**2. Current trends in residual stress experimental analysis and numerical evaluation**

The importance, and at the same time, the complexity, of obtaining an accurate experimental evaluation of the residual stress inside a material has led, along the years, to the development of many different approaches. Some of them such as the hole drilling method (HDM) and X-ray diffraction (XRD) are, nowadays, mature and widespread techniques ruled by specific standards [4, 5]. Even if they can be considered as benchmark techniques, it should be taken into account that, in many situations, they have some inherent limitations that cannot be discarded in some specific applications.

Hole drilling method, for example, cannot be conveniently used to determine the presence of residual stress too much far from the drilled surface. X-ray diffractometer is limited to the determination of superficial residual stress. Both the approaches cannot guarantee high surface resolution of the residual stress measurement.

More in general, it should be understood that each method developed for the residual stress evaluation has its specific advantages and drawbacks that make a specific approach appealing for a given application problem and inapplicable in some other situations. Having this in mind, however, it should be clear that current efforts are spent in the direction to increase the resolution of the measurement, both on the surface and in-depth, and to extend the capability to determine the residual stress present in the core of a given component. At the same time, also the development and improvement of nondestructive techniques for residual stress determination become of capital importance because this would expand the possibility to perform in situ evaluation.

Additionally, a further challenge is in the direction of adopting less expensive equipment and to reduce, at the same time, the cost and the time required for a single measurement [6]. A final direction of evolution of the research in the residual stress field is toward the RS characterization at the micro- and nanoscale. This is of interest by taking into account that, when dealing with high cycle fatigue and very high cycle fatigue conditions, the damage accumulation occurs at the grain level. Moreover, the introduction of micro- and nanomechanical systems requires an enhancement of the capabilities of characterization over this range scale. In the direction of improving the on-surface resolution, it worth mentioning the great effort that was done in the last years in the development of the contour method [7] that allows obtaining a complete map distribution of the residual stress along the cut plane, but it is limited to materials that can be cut by EDM machine, and it exhibits a higher level of uncertainty close to the edges of the measured surfaces. Magnetic methods such as magnetic Barkhausen noise [8] and acoustic Barkhausen noise [9] can be adopted as well to obtain residual stress map on a component, and current efforts are spent in the direction of accurate calibration procedures that take into account other factors such as the hardness, the crystallographic nature, or the complexity of the stress field that can impact on the measurement. Nevertheless, their adoption is limited to magnetic materials. The problem of increasing the depth of evaluation of residual stresses can be conveniently afforded, for example, by implementing synchrotron X-ray diffraction or neutron diffraction [10]; in both cases, however, very expensive equipment is required as well as long-term planning of the experimental campaign to access to the dedicated facilities. As an

**5**

Italy

**Author details**

*Introductory Chapter: New Challenges in Residual Stress Measurements and Evaluation*

alternative some modifications of the hole drilling method such as the deep hole drilling method [11] and the slitting method [12] are studied and under development for in-depth measurements. In the direction of developing new nondestructive

approaches, it appears now promising the possibility to combine optical and acoustic methods [13] to evaluate residual stress through the detection of the variation of the elasticity constant of the material. This is, incidentally, along another path of development followed by scholars in the last years aiming to combine multiple approaches in a single hybrid method (e.g., hole drilling/ring core). Capability to measure residual stress at the micro- and nanoscale is showing promising results, based, for example, on the adoption of the focused ion beam (FIB)-DIC micro-ring-core technique. This approach is very promising to be adopted for the evaluation of the type I, II, and III residual stresses. Raman spectroscopy, as well, is attracting a lot of interest in view of its capability to obtain microscale residual stress measurements [14]. It is worth mentioning, additionally, that efforts in improvement in experimental capability of residual stress evaluation impact also on the development and more accurate validation of modeling for numerical RS prediction. In this area the major challenges can be identified in the direction to extend the number of manufacturing processes and post-processes that can be modeled as well as to make the evaluation process faster and more accurate by introducing, for example, effects and features such as the kinematic

Caterina Casavola\*, Claudia Barile, Vincenzo Moramarco and Giovanni Pappalettera Dipartimento di Meccanica, Matematica e Management, Politecnico di Bari, Bari,

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

\*Address all correspondence to: caterina.casavola@poliba.it

provided the original work is properly cited.

hardening [15], phase transformation, and anisotropy.

*DOI: http://dx.doi.org/10.5772/intechopen.92620*

*Introductory Chapter: New Challenges in Residual Stress Measurements and Evaluation DOI: http://dx.doi.org/10.5772/intechopen.92620*

alternative some modifications of the hole drilling method such as the deep hole drilling method [11] and the slitting method [12] are studied and under development for in-depth measurements. In the direction of developing new nondestructive approaches, it appears now promising the possibility to combine optical and acoustic methods [13] to evaluate residual stress through the detection of the variation of the elasticity constant of the material. This is, incidentally, along another path of development followed by scholars in the last years aiming to combine multiple approaches in a single hybrid method (e.g., hole drilling/ring core). Capability to measure residual stress at the micro- and nanoscale is showing promising results, based, for example, on the adoption of the focused ion beam (FIB)-DIC micro-ring-core technique. This approach is very promising to be adopted for the evaluation of the type I, II, and III residual stresses. Raman spectroscopy, as well, is attracting a lot of interest in view of its capability to obtain microscale residual stress measurements [14]. It is worth mentioning, additionally, that efforts in improvement in experimental capability of residual stress evaluation impact also on the development and more accurate validation of modeling for numerical RS prediction. In this area the major challenges can be identified in the direction to extend the number of manufacturing processes and post-processes that can be modeled as well as to make the evaluation process faster and more accurate by introducing, for example, effects and features such as the kinematic hardening [15], phase transformation, and anisotropy.
