Microindentation tests

It has been noted in a round robin test of microcomposites that there arc large variations in test results for an apparently identical fiber and matrix system between 13 different laboratories and testing methods (Pitkethly et al., 1993). Table 3.1 and Fig 3.15 summarize the IFSS values of Courtaulds XA (untreated and standard surface treated) carbon fibers embedded in an MY 750 epoxy resin. It is noted that the difference in the average ISS values between testing methods, inclusive of the fiber fragmentation test, fiber pull-out test, microdebond test and microindentation test, are as high as a factor of 2.7. The most significant variation in ISS is obtained in the fiber pull-out /microdebond tests for the fibers with prior surface treatments, and the microindentation test shows the least variation. 

Desaeger, M. and Verpoest, I. (1993). On the use of the microindentation test technique to measure the interfacial shear strength of fiber reinforced polymer composites. Composites Sci. Technol. 48, 215-226. 

Mencik, J. Swain, M. Errors associated with depth-sensing microindentation tests. Journal of Materials Research 10, 1491-1501 (1995). 

Other methods to evaluate the IFSS have been proposed mainly for synthetic fibers. Among these, the microdebond, also known as the microindentation test, first proposed by MandeU et al. [54], has been theoretically investigated [34, 35] and applied to synthetic and metallic fibers. To the knowledge of the authors of this chapter, the microindentation test has not yet been used to measure the IFSS of lignocellulosic fibers embedded in polymeric resins. In spite of advantages listed by Herrera-Franco and Drzal [34], they also emphasized the following drawbacks for this test ... 

The microindentation test is run on an individual selected fiber oriented normal to a polished cross-section of a high fiber volume fraction composite. Instead... 

Carbon fibers in the cured epoxy was the composite system for the international round robin program on interfacial test method. The comparison of four test methods is given in Table 9.12. The results indicate that within each laboratory the reproducibility is good, with coefficient of variation (CV) less than 10%. However, the CV among the laboratories can be as high as 25%. The indentation method has the smallest percent of CV because two of the laboratories used the same commercial equipment. The sized fibers show 20 MPa higher IFSS than untreated fibers in pull-out, microbond, and fragmentation methods. The microindentation test method is insensitive to the surface treatment of the fiber. 

Eiber lengths between 5 and 10 mm are conveniently selected for the microindentation test (111). Eor a carbon fiber/epoxy system, as the fiber volume fraction increases from 10 to 50 vol%, the indentation displacement distance decreases from 44 to 36 pm but the interfacial shear strength increases from 33 to 46 MPa. When the interphase-to-matrix modulus ratio increases from 1.0 to 7.5, the interfacial shear stress increases by only 10%. Likewise, the interphase thickness and fiber diameter have marginal effects on the interfacial shear stress. Three types of thermoplastic polymers (polyester, polyamide, and polypropylene) were tested for their interfacial shear strength to the glass fiber by Desaeger and... 

Li, X., Wang, X., and Yang, M. (2008) Assessment of mechanical properties of fluoroelastomer and EPDM in a simulated PEM fuel cell environment by microindentation test. Mater. Sci. Eng. A, 496, 464-470. 

Gilbert JL, Cumber J, Butterfield A. Surface micromechanics of ultra-high molecular weight polyethylene microindentation testing, ctosslink-ing, and material behavior. /BiomedMater Res 2002 61(2) 270-81. 

The microindentation tests are performed to measure the hardness and the elastic modulus of small specimens with low applied loads. One of the most difficult problems in the microindentation test is to determine the contact radius (Fischer-Cripps 2000). In this case, the contact radius is estimated by the following equation. 

The microhardness technique is used when the specimen size is small or when a spatial map of the mechanical properties of the material within the micron range is required. Forces of 0.05-2 N are usually applied, yielding indentation depths in the micron range. While microhardness determined from the residual indentation is associated with the permanent plastic deformation induced in the material (see section on Basic Aspects of Indentation), microindentation testing can also provide information about the elastic properties. Indeed, the hardness to Young s modulus ratio HIE has been shown to be directly proportional to the relative depth recovery of the impression in ceramics and metals (2). Moreover, a correlation between the impression dimensions of a rhombus-based pyramidal indentation and the HIE ratio has been found for a wide variety of isotropic poljuneric materials (3). In oriented polymers, the extent of elastic recovery of the imprint along the fiber axis has been correlated to Young s modulus values (4). 

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