In daily testing, in addition to the accuracy parameters of the equipment itself, have you ever considered the impact of sample size measurement on the test results? This article will combine standards and specific cases to give some suggestions on the size measurement of some common materials.
1. How much does the error in measuring the sample size affect the test results?
First, how big is the relative error caused by the error. For example, for the same 0.1mm error, for a 10mm size, the error is 1%, and for a 1mm size, the error is 10%;
Second, how much influence does the size have on the result. For the bending strength calculation formula, the width has a first-order effect on the result, while the thickness has a second-order effect on the result. When the relative error is the same, the thickness has a greater impact on the result. For example, the standard width and thickness of the bending test specimen are 10mm and 4mm respectively, and the bending modulus is 8956MPa. When the actual sample size is input, the width and thickness are 9.90mm and 3.90mm respectively, the bending modulus becomes 9741MPa, an increase of nearly 9%.
2. What is the performance of common specimen size measurement equipment?
The most common dimension measuring equipment at present are mainly micrometers, calipers, thickness gauges, etc.
The range of ordinary micrometers generally does not exceed 30mm, the resolution is 1μm, and the maximum indication error is about ±(2~4)μm. The resolution of high-precision micrometers can reach 0.1μm, and the maximum indication error is ±0.5μm.
The micrometer has a built-in constant measurement force value, and each measurement can get the measurement result under the condition of constant contact force, which is suitable for the dimension measurement of hard materials.
The measuring range of a conventional caliper is generally no more than 300mm, with a resolution of 0.01mm and a maximum indication error of about ±0.02~0.05mm. Some large calipers can reach a measuring range of 1000mm, but the error will also increase.
The clamping force value of the caliper depends on the operator’s operation. The measurement results of the same person are generally stable, and there will be a certain difference between the measurement results of different people. It is suitable for the dimensional measurement of hard materials and the dimensional measurement of some large-sized soft materials.
The travel, accuracy, and resolution of a thickness gauge are generally similar to those of a micrometer. These devices also provide a constant pressure, but the pressure can be adjusted by changing the load on the top. Generally, these devices are suitable for measuring soft materials.
3. How to choose the appropriate specimen size measuring equipment?
The key to selecting dimensional measuring equipment is to ensure that representative and highly repeatable test results can be obtained. The first thing we need to consider is the basic parameters: range and accuracy. In addition, commonly used dimensional measuring equipment such as micrometers and calipers are contact measuring equipment. For some special shapes or soft samples, we should also consider the influence of probe shape and contact force. In fact, many standards have put forward corresponding requirements for dimensional measuring equipment: ISO 16012:2015 stipulates that for injection molded splines, micrometers or micrometer thickness gauges can be used to measure the width and thickness of injection molded specimens; for machined specimens, calipers and non-contact measuring equipment can also be used. For dimensional measurement results of <10mm, the accuracy must be within ±0.02mm, and for dimensional measurement results of ≥10mm, the accuracy requirement is ±0.1mm. GB/T 6342 stipulates the dimensional measurement method for foam plastics and rubber. For some samples, micrometers and calipers are allowed, but the use of micrometers and calipers is strictly stipulated to avoid the sample being subjected to large forces, resulting in inaccurate measurement results. In addition, for samples with a thickness of less than 10mm, the standard also recommends the use of a micrometer, but has strict requirements for the contact stress, which is 100±10Pa.
GB/T 2941 specifies the dimensional measurement method for rubber samples. It is worth noting that for samples with a thickness of less than 30mm, the standard specifies that the shape of the probe is a circular flat pressure foot with a diameter of 2mm~10mm. For samples with a hardness of ≥35 IRHD, the applied load is 22±5kPa, and for samples with a hardness of less than 35 IRHD, the applied load is 10±2kPa.
4. What measuring equipment can be recommended for some common materials?
A. For plastic tensile specimens, it is recommended to use a micrometer to measure the width and thickness;
B. For notched impact specimens, a micrometer or a thickness gauge with a resolution of 1μm can be used for measurement, but the radius of the arc at the bottom of the probe should not exceed 0.10mm;
C. For film samples, a thickness gauge with a resolution better than 1μm is recommended to measure the thickness;
D. For rubber tensile specimens, a thickness gauge is recommended to measure the thickness, but attention should be paid to the probe area and load;
E. For thinner foam materials, a dedicated thickness gauge is recommended to measure the thickness.
5. In addition to equipment selection, what other considerations should be made when measuring dimensions?
The measurement position of some specimens should be considered to represent the actual size of the specimen.
For example, for injection molded curved splines, there will be a draft angle of no more than 1° on the side of the spline, so the error between the maximum and minimum width values can reach 0.14mm.
In addition, injection molded specimens will have thermal shrinkage, and there will be a large difference between measuring in the middle and at the edge of the specimen, so the relevant standards will also specify the measurement position. For example, ISO 178 requires that the measurement position of the specimen width is ±0.5mm from the thickness centerline, and the thickness measurement position is ±3.25mm from the width centerline.
In addition to ensuring that the dimensions are measured correctly, care should also be taken to prevent errors caused by human input errors.
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