Laboratory tests can accelerate cavitation erosion effects on composites and coatings for emerging industrial applications.
With the recent increase of industrial applications for composites and various coatings, cavitation erosion resistance of these materials is of interest to designers and engineers. In the absence of historical data on material performance in the real cavitating flow field, selection of the material relies on laboratory controlled testing.
Cavitation erosion results from the repeated impulsive loading of the material by localized high-intensity, short-duration pressure pulses due to the collapsing bubbles. Several laboratory techniques to generate cavitation can be used to study cavitation erosion in a controlled environment and in an accelerated manner. Accelerated erosion tests involve subjecting the considered material to an erosion field that is significantly more repetitive than the actual cavitation field to which the material will be subjected.
Dynaflow Inc., Jessup, Md. conducts studies on cavitation resistance of various materials including composites and coatings using several cavitation erosion facilities. The two most popular methods for accelerated cavitation erosion testing are the ultrasonic vibration method and the cavitating jet technique.
Procedures for both of these are standardized by the American Society for Testing and Materials (ASTM) as the G-32 Test Method for Cavitation Erosion Using Vibratory Apparatus and G-134 Test Method for Erosion of Solid Materials by a Cavitating Liquid Jet.
The G-32 method uses the cavitation generated by a vibratory device employing a magnetostrictive ultrasonic horn. In the standard method, a button-shaped sample of the material is affixed to the end of a horn and is subjected to cavitation resulting from the vibrations of the horn. Since most composite materials are difficult to attach to the vibrating horn, a so-called “alternative” method is also prescribed in G-32. In this alternative method—also known as the stationary specimen method—the material sample is placed at a small distance from the flat horn tip surface. The cavitation bubble cloud is generated in between the sample and the face of tip of the horn, which is equipped with a cavitation-resistant button such as titanium. In the standard G-32 test, the temperature, liquid beaker volume, horn tip submergence beneath the free surface, frequency, and amplitude of the oscillations are all prescribed by the ASTM method. In the alternative method, the ultrasonic flow field is less affected by the compliance of the material than in the direct method.
The usual test procedure is as follows:
- Expose the sample to the cavitation for a predetermined period of time;
- Interrupt the test, and remove the sample for examination;
- Record the damage (weight, depth, photographs, etc.); and,
- Return the sample for additional testing, and repeat the above process.
For metallic materials, weight loss as a function of time is usually calculated from the measured weight. However, for composite materials, weight measurement often poses a problem due the potential of such materials to absorb water during the test, which makes it difficult to get weight information, even using sample drying techniques. In this case, erosion depth as a function of time is used to quantify the test results.
Figure 1 shows a typical erosion progression on a composite material tested by the G-32 alternative method. The erosion starts with tiny pits on the surface of resin. As these tiny pits grow, the underling fibers become exposed to cavitation; the cavitation makes its way into the resin around the fibers. Then the fibers start to fail, and the erosion progresses into the material in a layer-by-layer fashion.
One should be careful in using the G-32 ultrasonic method for some materials. The Annual Book of ASTM Standards – Section 3 Material Test Methods and Analytical Procedures (2010) warns that this test is not recommended for evaluating elastomeric or compliant coatings. The document reads, “This is because the compliance of the coating on the specimen may reduce the severity of the liquid cavitation induced by its vibratory motion. The result would not be representative of a field application, where the hydrodynamic generation of cavitation is independent of the coating.” Another caution with the G-32 method is that the strength of the cavitation field it generates is fixed, which could be different from the strength in the actual application. The test results could be misleading as many composites and elastomers respond very differently to the level of cavitation intensity.
The cavitating jet method is a better choice for composites and elastomers. The cavitation intensity can be varied by controlling the jet pressure. A jet cavitation erosion testing facility is composed of a cavitating nozzle, a sample holder, a test tank, and a pump (Figure 2). The sample holder ensures that the sample is placed precisely at the same location after each test when it is returned from periodic examination. ASTM standardized this testing method as G-134 by specifying the nozzle geometry. The test method using the DynaJets nozzle at Dynaflow is similar, but has more flexibility in selecting nozzle and environment flow conditions.
A series of preliminary tests are conducted first to determine appropriate jet conditions to be used in the main tests. These need to be such that full erosion evaluation history on each sample could be obtained in a reasonable time span—from a few minutes to less than a couple of hours. The operating pressure of the DynaJets cavitating nozzles, which ranges from the mid hundreds up to 1,000 psi, is appropriate for most composites and elastomers. Once the test conditions are determined, the test procedure is similar to that of the ultrasonic cavitation tests.
The erosion progression of an elastomeric coating material tested by DynaJets is shown in Figure 3. During the initial incubation time, the coating material does not lose any weight. Then, the material starts to be removed, and the erosion damage grows larger and deeper over time.
The jet cavitation test has the advantage that the cavitation intensity is easily controlled by the jet pressure, the type of nozzle, the standoff distance, and the diameter of the nozzle. This intensity level is not influenced by the response of the elastomeric material. It also enables testing the same material at different intensities, which can provide a broad perspective (a range of cavitation intensity levels) in evaluating overall performance of the tested material.