PSI - Issue 36

V.V. Kharchenko et al. / Procedia Structural Integrity 36 (2022) 59–65 V. V. Kharchenko et al. / Structural Integrity Procedia 00 (2021) 000 – 000

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Indentation at the macro level is performed when examining macro-volumes of material, whereas indentation at the micro- and nanoscale is applied when examining thin films, coatings, and reinforcing layers, as well as individual phases or inclusions in micro- and nanostructured materials. Therefore, the attempt to provide all requirements for such tests at fundamentally different levels in one standard is not very convenient, especially for its practical application, since the equipment requirements and other important aspects of this type of test at such levels can differ significantly. When determining the mechanical characteristics, the indentation range plays an important role. Indeed, when extending the service life of critical equipment, it is more reasonable to determine the strength characteristics at the macro level. But a proper balance must be maintained – the indentation imprint should not lead to huge damage in the structure, and the deformed material must characterize the structure as a whole. The authors believe that a spherical indenter is the most appropriate for indentation. When tips of different shapes are indented into the material under investigation, different stress-strain states are formed in the indentation zone. When a cone or pyramid is indented to different depths, similar imprints are formed creating identical strain fields within the imprint zone, shown by Bakirov and Potapov (2000). The average pressure on the surface of the imprint appears to be a constant value. In the process of ball indentation to different depths, the average pressure in the imprint varies depending on the angle of indentation or the degree of deformation. The ball indentation under different loads produces an indentation diagram that provides significantly more information than a cone or pyramid indentation denoting a single point on that diagram. The construction of hardness diagrams using a conical or pyramidal indenter requires several indenters with a variable angle at the tip. A ball is considered as a universal indenter with a variable sharpening angle that increases with depth, pointed out by Bakirov and Potapov (2000). However, standard ISO 14577 1 (2015) pays almost no attention to indentation using a spherical indenter and consequently does not regulate the Brinell hardness determination. This is a significant drawback since it is a key mechanical characteristic of materials and is widely used in determining other characteristics using indirect methods. In addition, the Brinell hardness along with the strength characteristics serves as an important indicator of the quality of the material during its approval and the design of structures. Different approaches are used to determine the yield and strength limits by instrumented indentation, the main of which are illustrated in Fig. 3.

Haggag (1989), Ahn et al. (2001), etc.

n n   =     К e

F d

f    =     

 

  

( ) f   =

f = 





в 

2



( ) * , c d f h n =

( ) , , p d f F h d =

ISO/TR 29381:2008 (2008), Ahn et al. (2001)

( ) n K   =

2 f F d   =       =     f d D

( ) * , c d f h n =

(0, 002)

n

K

0,2  =



(

)

( ) 1 1 n +

K n

* t r h f h h = , c

в  =



n

Haggag (1989), George et al. (1976), Bakirov et al. (2000), etc.

0,2 m A   =

2 n F d А d D   =    

2 ( t t d h D h = −

)

Fig. 3. Scheme of the main approaches to the determination of yield and strength limits via instrumented indentation.