Charpy Impact Steel Testing: Part Two
The Charpy V- notch test is a standardized test which measures the impact energy of materials during fracture which is carried out by pendulum.
Temperature is often an important factor to understand the real potential for a material or structure to succumb to brittle fracture under specific conditions.
Charpy (CVN) toughness tests are videly used to determine the effect of temperature on the propensity of structural steels to brittle fracture. Notched specimens are submitted to the impact of a hammer with the kinetic energy of 300 J. The fracturing occurs in ductile, mixed or brittle mode and, accordingly, a very different quantity of energy is consumed. Generally, in the upper shelf CVN ductile region, the energy consumed for the fracture of structural steels is 150 J and more and it is different for different steels. The energy consumed for brittle cleavage fracture below the lower shelf threshold is below 10 J and very similar for steels with different microstructure, a wide range of grain size and yield stress as well as for as delivered and strain aged steels (see Figure 1).
Figure 1: Charpy notch toughness for structural steels with different microstructure and yield stress in the range of transition temperature
The fracturing time depends on the fracturing energy and it is very different, it is of 11 ms with the energy of 250 J and of 1 ms with the energy of below 10 J. Also the volume of the plastically deformed metal is very different, it amounts to several hundreds of mm3 in the ductile range and it is negligible in the brittle range. Virtually 90 % of the energy consumed for the plastic deformation and fracturing is dissipated as heat. For this reason, the fracturing in the upper shelf and transition range occurs above the nominal testing temperature.
In brittle range virtually no plastic deformation heat is generated and the fracturing occurs at the nominal testing temperature. Therefore, the temperature, which is used to show graphically the dependence CVN toughness versus testing temperature, is not the real fracturing temperature for the vhole testing interval. The time and volume distribution of deformation and fracturing temperature complicate additionally the proper understanding of the Charpy ductile fracturing mechanism.
Also, the important parameters for the assessment of critical structures are different fracture mechanical parameters which are capable of describing a components resistance against flaws. One frequently applied parameter is the fracture toughness KIC.
It´s widely recognized that Charpy CVN impact energy values can be converted to KIC using KIC-CVN correlations. In this context, plane-strain fracture toughness (KIC) is an important material property in the prediction and prevention of fracture, and for damage tolerance assessment of brittle materials. The KIC in the linear elastic fracture mechanics (LEFM) is the size of the stress intensity factor at the tip of the crack if the strain in the body is elastic. The ASTM E-399 standard is used to obtain KIC values in plane-strain for the displacement mode of the opening cracking. However, it is not always possible to prepare such specimens when the analyzed material does not have the proper dimensions, even at room temperature, standard tests for KIC are difficult, time-consuming and costly.
Charpy impact energy CVN is utilized to indirectly estimate CVN data for KIC values. Then, to obtain the KIC values from the CVN data, it is necessary to select the behavior from CVN impact results, according to the interest zone, σYS of the material and the CVN energy values. Although this CVN impact test data does not represent the real fracture toughness data, this data can be used as a series of points to estimate toughness in an evaluation of fracture mechanics. Nevertheless, there are studies in the literature about estimating KIC values from correlations of CVN impact data. KIC-CVN correlations available in published literature are applied to lower-shelf, transition zone and upper-shelf temperatures and are not applicable to wet welding. However, many of these correlations are based on the yield stress (σYS) of the material, Young´s Module (E), and correlations applied to different zones in a Charpy transition temperature curve. Thus, these correlations could be applied to different metals and wet welding.
The purpose of the study of G. Terán Méndez et al. was to estimate the fracture toughness (KIC) of CVN data from different authors who employed wet welding in A36 steel and compared with CVN values obtained in this study. A36 steel and E6013 electrodes were utilized to construct T-welded connections. Then, a rectangular grinding was carried out at the weld toe with two depths, 6 mm and 10 mm. The rectangular grinding was filled with wet welding simulating seawater at depths of 50, 70 and 100 m. Standard Charpy specimens were extracted to obtain energy values. In addition, a search of Charpy CVN values using A36 steel and E6013 electrodes was performed with wet welding. Finally, KIC -CVN correlations were presented to measure the fracture toughness.
As conclusions they drawn that CVN impact energy and KIC values decrease as the sea depth increases. It is attributed to various discontinuities in wet welding, such as porosity, slag-inclusion, non-metallic inclusion and cracking. Although the equations of KIC-CVN correlations did not specify that they can apply to wet welding, the KIC values obtained by Barsom could be considered the closest values in order to compare them with the ASTM E369 standard specimens. This is because they use values from Charpy (CVN) in all zones of the absorbed energy curve, and the yield stress of the material. It is necessary to conduct or develop further correlation equations to introduce different mechanical properties, such as, σYS, σUTS, hardness (HRC) and define microstructure. It is noted that there is no singles fracture toughness value for steels, nor a fixed temperature, Charpy CVN data and water depths. A relatively accurate KIC -CVN correlation can be a useful tool in the MFLE. CVN testing is less demanding than KIC testing in terms of experimental complexity, speed, and cost. Then, traditional equations to estimate KIC data will be used. It is due to KIC representing a mechanical material property that it could be transferable from a test specimen to a structure.