PSI - Issue 14

Digendranath Swain et al. / Procedia Structural Integrity 14 (2019) 207–214 Author name / Structural Integrity Procedia 00 (2018) 000–000

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1. Introduction 15CDV6 ESR steel is envisaged to be a futuristic material for the replacement of the existing solid stage motor casings material for Indian launch vehicles (Suresh (2011)). The low cost, abundant availability and comparable strength of this material are the main merits of its selection. However, this material lacks a consistent weld failure behavior. Therefore, it becomes problematic for the evolution of failure criteria at the weld joints through fracture based designs. Recently, the microstructure of this material has been improved (Suresh (2011)). This improved material is used for fabrication of few segments of a typical motor case so that failure criteria for design can be evolved through proof pressure testing (PPT). During PPT, Acoustic Emission Technique (AET) is generally employed in ISRO to access the structural integrity of various pressure vessels for the past three decades. AE technique helps in aborting the test in real-time before any catastrophic failure occurs depending on the severity of signals received from various AE channels. The locations of critical AE signals are provided to the designers for post-test inspection and further design upgradation. However, the main drawback with AE is that it provides only qualitative data; cf. any trend change in signal amplitude, duration, energy and hit counts, rather than any quantitative understanding of structural integrity. Here, it becomes imminent that the mechanical nature of material degradation and failure is properly understood so that strategies for improvements in design and fabrication can be chalked out. Towards this end, the modern full-field optical techniques may play an assisting role in assimilating the valuable AE data based on quantitative mechanical behavior. In this paper, DIC is used as a complementary full field measurement tool to comprehend the AE signals. 1.1. Literature survey and scope of work DIC today is considered to be the leader in optical techniques, which has been widely applied to crucial mechanics problems irrespective of material, loading, and physical conditions (Sutton et al. (2009), Reu (2015)). The structural integrity evaluation (Swain et al. (2014a, 2016)) and quantitative NDE capability (Swain et al. (2014b)) of this technique is already demonstrated. DIC today is self-sufficient in characterizing the surface deformation behavior of any object. However, DIC cannot predict the internal (bulk) microscopic behavior if its response is not reflected on the surface. On the other hand, AE is popular for its sensitivity to pick microstructural level changes in materials subjected to mechanical loading. AE technique also has been widely used to study many engineering materials and structures (Gholizadeh et al. (2015)). However, until now this technique, as a structural integrity evaluation tool, suffers from providing a succinct picture of failure behavior (Carmi et al. (2013)). Therefore, both of these techniques complement each other in terms of their capabilities. Very recently, a spurt in research is seen in the NDT, structural integrity, and experimental community to take advantage of the integration of these two techniques (Kontsos et al. (2015)). The objective of the current paper is elaborated below in the perspective of existing literature. Current literature reveals that AE-DIC technique have not only been used for studying metals (McCrory (2015), Sharma et al (2014, 2018), Tanvir et al. (2018), Vanniamparambil et al. (2015)), but for composites (Carmi et al. (2013), Faci and Baccouche (2016), Flament et al. (2014), Jefferson et al. (2015), Wilderman et al. (2015)), ceramic composites (Whitlow et al. (2016)), adhesives (Zarouchas and Hemelrijck (2014)), concrete (Fischer and Bohse (2014), Koschitzki et al. (2011)) and rocks (Cheng et al (2017), Lin and Labuz (2011)) also. The research trend indicates that combined AE and DIC have the potential to understand the structural integrity behavior with greater detail. Until now few studies have been carried out to understand weld failures under monotonic loading except Sharma et al. (2018). However, Sharma et al. (2018) did not consider any crack in the weld. Moreover, data related to failure of 15CDV6 ESR steel using AE-DIC is not available elsewhere. Furthermore, only compact tension (CT) and full thickness centre pre-cracked specimens have been studied using AE-DIC technique without paying much attention to surface cracks. In this paper the main objective is to interpret AE data by correlating the full field DIC data for SCT specimens (ASTM E740-03) and plain tensile specimens without any weld joints. AE parameters such as amplitude, duration, energy and hit counts were analyzed whereas quantitative parameters such as centre crack opening displacement (CCOD) and strain field near the crack tips were derived from DIC. CCOD in SCT specimens is similar to crack mouth opening displacement (CMOD) in edge cracked specimens. The experimental details and the results are presented in the following sections before concluding the study.