PSI - Issue 13

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

399

2

assessment and safe life predictions. However, I remember not a single task or project where standard fracture toughness values of the involved materials were available or specified, or where it was adequate to perform the corresponding tests. In most cases, Charpy-V- (CV-) impact tests were the only source of toughness-related data, or the most suitable ones if additional testing was required, respectively. As a matter of fact, toughness requirements for structural steel are still formulated in terms of CV impact toughness. This is amazing, since fracture toughness is considered to be preferable, compared to CV-testing, because it is better founded, more informative, transferable, thus expected to be more valuable in an FM analysis. The ongoing popularity of CV-testing is mainly due to the fact that it is simple and inexpensive, compared with standard FT testing, but still serves the purpose of giving useful toughness-related information. Indeed, according to my experience, in most cases of practical application the CV data are sufficient to solve the actual problem to a satisfying degree. Compared with CV-testing, standard FT testing is complex and demanding in terms of costs, test material and expertise of the laboratory staff. Furthermore, there are several validity criteria that have to be fulfilled, with the corresponding risk of ending up with “invalid” resul ts. The complexity of the standards is a result from the frequent revisions that the standards underwent in recent years, in order to keep up with the development in the field of FM and to respond to difficulties or inaccuracies encountered by the users. Correspondingly, the standards became more and more detailed and complex. By today, they are understandable only by experts in FM and only a few specialized laboratories still offer these tests. It seems that FT-testing standardization is on a wrong track and requires a fundamental change of direction. Simplifications instead of complications are required. A few possibilities are outlined in this paper. E Young’s modulus EDM electrical discharge machining FM fracture mechanics FT fracture toughness; resistance against fracture in a general sense, in terms of KI, J, or related parameters J Ic fracture toughnss in terms of J-integral, as defined in ASTM E 1820 or E1921 K Ic , K Jc fracture toughness in terms of KI, as defined in ASTM E 1820 or E1921 KV absorbed energy in Charpy-test according to ISO 148-1 or ASTM E23 R m ultimate tensile strength according to ISO 6892-1 R p0.2 technical yield stress according to ISO 6892-1 SENB Single edge notch bend Z reduction of area in the uniaxial tensile test according to IS0 6892-1 The theory of FM is generally accepted and indispensable in structural engineering, since it allows crack loading to be quantified by parameters like K I , J or CTOD. Their critical values, K IC , J Ic , or CTOD c , called fracture toughness (FT), characterize the resistance of the material against crack extension and are required in an FM analysis. In contrast to “classical” toughness-related characteristics like CV impact energy, FT is claimed to be “transferable” under certain conditio ns, which means that the same value of K I or J is required to cause crack extension in a test specimen and in a structural component in service. Correspondingly, the present FT testing standards were developed under the premise that transferable plane strain J Ic or K Ic values should be obtained, independent on the specimen shape. Consequently, the standards do not prescribe the shape and size of the specimens. Instead, they include several restrictive “validity criteria”, i.e. conditions concerning specimen size, crack configuration, preparation of the specimens, in order to make sure that plane strain was achieved. As a consequence, it happens quite often that one or more of the validity criteria are not 2. Fundamental misconceptions in standardization of fracture toughness testing Nomenclature and abbreviations A g uniform fracture strain in the tensile test CV CT Charpy V-notch impact test compact tension DBT ductile-to-brittle transition