Selecting the right pressure test for verifying a tubular component's integrity
March 11, 2008
Although it is used primarily for forming, hydroforming equipment also is useful in many pressure testing applications (burst testing, pressure pulsation, leak testing, autofrettage).
It's one thing to manufacture a tube; it's another to test it to verify its characteristics. Regardless of the raw material or the production method used to make the tube, pressure testing can be a necessary step in verifying the tube's quality. Whether the tubing is used for structural components or fluid transport systems, its strength and integrity are critical to knowing that the tube will hold up when it is put into service.
Tube stock isn't the only form that should be tested. Tubular components that have additional features created by end forming, hydroforming, and other processes, or components added by welding or brazing, also should be tested. Improper or inadequate testing is a big step on the path to a product failure, which can lead to safety and environmental hazards.
Two common types of test are tensile and pressure. While tensile tests determine the properties of the material and the tubing itself, the focus of this article is pressurization tests to evaluate tubular materials' quality and suitability for purpose.
Before pressure-testing a tube or tubular component, it is critical to gain an understanding of the various types and applications of pressure tests.
Burst and bulge tests involve increasing the pressure until the specimen fails or reaches some level of expansion. The test equipment controls one of two parameters: the pressure increase rate or the fluid flow rate (for a less dramatic failure). Either case requires that you monitor the pressure.
A burst test determines a maximum pressure at which the specimen fails. The failure may be in a weld or the material itself. Therefore, this sort of test is a way of evaluating the material and tube-making process. Because it is relatively simple to perform, the burst test is useful as a quality check of either incoming material or finished parts (see Figure 1).
Unlike the burst test, which stresses the tube until it fails, the bulge test evaluates the tube's deformation properties. Therefore, it is well-suited for evaluating materials for hydroforming applications. In addition to monitoring pressure, a bulge test includes some means of characterizing the tube deformation. This may be done in real time or by measuring deformed parts at various stages (see Figure 2).
The deformation properties determined by bulge testing depend on the end condition which, in turn, is based on the application. If the ends are free to move, the tube will self-feed, drawing material in as the pressure builds. If the ends are fixed, two-dimensional tensile strains will develop. If the ends are fed in, tensile and compressive strain fields will develop. This allows evaluation of the strain states encountered in tube hydroforming applications.
Testing is augmented by computer-based mathematical analysis. For example, models of the deformation process can be used to calculate the material properties. Another approach is using optimized finite element simulations to match the observed results to identify the material parameters. Still another is to put the tube under biaxial stress.1
Tube and components used in fluid systems frequently require leak testing for safety and environmental reasons. Leak tests are useful also for verifying the integrity of structural components by identifying defective welds or improperly installed parts.
For parts intended to operate at pressures less than 100 pounds per square inch (PSI), a leak test using a gas is appropriate. Components used in higher-pressure applications, such as fuel lines and brake lines, require leak testing at higher pressures. While low-pressure tests will reveal holes or open cracks, high-pressure tests will reveal flaws or defects that aren't visible but will open when pressurized.
The usual method for leak testing is pressure decay, in which the part is pressurized and sealed, and the equipment monitors the pressure drop rate. The baseline pressure drop rate is associated with thermal and other effects, which can be measured using reference parts known not to leak. A pressure decay rate above this baseline indicates a leak or leaks.
The key element in the sensitivity of this test is the resolution of the pressure transducer and data acquisition system. The resolution determines how small the minimum detectable leak is. This type of test is subject to a tradeoff between test time and sensitivity. Achieving a higher level of test sensitivity is a matter of allowing more time for a longer test period. The time allowed in a production situation is likely to be somewhat less than the time allowed when prototyping the part, but it must be sufficient to provide valid test results.
Analyzing the decay curve characteristics provides more consistent results than looking at individual pressure values.
Most fluid systems experience some type of cyclic pressure. The cyclic stresses induced by these pressure fluctuations can fatigue the tubular components. Features such as welds, end forms, and bends can act as stress risers, which can initiate fatigue cracks. Fatigue becomes a more significant consideration as operating pressures increase. Increases in pressure can significantly shorten a component's service life (see Figure 3).
Fatigue tests require cyclic pressures, usually sinusoidal, at a specified frequency. The maximum and minimum pressures must be controlled precisely, which requires a closed-loop control system. In this type of test setup, a servo valve controls the pressure directly or through an intensifier. The operator specifies the minimum pressure, maximum pressure, and frequency. Once started, the system runs unattended. A part failure results in a pressure control error—that is, the test setup no longer builds pressure and the test equipment cannot fulfill one of the test parameters. It is important that all components have fatigue life ratings higher than the expected duration of the test.
Service life of tubular products can be increased by a pressurization technique known as autofrettage. Typically it is applied to thick-walled tubes exposed to high cyclic stresses. Autofrettage applies an internal pressure that is high enough to plastically strain the material on the inside of the bore. When the pressure is released, the outer layer of material, which is still in an elastic state, springs back and compresses the inner layer. This creates a compressive bias to the inner wall, reducing the tensile stress peaks from the service pressure cycles (see Figure 4).
Implementing an autofrettage process requires a thorough analysis of the tubing and expected pressures to determine the extent to which the inner layer of material should be strained plastically. Then accurate pressure control is required to achieve the proper amount of deformation. The pressures used in autofrettage can range from 20,000 to more than 120,000 PSI. A measurement of the external expansion can be used as a supplement to the pressure control.
The four main considerations in developing a pressure system for these testing applications are the pressure source and control equipment, sealing, fluid selection, and safety.
Pressure Sources and Controls. The most basic equipment setup is a transducer and variable-speed control to regulate the pressure output from a pump. While this type of system can reach a set pressure, it doesn't provide sophisticated control for applying and releasing the pressure. Furthermore, a pump's ability to build pressure has a practical limit.
A more precise and flexible approach for low- to medium-pressure applications is to use a pump and control its output with a servo valve. Such a setup should include a pressure transducer, which provides feedback to a controller that drives the servo valve. The limitation to this approach is that readily available servo valves are limited to 5,000 PSI (see Figure 5).
An intensifier is necessary to achieve high to ultrahigh pressures. Intensifiers fall into two categories: single-acting or reciprocating. Single-acting intensifiers provide a more measurable and controllable volume, whereas reciprocating intensifiers provide a great available volume.
Intensifiers have a drive cylinder that operates at standard hydraulic pressure, usually 3,000 to 4,500 PSI, and a high-pressure section where the working fluid is compressed either by an intrusion rod or a piston. A servo valve, proportional valve, or regulator controls the input to the drive cylinder.
Sealing. Every application requires some form of sealing. For tests that may not require high throughput, such as burst tests and fatigue tests, using a plug and clamps to seal the tube is sufficient. For production operations, leak tests, and autofrettage, remotely operated seals are necessary.
Sealing method selection takes into consideration the part and the pressure. A part analysis determines the faces or edges that are available for sealing and a tolerable amount of deformation of the surfaces. Elastomeric seals work for low to moderate pressures. Fixing the seal in place might require expanding it in a bore or clamping it to an outer surface. High pressures generally require metal-to-metal seals.
Automated sealing devices are necessary for production leak testing for fuel lines, brake lines, power steering lines, and so on (see Figure 6). A gripper also is necessary for autofrettage of lines. Where damage to the sealing faces is a concern, seal actuation forces can be synchronized with the pressure to minimize the contact force).
Fluids. Fluid choice is based on many criteria. Regarding the part being manufactured, fluid considerations include chemical compatibility, corrosion, and cleaning.
Second, the fluid must be suitable for the pressure range. The fluid's properties change as the pressure changes. For example, at the pressures typical in autofrettage, some fluids freeze and do not transmit the pressure uniformly. Also, if the part is not cleaned at that stage, the fluid must be compatible with the next manufacturing step.
Some cases require testing the part with the fluid it will encounter later when it is put into service. For example, a diesel fuel equivalent is suitable for leak-testing components for a diesel fuel injection system.
The choice of fluid also affects the design of the pressure system components, pumps, valves, and intensifiers. Factors such as viscosity and chemical compatibility with seal materials come into play.
Conditioning the fluid is important. In leak tests a constant fluid temperature provides more consistency in the results. Also, fluid cleanness is just as important in testing as it is in manufacturing. It is necessary to circulate the fluid through a filtering system before reusing it.
Safety. Safety is a serious consideration in the design of pressurization systems. Pressurized fluids can store a tremendous amount of energy. It is imperative that a suitable protective barrier separates the testing system from the testing personnel. Interlocks are necessary so that the access doors cannot be opened when pressure is applied and pressure cannot be applied when the door is open.
The flammability of some process fluids is of concern because mists may be created during the testing process. Regardless of the type of fluid used, any mist that develops should be contained and routed through a mist collector.