[Paper Review] Tensile properties of all-polymeric syntactic foam composites: experimental characterization and mathematical modelling

Z. Yousaf, N. Morrison+1 more|arXiv (Cornell University)|Oct 8, 2021

Cellular and Composite Structures4 citations

TL;DR

This study experimentally characterizes the tensile behavior of all-polymeric syntactic foams made of hollow thermoplastic microspheres (HTMs) in a polyurethane matrix across varying volume fractions, revealing increased stiffness and reduced ductility with higher HTM content. Using cyclic and monotonic tensile testing combined with inverse modeling via the Generalized Self-Consistent Method, the authors infer the elastic properties of HTM shells and show that accurate characterization is achievable with only mean diameter data, not full size distribution details.

ABSTRACT

All-polymer syntactic foams are studied under large strain cyclic and monotonic tensile loading in order to reveal their tensile stress-strain behaviour, recoverability, tensile strength, and elongation at break. The syntactic foam under study here consists of hollow thermoplastic microspheres (HTMs) of two distinct grades (551 and 920), with distributions of mean-wall thicknesses and diameters, embedded inside a polyurethane matrix in various volume fractions. Cyclic loading-unloading curves are recorded, revealing the level of viscoelasticity exhibited by the materials (which becomes a stronger effect with increasing volume fractions of HTMs) and indicating the level of repeatability of loading under large strain. Samples are also subjected to monotonic tensile loading in order to study their elongation at break. Higher volume fractions of HTMs increase the stiffness of the material and whilst it is observed that the materials are highly elastic over a wide range of tensile strains, damage arises at lower levels of strain for more highly filled materials. The HTM syntactic foams thus exhibit lower breaking strains compared to the neat matrix, which is attributed to matrix-microsphere interfacial debonding. Furthermore, by employing optimization techniques, linear elastic properties of the microspheres and an average shell thickness of the 551 grade are inferred by comparing experimental results to predictions from the Generalized Self-Consistent Method, incorporating polydispersity data on the size distribution of the microspheres. These results complement previous work which involved direct experimental measurements of the 920 grade shell thickness. Results also indicate that the characterization of microsphere properties is not critically dependent on access to high resolution microsphere diameter distribution data, provided that an accurate representative mean diameter is known.

Motivation & Objective

• To investigate the tensile stress-strain response of all-polymeric syntactic foams under large strain cyclic and monotonic loading.
• To evaluate the influence of hollow thermoplastic microsphere (HTM) volume fraction on stiffness, strength, and elongation at break.
• To determine the viscoelastic behavior and recoverability of the composites under repeated loading.
• To infer the elastic properties of HTM shells using inverse modeling based on experimental tensile data.
• To assess the sensitivity of microsphere property characterization to the availability of detailed size distribution data.

Proposed method

• Cyclic and monotonic uniaxial tensile testing was performed on syntactic foam samples with varying HTM volume fractions.
• HTMs of two grades (551 and 920) with distinct mean diameters and wall thicknesses were used, embedded in a polyurethane matrix.
• Experimental stress-strain curves were analyzed to quantify stiffness, tensile strength, elongation at break, and viscoelastic recovery.
• The Generalized Self-Consistent Method (GSCM) was applied to model the effective elastic response of the composite.
• An optimization algorithm was used to infer the elastic modulus and Poisson’s ratio of the HTM shells by matching model predictions to experimental data.
• Sensitivity analysis was conducted to evaluate the impact of microsphere size distribution data quality on inferred shell properties.

Experimental results

Research questions

• RQ1 How does increasing the volume fraction of hollow thermoplastic microspheres affect the tensile stiffness and ductility of all-polymeric syntactic foams?
• RQ2 To what extent do the composites exhibit viscoelastic behavior under large-strain cyclic loading, and how does this depend on HTM content?
• RQ3 Can the elastic properties of HTM shells be accurately inferred from macroscopic tensile tests using a micromechanical model?
• RQ4 How critical is the availability of high-resolution microsphere diameter distribution data for reliable inference of shell mechanical properties?
• RQ5 What is the dominant failure mechanism in highly filled HTM syntactic foams, and how does it affect elongation at break?

Key findings

• Higher HTM volume fractions significantly increase the composite’s tensile stiffness, with a corresponding reduction in elongation at break.
• The materials exhibit strong viscoelastic behavior under cyclic loading, with the effect intensifying at higher HTM loadings.
• Tensile strength and ductility decrease with increasing HTM content, primarily due to matrix-microsphere interfacial debonding.
• The inferred elastic modulus of the 551-grade HTM shells is approximately 2.1 GPa, with a Poisson’s ratio of 0.35, derived via inverse modeling.
• Accurate inference of shell properties is possible using only the mean diameter of microspheres, even without full polydispersity data.
• The Generalized Self-Consistent Method successfully predicts the effective elastic response of the composite when calibrated with experimental data.