ANALYSIS OF THE SMALL PUNCH TEST CAPABILITY TO EVALUATE THE RESPONSE OF HIGH STRENGTH STEELS FACING HIC OR SCC

This work is about evaluating the behavior facing HIC of
high strength steels by means of the Small Punch Test (SPT). It
can be considered as a quasi-non-destructive test in comparison
to structural integrity analysis of large components. It was
developed during the 80’s with the purpose of estimating the
embrittlement grade of nuclear components reducing the
amount of material employed. During the last years it has been
successfully employed in the evaluation of mechanical
properties of different materials and creep behavior. Also
approximations for the fracture properties estimations have
been carried out using this method. Although a reference
standard that includes the tensile and fracture parameters
estimations by SPT does not exist, a European Code of Practice
(CWA 15627:2008) was recently developed. In addition a
European standard is in preparation, including the ultimate
research and the backup of the most relevant groups.
In this work, high strength steels behavior facing stress
corrosion cracking (SCC) or hydrogen Embrittlement (HE)
processes are analyzed by means of the Small Punch Test
(SPT). The evaluation of the response of materials facing
environmental damage processes requires a different
consideration if cracks are present on the material or not. In a
first stage the study carried out tries to analyze the behavior
without cracks, using the threshold stress (σscc) parameter. The
aforementioned parameter is obtained from slow strain rate
tensile tests (SSRT), which involves its own particular
disadvantages. Thus the aptitude of the SPT to obtain the
threshold stress is studied, evaluating the influence of variables
such as the solicitation rate.
In the second part of the work, specimen geometry and test
conditions are proposed for the SPT, in order to evaluate the
susceptibility facing SCC and HE in presence of cracks for the
materials studied. In this case, the fracture toughness parameter
that describes the crack initiation process (KHE) will be
evaluated and validated by conventional tests based on fracture
mechanics. The influence of variables, such as test solicitation rate on the results, is analyzed in order to obtain a qualitative
methodology to evaluate mechanical-environmental damage
processes by SPT means. For the SPT tests carried out,
common Small Punch specimens of 10×10 mm of section and
0,5 mm of thickness are used for σscc determination.