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Commonly performed compression, tensile and flex tests only provide information about how a material will perform when subjected to a static load. The ceramic under test is allowed to absorb the load slowly and, possibly, over an extended period of time. While appropriate for providing information about the material’s basic properties, such as stress and strain, these tests do not necessarily provide useful information as to how the material or product will react in real-life situations.
Understanding Impact TestingImpact tests expose materials and products to dynamic events, forcing the material to absorb loads quickly. The information provided by this type of testing is useful to understand how that material will perform in real life. Inevitably, all products will experience some type of blow, collision, drop or impact from a falling object during their lifetime.
Products are more likely to fail when subjected to higher-than-expected forces. The purpose of impact testing is to simulate these real-life conditions in an effort to prevent the product from breaking, and, depending upon the product’s final use, to ensure personal safety.
Pendulum impact machines consist of a rigid base, a pendulum (either single arm or segmented in design), a hammer, a striker (whose geometry will vary in accordance to the test standard being used), and an analog dial with a pointer or tell-tail attached. The mass and drop height determine the potential impact energy of the hammer. Extra weight can be added to the hammer to increase the impact energy. A specimen support fixture located on the base is also used.
To run a test, the pendulum (hammer) is raised to the desired drop height and latched in place. A test sample is installed on the support fixture and the pendulum is released to impact the sample. The pendulum will continue to swing up after impacting the sample to a height somewhat lower than that of the latched location. During the swing, a tell-tail or pointer is engaged by the pendulum arm and is “left behind” as the pendulum hammer swings back toward its latched position. The engineer can use this lower final height point to calculate the energy that was lost in breaking the specimen.
A digital display can be used in place of the pointer, or the machine can be equipped with a data acquisition system to collect load-time-displacement data. Data acquisition systems consist of software that works with a tup (instrumented striker) and velocity flag to capture, display and analyze impact performance data.
While pendulum impact systems have a mass that revolves around a central pivot point and strikes the sample in a horizontal direction, a drop tower’s weight is released to fall along rails in a vertical direction. Attached to the bottom of the weight is a tup that features geometry defined by the test standard, while a specimen support fixture is affixed to the rigid base.
As long as the drop height and weight are defined, the potential (or impact energy) value can be established. If uninstrumented (i.e., results are determined by a visual inspection), the only result that can be obtained is pass or fail. If the falling weight is stopped dead on the test specimen, the result is a pass, while a fail would be achieved if the specimen is destroyed completely by the striker passing through.
Additional details about the failure mode of the sample can be gathered by instrumenting the test. In an instrumented impact test, the load on the specimen is continuously recorded as a function of time and/or specimen deflection prior to fracture. All of the above impact tests can be retrofitted or designed with electronic sensing instrumentation.
The best systems record load vs. time, or deformation for the entire period of the impact event. This gives a much more complete representation of an impact than a single calculated value. Another area of improvement with instrumentation is time. Test times can be reduced and automation can even be incorporated into the testing.
Focused TestingSince ceramic materials are so diverse and used in many areas that affect our lives, it is important for manufacturers to understand what effect a real-life impact may have. For example, floor/roofing tile and water/sewer pipe are made from traditional ceramic materials. These four products are routinely subjected to impact events; floor and roofing tile are repeatedly impacted during daily use, while water and sewer pipe are preferably only impacted as they are covered with gravel and soil after installation.
It is critical that the manufacturers of these products understand how strong they are when subjected to high or heavy impacts, and to light impacts. Cracks in any of these products will result in moisture traveling into the surrounding area. By performing a series of increasing energy impact tests, the product manufacturer can establish the amount of impact force the materials should be able to withstand.
Following are some typical test scenarios for various segments of the ceramic and related industries.
Ceramic tableware experiences daily impacts from serving utensils, cleaning and being stacked in cupboards. To understand how resistant their tableware is to chipping and cracking, manufacturers conduct pendulum impact tests as detailed in ASTM C368–Impact Resistance of Ceramic Tableware.
By impacting the center of the specimen, the manufacturer can determine how much energy is required to produce an initial fracture, as well as the amount of energy necessary to produce complete failure of the specimen. By repeatedly striking the rolled edge of a tableware specimen while increasing the amount of force (energy) used with each impact until full failure, the manufacturer can calculate the strength of the product. These results can be used to predict the product’s resistance to breakage while in service.
Glass and its end products also illustrate the importance of impact testing. To utilize glass lenses, for example, the eyeglass industry has standards on testing the impact resistance of lenses by subjecting them to a ball drop impact test. A steel ball with a specified diameter and weight is dropped from a defined height onto the outer surface (concave) of the glass lens. The lens is then visually examined for cracks or detached material.
In order for this product to pass, no particle on the interior surface (convex) may be detached. When using a data acquisition system and performing this test with a small tabletop drop tower, information on any crack initiation can be found in the load data.
Structural ceramics are often used for applications such as industrial wear parts, engine components and cutting tools, all of which are often intentionally impacted while in use. For example, polycrystalline diamond cutters are used for oil, natural gas and other mining applications. These bits made from industrial diamond are manufactured under conditions of extreme temperature and pressure to obtain maximum hardness and durability.
Damaged or worn bits are not as effective at removing material and ultimately need to be replaced, causing a loss of revenue and downtime in the drilling operation. To reduce costly maintenance and improve drilling efficiency, bit manufacturers have included impact testing in their design process. Using a drop tower with a data acquisition system to repeatedly impact the polycrystalline cutter establishes the energy level at which the cutter begins to sustain damage. Changes can then be made to the shape/design of the cutter profile and the manufacturing process as well, if needed, to improve the longevity of the cutter.
Due to their resistance to wear and inherent stability, the use of advanced ceramics in the medical market is increasing rapidly. Demand is expected to continue growing through 2010 due to the increased use of ceramics in joint replacements and dental procedures. The advanced ceramic orthodontic brackets available today are tested for impact resistance using a ball drop impact test similar to that used for glass lenses.
Though the majority of testing on crowns and ceramic implants focuses on flex tests to establish the material’s flex strength, these same parts are subjected to impacts as well since failures can occur due to impact with opposing teeth. Knowing the level at which impact energy will cause a crack in a crown or dental implant allows for improvements to be made in material selection, product design and manufacturing processes.
When used in the human body as implants or coatings attached to metal replacements, ceramic materials can help with the stimulation of bone growth, support the formation of tissues, and provide protection from the immune system. A new artificial cervical disc assembly (a ball and socket configuration made from titanium and a ceramic composite) is currently undergoing clinical trials. In addition, producers of these assemblies use drop towers to perform tests to see how the artificial discs hold up to an impact.
By increasing the energy used to test the disc, the manufacturers are able to establish at what force levels the components of the assembly will begin to suffer damage. Areas of interest include splintering or chipping in the ceramic ball possibly caused by the edge of the titanium cup as the assembly is impacted, which may cause problems that could necessitate a second operation. By increasing the impact energy, the process used to attach the ceramic ball to its metal base can also be observed for signs of failure.
Full ImpactFrom our tables to our homes to medical improvements in our lives, ceramic materials and the products made from them are a key part of our lives. Through impact testing, manufacturers can gain insight into how real life can affect ceramics.
For additional information regarding impact testing, contact Instron, 825 University Ave., Norwood, MA 02062; (800) 564-8378; fax (781) 575-5770; firstname.lastname@example.org; or visit www.instron.com.