DIC-hole drilling method for in-situ residual stress measurement

Residual stress measurement carries an important significance in ensuring safety and reliability of steel structures. In order to simplify the measurement procedure and enhance flexibility of the conventional hole drilling method to adopt in in-situ residual stress measurement, digital image correlation (DIC) is applied to measure the displacement field caused by the localized stress relief associated due to hole drilling. It is referred to as DIC-hole drilling method. The residual stress theoretical expressions of the DIC-hole drilling method are discussed. The requirements of drilling device, camera and lens are determined by accounting for the accuracy of the in-situ residual stress measurement. A benchmark experiment by using steel beam specimens is developed to verify the feasibility and reliability of DIC-hole drilling method. Test data are compared with theoretical calculations and FEM results. The comparison indicates the DIC-hole drilling method has enough accuracy for the in-situ residual stress measurement. The displacement field in the regions centred at 2 to 2.5 times drilling hole radius far from the hole is proposed for the accurate residual stress measurement.


Introduction
The techniques of destructive detection and nondestructive detection to detect residual stress have been widely accepted [1]. Although the theoretical background is perfect, the destructive detection method causes great damage to the tested components.
Nondestructive method has higher accuracy [2]. However, the testing conditions are harsh, and set up equipment in the field is difficult, it is unsuitable for in-situ rapid detection [3]. In order to realize the quick detection of residual stress in the field, the DIC-hole drilling method for in-situ measurement has been put forward.  [8].
In order to improve the limitation of the existing methods, it is urgent to study the in-situ application of DIC-hole drilling method. DIC method can be used to record the images before and after drilling, and the full field displacement can be calculated by these images. The DIC method is not sensitive to vibration, and does not need vibration isolation device. It can work under ordinary white light conditions and is suitable for rough surface. In terms of accuracy control, the existing research of DIC-hole drilling method only stays in the verification stage, and lacks a systematic evaluation of its accuracy [9]. Therefore, using DIC method to develop an economical, portable, accurate and real-time in-situ residual stress detection method is significant.

Calculation principle
The residual stress of blind-hole method is calculated by measuring the strain after releasing stress. However, the accuracy of DIC measurement is lower than that of releasing displacement, so it is necessary to reestablish the method. A schematic diagram of the numerical calculation method is shown in Figure 1 [10], which directly calculates the residual stress of the measuring point by displacement measurement. U1, U2 and U3 are 0°, 45°and 90°respectively, and the radius is r (r is larger than the aperture 0.75mm). The corresponding formula are formula (1) and formula (2).
In the formula, r is the measuring line radius of displacement (r is greater than r0). U1(r,0°), U2(r,45°) and U3(r,90°) are relative displacements between two ends of the measuring line after releasing stress. φ is displacement measuring line rotation angle (0°<φ<180°). σx(φ), σy(φ), τxy(φ) are all the stress components of the measuring points before releasing stress. A(r) and B(r) are calibrated releasing coefficients when the measured line radius is r.

Fig. 1. Calculation principles of DIC-hole drilling method
In order to reduce the systematic errors and accidental errors, the residual stress can be calculated by using multiple groups of displacement data [11]. Taking the average value of these data for correction can effectively reduce these errors.

Selection of device configuration
In order to ensure the working distance of the DIC binocular camera and prevent drilling device from scratching the random speckles, the JHZK type precision residual stress drilling device is adopted, and the aperture is D=1.5mm. The field range is 30cm×30cm centered on blind hole to meet the requirement of displacement measurement accuracy.
The experiment uses the VIC-3D system developed by CSI company, set up the VIC-3D system in the laboratory, as shown in Figure 2. four-point bending rebound test of hot rolled steel beam is designed to calculate the rebound stress distribution. Figure 3, the theoretical value of residual stress can be calculated. The measuring points are separated by 30mm along the high direction of the beam, selecting point 1 to discuss, the distance from the lower flange is 84.5mm.

L1
L2 L1 Loading point Loading point In-situ testing procedure as follows:

Fig.4. Testing procedure
Here, in particular, need to note that the image after drilling is taken as the center of the blind hole and the coordinate system is established to determine the angle and measuring radius.

Finite element calibration of releasing coefficient
ABAQUS is used to calibrate the releasing coefficient  Table 1.   Table 2.  Relative error/% 9.9 3.8

Conclusion
(1) DIC-hole drilling method is simple in surface treatment and fast in operation, so it is convenient for commercialization of equipment and software.
(2) DIC-hole drilling method with full displacement field data can reduce the influence of accidental error on detection results.
(3) The accuracy of measuring circle is high in the range of r=2ro~2.5ro by DIC-hole drilling method.

Acknowledgements
The authors wish to thank the Natural Science