Because ball pressure testing is affected by many external factors, such as temperature and time, and because the indentation diameter is sometimes within 1 mm, different choices of measurement points and different measuring instruments can yield different test data for very small distances. Therefore, ball pressure testing is often used as a comparative test between laboratories and internationally. However, commonly used standards for low-pressure testing laboratories, such as GB16917.1—2003 and IEC61009—1∶1996, clause 9.14 on heat resistance testing, do not explain the process of ball pressure testing and the method for measuring the indentation diameter in detail. This article will provide a detailed explanation and analysis of ball pressure testing and discuss methods for determining the indentation boundary.
1. Instrument preparation and test procedure for ball pressure test
1.1 Instruments and Equipment
Test equipment meeting the standard requirements should be prepared, including: ball indentation testing apparatus, forced-air drying oven, temperature data recorder, microscope or projector for measuring indentation, electronic stopwatch or other effective timing instrument for recording time. All equipment must be used within its validity period.
It should be noted that the end of the ball pressure testing apparatus should be a complete sphere, not an incomplete sphere, as shown in Figure 1. The previously used ball pressure testing apparatus, as shown in Figure 2, had an incomplete sphere at its end, so the fixed conical surface significantly affected the size of the indentation. The volume/capacity of the forced-air drying oven should also be recorded in detail.
1.2 Experimental Procedure
Place the ball compression testing apparatus and sample holder into the heating chamber. Position the thermocouple at the designated sample placement location. Thermocouples are used because the indicated temperature of many heating chambers does not match the actual temperature inside, especially near the sample, where accurate measurement is crucial. Set the heating chamber temperature to the required temperature and the operating time to 24 hours. Laboratory experience also stipulates that for temperatures above 200°C, a preheating time of at least 13 hours is often required. During the heating chamber's operation, the inspector should repeatedly record the actual temperature changes inside the chamber to determine when thermal equilibrium is reached.
The purpose of placing the support in the heating chamber beforehand is to reduce the influence of the support on the test results. If the support is placed in the heating chamber after it has reached thermal equilibrium, the support will absorb a lot of heat, prolonging the time it takes for the heating chamber to reach thermal equilibrium again, reducing the time the sample is exposed to the test temperature, and reducing the severity of the test conditions.
Record the temperature inside the heating chamber frequently. Once thermal equilibrium is reached, open the heating chamber at time t+10s, place the sample onto the support inside the chamber, and secure the ball pressure device. Continue recording the temperature change process. IEC 60695—10—2∶2003 has strict requirements for test samples, so the sample block placed in the chamber should meet the following requirements: thickness ≥ 2.5 mm; the test sample should be a square with a side length ≥ 10 mm or a circle with a diameter ≥ 10 mm, a thickness of (3.0±0.5) mm, and a smooth surface.
After opening the chamber, the temperature inside drops sharply. After closing the chamber, the temperature needs to be readjusted and balanced. To minimize temperature fluctuations caused by opening the chamber, the opening time should be minimized, and the initial temperature surge upon reaching the set temperature should be reduced. Specifically, after closing the chamber, the set temperature can be adjusted slightly below the test temperature. Once the specified temperature is reached, the set temperature should be set to the test temperature. When placing the sample, ensure the indenter does not move on the sample. In other words, the hot air circulation in the forced-air drying chamber should not interfere with the central position of the sample; the hot air should flow along the edge of the chamber, and the temperature of the placement area must be uniform. Closing the chamber door should not cause displacement of the indenter on the sample; otherwise, the indentation will be overprinted, affecting the accuracy of the test results.
2. Measuring indentations
The most important and difficult step in the experiment is indentation measurement. After the prescribed test time, the ball indentation testing device is removed from the sample, and the sample is immersed in water at room temperature within 10 seconds (note the time). To ensure the sample is close to room temperature, the cooling time in the water should not exceed 10 minutes. After removing the watermark, the indentation diameter d should be measured within 3 minutes of removing the sample from the water. Theoretically, the indentation diameter is the maximum distance between two tangent points, where the tangency point is the geometric tangency between the spherical surface at the end of the ball indentation device and the concave surface of the indentation. Common indentation forms include circular and non-circular, as shown in Figures 3 and 4. Determining the indentation diameter depends on determining the indentation boundary, which in turn depends on the measurement method: the results obtained using digital vernier calipers have large dispersion; precision projectors are designed for cross-sections, and ball indentation samples that cannot be cross-sectioned can only be measured by transmission projection, where the boundary of the circular shadow on the projection screen is still difficult to determine. Ideally, a reading microscope with appropriate illumination (luminous intensity, incident angle) is used. For example, the measuring instrument should have 10x magnification, a measuring platform equipped with the caliber or orthogonal displacement line to be measured, and a lamp to illuminate the indentation surface. Since the indentation diameter must be measured within 3 minutes of removal from the water, the allotted time is very short. The microscope should be adjusted before sample removal to ensure successful indentation measurement within the specified time. Note the time range for each step of sample removal and record it with a stopwatch.
Indentation measurements should be taken at the point where the pressure ball and the sample surface are tangent, and the dimensions of the material deformation should be deducted during measurement. For non-circular indentations, such as those with a cross-section that may be "U/V/M" shaped, the indentation diameter should be measured at the point of maximum diameter.
From practical experience, we know that the cross-sectional views shown in Figures 3 and 4 represent idealized measurement conditions. In actual operation, it is impossible to have longitudinal or transverse cross-sectional views to determine the boundary of the indentation. Therefore, we propose three methods to determine the boundary of the indentation: ① Apply colored ink to the indentation to reveal the boundary, or cover the indentation surface of the sample with a thin layer of white paper, lightly trace the edge with a pencil, measure the maximum and minimum values with a reading microscope, and then take the median value as the test result. ② From a certification and accreditation perspective, the most stringent method is adopted: take the maximum indentation value as the final result (this method is basically used in current international comparative tests). The actual indentation must be within this maximum value. ③ Devices that can directly and accurately image and read the indentation diameter data, such as hardness testers, can be used. However, such non-destructive testing equipment is generally expensive. If the testing institution happens to have such equipment, it can be used to measure the indentation; otherwise, the former method should be used as a reference to select a microscope or calipers with appropriate magnification, select the accurate edge of the indentation, and determine the diameter range.
Since the standard requires measuring the indentation diameter within 3 minutes of removing the material from the water, it is crucial to prepare the reading microscope and projector hardware. The operator should familiarize themselves with the operation of the microscope or projector multiple times to ensure they are ready to take measurements immediately after the experiment. Ideally, a (hand-drawn) schematic diagram of the material deformation should be recorded after the experiment.
3. Conclusion
While ball pressure testing may seem simple, it is not. To ensure the accuracy of the results, it is essential to strictly adhere to standard requirements, prepare equipment and records in advance, and carefully consider the impact of various factors on the test results, especially the determination of the indentation boundary. For tests with time and temperature requirements, effective and feasible solutions should be sought to address potential influences. For difficult measurement points, multiple approaches should be explored, and practical experience from within the industry and internationally should be consulted to better complete the test.
