Microhardness macroscopic mechanical property correlation

These include cold drawn, high pressure oriented chain-extended, solid slate extruded, die-drawn, and injection moulded polymers. Correlation of hardness to macroscopic properties is also examined. In summary, microhardness is shown to be a useful complementary technique of polymer characterization providing information on microscopic mechanical properties. 

Figure 5.2 shows the linear relationship found between the microhardness and the yield stress for the two series of blends A /B and A2/B. However, as we will discuss later, the slope of this plot differs from that found for other PE samples (Baltd Calleja, 1985). The simultaneous increase of both H and tensile yield stress Yt with increasing content of the HOPE component results in a linear correlation between these two mechanical properties with a ratio H/Yf 2 2 (Fig. 5.2). This ratio is significantly smaller than that previously found for PE (H 3Tf) (see Section 4.7.3). The smaller H/Yt ratio found for these materials can be related to the relatively high deformation speed (50 mm min ) used in a macroscopic measurement, which is about 40 times higher than that used in a microhardness test (see Balta Calleja et al., 1995).

The indentation test is one of the simplest ways to measure mechanical properties of a material. The micromechanical behavior of polymers and the correlation with microstrnctnre and morphology have been widely investigated over the past two decades (23). Conventional microindentation instruments are based on the optical measnrement of the residual impression produced by a sharp indenter penetrating the specimen surface under a given load at a known rate. Microhardness is obtained by dividing the peak load by the contact area of impression. From a macroscopic point of view, hardness is directly correlated to the yield stress of the material, ie, the minimnm stress at which permanent strain is produced when the stress is snbseqnently removed. 

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