Guide for Recommended Practices to Perform Crack Tip Opening Displacement Tests in High Strength Low Alloy Steels

Soldagem & Inspeção. 2016;21(3):290-302 http://dx.doi.org/10.1590/0104-9224/SI2103.05 Technical Papers Guide for Recommended Practices to Perform Crack Tip Opening Displacement Tests in High Strength Low Alloy Steels Julián A. Ávila1,2*, Vinicius Lima2, Cassius O. F. T. Ruchert3, Paulo Roberto Mei1, Antonio J. Ramirez1,2,4 Universidade Estadual de Campinas – UNICAMP, School of Mechanical Engineering, Campinas, SP, Brazil. Brazilian Nanotechnology National Laboratory, Campinas, SP, Brazil. 3 Universidade de São Paulo – USP, School of Engineering, São Carlos, SP, Brazil. 4 The Ohio State University, Columbus, OH, USA. 1 2 Received: 26 Feb., 2016 Accepted: 17 June, 2016 *E-mails: (JAA), (AJR) Abstract: Fracture mechanics approach is important for all mechanical and civil projects that might involve cracks in metallic materials, and especially for those using welding as a structural joining process. This methodology can enhance not only the design but also the service life of the structures being constructed. This paper includes detailed consideration of several practical issues related to the experimental procedures to assess the fracture toughness in high strength low alloy steels (HSLA) using the crack tip opening displacement (CTOD) parameter, specifically pipeline steels for oil and gas transportation. These considerations are important for engineers who are new in the field, or for those looking for guidelines performing different procedures during the experimentation, which usually are difficult to understand from the conventional standards. We discuss on topics including geometry selection, number of replicate tests, fatigue precracking, test procedure selection and realization, reports of results and other aspects. Key-words: CTOD; Experimental evaluation; Fracture toughness; HSLA steel; Pipeline steels. 1. Introduction Selection of a fracture toughness parameter and type of specimen to assess materials with elasto-plastic behavior, such API-5L steels and different HSLA steels, depends on the level of approximation of the test conditions to a hypothetical crack in the actual structure. Aspects such as geometry of the specimen, load application, environmental and mechanical properties are the key factors to perform fracture toughness tests. CTOD and J-integral are the most common parameters used in the industry because the tests are practical and the methods are standardized [1]. These tests are conducted regularly on precracked specimens, where the initial state of the crack front might represents an actual or hypothetical crack in any structure element. Regarding the reproducibility of real work conditions of a structure in lab conditions, the definition of the stress-strain state at the crack tip (or constraint) is fundamental to properly assess the fracture toughness [2], because it is basically the experimental assumption of how the stress is working in a real structure. That issue has been sorted by using different standardized specimens, the available technical standards offers different geometries and load modes, such as bending or tensile configurations. In addition, it is usual to find that those specimens offer a high constraint state and consider a crack growth under the plane-strain condition [3-6]. Although cracks in real structures do not always fulfill the requesting standard conditions, for example, residual stresses induced by welding procedures could change the constraint state and cause uneven fatigue cracks fronts [7,8], very shallow cracks are not considered standard, and some constraint conditions are far away from the options presented by the standards [9]. This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License which permits unrestricted non-commercial use, distribution, and reproduction in any medium provided the original work is properly cited. Even though it is not often used in the engineering projects, the core of fracture mechanics as we know was developed during the 1970s through the first part of the 1980s [10]. The bibliography on fracture mechanics is long, many important reviews have been published [2,10,11], books have been written addressing theoretical and experimental approach [12-14], and classic literature has also focused on the experimental assessment of welded structure [15]. A new user might struggle with problems associated with the proper selection of the geometry of the specimen, test preparation and testing, as well Guide for Recommended Practices to Perform Crack Tip Opening Displacement Tests in High Strength Low Alloy Steels as results interpretation. The aim of this guide is to present procedures from the experimental viewpoint, where different problems-solutions have been addressed in order to properly perform a fracture toughness assessment using the CTOD parameter. 2. Guidelines 2.1. Initial considerations The BS-EN-ISO-15653 standard [8], at item 6.3 presents a flow diagram for performing the fracture toughness test. It starts by defining the interest region of study, followed by the selection of the specimen which better represents the constraint of the real application and finally, the choice of the direction, sense and depth of the initial notch. The rate of invalid specimens can be about 10 to 15% of the total number. In order to decrease that rate, it is recommended to perform a preliminary matrix of tests which allows early correction of the problems. Most of the difficulties are related to the unknown fatigue crack and test parameters for each kind of material or welding conditions. Other problems are related to incorrect position of the specimen on the fixtures, improper adjustment of the fixtures or sensors, incorrect specimen sizing, excessive brittle or plastic behavior and residual stresses. 2.2. Number of tests For the resistance curves Δa-R, 6 specimens in each evaluated condition are needed. The DNV-OS-F101 standard [16] recommends three repetitions of each evaluated condition for the CTOD, J and KIC parameters; if one of the three results is unsatisfactory, performing more 3 tests is recommended, then choosing the lowest result from the 5 results as the critical one [17]. The BS-7910 standard [18], item 7.1.5.6, recommends for levels 2 and 3 assessments for CTOD and the critical stress intensity factor (KIC), use of the minimum value of three tests as the materials toughness. More detailed and statistical data treatment can be found in Annex k of the same standard [18]. 2.3. Geometries of specimen The suggested geometries for the fracture toughness assessment by the standard methods produce conservative results, because of the high constraint state consideration [19]. In addition, there are geometries which represent the crack tip constraint better than the usual standard geometries; however, geometry selection depends on the features of the project. The majority of the standardized specimens recommends specific rela (...truncated)