Abstract:
An in-situ specimen fixture particularly adapted for prestressing rod-type SNTT-type specimens comprising a tube and end cap wherein the specimen is secured at one end to the tube, and at the opposite end to the end cap. The end cap is rotatable relative to the tube, and may be fixedly secured for creating a torsional force prestressing the specimen enclosed within the tube.

Description:
GOVERNMENT RIGHTS 
     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. 
    
    
     FIELD 
     This disclosure relates to the field of in-situ test specimen fixtures. More particularly, this disclosure relates to in-situ test specimen fixtures for rod-type Spiral Notch Torsion Test (SNTT)-type specimens. 
     BACKGROUND 
     Various methods of determining fracture toughness values of metallic and ceramic materials have been established by the American Society of Testing Materials (ASTM) and these standard methods are widely accepted by the technical community. Accordingly, a wealth of test data obtained by such protocols has been reported and evaluated for many types of these materials. In spite of the adherence to these standard methods, the test data obtained can still be scattered and inconsistent even within a family of the same material type, resulting in irreconcilable test data. Differences in the size of specimens, the inhomogenity of the specimen material and other inherent specimen factors which are not standardized can result in such inconsistencies. 
     Additional difficulties in determining fracture toughness occur when evaluating weldments, which inherently consist of three different phase zones: weld; heat affect; and base material. Each of these zones is likely to manifest a characteristically different microstructure and mechanical properties. As is well known, the fracture behavior of the fusion line that lies between the solidified weld puddle and the heat affect zone still remains unexplored because of the lack of a standardized test method. 
     Each of these difficulties is further complicated when evaluating the fracture toughness of these materials for use in high pressure hydrogen environments. Such information is important and needed for many energy development programs, yet the influences of hydrogen on in-situ crack behavior of weldments are virtually unknown. The standardized or conventional testing protocols previously mentioned are neither physically suitable nor economically viable for in-situ testing in extremely high pressure hydrogen environments. ASTM recommended compact tension (CT) specimens, and their variations, are generally tested in open space, and are not tailored for in-situ testing in a controlled environment with an extremely limited space such as that which occurs in many desired applications for these materials. Small and thin CT specimens do not yield reliable data and are not effective for use in investigating fracture toughness or fracture cracking behavior of weldments. Accordingly, a spiral-notch torsion test system (SNTT) was invented by Jy-An Wang and Kenneth C. Liu, “FRACTURE TOUGHNESS DETERMINATION USING SPIRAL-GROOVED CYLINDRICAL SPECIMEN AND PURE TORSIONAL LOADING”, U.S. Pat. No. 6,588,283, the disclosure of which is hereby incorporated by reference, which utilizes a rod-type specimen having a helical groove with a 45-degree pitch to effectively simulate the fracture failure behavior of a thick CT specimen with a thickness equal to the total length of the groove line. This SNTT test method provides a small volume test specimen which is independent of the size effect previously encountered, and facilitates the testing of textured materials in any desired orientation. 
     SUMMARY 
     The present disclosure provides an in-situ specimen fixture particularly adapted for prestressing rod-type SNTT-type specimens comprising a tube and end cap wherein the specimen is secured at one end to the tube, and at the opposite end to the end cap. The end cap is rotatable relative to the tube, and may be fixedly secured for creating a torsional force prestressing the specimen enclosed within the tube. 
     In accordance with one embodiment, an in-situ test fixture includes a frame holding a test specimen, and a receiver disposed on a first frame end is configured for receiving a first end portion of the test specimen. The receiver also defines one end of a torsion axis and applies a torsion force to the test specimen about the torsion axis. A cap has an opening that is configured for receiving the second end portion of the test specimen and the cap also prevents rotation and applies a torsion force to the specimen. A rotational lock mechanism is formed on the cap and the second end of the frame for rotationally locking the end cap in a fixed rotational position relative to the second end of the frame. To apply and hold a desired torsion force, the specimen is inserted into the receivers on the frame and cap, and the cap is rotated about the torsion axis relative to the frame. When the desired torsion has been applied to the specimen, the end cap is rotationally locked relative to the frame to thereby hold a torsion force on the specimen. The frame and the specimen may both be placed in a desired environment such as a high pressure hydrogen environment for fracture testing. 
     The in-situ test fixture may also include a translational lock formed between the cap and the second end of the frame for locking the cap onto the frame and preventing translational movement between the cap and the frame, where translational movement is defined as movement parallel to the torsion axis. The translational lock may be a plunger mechanism formed in the end cap with a plunger that is extensible to an extended position and retractable to a retracted position. An indent may be formed in the second end of the frame, and it receives the plunger when the plunger is positioned in the extended position. The plunger in the indent locks the cap and frame together and prevents translational movement. When in the retracted position, the plunger disengages from the indent and allows translational movement between the cap and frame. 
     In particular, the plunger mechanism may be a threaded bore in the cap and the plunger may be a threaded bolt. The rotational lock may be a cap wedge formed on the cap and a frame wedge formed on the second end of the frame. The cap wedge is oriented oppositely from the frame wedge and is configured such that an abutment face of the cap wedge abuts an abutment face of the frame wedge to rotationally lock the cap on the second end of the frame. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various advantages are apparent by reference to the detailed description in conjunction with the figures, wherein elements are not to scale so as to more clearly show the details, wherein like reference numbers indicate like elements throughout the several views, and wherein: 
         FIGS. 1A-1D  illustrates an SNTT specimen design configuration suitable for use in the in-situ test specimen fixture where  FIG. 1A  is an end view,  FIG. 1B  is a side view,  FIG. 1C  is a detailed view of a groove in the specimen, and  FIG. 1D  is a sectional view of the groove through section line  1 D- 1 D; 
         FIGS. 2A-2D  illustrates a torque applicator tube and cap of the in-situ test specimen fixture for receiving a test specimen where  FIG. 2A  is an end view of the tube,  FIG. 2B  is a side cross sectional view of the tube,  FIG. 2C  is an end view of the cap that fits on the end of the applicator tube, and  FIG. 2D  is a detail view of a groove in the tube; 
         FIG. 3  illustrates the end cap for the torque applicator tube of  FIG. 2  whereby a predetermined torque is applied to the specimen for locking the specimen in permanent torsion where  FIG. 3A  is an left end view of the cap,  FIG. 3B  is a side cross sectional view of the cap, and  FIG. 3C  is a view of the right end of the cap; and 
         FIG. 4  is a schematic diagram of a full torsion bridge formed by four active strain gages on the in-situ test specimen fixture. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description of a preferred embodiment, reference is made to the accompanying drawings, which form a part hereof, and within which are shown by way of illustration the practice of a specific embodiment of the in-situ test specimen fixture. It is to be understood that other embodiments may be utilized, and that structural changes may be made and processes may vary in other embodiments. 
     Referring now to  FIG. 1 , there is illustrated one embodiment of an SNTT specimen  10  for use in the in-situ fixture  100  to which a predetermined torsional force can be permanently applied for in-situ testing in, for example, an extremely high-pressure environment of hydrogen to test for hydrogen embrittlement. The specimen  10  has a spiral V-groove  11  formed on a uniform gage mid-section thereof, and squared end sections  12  for receiving in a manner hereinafter described. 
     Referring to  FIGS. 2 and 3  the in-situ fixture  100  comprises two portions, a heavy-walled tube  20  acting as a torque anchor, and an end cap  30  which is rotatable relative to the tube  20  to apply a torsional fracture force to a specimen  10  placed into the tube  20  and engaging the end cap  30 . The tube  20  may be regarded as a frame; its primary function is to hold the specimen  10 . In alternate embodiments an open frame may be used that does not necessarily enclose the specimen  10  and is not tubular. 
     The tube  20  is preferably made, for example, from a 0.55″ ID by 1.25″ OD high grade stainless steel tube  21  with a coaxial square hole  22  formed at one end. The hole  22  is sized for snuggly receiving one of the squared end sections  12  of the specimen  10 . In this manner, once an end of the specimen  10  is inserted longitudinally through the tube  20  and into the square hole  22 , the tube  20  acts as a torque anchor for the specimen  10 . A pair of parallel flat portions  23  is formed in the tube wall adjacent to the square hole  22  for securing the tube  20  in a vise or the like. 
     The end of tube  20  opposite to the square hole  22  is castellated forming three 65 degree arc angle wedge sections  25  with a 55 degree arc angle space there between, as best illustrated in  FIG. 2 . Each of the three wedge sections  25  is formed with a threaded bore for receiving there through a threaded bolt  29  to lock the positioning of the tube  20  and end cap  30  after a torsional force has been applied to the specimen  10 . 
     As best illustrated in  FIG. 3 , the end cap  30  is formed from the same steel as the tube  20 , with a coaxial square hole  32  extending there through. The end cap hole  32  also is sized for snuggly receiving a squared end section  12  of the specimen  10  which will extend outwardly from the tube  20  beyond the castellated wedge portions  25  when seated in the square hole  22  formed in tube  20 . In this manner, once one end of the specimen  10  is received into the square hole  22  of the tube  20 , the other end of the specimen  10  will extend outwardly there from to be received into the end cap hole  32  of the end cap  30  so that a rotational force can be applied to the specimen  10 . A matching complimentary set of three castellated wedge sections  35  is formed on the mating face of end cap  30 , which in combination with the wedge sections  25  on the tube  20  can hold an applied torsional force to the specimen  10 . The castellated wedge sections  25  and  35  may be considered a rotational lock because the wedge sections lock the end cap  30  against rotational movement relative to the tube  20 . A suitable ring groove  26 / 36  is formed in the respective castellated wedge sections  25  and  35  so that when the end cap  30  has been placed on the tube  20 , a “C” ring can be inserted into the grooves to secure the end cap onto the tube. A pair of parallel flat portions  33  is formed on the free end of the end cap  30 , adjacent to the square hole  32 , through which a rotational force can be applied to the end cap  30 . Each of the castellated wedge sections  35  is formed with a spherical indent  39  which is to be engaged by the end of a respective bolt  29 , threaded through the castellated wedge sections  25 , to lock the relative rotational and translational position of the tube  20  and end cap  30 . 
     Vents are provided in the tube  20  to allow the pressure inside and outside the tube  20  to equalize. In this embodiment, holes  41  are formed in the left side of the tube to allow gas to migrate into and out of tube  20 , and a gap is formed between the face  43  of the tube  20  and the cap  30 . The gap will likewise function as a vent between the inside and outside of the tube  20  when the specimen  10  is held in the tube  20 . 
     The amount of torsional force to be applied to the specimen  10  is monitored by the use of strain gauges, instrumented on the frame  20  or on the specimen  10 . The full torsion bridge composed of R 1 , R 2 , R 3  and R 4  as illustrated in  FIG. 4  is used on the frame  20 . In this manner once the bridge on load frame  20  is calibrated, re-calibration for each tested specimen will not be necessary. While using a strain-gage full-bridge system on individual specimens also is suitable, such a bridge will require recalibration for each tested specimen. The illustrated four active strain gages R 1 , R 2 , R 3  and R 4  gages aligned in a torsion bridge on frame  20  eliminates the necessity for recalibration for each specimen. In operation the readings of voltmeter M are calibrated to correspond to the torsion applied to the tube. In this manner a torsional strain is developed in the specimen  10  by rotating the end cap  30  relative to the tube  20 , the voltmeter M readings can be calibrated to provide a reading corresponding to the level of stress and strain on the specimen  10 . The torsion force on the specimen  10  and the tube  20  will be the same, but in opposite directions. Thus by measuring stress or strain on either specimen  10  or tube  20 , the stress and strain on either or both the specimen  10  and the tube  20  may be determined. 
     Referring again to  FIGS. 1-3  the threaded bolts  29  in the castellated wedge sections  25  are tightened to bear against their respective indents  39  of the castellated wedge sections  35  to fixedly lock the relative rotational position between the tube  20  and end cap  30 , and to set the stress and strain on the specimen  10 . As the bolts  29  are rotated to extended positions, they engage the indents  39  and with continued rotation of the threaded bolts, the bolts will push against the indents and rotate the cap  30  relative to the tube  20  to thereby impose the desired torsion force on the specimen  20 . The in-situ test specimen fixture  100  can impose a desired torsion stress and strain on a specimen  10  and then the complete fixture  100  can be placed in any desired environment for long term testing without requiring costly large-space testing facilities. In particular, the fixture may be disposed in high pressure hydrogen (e.g. 1,000 to 10,000 psi) and the specimens are thereby tested for fracture resistance under torsion stress-strain conditions. 
     The foregoing descriptions of embodiments have been presented for purposes of illustration and exposition. They are not intended to be exhaustive or to limit the embodiments to the precise forms disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiments are chosen and described in an effort to provide the best illustrations of principles and practical applications, and to thereby enable one of ordinary skill in the art to utilize the various embodiments as described and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.