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TECH SPOTLIGHT Impact Testing notched side by the moving striker by the metrology of the parts that are Wayne Hayward pendulum, and the energy needed to subject to the most wear and tear, and Tinius Olsen Testing Machine Co. break off the free end is the measure some machines were manufactured Horsham, Pennsylvania of the impact strength. This article before strict adherence to the specifi- discusses the equipment, calibration, cations in ASTM E23 was deemed to and procedures for Charpy testing. be critical. Accurate results are not pos- he purpose of impact testing is to sible if the equipment is not operating Tdetermine the toughness of a ma- Impact testing equipment according to the specification; there- terial by measuring the amount of en- Instruments for impact testing of fore, it is strongly recommended that ergy absorbed by a specimen as it frac- metals have been manufactured com- the impact tester be calibrated and/or tures while being struck by a striker mercially since the 1900’s, and several verified by an accredited calibration pendulum moving at high speed. The manufacturers of testing equipment service at regular intervals. impact strength is defined as the max- meet the requirements of ASTM E23, imum amount of energy that can be “Standard Test Methods for Notched Instrument calibration absorbed by the specimen without Bar Impact Testing of Metallic Mate- A calibration swing is performed to fracture. rials.” The standard provides dimen- ensure that energy losses due to In the Charpy impact test, a notch sion and tolerance requirements for windage and friction in the bearings is placed in the specimen. The notch test specimens, the anvil supports and and axle are within acceptable toler- may be shaped like a V, keyhole, or U. the striker, the pendulum action of the ances. This calibration procedure in- The test piece is supported horizon- test machine, the testing procedure, volves 11 half-free swings of the striker tally at both ends by anvils, and is machine verification, and the deter- pendulum, during which the frictional struck behind the notch by a striker mination of fracture appearance and forces incurred are measured. This mounted at the lower end of a bar that lateral expansion. short procedure is often repeated daily swings like a pendulum, Fig. 1. The Over the years, the basic physical to make sure that energy losses within energy that is absorbed during frac- characteristics of the test machine have the system are consistent, current and ture is calculated by comparing the stayed the same, but the means of data corrected for. pendulum release height to the height acquisition have changed significantly. Direct verification: Additionally, the to which the hammer rises after As a result, many types of equipment, pendulum itself must be verified, since striking the specimen. ranging from analog dial and pointers, the amount of available energy is cal- In the Izod impact test, the specimen to digital readouts, to computer inter- culated from both theoretical and prac- has a V-shaped notch and is mounted faces, are found in various labs tical calculations. In simple terms, a vertically, Fig. 2. The specimen is throughout the country. full dimensional check is performed clamped at one end, with the top of Old equipment is not necessarily ob- to determine the correct amount of the clamp corresponding to the loca- solete. On the other hand, it may not stored potential energy in the latched tion of the notch. It is struck on the be acceptable; results are influenced pendulum, and also to ensure that the

Fig. 1 — In the Charpy test, the specimen is held horizontally and is Fig. 2 — In the Izod impact test, the specimen is held vertically on one mounted at both ends. The pendulum moves at high speeds and strikes end, and the specimen is struck on the front of the notch by the moving the specimen behind the notch. striker pendulem. ADVANCED MATERIALS & PROCESSES/APRIL 2004 31 impact.qxd 3/12/04 11:45 AM Page 2

maximum amount of available energy Charpy impact values for selected alloys is imparted to the sample. This method of calibration is called direct Charpy impact energy, verification. Alloy ft-lb (Joules) Indirect verification: The testing of AISI 1020 Steel, as rolled 12.5 (17) specimens with certified values to AISI 4320 Steel, quenched 900°C (1650°F), 425°C (800°F) temper 5.16 (7.0) verify the accuracy of Charpy impact Aluminum 2048 7.6 (10.3) machines is called an indirect verifica- Aluminum 201.0-T6 Casting Alloy 3.69 - 11.1 tion, and this is also required. These (5.0 - 15.0) specimens can be NIST Standard Ref- UNS C95800, Copper Casting Alloy 9.59 (13.0) erence Materials, or can be samples ASTM 897 Grade 1 (125-80-10), Austempered Ductile Iron 8.85 (12.0) that have been verified on NIST-certi- Beryllium, Be 1.11 - 4.06 fied Charpy impact machines. (1.5 - 5.5) Magnesium AM60B-F, Die Cast 2.07 (2.8) Test procedure As with most precise activities, the Niobium Nb-1Zr (Wah Chang WC-1Zr; Fansteel 80), Reactor Grade 92.9 (126) bulk of the critical work is done in the ASTM A131 Steel, grade DH32 25.8 (35.0) preparation, while the actual test is Titanium Ti-5Al-2.5Sn 17.7 (24.0) completed extremely quickly. In fact, Titanium IMI 834 11.1 (15.0) a typical impact test takes approxi- Carpenter Custom 450® Stainless Steel, Annealed 98.1 (133) mately 10 milliseconds. The procedure Carpenter Pyrowear Alloy 53 Tool Steel, Core Tensile Properties 95.9 (130) is as follows: ECOMold® 95 Copper Mold Material 7 (9.5) 1. Latch the pendulum in its upright Haynes Hastelloy S alloy, plate 12.7 mm (1/2 in.) thick 140 (190) position. Special Metals Inconel 725, Nickel Superalloy (UNS N07725) 2. Place the specimen on the anvils Age Hardened Strip 61.2 (83.0) with special Charpy tongs. These Allegheny Ludlum Stainless Steel Type 301, annealed (UNS S30100) 111 (150) tongs typically have a fill-in section for Allegheny Ludlum AL 276 Nickel-Base Alloy, UNS N10276 240 (325) the notched specimen, and are usually Nitronic 50 Austentic Stainless, Annealed 232 (315) just slightly smaller than the width be- tween the anvils — this way the AISI Type 304 Stainless Steel 240 (325) sample is easily placed in the center of the test area. The sample is placed such and placed on the anvils. Then the test has broken the sample. Typically, a that the notched surface is facing away must be performed within five sec- pointer and analog scale will allow a from the direction of impact. onds of removing the sample from the direct, corrected, measurement of ab- 3. After making sure that the swing cooling medium. sorbed energy, although more recent plane of the pendulum arm is clear, re- An alternative to this is a heating machines have digital shaft encoders lease the pendulum and allow it to and/or cooling system that can bring that can provide the same information. break the specimen. the sample to test temperature on the Machines are also available with 4. Once the test is complete, allow support anvils — in fact, the anvils are strain gaged strikers that can be con- the pendulum to swing back close to parts of the heating and cooling nected to high-speed data capture the original release point, and push the system. This can be an expensive op- equipment. This can be connected to pendulum back into the latched posi- tion, but it has many benefits. It allows a computer running data analysis tion. Some models have a motorized accurate placement of the test spec- software that can show a graphical return capability in which a brake is imen, rapid cooling or heating of the representation of the impact in terms activated, stopping the swinging pen- sample and, since no time constraint of force versus time. dulum. The motor then returns the is imposed on completion of the test, All commercial acceptance testing pendulum back into its original a subsequent faster throughput of ac- must report the following information: latched position. curately tested samples. • Specimen type, • Test temperature, Alternate temperatures Interpretation of results • Absorbed energy, and Most Charpy impact testing is per- The most common test result that • Other contractual requirements. formed at other-than-room-tempera- the machine gives is the absorbed en- Additionally, other results can be ture conditions, typically lower tem- ergy. This is taken as the difference be- reported for non-commercial accept- peratures, so that the transition tween the amount of available energy ance. These may include: temperature and the upper and lower in the striker immediately prior to im- • Lateral expansion shelf regions may be analyzed. In these pact, and the energy remaining after • Unbroken specimens cases, the ASTM standard states that breaking the sample, after correction • Fracture appearance the sample has be at the test tempera- for windage and friction losses. • Specimen orientation ture for a minimum of five minutes The amount of energy available • Specimen location prior to testing. Additionally, the lo- prior to impact is the amount of po- For more information: Wayne Hayward, cating tongs, used to put the sample tential energy in the pendulum at its Tinius Olsen Testing Machine Co., 1065 in place on the support anvils, must starting/latched position. The amount Easton Road, Horsham, PA 19044; tel: be kept at the test temperature. When of energy remaining after the sample 215/675-7100; fax: 215/441-0899; e-mail: the system is ready, the sample must break is measured by recording the [email protected]; Web site: be removed from the cooling medium height of the pendulum swing after it www.TiniusOlsen.com. 32 ADVANCED MATERIALS & PROCESSES/APRIL 2004