Wiroj Nantasetphong

Determining the Shear Relaxation Modulus and Constitutive Models for Polyurea and Polyurea-Based Composite Materials from Dynamic Mechanical Testing Data

Polyurea and polyurea-based composite materials are widely used due to their excellent mechanical properties. In order to facilitate large-scale computational studies for this group of materials, a robust and standard method is needed to extract their viscoelastic constitutive parameters. In this study, frequency-domain master curves which cover a wide range of frequencies are developed using the data of dynamic mechanical analysis through time-temperature superposition (TTS). The quality of the master curves is assessed both by Kramers-Kronig relations and by comparing with the ultrasonic wave testing data. Then the time-domain relaxation modulus is obtained by the high-resolution Prony series approximated from the relaxation spectrum. To reduce computational cost, 4 to 8-term Prony series are then fitted from the time-domain relaxation modulus for a limited frequency range of interest. Both the high and low-resolution Prony series are converted back to frequency domain to compare with the master curves developed by TTS and show good agreements. This method is not limited to polyurea and polyurea-based composites and it can be applied to other similar polymer systems as well.

Polyurea-Based Composites: Ultrasonic Testing and Dynamic Mechanical Properties Modeling

Many scientists and researchers study polyurea due to its excellent blast-mitigating properties. In this work, we have studied two polyurea composite systems with filler materials intended to improve dynamic mechanical properties. The two filler materials are milled glass and fly ash. The shape and quantity of filler significantly affect the dynamic mechanical properties of the composite. Ultrasonic tests were conducted on samples with both fillers. The volume fraction of the inclusions was varied to study the effect of filler quantity on mechanical properties. Moreover, computational models based on the methods of dilute-randomly-distributed inclusions and periodically-distributed inclusions were created to improve our understanding of polyurea-based composites and serve as tools for estimating the dynamic mechanical properties of similar composite material systems. The experimental and computational results were compared and show good agreement. The experiments and modeling have been conducted to facilitate the design of new elastomeric composites with desirable impact- and blast-mitigating properties.

Storage and Loss Moduli of Low-Impedance Materials at kHz Frequencies

Standard Dynamic Mechanical Analysis (DMA) is generally used to measure the mechanical properties of polymers at frequencies around and below 100 Hz. Ultrasonic (US) techniques measure wave speeds and impedances at higher frequencies. However, both approaches run into issues between the two regimes. DMA systems become less reliable due to the dynamic response of the frames and load path as one tries increasing the frequency. On the other hand, the internal multiple reflections in the wave propagation techniques introduce challenges in clean measurements and require careful analysis. In this presentation, we introduce a robust procedure for determining the storage and loss moduli of low-impedance materials, where a cylindrical sample is placed between two long metal bars, similar to SHPB technique. However, unlike SHPB, the incidence signal is created by a very light impact, to ensure that the sample does not experience permanent or large deformation. Furthermore, due to the length of the specimen, dynamic equilibrium is neither guaranteed nor intended. The reflected and transmitted pulses are measured using semi-conductor strain gages. The wave speed may be determined using a phase spectral analysis of the time-resolved signals. Determination of the material loss requires a more thorough transfer matrix analysis. The method was applied to a soft polyurea elastomer that was tested in a temperature-control chamber and results were compared with DMA and US data using time-temperature superposition (TTS). While the predictions of the storage modulus using DMA and TTS matched very well with the direct measurements, the DMA/TTS predictions generally underestimate the material loss at higher frequencies. We expect that this method may be applied successfully to other low impedance materials including foams and metamaterials.

9 – Modification and Engineering of HSREP to Achieve Unique Properties

Polyurea is a microphase-separated block copolymer that exhibits unique properties as a result of its phase-separated morphology. Experimental results suggest and computational simulations support that judicious arrangement of steel and polyurea can enhance shock mitigation. Schlieren imaging techniques can be used to qualitatively investigate the behavior of shock waves. The properties and performance of polyurea can be significantly modified by the integration of additives, and micromechanical modeling can aid in the design of such composites with specific properties. For polymers and their composites, temperature, pressure, and strain rate usually affect the mechanical properties of the material at the same time. However, direct measurements of the mechanical properties for the high-rate loadings are often difficult to perform. Thus, alternative methods are used to predict and estimate the mechanical properties. Full-scale head–neck manikins were fabricated in order to assess comparative intracranial pressure and acceleration from explosive blast. A range of standoffs to the explosive were used. Three helmet variations were examined: the standard advanced combat helmet (ACH), an ACH helmet with a polymer coating, and a standard World War II, M1 helmet. Use of a helmet decreased the pressure at different locations within the brain. All of the helmets resulted in similar peak pressure and resultant acceleration values. However, the ACH helmet with a polymer coating decreased the time of pressure application significantly, lowering the applied impulse at the closest standoffs. The lower impulse would also contribute to a decrease in applied power density in these portions of the brain. Use of a polymer coating provides a basis to explore other helmet variations to provide protection from traumatic brain injury.

Low-Density, Polyurea-Based Composites: Dynamic Mechanical Properties and Pressure Effect

In this study, we explore the fabrication, characterization and modeling of low-density polymeric composites to understand their acoustic responses. Polyurea is chosen as the matrix of the composites due to its excellent properties and advantages, i.e. blast mitigation, easy casting, corrosion protection, abrasion resistance, and various uses in current military and civilian technology. Two low-mass-density filler materials of interest are phenolic and glass microballoons. They have significant differences in their mechanical properties and chemical interactions with the matrix. Ultrasonic tests are conducted on samples with different volume fractions of fillers and variable pressure. Computational models based on the methods of dilute randomly distributed and periodically distributed inclusions are created to improve our understanding of low-density polymer-based composites and serve as tools for estimating the dynamic mechanical properties of similar composite material systems. The experimental and computational results are compared. The results are expected to facilitate the design of new elastomeric composites with desirable densities and acoustic impedances. These new composites will be useful in developing layered metamaterial structures. Furthermore, we seek to find out whether such inclusions may substantially affect the time-dependent response of the composite by introducing new resonant modes.

Pressure-invariant non-reflective and highly dissipative acoustic metamaterials

Metamaterials have shown great potential to transform the design of acoustic components in many applications. Many composites with extreme properties have been envisioned, designed, fabricated, and experimentally verified. The next step involves testing such composites in realistic application environments. Oceans are one such environment in which the mechanical effects of water pressure and flow become important factors in any acoustic design, particularly for soft dissipative shells. We have designed a layered metamaterial composite that not only shows very high dissipation but also matches the acoustic impedance of water. Furthermore, we have experimentally verified that the relevant properties of the constituents of this layered design do not change under pressure levels that exist down to significant depths. We are in the process of fabricating this composite to test its acoustic properties under pressure. The metamaterial composites lend themselves naturally to multi-component designs, examples of w...

Modifying the acoustic impedance of polyurea-based composites

Acoustic impedance is a material property that depends on mass density and acoustic wave speed. An impedance mismatch between two media leads to the partial reflection of an acoustic wave sent from one medium to another. Active sonar is one example of a useful application of this phenomenon, where reflected and scattered acoustic waves enable the detection of objects. If the impedance of an object is matched to that of the surrounding medium, however, the object may be hidden from observation (at least directly) by sonar. In this study, polyurea composites are developed to facilitate such impedance matching. Polyurea is used due to its excellent blast-mitigating properties, easy casting, corrosion protection, abrasion resistance, and various uses in current military technology. Since pure polyurea has impedance higher than that of water (the current medium of interest), low mass density phenolic microballoon particles are added to create composite materials with reduced effective impedances. The volume fraction of particles is varied to study the effect of filler quantity on the acoustic impedance of the resulting composite. The composites are experimentally characterized via ultrasonic measurements. Computational models based on the method of dilute-randomly-distributed inclusions are developed and compared with the experimental results. These experiments and models will facilitate the design of new elastomeric composites with desirable acoustic impedances.

Experimental Investigation of Dynamic Mechanical Properties of Polyurea-Fly Ash Composites

Polyurea has been the material of choice in many applications due to its thermomechanical properties. These applications span a wide spectrum from abrasion-resistant coating to reinforcement against blast damage in structures, ships, and vehicles. The improved observed performance is linked to its microstructure, a lightly crosslinked (elastomeric) block copolymer. The constitutive modeling of such materials is generally based on a wide variety of experimental measurements, including dynamic mechanical analysis. Furthermore, the response under stress-wave propagation may be measured through ultrasonic tests. In this work we present our research towards improvement and modification of the dynamic mechanical properties of polyurea through inclusion of various size and distributions of fly ash hollow spherical particles. The extensive experimental results are reported and a micromechanical homogenization model is presented. The extent of application of such model will be established and alternative modeling techniques which include the possible inertia effects at high rates of deformation will be surveyed.