Cross-head rate

Test rate and property The test rate or cross-head rate is the speed at which the movable cross-member of a testing machine moves in relation to the fixed cross-member. The speed of such tests is typically reported in cm/min. (in./min.). An increase in strain rate typically results in an increase yield point and ultimate strength. Figure 2-14 provides examples of the different test rates and temperatures on basic tensile stress-strain behaviors of plastics where (a) is at different testing rates per ASTM D 638 for a polycarbonate, (b) is the effects of tensile test-... 

The compressive stress-strain measurements are performed in an Instron Universal Test Machine. Pad specimens (Figure 1) are loaded to the bottomed deflection (Figure 2) at 1.1 in. and unloaded without pause. A cross-head rate of 2.0 in./min which is sufficiently slow as to give essentially a static loading condition is employed. Compressive stress data are reported for deflections of 0.2, 0.4 and 0.6 in. 

The plastic strains are measured by means of point marking method. The markings with 1 mm interval in a direction of tensile axis were made by a micro-vickers hardness tester before tensile tests. Tensile tests were conducted at room temperature at a cross-head rate of 0.5 mm/min with an Instron-type testing machine. After the tensile deformation, the relative displacements between markings were measured under a precision machinery microscope, and the corresponding local strains in the direction of tensile axis were calculated. Then deformed specimens were cut into pieces of 1 mm width by a low speed cutter perpendicular to the tensile axis. The saturation magnetization of each piece was measured by magnetic balance at room temperature. 

Jfolded microdumbbell specimens with a gauge length of 2.5 cm and a cross-sectional area of about 3 were fitted with an extensometer to acctorately measure strain. A cross-head rate of 0.05 cm/min (an initial strain rate of 3.4 x 10 /sec) was employed for all testing. The tensile data were analyzed assuming that the Poisson s ratio was 0.35. 

All of the tests were performed at a cross-head rate of 0.05 cm/ min. The fracture energy 2J determined from the SEN specimens was calculated from the relationship (35)... 

Specimens were fracture tested at constant cross-head rate in a tensile testing machine. The specimens were initially precracked using a razor blade as a wedge. Specimens were then mounted in the test machine using... 

Tear-test specimens are generally pulled on a tensile tester at a cross-head rate of 8.5 mm/sec (20 in/min). The maximum force required to initiate or propagate tear is recorded as force per unit thickness. Tear strength measurements can indicate gross differences in performance in some applications. They are also useful in production control. 

The test rate or cross-head rate is the speed at which the movable cross-member of a testing machine moves in relation to the fixed cross-member. The speed of such tests is typically reported in cm/min. (in./min.). 

Peel strength to aluminum, N/cm width at cross-head rate of 0.85 mm/s... 

Therefore, based on classic shear lag analysis (also based on definition of shear strain), it can be shown that for a fixed strain rate (function) the effect of increased cross-head rate can be induced by increasing the interphase thickness (i.e., application of fiber sizing). Indeed, the experimental results reveal shorter fragment lengths for increased cross-head rate and/or presence of fiber sizing. Based on this premise, the following superposition relations can be written for the interfacial strength, Tc ... 

For test equipment driven by computer software, enter required information—e.g., specimen dimensions, cross-sectional area, desired calculations, and the rate at which the sample will be deformed (cross-head speed or deformation rate). 

Extrusion of tubes, hose, and profiles is done on standard extruders for rubber. The usual temperature pattern is a gradual increase of temperature from the feed zone to the die. The die temperature is typically 100°C (212°F), and the screw temperature is approximately the same as the temperature of the feed zone [44]. Processing aids are almost always required to improve the surface appearance and to increase the extrusion rate. Extrusion represents only a small proportion (about 10%) of the total consumption of fluorocarbon elastomers [52].Cold feed extruders typically used for extrusion of rubber stocks are also suitable for the extrusion of fluorocarbon elastomers. Extrusion should be carried out at temperatures below 120°C (250°F) to avoid scorch. In most cases a breaker plate and screen pack are used to generate backpressure on the screw and to remove foreign particles from the stock. A straight head is used for the extrusion of profiles or tubing, whereas a cross-head die is used for... 

Fig. 1. Load-displacement diagram for DENT specimens at various ligament lengths tested at a cross-head displacement rate of Imm/min and at a temperature of 23 °C.

Harwood and Payne (210) have demonstrated a simple, general relationship between the work input at break, Ub, and the hysteresis at break, Hb. The latter is defined as the area of the hysteresis loop formed between the extension and retraction branches of the strain curve, obtained in uniaxial tension at constant strain rate. Hb may be obtained by measuring hysteresis loops for various strain ranges (0 to e) and extrapolating the area of the loop to the breaking strain, eb. In a single experiment, the cross-head travel must be reversed just short of eb, which is not easily done in view of the stochastic nature of the failure process. The empirical relationship between Ub and Hb is... 

In evaluating gel structure, fundamental characteristics such as brittleness, hardness, and elasticity can be quantitatively measured and related to sensory attributes such as chewiness and gumminess. Sherman (56) found sample dimension and cross head speed affect these readings. Gels were not linear In their force-compression behavior. Slow cross head speeds can lead to stress relaxation, so from low compression test rates. It was Impossible to predict how a gel would behave at high compression rates as In the mouth. 

B curves, two load values are exhibited P, the crack initiation load at the point of instability used in the calculation of Gj and P, the crack arrest load used for estimating G,j,. For rate-insensitive materials, Gjc and Gj are identical, and the crack growth a is dictated by the cross-head speed, e. 

The sample is preferably prepared using a lathe and the conically shaped ends fit into specially shaped holders that make the fracture within the gauge marks more probable. The standard speed of test machine jaw separation should be between 3 to 6 minutes to rupture with a suggested rate of cross-head movement set at 1.3 mm minute for each 25.4 mm of test section gauge length. This test method also can be used to determine the tensile adhesion properties. 

---