Charpy Impact Steel Testing: Part Two

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Introduction to Charpy Impact Testing

The Charpy V-notch test is widely used to determine how temperature affects the susceptibility of structural steels to brittle fracture. In this standardized test, notched specimens are subjected to the impact of a hammer with a kinetic energy of 300 J. The fracturing process occurs in ductile, mixed, or brittle modes, each consuming significantly different amounts of energy.

In the upper shelf CVN ductile region, structural steels typically consume 150 J or more during fracture, with specific values varying between different steel types. Conversely, brittle cleavage fracture below the lower shelf threshold consumes less than 10 J and remains relatively similar across steels with different microstructures, grain sizes, yield stresses, and conditions (as delivered or strain aged), as illustrated in Figure 1.

Figure 1: Charpy notch toughness for structural steels with different microstructure and yield stress in the range of transition temperature

Temperature, Time, and Energy Relationships in Fracture Testing

The fracture time varies considerably depending on the fracturing energy. For instance, it takes approximately 11 ms with an energy of 250 J but only 1 ms with an energy below 10 J. The volume of plastically deformed metal also differs significantly—several hundreds of cubic millimeters in the ductile range but negligible in the brittle range.

Approximately 90% of the energy consumed during plastic deformation and fracturing dissipates as heat. Consequently, fracturing in the upper shelf and transition range occurs above the nominal testing temperature. In the brittle range, however, virtually no plastic deformation heat is generated, so fracturing occurs at the nominal testing temperature. This means that the temperature used in graphical representations of CVN toughness versus testing temperature is not the actual fracturing temperature throughout the entire testing interval. The time and volume distribution of deformation and fracturing temperature further complicate our understanding of the Charpy ductile fracturing mechanism.

Fracture Toughness and Its Correlation with Charpy Testing

For critical structure assessment, various fracture mechanical parameters are used to describe a component's resistance to flaws. One frequently applied parameter is fracture toughness (K_IC).

It is widely recognized that Charpy CVN impact energy values can be converted to KIC using established K_IC-CVN correlations. Plane-strain fracture toughness (K_IC) represents a critical material property for predicting and preventing fracture and for damage tolerance assessment of brittle materials. In linear elastic fracture mechanics (LEFM), K_IC indicates the stress intensity factor at the crack tip when the strain in the body remains elastic.

While the ASTM E-399 standard is used to obtain K_IC values in plane-strain for the displacement mode of opening cracking, preparing such specimens is not always feasible when the analyzed material lacks proper dimensions. Even at room temperature, standard tests for K_IC are often difficult, time-consuming, and costly.

Applications in Wet Welding Environments

G. Terán Méndez and colleagues conducted a study to estimate the fracture toughness (K_IC) from CVN data collected by different researchers who employed wet welding in A36 steel. They compared these values with CVN measurements obtained in their own study.

In their methodology, they constructed T-welded connections using A36 steel and E6013 electrodes. Rectangular grinding was performed at the weld toe with depths of 6 mm and 10 mm. This grinding was filled with wet welding simulating seawater at depths of 50, 70, and 100 m. Standard Charpy specimens were then extracted to obtain energy values. Additionally, the researchers compiled Charpy CVN values from previous studies using A36 steel and E6013 electrodes with wet welding. Finally, they presented K_IC-CVN correlations to measure fracture toughness.

Conclusions and Implications

The researchers concluded that both CVN impact energy and K_IC values decrease as sea depth increases. This decline is attributed to various discontinuities in wet welding, including porosity, slag inclusion, non-metallic inclusion, and cracking.

Although existing K_IC-CVN correlation equations were not specifically designed for wet welding applications, the K_IC values obtained using Barsom's method were considered the closest approximation to those obtained with ASTM E369 standard specimens. This is because they utilize values from across all zones of the absorbed energy curve and incorporate the material's yield stress.

The study highlights the need to develop more comprehensive correlation equations that factor in additional mechanical properties such as yield strength (σYS), ultimate tensile strength (σUTS), hardness (HRC), and microstructure. Importantly, there is no single fracture toughness value for steels that applies across all temperatures, Charpy CVN data points, and water depths.

A reasonably accurate K_IC-CVN correlation can serve as a valuable tool in linear elastic fracture mechanics (LEFM). CVN testing offers advantages over K_IC testing in terms of experimental simplicity, speed, and cost-effectiveness. Traditional equations for estimating K_IC data remain useful because K_IC represents a mechanical material property that can be transferred from a test specimen to a full structure.