Kim Wallin

The scatter in KIC-results

Abstract Scatter in KIC-results can often be quite extensive, and to make reliable interpretations of the results it is of great importance to understand the nature of it. Cleavage fracture in steels is of a statistical nature and therefore the scatter in KIC-results will behave similarly. Two different approaches, one based on a microstructural statistical model and an other based on the Weibull distribution are applied to evaluate the theoretical scatter in KIC-results. With both methods it is shown that the theoretical value of the relative scatter described through the Weibull slope factor is constant and equal to four. The reason for the discrepancy between the theoretical value and the experimentally determined values of the slope factor is shown to be caused by inadequate number of experimental KIC-measurements. The existence of a lower limiting Kmin value is verified and a simple procedure for conservative estimation of the KIc-mean and lower bound values is presented.

The size effect in KICresults

Abstract Fracture toughness results are often connected with varying thickness effects. Many results are contradictory, with some results indicating no size effect, some indicating increasing toughness with decreasing thickness and others indicating decreasing toughness with decreasing thickness. In this paper the causes for different thickness effects are discussed. It is shown that most of the observed size effects are due to invalid tests. The only test parameters to be regarded as valid are those that correspond to the initiation of crack extension. The theoretical thickness effects for both ductile and brittle fracture initiation are evaluated. It is shown that ductile fracture initiation is thickness independent if B ≥α(J/σy). The thickness effect in brittle fracture is explained by applying a mechanism-based statistical cleavage fracture model. A theoretical thickness correction for cleavage fracture is derived, and its validity is confirmed for a variety of materials.

Master curve analysis of the “Euro” fracture toughness dataset

Abstract Brittle fracture in the ductile to brittle transition regime is connected with specimen size effects and – more importantly – tremendous scatter of fracture toughness, which the technical community is currently becoming increasingly aware of. The size effects have the consequence that fracture toughness data obtained from small laboratory specimens do not directly describe the fracture behavior of real flawed structures. Intensive research has been conducted in the last decade in order to overcome these problems. Different approaches have been developed and proposed, one of the most promising being the master curve method, developed at VTT Manufacturing Technology. For validation purposes, a large nuclear grade pressure vessel forging 22NiMoCr37 (A508 Cl.2) has been extensively characterized with fracture toughness testing. The tests have been performed on standard geometry CT-specimens having thickness 12.5, 25, 50 and 100 mm. The a / W ratio is close to 0.6 for all specimens. One set of specimens had 20% side-grooves. The obtained data consists of a total of 757 results fulfilling the ESIS-P2 test method validity requirements with respect to pre-fatigue crack shape and the ASTM E-1921 pre-fatigue load. The master curve statistical analysis method is meticulously applied on the data, in order to verify the validity of the method. Based on the analysis it can be concluded that the validity of all the assumptions in the master curve method is confirmed for this material.

Quantifying Tstress controlled constraint by the master curve transition temperature T0

Abstract Specimen size, crack depth and loading conditions may effect the materials fracture toughness. In order to safeguard against these geometry effects, fracture toughness testing standards prescribe the use of highly constrained deep cracked bend specimens having a sufficient size to guarantee conservative fracture toughness values. One of the more advanced testing standards, for brittle fracture, is the master curve standard ASTM E1921-97, which is based on technology developed at VTT Manufacturing Technology. When applied to a structure with low constraint geometry, the standard fracture toughness estimates may lead to strongly over-conservative estimate of structural performance. In some cases, this may lead to unnecessary repairs or even to an early “retirement” of the structure. In the case of brittle fracture, essentially three different methods to quantify constraint have been proposed, J small scale yielding correction, Q-parameter and the Tstress. Here, a relation between the Tstress and the master curve transition temperature T0 is experimentally developed and verified. As a result, a new engineering tool to assess low constraint geometries with respect to brittle fracture has been obtained.

The effect of ductile tearing on cleavage fracture probability in fracture toughness testing

Abstract Fracture toughness testing in the transition region often causes ductile tearing to precede cleavage fracture. The effect of ductile tearing on the fracture probability has in recent years become a subject of great interest. Bruckner and Munz have developed a Weibull statistics based model to describe this effect. In this paper a similar model, based on the WST-model, is derived. The model is applied to several different sets of brittle fracture data. The differences between the present model and the B & M model is discussed. Finally the effect of loss of constraint on the models is noted.

Applicability of miniature size bend specimens to determine the master curve reference temperature T0

Abstract The master curve method enables characterisation of the brittle fracture toughness based on a few relatively small specimens. Presently the general view is that pre-cracked Charpy-V specimens constitute, effectively, the smallest specimens that can be used with the master curve. However, even though the method includes a specific measuring capacity limit for the specimen, it does not specify a minimum specimen size to be used. In this work, the applicability of miniature specimens, smaller than the normal Charpy size bend specimen, are investigated by comparing the test results of miniature and normal Charpy size specimens. Furthermore, the possible differences in estimates from CT- and 3PB-specimen tests are examined.

Validity of Small Specimen Fracture Toughness Estimates Neglecting Constraint Corrections

Historically, size criterions have been developed with the aim of guaranteeing specimen size independent fracture toughness values. The basis for the criterions have been different assumptions regarding the specimen constraint. Presently, it has become evident that for cleavage type fracture there exists a statistical size effect affecting the measured toughness in addition to constraint. In this work existing data bases are analyzed, considering the statistical size effect, but neglecting any corrections for constraint. It is shown that the validity of small bend specimen fracture toughness estimates is excellent, even with specimens that clearly violate even the least stringent size requirements.

Master Curve implementation of the warm pre-stress effect

The Master Curve (MC) methodology has evolved from only being a brittle fracture testing and analysis procedure to a technological tool capable of addressing many more structural integrity issues like constraint and parameter transferability. One issue that not yet has been covered by the MC is the warm pre-stress (WPS) effect. This effect, which is known to produce an effective increase in fracture toughness, may be of great importance for some structural safety assessment situations where thermal transients are involved. Here, the WPS effect is re-examined and implemented into the MC methodology, by introducing a new simple WPS equation.

The effect of ligament size on cleavage fracture toughness

Abstract A commonly encountered problem in fracture toughness testing is the question regarding the valid specimen size. Different testing standards have different requirements for the different minimum specimen dimensions. These requirements are, however, not always realistic nor correct. In this paper the effect of ligament size on cleavage fracture toughness in the elastic-plastic regime is considered. The consideration is performed within the framework of a statistical cleavage fracture model (WST). A new specimen ligament size requirement for elastic-plastic cleavage fracture toughness testing is presented, together with a correction function to validate invalid test results.

Improved crack growth corrections for J–R-curve testing

Abstract The ASTM J–R-curve testing standard contains an incremental crack growth correction to J, whereas the ESIS P2 test procedure contains a correction for the total crack growth. The ASTM expression is only applicable to single specimen J–R-curve testing in contrast to the ESIS expression which is also applicable for multi-specimen tests. The ASTM expression is, however, considered to be more accurate than the ESIS expression. It is of interest to unify the test procedures so that irrespective of standard, the same result would be obtained. Having this goal in mind, the different crack growth corrections have been examined and compared both with each other as well as with other analytical and numerical estimates, applicable both in incremental as well as integral form. As a result a new correction is proposed, which is capable of unifying the different testing standards.