Parallel-sided specimen

Although the Rockwell test is intended to be used on flat parallel-sided specimens, its use can be extended to rounded surfaces by using a curvature correction factor. Compound surfaces such as gear teeth can be tested but the results must be corrected for curvature. 

For brittle thermosets, parallel-sided dumb-bell or waisted dumb-bell specimens are preferred simple parallel-sided specimens often fail in the jaws of the testing instrument. Moreover, in our experience, sample preparation is absolutely critical. Even minor imperfections and scratches on the surface of the specimen can lead to failure occurring in times several orders of magnitude earlier than expected. 

Fig. 5. 23 shows an idealized stress-strain curve for a ductile polymer sample. In this case the nominal stress (r is plotted against the strain e. The change in the cross-section of a parallel-sided specimen is also sketched schematically at different stages of the deformation. Initially the stress is proportional to the strain and Hooke s law is obeyed. The tensile modulus can be obtained from the slope. As the strain is increased the curve decreases in slope until it reaches a maximum. This is conventionally known as the yield point and the yield stress and yield strain, Oy and Cy, are indicated on the curve. The yield point for a polymer is rather difficult to define. It should correspond to the point at which permanent plastic deformation takes place, but for polymers a permanent set can be found in specimens loaded to a stress, below the maximum, where the curve becomes non-linear. The situation is further complicated by the observation that even for specimens loaded well beyond the yield strain the plastic deformation can sometimes be completely recovered by annealing the specimen at elevated temperature. In practice, the exact position of the yield point is not of any great importance and the maximum point on the curve suffices as a definition of yield. The value of the yield strain for polymers is typically of the order of 5-10% which is very much higher than that of metals and ceramics. Yield in metals normally occurs at strains below 0.1%. 

Fig. 3.1. Single fiber compressive tests with (a) parallel-sided and (b) curved-neck specimen.

Stress-strain tests were mentioned on page 24 and in Fig. 11-12. In such a tensile lest a parallel-sided strip is held in two clamps that are separated at aconstant speed, and the force needed to effect this is recorded as a function of clamp separation. The test specimens are usually dogbone shaped to promote deformation between the clamps and deter flow in the clamped portions of the material. The load-elongation data are converted to a stress-strain curve using the relations mentioned on p. 24. These are probably the most widely used of all mechanical tests on polymers. They provide useful information on the behavior of isotropic specimens, but their... 

The shear component of the applied stress appears to be the major factor in causing yielding. The uniaxial tensile stress in a conventional stress-strain experiment can be resolved into a shear stress and a dilational (negative compressive) stress normal to the parallel sides of test specimens ofthe type shown in Fig. 11-20. Yielding occurs when the shear strain energy reaches a critical value that depends on the material, according to the von Mises yield criterion, which applies fairly well to polymers. 

Screw dislocation. The simplest case to start with is that of a straight screw dislocation of Burgers vector b parallel to the surface of a thin parallel-sided crystal foil, as shown in Figure 5.10. Using the coordinate system defined there, the dislocation AB is parallel to y and at a depth z below the top surface. The dislocation causes a column CD of unit cells parallel to z in the perfect crystal to be deformed. If we assume that the atomic displacements around the dislocation are the same in the thin specimen as in an infinitely large, elastically isotropic crystal, then the components u, v, w of the deformation of the column along thex, y, and z directions will be... 

Tension Perhaps the best indication of the properties of a material is obtained from a tensile test in which a specimen with parallel sides is caused to extend. Compression The compression test is essentially similar to the tension test except that the force is in the opposite sense (i.e., push instead of pull). An additional complication arises because of a further failure mode for slender specimens buckling may occur. Most standard test techniques employ some form of anti-buckling guide to suppress this failure. 

HP-LCP exhibits a quite attractive mechanical performance when incorporating fillers or fibers. Table 2.1(B) shows the physical properties of HP-LCP composite and LCP-E composite. For measuring mechanical properties Dumbbell-shaped specimens according to the JIS K7113 1(1/2) standard were used. Their overall length was 75 mm, the length of the narrow parallel-sided portion was 30 mm, the width of narrow portion was 5 mm and the thickness was 0.5 mm. 

The fracture strength of an un-notched parallel-sided sheet of a brittle polymer of thickness 5 mm and breadth 25 mm is found to be 85 MN m . If the critical stress intensity factor, Kc, for the polymer is determined from a separate experiment on a notched specimen to be 1.25 MN calculate the inherent flaw size for the polymer. (The value of Kc for a single-edge notched sheet specimen may be determined from the appropriate expression in Table 5.5.)... 

We prepared a strip-type specimen (100 X 50 X 2 mm) from the rubber sheet of SBR filled with HAH carbon black (50 phr), vulcanized for 30 min at 155°C. On the specimen, a slit of different lengths (si = 30 mm, S2 = 20 mm, S3= 10 mm) parallel to the extension direction and a notch of different lengths (2 or 5 mm) at the center of the side surface of the specimen perpendicular to the extension direction were made by razor-cutting (see the inserted figure in Figure 18.14). The distance 8 between slits and between the slit Si and the tip of notch was 1 and 3 mm, respectively. The no-slit specimen means that it only includes a notch, without slits. 

Lateral Expansion Requirements. Other carbon and low alloy steels having specified minimum tensile strengths equal to or greater than 656 MPa (95 ksi), all bolting materials, and all high alloy steels (P-Nos. 6, 7, and 8) shall have a lateral expansion opposite the notch of not less than 0.38 mm (0.015 in.) for all specimen sizes. The lateral expansion is the increase in width of the broken impact specimen over that of the unbroken specimen measured on the compression side, parallel to the line constituting the bottom of the V-notch (see ASTM A 370). 

Eressive stress parallel with the crack path which helps to keep the crack om running off to the side of the specimen. The critical separation is determined by measuring with a micrometer the total distance across the top of the specimen, dCi and by subtracting the thickness of the beams, 2h. The crack length, c, from where the stress is applied at the top of the specimen is measured with a cathetometer. 

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