Craze-kinetics and Fracture Processes in Injection Moulded Standard polystyrene - Tensile Stress at Different Strain Rates

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Video in TIB AV-Portal: Craze-kinetics and Fracture Processes in Injection Moulded Standard polystyrene - Tensile Stress at Different Strain Rates

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Craze-kinetics and Fracture Processes in Injection Moulded Standard polystyrene - Tensile Stress at Different Strain Rates
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Deformations- und Bruchverhalten von spritzgegossenem Standard-Polystyrol - Einfluß der Verformungsgeschwindigkeit bei Zugbeanspruchung
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Film, 16 mm, LT, 116 m ; SW, 10 1/2 min

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Plastics are subjected to tensiletensile stress at different strain rates. The different flaw mechanisms are demonstrated on the example of injection moulded polystyrene with quick, impact, and constant tensile load. Flow zones, brittle fracture, ductile fracture, shear bands, shear elongation, normal stress crazes. Simultaneous shots, slow-motion and time-lapse.
Keywords plastics / breaking plastics / tensile stress polystyrene / tensile stress material testing
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Kinetics of stress-crazing and fracture processes in injection moulded polystyrene at different strain rates. (Standard dumb-bell test pieces. Width 10 mm; thickness 3 mm; injection temperature 200 degrees centigrade; stress direction horizontal.) (Short time tensile tests at medium strain rates.) (Strain rate 0.2 %/min.)
At room temperature the dumb-bell shaped test pieces are strained up to breaking point. They consist of transparent polystyrene dulled on the reverse side. Directly below the specimen, an analogue display informs about the actual tensile stress. At first, uniform deformation without any structural changes. Inhomogeneous deformation by crazing. Other crazes follow at relatively long time intervals. Because of process initiated internal stress and molecular orientation distributions, the highly oriented material near the surface remains still uncrazed. The chosen strain rate of only 0.2 %/min effects relatively low craze concentration. The brittle fracture will take place at a stress of 610 kp/cm^2 - that is 61 Newton/mm^2 - just over the right end of the analogue display.
(Strain rate 0,5%/min.)
In general, raising the strain rate by the factor 2.5 does not change the deformation behaviour. Craze concentration and growth velocity increase slightly. However, the oriented surface material remains uncrazed. Again the sample will break over the right end of the analogue display, but now at a fracture stress of about 670 kp/cm^2 - that is 67 Newton/mm^2.
(Strain rate 1.5%/min; normal shots and slow motion.) A triplication of this higher strain rate effects an enormous craze concentration. At stresses higher than 700 kp/cm^2 - that is 70 Newton/mm^2 - the crazes grow partially through the oriented regions near the surface. In the area near the surface, the racture shows significant plastic deformations.
The growth of crazes in the surface region can be observed at higher magnification and roughly triple slow motion frequency. At first, the ends of crazes become dumb-bell shaped. The crazes grow slightly into the highly oriented regions and finally penetrate them in places.
The fracture-initiating deformation processes in slow motion at about 140 : 1. On the left, the increasing craze concentration causes decreasing transparency and slight necking of the sample. Fracture now takes place. Ductile fracture in the oriented surface regions - brittle type fracture internally. (Impact tensile tests. Kinetics of stress-crazing; slow motion rate about 38.000 : 1 (with repetitions).) The pendulum impact velocity of 3.85 meters per second generates initial strain rates in the range of some 100,000 percent per minute. Craze-formation internally.
Here the frame series of the high speed rotating mirror camera ends just before breaking.
Once more, the same scene.
And a further repetition. In real time the deformation process shown takes 170 microseconds, that is: 170x10^-6 seconds. (Fracture processes. Slow motion rate about 125.000 : 1 (with repetitions).)
Some moments before cracking under the same experimental conditions the crazes penetrated into the surface region. Enormous craze concentration and small transparency in the area of the expected crack. Here Crack-formation at the lower sample surface in the middle of the picture.
Repetition of this scene. Ahead of the Crack front, an area of strongly deformed material moves across the sample.
By measurements on the original single frames shot at a frequency of one million frames per second the maximum crack velocity amounts to approximately 970 meters per second.
A further repetition.
In real time, Crack-propagation through the 10 mm wide sample takes 15 microseconds, that is: 15 x 10^-6 seconds.
(Creep tests.
Tensile stress 42.5 N/mm^2 ; fracture time 18 hours; time lapse rate about 1 : 17.000 (with repititions).) Extremely low strain rates are verified in creep tests.
Repetition. Cracking is announced by only one craze.
Repetition. After breaking, the right end of the specimen shows slight necking.
A further repetition of the same scene.
(Tensile stress 41 N/mm2; fracture time 45 hours; time lapse rate about 1 : 8.600 and 1 : 180 (with repetitions).) After stress reduction of only 1.5 Newton/mm^2 a microscopic craze does not occur. After a long period of homogeneous deformation shot at a fast motion rate of 8.600 : 1, the motion rate is reduced to 180 : 1 in order to get a better time resolution of deformation changes, starting with slight diminution of section. Instead of microscopical crazing after 44 hours under load in the middle of the specimen, there appears - after formation of shear bands - a clearly visible shearing strain. It leads to macroscopical necking, to normal stress-initiated small failures, and finally to the crack.
In this partial repetition, the main points of inhomogeneous deformation are again seen: - slight diminution of section, - shear bands, - shearing strain, - lateral growth of the macroscopic necking zone, - normal stress-initiated failures, and ductile fracture.