Notched tensile test notch effects

The concept of a ductile-to-brittle transition temperature in plastics is likewise well known in metals, notched metal products being more prone to brittle failure than unnotched specimens. Of course there are major differences, such as the short time moduli of many plastics compared with those in steel, that may be 30 x 106 psi (207 x 106 kPa). Although the ductile metals often undergo local necking during a tensile test, followed by failure in the neck, many ductile plastics exhibit the phenomenon called a propagating neck. Tliese different engineering characteristics also have important effects on certain aspects of impact resistance. 

Zhou and co-workers [27] studied the effect of surface treatment of calcium carbonate with sulfonated PEEK on the mechanical properties of the polymer. Tests used included tensile tests, flexural tests, notched Izod impact tests, TGA, DSC and SEM. The modulus and yield stress of the composites increased with CaCOs particle loading. This increase was attributed to the bonding between the particles and the PEEK matrix, was proved by the SEM of the tensile fracture surface of the composites. The treated fillers were found to give a better combination of properties, which indicated that the sulfonated PEEK played a constructive role in the calcium carbonate/PEEK composites. 

Tensile and notch-tensile tests of AISI 304 were conducted at the four test temperatures listed above. In addition, two other variables, strain rate and thermal cycling, were introduced. The strain rates employed were those resulting from crosshead speeds of 0.005, 0.02 and 0.2 in. per min. at each test temperature. To determine the effect of thermal cycling on mechanical properties of AISI 304 in the absence of spontaneous martensitic transformation, one half of all the specimens were subjected to temperature cycling between 300° and 76°K (holding at 76°K) for a period of one year [2]. 

Fig. 5.18. Time-to-failure t in the long-term tensile test (NCTL test) on cold drawn test specimen as a function of the degree of orientation X. Circles tensile stress parallel to the direction of stretching and the notch across it, rectangle applied tensile stress across the direction of stretching and the notch parallel to it. In the first case the time-to-failure increases exponentially with the degree of orientation, in the second case it decreases exponentially. Orientation has a drastic effect on stress crack resistance. Source (Zhou and Brown 1994)...

In addition to thickness reduction, the rheological process obviously is characterized by the size of the squeeze-out of the seam, which is produced during melt flow. Type and form of the squeeze-out at the same time influence the notch effect in the welded area and thus the time-to-failure in the long-term tensile test. Measurements of form and area of the squeeze-out were also used for the description of the quahty of dual hot wedge seams, see (Bielefeldt et al. 1991 Corbet and Peters 1996). This quantity alone, however, cannot completely and unambiguously characterise the welding process. 

The specimens are shown in Fig. 1. In each case, the tests were run over the temperature range from 75 to -320 F. The tensile tests evaluated the effect of only one embrittling service condition—low temperatures. The unnotched tensile impact tests added a second embrittling condition—high strain rate. The notched tensile impact tests and the Charpy keyhole impact tests added the third embrittling service condition—stress concentration. 

Fig. lOa-d TEM images from a first generation HDPE tested to failure with an initial K of 0.22 MPa m1/2 in air showing the effect of increasing K as the crack-tip advances, a Section from just in front of the notch tip. b Section from about 2 mm from the notch tip. c Section from close to the centre of the specimen, d Section from the centre of the specimen (embedded in epoxy and stained in Ru04, tensile axis as indicated by the arrows) [73]... 

The tests discussed thus far, while important in defining the effects expected from an alkali metal-water reaction, have involved large quantities of water and comparatively little alkali metal. In the event of water leakage into sodium in a heat-transfer system, the sodium would be in excess. To investigate the effects of a sodium-water leak in heat transfer systems, a small test system was operated at KAPL. The system consisted of a water boiler using one double-walled tube with flowing sodium on the tube side, boiler water on the shell side, and mercury as the third fluid. In each test run a hole was fabricated in the outside tube. After the boiler reached equilibrium conditions, steam pressure of 100 pounds per square inch and 475 to 500 F. sodium temperature, the inner tube was completely parted by a tensile load applied to a preformed peripheral notch. 

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