*3.2.2 Observation 2*

The shear wave velocities (commonly for the *x*- and *y*-waves) show the following features. On the V30 side, the velocity is slightly higher than the nominal value for the entire horizontal span. On the skh51 side, the velocities are clearly higher than the nominal values in the near-joint region (20<*x*<0 mm) and approximately the same as the nominal value for the rest of the region. These features indicate the following characteristics in the in-plane residual stresses. The V30 side experiences slight compressive residual stress uniformly over the horizontal span. On the skh51 side, the near-joint region experiences compressive residual stress. In the region toward the cold end, the material experiences tensile residual stress at a low level around reference lines b and c. Around reference line a, the residual stress in the *x*-direction is considerably compressive.

The above observations indicate the following overall residual stresses. On the V30 side, the residual stress is tensile along the thickness and slightly compressive along the surface plane. In both cases, the residual stress is uniformly distributed over the entire horizontal span. On the skh51 side, the residual stress is concentrated in the near-joint region where the residual stress is compressive in all directions. Toward the cold end, the residual stress is slightly tensile in all directions around reference lines b and c. Near reference line a, the residual stress is more tensile in the *z*-direction and compressive in the *x*-direction.

Possible explanations of these features found in the residual stresses are as follows:


*Opto-Acoustic Technique for Residual Stress Analysis DOI: http://dx.doi.org/10.5772/intechopen.90299*

**Figure 7.**

(0 <*x*< 50 mm). On the skh51 side, the velocity is clearly higher than the nominal value in the near-joint region (20<*x*<0 mm) and slightly lower than the nominal value toward the cold end (*x*< 20 mm). These features can be translated into the following characteristics in the residual stress in the *z*-direction. The V30 side experiences tensile stress uniformly over the entire horizontal span. The skh51 side experiences compressive residual stress near the joint and very slight tensile stress for the rest of the horizontal span. Along reference line a, the situation toward the cold end on the skh51 side is slightly different. The material experiences clearer

*New Challenges in Residual Stress Measurements and Evaluation*

The shear wave velocities (commonly for the *x*- and *y*-waves) show the following features. On the V30 side, the velocity is slightly higher than the nominal value for the entire horizontal span. On the skh51 side, the velocities are clearly higher than the nominal values in the near-joint region (20<*x*<0 mm) and approximately the same as the nominal value for the rest of the region. These features indicate the following characteristics in the in-plane residual stresses. The V30 side experiences slight compressive residual stress uniformly over the horizontal span. On the skh51 side, the near-joint region experiences compressive residual stress. In the region toward the cold end, the material experiences tensile residual stress at a low level around reference lines b and c. Around reference line a, the residual stress

The above observations indicate the following overall residual stresses. On the V30 side, the residual stress is tensile along the thickness and slightly compressive along the surface plane. In both cases, the residual stress is uniformly distributed over the entire horizontal span. On the skh51 side, the residual stress is concentrated in the near-joint region where the residual stress is compressive in all directions. Toward the cold end, the residual stress is slightly tensile in all directions around reference lines b and c. Near reference line a, the residual stress is more tensile in

Possible explanations of these features found in the residual stresses are as follows:

thermal conductivity (**Table 1**). The heat input from the joint flows relatively easily to the cold end on the V30 side. Consequently, the thermal effect is uniform on this size. Contrastively, due to the poor thermal conductivity, the heat input is confined in the near-joint region 20<*x*< 0 mm (called the heat-

2.The compressive residual stress in the HAZ of skh51 (20<*x*<0 mm) results from the following effects. In the heating phase, the HAZ experiences bodycentered cubic (bcc) to face-centered cubic (fcc) phase transformation. This process is accompanied by an increase in the density. Consequently, the thermal expansion of the HAZ is smaller as compared with the non-heataffected zone (the non-HAZ) in the 50<*x*< 20 mm region. In the cooling

phase, the HAZ of skh51 experiences fcc to bcc phase transformation.

3.The above phenomena affect the HAZ of the skh51side differently along the *x*-axis than the *y*- and *z*-axes. Along the *x*-axis, the following events take place. In the heating phase, the HAZ is compressed by the non-HAZ of the skh51 side and the V30 side. The non-HAZ undergoes larger thermal expansion because

Consequently, it undergoes relatively smaller shrinkage.

1.The more uniform feature observed on the V30 side is due to the higher

tensile residual stress.

in the *x*-direction is considerably compressive.

the *z*-direction and compressive in the *x*-direction.

affected zone, HAZ) on the skh51 side.

**20**

*3.2.2 Observation 2*

*Deformation induced by thermal load due to brazing and other constraints. Inward arrows represent compressive residual stress and outward arrows tensile residual stress. Sizes of arrows represent the magnitude of residual stress.*

the phase transformation does not take place. The V30 side experiences uniform thermal expansion with the cool end constrained by the table of the welding setup. The gravity acts in favor of this compression experienced by the HAZ. The greater elastic modulus of V30 also helps this compressing mechanism. In the cooling phase, the HAZ of skh51 shrinks less than the other regions (the non-HAZ of skh51 and the V30 side). This makes the HAZ of skh51 tend to be stretched by the other regions. However, this time the stretching force is against the gravity. Consequently, the compressive stress formed in the heating phase remains in the HAZ on the skh51 side.


**Figure 7** illustrates this observation schematically with some exaggeration.
