Abstract
Undispersed filler agglomerates or other substantial inclusions/contaminants in rubber can act as large crack precursors that reduce the strength and fatigue lifetime of the material. To demonstrate this, we use tensile strength (stress at break, σb) data from 50 specimens to characterize the failure distribution behavior of carbon black (CB) reinforced styrene-butadiene rubber (SBR) compounds. Poor mixing was simulated by adding a portion of the CB late in the mixing process, and glass beads (microspheres) with 517 μm average diameter were introduced during milling to reproduce the effects of large inclusions. The σb distribution was well described with a simple unimodal Weibull distribution for the control compound, but the tensile strengths of the poor CB dispersion material and the compounds with the glass beads required bimodal Weibull distributions. For the material with the lowest level of glass beads—corresponding to less than one microsphere per test specimen—the bimodal failure distribution spanned a very large range of σb from 13.7 to 22.7 MPa in contrast to the relatively narrow σb distribution for the control from 18.4 to 23.8 MPa. Crack precursor size (c0) distributions were also inferred from the data, and the glass beads introduced c0 values in the 400 μm range compared to about 180 μm for the control. In contrast to σb, critical tearing energy (tear strength) was unaffected by the presence of the CB agglomerates and glass beads, because the strain energy focuses on the pre-cut macroscopic crack in the sample during tear testing rather than on the microscopic crack precursors within the rubber. The glass beads were not detected by conventional filler dispersion measurements using interferometric microscopy, indicating that tensile strength distribution characterization is an important complementary approach for identifying the presence of minor amounts of large inclusions in rubber.
Highlights
Rubber fracture mechanics can be used to predict fatigue lifetime of an elastomer product, and the essential inputs are the crack growth rate law and the size of crack precursors [1,2,3]
In contrast to σb, critical tearing energy was unaffected by the presence of the carbon black (CB) agglomerates and glass beads, because the strain energy focuses on the pre-cut macroscopic crack in the sample during tear testing rather than on the microscopic crack precursors within the rubber
The addition of 0.5 mm diameter glass beads at two levels and the impact of intentional poor mixing of carbon black were all successfully diagnosed in CB-filled styrene-butadiene rubber (SBR) compounds using tensile strength testing of 50 replicate specimens
Summary
Rubber fracture mechanics can be used to predict fatigue lifetime of an elastomer product, and the essential inputs are the crack growth rate law and the size of crack precursors [1,2,3]. Tensile strength of rubber is sensitive to the size of crack precursors, with larger precursors from poorly dispersed raw materials leading to lower strengths They used Weibull statistics to fit the tensile strength (stress at break, σb ) data from testing of numerous replicate rubber specimens, with 50 specimens determined to be an efficient population size for characterizing the failure distribution to give quantitative insights into dispersion effects [10]. Our research further investigates the utility of tensile testing of 50 specimens to quantify the σb distribution in rubber, and we expand the approach to include assessment of the related crack precursor size (c0 ) distribution This effective characterization of c0 distribution is key to fatigue lifetime modeling efforts to predict the reliability of rubber products, as will be discussed. We compare the resulting crack precursor size distributions with defect size distributions measured using microscopy to illustrate that assessing c0 distribution from tensile strength testing is an important complementary approach for identifying the presence of minor amounts of large inclusions in rubber that cannot be detected by conventional filler dispersion measurements
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
