PSI - Issue 13

Hans-Jakob Schindler / Procedia Structural Integrity 13 (2018) 398–403 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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fulfilled, which means that the test is “invalid” . These restricting validity conditions are one of the main reasons why FT-testing is inefficient and complicated, since they can be checked only post-mortem.. Actually, it is a fundamental misconception of standardization to go for size-independent plane-strain fracture toughness values. In practical application, the conditions for transferability of FT are often not fulfilled anyway, meaning that corrections of FT are required depending on specimen thickness, crack-size, crack-shape, notch- or crack sharpness, loading rate, temperature and other influencing factors. Actually, a well-defined size-dependent apparent FT can be as useful as a standard plane strain FT for practical purposes. Conceptually, the issue of transferring toughness-related test data from the specimen to the structural component is a matter of engineering application of FM, not of testing. Today, the theoretical knowledge and the numerical tools are available to cope with any kind of toughness-related data, irrespective of it s “validity” . The primary task of material testing is to deliver well-defined toughness-related material data, but not necessarily lower bounds. The latter can be derived from the former on theoretical basis by the user of the data. In principle, from any fracture test that exhibits cleavage fracture, K Ic in the transition regime can be derived, and from any tests that exhibits ductile tearing, upper shelf fracture toughness can be derived. It is just a matter of analytical relations or empirical correlations to account for the differences in constraints, sizes, loading rates, and temperature. Besides, from a philosophical point of view, there is no need to standardize the determination of parameters that are theoretically well defined, such as K Ic or J Ic , since the specialists who are interested in them know how to measure or identify them experimentally. What needs to be standardized are simple practical tests such as the CV impact test, in order to make the test results unambiguous and comparable. In the following, the common practice of evaluation of FT in terms of K Ic or J Ic is briefly recapitulated and discussed by the example of ferritic/bainitic steels. Depending on the temperature, the fracture behavior of these steels varies from purely ductile tearing (“upper shelf” ) to completely brittle (“lower shelf”), with the transition range in between. In each of these toughness ranges, fracture toughness can be determined either directly by standard testing, or by correlations with Charpy V-data, as discussed below. 3.1. Upper shelf In the upper shelf, crack extension occurs by ductile tearing, which is characterized by the J-R-curve. However, it has become common to consider only one distinct point of the curve, namely the so-called technical initiation value J Ic according ASTM E 1820 or J 0.2Bl according to ISO 12235, defined as the intersection with the 0.2mm-offset blunting line. The corresponding K I -value, K Jc , is expected to be relatively close to plane strain fracture toughness. The main difference between the two mentioned standards is the slope of the blunting line, which can lead to major differences in K Jc . Anyway, determination of J Ic or J 0.2Bl according to both standards is relatively time-consuming and costly. However, on the upper shelf there usually is no need for very precise K Jc . Approximations usually serve the practical needs as well. To estimate FT, several correlations between K Jc and CV-data can be found in the literature. Schindler (2000), for example, derived the following formula to estimate the J-R-curve from the CV impact energy KV: 3. Standard fracture toughness vs. CV-estimates

p

1



A

[%]

  

   

Z

  

    

[%]

(in J/mm)

(1)

g

p

J a

KV J

R MPa [

a mm

( ) 11.4  

[ ] 0.02 

]

( [

])







m

Z

100

100

[%]



with

   

   

100

(2)

p

0.75





g A

100

[%]



The second term in the second brackets of (1) represents a rough correction for the finite notch root radius  = 0.25 mm) of the CV specimen based on Schindler, Kalkhof and Viehrig (2014). From (1) and (2), near-initiation FT comparable to J Ic or J 0.2Bl can be determined by the intersection with the 0.2mm-offset blunting line according to ASTM E1820 or ISO 12135.