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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1. Introduction High Density Polyethylene (HDPE), thanks to its good properties, such as corrosion resistance and lightness, is still one of the most adopted material for the realization of water and gas distribution pipeline. Given the fact that it is a viscoplastic material, it also allows the joining of different section of pipe by welding. Several studies exist in literature regarding the base material, both under static and fatigue loads. Deveci and Fang (Deveci and Fang, 2017) discussed the correlations between chemical properties and slow crack growth resistance in notched pipe tests. Other authors investigated the fatigue damage of HDPE adopting several damage model, stress and energy based (Bourchak and Aid, 2017; Djebli et al., 2016). Risitano and Santonocito for the first time, applied infrared thermography to investigate in a rapid way static and fatigue properties of HDPE (Risitano et al., 2020, 2018). Galchun et al. and Kovalchuk et al. (Galchun et al., 2015; Kovalchuk et al., 2019) investigated the thermal properties of dissimilar welded joint made of PE-80 and PE-100. They show how during the welding process, restructuring of crystalline phases occurs, leading to higher mechanical and thermal properties. Demchenko et al. (Demchenko et al., 2017) observed a relaxation behavior in butt weld joints: after a year relaxation occurs both on the amorphous and crystalline phase, with a relative change in the PE properties. Dai and Peng (Dai and Peng, 2017) analyzed the welding process, showing how a molecule reorientation occurs and crystallinity is severally affected by welding pressure. Li et al. (Li et al., 2016) studied the welding effect on crack growth adopting the Cracked Round Bar test. They observed how thermal welding process has a little influence on crack propagation characteristic of PE but, on the other hand, it has a greater influence on its lifetime. Mikula et al. (Mikula et al., 2015) adopted fracture mechanics approach to investigate the crack behaviour of welded polyolefin pipes. As the authors are aware, no study exists regarding the fatigue properties of HDPE welded details. Usually, fatigue tests require a large amount of time to derive the fatigue limit and the SN curve of the material. Given the fact that fatigue damage is an energy dissipative phenomenon, adopting infrared thermography could give us important information, shortening the testing time. Following the first formulation by Risitano and co-workers (Curti et al., 1986), La Rosa and Risitano proposed an innovative approach for the rapid assessment of the fatigue limit and the S-N curve of the material adopting a very limited number of tests: the Thermographic Method (TM) (La Rosa and Risitano, 2000). In 2013, Risitano and Risitano (A. Risitano and Risitano, 2013) proposed the Static Thermographic Method (STM) as a rapid test procedure to assess the first damage initiation within the material, evaluating the end of the thermoelastic effect during a static tensile test. Thermography has been adopted on several materials under different loading conditions: notched and plain steel specimens under static and fatigue tests (Corigliano et al., 2020, 2019; Foti et al., 2020; Ricotta et al., 2019; Rigon et al., 2019; Antonino Risitano and Risitano, 2013), glass fibre reinforced composites under static and fatigue loading (Crupi et al., 2015b), steels under high cycle (Amiri and Khonsari, 2010; Curà et al., 2005; Meneghetti et al., 2013) and very high cycle fatigue regimes (Crupi et al., 2015a; Plekhov et al., 2015), high strength concrete (Cucinotta et al., 2021). More recently, Santonocito applied infrared thermography on 3D printed plastic and metals (Santonocito, 2020; Santonocito et al., 2021). The aim of this research activity is the application of the STM and of the TM during static tensile and fatigue tests for the assessment of the fatigue life of HDPE welded specimens. In addition, constant amplitude (CA) fatigue tests are carried out to evaluate the fatigue limit in a traditional way and compared it with the ones obtained by STM and TM.

Nomenclature c

specific heat capacity of the material [J/kg.K]

f

test frequency [Hz]

k

inverse slope of the SN curve thermoelastic coefficient [MPa -1 ]

K m

R

stress ratio test time [s]

t

T, T i

instantaneous value of temperature [K]

T 0

initial value of temperature estimated at time zero [K]