Lidvin Kjerengtroen

Determining the interphase thickness and properties in polymer matrix composites using phase imaging atomic force microscopy and nanoindentation

In polymer matrix composites, the interface between the reinforcing phase and the bulk phase is paramount to the overall performance of the composite as a structural material. This interface is now thought to be a distinct, three-dimensional phase surrounding the reinforcing phase called the interphase. The developments of the atomic force microscope and nanoindentation devices have facilitated the investigation of the interphase. Previously, force modulation atomic force microscopy (AFM) and nanoindentation were the primary methods used to determine the size of the interphase and its stiffness relative to the bulk phase. The present investigation utilized phase imaging AFM and nanoindentation to examine the interphase in a glass fiber-reinforced epoxy matrix composite. Nanoindentation experiments indicated that the relatively stiff fiber might have caused a gradient in the modulus across the interphase region. Specifically, the modulus next to the fiber approached that of the fiber and decreased to that of the bulk polymer as the distance away from the fiber increased. Once the fiber was removed by chemical etching, this gradient reversed itself; hence, nanoindentation, due to the fiber bias, was not found to be adequate for measuring actual interphase properties. It was found that phase imaging AFM was a highly useful tool for probing the interphase, because it involves much lower interaction forces between the probe and the sample than force modulation or nanoindentation. The interphase in the model composite investigated was found to be softer than the bulk phase with a thickness of 2.4-2.9 μm, and was independent of fiber silane pretreatment, for silane pretreatments between 0.1% and 5.0% (initial aqueous concentration).

Interphase variation in silane-treated glass-fiber-reinforced epoxy composites

The interphase region of an epoxy/glass fiber model composite was examined by atomic force microscopy phase imaging (AFM-PI) and nanoindentation. The interphase thickness was determined by AFM-PI as a function of γ-aminopropyl silane coupling agent concentration. With no silane, no measurable interphase was observed. With adsorption from 5 wt% solution, the observed interphase was 888 ± 30.3 nm thick. Coupling agent adsorption was also performed from 0.1, 1 and 3 wt% silane solutions. The interphase thickness was found to increase with increasing silane solution concentration from 110 to 210 to 375 nm, respectively. Nanoindentation of these same interphases showed that only the 3 wt% and 5 wt% interphases were sufficiently thick enough to not include a significant fiber bias effect. For these two interphases, the indentation depths in the interphase were 8.3% and 42% greater, respectively, than the indentation depth in the matrix.

Finite element evaluation of the microbond test: meniscus effect, interphase region, and vise angle

Abstract An axisymmetric finite element model (FEM) was used to determine the stresses that develop during a microbond test of a glass fiber/polymer matrix composite system. The complete bead shape including the meniscus that occurs as the bead wets to the fiber was included in the FEM to determine the influence of the bead geometry on the resulting stresses. Significant differences in the stress fields were found near the fiber/bead contact point due to the geometry and resulting z-location of the vise. In addition, the vise angle and interphase properties were varied to examine their influence. As the vise angle increases, the combined state of stress decreases considerably. The effect of the interphase, while not as significant as the effect of the vise angle, did produce a 10% variation in von-Mises stress at the fiber/bead contact point for the properties used. It was found that the ratio of the maximum shear to average shear equaled 4 near the fiber/bead contact point. At this location the interfacial material has yielded with a von-Mises stress 8× the average shear. Finally, results are provided showing the average interfacial shear stress value obtained by the FEM is within 2% of the theoretical solution.

MICROHARDNESS TESTING OF FIBER-REINFORCED CEMENT PASTE

Microhardness testing of polyolefin fiber-reinforced Type III cement paste was performed. Type III cement with two water-to-cement ratios (w/c)--0.38 and 0.46--was tested. For 0.38 w/c cement aged for 7 days, the mean Vickers microhardness found with a 0.981 N load was 620 MPa. For 0.46 w/c cement, the mean Vickers hardness with a 100 gf load was determined to be 435 MPa. The Vickers microhardness data obtained at both w/cs were not normally distributed, but were, instead, distributed according to a lognormal distribution. The data at each w/c, however, displayed the same mean and variance at 95% confidence, as determined by the Kruskal-Wallis test. The indentation size effect was quantified, and the exponent n was found to be 2.24; while the load necessary to give a 2 micrometer indentation diagonal KL was 0.062 N. The minimum edge-to-edge distance between indentations at which the indentations did not affect each other was measured and found to be less than 12.5 micrometers, which was the smallest distance that could be measured with the microhardness tester utilized. Also, the Youngs modulus was estimated from Knoop microhardness tests and was found to be approximately 11 GPa for w/c = 0.38, and 18 GPA for w/c = 0.46, although there was no significant difference between the two values at 95% confidence. The second part of the experimental work consisted of microhardness testing in the interfacial transition zone (ITZ) of polyolefin fiber-reinforced Type III cement paste. The Knoop microhardness of the ITZ was measured using 0.245 N load. The mean Knoop microhardness (HK25) of the bulk paste was found to be 542 MPa. Within the ITZ, a statistically less hard region was found to exist approximately 40-65 micrometers from the polyolefin fiber surface. The mean HK25 of this region was 393 MPa. The region less than 40 micrometers from the surface could not be investigated because of the indenter head impinging on the fiber.

The effect of interphase curing on interphase properties and formation

Fiber-optic evanescent wave FTIR spectroscopy was combined with phase imaging AFM to examine two thermosetting polymer matrix composite systems. The epoxy/NMA system data from the fiber-optic, evanescent wave FTIR analysis showed incomplete curing (∼75% complete) in the region near the fiber, but essentially complete (∼95% complete) curing in the bulk. Conversely, the unsaturated polyester system exhibited essentially complete curing (∼95% complete) both near the fiber and in the bulk material. For the same samples, phase imaging AFM indicated that the epoxy/NMA system had an ∼2.5 micron thick interphase, while the unsaturated polyester system showed no interphase between the fiber and the matrix. Therefore, the presence of the interphase in the epoxy/NMA system can be attributed to the incomplete curing next to the fiber. In addition, the systems chosen allowed the reactivity of adsorbed γ-APS coupling agent to be assessed simultaneously with polymer curing. For the epoxy/NMA system, the amine band decreased about 54% during curing. For the polyester system, the amine band decreased 43% during curing.

Achieving Dimensional Stability Using Functional Fillers

Several materials exhibiting negative coefficients of thermal expansion have recently been identified. Such materials incorporated as filler in a polymer matrix should result in a composite with reduced, if not zero, expansion when exposed to elevated temperatures. One such filler material, ZrW2O8, has a highly negative coefficient of thermal expansion and its cubic structure leads to isotropic thermal behavior. Consequently, Zr2WO8 was investigated as functional filler for polymer matrices, with the goal being improved dimensional stability of the filled system. The choice of polymeric matrix material is also crucial when seeking dimensional stability, and fabrication of a useful end product. In this regard, several aerospace grade epoxies along with a novel cyanate ester resin were investigated. The dynamic mechanical properties along with thermal behavior of the filled systems were compared. A cyanate ester/ZrW2O8 system exhibited the lowest coefficient of thermal expansion (CTE), and consequently was investigated in most detail. Cylindrical samples of the cyanate ester/ZrW2O8 system were molded and cured per schedule. The ZrW2O8 particles were allowed to settle at their natural rate. Thus, the final samples contained various weight percentages and sizes of ZrW2O8. The sectioned samples allowed investigation of filler percent and how it impacts overall CTE. Decreases in CTE of ~80% are reported for samples containing ~82 wt% ZrW2O8. The lowest value observed for composite CTE was 2.8 ppm/K at 32 °C for a sample having 83% zirconium tungstate, which had been subject to one temperature cycle..

Strain monitoring by evanascent wave spectroscopy

Prior interferometric fiber sensors and evanescent wave fiber sensors have proven useful in obtaining information about portions of the lifetime of a composite materials. The overall goal of this research is to develop an IR evanescent wave sensor system that can be used to monitor lifetime of a polymer matrix composite. In this regard, a single fused silica core fiber was placed across a miniature materials tester, while simultaneously having the fiber ends attached to an IR spectrometer. The fiber was strained in increments by the MINIMAT, while the IR spectrometer allowed simultaneous determination of the IR spectrum. An increase in baseline absorbance across the entire IR spectrum occurred as the strain increased. The increase in absorbance is relate to an increase in strain in the fiber. From regression analysis of independent measurements of fiber strain and absorbance, a strong relation between the change in absorbance and change in strain energy was found. Future work will involve incorporation of the strain sensing approach with evanescent wave chemical sensing to allow total lifetime monitoring of polymer matrix composites.

Impact and Lap Shear Properties of Ultrasonically Spot Welded Composite Lap Joints

Ultrasonic spot welding (USSW) is a widely used technique for joining thermoplastics where high frequency, low amplitude vibrations are applied through an ultrasonic horn resting on the polymer surface to create frictional heat, producing a solid state joint between polymer sheets. Advantages such as short weld cycle time, fewer moving components and reproducibility make this technique attractive for automation and industrial use. The goal of this work was to evaluate the feasibility and analyze the lap shear and impact strength of a composite material joint created using ultrasonic spot welding. The base material used for the joints was a composite consisting of a polycarbonate matrix with chopped glass fibers. The strength of the lap joints was determined through experimental lap shear and impact testing. A finite element analysis was conducted for more thorough insight into the stress patterns in the lap joints. Experiments showed that the ultrasonically spot welded joints tested in tensile lap shear loading carried a load 2.3 times higher than adhesive joints and the impact tested joints had an impact strength 3.5 times higher than adhesive joints. The results of this work suggest ultrasonic spot welding as a viable joining method for thermoplastic composite materials.

Estimation of the True Interfacial Shear Strength for Composite Materials With the Microbond Test

Conservative estimates of the true interfacial shear strength (IFSS) were obtained by applying the Coulomb-Mohr failure theory to microbond test data in this combined experimental and computational study. Experimentally, interfacial strengths of unsaturated polyester on untreated and silane treated optical glass fibers were measured with an axisymmetric microbond test system. Commonly reported average IFSS values were 6.51 MPa for untreated fibers and 8.01 MPa for silane treated fibers with coefficients of variation that ranged from 9.7–22%, which was an improvement from the previous parallel blade system that ranged from 17–66%. Axisymmetric finite element analysis (FEA) was completed with the aid of a Microbond Input Generator program that reduced model development and analysis time by 97% from 1000 minutes to under 30 minutes making this feasible to perform FEA on every experimental microbond sample. FEA models include the complex microbond bead geometry, contact loading conditions, and constituent material properties measured according to ASTM standards. Application of the Coulomb-Mohr failure theory revealed a conservative estimate of the true IFSS in the absence of compressive radial stresses to be 19.2 MPa for untreated fibers and 25.1 MPa for silane treated fibers. Comparisons are also made with the maximum IFSS value.Copyright

The Effect of Induced Vibration on Thermoplastic Composite System

Thermoplastic composite manufacturing is often difficult due to high viscosity of the matrix materials. Coupling a high level of mechanical properties with simple, low-cost processing technique is a difficult subject, but an important task for any state-of-the-art impregnation processes. In this paper, Thermoplastic Prepreg Fabrication Technology is utilized to prepare thermoplastic tapes and the technologys effect on strain energy absorption was investigated. The tape was prepared under three categories: first, with induced vibration and no fiber preheat (NV), second, without vibration and fiber preheat (HN) and last, with both fiber preheat and vibration (HV). For the purpose of comparison, all other variables such as pulling speed, fiber tension, fiber preheat and processing temperature were kept constant. The HV category showed improvement in the strain energy absorption by 10 and 23% when compared to HN and NV, respectively. In addition, HV had better wetting, fiber spread and dispersion. Fiber preheating is important as it worked well with vibration possibly due to good fiber spread on the HV category (widest tape). Also, HV had the least fiber volume fraction as it takes more matrix volume when exiting the die plate.