Martin Y.M. Chiang

Combinatorial and high-throughput measurements of the modulus of thin polymer films

We describe the design and refinement of a high-throughput buckling-based metrology for ascertaining the mechanical properties (e.g., modulus) of combinatorial thin polymer film libraries. We provide critical details for the construction of a suitable strain stage, describe sample preparation, and highlight methods for high-throughput data acquisition and data analysis. To illustrate the combinatorial and high-throughput capability of this metrology, we prepare and evaluate films possessing a gradient in the elastic modulus and compare the results with an analytical expression derived from composite beam theory. Application of this metrology is very simple and practically any laboratory, academic or industrial, can perform such measurements with only modest investment in equipment. Although developed as a platform for investigating combinatorial libraries, researchers can take advantage of the high-throughput nature of this metrology to measure noncombinatorial film specimens as well.

A microshear test to measure bond strengths of dentin-polymer interfaces

The microbond test. a single fiber shear test, has been adapted to be a microshear test for the measurement of the adhesion of resin-based dental materials to dentin and enamel. The objective of this study is to improve the design of this microshear test so that it can provide accurate and reliable shear bond strength data. In the current design of the microshear test apparatus, the bonding diameters of the specimens have been as small as 0.70 mm. The smaller diameters give researchers the ability to test several bonded specimens on one flat dentin or enamel surface, thus allowing both for the regional mapping of the mineralized surface and the conservation of extracted teeth needed to provide the necessary substrates. The test corfiguration used in earlier studies has been modified through finite element analysis to address concerns in the test methodology. The results of this study show that the microshear bond test can be a useful tool in helping to understand the complex interactions that occur at the interface between dental composites and dentin and/or enamel surfaces, especially at interfacial sites not amenable to macroshear testing.

Plastic deformation analysis of cracked adhesive bonds loaded in shear

Abstract The plane strain elastoplastic stress field around an interface crack in adhesively bonded joints deforming in shear was determined from a large strain, incremental plasticity finite element analysis. Two particular specimens were analysed, i.e. the end-notched flexure and the end-loaded split, with the bond thickness varying from 18 μm to 0.4 mm. The yield behavior of the adhesive was modeled by the von Mises ( J 2 ) and the extended Drucker-Prager (EDP) material models, the latter being more appropriate to polymeric adhesives. Associated and non-associated flow rules were considered for the J 2 and EDP models, respectively. The adhesive stress-strain response was assumed to be elastoplastic, and it incorporated various levels of strain hardening. The analysis shows that the stresses at the crack tip are triaxial, with the deformations dominated by the shearing component, the latter being localized at the very edge of the crack tip, an effect which tended to increase with increasing bond thickness or decreasing degree of strain hardening. The numerical predictions of the length of the plastic zone that developed ahead of the crack tip and of the distribution of average shear strain across the bond within that zone agreed well with experimental results. In contrast with the behavior for the analogous mode I loading case, the mean stress declined monotonically with increasing distance from the crack tip.

Cell morphology and migration linked to substrate rigidity

A mathematical model, based on thermodynamics, was developed to demonstrate how substrate rigidity influences cell morphology and migration. The mechanisms by which substrate rigidity are translated into cell-morphological changes and cell movement are described. The model takes into account the competition between the elastic energies in the cell-substrate system and work of adhesion at the cell periphery. The cell morphology and migration are dictated by the minimum of the total free energy of the cell-substrate system. By using this model, reported experimental observations on cell morphological changes and migration can be better understood with a theoretical basis. In addition, these observations can be more accurately correlated with the variation of substrate rigidity. This study indicates that the activity of the adherent cell is dependent not only on the substrate rigidity but also is correlated with the relative rigidity between the cell and substrate. Moreover, the study suggests that the cell stiffness can be estimated based on the substrate stiffness corresponding to the change of trend in morphological stability.

A CRACK PROPAGATION CRITERION BASED ON LOCAL SHEAR STRAIN IN ADHESIVE BONDS SUBJECTED TO SHEAR

Direct observations show that the fracture of thin adhesive bonds subjected to shear is characterized by stable crack propagation along the interface followed by catastrophic growth. Other failure processes are observed during the stable growth phase, including crack kinking, development of a large void at the crack tip and formation of a detrimental interface micro-debond ahead of the main crack. The specific choice depends on factors such as the position of the precrack plane within the adhesive layer, the layer thickness and the loading level. Regardless of the specimen geometry, the entire interfacial crack propagation event is controlled by a single parameter-a critical shear strain at the crack tip that is independent of the bond thickness and the shearing direction but is a (decreasing) function of the crack velocity. The results can be useful in the application of fracture mechanics to the design of microlaminates, composites, traditional adhesive bonding and other technologies in which thin adhesive layers are used. Published by Elsevier Science Ltd

An analytical assessment of using the losipescu shear test for hybrid composites

Abstract A theoretical evaluation of the applicability of the Iosipescu test (v-notch shear test) has been conducted for hybrid composites having unidirectional glass and carbon fiber tows that are intimately mixed, instead of being arranged in separate lamina. The v-notch specimen of hybrid composites was analyzed using the finite element method based on the fiber tow properties to evaluate the effect of varied microstructures in hybrids on the shear stress and strain states. The analyses were conducted to determine how closely the test would meet the requirement of an ideal shear test that there should be pure and uniform stress and strain distributions in the test region. The study shows that, theoretically, the v-notch test can be used to determine the shear modulus of the hybrid composites studied when it is correctly used. However, practically, premature failures caused by the stress concentrations near the notch roots can make the test undesirable for determining the shear strength of the hybrid composite.

Bond strength measurement in composites—Analysis of experimental techniques

Abstract Two of the most common techniques used to measure fiber-matrix interfacial shear strength, the single-fiber fragmentation test and the microbond, have been analyzed and compared. Photoelastic and finite element analyses were performed to obtain the stress distribution at the fiber-matrix interface and its dependence on the loading and geometrical parameters. The effect of a penny-shaped crack in the fiber, in the fiber fragmentation test is shown to be one of the parameters governing the interfacial failure mode. It is also shown that loading conditions, meniscus formation, and fiber free length have a large effect on the distribution of interfacial stresses in the case of the microbond, which may explain the large observed scatter of experimental results. Furthermore, it is shown that the effect of the stress distribution is highly non-uniform, thus making the calculation of shear stress very inaccurate when single averages are considered. It appears that the single-fiber fragmentation test is more reliable than the microbond test because of its simplicity and the smaller number of parameters involved in its analysis.

Simultaneous measurement of polymerization stress and curing kinetics for photo-polymerized composites with high filler contents

OBJECTIVES Photopolymerized composites are used in a broad range of applications with their performance largely directed by reaction kinetics and contraction accompanying polymerization. The present study was to demonstrate an instrument capable of simultaneously collecting multiple kinetics parameters for a wide range of photopolymerizable systems: degree of conversion (DC), reaction exotherm, and polymerization stress (PS). METHODS Our system consisted of a cantilever beam-based instrument (tensometer) that has been optimized to capture a large range of stress generated by lightly-filled to highly-filled composites. The sample configuration allows the tensometer to be coupled to a fast near infrared (NIR) spectrometer collecting spectra in transmission mode. RESULTS Using our instrument design, simultaneous measurements of PS and DC are performed, for the first time, on a commercial composite with ≈80% (by mass) silica particle fillers. The in situ NIR spectrometer collects more than 10 spectra per second, allowing for thorough characterization of reaction kinetics. With increased instrument sensitivity coupled with the ability to collect real time reaction kinetics information, we show that the external constraint imposed by the cantilever beam during polymerization could affect the rate of cure and final degree of polymerization. SIGNIFICANCE The present simultaneous measurement technique is expected to provide new insights into kinetics and property relationships for photopolymerized composites with high filler content such as dental restorative composites.

Finite element analysis of interfacial crack propagation based on local shear, part II—Fracture

Al~tract--The mechanics of fracture of a stably extending interface crack in polymeric adhesive bonds undergoing very large shear deformation is studied using a rate-del~ndent finite element analysis. Plane-strain and J2 plasticity conditions are considered. Based on recent experimental observations, it is assumed that the local engineering shear strain at a certain distance (i.e. for the polymer adhesive studied, approximately a tenth bond thickness) straight ahead of the crack tip remains constant during the crack propagation. This critical strain is rate dependent, being a function of the crack velocity. The proposed fracture criterion is applied to several experimental crack growth histories pertaining to different specimen geometries, bond thicknesses and crack velocities. Although the analysis is highly sensitive to rate effects and other material characteristics, the comparison is generally reasonably successful. The analysis also provides quantitative insight into the mechanics of other failure modes observed in the experiments. In particular, the growth of a detrimental micrndebond which is formed several bond thicknesses ahead of the crack tip seem to be controlled by the bond-normal tensile stress while hydrostatic tensile stresses appear responsible for the development of a kink or a large void at the crack tip which temporarily arrests the crack. All these and other failure modes are activated under large strains, which manifests the important rule of plasticity in the fracture of polymeric joints.

Relationships among cell morphology, intrinsic cell stiffness and cell–substrate interactions

Cell modulus (stiffness) is a critical cell property that is important in normal cell functions and increasingly associated with disease states, yet most methods to characterize modulus may skew results. Here we show strong evidence indicating that the fundamental nature of free energies associated with cell/substrate interactions regulates adherent cell morphology and can be used to deduce cell modulus. These results are based on a mathematical model of biophysics and confirmed by the measured morphology of normal and cancerous liver cells adhered on a substrate. Cells select their final morphology by minimizing the total free energy in the cell/substrate system. The key mechanism by which substrate stiffness influences cell morphology is the energy tradeoff between the stabilizing influence of the cell-substrate interfacial adhesive energy and the destabilizing influence of the total elastic energies in the system. Using these findings, we establish a noninvasive methodology to determine the intrinsic modulus of cells by observing global changes in cell morphology in response to substrate stiffness. We also highlight the importance of selecting a relevant morphological index, cell roundness, that reflects the interchange between forms of energy governing cell morphology. Thus, cell-substrate interactions can be rationalized by the underlying biophysics, and cell modulus is easily measured.