Charpy Impact Testing

The Charpy impact test measures the energy absorbed (in joules) when a standardised notched specimen is fractured by a single pendulum blow. This energy, denoted K_V, is a measure of the material's notch toughness — its ability to resist brittle fracture at a stress concentration under impact (high strain rate) loading. The test is particularly sensitive to temperature: most steels show a transition from high-energy ductile fracture at elevated temperatures to low-energy brittle fracture at low temperatures, making Charpy testing at specified design temperatures the standard method for verifying fitness for low-temperature service.

K_V is the absorbed energy in joules — the primary result of every Charpy test. Shear area is the percentage of the fracture surface that shows ductile (fibrous) fracture mode, as opposed to brittle (cleavage) fracture. High shear area indicates ductile behaviour; low shear area indicates brittle behaviour. Lateral expansion measures the permanent bulge on the compression face of the tested specimen perpendicular to the notch, in millimetres. It is an independent measure of ductility that is particularly useful when K_V values are scattered or when the specified acceptance criterion is lateral expansion rather than energy (common in NACE MR0175/ISO 15156 for sour service material qualification).

FATT stands for Fracture Appearance Transition Temperature — the temperature at which the Charpy fracture surface shows exactly 50% ductile (fibrous) fracture and 50% brittle (cleavage) fracture. To determine FATT, tests are run at a series of temperatures spanning the transition zone, and the shear area at each temperature is plotted. The FATT is found by interpolation at the 50% shear area level. Similarly, T_27J, T_40J, and T_68J are the temperatures corresponding to K_V = 27, 40, or 68 J respectively, determined by interpolation from the K_V–temperature transition curve. These values are widely used in ECA (Engineering Critical Assessment) and fracture mechanics assessments as correlations to fracture toughness.

For weld Charpy specimens, the notch must be positioned precisely within the weld metal, at the fusion line, or at specified distances into the HAZ (heat-affected zone) — typically at 1 mm, 2 mm, and 5 mm from the fusion line. Achieving accurate notch placement requires either careful specimen layout with macroscopic examination of the weld cross-section, or EDM (electro-discharge machining) notching where the notch is placed with micrometric precision after metallographic identification of the fusion line. SMT provides full weld HAZ sampling programmes, including the macroscopic examination and notch position verification required by EN ISO 15614-1, ASME IX, and pipeline weld procedure qualification standards.

At −196°C, specimens are conditioned by immersion in liquid nitrogen contained in a Dewar flask. The specimens are held in the liquid nitrogen until thermal equilibrium is reached (minimum 5 minutes per ISO 148-1, with the thermocouple on the control specimen confirming stable temperature), and then transferred to the anvil within the 5-second maximum transfer time specified by the standard. The −196°C test is used to determine lower shelf energy for the Charpy transition curve, and is specified for nuclear RPV surveillance programmes and for titanium alloy testing where −196°C represents the cryogenic service temperature.

Most product standards specify a minimum of three specimens per test condition (three specimens at each test temperature and specimen position). The result is reported as the average of the three individual values, with individual values subject to the condition that no more than one value may fall below the specified minimum. For transition curve determination, a minimum of five to eight temperatures are tested with three specimens each — typically 15–24 specimens for a complete programme. For statistical characterisation or surveillance programmes, larger lots (10+ specimens per condition) are tested and the data fitted to a Boltzmann or tanh curve to extract FATT and upper/lower shelf values with confidence bounds.

ASTM A923 Method B is a Charpy impact test specifically designed to detect the presence of detrimental intermetallic phases (sigma phase, chi phase, R phase) in duplex and superduplex stainless steels. These phases precipitate in the temperature range 600°C–1000°C during slow cooling or inadequate solution annealing, causing severe embrittlement with K_V values falling to near zero. The test is performed at −40°F (−40°C) with pass criteria of K_V ≥ 54 J (40 ft-lb). It is required by ASTM A790/A928 (duplex pipe and tube), many oil and gas customer specifications for duplex piping, and NORSOK standards for offshore duplex material qualification. SMT is ACCREDIA accredited for A923 Method B with the 750 J pendulum.

Several empirical correlations exist between Charpy K_V and plane-strain fracture toughness K_Ic or CTOD (δ_c), most notably the Barsom–Rolfe correlations, the BSI PD 6493 correlations, and the more recent master curve approach (ASTM E1921). These are used when direct fracture toughness testing is impractical (e.g. due to material availability) and K_V data from the transition curve is available. SMT can provide combined Charpy and direct fracture toughness (CTOD, K_Ic) testing on the same material, allowing the correlations to be validated against measured toughness for the specific material batch, which is the preferred approach for critical ECA work.