Advanced Materials & Processes

NOV-DEC 2013

Covers developments in engineering materials selection, processing, fabrication, testing/characterization, materials engineering trends, and emerging technologies, industrial and consumer applications, as well as business and management trends

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Page 16 of 92

ASTM E1290, BSI 7448 Parts I and II, and ISO 12135 and 15653[1-5]. These test procedures were largely developed for base metals and did not provide specific guidance for characterizing the toughness of welds and heat-affected zones (HAZs). In addition, application of these test methods was largely accomplished using single-edge notched bend (SENB) and compact tension (C-T) specimen geometries. These geometries are presented in Figs. 1 and 2. In each figure, B corresponds to material thickness. The SENB specimen shown in Fig. 1 is loaded in threepoint bending, whereas the C-T specimen shown in Fig. 2 is loaded in uniaxial tension. These specimen geometries are frequently used to determine single-point fracture toughness properties, i.e., toughness at either the onset of unstable crack extension or upon achieving fully ductile behavior where the specimen reaches a maximum load plateau without unstable fracture. One advantage of the CT specimen is that it typically requires less material to produce compared to the SENB specimen. In addition, test specimen fixturing is less complicated with SENB geometry because the C-T specimen typically requires several clevis fixtures of various sizes to test a wide range of specimen thicknesses. In most cases, results obtained from either specimen can be correlated and compared with one another, and equations exist to permit characterizing toughness in terms of J or CTOD with either geometry. Regardless of design features, specimen preparation includes machining a notch via milling, cutting, or electrodischarge machine (EDM) methods. The specimen is then loaded cyclically, usually in a conventional servo-hydraulic 0.1 W W ± 0.005 W 0.2 W 2.25 W min B= 0.5 W 2.25 W min Fig. 1 — Single edge notched bend (SENB) geometry (reproduced from [1]). 0.275 W ± 0.005 W a W± 0.005 W 1.25 W ± 0.010 W 0.6 W ± 0.005 W 0.275 W ± 0.005 W 0.6 W ± 0.005 W 0.25 W ± 0.005 W diameter 2 Holes B B = (W/2) ± 0.010 W Fig. 2 — Compact tension (C-T) geometry (reproduced from [1]). 14 ADVANCED MATERIALS & PROCESSES • NOVEMBER-DECEMBER test frame, to initiate a fatigue crack from the root of the machined notch. Test standards provide guidance on applied loads to be used during this fatigue precracking operation and place limits on fatigue crack length. Fatigue crack provides a sharp discontinuity from which either unstable fracture or stable tearing will initiate during the fracture toughness test. Correct placement of the fatigue crack is of great importance in order to sample the material or microstructure of interest, particularly when characterizing the toughness of weld HAZ regions. More recently, increased use of alternative specimen geometries, particularly the single edge notched tension (SENT) geometry, has become common. This geometry supports strain-based design criteria, principally for pipeline applications, where applied strain in service may exceed the material's yield strength and conventional stress-based design methods are unsuitable. In these situations, the material may be subjected to stresses that will cause permanent deformation and therefore, traditional fracture toughness test methods addressing initiation toughness are not applicable. Typical applications include pipelines operating in cold climates where frost heave may occur, in seismic zones where large soil displacements are possible, or in zones at risk of landslides. This article highlights some issues from a practical test perspective that should be considered when attempting to characterize the fracture toughness properties of base metal, weld, or HAZ regions. Various specimen designs are also discussed, such as the SENB and SENT geometries, including guidance on when to choose one versus another. This article also focuses on issues relevant for carbon steels and high-strength low alloy (HSLA) materials in wrought, cast, or forged conditions. Specimen size and constraint effects For ferritic materials, specimen size and level of constraint imposed on the crack tip can significantly influence toughness results, particularly for test temperatures within the material's ductile-to-brittle temperature transition regime. These effects are only addressed in a limited fashion in most test standards and test laboratories must have some understanding of these issues when assessing fracture toughness test data. The primary factors that affect fracture toughness of ferritic steels are chemistry, grain size, material processing methods, heat treatment, heat input during welding, test temperature, and strain rate. However, more subtle factors associated with geometry and loading also may influence fracture toughness measurements of carbon steels. These factors influence the stress state at the crack tip (constraint), and therefore should be considered when developing a test plan to measure fracture toughness. Specimen parameters such as size, a/W ratio (represented as crack depth-to-specimen width ratio), and specimen aspect ratio all control constraint in the sample, which can increase or reduce fracture toughness of the material being tested, depending on test conditions. These influences have been evaluated by many studies over the years[6-9] and have a

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