Apparatus and method for fatigue testing of a material specimen in a high-pressure fluid environment

The invention provides fatigue testing of a material specimen while the specimen is disposed in a high pressure fluid environment. A specimen is placed between receivers in an end cap of a vessel and a piston that is moveable within the vessel. Pressurized fluid is provided to compression and tension chambers defined between the piston and the vessel. When the pressure in the compression chamber is greater than the pressure in the tension chamber, the specimen is subjected to a compression force. When the pressure in the tension chamber is greater than the pressure in the compression chamber, the specimen is subjected to a tension force. While the specimen is subjected to either force, it is also surrounded by the pressurized fluid in the tension chamber. In some examples, the specimen is surrounded by hydrogen.

THE NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present disclosure relates to fatigue testing of material specimens, and more specifically to apparatuses and methods for in-situ fatigue testing of material specimens under high fluid pressure conditions.

2. Description of the Related Art

Hybrid fuel cell/electric vehicles convert the chemical energy of hydrogen gas into electrical energy to power the vehicle's electric motor. In order to make these vehicles a viable for everyday transportation, decentralized hydrogen filling stations are needed to ensure hydrogen is available where consumer-demand is. In order for economic distribution, hydrogen must be piped from its point of production to its point of demand. An extensive pipeline infrastructure is thus needed to distribute the hydrogen from the generation plants to the filling stations.

The ASM Materials Handbook lists five specific types of hydrogen induced damage to metals and alloys. These types are: hydrogen embrittlement, hydrogen-induced blistering, cracking from precipitation of internal hydrogen, hydrogen attack, and cracking from hydride formation. Except for hydrogen embrittlement, a phase transformation is coupled to each of the listed hydrogen damages. Hydrogen embrittlement is the result of hydrogen atoms diffusing through the surface of certain materials. The hydrogen atoms can accumulate within the material's microstructure causing increased subsurface pressure and eventually cracks to form. Hydrogen embrittlement is a major concern for hydrogen pipeline material designers, since even a small leak in a pipe wall, a welded connection, a flange or a fastener could lead to a dangerous situation.

Fatigue effects in materials due to hydrogen contact may cause defects which can remain undetected until a catastrophic failure occurs without warning. Applying tensile, compressive, low cycle fatigue and high cycle fatigue loads to characterize the strength of materials is known. Novel apparatuses and techniques for testing materials under adverse conditions such as hydrogen gas contact are presently needed.

BRIEF SUMMARY OF THE INVENTION

Provided are several examples of apparatuses and methods for applying loads to material specimens in pressurized fluid environments. The apparatuses use the fluid pressure as the force for applying the loads to the specimens.

According to an example, an apparatus has a vessel-shaped body with an interior surface defining a volume for holding the pressurized fluid. In some examples, the fluid is highly pressurized hydrogen gas, but other fluids may be also used. The interior surface has a specimen receiver configured to accept a first end of a material specimen. A piston assembly is disposed in the vessel; the piston assembly has a specimen receiver configured to accept a second end of the material specimen. An exterior surface of the piston assembly cooperates with the vessel's interior surface to form a seal. The seal partitions the volume into two chambers: a tensile chamber and a compression chamber.

A first branch conduit (e.g., tensile branch) conveys the fluid from a source into the tensile chamber at a first pressure (P1), and a second branch conduit (e.g., compression branch) conveys the fluid from the source into the compression chamber at a second pressure (P2). A pressure controller regulates the fluid pressures in the first and second branch conduits.

The piston assembly is movable in relation to the vessel when acted upon by the pressurized fluid. The receivers for holding a material specimen move away from one another when the fluid pressure (P1) in the tension chamber is greater than the fluid pressure (P2) in the compression chamber, and the receivers move toward one another when the fluid pressure (P2) in the compression chamber is greater than the fluid pressure (P1) in the tension chamber. By alternating the (P1) and (P2) pressures with the controller, alternating tension and compression loads are applied to a loaded specimen.

DETAILED DESCRIPTION OF THE INVENTION

With reference toFIGS. 1 and 2, an example of an apparatus10for fatigue testing of a material specimen S in a high-pressure, fluid F environment is illustrated. As used throughout this disclosure, the term “fluid” encompasses any continuous amorphous substance whose molecules move freely past one another and that assumes the shape of its container; a liquid or a gas. The apparatus10utilizes the pressure of the fluid F to impart tensile, compressive and cyclic, fatigue loads in the specimen S. The fluid F imparts the loads while the specimen S is being exposed to the fluid F.

A pressure vessel12includes a body14and end caps16A and16B, which are generally affixed to the body14with tie rods, studs, bolts, clamps, threads, welds or other fastening means. In other examples, at least one of the end caps16A and16B is integrally formed with the body14. The vessel12has an interior surface18, defining an enclosed volume, for confining the fluid F such as hydrogen gas for example. In one example, the interior surface18is a bore with a cylindrical shape. Although a cylindrical-shaped, thick-walled body14and circular-shaped end caps16A,16B are commonly used for making pressure vessels, other shapes and configurations are also contemplated in the present examples. The materials, thicknesses and manufacturing methods used to manufacture the body14and end caps16A,16B are engineered to safely handle the pressure loads imparted by the pressurized fluid F. Pressure vessel design criteria are available through the American Society of Mechanical Engineers (ASME) boiler and pressure vessel code.

A specimen receiver20is disposed in at least one of the end caps16A and16B, and is configured to accept one end of a mounted specimen S. The receiver20may be configured to accept one end of a standard specimen S (e.g., 0.750 inch NC threads), or the receiver20may be configured to accept one end of a standard test strip or a specimen S of custom size and shape.

Disposed within the pressure vessel12is a piston assembly22for partitioning the enclosed volume into two pressure chambers: a tensile chamber24and a compression chamber26. The piston assembly22includes a piston body28with an external surface30that is complementary to the shape of the interior surface18, and in the example shown; the piston body28is cylindrical in shape. A receiver20is disposed in the piston body28and is configured to accept a second end of a loaded specimen S.

A clearance gap32, formed between the piston assembly22and the interior wall18, permits the piston assembly22to move in relation to the interior wall18. Sealing elements34A and34B are disposed respectively in glands36A and36B formed in the external surface30of the piston body28. The sealing elements34A and34B span across the clearance gap32, interacting with the piston body28and interior wall18, to create a fluid F seal. The seal discourages leakage of fluid F between the tensile chamber24and the compression chamber26. The cross section of the sealing elements34A and34B may be square, rectangular (shown), circular, oval, or some other shape known in the sealing art. The sealing elements34A and34B may be full annular, or segmented annular in form. The material of the sealing elements34A and34B is chosen for its fluid compatibility, lubricity, temperature, and pressure capabilities. A material such as polyurethane or carbon provides adequate properties for this particular application.

Fluid F at pressure P1 in the tensile chamber24and at pressure P2 in the compression chamber26imparts loads on piston faces38A and38B of the piston assembly22. A bearing set40centers the piston assembly22with the interior surface18, maintaining a fairly constant clearance gap32as the piston assembly22moves in relation to the vessel body14. The bearing set40is disposed in a single groove42or individual grooves (e.g., pockets) formed in the piston body28. The bearing set40may be full annular, or segmented annular in form. A material such as DuPont TEFLON brand fluoropolymer provides adequate strength and lubricity properties for this particular bearing application.

A fluid F supply source44(e.g., tank or bottle) stores the fluid F, for example hydrogen gas, and provides the fluid F to an attached pressure intensifier46via a low pressure conduit. The pressure intensifier46increases the pressure of the fluid F supply for use in the pressure vessel12. Within the intensifier46, fluid F supplied from the supply source44acts on a larger piston48, a force is transferred mechanically through a connecting rod50to an adjoined smaller piston52. The smaller piston52area acts on the fluid F, increasing the pressure with the pressure ratio being inversely proportional to the ratio of the two piston areas. The fluid F exits the fluid intensifier46via a high pressure conduit to a one-way valve54A, thus forcing the high pressure fluid F in a direction out of the fluid intensifier46and thus preventing back flow.

Downstream of the one-way valve54A, the fluid F is directed into two separate, high pressure branches: a tension branch56and a compression branch58. The tension branch56delivers a first portion of the fluid F to the tension chamber24through end cap16A and the compression branch58delivers a second portion of the fluid F to the compression chamber26through end cap16B. The fluid F pressure within the tension chamber24and the compression chamber26acts on the piston assembly22to apply tension and compression loads to a loaded specimen S. In some examples only a tension load is applied. In other examples only a compression load is applied. In yet other examples, alternating tension and compression loads are applied.

Disposed within the tension branch56and compression branch58, are four-way valves60A and60B for modulating the fluid F pressures within the tension chamber24and compression chamber26respectively. Low pressure return branches convey low pressure fluid F from the four-way valves60A and60B back to an attached gas collector62. In turn, the gas collector62is attached to the supply source44and pressure intensifier46through low pressure conduits and one-way valves54B and54C.

A control system includes a processor64(e.g., a personal computer) attached to an electronic fluid pressure controller66. The pressure controller66, in turn, is attached to one or more fluid F pressure transducers68A,68B, one or more pressure regulators70A,70B, and one or more specimen S strain monitors72(e.g., strain gages). In other examples, temperature and/or humidity monitors may also be installed (not shown). A laboratory monitoring and control software program such as LabVIEW, available from National Instruments, may be installed on the processor64to allow an operator to easily view a schematic of the apparatus10, monitor the various pressure transducers68A,68B and adjust valve regulators70A,70B.

The processor64monitors the magnitude of specimen S loading with the strain monitor72while simultaneously modulating the four-way valves60A and60B with feedback from the pressure regulators70A,70B. The processor64also monitors the fluid F pressures of the tension chamber24and the compression chamber26with the pressure transducers68A and68B respectively. The specimen S is loaded in tension when the fluid F pressure in the tension chamber24exceeds the fluid F pressure in the compression chamber26; and the specimen S is loaded in compression when the fluid F pressure in the compression chamber26exceeds the fluid F pressure in the tension chamber24.

The apparatus10generates in-situ tensile, compressive or cyclic fatigue loading on a specimen S while it's subjected to a high-pressure fluid F environment. The pressure of the fluid F acting on the piston assembly22provides the load source for loading the specimen S in tension and compression. No other mechanical means (e.g., lead screws, actuators, etc . . . ) are used for loading the specimen S during testing. The fluid F may be in a liquid or a gas state and in the illustrated example gaseous hydrogen is used.

Referring now to the flow diagram ofFIG. 3, a method100for fatigue testing of a material specimen in a high pressure fluid environment with an apparatus that utilizes the fluid as the load source is now described. In the first process step block labeled101, an apparatus10as previously described above is provided. Next, in the process step block labeled102, a material specimen S is loaded into the specimen receivers20. Next, in the process block labeled103, the fluid pressures in the tension and compression branch conduits56,58are modulated with the pressure controller66such that the fluid pressure (P1) in the tension chamber24alternates between being greater than and less than the fluid pressure (P2) in the compression chamber26.

Step103may be accomplished by modulating a four-way valve60A and60B disposed in each of the tension56and the compression branch circuits58.

The method100may also include a step104for monitoring pressure transducers68A and68B disposed between said pressure controller66and each of the tension chamber24and the compression chamber26with the pressure controller66. Step104may also include monitoring at least one strain measurement from a strain monitor (e.g., strain gage)72disposed between the pressure controller66and the material specimen S with the pressure controller66.

While this disclosure illustrates and enables specific examples in the field of material specimen testing, other fields may also benefit. Accordingly, the invention is intended to embrace those alternatives, modifications, equivalents, and variations as fall within the broad scope of the appended claims. The technology disclosed and claimed is available for licensing in specific fields of use by the assignee of record.