Transactions, SMiRT 21, 6-11 November, 2011, New Delhi, India Div-II: Paper ID# 385 1 EVALUATION OF CRACK ARREST LENGTH AND CRACK ARREST TOUGHNESS OF COLD-WORKED AND AGED MODIFIED 9CR-1MO STEEL BY APPLICATION OF KEY-CURVE METHOD S.Sathyanarayanan, A.Moitra, G.Sasikala, A.K. Bhaduri and V.Singh* Materials Technology Division Indira Gandhi Centre of Atomic Research Kalpakkam, India -603102 *Department of Metallurgical Engineering Institute of Technology, Banaras Hindu University Varanasi, India 521005 E-mail of corresponding author: [email protected] ABSTRACT Crack Arrest Toughness (K IA ) is being proposed as a characteristic material toughness below which cleavage fracture does not occur. K IA is usually obtained by means of large specimens like the Compact Crack Arrest (CCA) specimens used in ASTM standard E1221. Obtaining K IA from small size Charpy specimens would be of utility to the nuclear industry because of their usage as nuclear surveillance specimens. Literature reports indirect methods of obtaining K IA from Charpy specimens by empirical correlation with results obtained from larger specimens. In a recent study, the authors have proposed a method for direct evaluation of K IA from instrumented impact tests of pre-cracked Charpy specimens and applied it to modified 9Cr-1Mo steel. In the present study, this method is extended to the same steel in thermally aged condition. The load-displacement traces obtained from instrumented Charpy-V impact tests with fatigue pre-cracked specimens of a modified 9Cr-1Mo steel in 0%, 10% and 15% cold worked and aged (650 °C/10000h) condition in the transition temperature regime bear signatures of crack arrest, because after crack initiation the load does not fall to zero level as observed for total cleavage fracture. While the arrest load can be determined from the load- displacement traces, assessment of crack length at arrest is an uncharted domain. This is mainly because no unequivocal signature of arrest can be found on the fracture surface enabling direct measurement. In this study, the Key-Curve technique has been effectively used to assess the crack arrest length that is subsequently used for estimation of the crack arrest toughness (K IA ). The Key-Curve commonly used for ductile fracture, expresses the load (P) as a separable and multiplicative function of ligament length (b) and displacement (d) in the form P = B*W*(b/W) η *H(d/W), where B and W are standard specimen dimensions and η = 2 for 3PB specimens and H is a power law function. In this study, this has been applied for cleavage fracture. The constants in the Key-Curve expression were obtained by fitting the load– displacement pairs between yield point and peak load preceding cleavage fracture and used to obtain crack arrest length. The results were validated by comparing with final crack length measured optically on unbroken specimens of 15% cold worked material aged for 650°C/5000h. Using the arrest load and the arrest length, K IA has been determined using the ASTM E399 expression. The average K IA for 0, 10 and 15% cold-worked and aged (650°C and 10000 h) datasets are 62.7, 65.8 and 69.9 MPa.m 0.5 respectively. The datasets indicate substantial scatter in crack initiation phenomenon contrary to the arrest. The scatter in crack initiation toughness was characterized by the three parameter Weibull distribution used in the reference temperature based Master Curve approach. The corresponding median K Jd , where K Jd is the dynamic crack initiation toughness, for the three datasets are 74.3, 72.2 and 81.2 MPa.m 0.5 respectively. Keywords: Crack arrest, key curve, cleavage fracture, DBTT, ferritic steel. 1. INTRODUCTION Crack arrest toughness [1, 2] is being proposed as a characteristic material toughness below which cleavage fracture does not occur. The standard test method [3] for determining crack arrest toughness uses Compact Crack Arrest (CCA) specimens that are much larger in size than Charpy specimens used for nuclear surveillance studies. Crack arrest phenomenon has been studied using instrumented impact tests of Charpy
EVALUATION OF CRACK ARREST LENGTH AND CRACK ARREST TOUGHNESS OF COLD-WORKED AND AGED MODIFIED 9CR-1MO STEEL BY APPLICATION OF KEY-CURVE METHOD
May 30, 2023
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