WO2017122080A1

WO2017122080A1 - Torsional testing apparatus and method

Info

Publication number: WO2017122080A1
Application number: PCT/IB2017/000010
Authority: WO
Inventors: Donald Ray METZGER; Andre Roger GAGNON
Original assignee: Candu Energy Inc.
Priority date: 2016-01-12
Filing date: 2017-01-12
Publication date: 2017-07-20

Abstract

An apparatus for torsion testing of a helical specimen. A headstock includes a rotatable spindle. A tailstock is aligned with the spindle along a central axis. A pair of gripping assemblies grip first and second ends of the specimen, respectively, and are supported by the headstock and the tailstock. Each of the pair of gripping assemblies includes a mandrel having a portion which tapers toward a first end which is insertable into the helical specimen, a cup having a tapered bore configured to receive a second end of the mandrel along with the corresponding end of the helical specimen, and a fastener configured to engage the second end of the mandrel and pull the mandrel into the cup to pinch the helical specimen. A camera is aimed between the pair of gripping assemblies and operable to capture images of the helical specimen.

Description

TORSIONAL TESTING APPARATUS AND METHOD

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent Application No. 62/277,889, filed January 12, 2016, the entire contents of which are hereby incorporated by reference.

FIELD OF THE INVENTION

[0002] The present invention relates to a torsion testing apparatus and method for evaluating the material properties of helical specimens. Some aspects of the invention also relate to testing of ex-service garter springs used as annulus spacers between fuel containing pressure tubes and calandria tubes in CANDU-type nuclear reactors.

CANDU-type reactor operators typically must demonstrate that a gap is maintained between the pressure tube and calandria tube. This in turn leads to the need to demonstrate that the annulus spacers retain structural integrity through operating life.

BACKGROUND OF THE INVENTION

[0003] Conventional torsion testing rigs apply a torsional load or torque to a specimen in order to determine the material properties of the specimen, by measuring the strain through the angle of deflection and comparing it against the load applied. Commercial equipment is available for testing both solid sections of material as well as torsional springs. However, in the case of torsional springs, the specimens require straight segments on both ends in order for the test rigs to be able to grip the specimen before applying the torsional load. Additionally, in order to test both the ultimate strength and the fatigue life of the annulus spacers, two separate test rigs are required.

[0004] In a CANDU-type nuclear reactor, a calandria includes an array of tubes including pressure tubes, each positioned within a corresponding calandria tube. Fuel for the nuclear reaction is positioned within each of the pressure tubes and heavy water coolant flows through the pressure tubes. Each pressure tube is positioned within a calandria tube. The calandria tubes extend through the calandria, which contains a heavy water moderator. An annular spacing containing gas rather than liquid is maintained between the outside of each of the pressure tubes and the inside of the corresponding calandria tube for insulation. Annulus spacers in the form of garter springs are provided at several locations along the length to counteract sagging and maintain the annular spacing between the pressure tubes and the calandria tubes. The structure of the garter springs manufactured for use as annulus spacers in the CANDU-type nuclear reactor does not facilitate the ability to perform torsion testing with conventional equipment, and testing on ex-service annulus spacers is further complicated by the fact that the annulus spacers are irradiated during use in the reactor.

SUMMARY OF THE INVENTION

[0005] The invention provides, in one aspect, an apparatus for torsion testing of a helical specimen. The apparatus includes a headstock including a rotatable spindle, and a tailstock aligned with the spindle of the headstock along a central axis. The apparatus also includes a pair of gripping assemblies configured to grip a first end and a second end of the helical specimen, respectively, and configured to be supported by the headstock and the tailstock, respectively. Each one of the pair of gripping assemblies includes a mandrel having a portion which tapers toward a first end which is insertable into the helical specimen, a cup having a tapered bore configured to receive a second end of the mandrel along with the corresponding end of the helical specimen, and a fastener configured to engage the second end of the mandrel and pull the mandrel into the cup to pinch the helical specimen. The apparatus further includes a camera aimed between the pair of gripping assemblies and operable to capture images of the helical specimen.

[0006] The invention provides, in another aspect, a method of operating a torsion testing apparatus to perform a material test on a helical specimen. The helical specimen is loaded into a pair of gripping assemblies and fasteners of the gripping assemblies are tightened. The pair of gripping assemblies is supported by a headstock and the tailstock. A measured load is applied to rotate a spindle of the headstock and induce torsional deflection in the helical specimen. A camera is operated to capture images of the helical specimen.

[0007] Other features and aspects of the invention will become apparent by

consideration of the following detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Fig. 1 is a side view of a torsional test rig in accordance with an embodiment of the invention, illustrating a camera attached to a rotary support and a load cell attached to a tailstock of the torsional test rig.

[0009] Fig. 2 is a view of a pair of gripping assemblies holding a helical specimen alongside a pair of collet chucks.

[0010] Fig. 3 is a perspective view of the helical specimen and a gripping assembly.

[0011] Fig. 4 is a side view of the helical specimen and the mandrel.

[0012] Fig. 5 is a perspective view of the helical specimen and the gripping assembly of Fig. 3 disassembled.

[0013] Fig. 6 is a perspective view of a configuration of the torsional test rig of Fig. 1 with a fixed camera and an external load measuring device.

[0014] Fig. 7 is an alternate perspective view of the torsional test rig of Fig. 6.

[0015] Fig. 8 is a detail view of the camera positioned adjacent to the specimen, loaded in the test rig of Fig. 6. [0016] Fig. 9 is an exemplary image showing the helical specimen of Fig. 4 from the camera.

[0017] Fig. 10 is a schematic axial end view illustrating camera movement around the helical specimen.

[0018] Fig. 1 1 is a side view of the helical specimen of Fig. 10 from the perspective of the camera, without a torsional load applied.

[0019] Fig. 12 is a side view of the helical specimen of Fig. 10 from the perspective of the camera, with a torsional load applied.

[0020] Before any embodiments of the present invention are explained in detail, it should be understood that the invention is not limited in its application to the details or construction and the arrangement of components as set forth in the following description or as illustrated in the drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. It should be understood that the description of specific embodiments is not intended to limit the disclosure from covering all modifications, equivalents and alternatives falling within the spirit and scope of the disclosure. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting.

DETAILED DESCRIPTION

[0021] Fig. 1 illustrates a torsional test rig 20 for helical specimens (e.g., single coils of ex-service annulus spacers). The test rig 20 is operable to, among other things, provide failure strain data from which bounding values of failure stress can be calculated. A single coil test can significantly improve the number of tests that can be performed with specimens that are in very limited supply, such as ex-service material from an annulus spacer used in a CANDU-type nuclear reactor, between a pressure tube and a calandria tube. Also, the ability to resolve spatial differences in properties is improved, and testing at operating temperature is more practical. [0022] The torsional test rig 20 includes a bed 24 to support a headstock 28, a tailstock 32 and a carriage 36. The headstock 28, the tailstock 32 and the carriage 36 may be supported by riser blocks 40. The torsional test rig 10 further includes a pair of gripping assemblies (not visible in Fig. 1), that may be similar to the gripping assemblies 100 shown in Figs. 2-5, to grip a first end and a second end of a helical specimen 44, and first and second collet chucks 48, 52 to receive the respective gripping assemblies. The chucks 48, 52 can be adjustable or non-adjustable chucks of any particular structure for engaging and securely holding the respective gripping assemblies. The chucks 48, 52 are positioned across from one another along a central axis 56 shared by the test rig 20 and the helical specimen 44. It should be appreciated that the term "helical specimen" refers to a non-solid specimen, i.e. void of a central section. For example "helical specimen" is defined as a helix, spring, spiral, coil, screw or any other similarly shaped convolute structural specimen. The illustrated helical specimen 44 includes multiple coils 46 {see Figs. 3-5), and may be entirely helical from end to end. In other constructions, the helical specimen 44 may have non-helical portions, for example at one or both ends. As used herein, the term "single coil" can refer to a portion of a helical specimen or to a portion of a spring, spiral, coil, screw or any other similarly shaped convolute structural specimen.

[0023] The first collet chuck 48 is coupled for rotation with a spindle 60 that is supported by the headstock 28 for rotation relative to the headstock 28. The second collet chuck 52 can be fixedly coupled to the tailstock 32 by a tailstock adaptor 64 and a chuck adaptor 68. Other structural arrangements, with or without adaptors, are optional. In some constructions, the second collet chuck 52 can be coupled for rotation with a spindle supported by the tailstock 32 for rotation relative to the tailstock 32. A rotary table 72 may be coupled to the spindle 60 by a coupler 76 to rotationally drive the spindle 60 and the first collet chuck 48 to apply a torsional load to the helical specimen 44 in a first coiling direction or clockwise rotational direction and/or a second coiling direction or counter-clockwise rotational direction as the second end of the helical specimen 44 remains fixed by the tailstock 32. Alternately, the second end of the helical specimen 44 can be rotated about the central axis 56 in a direction opposite the first end. [0024] The helical specimen 44 has a coiling diameter that changes depending on the torsional load applied. When the helical specimen 44 is twisted in the first direction, the coiling diameter decreases and when twisted in the second or opposite direction, the coiling diameter increases. The rotary table 72, in some configurations, may be driven by a powered actuator, such as a servo motor, to provide precision computer controlled rotational adjustment. In other configurations, the rotary table 72 may be manually operable. A load cell 80 is coupled between the tailstock adaptor 64 and the chuck adaptor 68 to measure the torsional load applied to the helical specimen 44 by the rotary table 72 in both rotational directions. The load cell 80 may be replaced with another suitable load measuring device in other embodiments of the torsional test rig 20.

Whether an external load cell or measurement device takes a measurement of the applied load or a predetermined load is applied in a predetermined measured manner by a powered actuator, the test rig 20 is capable is applying a measured load for data analysis.

[0025] The carriage 36 supports a camera 84 aimed at an area between the first and second chucks 48, 52. Though not illustrated in Fig. 1 , when the pair of gripping assemblies 100 of Fig. 2 are inserted in the chucks 48, 52, the camera 84 is aimed at the area between the pair of gripping assemblies 100. As illustrated, the camera 84 is positioned between the first and second chucks 48, 52 and radially directed at the helical specimen 44 (i.e., aimed orthogonal to the central axis 56) of to capture images (i.e., image data) of the deflection in the helical specimen 44 as the helical specimen 44 is exposed to torsional loading. Other positions and/or orientations of the camera 84 are optional so long as the camera 84 is aimed to collect image data of the helical specimen 44 as desired. The captured image data may then be used for data analysis. The camera 84 may further be supported by a rotary support 88 to allow the camera 84 to rotate at least 180 degrees around the central axis 56, for example 360 degrees around the central axis 56. The camera 84 may in other configurations be supported statically as seen in Figs. 6-9. The rotary support 88 may be any rotary support where the angular position of the camera 84 can be effectively known. The camera 84 may be any suitable image capturing device, such as a video camera. The camera 84 and the load cell 80 are coupled to the data acquisition to capture and correlate torsional load data from the load cell 80 with image data from the camera 84. In some constructions, the torsional test rig 20 can include multiple cameras 84 (e.g., diametrically opposed or otherwise spaced about the central axis 56).

[0026] Fig. 2 illustrates one embodiment of the pair of gripping assemblies 100 that hold the helical specimen 44 at its opposite ends. Each gripping assembly 100 includes a mandrel 104, a cup 108 and a fastener 1 12. In this configuration, the fastener 1 12 is illustrated as a locking nut that is threaded onto the mandrel 104. The gripping assemblies 100 are configured so that they may be mounted within the chucks 48, 52 or other similar chucks or mounting fixtures of a test rig, such as the test rig 20 of Fig. 1.

[0027] Figs. 3-5 further illustrate one of the pair of gripping assemblies 100 of Fig. 2, with the understanding that the other gripping assembly 100 can be identical. The mandrel 104 includes a tapered portion 1 16 that tapers towards a first end 120, a cylindrical portion 124 that extends towards a second end 128, and a threaded portion 132 therebetween. The first end 120 of the tapered portion 1 16 of the mandrel 104 is inserted into the first end of the helical specimen 44. The mandrel 104 has an outer diameter that is approximately equivalent to an inner diameter of the helical specimen 44. The coils 46 of the helical specimen 44 are threaded onto the threaded portion 132 of the mandrel 104 as seen in Fig. 4.

[0028] Forcing a single spacer coil along the gently tapered mandrel 104 causes the radius of the specimen 44 to conform to that of the mandrel 104. The radius relates to strain through curved beam formulae. Failure strain is thus related to the radius at the position along the mandrel 104 where failure occurs. The trend of failure strain with dose is very useful in evaluating the load capacity of spacers, as loss of ductility directly contributes to loss of load carrying capacity. The test rig 120 can make use of specimens that are nominally a single coil; therefore, local effects on properties can be more thoroughly explored. Pure bending strain can then be analyzed. Particularly good accuracy is possible in determining macroscopic strain at failure. It is also possible to conduct the test at elevated temperatures with a preheating procedure.

[0029] As shown in Fig. 3, the cup 108 includes a tapered bore 136, a straight bore 140 that is axially aligned with the tapered bore 136, and an outer surface 144 that may be positioned within either one of the first and second chucks 48, 52. The straight bore 140 of the cup 108 is configured to receive the cylindrical portion 124 of the mandrel 104 through the tapered bore 136 such that the fastener 1 12 can be coupled to the cylindrical portion 124 of the mandrel 104. The fastener 1 12 is configured to engage the cylindrical portion 124. In some configurations the fastener 1 12 is a locking nut that is threaded onto external threads of the cylindrical portion 124, as seen in Fig. 2. In other configurations, the fastener 1 12 includes a screw portion with male threads received internally by corresponding female threads of the cylindrical portion 124, as seen in Figs. 3-5. In yet other constructions, the fastener 1 12 is not a threaded fastener, but takes an alternate form (e.g., load bar, strap, lever, cam) operable to exert a force that pulls the mandrel 104 into the tapered bore 136 of the cup 108. When the fastener 1 12 is tightened relative to the cylindrical portion 124 of the mandrel 104, the mandrel 104 is pulled into the tapered bore 136 of the cup 108 to pinch the helical specimen 44, thereby gripping the specimen 44 tightly within the gripping assembly 100. The fastener 1 12 is configured to abut an axial outboard end surface of the cup 108 during tightening.

[0030] For both the cup bore 136 and the mandrel 104, the angle of the taper with respect to the central axis 56 can be calculated to ensure a sufficient change in specimen diameter due to winding under torsional loading to achieve a desired strain range to query the material properties of the helical specimen 44. The bore and mandrel taper angles can also be calculated to ensure a sufficient change in specimen diameter due to winding under torsional loading to prevent premature failure of the helical specimen 44 due to discontinuities in the helical specimen 44 caused by being directly gripped.

[0031] Figs. 6 and 7 illustrate the torsional test rig 20, in an alternate configuration. In this configuration, the torsional test rig 20 generally includes the same components as illustrated in Fig. 1 , other than what is specifically outlined below. Specifically, the rotary support 88, to rotationally support the camera 84 in Fig. 1 , is replaced with a fixed camera support 148 to statically support the camera 84 radially adjacent to and directed orthogonally at the helical specimen 44. The load cell 80 is also replaced with a cantilever beam 152 coupled to a force measuring device or scale 156. However, as previously mentioned, in other configurations, a load cell (e.g., coupled between the tailstock 32 and second collet chuck 52) can replace the cantilever beam 152 and the load measuring device 156, so that the torsional load can be measured in both rotational directions. In other configurations, a second load measuring device and second cantilever beam may be added opposite the cantilever beam 152 and load measuring device 156 in order to measure torsional load in both rotational directions. In the configuration of the torsional test rig 20 shown in Fig. 6 and 7, the rotary table 72 is manually operable to apply the torsional load to the helical specimen 44. However, the rotary table 72 may be replaced by a powered actuator, such as a servo motor for applying one or various measured rotational loads (e.g., according to a preset program or pattern).

[0032] Figs. 8 and 9 further illustrate the test rig 20 of Figs. 6 and 7. Specifically, Fig. 4 shows that in this configuration, the first and second collet chucks 48, 52 directly clamp the helical specimen 44, though in other configurations the gripping assembly 100 seen in Figs. 2-5 can be used. As seen in Fig. 9, the helical specimen 44 is supported internally in the radial direction by the mandrel 104 to resist compression by the collet chucks 48, 52 which apply gripping force to corresponding outer surfaces of the helical specimen 44. The mandrel 104 reduces compression of the coils 46 of the helical specimen 44, reducing the possibility of premature failure of the specimen.

[0033] A visual inspection system can be used to detect failure as the specimen 44 is expanded over the mandrel 104. A means to eliminate the end effect has been devised. The end-effect approach can also be used to estimate the elastic strain at the time of failure. Over time, specific dimensional details can be used to quantitatively address stress/strain analysis based on curved beam formulae. [0034] Fig. 10 illustrates the rotary path of the camera 84 of Fig. 1, which can be swept circumferentially about the helical specimen 44 while pointed radially inward as the camera 84 is guided by the rotary support 88 configured such that the camera 84 can rotate 360 degrees around the helical specimen 44.

[0035] In the configuration shown in Fig. 10, the rotary support 88 is located such that the camera 84 maintains a fixed radial distance to the helical specimen 44 when positioned at any circumferential position on the rotary support 88. The camera 84 can be integrated, along with a load control and data acquisition system of standard torsional test units, with software functions to control the camera 84, and to acquire and analyze the image data captured by the camera 84. In other embodiments, the video camera 84 can be differently configured or mounted to accommodate alternate specimen geometries, the various travel distances required for applying appropriate quantities of strain to specific specimens, and the axial position accuracy required to establish suitable resolution in failure strain of specific samples. The camera 84 and data acquisition system may be radiation resistant digital systems.

[0036] Figs. 1 1 and 12 are exemplary images of the helical specimen 44 captured by the camera 84 during testing. Fig. 1 1 shows the helical specimen 44 without a torsional load applied. A set of marks 160 applied (e.g., drawn, etched, etc.) on the coils 46 are aligned as an initial reference position. When a torsional load is applied to the helical specimen 44, the set of marks 160 on the coils 46 are circumferentially offset relative to each other due to deformation by the torsional loading, as seen in Fig. 12.

[0037] These exemplary images demonstrate how the marks 160 on the helical specimen 44 move relative to each other when the helical specimen 44 deforms due to an applied torsional load. Including multiple coils on the helical specimen 44 improves accuracy by augmenting the relative motion. Alternatively, smaller pixels can be used with fewer coils, or a shorter sample length to improve accuracy. Though marks 160 are shown in the illustrated configuration, in other configurations, the marks 160 may not be required by the software in order to track relative motion. In other configurations, existing contrasting features, deliberately introduced marks or any visually identifiable feature(s) may be used to track relative angular motion and therefore determine an amount of deformation.

[0038] In preparation for a torsional load test, the first and second ends of the helical specimen 44 are coupled to or loaded into the gripping assemblies 100, as shown in Fig. 2-5. The helical specimen 44 is secured by axially inserting the first end 120 of the tapered portion 1 16 of the mandrel 104 into the first end of the helical specimen 44 and then threading the first end of helical specimen 44 onto the threaded portion 132 of the mandrel 104. The second end 128 of the cylindrical portion 124 of the mandrel 104 is inserted axially into the tapered bore 136 of the cup 108, so that the cylindrical portion 124 is protruding from the cylindrical bore 140. Then, the fastener 1 12 is engaged with the cylindrical portion 124 of the cup 108 by rotating the fastener 1 12 in a first direction with respect to the cup 108, tightening the fastener 1 12 and causing the mandrel 104 to be pulled back into the cup 108, thus pinching the helical specimen 44 into the cup 108 and securing the helical specimen 44 into the gripping assembly 100. This process is repeated for the second end 128 of the helical specimen 44 with the corresponding gripping assembly 100. Mounting the gripping assemblies 100 into the torsional test rig 20 is done by positioning the outer surface 144 of the cup 108 of each gripping assembly 100 into the first and second collet chucks 48, 52, respectively, as shown in Fig. 1 and tightening or locking the chucks 48, 52. Removing the helical specimen 44 from the gripping assemblies 100 is done by rotating the fastener 1 12 in a second direction, allowing the first end 120 of the tapered portion 1 16 of the mandrel 104 to move out of the tapered bore 136 of the cup 108, thereby loosening the grip on the helical specimen 44 and allowing for the helical specimen 44 to be threaded off of the threaded portion 132 of the mandrel 104. This process is repeated for the second end of the helical specimen 44 with the corresponding gripping assembly 100. Continuing to rotate the fastener 1 12 in the second direction removes the fastener 1 12 from the cylindrical portion 124 of the mandrel 104, completely disassembling the gripping assembly 100. [0039] Once the helical specimen 44 is secured in place on the torsional test rig 20, the torsional test rig 20 is operable for running a material test on the helical specimen 20. The camera 84 records an initial image of the helical specimen 44 (e.g., as shown in Fig. 1 1). If the test rig 20 is configured to include the rotary support 88, the initial image recording may also include a sweep, rotating the camera 84, about the central axis 56 and thereby the helical specimen 44. The sweep includes taking multiple orthogonal images of the helical specimen 44 at a series of set rotational increments along the rotary support 88 as the camera 84 moves at least 180 degrees (e.g., 360 degrees) around the central axis 56. The images are compiled to characterize the position of the helical specimen 44. In other configurations, the camera 84 may take continuous footage (i.e., at a designated frame rate) during the sweep to characterize the position of the helical specimen 44. Sweeping the camera 84 allows identification of any positional error due to bending or bowing of the helical specimen 44 away from the central axis 56 or any slipping or moving of the helical specimen 44 during testing, which would otherwise introduce inaccuracy to the image data collected from later camera images during the torsional test.

[0040] The rotary table 72 rotates the spindle 60 a fixed increment driving the first collet chuck 48 holding the first end of the helical specimen 44, where the fixed increment is a predetermined rotational distance or degree of motion, applying a first torsional load value to the helical specimen 44. The rotary table 72 may also rotate the spindle until reaching a predetermined incremental target torsional load that is read by the load cell 80 and communicated to the data acquisition. The rotary table 72 can rotate the spindle 60, to vary the coiling diameter of the helical specimen 44 according to a predetermined routine or pattern. The rotation amount, pattern, and/or direction may vary depending on what type of test is being run. Once the rotary table 72 rotates the spindle 60 the predetermined increment, the data acquisition system records another image through the camera 84 (e.g. Fig. 12), as well as torsional load measured by the load cell 80, and spindle position. [0041] Image analysis of the two recorded images or sweeps is done by determining relative motion of points on each coil 46 of the helical specimen 44 between the two images. This is done by identifying the same features or marks 160 in the images before and after the increment of rotational motion. Current image processing techniques can detect motion to within the size of a pixel. Therefore, the accuracy or resolution of the relative motion is determined by the number of pixels by which the mark 160 differs between images. A calibration constant determines the physical displacement, and the angular displacement is determined by dividing the physical displacement by the radius of the helical specimen 44. The analysis then determines the strain in each of the coils 46 by calculating the angular displacement between at least two points on adjacent coils 46, for example, applying curved beam formulae using the pre-determined dimensions of the helical specimen 44. A stress-strain analysis of the acquired torsional load data and the calculated strain is run based on curved beam theory relating the torsion vs. angle data to stress vs. strain data to determine effective material properties. During the analysis, the bounding values for stress may be obtained from elastic and plastic hinge assumptions for stress distribution, for the upper and lower bounds, respectively. This analysis can be automated through computational software, and once the analysis for a rotational increment and torsional load value is complete the data may be stored in a file and plotted in real time, displaying the progress of the test.

[0042] The spindle 60 is again rotated a predetermined increment, increasing or decreasing the torsional load relative to the previous, which may be by a predetermined rotational increment or a predetermined torsional load increment. The camera 84 records another image of the helical specimen 44, while the load cell 80 measures the torsional load value applied to the helical specimen 44. The data acquisition system then analyzes and compares the relative motion to the previous image, measuring angular displacement of at least two points on the helical specimen 44. This process is repeated until failure of the helical specimen 44 or another terminating condition, such as wrapping all of the coils 46 of the helical specimen 44 around the mandrel 104 or increasing the coiling diameter of the helical specimen 44 such that the coils 46 make contact with the tapered bore 136 of the cup 108. There are a variety of ways in which the data acquisition software can be coordinated with the loading sequence. For example, small load increments can be monitored at higher frequency with the camera 84 in a static position as shown in Figs. 6-9. For more accurate results, the test may periodically include one or more sweeps of the camera 84 about the helical specimen 44, allowing for correction of displacement data and to look for anomalous radial motion of the helical specimen 44. The sweeps are useful, because the data analysis process may assume that the helical specimen 44 stays centered about the central axis 56. By including the rotary support 88 and including periodic sweeps of the camera 84 during a test, absolute tracking of reference points can be used to correct for accumulated positional error during the data analysis process. By correcting for positional errors, the deflection measurement is improved.

[0043] Upon completion of the test, the software may archive the results for reprocessing, checking or analyzing in more detail. The entire test may be run under computer control, only requiring manual installation of the test equipment and insertion of the helical specimen 44 into the torsional test rig 20, though in some configurations these may also be automated or robotically assisted. The test may be run as either a static test to determine mechanical properties such as yield stress or ultimate strength, a dynamic test to determine fatigue properties, or any other suitable test and varies only by what automated program is run. Variables can be varied depending on the program run, such as how large of a rotational increment is used, or the rotational direction of the spindle 60 and thereby the helical specimen 44. Other factors can be varied depending on what type of test is desired to be run on the helical specimen 44. From the visual data, the deflection is computationally determined and from the deflection and other known data, the material properties are determined. Therefore, no strain gauge is required to be applied directly to the helical specimen 44. This is especially beneficial as one possible test method includes measuring the material properties of annulus spacers that have been exposed to substantial amounts of radiation. In addition to this, the entire process can be automated or remotely manipulated by a robot arm or "shielded box", thus making it possible to remove human contact with the sample, which can be advantageous whether or not the helical specimen 44 is radioactive. This is especially advantageous if the material test performed is on a helical specimen that has been in-service as an annulus spacer between a pressure tube and a calandria tube in a CANDU-type nuclear reactor.

[0044] The entire test can be run under computer control, with no hot-cell manipulation required other than installation of the test rig 20, and insertion of the helical specimen 44 into the test rig 20. Since the test rig 20 is designed for helical specimens rather than solid specimens, the size of the test rig 20 can be kept small compared to conventional test equipment. This allows the test rig 20 to fit easily into limited space available in a hot-cell at a nuclear reactor facility for example. Due to the image processing technique, a specimen of minimum length can also be used, which preserves scarce material.

[0045] Typical test rigs for helical specimens require the specimen to have a tab or other protrusion to be gripped in order to perform torsion testing, whether static or fatigue. Due to inherent instability of the geometry of a helical specimen, a standard chuck such as a collet or 3-jaw chuck, is insufficient to maintain a hold on a helical specimen, without deforming the helical specimen. Inserting the mandrel 104 inside the helical specimen 44 as described herein provides support to securely hold the helical specimen 44. The gripping assemblies 100 are also designed to mitigate failure due to how the specimen is clamped, by including the tapered portion 1 16 of the mandrel 104 and the tapered bore 136 of the cup 108, allowing the coiling diameter of the helical specimen 44 to increase or decrease, depending on the direction of rotational motion, without causing premature failure of the helical specimen 44, due to discontinuities in the helical specimen 44 caused by being directly gripped.

Claims

CLAIMS What is claimed is:

1. An apparatus for torsion testing of a helical specimen, the apparatus comprising: a headstock including a rotatable spindle; a tailstock aligned with the spindle of the headstock along a central axis; a pair of gripping assemblies configured to grip a first end and a second end of the helical specimen, respectively, and configured to be supported by the headstock and the tailstock, respectively, each one of the pair of gripping assemblies including a mandrel including a portion which tapers toward a first end which is insertable into the helical specimen, a cup having a tapered bore configured to receive a second end of the mandrel along with the corresponding end of the helical specimen, and a fastener configured to engage the second end of the mandrel and pull the mandrel into the cup to pinch the helical specimen; and a camera aimed between the pair of gripping assemblies and operable to capture images of the helical specimen.

2. The apparatus of claim 1, wherein the camera is supported for rotation of at least 180 degrees about the central axis.

3. The apparatus of claim 2, wherein the camera is supported for rotation of 360 degrees about the central axis.

4. The apparatus of claim 1, further comprising a load measuring device configured to measure a torsional load applied to the helical specimen held by the gripping assemblies.

5. The apparatus of claim 4, wherein the load measuring device is a load cell.

6. The apparatus of claim 5, wherein the load cell is coupled to a data acquisition system to correlate measured torsional load data with the image data from the camera.

7. The apparatus of claim 1, further comprising a powered actuator configured to apply a torsional load to the helical specimen held by the gripping assemblies.

8. The apparatus of claim 7, wherein the powered actuator is operable to apply a torsional load to the helical specimen in both of a first, coiling direction of the helical specimen and a second, uncoiling direction of the helical specimen.

9. The apparatus of claim 1, further comprising a first chuck supported by the spindle of the headstock, and a second chuck supported by the tailstock, wherein the cup of each of the pair of gripping assemblies further includes an outer surface positioned within the corresponding one of the first and second chucks.

10. The apparatus of claim 1, wherein the fastener of each of the pair of gripping assemblies comprises a threaded fastener.

1 1. The apparatus of claim 1 , wherein the camera is aimed to view orthogonal to the central axis.

12. The apparatus of claim 1 , wherein the one of the pair of gripping assemblies supported by the tailstock is held fixed against rotation about the central axis.

13. A method of operating the apparatus of any of the preceding claims to perform a material test on the helical specimen, including at least the following: loading the helical specimen into the pair of gripping assemblies and tightening the fasteners; supporting the pair of gripping assemblies by the headstock and the tailstock; applying a measured load to rotate the spindle of the headstock and induce torsional deflection in the helical specimen; and operating the camera to capture images of the helical specimen.

14. The method of claim 13, wherein the material test is run as a static test to determine mechanical properties of the helical specimen.

15. The method of claim 13, wherein the material test is run as a dynamic test, including releasing and re-applying the measured load, to determine fatigue properties of the helical specimen.

16. The method of claim 14 or 15, wherein the material test includes measuring angular displacement of at least two points on the helical specimen with the images from the camera at each of a plurality of measured loads.

17. The method of claim 16, wherein the method further includes rotating the camera about the central axis at least once during the material test to identify any positional error due to bending or bowing of the helical specimen away from the central axis.

18. The method of claim 13, wherein the helical specimen has been in-service as an annulus spacer between a calandria tube and a pressure tube in a CANDU-type nuclear reactor.

19. The method of claim 13, wherein the helical specimen is entirely helical from end to end.