*Use of Hybrid Methods (Hole-Drilling and Ring-Core) for the Analysis of the RS on Welded… DOI: http://dx.doi.org/10.5772/intechopen.102051*

also small errors in the strain measurements due to various spurious influence parameters lead to significant errors on the computed stresses. For this reason in general the use of about 6–8 non-uniform depth increments are advised by following the rule clear exposed in [11]. As an example, **Table 1** shows the optimum distribution of depth increments for the RCM by using a total steps included between 4 and 14 (from Ref. [14]); it is seen how the optimised depth increments are relatively larger at the first and specially at the last steps, whereas they are smaller at the intermediate steps. Similar optimized depth increments are provided in [11] for the HDM. Also, **Tables 2 and 3** show the relative optimized influence coefficients *aij* and *bij* for 8 total depth


**Table 1.**

*Optimum distribution of depth increment for RCM with total groove depth of 5 mm.*


**Table 2.**

*Influence coefficients relative to a thick component (>100 mm) with standard dimension of the core-rosette assembly (Do=14 mm, Lo = 5 mm).*


#### **Table 3.**

*Influence coefficients relative to a thin component (<15 mm) with standard dimension of the core-rosette assembly (Do =14 mm, Lo = 5 mm).*

increments, with refer to a thick (**Table 2**) and a thin component analysed by the RCM (**Table 3**).

Although for standard dimensions of the geometry variation properly introduced by the HDM, the relative influence coefficients are provided by the same ASTM standard [13], and similarly accurate evaluation of the coefficients for the RCM are provided in [14], in particular practical cases where the analysed component cannot be considered as an infinite plate subjected to a plane stress field, the relative influence coefficient have to be determined by specific numerical simulations, that consider the exact geometry of the component to be examined. As occur in many inverse problems, the evaluation of the RS from the strains relaxed on surface after each depth increment is influenced by several influence parameters that can introduce into the evaluated RS a significant uncertainty, so that an accurate RS evaluation needs to a reliable estimation of the corresponding uncertainty. For this reason, interesting study are reported in literature that deal with the evaluation of the accuracy and of the uncertainty that affect the principal RS computed by using both the HDM [16] and the RCM [17]. Such interesting works contain the formulas that the user can use to an accurate estimation of the uncertainty of the RS after the analysis and the correction of the main influence parameters, as the thermal effects due to machining, the zero depth offset, the rosette eccentricity, the stresses induced by machining and so on. Synthetically, such studies have been demonstrated that using modern apparatus as that shown in **Figure 3**, in good experimental conditions the uncertainty of the principal RS computed by the RCM in general falls in the range 10–55 MPa [17]

*Use of Hybrid Methods (Hole-Drilling and Ring-Core) for the Analysis of the RS on Welded… DOI: http://dx.doi.org/10.5772/intechopen.102051*

moving from the first to the last steps; similarly for the HDM the mean uncertainty is in general equal to about 25 MPa [16].
