A Methodology for Characterization of the Strain Rate-Dependent Behavior of PU Foam

Abstract

Dynamic characterization of foam i.e. finding stress-strain responses at strain rates higher than quasi-static is essential for predicting its behavior in many real-world applications such as energy-absorption under impact loads. However, generating stress-strain data at dynamic strain rates is often a challenging task due to the necessity of specialized equipment such as a Split Hopkinson Pressure Bar [1, 2, 3] which can be expensive and not ordinarily available in an engineering laboratory. This device, despite perhaps being the best resource for ascertaining material behavior at high strain rates, has limitations due to its indirect nature arising from reliance on one-dimensional wave theory for solids, difficulty in performing tensile tests, special material requirements for incident and transmission bars for testing of soft materials such as foam and fibre-reinforced composites, etc. Additionally, the SHPB is generally suitable for strain rates of the order of 1000 s −1 or higher [4] although SHPB tests on polymeric foams with acrylic bars have been reported for strain rates in the range of 500-2500 s −1 [5].Due to the relevance of rigid PU foam for impact safety and packaging applications, attention is paid here toward developing a simple methodology for determining the stressstrain responses of a rigid PU foam at low to medium strain rates. However, it may be pointed out that the methodology adopted is not dependent on the type of foam being considered and can be applied to cellular materials in general. ABSTRACTPolymeric foams are known to be sensitive to strain rate under dynamic loads. Mechanical characterization of such materials would not thus be complete without capturing the effect of strain rate on their stress-strain behaviors. Consistent data on the dynamic behavior of foam is also necessary for designing energy-absorbing countermeasures based on foam such as for vehicle occupant safety protection. Strain rates of the order of 100-500 s −1 are quite common in such design applications; strain rates of this range cannot be obtained with an ordinary UTM (universal testing machine) and a special test set-up is usually needed. In the current study, a unique approach has been suggested according to which quasi-static tests at low strain rates and low velocity drop tests at medium strain rates are utilized to arrive at an empirical relation between initial peak stress and logarithm of strain rate for a rigid closed-cell PU foam. Using a stress-scaling methodology and the empirical relation mentioned, foam stress-strain curves are obtained for a number of strain rates spanning low (from 0.00033 s −1 ) to high strain rates (up to1000 s −1 ). This data on foam material behavior is expected to be particularly useful in numerical modelling of foam-based countermeasures for impact energy absorption applications.

Numerical Study on Impact Resistance of Nonuniform Nacre-patterned Multi-layer Structures

“…이태희 ․ 고권환 ․ 홍정욱 한국전산구조공학회 논문집 제35권 제4호(2022.8) 221 고,         는 상대적 부피 변화이다.TPU 재료의 변형률 속도 효과를 고려하려면, 인장 시험의 결과로 얻어진 TPU 재료의 s-s 곡선(응력-변형률 곡선)이 사용 되어야 한다(Shivakumar et al, 2014). Guo 등(2016)은 준 정적…”