How do you design and perform a quality control test for hole drilling method?

1 What is hole drilling method?

Hole drilling method is based on the idea that drilling a small hole in a material will release some of the residual stresses around the hole, causing a deformation or strain in the surrounding area. By measuring the strain with strain gauges attached to the surface of the material, you can calculate the residual stresses using a mathematical formula or a calibration curve. Hole drilling method can be applied to various materials and geometries, as long as the hole is small enough compared to the thickness and the diameter of the material.

2 How to design a hole drilling test?

To design a hole drilling test, you need to consider several factors, such as the material properties, the hole size and shape, the strain gauge configuration, the drilling method and speed, and the data acquisition and analysis. You should choose a material that has a homogeneous and isotropic behavior, meaning that its properties are the same in all directions and locations. You should also select a hole size and shape that minimize the drilling-induced errors, such as burrs, cracks, or plastic deformation. A circular hole with a diameter of 1.5 to 2 mm is usually recommended. The strain gauge configuration should match the stress state of the material, such as uniaxial, biaxial, or triaxial. The most common configuration is a rosette strain gauge, which consists of three or more strain gauges arranged in a specific angle. The drilling method and speed should be chosen to avoid excessive heat generation, vibration, or damage to the material or the strain gauges. A high-speed electric drill with a carbide or diamond tip is usually preferred. The data acquisition and analysis should be done with a suitable instrument and software that can record and process the strain data and calculate the residual stresses using the appropriate theory or calibration curve.

3 How to perform a hole drilling test?

In order to perform a hole drilling test, you must first prepare the material and strain gauges. Clean the surface of the material and mark the center of the hole. Attach the strain gauges to the surface with a special adhesive and connect them to the data acquisition instrument. Check the resistance and alignment of the gauges and calibrate them, if necessary. Then, align the drill bit with the center mark and begin drilling with constant speed and pressure. Stop drilling when the hole reaches a depth of between 0.3 and 0.8 mm, without removing the drill bit until data acquisition is complete. After that, measure and record strain data before, during, and after drilling. The data acquisition instrument should capture strain changes at different depths of the hole, which correspond to different stress levels. Plot this data as a function of depth to check for any anomalies or outliers. Finally, use software or a formula to calculate residual stresses from strain data using an appropriate theory or calibration curve. This should account for material properties, hole size/shape, strain gauge configuration, and drilling method/speed. The residual stresses should be reported as principal stresses or shear stresses depending on the stress state of the material.

4 How to verify the quality of a hole drilling test?

Verifying the quality of a hole drilling test requires comparing the results with reference values or standards, such as theoretical values based on the material's manufacturing process or loading history, experimental values from other methods like X-ray diffraction or ultrasonic methods, and statistical values from multiple tests on the same material. By doing so, you can evaluate the accuracy and precision of the test, assess the agreement and consistency of the results, and analyze the variability and reliability of the results.

5 How to improve the quality of a hole drilling test?

To optimize the design and performance of a hole drilling test and improve its quality, you should select a suitable material and location that is free of defects, scratches, or coatings. Additionally, use high-quality equipment and materials such as a sharp and clean drill bit, a stable and precise drill machine, a calibrated and sensitive strain gauge, and reliable data acquisition instrument and software. Lastly, follow the standard procedures and guidelines of the ASTM E837 standard as well as the manufacturer's instructions for the strain gauges, drill machine, and data analysis software. Furthermore, always adhere to safety rules when handling and disposing of materials and equipment.