PSI - Issue 33

Dario Santonocito et al. / Procedia Structural Integrity 33 (2021) 724–733 Santonocito et al./ Structural Integrity Procedia 00 (2019) 000–000

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2.2. Static Thermographic Method

In 2013, Risitano and Risitano (A. Risitano and Risitano, 2013) proposed a very rapid procedure to assess the first damage within the material monitoring its temperature evolution during a uniaxial tensile test. During a static tensile test of common engineering materials, the temperature evolution, detected by means of an infrared camera, is characterized by three phases (Fig. 2): an initial approximately linear decrease due to the thermoelastic effect (obeying to Lord Kelvin’s law, Phase I), then the temperature deviates from linearity until a minimum temperature value (Phase II), therefore it experiences a very high further increment until material failure (Phase III). Under uniaxial stress state and in adiabatic test conditions, Equation 1 can be simplified as:

1  T K T m s   

(1)

Fig. 2. Temperature trend during a static tensile test

The use of high precision IR sensors allows to define experimental temperature vs. time diagram during static tensile test in order to define the stress at which the linearity is lost. In 2010, Clienti et al. (Clienti et al., 2010) for the first time correlated the damage stress σ lim related to the first deviation from linearity of ∆T temperature increment during static test (end of phase I) to the fatigue limit of plastic materials. If it is possible during a static test to estimate the stress at which the temperature trend deviates from linearity, that stress could be related to a critical macro stress σ lim which is able to produce in the material irreversible micro-plasticity. This critical stress is the same stress that, if cyclically applied to the material, will increase the microplastic area up to produce microcracks, hence fatigue failure. 3. Materials and Methods Static tensile tests and fatigue tests were carried out on dogbone specimens made of HDPE, according to the type 1A geometry of ISO527-2:1993 standard, with a nominal cross section of 10mm x 4mm (Fig. 3a). The specimens were retrieved from a pipe joined by means of the welding procedure prescribed by the Italian national standard UNI 10520. The welding process consist, firstly in heating the two sides of the pipeline, up to their fusion, adopting a thermoelement. Secondly, the two sides are joined together by applying a pressure force for a prescribed amount of time. All the tests were performed with a servo-hydraulic load machine ITALSIGMA (Fig. 3b) with a maximum load of 25 kN. The static tests were performed on three specimens under displacement control, adopting a crosshead speed of 5 mm/min. A series of stepwise fatigue tests was conducted on two specimens with a stress ratio R= 0.2 and