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How to test the mechanical properties of polymer materials?

Posted on Tue, 08 Mar 2016 09:49:24 GMT
First, the tensile properties test
The basic concept: In a tensile test, maintain this force state final, is to measure the tensile force until fracture so far, are exposed to the maximum tensile stress test known as the tensile strength of principle: Is the tensile test specimens along static tensile load is applied to the longitudinal direction, to break them. By elongating the distance between the measurement sample and the sample standard yield destructive force to obtain a sample of the yield strength, tensile strength and elongation.
definition:
Tensile stress: the sample within the measurement range of the gauge, the initial cross-section of the unit to withstand tensile loads.
Tensile strength: up sample until breakage, exposed to the maximum tensile stress in the tensile test.
Tensile Stress: tensile stress - strain curve, stress at break.
Tensile yield stress: tensile stress - strain curve, yield stress point.
Elongation at break: under tension, when the specimen fracture, marking ratio between the amount of the increase of the distance of the initial gauge length, expressed as a percentage.
Modulus of elasticity: in the proportional limit strain than the material and produce the stress response.
Stress - strain curve
The stress - strain corresponding values plotted corresponding to each other, usually stress value as ordinate, strain as abscissa. Stress - strain curve is generally divided into two parts: the elastic deformation zone and plastic deformation zone, the elastic deformation zone, elastically deformable material can be fully restored, the stress and strain was proportional relationship. The slope of the linear portion of the curve is the tensile elastic modulus, which represents a rigid material. The greater modulus of elasticity, the rigidity, the better. In the plastic deformation zone, the stress and strain increase was not proportional relationship, last seen fracture.
Experimental procedure:
① state regulation and testing of environmental samples according to national standards.
② in the middle portion of the sample parallel to do marking, showing gauge.
③ intermediate sample measuring thickness and width of the parallel portion, precise 0.01mm, width of the middle portion of the parallel type II specimens, accurate to 0.05mm, measuring 3:00, the arithmetic mean.
④ When the holder holding the sample, to make the upper and lower longitudinal axis specimen clamp the line of centers coincide, and elastic fit.
⑤ selected test speed, test.
⑥ record load, or breaking load and elongation between gauge the yield. When the specimen breaks outside the middle of the parallel portion of this sample set aside, the other to take up the sample do.
Factors experiment
(1) molding conditions: the sample itself is caused by microscopic defects and microstructure of different
(2) temperature and humidity:
(Three) stretching speed: belonging viscoelastic plastic material, which is closely related with the stress relaxation strain rate, the process requires a time
(4) Pretreatment: material in the process, due to the different time and speed of heating and cooling, easy to produce local stress concentration, after a heat treatment at a certain temperature or said annealing treatment can eliminate stress, increase strength
(5) material properties: crystallinity, orientation, and its molecular weight distribution, degree of crosslinking
(6) aging: decreased strength after aging
Second, the flexural strength test
Basic definitions
Deflection: bending test, fixed or bottom surface of the center span of the specimen deviates from the original position.
Bending stress: at any time during the bending of the sample, the maximum normal stress on the outer fiber of the central cross section.
Bending strength: the arrival at or before a predetermined deflection, the load reaches the maximum value of the bending stress.
Set flexible bending stress: deflection equal to 1.5 times the bending stress when the sample thickness.
Bending yield strength: the load - deflection curve on the load and deflection without increasing stress surge points.
Principle of the method
The test will be a predetermined shape and size of the sample is placed two sit, and applying a concentrated load at the midpoint of the two sitting, the specimen bending stress and deformation. This method is called static three-point bending test. (Another four-point loading method, not described here.)
Experimental Procedure
① use the middle of the vernier caliper specimen width and thickness, measuring three points, the mean value, accurate to 0.02mm.
② electronic universal testing machine before use warm up for 30 minutes.
③ regulate the span, the sample was put on the stand, on the line of contact indenter and specimen width shall be perpendicular to the longitudinal direction of the sample, the sample close to the ends of the stent ends.
④ Start button drop testing machine according to the parameters set to work. After contact with the indenter into the sample, the computer automatically starts recording the sample suffered displacement data and the resulting load. To sample the yield point is reached or broken, stop immediately.
⑤ save data, and based on data for the bending load - displacement curve and save. Graphical analysis of the sample according to the bending behavior.
Factors experiment
① thickness and width of the sample size and sample processing are related to the bending strength and deflection.
② loaded indenter radius and if the load bearing surface radius indenter radius is small, the sample likely to cause large shear force and impact bending strength. Bearing surface radius may affect the accuracy of the sample span.
③ strain rate and strain rate dependent bending strength at a low strain rate, the bending strength is low.
④ When the test span across thickness ratio increases, various materials have shown the shear force is reduced with increased visibility across the thickness ratio reduces shear stress, the three-point bending test is closer to pure bending.
⑤ temperature is the same material, the yield strength is affected by temperature strength larger than brittle. Current national standards for plastic bending performance test GB / T9341-2008.
Determination of Three, impact strength
Impact test is used to measure the toughness of the material in the state of high-speed impact or resistance to breakage.
General impact experiments using three methods:
(1) pendulum: Test placed in the form of simply supported beam (charpy) - supporting both ends of the sample and the impact of the middle; Izod (Izod) - end of the specimen holder and the impact of the free end. (2) falling ball.
(3) high-speed drawing method. Although this method is desirable, it can be directly converted into the stress - strain curve, calculate the area under the curve, the impact strength can be obtained, but also qualitative judgments of brittle fracture or ductile fracture, but the higher requirements of the Rally.
Charpy impact test more common, the principle as follows:

Another notch impact test samples, as shown below:

Factors experiment

Energy (1) the impact of the consumption process
① specimen elastic and plastic deformation of the energy needed to happen.
② the sample cracks and crack propagation fracture energy required.
③ After specimen fracture propagation energy required to fly.
④ pendulum and the support shaft, pendulum friction between the blade and the specimen loss of energy.
⑤ When the pendulum motion, the test machine inherent energy loss.
(2) Impact test temperature and moderate material properties dependent on temperature.
(3) the sample size using the same formula and the same molding conditions and different material thicknesses impact test, the impact strength of the resultant different.
(4) the value of impact strength Charpy impact velocity of the impact speed is high but lower.
Fourth, universal testing machine
Mechanical properties used more universal testing machine, universal testing machine, also known as universal testing machine, or pulling, double screw series, control, measurement, operating integrated structure, melting contemporary advanced technology in one, with high precision , wide speed range, compact structure, convenient operation, stable performance advantages. Electronic universal testing machine to meet GB / T1040,1041,8804,9341,9647, ISO7500-1, GB16491, GB / T17200, ISO5893, ASTM D638,695,790 and plastic tubing, and other standards. Suitable for stretching plastic, waterproof materials, textiles, paper products and rubber materials and products samples, compression, bending, creep test and with a large pressure plate can be directly pipe flat compression (compression recovery), ring stiffness ( resistance to external load), creep rate, ring tensile test.
Practice
⒈ bringing the total switching power.
⒉ according to the sample, the choice of the measuring range, put the rod in the hanging pendulum thallium and adjust or remove the cushion valve handle alignment marking.
⒊ according to the shape and size of the sample loaded with the corresponding upper and lower jaw chuck seat.
⒋ depicted on the drum, good recording rolled paper (graph paper), this is only when required.
⒌ start the pump motor, unscrew the oil delivery valve test station so that the rise of paper 10 mm, and then close the valve, if the test station has been raised position without making a start and pump the oil feed will only send the oil valve can be closed.
⒍ one end of the sample holder on the jaws.
⒎ start the pump to adjust the alignment pointer dial zero.
⒏ start the motor lower jaw and the lower jaw lift to the appropriate height, the other end of the specimen is clamped in the lower jaw, should be noted that the vertical sample.
⒐ push rod down into the paint pen depicting readiness (depicted only need be).
⒑ Press loading speed test requirements, slowly unscrew the oil feed valve loading test.
⒒ specimen after fracture closed valve and stop the pump motor.
⒓ records need to value and description.
⒔ open the oil return valve, after unloading passive needle dial back to zero.
⒕ remove the specimen after fracture.
⒖ compression and bending tests can refer to the above instructions.

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