Crack tip opening displacement CTOD

In the crack tip opening displacement method (or CTOD-method for short), it is assumed that crack propagation is not determined by the stress intensity factor, but by the amount of plastic deformation near the crack tip. This can be measured by the opening 5t of the crack tip. If this reaches a critical value Jc, the crack propagates. 

The crack tip opening displacement can be defined in different ways. They all have in common that it is assumed that the crack tip is blunted by the 

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One way of looking at the fracture characteristics of a ductile material is by measuring the amount of plasticity at a crack tip prior to crack propagation (Fig. 8.84). One test which measures this is the crack-tip opening displacement (CTOD), 5. Wells has found that 6 can be related to the strain energy release rate, G, by the formula ... 

We obtained full field solutions for an axial crack on the ID surface of a pipe under plane strain conditions. The pipe has an outside diameter of 40.64 cm and a wall thickness h = 9.52 mm. The depth of the axial crack is a = 1.9 mm (a/h = 0.2) (Fig. 2). In the absence of hydrogen, we assume a finite crack-tip opening displacement (CTOD), ha = 2 pm. 

The crack-opening displacement (COD) at any point along the crack is defined as twice the displacement of the crack surface at that location, i.e., COD = 2v -r, 0). The quantity crack-tip-opening displacement (CTOD), however, is given a special designation as the opening displacement at the actual crack tip, which is assumed to be correctly located at a distance x = -Vy from the effective crack tip based on Irwin s approximation. As such, from Eqns. (3.28), (3.37), and (3.49), one obtains ... 

Fracture Toughness by Crack Tip Opening Displacement (CTOD)... 

Wells [11] first proposed the crack opening displacement as a measure of fracture toughness for tough metals such as the structural steels where LEFM is not valid. The concept became a standard in 1979 in the form of British Standard (BS) 5762 [12], which is now-superseded by BS 7448 Part 1. 1991. Method of determination of Afi. critical CTOD and critical J values of metallic materials [13], More recently. ASTM E1290-9,3, Crack-tip opening displacement (CTOD) fracture toughness measurement [14] was issued. It is stated that this method may be used to characterize the toughness of materials that arc (a) too ductile or fail the size conditions to be tested for or (b) likely to produce unstable crack extension that would invalidate Ji tests in accordance with the requirements of ASTM E81.3 [15]. 

ASTM El290. 1993. Crack-tip opening displacement (CTOD) fracture toughness measurement. 

When yielding causes large departures from linearity in the force-displacement curve, such that valid Kiq data cannot be obtained, it is still possible to make geometry-independent measurements of the fracture resistance of the material, using the methods of elastic-plastic fracture mechanics. These usually require additional information to determine whether non-linearity is due to crack tip plasticity alone, or to a combination of plasticity and crack growth. Several different approaches have been developed, of which we will discuss onfy two the crack tip opening displacement (CTOD) and /-integral methods. 

VI.22] AMERICAN SOCIETY FOR TESTING AND MATERIALS, Standard Test Method for Crack Tip Opening Displacement (CTOD) Fracture Toughness Measurement, ASTM E1290-93, Annual Book of ASTM Standards, ASTM, PhUadelphia, PA (1993). 

In general, the experimental approach to the problems of crack propagation in non-linear and non-elastic materials is based on the observation and recording of behaviour of specimens with initial notches, and COD measurements were developed by several authors. Later, crack tip opening displacement (CTOD) was determined as a function of crack mouth opening displacement (CMOD) (Velasco et al. 1980), which was directly measured and recorded, for example, according to EN ISO 12737 1999 and ASTM E 399-06 (2006), (cf. Figure 10.5). 

Another useful relationship connects / to the crack tip opening displacement (CTOD), 6, which defines the distance that the crack faces are pulled apart near the crack tip and is a measure of toughness and ductility. This relationship, shown as Equation 17, holds well beyond the limits of LEFM [27], where oy is the flow stress of the material at the crack tip. 

Recent investigations showed that the critical crack tip opening displacement CTOD c could be taken as a material parameter to judge the unstable fracture in concrete. It is very convenient that the laser spekle photography is used to... 

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