Introduction to Automated Macro Indentation Tester

Industrial infrastructure components undergo time-dependent material and property degradation associated with thermomechanical loads (temperature, pressures, dynamic loads) and operating environments (corrosive media, H2) which influences the structural integrity, and in rare scenarios lead to failure. The traditional approach has been to schedule periodic inspections to evaluate defect size, location and statistics which is represented as solid line in bath tub curve.

Improved operational life by using AMIT (knowledge-based NDT)

The emerging trend is to go beyond traditional risk-based inspection (RBI) to knowledge based inspection (KBI) which aims to understand mechanisms and use information to schedule the next inspection, represented by the dotted line in the bath tub curve. The benefit of this approach is an increase in plant availability factor as well as extended operational life beyond design limits.

AMIT (automated macro indentation test) falls in the category of knowledge-based inspection system.

What is AMIT ?

AMIT, automated macro indentation test, is a technique used for in-situ determination of key mechanical properties of components, such as yield strength (YS), ultimate tensile strength (UTS), strain hardening exponent (n), hardness (BHN) and fracture toughness (KIC). This technique was jointly developed by Ducom Instruments in partnership with NML, Jamshedpur.

This is an instrumented indentation technique of continuous measurement of indentation force and corresponding penetration depth throughout the loading cycle. It involves indenting the test piece with a spherical indenter (typically a 0.5 – 1.5 mm diameter, tungsten carbide ball) at a given location usually under load control. The test is conducted with repeated loading-partial unloading-reloading steps up to the maximum load, typically 1-2 kN. The indent size covers 5 to 10 grain diameters, thereby enabling derivation of macroscopic properties. Partial unloading enables determination of the elastic strain. Progressively increasing loading with a spherical indenter leads to increasing plastic strain thereby enabling determination of the complete stress vs. strain curve of the material.

Ducom AMIT software with load vs. displacement curves during indentation test.

The correlation between indentation load-penetration depth and material stress-strain is based on the following theories

(a) indentation strain correlates with true-plastic strain from a uniaxial tensile test [Tabor, 1951]

(b) mean ball indentation pressure correlates with true flow stress in uniaxial tensile test [Meyer, 1908] (c) tri-axial stresses under indentation are like stress state ahead of crack tip.

These correlations help us in determining the stress-strain curve and key mechanical properties from AMIT  

Where is AMIT currently being used?

There are several industries where AMIT technique is useful and few examples are given below.

(a) pipelines – evaluation of tensile properties of welds and HAZ of pipes carrying natural gas

(b) railways – evaluation of fracture toughness of railway tracks

(c) power generation – assessment of mechanical properties of high temperature components for residual life programs

(d) manufacturing – evaluation of mechanical properties and quality assurance of forged components as well as life assessment of forging dies

Ducom AMIT used in testing of pipes in the field and in the laboratory.

Does AMIT qualify to be a non-destructive technique (NDT)?

Yes. It is due to the barely visible hemi-spherical impression on surface < 0.2mm diameter after indentation, the impression has no sharp edges (hence no cracks initiated) and the structural integrity of component is not affected.

Benefits of AMIT compared with other NDT techniques?

Currently available NDT methods such as ultrasound, eddy current, thermography is used for inspection of components to identify defects, hotspots, wall thinning, from safety and reliability perspective and does not provide information on material degradation due to thermomechanical loads (creep, fatigue, microstructure evolution). AMIT is a unique technique which can estimate several critical mechanical properties such as yield strength, ultimate tensile strength and fracture toughness through a single non-destructive test at a given location. Moreover, there is no test sample preparation required and a test can be completed within 5 mins. As the test is localized, it is useful for characterizing welds and HAZ (heat affected zone). AMIT together with in-situ replica (another established NDT method) form a set of complementary techniques providing comprehensive information on the actual microstructure as well as corresponding mechanical properties of components based on their service history.

What are the competitive advantages of AMIT?

In addition to offering a high value at affordable price, we provide the following advantages:

(a) less than 10% difference in estimated properties compared to conventional tensile tests

(b) location specific data tags during field tests and data portability to cloud storage

(c) mini compressed air canister portable energy source for instrument operation

(d) indenter balls of different sizes from 0.5 – 1.5 mm diameter providing flexibility for testing component/pipe with thicknesses from few mm to 10’s of mm

(e) successful in understanding structural integrity of wide range of engineering materials e.g., aluminum alloys, titanium alloys, steels and superalloys

How satisfied are the customers using the AMIT?

Leading scientific organizations in India such as Baba Atomic Research Center (BARC) and National Thermal Power Corporation (NTPC) have used Ducom AMIT successfully both in the lab for material property evaluation and database development as well as in the field for component structural integrity evaluation. Dr. Gautam Das (NML, National Metallurgical Lab), Dr. Kamal Sharma (BARC, Bhabha Atomic Research Center) and Dr. Avijit Mondal (NTPC, National Thermal Power Corporation) are few of the technical experts on this technique, who can provide more insights about Ducom AMIT.

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