Understanding the Dimension Measurement of Specimens in Material Mechanics Testing

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 1m, and the maximum indication error is about plus-minus (2~4)m. The resolution of high-precision micrometers can reach 0.1m, and the maximum indication error is plus-minus 0.5m.

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 plus-minus 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



This release was published on openPR.