MANUALLY OPERABLE FOUR-POINT BEND FIXTURE FOR RESIDUAL STRESS MEASUREMENT VERIFICATION

FIELD OF THE INVENTION

The present invention relates to manually operable four-point bend fixture for residual stress measurement verification. More particularly, the present invention relates to a bend fixture, its construction and the method of use of the fixture to bend a thin flat sample to measure residual stress experienced by the sample using XRD and compare with the theoretical value of residual stress.
BACKGROUND OF THE INVENTION
Residual stress is defined as the stress that remains in a body that is not being subjected to external forces. Residual stress is caused by forming or processing operations, or by the service environment. Measurement of residual stress is important in predicting service life, analyzing distortion, and determining the reasons for failure.
All stress measurement techniques can be divided into one of two classes: One measures actual strain, and the other measures changes in strain. Residual stress measurement techniques may also be classified by whether the results are qualitative or quantitative. The choice of a method of measurement should be based on the kind of information needed, but economic concerns are frequently the overriding factor. In addition, it is important to recognize the limitations of each technique.
In this study, the technology of XRD is made use for measuring residual stresses in a small rectangular strip shaped sample. The measurement of residual stress using XRD was driven from the limitations of using strain gauge based techniques i.e. for measuring residual stress in nickel based alloys, the strain gauge required is unique in built and the conventional strain gauge applicable for steels may not be appropriate. In measuring residual stress using X-ray diffraction (XRD), the strain in the crystal lattice is measured and the associated residual stress is determined from the elastic constants assuming a linear elastic distortion of the appropriate crystal lattice plane. Since X-rays impinge over an area on the sample, many grains and crystals will contribute to the measurement. The

exact number is dependent on the grain size and beam geometry. Although the measurement is considered to be near surface, X-rays do penetrate some distance into the material: the penetration depth is dependent on the anode, material and angle of incidence. Hence the measured strain is essentially the average over a few microns depth under the surface of the specimen. When making a residual stress using XRD, the values need to be validated by some means for this particular material. So, a strip sample is taken and a residual stress is induced by bending the sample. This stress induced is measured using XRD and is compared with the calculated theoretical stress value based on the deflection of the strip sample. Hence in order to bend the strip a four-point bend fixture is fabricated which can also facilitate the measurement of residual stress using XRD.
PRIOR ART SEARCH
US 4986132 A. A fully articulated four-point-bend loading fixture for MOR and fracture toughness specimens utilizes an upper loading plate in combination with a lower loading plate. The lower plate has a pair of spring loaded ball bearings which seat in V-shaped grooves located in the upper plate. The ball bearings are carried in the arms of the lower plate. A load is applied to the specimen through steel rollers, one large roller and one smaller roller each located on both the upper and lower plates. The large rollers have needle roller bearings which enable a single loading roller to rotate relative to the plate to which it is attached.
US 4941359 A. An articulating fixture for the flexure testing of ceramics, having a lower structure for supporting the bottom of a specimen to be tested and an upper structure for placing a load into the top surface of the specimen. Each such structure includes at least one articulated cooperating assembly which allows rotation in the direction of the surface of the specimen whereby when the specimen is twisted, it will adapt to the twist and still properly apply the load to the specimen. In one arrangement, each of the structures includes two lines of pressure provided by two loading pins so that there are

four such points in the fixture. In another arrangement, the upper structure includes one line of pressure provided by one loading pin, and the bottom structure includes two lines of pressure provided by two loading pins so that there are three such points in the fixture. The assembly for the lower structure includes a lower support cradle having an arcuate groove extending in the same direction as the specimen, and a swivel bearing support having a convex arcuate surface which mates with the surface of the groove so that the cradle and swivel bearing support are slidable with the swivel bearing support rotating within the groove to accommodate twisted specimens. The loading pin may contact the swivel bearing support on one side and the specimen at another side opposite the first side.
US 8177953 B2. A method of verifying performance of a coated part includes calculating a deflection value as a function of a predetermined strain threshold value and a total thickness of a test coupon that comprises a coating on a substrate. The coating of the test coupon is co-deposited in a deposition process for producing a coated part. The test coupon is bent in an amount equal to the calculated deflection value and then evaluated as an indication of whether a mechanical characteristic of the coated part meets a specified level.
With reference to the above prior art search, all the above are used to cause some particular amount of deflection to the sample. Specifically, for the first two (US 4986132 A, US 4941359 A) the fixtures cannot be operated manually, in fact it requires the assistance of a machine to cause deflection in the sample using the said fixtures. Whereas the third one (US 8177953 B2) the fixture is operated manually and has got lot of similarities to the present study / invention. But, the prior invention cannot be used to placing a goniometer (used for measuring RS using XRD) over the top of the sample as there is no access.

OBJECTS OF THE INVENTION
Objective of the invention is to perform a step-by-step bending in the hogging position to cause a calculated deflection to the strip shaped sample manually and then using XRD (X-ray diffraction) to measure the stress experienced by the sample by placing goniometer over the bent surface. The measured value is finally compared with the calculated theoretical flexural stress experienced by the sample due to the deflection.
SUMMARY OF THE INVENTION
According to the invention, there is a device used to cause a calculated hogging deflection to a strip shaped sample of rectangular cross-section. The invention uses a platform which supports a square threaded rod with a roller plate and a nut on either ends. The threaded rod is connected to the roller plate (a plate with two fixed roller pins) through a female threaded adapter and a ball bearing. This entire set up is rigidly mounted on table like platform of height adequate enough to allow the movement of threaded rod to and fro. A bearing is fixed to the roller plate by welding the bearing through its circumference. This provides rotational freedom for the roller plate with the movement of threaded rod. Once the threaded rod is moved up, using the welded nut at its bottom end, the roller plate tend to move up without rotation. Now when a strip sample is place on the roller plate by providing two-point support between the sample and roller pins, the sample also move along with the roller pin. In order to cause hogging bend, the ends of the sample are held tight to the shoulder clamps which in turn is clamped to the same platform. Since the rod is threaded it provides a self-locking feature there by restricting the movement of rod down due to the resilience developed by the sample. The entire assembly can be taken to a single platform and can be aligned using a C-clamp or appropriate holding technique thereby making the base a fixed member. Hence, once the threaded rod is moved up by turning it manually, the center of the sample is pushed up by the roller plate where the ends are held tight in the shoulder clamp plates, causing the strip sample to bend and hog.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The proposed invention will be better understood by the following description with reference to the accompanying drawings:
> Fig 1: Figure of the fabricated four-point bend fixture in normal condition.
> Fig 2: Figure of the fabricated four-point bend fixture in the bent condition of sample.
> Fig 3: Correlation between the theoretical and measured values of residual stress.
> Fig 4: Shows the bent specimen using the fixture and a direct measurement using XRD.
> Fig.5: Shows photographic view of the step up
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
Accordingly referring to the figure 1, provided a device consisting of a square threaded rod (2) assembled on a table platform (1) to cause a calculated deflection for a strip sample (8). The threaded rod is guided up and down through a female threaded adapter (item-4) with a ball bearing (5) welded at the top end of the threaded rod. The welding is done between the annular region of the bearing and outer surface of the threaded rod in order to restrict the rotational freedom of threaded rod. That is when the outer of the bearing is arrested the thread rod will rotate along with the annular part of the bearing (5). The circumference ball bearing is further welded with the roller support plate (3) by making a groove on roller support plate (3) prior to welding, so that the rotational freedom of the roller support plate is restricted. The rotation of the threaded rod is done by using the welded nut (7) at the bottom end of the rod. The nut 7 can be rotated manually (by hand). Since the rod (2) is threaded it is constrained only to move up or down. Once the nut (7) is rotated clockwise, the rod (2) moves up and once rotated anti-clock wise the rod (2) moves down. Once the rod (2) move up, referring to the fig.1, one can make out

that the sample (8) will be subjected to bending. The extent to which the rod (2) moves up, bend is related to this movement. A set of shoulder clamp plates (6a & 6b) are used to hold tight the sample from either ends by using bolts (9a & 9b) fastened at these respective ends. XRD is an instrument which can measure residual stress on ANY surface using the principle of X-ray diffraction. Basically, any sample strip held at some curvature will experience a stress. As the go on increasing the extent of bent, the stress is bound to increase. This stress is measured using XRD at every stages of bending. X-ray diffraction of residual stress measurement uses the distance between crystallographic planes as a strain gage. The deformations cause changes in the spacing of the lattice planes from their stress free value to a new value that corresponds to the magnitude of the residual stress. Because of Poisson’s ratio effect, if a tensile stress is applied, the lattice spacing will increase for planes perpendicular to the stress direction, and decrease for planes parallel to the stress direction.
In short, XRD act as a black box which will directly give the residual stress experienced by the sample as an output, experimentally. Referring to figure 1, the XRD will give an output as zero residual stress and for the bent condition in figure 2, XRD reads some amount of residual stress. The invention is not focused on XRD technique. Suppose, we use a Ni based alloy as sample strip and bend it. An amount of residual stress is experienced by the sample at every stages of bending.
So, this will be the residual stress experienced by the sample strip, calculated theoretically. These values can be compared for verifying the correctness of XRD in measuring residual stress.

The tabulation shows the calculated theoretical value and the measured (experimental) value of residual stress for different amount of deflection.
deflection in theoretically Measured
the sample, calculated using XRD,
mm MPa MPa
0 _ 48
2 129 132
5 220 224
7 282 268
9 348 341
11.5 382 363
The correlation between the theoretical and measured values are shown in figure-3.
Advantages:
1. The fixture can be operated manually without any requirement of any machine to create a bend.
2. The setup is portable and taken to any location.
3. It can provide access to the residual stress measurement using XRD.
4. A square thread is used which is self-locking in nature and hence avoids slip.

LITS OF REFERENCE NUMERALS
1 - The table platform
2 - Square threaded rod
3 - Roller support plate
4 - Female thread adapter
5 - Ball bearing
6a – Shoulder clamp plate right side 6b – Shoulder clamp plate left side
7 – Welded nut
8 – Strip sample
9a- Bolt to fasten the sample to right side shoulder clamps
9b- Bolt to fasten the sample to left side shoulder clamps
10 – XRD equipment
10a – Display of the XRD reading the residual stress (RS) directly

WE CLAIM
A four-point bending fixture for causing a calculated hogging bend for a flat sample comprising:
- a square threaded rod (2) assembled on a table platform structure (1);
- the said threaded rod (2) is guided up and down through a female threaded adapter (4) welded on to top support of platform (1) and ball bearing plate (5) welded on to a roller support plate (3);
- the said ball bearing plate (5) being welded with the roller support plate (3) on the outer circumference of the ball bearing plate (5) and between the outer side of the threaded rod (2) and on the inner circumference of ball bearing plate (5) such that when the screw rod (2) is rotated by the welded nut (7), the screw rod (2) will automatically move in upward and downward direction being guided between the female threaded adapter (4) and the ball bearing plate (5);
- the threaded rod (2) is connected to the roller plate (3) which consists of a plate with two fixed roller pins on it;
- a strip sample (8) of rectangular cross section placed over the roller pin;
- the said strip sample being held tightly from either side by a pair of shoulder clamps (6a, 6b) and fastening bolts (9a,9b);
- an XRD equipment (10) connected to the top of strip sample (8) for measuring the residual stress (RS) directly in a display recorder (10a) the said arrangement being made in a manner so that when the threaded rod (2) is rotated with the help of a nut (7), the rod (2) moves up and down exerting upward pressure on the roller support plate (3) which causes the strip sample (8) to deflection.

2. The bending fixture as claimed in claim 1, wherein the nut (7) is welded to the threaded rod (2) at the bottom end.
3. The bending fixture as claimed in claim 1, wherein the threaded rod (2) has square
thread which act as self-locking.
4. The bending fixture as claimed in claim 1, wherein the strip sample is restricted to
Ni-based component.
5. A process for creating the deflection (ɗ ) in a strip sample (8) in a four-point bend
fixture as claimed in claim 1 comprising:
- holding the strip sample (8) tightly by a pair of clamp shoulders (6a,6b) with the help of fastening bolts (9a,9b);
- rotating the nut (7) for creating an upward movement of the threaded rod (2) for causing deflection of the strip sample (8);
- rotating down the deflection values (ɗ) at different ranges and calculating the theoretical values.
- observing the XRD values from the recorder (10a) of the XRD equipment (10)
and then validating the experimental values of XRD.