IMPACT: Which Test to Use? Which Instrument to Buy?
Impact testing is a complex and controversial subject, and one that is in the midst of change. These factors add challenge to selection of the most appropriate impact instrument for a processor's lab.
Impact testers are among the first instruments that plastics compounders, extruders, and molders consider when outfitting a lab. Historically, the choice between traditional falling-weight and pendulum impact testers seemed relatively simple. It was determined by the material, end-use application requirements, and the customer's preference for a particular type of test data. These traditional instruments remain popular due to their simplicity, affordability, and long history of use. That is particularly true of pendulum devices for notched-Izod impact, the test most often cited in the U.S.
But traditional tests are coming under challenge. Researchers at some materials suppliers disparage these simple tests as low in accuracy and repeatability. New instrument modifications are offered to remedy some of these limitations. What's more, a movement has emerged to abandon Izod impact reporting (as per the ASTM D256 test protocol) in favor of the Charpy test (ISO 179), another pendulum impact method that is dominant in Europe. This shift is being driven mainly by the automotive industry as part of its global standardization efforts.
What's more, many material suppliers, compounders, and additive masterbatch suppliers are turning to instrumented impact tests that use devices outfitted with load sensors, which provide more detailed information about materials' response to impact loads.
The trend toward "real-life" testing further complicates the picture. "Testing the material for a car bumper is not the same as testing an actual finished car bumper," says product manager Frank Lio at Instron Corp.
At present, sophisticated information from both instrumented impact and "real-life" testing is mainly used internally by material suppliers and compounders. Traditional Izod and dart impact tests continue to dominate material-specification data requested by processors and their customers. In fact, some sources see a growing number of plastics processors performing their own impact tests to ensure product consistency, rather than just relying on data provided by their material suppliers. "Where you do see impact testing at the processors' level is generally with products that undergo severe service and must have some structural quality," notes Bob Elston, styrenics technologist at Pittsburgh-based Nova Chemicals. He cites producers of gas pipe, PVC siding, automotive components, sports helmets, ski boots, medical equipment, and even toys.
Even traditional test instruments have seen some evolution in design over the past decade. There has been a trend from analog dials to digital readouts in pendulum testers, as well as more options for test automation, but the great majority of systems sold do not have all the bells and whistles. Instrumented falling-weight and pendulum testers incorporate more innovations. But fully automated systems with automatic specimen loading are still rare, except in those few labs where hundreds of tests are performed daily.
Differentiating the tests
What is an impact test? It is applying a load to a specimen at high speed, then measuring the response of the specimen. Breaking the sample is a two-step process: Energy is needed to create a crack, and more energy is needed to enlarge the crack to failure, explains Harry Yohn, marketing manager at Tinius Olsen Testing Machines.
Of the two basic types of plastics impact testers, pendulum types for Izod, Charpy, and tensile impact measure the energy absorbed by the specimen to cause failure. The other category is falling-weight tests—Gardner falling weight for rigid materials and dart drop for film. These are typically pass/fail tests: They give the average impact energy that breaks the sample 50% of the time.
While these traditional tests are adequate for quality control, they do not provide good information on the mechanism of failure or the cause of a fracture in an end-use application. Whereas the non-instrumented impact tests just measure the energy necessary to break a specimen, instrumented impact tests provide curves of high-speed stress/strain data that distinguish ductile from brittle failure and crack-initiation from crack-propagation energy. The latter give a more nuanced picture of the "toughness" of a specimen, explains Yohn.
In addition to the limitations of traditional tests, there are other factors driving the trend to instrumented impact testing. A key one is concern about product liability for an increasing range of products, from medical and automotive components to toys or pipe. A case in point cited by Instron's Lio involves a plastic ladder company that learned the hard way about testing its product for the ductile-to-brittle-failure transition—the temperature at which the material is no longer flexible enough to be resilient. The firm supplied ladders to a city for use in sewer systems. The ladders worked well until winter came, at which point they started shattering when they were being pounded into their foundations. As a result, workers were falling off rungs that could not support them. While the company had run tensile tests, it had not performed impact tests at realistic temperature conditions. It was an expensive lesson: The city sued, won, and forced the processor to pay damages and replace all the ladders.
Yohn notes that impact results are sensitive to numerous factors besides temperature:
- Humidity or moisture content.
- Impact velocity or strain rate.
- Total kinetic energy of the dart or pendulum.
- Impact geometry—shape and dimensions of the sample and the impact device, as well as the angle and direction of impact.
- How the sample is prepared (molded, extruded, or fabricated)
- Sample-notching procedure.
- Sample mounting in the tester.
Roughly 20% of pendulum impact tests are done on cold specimens. Most of the time, the samples are conditioned in a freezer and transferred to the pendulum (at room temperature) for testing as quickly as possible. However, even in the short time the sample sits in the specimen supports at room temperature, it can warm up and appreciably change the results.
For this reason, auto companies (each of which has its own impact-test specifications) prefer that the specimen supports be cooled. As a result, many test labs are installing a "cryobox" that encloses the Izod vise or Charpy supports.
Several other factors contribute to the growing dissatisfaction with the Izod and Charpy tests—particularly among materials suppliers. According to Gerard Nelson, area sales representative for Ceast USA, they include inaccurate or improper notching techniques, as well as subjective judgments of brittle versus ductile failure. The different specimen sizes, impact velocities, and hammer energies for the ISO and ASTM standards only make things worse.
Most impact testers in use today are still not instrumented. Probably the biggest reason is historical. The industry holds a vast amount of data based on notched-Izod testing without instrumentation. The most commonly used impact-test standards do not call for instrumentation. Also, an instrumented system costs significantly more and requires additional technical expertise.
What to use when
Your choice of impact-testing equipment depends on what you will use it for—R&D, quality control, material characterization, checking incoming material, or product design. In most cases, your customers dictate what is required. Automotive specs, for example, can cite ASTM, ISO, or SAE test methods or each automaker's own proprietary standards. Says Instron's Lio, "A GM vendor most likely would need to run the GM impact-test standards, which vary a bit from the ISO or ASTM versions."
The ASTM, ISO, and other test standards are very clear on what plastics they pertain to. For example, thin-films may be tested according to ASTM D1709, while polycarbonate used in automotive parts would be covered by either ASTM D3763 or ISO 6603-2.
In addition, there is a whole realm of non-standard tests with no industry specifications. These might be tests on actual products or assemblies (e.g., plastic gas tanks). Such cases require understanding of the end-use conditions. For example, a manufacturer of food trays tests them at subambient temperatures to simulate freezer storage.
For QC of finished products that are not covered by a particular test requirement, a processor may opt for a basic pendulum or falling-weight impact tester. In general, falling-weight tests are easier to use because no notching of samples is required. Yohn from Tinius Olsen says, "A falling weight, such as Gardner impact, may be used for relatively flat objects like plastic sheet or vinyl siding. For testing say, a toothbrush, a pendulum-style unit may be used, but a method of holding the part securely has to be developed on a case-by-case basis."
Nova's Elston, who is ASTM section chair for static properties, sees no appetite in the industry for migrating from ASTM notched Izod to Charpy ISO standards for testing commodity resins. However, for engineering and specialty resins, there is indeed a move toward Charpy impact testing. "The automotive people are now on the ISO/CAMPUS bandwagon," he says, referring to the Consortium for Computer-Aided Preselection by Uniform Standards (CAMPUS), a global alliance of materials suppliers committed to ISO standards.
Stephen Sinker, development associate at Ticona Corp. in Summit, N.J., notes that his firm is moving away from Izod toward Charpy testing in response to requests from a wide range of customers. On the other hand, Greg Jarrell, research assistant at LNP Engineering Plastics in Exton, Pa., says his firm has received very few requests to switch.
In any case, Ticona's Sinker anticipates that drop-weight impact testing eventually will become more widely used as a replacement for pendulum impact tests. He says falling-weight testers are generally used on materials like polyolefins that exhibit ductile behavior. According to Sinker, pendulum-type testers are best for brittle-fracture materials, like many engineering thermoplastics that go into metal-replacement applications.
LNP's Jarrell agrees that the material can dictate which impact test you use—but it's not the only factor. LNP uses its instrumented falling-dart tester with high-impact, glass-reinforced compounds, such as its Verton long-glass nylon 66 and PP materials. But the company's non-instrumented pendulum impact unit gets a lot more use for running Izod tests as a QC indicator when testing 40 different formulations of PP, for example.
Why instrumented impact?
Instrumented impact tests are becoming more widespread, particularly for R&D at compounding operations or anywhere there is a need to examine in detail how the material fractures. Tinius Olsen's Yohn says the auto industry is showing new interest in instrumented impact tests because they are looking to develop impact data for plastics that better simulate real-life conditions.
Instrumented impact can be performed either on a falling-weight tester, such as those supplied by Instron and Ceast, or on a pendulum-type instrument like those offered by Tinius Olsen and Atlas. Most instrumented drop-weight testers allow users to also perform the Izod and Charpy tests by changing the impact striker and fixture on the instrument. However, Ceast's Nelson notes that even with these fixtures, the drop-weight tests do not comply with ASTM Izod or ISO Charpy standards. Furthermore, an ASTM task group has recently debated evidence that data on some materials tested this way do not correspond well to impact data derived from a traditional pendulum.
With instrumented impact, the falling dart's tip or the pendulum's hammer is fitted with a load cell. The force-time data during the actual impact are stored by a high-speed data-acquisition system. These data can be used to generate curves showing force, energy, velocity, and deformation versus time. By analyzing these curves, one can learn the force, energy, and deformation necessary to initiate a crack and then to cause total failure; the rate sensitivity of a material to impact loading; and the temperature of a material's transition from ductile to brittle failure mode.
Instrumented impact tests are performed according to ASTM D3763 and ISO 6603 and 7765 for drop-weight instruments, and ISO 179 Part 2 or ISO 180 Part 2 for pendulum testers. Prices of instrumented impact testers start at around $20,000 for a basic model to $40,000-70,000 for advanced systems with additional sensors and environmental chambers, and over $100,000 for fully automated systems.
Instrumented falling-weight tests can be performed on films, plaques, pipe sections, and finished products such as safety helmets. Instrumented pendulum (Izod and Charpy) tests can be done on standard specimens or on sections cut from injection molded or compression molded finished parts. Some falling-weight instruments have a support table on the base of the unit that allows testing of larger whole parts or assemblies.
Says Nova's Elston, "I don't see any interest in instrumented impact tests on commodity-resins. Basic falling-weight or Izod impact are sufficient for products such as CD cases. But instrumented impact is desirable for rigid structural products."
But that could be changing. The advent of piezoelectric sensors for instrumented impact testers is said to provide greatly increased sensitivity, allowing for testing of very light films, foams, and most other materials used in packaging.
Choosing a pendulum unit
The most common impact testers sold today are pendulum units. Most of these accept different accessories (striker heads and specimen supports) in order to perform Izod, Charpy, and tensile-impact tests.
Izod and Charpy tests are similar in many respects. Both use test specimens that are either molded to size or cut from a larger "dog-bone" tensile-test sample. Specimen size for Izod testing is 2.5 x 0.5 in., while Charpy uses 5 x 0.5 in. specimens. In both tests, sample thickness depends on the specifications for the material being tested (typically 1/8 in. for Izod tests). Specimens are notched and conditioned with temperature and humidity before testing. At least 10 specimens are tested and the results are averaged. Units are ft-lb/in. for Izod and joule/m2 for Charpy.
Pendulum impact machines consist of a base, a pendulum of either single-arm or "sectorial" design, and a striker rod (also called a hammer), whose geometry varies in accordance with the testing standard. The mass and the drop height determine the potential energy of the hammer. Each pendulum unit has provisions to add extra weight. There is also a specimen support—a vise for the Izod test and an anvil for the Charpy test.
A relatively new option for plastics testing is the sector pendulum design, offered by Instron and Tinius Olsen. An ASTM task group has deemed it equivalent to a conventional pendulum, which consists of a slender rod with a concentrated end-mass. The sector pendulum resembles a thin, flat pie wedge. It is fastened at the top to a bearing, and the striking nose is centered at the bottom, rounded portion. Because this flat metal wedge lies in the plane of the impact swing, it is extremely stiff in the direction of the impact. This reduces machine vibration and is said to improve the accuracy of the results.
Ceast's Nelson offers this caution about sector pendulums: "Although the sector is stiff in the plane of impact, it is very thin and therefore weak in the lateral plane. When you impact flexible specimens that do not break but simply bend and twist, this can set up a lot of vibration in the sector." For this reason, Ceast abandoned the sector design.
Very basic pendulum units without electronics can be bought for around $5000. These units have just a pointer to mark how far the swinging pendulum travels after striking the sample. The pointer is moved by the swinging pendulum and remains at the highest point of the arc after the pendulum swings back the other way. A small amount of the pendulum's energy is lost to friction in moving the pointer.
More advanced units cost between $10,000 and $14,000. They have an encoder on the pivot point that electronically records the movement of the arm for greater accuracy. These microprocessor-controlled units automatically drop the pendulum and collect the data. Says Richard Young, director of sales at Testing Machines Inc., "You no longer have to worry about the friction caused by the pointers, and you get better resolution." Fully automated pendulum units cost upwards of $50,000.
The notching device required for the most popular impact tests—notched Izod and notched Charpy—is sold separately from the test instrument. Notchers cut away a V-shaped section of the sample. The notch size and shape are specified by the test standard. The purpose of the notch is to mimic part-design features that concentrate stress and make crack initiation easier under impact loads. Notchers sell for around $4000-6000 for a basic unit and as much as $30,000 for a computerized unit that makes the notch automatically. A notch-verification device is necessary to check the notching accuracy.
Before testing, Izod specimens are clamped in a vise, while Charpy samples are placed on an anvil without a clamp. A weakness of the Izod test is that the force used to clamp the sample can vary and can add significant stress to the specimen. Both can cause erratic results and lower total-energy readings. Industry sources reply that more consistent Izod results can be obtained with repeatable clamping force. An air-driven clamp or a torque wrench will help. Most suppliers now offer an Izod vise with an integral load cell that allows direct monitoring of the clamping force.
Some in the industry argue that the primary cause of error in pendulum impact tests is incorrect or inconsistent notching of test specimens. Notch geometry defines the degree of stress concentration, especially in "notch-sensitive" materials. Notes Instron's Lio, "Some notch cutters heat up the surrounding notch area, which changes the properties of the test specimen."
Notching can be done with specialized notchers or a standard milling machine. Correct notching requires both the right cutting tool and proper technique. Tinius Olsen's Yohn warns that the same cutting tool can produce different notches in different materials.
Industry critics also say tests on notched specimens measure only propagation energy, not crack-initiation energy, and thus do not give a true indication of the specimen's impact resistance. However, pendulum impact units can also perform unnotched Izod and Charpy tests. Though not widely used, these unnotched tests are believed to give an indication of the energy both to initiate and propagate a crack. Neither is another unnotched pendulum test, known as tensile impact, which is more like a high-speed tensile test, according to Instron's Lio.
Falling-weight instruments, including the traditional Gardner dart drop and instrumented drop towers, can be used to determine the amount of energy that is needed to cause a failure on a plaque, sheet, film, pipe, profile, or molded product. The simplest and most inexpensive versions are the Gardner falling-weight test (ASTM D5054) for rigid plastics and dart-drop impact testers for thin films and flexible sheet (ASTM D1709). These units have a weight placed at the end of a nub or dart that is raised to a specific height and dropped on the secured sample. Weights are typically 2, 4, and 8 lb for a basic unit and up to 50 lb or more for an instrumented drop tower.
The procedure is incremental, requiring the destruction of a relatively large number of samples—typically 30. Calculations of energy absorbed in ft-lb, in.-lb, or grams are based on the radius of the impact tester, weight dropped, and the height from which it was dropped. Drop heights and dart geometry are still rather arbitrarily selected, according to Instron's Lio. "Polymers can be strain-rate dependent. A 10-lb weight dropped from a 2-ft height and a 2-lb weight dropped from a 10-ft height both impart 20 ft-lb of energy to a sample. But the effect may be different because of the different velocity of impact."
Dart-drop instruments are popular with large film producers and their resin suppliers. John DeChristofaro, sales and marketing manager at Dynisco Polymer Test, estimates that more than 50% of film processors use this test to monitor production quality. The company offers units with either a manual specimen clamp or a new pneumatic clamp.
In the last couple of years, Atlas has offered the Total Energy Option for its dart-drop film testers, which complies with the newer standard ASTM D4272. Bob Lattie, manager of Atlas' Polymer Evaluation Products Div., says this photoelectric system (a box with photodiodes placed beneath the film) captures the change of velocity of the dart and automatically calculates the amount of energy absorbed by the film. Particularly popular with large film makers, it provides a continually updated calculation of standard deviation.
The cost of conventional falling-weight testers ranges from under $2000 for the simplest Gardner type to $10,000 for units that can test pipe, profiles, or automotive components (ASTM D3763). Dart-drop testers for films cost from $2800 to $5000, though options such as total-energy readings can run the price up to around $10,000.
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