Raymond L. Santoyo

Evaluation of Bias for Two Charpy Impact Machines with the Same Instrumented Striker

Two Charpy machines were used to test NIST verification specimens at three energy levels: low energy (∼15 J at −40°C), high energy (∼100 J at −40°C), and super-high energy (∼240 J at room temperature). The study evaluates the differences observed for the bias between two impact machines and the variation in test data for instrumented versus non-instrumented impact tests. The machines used for testing were of very similar design, and all tests were performed with the same instrumented striker (switched between machines). After testing, the raw force/time data were used for the analyses, without correcting instrumented data by matching absorbed energies measured by the machine encoder (KV) and calculated under the force/deflection test record (Wt). The characteristic forces at general yield (Fgy) and the maximum forces (Fm) were determined in accordance with ASTM E2298-09 from the instrumented impact record that was used to calculate the total impact energy (Wt). The findings show the following: (1) one machine consistently produced higher absorbed energy values than the other machine; (2) the variation in Wt is significantly lower than the variation in absorbed energy measured in the non-instrumented test (KV) for a given machine and energy level; (3) the relative differences between KV and Wt increased with increasing absorbed energy levels; (4) variations in maximum force are lower than variations in absorbed energy values; (5) instrumented data indicate that the variation in the curves is very small up to maximum force, and that differences in absorbed energy mainly occur during fracture propagation (post-maximum force data); (6) data from these two independent measures of absorbed energy indicate that scatter is due primarily to material variability; and (7) the bias between the two machines is significantly reduced when the same striker is used for testing.

Instrumented Impact Tests: Effects of Machine Variables and Specimen Position

An investigation has been conducted on the influence of impact machine variables and specimen positioning on characteristic forces and absorbed energies from instrumented Charpy tests. Brittle and ductile fracture behavior has been investigated by testing NIST reference samples of low-, high-, and super-high-energy levels. Test machine variables included tightness of foundation, anvil and striker bolts, and the position of the center of percussion with respect to the center of strike. For specimen positioning, we tested samples that had been moved away or sideways with respect to the anvils. In order to assess the influence of the various factors, we compared mean values in the reference (“unaltered”) and “altered” conditions; for machine variables, t-test analyses were also performed in order to evaluate the statistical significance of the observed differences. Our results indicate that the only circumstance which resulted in variations larger than 5 % for both brittle and ductile specimens is when the sample is not in contact with the anvils. This should be taken into account in future revisions of instrumented Charpy test standards.

Fracture Appearance of Steels in Transition: Experimental Observations and Measurements

Measuring the shear fracture appearance (SFA) of steel in the ductile-to-brittle transition region is challenging, largely because many modern steels have complicated mixtures of brittle and ductile fracture features that the carbon steels used to develop SFA measurements did not possess. Features of Charpy impact specimen fracture surfaces for nine different steels are reported here. The mixtures of ductile and brittle fracture surface features observed in the transition regions of these steels are presented in context with their instrumented impact data and fractographs illustrating the SFA measurements. The study then discusses the accuracy of the SFA measurements and suggests ways to improve them. These data can be used for future work focused on improving SFA measurements and will be of use to engineers interested in the ductile-to-brittle transition in steel and the fracture surface features related to it.

Overview of NIST Activities on Subsize and Miniaturized Charpy Specimens: Correlations With Full-Size Specimens and Verification Specimens for Small-Scale Pendulum Machines

NIST in Boulder, CO investigated the correlations between impact test results obtained from standard, full-size Charpy V-notch (CVN) specimens and specimens with reduced thickness (subsize Charpy V-notch specimens (SCVN)) or reduced or scaled cross section dimensions (miniaturized Charpy V-notch specimens (MCVN)). A database of instrumented impact test results was generated from four line pipe steels: two quenched alloy steels, a tempered alloy steel, and a 18 Ni maraging steel. Correlations between specimen types were established and compared with the previously published relationships, considering absorbed energy (KV), ductile-to-brittle transition temperature (DBTT), and upper shelf energy (USE). Acceptable correlations were found for the different parameters, even though the uncertainty of predictions appears exacerbated by the expected significant experimental scatter. Furthermore, we report on the development of MCVN specimens for the indirect verification of small-scale pendulum machines (with potential energies between 15 J and 50 J), which cannot be verified with full-size verification specimens. Small-scale pendulum machines can now be verified at room temperature with certified reference specimens of KLST type (3 mm × 4 mm × 27 mm), supplied by NIST at three certified KV levels (low energy (LL), 1.59 J; high energy (HH), 5.64 J; and super-high (SH) energy, 10.05 J). These specimens can also be used to verify the performance of instrumented Charpy strikers through certified maximum force values. Certified reference values for both KV and maximum force were established by means of an interlaboratory comparison (Round-Robin), which involved nine qualified and experienced international laboratories.

Overview of NIST Activities on Sub-Size and Miniaturized Charpy Specimens: Correlations With Full-Size Specimens and Verification Specimens for Small-Scale Pendulum Machines

NIST in Boulder Colorado investigated the correlations between impact test results obtained from standard, full-size Charpy specimens (CVN) and specimens with reduced thickness (sub-size Charpy specimens, SCVN) or reduced or scaled cross-section dimensions (miniaturized Charpy specimens, MCVN). A database of instrumented impact test results was generated from four line pipe steels, two quenched and tempered alloy steels, and an 18 Ni maraging steel. Correlations between specimen types were established and compared with previously published relationships, considering absorbed energy, ductile-to-brittle transition temperature, and upper shelf energy. Acceptable correlations were found for the different parameters, even though the uncertainty of predictions appears exacerbated by the expected significant experimental scatter.Furthermore, we report on the development of MCVN specimens for the indirect verification of small-scale pendulum machines (with potential energies between 15 J and 50 J), which cannot be verified with full-size verification specimens. Small-scale pendulum machines can now be verified at room temperature with certified reference specimens of KLST type (3 mm × 4 mm × 27 mm), supplied by NIST at three certified absorbed energy levels (low energy, 1.59 J; high-energy, 5.64 J; super-high energy, 10.05 J). These specimens can also be used to verify the performance of instrumented Charpy strikers through certified maximum force values. Certified reference values for both absorbed energy and maximum force were established by means of an interlaboratory comparison (Round-Robin), which involved nine qualified and experienced international laboratories.Copyright