PSI - Issue 39

Muhammad Ajmal et al. / Procedia Structural Integrity 39 (2022) 347–363 Author name / Structural Integrity Procedia 00 (2019) 000–000

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yielding retains the advantage of Linear Elastic Fracture Mechanics (LEFM) (Paris and Erdogan 1963). The use of SIF is also helpful to quantify the effect of crack size and loading conditions on stress singularity. However, there are some limitations in the use of stress intensity factor range as a driving force for FCGR: i) Inability to explain load ratio and variable amplitude effects, and its inconsistent behavior observed for short cracks(Antunes et al. 2019) and ii) da/dN- ∆K relations do not add unders tanding as a driving mechanism for fatigue crack growth because the units of both parameters are totally different. Therefore, to overcome these limitations different concepts have been introduced. Elber (Elber 1971) introduced the concept of crack closure assuming that the part of load cycle during which the crack flanks are in contact does not contribute to fatigue crack growth and modified the Paris Law by replacing ∆ K with ∆ K eff as follows: ( ) m da eff dN C K = ∆ (2) Where ΔK is the SIF range (between the opening load and maximum load) This concept has been applied successfully to address the effects of load ratio, load history, short cracks and stress state, but the quantification of crack closure levels are dependent on measurement procedures, along with according to several authors its irrelevance under plain strain conditions (Vasudevan, Sadananda, and Louat 1992). Similarly, several other solutions were proposed like the concept of partial crack closure (Donald and Paris 1999)(Kujawski 2001b), the use of K max along with ∆ K (Kujawski 2001a)(Noroozi, Glinka, and Lambert 2005), T stress (Lugo and Daniewicz 2011)(Larsson and Carlsson 1973) (Miarka et al. 2020a)(Miarka et al. 2020b) to consider the effect of specimen geometry and CJP model (Christopher et al. 2007) proposing four parameters to describe the stresses around the crack-tip. However, ∆K has limited application to describe fatigue crack growth, being an elastic parameter while crack growth is believed to be controlled by non-linear and irreversible parameters occurring at the crack-tip. Consequently, many researchers focused themselves to study FCG based on stress and strain fields (Noroozi, Glinka, and Lambert 2005), energy dissipated at crack-tip (Zheng et al. 2013), and cyclic J-Integral (Ktari et al. 2014). Crack-tip plastic deformation can be better described by two parameters naming J-Integral and crack tip opening displacement (CTOD). It is believed that CTOD serves better being a local parameter. The concept of da/dN- ∆CTOD p (plastic component range of CTOD) was originally presented by Antunes et al. (Antunes et al. 2016)(Antunes et al. 2017b)(Antunes et al. 2018) making use of numerical data for the extraction of ∆ CTOD p . The main objective of the present work is to obtain a da/dN- ∆CTOD p model for austenitic stainless steel from full-field displacement data collected by using Digital Image Correlation (DIC). The material used in the study is 316L austenitic stainless-steel alloy having a Young’s modulus, E around 195 GPa and yield stress, σ 0 = 304 MPa. Fatigue crack growth specimens were prepared following a standard CT specimen configurat ion according to (ASTM E647−13 2014) . The specimen to be considered as thick is specified according to the ratio of the thickness, B to the uncracked ligament length, ( ) W a − , as ( ) / 1 B W a − ≥ . Due to experimental limitations, the ratio of the thickness to the uncracked ligament length was chosen to be 0.5 for thick CT ( B = 12 mm) specimen. 2.2. Fatigue crack growth experiments Four specimens were used in this experimental study. For reference purposes the critical fracture toughness of these thin specimens was determined to be Kc 35 MPa m = (Yusof, Lopez-Crespo, and Withers 2013). The experiments were conducted at room temperature for fatigue crack growth on a servo-hydraulic testing machine with a ±10 kN loading range. A schematic diagram of the experiment is shown in Fig. 1. The specimen was fatigued at a frequency, 2. Experimental Details 2.1. Materials and specimen