**6. Conclusions**

The above reported applications of the HDM show that in principle the HMs methods can be advantageously used for the experimental RS analysis in welded joints. As above mentioned the HDM allows to detect the RS until depth of about 1.5– 2 mm under the component surface, whereas the RCM allows to reach depths until 4– 5 mm. In detail, the accurate use of such methods need the use of modern systems equipped with a high speed machining cutter to minimize the RS due to machining, and a proper microscope to minimize the rosette eccentricity error. The main limitation of the HMs are related to the possible surface curvature of the welded bead and to the particular geometry of the joint. Obviously, in case of non-plane surface and nonplane welded components, the influence coefficients should be computed properly by specific and accurate numerical simulations. In general, to limit the experimental work it is convenient to apply the HMs directly into the zone where other independent considerations (as coarse numerical simulation or theoretical consideration s etc.)

**Figure 15.** *Ultimate tensile stress of AA6082-T6 joints, with HSTC, MSTC and LSTC.*

**Figure 16.** *Fatigue strength of AA6082-T6 joints, welded with HSTC, MSTC and LSTC.*

indicate as the more stressed zone. As an example, as above clearly shown, in case of a butt joint the more stressed zone coincides with the zone near the central point, whereas in case of FSW butt joint the more stressed point is that close to the tool shoulder border in the advancing side of the joint. In such a manner the use of a unique experiment allow**s** the user to evaluate the maximum residual stress that influence not only the static strength but also the fatigue strength of the welded joint. As an example **Figures 15 and 16** shows the correlation between the maximum RS (*σmax*) computed by applying the HDM and the static (**Figure 15**) and the fatigue (**Figure 16**) strength of an AA6082 – T6 FSW butt joint, varying the STC.

It is seen a linear relationship between the maximum RS (*σmax*) and the mechanical strength. Such a linear relationship is in accordance with the Goodman criterion commonly used in the mechanical design to estimate the mean stress effect on the metal fatigue resistance. Moreover, the generic good accordance relieved in the above exposed application (as in many other application here not mentioned for brevity sake), permits to establish that the HMs, widely employed in the industrial field due to its simplicity and low cost, allows the user an accurate estimation of the maximum residual stresses that occur in an a generic welded joint realized by traditional techniques (TIG, MIG etc.) or by innovative techniques (friction stir welding etc). Obviously, in general the HMs can be advantageously used to optimize the parameters that govern the welding process in order to minimize the maximum RSs and, consequently, the mechanical properties of the examined welded joint.
