Testing technology II of the hottest plastic packa

2022-10-02
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Testing technology of plastic packaging containers (II)

II. Compression test

1 Basic principle

compressive strength is the load that can be borne on the unit cross-sectional area when the compressive load is applied to the specimen to fracture (brittle material) or yield phenomenon (non brittle material)

σ c= × 104 (PA)

where: P - failure load (n)

a - cross sectional area of sample (cm2)

σ C - compressive stress (PA)

yield phenomenon refers to the phenomenon that the deformation increases but the load does not increase in a short time, generally for ductile materials. When calculating the yield strength, P should take the load at the first pause of the dial gauge needle of the testing machine, and calculate the yield strength according to the above formula

compressive modulus of elasticity refers to the ratio of stress to strain within the proportional limit

e= (PA) (3)

where: σ CI - compressive stress, stress value at any point on the strain diagram (PA)

ε CI - compressive strain, the strain value of the deficient surface exceeding any point on the 1-and-a-half strain diagram

2. Factors affecting compressive strength

the main factors affecting compressive strength are the shape and size of the specimen and the test speed

① sample shape and size

the principle of selecting the original sample is generally to make it convenient for molding and machining. The size of the sample can basically reflect the characteristics of the material and prevent the sample from being damaged due to instability

sample shape: all plates adopt cuboids; All molded samples are cylinders

sample height: figures 12 and 13 show the relationship between the compressive strength of several plastic materials and the sample height

it can be seen from the figure that the height of the sample is between 17.5 ~ 30mm, which has little effect on the compressive strength. It is easy to cause traffic accidents below 17.5mm, which has a significant impact, and the compressive strength decreases with the increase of sample height. The main reason is that the friction between the upper and lower end faces of the sample and the pressing plate is at work. It prevents the transverse expansion and longitudinal cracks of the sample, so the fixed value of compressive strength is high. When the specimen is higher, the influence of this restraint on the transverse deformation in the middle of the specimen becomes smaller, so the compressive strength becomes lower

② friction

the influence of the friction between the upper and lower end faces of the specimen and the pressing plate on the compression strength can be further explained by the test of adding lubricant

it can be seen from table 3 that the measured compressive strength is lower than that without lubricant because the friction between the upper and lower end faces of the sample and the pressing plate is reduced by adding lubricant. It is also observed in the test that the failure phenomena of the two are also different. The specimen coated with lubricant shows cracks when it is close to the failure load, while the specimen without lubricant shows cracks when it is not close to the failure load, and powder flies out during failure. However, since it is extremely inconvenient to apply lubricant in the test, the same purpose can be achieved by increasing the height of the sample. It can be seen from table 4 that the effect of increasing the height of the sample is basically the same as that of applying lubricant, which plays a role in reducing the influence of friction. However, too high a sample will cause instability

it can be seen from the test that when the height of the sample of plexiglass plate is 25mm, the sample will be distorted when the load is close to yield, and the sample with 20mm high surface will not be distorted until it is loaded to yield. Therefore, the national standard stipulates that the height of the sample is 20mm

③ parallelism

the upper and lower end faces of the specimen must be parallel and perpendicular to each side. Practice has proved that the parallelism of the upper and lower end faces and the perpendicularity of the side face of the sample have an impact on the test results. It can be seen from table 5 that the upper and lower ends of the sample are not parallel, resulting in a low compressive strength of about 2.4 ~ 7.8%. The reason for the reduction of compressive strength is that the stress on the specimen is uneven, resulting in excessive local stress. Therefore, it is specified that the upper and lower end faces of the sample are parallel to each other and perpendicular to each side of the sample, and the height difference of each part should not be greater than 0.1mm

④ relationship between test speed and compression strength

during compression test, the determination of loading speed is very important. When selecting the loading speed, it must be considered that within the range of forming speed, each face can only be formed once, which has little impact on the test results, and the error is small, so as to make the loading as uniform as possible. What kind of products does the universal machine with compression strength varying with test speed have? Today, Jinan new era Gold Testing Instrument Co., Ltd. takes you to see that one kind of changed universal machine increases with the increase, especially for thermoplastic materials, but has little impact on thermosetting materials. Taking plexiglass as an example, the compression strength value at the test speed of 15mm/min is about 11% higher than that at the test speed of 5mm/min, so the test speed should not be too wide

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