Detection of local stress concentration zones in engineering
products—the lacking link in the non-destructive testing system
Received: 22 December 2016 /Accepted: 4 January 2018 /Published online: 18 January 2018
International Institute of Welding 2018
It is known that one of the main sources of damage during operation of machine-building products are local stress concentration
zones (SCZ) that form under the action of workloads primarily on defects of metallurgical and process nature. Sizes of these SCZ
vary from several tens of microns to several millimeters. Furthermore, it is unknown where these local areas are located and how
they can be found. At the same time, the reject levels of the applied conventional non-destructive testing (NDT) methods at
manufacturing plants significantly exceed the sizes of some metallurgical defects. At present, a fundamentally new NDT method
based on the use of the magnetic memory of metal (MMM) is more commonly applied in practice. It uses the natural magne-
tization formed during the products fabrication. The article considers the capabilities of the MMM method for inspection of
machine-building products quality by structural inhomogeneity and residual stresses.
Metal magnetic memory method
Stress concentration zone
Failure due to the combined effects of stress and other causes
is included in the general types of mechanical failure .
Local stress concentration zones (SCZ) appear due to effect
of working loads, first of all, on metallurgical and process
defects . Presence of the SCZ is one of the sources of
damaging during critical equipment operation.
Metallurgical and process manufacturing defects are
known to cause high levels of residual stress (RS) in local
zones of the product. The RS examination at some produc-
tions is performed on a selective basis. In this case, the average
(volumetric) level of the RS is inspected, and local RS zones
due to internal defects of the metal, as a rule, are not inspected
and omitted. Besides, location of these local zones and meth-
od of their detection are unknown.
As a rule, the RS examination during the incoming inspec-
tion is not performed. For these reasons, during the very first
years of products operation under the working load failure can
occur. Process and metallurgical defects, causing the high lev-
el of RS in local zones of products at unfavorable combina-
tions with stresses due to working loads, cause accelerated
development of damages.
It is known that conventional non-destructive testing
(NDT) methods (radiography, ultrasonic testing, eddy-
current testing, magnetic powder inspection, dye penetration)
are aimed at searching and detection of pronounced defects.
Internal casting defects, various types of structural inhomoge-
neity as well as manufacturing process defects (welding,
rolling, bending, heat treatment defects, etc.) could remain
undetected in products due to the lack of 100% quality inspec-
tion at most of the plants as well as due to imperfection of
NDT methods applied. Moreover, these rejection standards of
NDT methods used at products manufacturing plants are
aimed at detection of defects with sizes that many times ex-
ceed those of metallurgical defects. As practice shows, metal-
lurgical defects of smaller sizes, when exposed to working
loads, are one of the main sources of operational damages.
In conditions of products operation, practically all NDT
methods are aimed at detection of discontinuities with larger
sizes deemed by referencing code to be detrimental to service.
Recommended for publication by Commission V - NDT and Quality
Assurance of Welded Products
* Sergey Kolokolnikov
Energodiagnostika Co. Ltd., Moscow, Russia
Welding in the World (2018) 62:301–309