Citation: Rebora, A.; Torre, G.;
Vernassa, G. Stress Concentration
Factors in Excavation Repairs of
Surface Defects in Forgings and
Castings. Materials 2022, 15, 1705.
https://doi.org/10.3390/ma
15051705
Academic Editors: Chunsheng Lu
and Arkadiusz Gola
Received: 20 December 2021
Accepted: 21 February 2022
Published: 24 February 2022
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Article
Stress Concentration Factors in Excavation Repairs of Surface
Defects in Forgings and Castings
Alessandro Rebora
1
, Giorgio Torre
2,
* and Gianluca Vernassa
1
1
Department of Mechanical Engineering, University of Genoa, Polytechnic School, Via All’ Opera Pia 15A,
16145 Genoa, Italy; rebora@unige.it (A.R.); s4256627@studenti.unige.it (G.V.)
2
SBM Offshore, 11 Avenue Albert II, 98000 Monaco, Monaco
* Correspondence: giorgio.torre@sbmoffshore.com; Tel.: +377-92051783
Abstract:
This paper provides an analytical formula for the theoretical stress concentration factor in a
common type of excavation repair for large forgings and castings. Mechanical components obtained
with these processes are often subjected to superficial defects. As the rejection of such pieces is out of
question, given the relevant size and costs associated with them, usual industrial practice consists in
the removal of the defect and a portion of the surrounding material through milling processes. The
authors have selected a reference geometry of the excavation to be left on the mechanical pieces, which
can be easily controllable in practice by three operating parameters. Then, the domain of existence
of such a repair was investigated on a sequence of discrete points, by means of FEA, obtaining for
each, the values of the stress concentration factor K
t
. Finally, through polynomial regression, the
K
t
functions have been accurately approximated by a sixth degree polynomial formulation, which,
given a triplet of dimensional geometric parameters, is able to compute the stress concentration factor
K
t,
with an error that never exceeds 8%.
Keywords: defects; stress concentration; repair; excavation; FEM
1. Introduction
Large mechanical components obtained by forging or casting can often be affected by
different types of superficial defects. The most common are bubbles and cracks [
1
], which,
depending on their shape and size, may lead to relevant stress concentration, compromising
the long-term life of the part. To overcome this issue, one of the most common industrial
practices consists of the simple technique of removing the defects and a portion of the
surrounding material, through manual milling operations, made with a conventional disc
cutter or with a ball nose cutter. Sometimes, after milling, the emptiness left on the surface
is also welded, aiming to fill it completely with additive metal. However, not all ferrous
materials may be easily subjected to this treatment. In addition, the time-consuming
welding operations may have a negative impact on final delivery times. In all such cases,
the remedy of sole milling does not bring the defective part to its ideal shape, since a small
portion of the material is still removed from its surface, but leaves an imperfection of a
more controlled shape. The stress amplification, therefore, persists, but at lower intensities
and can be grasped quantitatively through accurate FEM analyses. However, these require
the definition of numerical models for the specific purpose, where the detailed shape of the
new defect must be added to the original complexity of the system. Models usually turn
out to be very heavy computationally and the results are always a compromise between
accuracy and time. A practical solution of engineering interest is the identification of a
theoretical stress concentration factor K
t
, defined as the following ratio:
K
t
=
σ
max
σ
n
(1)
Materials 2022, 15, 1705. https://doi.org/10.3390/ma15051705 https://www.mdpi.com/journal/materials
