR) are considered. PVC foams with three densities: 75, 130, and 260 kg/m3 (labeled as R75, HD130 and R260) and two different levels of cross-linking (labeled as H130 and HD130), and PUR foams with density of 240 kg/m3 (labeled as PUR240) are examined. H130 foams have higher cross-linking than HD130 foams; both are PVC foam and have the same density of 130 kg/m3. Special blowing agents and
Tension tests
Tension tests are performed using prismatic bar specimens with a gage length of 100-mm and a cross section of 25-mm by 10-mm (width and thickness). Test specimens are prepared from foam panels both in the flow and rise direction. Specimens with flow direction are prepared from 12.7-mm thick foam panels. Specimens with rise direction are prepared only from the H130 foam panel of 25-mm thick by assembling and gluing of six prismatic pieces each 12.7-mm by 25-mm cross-section using Hysol EA 9309.
Dynamic fracture tests
Dynamic fracture tests are performed on a SENB specimens using an instrumented drop tower test setup (Instron-Dynatup-8210) with a hemispherical tup of 12.7 mm. Test specimens are placed with edge crack opposite to the loading site in between the two rectangular support plates with 76-mm circular opening and clamped to have clamped boundary conditions. A schematic of the test specimen and fixture during the dynamic fracture test is shown in Fig. 4b. Specimens are subjected to drop load (impact
Tensile behavior
Typical tensile stress–strain responses of PVC foams with different density in the flow direction are shown in Fig. 5. It is observed that the stress–strain curves are linear for all the foams. The tensile behavior of H130 and HD130 foams is found to be very similar indicating that level of cross-linking has a minimal effect on the tensile strength and stiffness. However, the effect of foam density on the tensile strength and stiffness is evident; higher tensile strength and stiffness for
Conclusions
Tension, quasi-static fracture, and dynamic fracture behavior of polymer foams with different densities, microstructures, polymer materials, and loading rates are examined. The tensile tests are performed using the prismatic bar specimens, whereas the fracture toughness tests are performed using the single-edge-notch-bend (SENB) specimens. The dynamic fracture tests are performed using an instrumented drop-tower test setup under clamped boundary conditions. The tensile and quasi-static fracture
Acknowledgement
The authors acknowledge with appreciation the National Science Foundation (NSF) under CREST program for supporting this work.
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