Evolution of Prony Parameters for Underfills Subjected to High Temperature Long Term Aging

Advanced drive assist systems and support power systems reside underhood where operating temperatures are much higher than in traditional consumer applications. Temperatures in automotive underhood electronics may range from −40 °C to +150°C for long periods of time during operation. Much of the advanced functionality is enable through the use of advanced architectures including flip-chip ball-grid arrays. Underfills are used to enhance the solder joint reliability between the chip and the substrate. However, there is insufficient information about the viscoelasticity of Underfills stored in sustained high temperature for long period of time. In this paper, two different types of Underfills have been cured and aged under two different temperatures: 100 °C and 150 °C. Multi-frequency scan dynamic mechanical analyzer (DMA) test has been conducted to study the viscoelasticity evolution from pristine, 30 days, 60 days, 90 days and 120 days. The master curve has been obtained and the pony pairs of UFs have been calculated. The linear viscoelastic behavior of two kinds of Underfills as the function of aging time and aging temperature has been investigated. Elastic modulus, loss modulus and glass transition temperature are extracted from the results of dynamic loading tests. The aging effect of linear viscoelasticity has been discussed.

Viscoelastic Behavior of an Epoxy Resin Modified with Recycled Waste Particles Analyzed through a Fractional Model

It is well-known that the addition of randomly dispersed particles in polymers influences their linear viscoelastic behavior and dynamic mechanical properties. The aim of this study was to describe the viscoelastic behavior of an epoxy resin modified by waste glass and rubber particles using the linear fractional spring-pot model. Unlike complex classical exponential models, fractional models, being only two-parameter dependent, make it easier to characterize the viscoelastic behavior of materials. Isothermal relaxation and single frequency sweep temperature dynamic tests were carried out in a dynamic mechanical analyzer DMA150 by varying the content of the particles from 0 to 20% by weight. Overall, the results of this study evidence that using waste materials as additives for polymer compounds is a practical and sustainable possibility when it comes to modifying their viscoelastic properties.

Thermo-Viscoelastic Response of Protein-Based Hydrogels

Because of the bioactivity and biocompatibility of protein-based gels and the reversible nature of bonds between associating coiled coils, these materials demonstrate a wide spectrum of potential applications in targeted drug delivery, tissue engineering, and regenerative medicine. The kinetics of rearrangement (association and dissociation) of the physical bonds between chains has been traditionally studied in shear relaxation tests and small-amplitude oscillatory tests. A characteristic feature of recombinant protein gels is that chains in the polymer network are connected by temporary bonds between the coiled coil complexes and permanent cross-links between functional groups of amino acids. A simple model is developed for the linear viscoelastic behavior of protein-based gels. Its advantage is that, on the one hand, the model only involves five material parameters with transparent physical meaning and, on the other, it correctly reproduces experimental data in shear relaxation and oscillatory tests. The model is applied to study the effects of temperature, the concentration of proteins, and their structure on the viscoelastic response of hydrogels.

Strain Sweep Fatigue Testing of Sand Asphalt Mortar to Investigate the Effects of Sample Geometry, Binder Film Thickness, and Testing Temperature

This study evaluated the viscoelastic fatigue behavior of binder using strain sweep fatigue testing of cylindrical sand asphalt mortar (SAM) samples. The SAM samples can represent the realistic film thickness (such as 10–70 μm thick) of the binder in mixtures, while testing repeatability-efficiency can still be met as a result of the use of a standard sand as a load carrier between binder films. A proper testing protocol is still under development, and one of the unknowns in the field is a set of testing conditions that can provide repeatable and case-sensitive test results. Toward that end, SAM samples with different dosages of binder in three geometries were tested at varying temperatures in this study. A PG 64-34 binder was used, and a strain sweep test using a dynamic shear rheometer was conducted to compare the test results from the different cases. Four parameters resulting from the SAM testing were examined: two accounting for material linear viscoelastic behavior and two accounting for fatigue damage characteristics. The parameters were incorporated with statistical analyses to quantitatively evaluate data variability and sensitivity influenced by the binder film thickness, SAM specimen geometry, and testing temperature. The coefficient of variation was less than 20% for all the cases, which indicated the validity of the SAM method attempted in this study. Results also indicated that fatigue behavior was independent of the SAM geometries used in this study, while binder film thickness and testing temperature significantly affected test results.

Structural response and sensitivity analysis of granular and asphaltic overlayment track considering linear viscoelastic behavior of asphalt

This study investigated the structural response of granular and asphaltic overlayment of rail track considering the linear viscoelastic behavior of asphalt. The calculation of the tensile strains at the bottom of the asphalt layer, the compressive stresses at the top of the subgrade layer, and the service life of the granular and the asphaltic overlayment rail track were conducted using the KENTRACK software. Furthermore, the sensitivity analysis by changing different factors was studied in this paper. The results of this study indicate that the asphaltic overlayment rail track structure has a much longer predicted service life than the granular rail track. It was also shown that the sub-grade compressive stress is more sensitive to the change in subgrade modulus than the change in ballast-sub-ballast-asphalt layer thickness and the change in binder type, respectively. In addition, the asphalt tensile strain is more sensitive to the change in asphalt layer thickness than the change in subgrade modulus and the change in binder type, respectively. These findings also enhance our understanding that subgrade compressive stress and asphalt tensile strain in the asphaltic overlayment track are more sensitive to the change in asphalt layer thickness than the change in binder type.

Keratins determine network stress responsiveness in reconstituted actin-keratin filament systems

The cytoskeleton is a major determinant of cell mechanics, a property that is altered during many pathological situations. To understand these alterations, it is essential to investigate the interplay between the main filament systems of the cytoskeleton in the form of composite networks. Here, we investigate the role of keratin intermediate filaments (IFs) in network strength by studying in vitro reconstituted actin and keratin 8/18 composite networks via bulk shear rheology. We co-polymerized these structural proteins in varying ratios and recorded how their relative content affects the overall mechanical response of the various composites. For relatively small deformations, we found that all composites exhibited an intermediate linear viscoelastic behavior compared to that of the pure networks. In stark contrast, the composites displayed increasing strain stiffening behavior as a result of increased keratin content when larger deformations were imposed. This strain stiffening behavior is fundamentally different from behavior encountered with vimentin IF as a composite network partner for actin. Our results provide new insights into the mechanical interplay between actin and keratin in which keratin provides reinforcement to actin. This interplay may contribute to the overall integrity of cells, providing an explanation for the stability of stressed epithelial tissues due to their high keratin contents. Additionally, this helps us to understand the physiological necessity to exchange IF systems during epithelial-mesenchymal transition (EMT) in order to suppress strain stiffening of the network, making cells more elastic and, thus, facilitating their migration through dense tissues.

Oscillating shear capillary rheometry (OSCAR) for polymer melts

Knowing the flow parameters of a polymer melt under steady state condition is required to assess the performance of the material in die and mold design. Often, however, this is not sufficient for a full understanding of the polymer processing behavior, and information on the linear and non-linear viscoelastic behavior is needed. In this paper, the non-linear viscoelastic behavior of a polymer under shear flow has been investigated by measuring the stress response when a cyclic oscillating shear rate in a capillary rheometer is applied. The time-dependent wall shear stress has been decomposed into in-phase viscous and elastic components. A model to interpret the experimental results is presented and applied to a well-characterized polystyrene and two polyethylenes with similar rheology but different molecular structure (HDPE and LLDPE). The relevant characteristics resulting from the model, such as the generalized elastic and viscous modulus under shear, are compared and discussed.

High-Temperature Performance Evaluation of Asphaltenes-Modified Asphalt Binders

Asphalt binder comprises four main fractions—asphaltenes (A), saturates (S), aromatics (A), and resins (R)—referred to as “SARA”. Asphaltenes plays an important role in determining the linear viscoelastic behavior of asphalt binders. In this research, asphaltenes are added as a distinct modifier to improve the performance properties of asphalt binder. The modified binders are aged using a rolling thin film oven. A dynamic shear rheometer is then used to measure the rheological properties of the binders at high temperatures. Changes in the chemical composition of the modified binders are also studied through the determination of SARA fractions, using precipitation and gravity-driven chromatography methods. The rheological results show that asphaltenes improve the stiffness and elasticity of asphalt binder. It is also shown that the addition of asphaltenes raises the high Performance grade (PG) temperature of the asphalt binder, with every 6% of asphaltenes added resulting in a one-interval increase in high PG temperature grade. SARA analysis shows that the increase in polar fraction content due to the addition of asphaltenes causes the stiffness, elasticity, and viscosity of asphalt binders to increase. The results indicate that asphaltenes are an effective yet inexpensive additive to improve asphalt binder properties at high temperatures.