There are many factors affecting the mechanical properties of plastics, such as the influence of polymer structure (such as: polymer type, molecular weight and its distribution, whether it is crystallized, etc.), which has the influence of molding processing (such as the molding processing method and processing conditions leading to crystallinity, Changes in orientation, defects in the sample, etc.; there are test conditions (such as: test temperature, humidity, speed, etc.), which can lead to defects such as poor repeatability of the test, so the test of mechanical properties has strict test standards, such as GB1042-92 stipulates that the ambient temperature is 25±1°C and the relative humidity is 65±5%. The size and shape of the sample are uniformly defined, and the experimental results are often averaged more than five times.
The purpose of tensile testing
Master the testing principle and test method of tensile strength of plastics, and analyze the influencing factors; deepen the understanding of stress-strain curve, and find useful various mechanical performance data; observe the yielding when stretching, Crack, whitening and other phenomena.
The testing principle of common mechanical properties of plastics
The tensile test is to apply a static tensile load to the sample along the longitudinal axis to cause damage. The yield strength, tensile strength and elongation of the sample were determined by measuring the yield strength, the breaking force, and the elongation between the sample gauge lengths.
Testing of conventional mechanical properties of plastics
• Tensile stress: The tensile load of the specimen on the initial cross section of the specimen within the gauge length range.
• Tensile strength: The maximum tensile stress experienced by the specimen until it breaks during the tensile test.
• Tensile fracture stress: The stress at break on the tensile stress-strain curve.
• Tensile yield stress: The stress at the yield point on the tensile stress-strain curve.
• Elongation at break: The ratio of the increase in the distance between the lines to the initial gauge length when the sample breaks under tensile force, expressed as a percentage.
• ε off = (L-L0) / L0 × 100%
• In the formula: L0------ the distance between the sample lines, mm
• L------- the distance between the marking lines when the sample breaks, mm
• Modulus of elasticity: The ratio of the stress experienced by the material to the strain that produces the response within the proportional limits.
• The corresponding values of the stress-strain are plotted against each other, usually with the stress value as the ordinate and the strain value as the abscissa. The stress-strain curve is generally divided into two parts: an elastic deformation zone and a plastic deformation zone. In the elastic deformation zone, the material undergoes elastic deformation that can be completely recovered, and the stress and strain are proportional. The slope of the straight line portion of the curve is the value of the tensile elastic modulus, which represents the rigidity of the material. The greater the modulus of elasticity, the better the rigidity. In the plastic deformation zone, the stress and strain increase are not proportional, and finally the fracture occurs.
The tensile test is to apply a static tensile load to the specimen in the longitudinal direction to cause damage. The yield strength, tensile strength and elongation of the sample were determined by measuring the yield strength, the breaking force of the sample, and the elongation between the gauge lengths between the samples.
– Mark the line in the middle of the sample. This line should have no effect on the test results.
– Measure the width and thickness of the parallel part of the sample. Three points are measured for each sample and the arithmetic mean is taken.
– The stretching speed is generally selected according to the material and the type of the sample.
– When the clamp holds the specimen, the longitudinal axis of the specimen coincides with the center line of the upper and lower clamps, and prevents the specimen from slipping off or breaking in the fixture.
– When the sample breaks outside the middle parallel part, another sample should be taken.
Instrument and sample used
• One punching machine; one piece of plastic sheet
• Or use the injection molding mechanism to get more than five standard samples
Experimental steps and data processing
• Sample preparation
• Use a dumbbell-shaped standard cutter to punch a sample of plastic sheet on the punching machine, take five strips in the longitudinal and transverse directions, accurately measure the width and thickness of the neck of the specimen, and mark the length in the thin neck portion. Standard test strips can also be molded using an injection molding machine.
• Select the test machine load so that the load is within the range of 1/3 to 4/5 of the dial when it is broken.
• Select to adjust the lowering speed of the lower jaw of the test machine. For soft thermoplastics, the stretching speed can be 50mm/min, 100mm/min, 200mm/min, 500mm/min.
• Install the sample on the fixture. When using the clamp, fix the upper clamp with a retainer to prevent damage to the knife edge of the instrument. After the specimen is clamped, loosen the holder.
• When the start button is pressed, the motor starts to run, the lower clamp starts to descend, and the pointer starts to indicate. During this process, the two rulers on the scale are manually controlled so that the △-shaped pointer moves with the two marks on the neck of the sample until the sample breaks. Record the disc reading and the distance between the two rulers.
• Press the return switch to return the lower clamp to its original position and turn the dial pointer back to zero to start the second test.