FIELD OF INVENTION
The present invention relates to a method of inspection of Mechanical testing specimens for detection of surface and sub-surface defects using vibro-thermography. More particularly the invention relates to a method of nondestructive testing of mechanical testing specimens to detect surface and sub-surface defects using vibro thermography. Custom designed fixture is used for holding of up to 10 number of specimens simultaneously during inspection. These specimens are prepared from castings as well as forgings to assess the material properties at both room temperature and high temperature by tensile, low cycle fatigue and high cycle fatigue testing. Using this nondestructive testing method, specimens can be checked for surface/volumetric imperfections present in the specimens which will hamper the test results during mechanical testing.
BACKGROUND OF INVENTION
It is essential to carry out large number of mechanical testing to generate mechanical property data to cater the designer requirements especially when novel materials are being used. Under Advanced Ultra Super Critical (AUSC) program, it is envisaged to use Ni-base superalloy materials which can with stand higher temperature and operating pressure. In order to use these novel materials for power plant applications, it is required to generate material property data. Large number of

specimens are being prepared from Ni-base superalloy materials such as Alloy 625, Alloy 617M to carry out tensile test at both RT and HT, Fatigue properties (Low Cycle Fatigue (LCF)/High cycle fatigue (HCF). To get consistent reliable results, the test specimens prepared out of these materials need to be completely free of any surface/ sub surface imperfections/defects/flaws.
The specimens are prepared from castings, forgings, similar and dissimilar welds, etc. for mechanical testing under AUSC program. It is possible that these specimens may contain porosity, micro cracks inclusions, weld cracks and machining defects at sample level though they are tested at block level, small defects may not be detected accurately These defects at component level may be acceptable. But the same kind of defects are not acceptable at specimen level as the test results are used for design of AUSC power plant components. Conventionally, these completed and ready to test specimens are subjected to visual inspection only. Lot of care is taken while preparation of these specimens especially the gauge length portion which is polished to a mirror finish to prevent premature failure due to cracks originating from surface imperfections.
Dye penetrant testing can be used for checking these specimens for surface defects. In this method, the surface area will be treated with a dye penetrant so that the dye enters into any cracks or defects, if any which are present on the surface, due to capillary action. The specimen surface is then cleaned, and treated with a

developer that causes the dye remaining in the cracks to spread into the developer making the cracks/defects visible. An ultraviolet (UV) light source is used to illuminate the defects in case of fluorescent dye penetrant testing. This technique is highly inspector intensive and it can only detect defects which are open to surface.
Magnetic particle inspection (MPI) method can be used for detecting surface as well as sub surface defects in these mechanical testing specimens. The limitation of this method is that, only ferromagnetic material such as iron, cobalt, nickel or their alloys only can be inspected. Specimens prepared from Ni-base superalloys cannot be inspected using MPI method.
Radiography testing (RT) can be used for detecting internal defects in the specimens. It is possible to carry out simultaneous testing of multiple specimens using RT technique. This technique is very much sensitive to internal defects and inclusions. It is time consuming and also not safe considering the usage of high energy radiation.
In the US Patent Titled “Infrared imaging of ultrasonically excited subsurface defects in materials” (Patent No: US 6,236,049 B1 dated 22/05/2001) a technique is disclosed for infrared or thermal imaging of ultrasonically excited subsurface defects in a material. An ultrasonic source is connected to a specimen being inspected through a coupler that transmits the ultrasonic waves into the material with minimum attenuation. The ultrasonic source emits a single ultrasonic pulse having

aconstant frequency amplitude for a predetermined period of time. A suitable thermal imaging camera is used to image the specimen when it is being excited by the ultrasonic source to generate heat in the defect region due to friction in the mating surfaces of the defects. A control unit is used to control the operation of the ultrasonic source and the infrared camera for timing purposes. However, this invention does not mention about using vibro lock-in thermography for inspection of mechanical testing specimens.
In the US patent titled “Vibro-thermographic weld inspections” (Patent No: US 2012/0288049 A1, dated 15/11/2012), a method of inspecting the quality of J-groove welds of a nuclear reactor pressure vessel (RPV) head and the RPV bottom mounted nozzles (BMN) using vibro-thermography. The Weld to be inspected is subjected to a transient sonic excitation while the weld area is monitored using a remote infrared camera. The sonic excitation induces mechanical vibration, which causes heat generation at any crack in the weld. The infrared camera detects any temperature differentials in the weld, indicating the presence of a crack/defect. This invention is only applicable for quality inspection of J-groove welds and bottom mounted nozzle in RPV.
The present invention relates to a method of inspecting the mechanical testing specimens made of materials such as Ni-base superalloy castings and forgings, stainless steel forgings, etc (Non-magnetic materials) using vibro-thermography. In this

technique, the specimen to be inspected is subjected to a transient sonic excitation and simultaneously monitored using a remote infrared camera. The sonic excitation induces mechanical vibration, which causes to generate heat in the defect region due to friction in the mating surfaces of the defects in the specimen. The infrared camera images the temperature gradient in and around the defect region in the specimens, indicating the presence of a porosity, weld crack, inclusion, etc. Since the heat source in vibro-thermography is the discontinuity itself, the identification of defects is much simpler. Using the special fixture developed for this testing, a batch of 10 specimens can be simultaneously loaded for inspection. Hence the time taken for inspection of a specimen is also much lesser compared to DPT or MPI. The sonic generator is capable of generating sound waves with max 2 kW energy. Since the diameter of the gauge length portion varies from 5mm to 10mm, the acoustic energy level can be varied to suit the specimen size. Using this technique, full volumetric inspection of the specimen can be performed especially covering the gauge length portion to detect any defects/discontinuities which are surface or sub surface in nature.
OBJECTS OF THE INVENTION
Therefore, it is an object of the invention to propose a method of inspection of Mechanical testing specimens for detection of surface and sub-surface defects using vibro-thermography which is capable of transmitting an ultras sound energy to the specimen for excitation to allow capturing the thermal image by an infrared camera.

Another object of the invention is to propose a method of inspection of Mechanical testing specimens for detection of surface and sub-surface defects using vibro-thermography which is capable of carrying out the testing of 10 specimen at a time.
A still another object of the invention is to propose a method of inspection of Mechanical testing specimens for detection of surface and sub-surface defects using vibro-thermography which is able to use a pneumatic actuator to hold to hold a sound transducer firmly on the specimen holder service for applying ultrasonic energy to the specimen holder.
SUMMARY OF THE INVENTION
This invention relates to the method of nondestructive evaluation of mechanical testing specimens using vibro-thermography. In this novel method, maximum of 10 specimens are mounted on a mechanical fixture/holder which is then placed on a wooden block. The sonic transducer which is initially in the upward position is then lowered using a pneumatic actuator to rest on the specimen holder by maintaining sufficient contact pressure. Sound energy of 15-25 kHz frequency is applied on the holder specimen assembly with varying power output depending on the number of specimens and specimen diameter. Maximum power of 2kW is applied using the transducer in a pre-defined wave form. Simultaneously thermal images of the specimen

is recorded by infrared camera. Any surface/ sub-surface defect or discontinuity present in the specimen will generate heat by clapping action of ultrasound and is detected by checking thermal wave form output.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Fig.1: Shows a specimen for testing.
Fig.2: Shows a pair of specimen holders with 10 number of specimens mounted.
Fig.3: Shows a vibro-thermography ready to test.
Fig.4: an isometric view of vibro-thermography ready to test.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
Mechanical testing specimens are made from casting, forging, welds of similar and dissimilar metals, etc. to carry out mechanical testing to evaluate the properties of the material such as tensile strength, fatigue strength. etc. These specimens should be free of any surface or internal defects which otherwise can result in erroneous test results. Defects/flaws if any, especially in the gauge length region can cause premature failure of the specimen. Machining marks or hair line crack on the

specimen surface can cause stress concentration sites and lead to crack initiation. To avoid such situations, the specimens must undergo thorough testing after final machining and polishing. It is possible that these specimens may contain porosity, micro cracks inclusions, weld cracks and machining defects at sample level though they are tested at block level, small defects may not be detected accurately These defects at component level may be acceptable. But the same kind of defects are not acceptable at specimen level as the test results are used for design of AUSC power plant components.
Visual inspection is performed on the specimens to identify any surface defects. Machining marks, scratches, etc. can be checked by this method. Ultrasonic Testing (UT) can be performed on the un-machined specimen blocks which involves manual scanning of the blocks with a suitable probe for detecting defects. For castings such as Alloy 625, performing UT is difficult due to coarse grain structure. ECT and DP also can be performed on the specimens to detect surface defects. All these methods are time consuming and operator dependent.
To overcome these problems, a novel NDT technique, Vibro-thermography, which comprises ultrasonic/sonic energy excitation system coupled to an infrared camera for capturing the thermal images, is employed. In this technique, thermal distinction is created in the component using sonic/ultrasonic vibrational energy. The novelty of this invention is the method used to test the mechanical testing specimens (1). These specimens (1) are prepared from variety of materials such as

casting, forging or weldments in nature from non-magnetic materials. To check these specimens for surface/sub-surface defects, especially in the gauge length portion (2), these specimens will be mounted on the specimen holder (H). This holder (H) has one top piece (3) and one bottom piece (4). There are slots to hold up to 10 specimens at a time in this holder. The specimens (1) are tightened to the holder (H) using allen screw (5) provided in the top and bottom piece. The specimen holder (H) is then placed on a wooden fixture (7) which is placed below the sound transducer (5). This wooden fixture helps to damp vibration and keep the specimen holder steady during the excitation. This transducer is mounted on a pneumatically operated actuator(6) which can move the ultrasound generator up or down and also introduce sufficient contact pressure on the specimen holder (H) top surface. This contact pressure is required for transferring the ultrasound energy which is essential for performing the examination. This active excitation system or sound transducer operates in 15 kHz to 25 kHz frequency range. During testing the sound transducer is lowered with the help of pneumatic actuator till it touches the upper portion of the specimen holder. This pneumatic actuator is attached to a actuator mount (8) which can be moved along the horizontal cross head (9) for accurate positioning of the sound generator during testing. This cross head is supported by two vertical columns (10) which are bolted to a base plate (11) firmly.
The infrared camera used for acquiring the images will be kept away from this set up at about 2ft distance focused on all the 10 specimens. The position of the camera can be changed as per convenience and hence it is not shown here in the set

up. During the inspection process, sound transducer is excited using a signal generator for a given period of time and simultaneously the infrared camera is triggered to acquire thermal images of the specimens with set rate of acquisition. The thermal image acquisition will continue as long as the sonic energy is fed to the specimen holder assembly. Defects such as porosity, micro cracks, inclusion, etc. which are present in the specimen will undergo clapping/rubbing action due to high power sonic/ultrasonic vibrations and produces heat signals which is seen from the infrared images after processing all the images acquired. This processing is done using proprietary software. These signals indicate the presence of defects in the specimen. During inspection, defects/flaws, if any, is found in the specimen, it will be rejected, and will not be used for mechanical testing. This method of inspection can be carried out batch wise for number of specimens before mechanical testing to bring about uniformity and reliability in the test results. The inspection method is employed to handle specimens of diameter 5 mm to 10 mm.

WE CLAIM
1) A method of inspection of mechanical testing specimens for detection of surface and sub-surface defects using vibro-thermography, the said method comprising the steps of:
preparing a plurality of specimens from variety of materials such as casting forging or weldments to check surface or sub surface defects;
mounting ten number of specimen (1) on a specimen holder (H) between top (3) and bottom (4) pieces of the said holder;
fixing the specimen (1) in the holder (H) by allen screws (S) provided in the top (3) and bottom (4) pieces;
placing the specimen holder (H) on as wooden fixture (7) disposed in a vibro-thermography set up (V) to arrest vibration and keep the specimen holder (H) steady during excitation;
mounting a sound transducer (5) on a pneumatically operated actuator (6) attached to a actuator mount (8) capable of moving along a horizontal cross head (9) to position the sound generator (5) accurately during testing;
supporting the horizontal cross head (9) by two vertical columns (10) bolted to a base plate (11) firmly;
Characterized in that
the ultrasound generator (5) capable of moving up and down by the actuator (6)

is lowered with help of pneumatic actuator (6) till it touches the top surface of the specimen holder (H) exerting sufficient contact pressure on the top surface (12) of the specimen holder (H) to transfer the ultrasound energy wherein surface/sub-surface defects or discontinuity present in the specimen generates heat by clapping action of ultrasound and is detected by checking thermal wave from output when thermal images of the specimen is recorded by a infrared camera configured near the vibro-thermography set up, wherein, the signal indicates the presence of defects in the specimen.
2) The method as claimed in claim 1, wherein the sound transducer operates in 15 kHz to 25 kHz frequency range with maximum power of 2 KW.
3) The method as claimed in claim 1, wherein the inspection method is employed to handle specimen of diameter 5 mm to 10 mm.