**3.2 CM measuring principle**

On the basis of the Bueckner's superposition principle, the CM is applied for evaluating residual stress existing in metallic parts or structures. The ideal theoretical implementation of CM used for weld is displayed in **Figure 2**. Step A in **Figure 2** is the undisturbed welded joint and the residual stress that one wishes to determine. In step B, the part was cut in two on the plane x = 0 and the cutting plane deformed due to the near-surface residual stress fully released by the cut; therefore stress in the plane x = 0 was zero. Step C is an analytical step, in which the deformed cut surface is forced back to its original shape; the resulting change in stress is determined. Superimposing the stress state in B with the change in stress from C gives the original residual stress throughout the part. And for arbitrary plane in weld, its residual stress can be determined by the following general expression:

$$
\sigma^A\_{(\mathbf{x},y,x)} = \sigma^B\_{(\mathbf{x},y,x)} + \sigma^C\_{(\mathbf{x},y,x)} \tag{3}
$$

cutting process relaxes the residual stress elastically and does not induce any stress into the material. At present, cutting process is implemented by using the wire electrical discharge machining (WEDM) as it can generate a perfectly straight cut and does not remove any further material from the cut surfaces. "Skim cut" setting is used in order to minimize the effect of cutting process on the contour displacement during the relaxation of residual stress. The "skim" mode of WEDM cutting is preferred for contour cuts because of the lower roughness this setting produces. After setting the cutting parameters, the welded specimen is submerged in temperature-controlled deionized water during cutting. In this study, the Sodick AQ400LS with the wire diameter of 0.25 mm is used, and the cutting speed is

*Residual Stress Evaluation with Contour Method for Thick Butt Welded Joint*

*DOI: http://dx.doi.org/10.5772/intechopen.90409*

Additionally, in order to minimize the amount of cutting deviates from the original plane, welded specimens should be constrained from moving as stresses are released during the cutting. In conventional (**Figure 3a**) cut configuration, the effects of cutting errors and cutting artifacts cannot be neglected. In Hosseinzadeh's study, plasticity-induced errors in contour measurements can be mitigated by controlling the magnitude of the SIF during cutting. This can be done by choosing an appropriate cutting and restraint strategy. And the SIF is reduced by undertaking an "embedded cut" [20, 21]. Therefore, to mitigate the effects to some extent by restraining the "mode I" opening of cut surfaces during cutting, the embedded cut

After the welded specimen cutting, the out-of-plane displacement can be implemented through contact measurement and non-contact measurement. Optical machines such as triangulating laser probes, confocal microscopes, et al. are useful for surface measurement. However, handling the large data sets, these systems produced can be problematic, usually requiring some sort of data reduction process. Comparing to non-contact measurement by optical machine, the coordinate measuring machine (CMM) with the uniform measured data is extensively employed for the contour measurement because the regular measured point can be obtained. In this study, the Hexagon micro plus is equipped with a 5-mm diameter touch probe, as is illustrated in **Figure 4**. And each cut surface was sampled with a

The procedure of measured data processing is data alignment, data smoothing, averaging of the two data sets, and fitting of the two data sets. The data alignment is

*Schematic drawing of contour cut configuration for welded specimens: (a) conventional contour cut*

0.3 mm/min.

(**Figure 3b**) is used in this study.

*3.3.2 Surface contour measurement*

measurement point spacing of 1 1 mm.

*3.3.3 Data processing*

**Figure 3.**

**111**

*configuration; (b) embedded cut.*

#### **3.3 CM measurement procedure**

#### *3.3.1 Welded specimen cutting*

Welded specimen cutting is the first and most important step during the CM procedure as the subsequent steps of surface measurement, data processing, and stress calculation rely on the cutting quality of surface contour. It assumes that the

**Figure 1.** *Weld groove and dimensions.*

**Figure 2.** *Schematic diagram of the CM.*

*Residual Stress Evaluation with Contour Method for Thick Butt Welded Joint DOI: http://dx.doi.org/10.5772/intechopen.90409*

cutting process relaxes the residual stress elastically and does not induce any stress into the material. At present, cutting process is implemented by using the wire electrical discharge machining (WEDM) as it can generate a perfectly straight cut and does not remove any further material from the cut surfaces. "Skim cut" setting is used in order to minimize the effect of cutting process on the contour displacement during the relaxation of residual stress. The "skim" mode of WEDM cutting is preferred for contour cuts because of the lower roughness this setting produces. After setting the cutting parameters, the welded specimen is submerged in temperature-controlled deionized water during cutting. In this study, the Sodick AQ400LS with the wire diameter of 0.25 mm is used, and the cutting speed is 0.3 mm/min.

Additionally, in order to minimize the amount of cutting deviates from the original plane, welded specimens should be constrained from moving as stresses are released during the cutting. In conventional (**Figure 3a**) cut configuration, the effects of cutting errors and cutting artifacts cannot be neglected. In Hosseinzadeh's study, plasticity-induced errors in contour measurements can be mitigated by controlling the magnitude of the SIF during cutting. This can be done by choosing an appropriate cutting and restraint strategy. And the SIF is reduced by undertaking an "embedded cut" [20, 21]. Therefore, to mitigate the effects to some extent by restraining the "mode I" opening of cut surfaces during cutting, the embedded cut (**Figure 3b**) is used in this study.
