Stress at break

Fig. 3. Composite curve of tme stress at break at 40°C vs reduced time to break tjfor polypropylene fibers of three draw ratios (15) (—...

Definitions of the commonly measured tensile properties are as follows Unear density (tex) is the weight in grams of 1000 m of yam. Tenacity is the tensile stress at break and is expressed in force-per-unit linear density of unstrained specimen, N /tex. Knot tenacity is the tensile stress required to mpture a single strand of yam with an overhand knot tied in the segment of sample between the testing clamps. It is expressed as force-per-unit linear density and is an approximate measure of the britdeness of the yam. Toop tenacity is the tensile stress required to mpture yam when one strand of yam is looped through... 

Figure 13.5. Effect of temperature on the stress at break and elongation at break of PTEF. (Reproduced by permission of ICT Plastics Division)...

Appendix B General Properties and Data on Elastomers and Plastics 175 Table B.IO True Stress at Break of Selected Melt-Mixed Rubber-Plastic Blends ... 

B.IO True stress at break of selected melt-mixed rubber-... 

Coran and Patel [33] selected a series of TPEs based on different rubbers and thermoplastics. Three types of rubbers EPDM, ethylene vinyl acetate (EVA), and nitrile (NBR) were selected and the plastics include PP, PS, styrene acrylonitrile (SAN), and PA. It was shown that the ultimate mechanical properties such as stress at break, elongation, and the elastic recovery of these dynamically cured blends increased with the similarity of the rubber and plastic in respect to the critical surface tension for wetting and with the crystallinity of the plastic phase. Critical chain length of the rubber molecule, crystallinity of the hard phase (plastic), and the surface energy are a few of the parameters used in the analysis. Better results are obtained with a crystalline plastic material when the entanglement molecular length of the... 

A E s TENSILE STRENGTH AT BREAK ELONGATION AT BREAK B = TENSILE STRENGTH ATYIELD ELONGATION AT YIELD C = TENSILE STRESS AT BREAK ELONGATION AT BREAK D = TENSILE STRESS AT YIELD... 

The examples that we have shown here represent only a small fraction of all the variations possible, There is no such thing as a typical force versus elongation curve for polymers. Samples can break at extensions of only a fraction of a percent up to several thousand percent, with engineering stresses at break ranging from only slightly above zero up to more than 10 GPa,... 

Figure 5. True stress-at-break plotted on doubly logarithmic coordinates against the strain-at-break. Conditions 30°C extension rates from 9.4 X 103 to 9.4 min 1. Quantity A introduced for clarity.

Data are presented which illustrate that the tensile strength and elongation-at-break depend significantly on the extension rate even when the stress remains in equilibrium with the strain prior to fracture. A crude estimate was made of the threshold (lowest possible) values of the true stress-at-break and the elongation-at-break for the TIPA elastomer. The estimated quantities are about 26% less than those found at an extension rate of about 0.01 min-1 at 30°C. 

Physical characterization of polymers is a common activity that research and development technologists at the Dow Chemical Company perform. A material property evaluation that is critical for most polymer systems is a tensile test. Many instruments such as an Instron test frame can perform a tensile test and, by using specialized software, can acquire and process data. Use of an extensometer eliminates calibration errors and allows the console to display strain and deformation in engineering units. Some common results from a tensile test are modulus, percent elongation, stress at break, and strain at yield. These data are then used to better understand the capabilities of the polymer system and in what end-use applications it may be used. 

Failure of parts is led by the ratio of the actual mechanical stress on the part versus the mechanical stress at break of the weakest point. In Figure 3.2, the two materials (A) and (B) have the same average tensile strength, but one (B) has more widely dispersed individual values with a broader risk of failure because the lowest values of tensile strength are inferior to the actual stress in the part. 

---