Thomas A. Siewert

Historical background and development of the Charpy test

The impact test method based on a pendulum, generally called the Charpy test, is one of the more cost-effective material testing procedures, both with respect to acceptance of products and to surveillance. This contribution attempts to present a brief historical review about the general development of material testing, starting at the beginning of the intense industrialisation in the second half of the 19th century, and tries to point out the role and the position of impact testing during this period. Several periods in the evolution of impact testing based on a pendulum are discussed in detail.

The History of Instrumented Impact Testing

Pendulum impact testing is widely known to have a history that extends back to the turn of the 20th century. To many researchers today, instrumentation of the impact test to acquire a load-time history, and thereby to provide important data in addition to absorbed energy, is usually considered to be a relatively recent development. However, our literature review has shown that starting from the earliest test machine development work, researchers have been interested in designing equipment capable of measuring both the energy expended in fracturing the specimen, and the force-deflection and energy-deflection curves. This paper recounts the early history of instrumented impact testing, and shows that it also extends back over 100 years. In fact, the earliest known paper on instrumented impact testing predates the first pendulum test machine publication by one year.

Welding Consumable Development for a Cryogenic (4 K) Application

This paper summarizes the development and qualification of an appropriate welding consumable for a demanding cryogenic magnet application. This research shows that higher oxygen content in the weld manifests itself as inclusions, which have a severe detrimental effect upon the fracture toughness at 4 K. Also, welds enriched with manganese and nickel have demonstrated improved fracture toughness. These discoveries were combined in the development of a nitrogen- and manganese-modified, high-nickel stainless-steel alloy. It produced gas metal arc welds with superior cryogenic mechanical properties when welding procedures were modified to reduce the oxygen content.

Cryogenic material properties of stainless steel tube-to-flange welds

Abstract The mechanical properties of stainless steel tube-to-flange welds for a cryogenic piping application were measured. A planar specimen was developed to duplicate the constraint, loading and heat-sink properties of the circular joint, while reducing preparation time and cost. Specimens were evaluated containing welds between the tube material (21 Cr-6Ni-9Mn) and the three stainless steels being considered for the flange materials: type 304L, type 316L and 21 Cr-6Ni-9Mn. The mechanical property tests consisted of three phases: simple tensile testing to failure, tensile testing of notched specimens (where the notch simulated fabrication flaws) and fatigue testing of notched specimens for the 4 × 104 cycle design life of the structure. The type 316L stainless steel flange produced welds with the best combination of strength and ductility at 295 and 4 K in all three phases of testing.

A Process Sensor for Locating the Liquid-Solid Boundary through the Mold of a Casting

Accurate process control of single-crystal and directionally-solidified castings requires knowledge of the exact location of the solidifying front. If the front advances too rapidly, single crystal growth in a preferred orientation degenerates into the formation of polycrystals. A solidification front which moves more slowly than necessary is wasteful of the casting resources. A sensing technology is being developed which determines the location of the boundary between a solidifying crystal and liquid metal. The sensing method utilizes the ordered pattern of x-rays diffracted from the solid as an absolute indicator of the liquid-crystal interface.

Dynamic Apparatus for CTOA Measurement in Pipeline Steels

When a crack initiates and propagates in a pressurized pipe, the only thing that might stop this high-velocity event is the release of internal pressure (decompression), resulting in a deceleration in the crack-propagation rate. This deceleration can be achieved through the use of crack arrestors, or the ability of the pipeline material to resist ductile fracture. To evaluate the resistance to crack growth, the crack tip opening angle (CTOA) is used. Recent articles on the CTOA of pipeline steels at quasi-static rates with modified double cantilever beam specimens (MDCB), and at dynamic displacements rates by use of drop weight tear testing have provided data to support this need. These laboratory results from the literature, compared with results of full-scale tests, indicate that details of the fracture mode depend on the rate of fracture. To further study the dependence among the rate, fracture mode, and CTOA, a dynamic test apparatus was designed to perform CTOA testing of MDCB specimens, so that comparisons to quasi-static and full-scale results could be made. This new apparatus consists of a 500 kN uniaxial hydraulic test machine capable of stand-alone displacement rates of 300 mm/s, and a disc spring apparatus that is used to further accelerate the testing displacement rate. Initial results of the testing show that full slant fracture mode is observed at the highest rates tested for X65 and X100 steels. Maximum crack velocities approaching 10 m/s were recorded with highspeed photography. CTOA measurements were typically made at a position about 30 mm ahead of the pre-fatigue crack, over a distance of about 15 mm in the steady-state crack propagation regime. In this paper, we describe the high-speed apparatus, discuss the relationship among specimen configuration, crack speed, and CTOA, and present initial results on X65 and X100 pipeline steels.© 2008 ASME

CTOA Measurements of Welds in X100 Pipeline Steel

A suite of tests characterizing X100 pipeline steels was initiated at the National Institute of Standards and Technology (NIST) in Boulder. Part of the test matrix included testing the toughness of the base metal, welds, and heat-affected zones (HAZ) by use of modified double cantilever beam specimens for crack tip opening angle (CTOA) testing. The thickness of the test section was either 3 mm or 8 mm. Girth welds perpendicular to the growing crack, and seam welds and their HAZ parallel with the crack, were tested with a crosshead displacement rate of 0.02 mm/s (with the exception of one girth weld specimen for each thickness, which were tested at 0.002 mm/s). Analysis of the data revealed some general differences among the weld specimens. The tests where the crack ran perpendicular to the girth weld demonstrated changes in CTOA and crack growth rate as the crack moved through the base metal, HAZ, and weld material. We observed the values for CTOA increasing and the crack propagation slowing as the crack moved through the weld and approached the fusion line. The stress field appeared to be strongly influenced by the thin HAZ, the fusion line, and the tougher base material. Consequently, the CTOA of the HAZ associated with the girth weld was larger than that of the seam-weld HAZ. It was not possible to obtain CTOA data for the seam weld, with the crack parallel within the weld, because the crack immediately diverted out of the stronger weld material into the weaker HAZ. CTOA values from both girth welds and seam-weld HAZ were smaller than those of the base material. The 8 mm thick specimens consistently produced larger CTOA values than their 3 mm counterparts, introducing the possibility that there may be limitations to CTOA as a material property. Further tests are needed to determine whether a threshold thickness exists below which the constraints and stress field are sufficiently changed to affect the CTOA value.

High Energy X-Ray Diffraction Technique for Monitoring Solidification of Single Crystal Castings

X-ray diffraction has been used successfully to study metal solidification and temperature-dependent phase changes1–3. However, this research used very thin specimens (a few mm at most), furnaces with low attenuation x-ray windows (beryllium, graphite, or polyimide), and low x-ray energies (< 50 keV). Since the penetration depth of low-energy x-rays is shallow, traditional x-ray diffraction is thus unable to probe the interior of thicker structures. Others have used higher energies to study thicker samples. Work, including that by Green4 and by Kopinek, et al5, extended x-ray diffraction investigations to energies exceeding 150 keV.

Sizing Canted Flaws in Weldments Using Low-Frequency Emats

Techniques for detecting and sizing flaws with electromagnetic-acoustic transducers (EMATs), previously used successfully for normal planar flaws, were applied to canted flaws in steel plates. Comparisons were made between metallographic and ultrasonic measurements on specially prepared welds. Results indicated a high probability of detecting canted flaws (> 0.5-mm deep) with EMATs. The EMAT sizing was highly repeatable and, for the most part, very accurate. Some, as yet unexplained, inaccuracies did show up, however, in some weld sections. There is a possibility that the calibration curve may be more complex for canted flaws than for normal flaws.

Observations on differences between the energy determined using an instrumented striker and dial/encoder energy

Abstract Instrumented striker systems, dial indicators, and optical encoders are widely used for measurement of absorbed energy in both conventional and miniature Charpy tests. It has been observed that the total absorbed energy measured using these technologies, while generally in good agreement, sometimes differs by a significant amount. This paper presents experimental evidence from high speed photography of Charpy tests that the differences between dial/encoder energies and instrumented striker energies (measured with U-hammers) can largely be explained by post-fracture collisions with the striker. Further, experimental and numerical studies show that dynamic, elastic deformations of the pendulum and striker, as well as load cell errors due to contact load distribution and inertial effects, can also significantly contribute to the energy differences for some test machine designs. Experimental evidence from the literature that was originally expected to show the significance of residual vibrational energy in the test machine may actually show that low energy Charpy specimen behavior can be significantly affected by the design of the test machine. A simple dynamic, model of a specimen and test machine system shows that a 20% increase in the stiffness of the striker and hammer assembly can lead to a 9% decrease in the energy required to fracture a low energy Charpy specimen.