Abstract:
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.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
       [0001]    This invention was made with government support under Contract No. DE-AC05-00OR22725 awarded by the U.S. Department of Energy. The government has certain rights in the invention. 
     
    
     THE NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT 
       [0002]    None. 
       BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention 
         [0004]    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. 
         [0005]    2. Description of the Related Art 
         [0006]    Hybrid fuel cell/electric vehicles convert the chemical energy of hydrogen gas into electrical energy to power the vehicle&#39;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. 
         [0007]    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&#39;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. 
         [0008]    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 
       [0009]    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. 
         [0010]    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&#39;s interior surface to form a seal. The seal partitions the volume into two chambers: a tensile chamber and a compression chamber. 
         [0011]    A first branch conduit (e.g., tensile branch) conveys the fluid from a source into the tensile chamber at a first pressure (P 1 ), and a second branch conduit (e.g., compression branch) conveys the fluid from the source into the compression chamber at a second pressure (P 2 ). A pressure controller regulates the fluid pressures in the first and second branch conduits. 
         [0012]    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 (P 1 ) in the tension chamber is greater than the fluid pressure (P 2 ) in the compression chamber, and the receivers move toward one another when the fluid pressure (P 2 ) in the compression chamber is greater than the fluid pressure (P 1 ) in the tension chamber. By alternating the (P 1 ) and (P 2 ) pressures with the controller, alternating tension and compression loads are applied to a loaded specimen. 
         [0013]    Other systems, methods, features and advantages will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the following claims. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0014]    A more complete understanding of the preferred embodiments will be more readily understood by reference to the following detailed description when considered in conjunction with the accompanying drawings where like numerals indicate common elements among the various figures. 
           [0015]      FIG. 1  is a simplified schematic diagram illustrating an example of an apparatus for fatigue testing of material specimens in a high-pressure fluid environment. 
           [0016]      FIG. 2  is a detailed view of the area labeled  2  in  FIG. 1 . 
           [0017]      FIG. 3  is a flow diagram illustrating an example of various method steps. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    With reference to  FIGS. 1 and 2 , an example of an apparatus  10  for 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 apparatus  10  utilizes 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. 
         [0019]    A pressure vessel  12  includes a body  14  and end caps  16 A and  16 B, which are generally affixed to the body  14  with tie rods, studs, bolts, clamps, threads, welds or other fastening means. In other examples, at least one of the end caps  16 A and  16 B is integrally formed with the body  14 . The vessel  12  has an interior surface  18 , defining an enclosed volume, for confining the fluid F such as hydrogen gas for example. In one example, the interior surface  18  is a bore with a cylindrical shape. Although a cylindrical-shaped, thick-walled body  14  and circular-shaped end caps  16 A,  16 B 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 body  14  and end caps  16 A,  16 B 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. 
         [0020]    A specimen receiver  20  is disposed in at least one of the end caps  16 A and  16 B, and is configured to accept one end of a mounted specimen S. The receiver  20  may be configured to accept one end of a standard specimen S (e.g., 0.750 inch NC threads), or the receiver  20  may be configured to accept one end of a standard test strip or a specimen S of custom size and shape. 
         [0021]    Disposed within the pressure vessel  12  is a piston assembly  22  for partitioning the enclosed volume into two pressure chambers: a tensile chamber  24  and a compression chamber  26 . The piston assembly  22  includes a piston body  28  with an external surface  30  that is complementary to the shape of the interior surface  18 , and in the example shown; the piston body  28  is cylindrical in shape. A receiver  20  is disposed in the piston body  28  and is configured to accept a second end of a loaded specimen S. 
         [0022]    A clearance gap  32 , formed between the piston assembly  22  and the interior wall  18 , permits the piston assembly  22  to move in relation to the interior wall  18 . Sealing elements  34 A and  34 B are disposed respectively in glands  36 A and  36 B formed in the external surface  30  of the piston body  28 . The sealing elements  34 A and  34 B span across the clearance gap  32 , interacting with the piston body  28  and interior wall  18 , to create a fluid F seal. The seal discourages leakage of fluid F between the tensile chamber  24  and the compression chamber  26 . The cross section of the sealing elements  34 A and  34 B may be square, rectangular (shown), circular, oval, or some other shape known in the sealing art. The sealing elements  34 A and  34 B may be full annular, of segmented annular in form. The material of the sealing elements  34 A and  34 B 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. 
         [0023]    Fluid F at pressure P 1  in the tensile chamber  24  and at pressure P 2  in the compression chamber  26  imparts loads on piston faces  38 A and  38 B of the piston assembly  22 . A bearing set  40  centers the piston assembly  22  with the interior surface  18 , maintaining a fairly constant clearance gap  32  as the piston assembly  22  moves in relation to the vessel body  14 . The bearing set  40  is disposed in a single groove  42  or individual grooves (e.g., pockets) formed in the piston body  28 . The bearing set  40  may be full annular, of segmented annular in form. A material such as DuPont TEFLON brand fluoropolymer provides adequate strength and lubricity properties for this particular bearing application. 
         [0024]    A fluid F supply source  44  (e.g., tank or bottle) stores the fluid F, for example hydrogen gas, and provides the fluid F to an attached pressure intensifier  46  via a low pressure conduit. The pressure intensifier  46  increases the pressure of the fluid F supply for use in the pressure vessel  12 . Within the intensifier  46 , fluid F supplied from the supply source  44  acts on a larger piston  48 , a force is transferred mechanically through a connecting rod  50  to an adjoined smaller piston  52 . The smaller piston  52  area 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 intensifier  46  via a high pressure conduit to a one-way valve  54 , thus forcing the high pressure fluid F in a direction out of the fluid intensifier  46  and thus preventing back flow. 
         [0025]    Downstream of the one-way valve  54 , the fluid F is directed into two separate, high pressure branches: a tension branch  56  and a compression branch  58 . The tension branch  56  delivers a first portion of the fluid F to the tension chamber  24  through end cap  16 A and the compression branch  58  delivers a second portion of the fluid F to the compression chamber  26  through end cap  16 B. The fluid F pressure within the tension chamber  24  and the compression chamber  26  acts on the piston assembly  22  to 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. 
         [0026]    Disposed within the tension branch  56  and compression branch  58 , are four-way valves  60 A and  60 B for modulating the fluid F pressures within the tension chamber  24  and compression chamber  26  respectively. Low pressure return branches convey low pressure fluid F from the four-way valves  60 A and  60 B back to an attached gas collector  62 . In turn, the gas collector  62  is attached to the supply source  44  and pressure intensifier  46  through low pressure conduits and one-way valves  54 B and  54 C. 
         [0027]    A control system includes a processor  64  (e.g., a personal computer) attached to an electronic fluid pressure controller  66 . The pressure controller  66 , in turn, is attached to one or more fluid F pressure transducers  68 A,  68 B, one or more pressure regulators  70 A,  70 B, and one or more specimen S strain monitors  72  (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 processor  64  to allow an operator to easily view a schematic of the apparatus  10 , monitor the various pressure transducers  68 A,  68 B and adjust valve regulators  70 A,  70 B. 
         [0028]    The processor  64  monitors the magnitude of specimen S loading with the strain monitor  72  while simultaneously modulating the four-way valves  60 A and  60 B with feedback from the pressure regulators  70 A,  70 B. The processor  64  also monitors the fluid F pressures of the tension chamber  24  and the compression chamber  26  with the pressure transducers  68 A and  68 B respectively. The specimen S is loaded in tension when the fluid F pressure in the tension chamber  24  exceeds the fluid F pressure in the compression chamber  26 ; and the specimen S is loaded in compression when the fluid F pressure in the compression chamber  26  exceeds the fluid F pressure in the tension chamber  24 . 
         [0029]    The apparatus  10  generates in-situ tensile, compressive or cyclic fatigue loading on a specimen S while it&#39;s subjected to a high-pressure fluid F environment. The pressure of the fluid F acting on the piston assembly  22  provides 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. 
         [0030]    Referring now to the flow diagram of  FIG. 3 , a method  100  for 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 labeled  101 , an apparatus  10  as previously described above is provided. Next, in the process step block labeled  102 , a material specimen S is loaded into the specimen receivers  20 . Next, in the process block labeled  103 , the fluid pressures in the tension and compression branch conduits  56 ,  58  are modulated with the pressure controller  66  such that the fluid pressure (P 1 ) in the tension chamber  24  alternates between being greater than and less than the fluid pressure (P 2 ) in the compression chamber  26 . 
         [0031]    Step  103  may be accomplished by modulating a four-way valve  60 A and  60 B disposed in each of the tension  56  and the compression branch circuits  58 . 
         [0032]    The method  100  may also include a step  104  for monitoring pressure transducers  68 A and  68 B disposed between said pressure controller  66  and each of the tension chamber  24  and the compression chamber  26  with the pressure controller  66 . Step  104  may also include monitoring at least one strain measurement from a strain monitor (e.g., strain gage)  72  disposed between the pressure controller  66  and the material specimen S with the pressure controller  66 . 
         [0033]    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.