DESC:TECHNICAL FIELD
The present invention relates to a machine to conduct fatigue tests of materials. More specifically the present invention relates to an electro-mechanical machine (100) for conducting flexural fatigue tests. The machine (100) can be adopted for conducting three point flexural fatigue tests to assess the relation between stress and number of cycles for failure of different materials.
BACKGROUND AND PRIOR ART

Flexural fatigue is the ability of the material to withstand flexural forces applied perpendicular to its longitudinal axis. The fatigue test of a material informs about the
strength of the material when subjected to repetitive stress at constant or variable
loading cycles. The flexural strength analysis is important for many products which must resist flexural deflection when forces are applied to it.
The patent document CN103149021 provides a device for testing fatigue, but the device is restricted to testing of rubber products. The patent document CN102706759
provides a device for testing plurality of materials; however the analysis of the
flexural fatigue is complex, owing to the design of the machine with a plurality of eccentric wheels, clamps and associated parts.
The importance of the analysis of flexural fatigue of materials mandates the requirement of a machine which can be adopted for testing various types of materials in a facile manner and cost effectively.
SUMMARY OF INVENTION
The present invention provides a machine (100) for testing flexural fatigue of a material.
The machine (100) comprises – a base plate (101) onto which the rotary actuator (108), spline shaft (114), cam (131), rotary actuator (110) with integrated lead screw and nut (10.1), follower (20.8), load cell (104), optical non contact based displacement Sensor (4.3) assembly, specimen holder (122), linear bearings (115), springs (128), belt (136) and pulley (118; 119);
Wherein,
- the rotary actuator (110) comprising an assembly of an integral lead screw (10.1) and follower (20.8) is attached to cam (131) mounted on a spline shaft (114);
- the rotary actuator (108) is attached to gearbox (107) with a rotary actuator mount (106) and secured with clamp (135);
- the pulley (118) is mounted on the gearbox (107) shaft and pulley (119) is mounted on the spline shaft (114);
- the linear bearings(115) support the carriage assembly (120) rested on the four springs (128);
- the belt (136) connects the pulleys (118, 119);
- the load cell (104) comprises of flexure (4.2), sensor (4.3), housing (4.1), centre support pin (4.6), support pin carriage (4.5), and connecting pin (4.4);
and
- the machine (100) is connected to a display system with a microcontroller for data acquisition, control and display the result of a test of the specimen (134).
The present invention also provides a method of testing flexural fatigue of a material adopting the machine (100) comprising acts of -
a) placing the specimen (134) in the specimen holder (122) and inserting the supporting pins (124) to fix the specimen (134) position,
b) considering the initial load on the specimen (134) as zero,

c) recording the specimen (134) information such as geometry and material name,
d) considering the test parameters such as test control type-strain or stress, and
frequency as the input,
e) based on the test parameters, the test is controlled by controlling the rotary actuator (108) speed and the cam (131) position through the rotary actuator (110) for frequency and amplitude control respectively, the cam (131) actuates the follower (20.8) that in turn applies the required force or displacement to the specimen (134), live data is recorded and analysed using appropriate analysis software, and
f) test ends when the specimen (134) is ruptured/ yielded or has reached the programmed number of cycles.
BRIEF DESCRIPTION OF FIGURES

The appended figures form part of specification. The features of the present invention can be understood in detail with the aid of figures, in combination with the detailed description of the specific embodiments and claims presented herein. It is to be noted, however, that the appended figures illustrate only typical embodiments of this invention and therefore not to be considered limiting of its scope for the invention.
Figure 1A: Shows schematic view of the in-situ micro three point flexural fatigue test machine (100).
Figure 1B: shows schematic of the isometric view of the machine (100).
Figure 1C: shows schematic front view of the machine (100).
Figure 1D: shows schematic top view of the machine (100).
Figure 2: shows schematic view of assembly of load cell (104);
Figure 3: shows schematic view of components of load cell (104).
Figure 4: shows schematic view of Readhead (127), Encoder (125).
Figure 5: shows schematic view of a Readhead Mount (126).
Figure 6: shows schematic view rotary actuator (110) with nut (10.1).
Figure 7: shows schematic view of assembly of rotary actuator mount (105).
Figure 8: shows schematic view Spline shaft (114) with spacer and locknuts.
Figure 9: shows schematic view carriage (120) assembly with linear bearings, specimen holder assembly and the follower assembly.
Figure 10: shows schematic view front view of the assembly of carriage (120A).
Figure 11: shows the schematic view of assembly of cam (131) with the yoke(129).
Figure 12: shows the schematic of sectional view of the cam (131A) depicting the cam profile and the position of the bearings mounted.
Figure 13: shows the schematic of the assembly of specimen holder (122) with the specimen (134) and the three support pins positions.
Figure 14: shows the image depicting the direction of the force applied by the support pins on the specimen.
Figure 15: shows an example of graph of the load vs number of cycles or time.
Figure 16: shows the image of the live plot of the graph in the software for the Deflection vs Time and Load vs Time.
Figure 17: shows the stress amplitude vs number of cycles of a 1045 steel and 2024-T6 Aluminium specimen.
Figure 18: shows the block diagram of flow of data with the control system and data acquisition (DAQ) for the test control and data recording.

DETAILED DESCRIPTION OF INVENTION
The foregoing description of the embodiments of the invention is presented for the
purpose of illustration. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, as many modifications and variations are possible in light of this disclosure for a person skilled in the art in view of the figures, description and claims. It may further be noted that as used herein, the singular “a” “an” and “the” include plural reference unless the context clearly dictates otherwise. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by a person skilled in the art.
The present invention, a flexural fatigue testing machine (100) allows a user to determine the fatigue properties and observe the changes in the microstructure of a specimen (134) during the fatigue test in the in-situ mode. The weight of the machine (100) is approximately 10 Kg or less, hence can be accommodated with a wide array of other machines; for example the machine (100) can be adopted with a scanning electron microscope (SEM) and X-ray diffraction machine (XRD).
The machine (100) is a three point (124; 4.6) flexural fatigue test machine. The three point (124; 4.6) flexural test machine analyses a material specimen (134) supported on three (3) points (124; 4.6) at equal distances and being actuated either by two end support points (124) or the center support point (4.6). The material specimen (134) can be selected from a group comprising metals, ceramics, glass, composites, polymers, surface coatings and the like.
Figure 1A, shows the schematic of machine (100). The figures 1B, 1C and 1D show’s schematic of different views of the machine namely isometric view, front view and top view respectively. The machine (100) comprises of a base plate (101) onto which the rotary actuator (108), spline shaft (114), cam (131), rotary actuator (110), load cell (104), optical non contact based displacement Sensor (125; 127) assembly (of figure 4 and 5 respectively), specimen holder (122), linear bearings (115), springs (128), belt (136) and pulley (118; 119).
In an embodiment, the rotary actuator (108) controls the frequency and the other rotary actuator (110) controls the amplitude of either load or displacement. The rotary actuator (110) has an integrated lead screw assembly which is attached to a cam (131) using a yoke (129). The cam (131) mounted on a spline shaft (114) then controls the amplitude of either displacement or load.
In an embodiment, the rotary actuator (108) is attached to the gearbox (107) on the base plate (101) with a rotary actuator mount (106). The rotary actuator (108) is secured using a clamp (135) to avoid vibrations. Pulley (118) is mounted on the gearbox (107) shaft and another pulley (119) is mounted on the spline shaft (114). The belt (136) connects the pulleys (118, 119).
In an embodiment, the rotary actuator (108) and gearbox (107) generates the necessary speed and torque to deflect the specimen (134) and to achieve the required frequency i.e., maximum number of cycles, accordingly the rotary actuator (108) and gearbox (107) are chosen. The torque from the rotary actuator (108), is transferred to the spline shaft (114) through belt (136) and pulley (118; 119) drive. The spline shaft (114) is connected to the cam (131). The cam (131) has a variable geometry which transfers the desired loads or displacements to the follower (20.8) that is applied on the test specimen (134).
In an embodiment, the load cell (104) assembly as depicted in figure 2 and in figure 3 as (104A), comprises flexure (4.2), sensor (4.3), housing (4.1), centre support pin (4.6), support pin carriage (4.5), and connecting pin (4.4). The flexure member (4.2) is fixed on one end of the housing (4.1), and the sensor (4.3) is mounted on the other end of the housing (4.1). A connecting pin (4.4) is inserted between the inner end of the flexure (4.2) and the sensor (4.3). The centre support pin (4.6) is fixed on the support pin carriage (4.5) that is fixed on the outer end of the flexure (4.2).
In another embodiment, high resolution of load cell (104) is achieved through use of a single capacitance based load cell (104) for the complete load range. The Double cantilever based flexure design for load cell (104) will facilitate uniaxial movement. The stiffness of the flexure is selected such that the deflection and the induced stress are minimal. The induced stress in the flexural member for maximum deflection are below the endurance stress limit. After a specific number of cycles, the flexure is replaced.
In an embodiment, the figure 4 and figure 5 depicts displacement sensor assembly
and its components (125; 127) respectively. Accordingly, it comprises an incremental optical encoder (125) and readhead (127). The encoder (125) is mounted on the carriage (20.1). The readhead (127) is fixed at a specific position on the base plate (101) with a readhead mount (126). Due to relative displacement between the encoder (125) and the readhead (127); the read head (127) gives the deflection of the specimen (134).
In an embodiment, the spline shaft (114) design in the assembly ensures the amplitude control by moving the cam (131) on the spline (114) in the axial direction (figure 8). The spline shaft (114) is divided into three components, namely, the left shaft (113), right shaft (112) and the spline shaft (114) for ease of assembly. The cam (131) is mounted on the spline shaft (114), which ensures the cam (131) rotation and control the amplitude of the cam (131) moves axially along the axis of the shaft (14) with the help of the spline (114).
In another embodiment, the spline shaft (114) is supported by two bearing housings (102; 103), onto which the cam (131) is mounted. The cam (131) position is varied using a rotary actuator (110) with an integral lead screw and nut (10.1) arrangement. The rotary actuator (110) is mounted on the base plate (101) using a rotary actuator mount (105) and the cam (131) is connected to the rotary actuator through a rigid member called yoke (129).
In still another embodiment, the spline shaft (114) is split into three components for ease of assembly and to avoid mounting errors in the linear bearings (115). The left shaft (113) and the right shaft (112) of the spline are mounted on the bearing housings (102, 103). The shafts are coupled to the spline (114). To restrict the axial movement and avoid eccentricity, the left shaft (113) is clamped with a spacer (116) and a lock nut (117) to the bearing housing (103) and the right shaft (112) is clamped to the bearing housing (102) with a spacer (116) and the pulley (119).
In another embodiment, the follower (20.8) and the specimen holder assembly (20.3; 121; 122; 123; 124) are mounted on the carriage (28.1), which is supported by four parallel springs (128) and linear bearings (115) as depicted in figure 9. The cam (131) and a follower (20.8) are in continuous contact as a result of appropriate spring force. At a given frequency, the follower's (20.8), Cam (131) pushing causes the specimen (134) to deflect as needed, thus following and ensuring the three point (124; 4.6) flexural fatigue test.
In an embodiment, the follower (20.8) is designed such that the contact area between the cam (131) and the follower (20.8) is as minimal as possible thereby reducing the wear on the cam. The material chosen for the follower is that the hardness is less than the cam. The minimal contact ensures better control resolution for the load and displacement amplitude control. The follower (20.8) is mounted on two roller bearing supports (20.7) mounted on the carriage base (20.1) as in figure 9 and figure 10.
In an embodiment, the linear Bearings (115) are used as guides for the carriage (28.1) for oscillatory motion. The linear bearings (115) guide the carriage (28.1) as per the movement required by the cam (131), which in turn bends the specimen (134). The linear bearings (115) consist of a rail (15.1) mounted on the bearing housings (102; 103) and the carriage (15.3) mounted on the carriage (28.1).
In another embodiment, the linear bearings (115) assembly consists of three components, the guide rail (15.1), the carriage (15.3) and the wiper (15.4). The rails (15.1) are mounted on the bearing housings (102, 103) and the carriages (15.3) are mounted to the adjustment block (20.3) and the carriage holder (20.2) from the carriage assembly (120).
In an embodiment, the cam (131) geometry ensures the required load and displacement amplitudes are achieved. The cam (131) is designed in such a way that the amplitude can be continuously changed as needed. The cam (131) provides the variable amplitude with a maximum deflection range of 0 mm to 5 mm. Accordingly, the cam (131) profile is chosen for the required amplitude that can generate a sinusoidal wave pattern. The cam (131) geometry has a curved outline throughout the length of the cam. The cam (131) has variable geometry throughout the length and any specific position of the cam (131) along the length the runout is curved. The cam (131) geometry has position controllable from zero to the maximum deflection required. Since the cam (131) is eccentric without any sudden drops and is curved throughout the profile for the entire length, the cam (131) provides the sinusoidal wave pattern for any given amplitude.
In another embodiment, the cam’s (131) unique geometry ensures sinusoidal waveform. The cam (131) geometry is such that there are multiple circles overlapping with each circle in tangent with the other. Since, the cam (131) is eccentric without any sudden drops and is curved throughout the profile. The profile incorporates axial embedded geometries to support amplitude variation of the sinusoidal waveform. The unique cam follower (20.8) design ensures sacrificial mating to the cam (131) thereby ensuring no wear to the cam (131). The design enables precise amplitude control. The cam (131) is always in contact with the follower (20.8) by the force of spring (128) mounted in between the carriage (28.1) and the base plate (101), thereby ensuring sinusoidal waveform.
In an embodiment, the rotary actuator (110) for controlling the cam (131) position has an integrated lead screw and nut (10.1). The rotary actuator (110) is mounted on the base plate (101) using a rotary actuator mount (105) as depicted in figure 6 and figure 7. The amplitude can be adjusted by controlling the rotary actuator (110) in a closed loop system. The DAQ (Data acquisition) controller takes the feedback from the readhead (127) and controls the rotary actuator (110) with the driver which can do micro stepping. The micro stepping will help to reduce the step angle further to achieve fine amplitude control over the test.
In an embodiment, within the control system of the machine (100), the rotary actuator driver controls the rotary actuator (108) based on the feedback from the DAQ system. An absolute rotary encoder located inside the rotary actuator (108) housing will provide feedback to the rotary actuator (108) driver. The test frequency control is a result from the rotary actuator driver's utilisation of the feedback to operate the rotary actuator (108) in a control loop for precise RPM control. The microcontrollers of the DAQ and control system will command the rotary actuator driver and run the test according to the test parameters like amplitude of load or displacement and frequency.
In an embodiment, the frequency of the test depends on the speed of the rotary actuator (108). The gear ratio of the belt drive comprising -belt (136), pulleys (118; 119) and the gearbox (107) are considered while choosing the maximum speed of rotary actuator (108). The speed of the rotary actuator combined with the gear ratio of the gearbox (107) and the pulleys (118; 119) ratio determines the maximum frequency within which the machine can operate without any load i.e without any specimen (134). For example, when the maximum required frequency of the machine is 15 Hz, then for one minute the speed required at the spline shaft (114) is 900 RPM (rotations per minute) i.e., 15 Hz x 60 secs = 900 RPM. If the gear ratio is 2.4, then the total speed required from the rotary actuator is 2160 RPM i.e 900 RPM x 2.4 = 2160 RPM. The maximum speed of the rotary actuator (108) must be greater than the required speed for proper functioning of the machine.
In the above example, if the nominal RPM of the rotary actuator is considered as 3000, with a gearbox (107) ratio of 3:1, the speed will reduce to 1000 RPM. Due to the belt (136) and pulley (118; 119) ratio of 0.8, the final speed at the spline shaft (14) is 900 RPM. For further consideration the maximum torque is included in selection of the rotary actuator. If the torque rating of the rotary actuator (108) is 0.64 N-m up to nominal speeds, the torque at the spline shaft (114) due to gear reduction would be 1.536 N-m i.e 0.64 N-m x 2.4 (gear reduction). Said value reduces due to frictional and inertial forces, the torque selection for the rotary actuator (108) is based on the friction between the cam (131) and the follower (20.8), the maximum radius of the cam (131) profile, spring force, angular momentum and linear momentum. For Example, by considering a normal force of 500 N and the radius of the cam (131) at the maximum amplitude of 5 mm to be 14.5 mm with coefficient of friction 0.1, spring (128) force of 100 N and the inertial forces from the carriage assembly (120) to be 38 N, the torque (T1) is the multiplied value of coefficient of friction, maximum radius and the sum of all the forces (normal, spring and inertial). Hence, the torque required is 925 N-mm (0.925 N-m). The total torque must include the inertial forces from the cam (131), spline shaft (114) and the pulleys (118; 119). That is considered as Torque (T2) is equal to the moment of inertia multiplied by the angular acceleration and bearing friction. For 15 Hz frequency the angular velocity is 15 multiplied by 2?? that is equal to 94.25 rad/sec and the angular acceleration is nearly 2827.4 rad/sec2. Also, consider a bearing friction of 0.05 including the factor of safety and the moment of inertia of 23689 x 10-9 Kg/m2, then the torque (T2) required to overcome the inertial forces is 0.07035 N-m. Hence, the total torque (T) is the sum of torque (T1) and torque (T2) i.e., the total torque (T) at the spline shaft (114) is 0.995 N-m. The torque required at the rotary actuator (108) is calculated by considering the gear reduction in between the rotary actuator and the spline shaft. In the example, the belt (36) and pulley (118; 119) ratio of 0.8:1 and the gear ratio of the gearbox connected to the rotary actuator is 3:1 are considered. The total gear reduction is 2.4, so the total torque required at the rotary actuator is 0.415 N-m. A rotary actuator with rated torque more than or equal to 0.415 Nm is required in the example. Similarly, the rotary actuator (8) power required = (2 x ?? x Speed x Torque) / 60, considering the speed of the rotary actuator (8) to be 3000 RPM from the above example, then the required rotary actuator (8) power is 130 Watts.
In an embodiment, the load cell (104) consists of a sensor (4.3) with high resolution. For example, 500 Newtons load cell (104) can have 0.5 milli newtons. The deflection in the flexure member (4.2) causes the same deflection within the sensor (4.3). The deflection is converted to an analog voltage by using an instrumentation circuit built within the sensor (4.3). The instrumentation circuit is placed within the sensor to avoid any interference and effect of external noise. The analog output from the instrumentation board is read by a data acquisition system that can ensure the load data acquired is within a resolution of 1 ppm ( parts per million) . Care is taken to reduce the noise due to external factors such as power supply noise and EMI (Electromagnetic Interference).
In an embodiment, for measuring the displacement, the chosen sensor is a non contact type displacement measuring device. It consists of an encoder (125), readhead (127) and an interface. The interface is an integral part of the readhead through which the information is retrieved from the encoder & is shared to the data acquisition system. The optical encoder (127) is attached to the carriage base (20.1) and the readhead (127) is fixed to the base plate (101) with a readhead mount (126). The read head (127) and the encoder (125) are assembled such that the gap between the encoder (125) and readhead (127) has enough clearance as required. Depending on the displacement range required, the length of the encoder (125) is chosen; Consider a system in which the maximum displacement is 5 mm, then the encoder can be chosen according to the requirement.
In an embodiment, the linear bearings(115) provide support to the carriage assembly (120) that are supported on the 4 springs (128). It guides the specimen holder (122). There are 4 linear bearings (15.3) supported on 4 guide rails (15.1) mounted diagonally. In another embodiment, the stiffness of the springs (128) is considered such that the follower (20.8) is always in contact with the cam (131) and has a solid length of more than the maximum deflection required. The ends of the springs (128) are closed and grounded, and it is mounted on a nylon bush (111), so that the springs (128) are fixed in its position.
In an embodiment, the specimen holder (122) is designed such that specimen (134) is placed for easier access for the in-situ applications. The specimen holder (122) has provision for accommodating a specimen (134) of different thickness and length.
In an embodiment, the specimen holder (122) assembly as depicted in figure 13, includes three components, the adjustment blocks (20.3; 20.4) is rigidly mounted on the carriage base (20.1) and connected to the two of the four linear bearings assembly (115). The adjustment block acts as a guide and rigidly holds the specimen (134) with respect to the three support points (124; 4.6) for the flexural test.
In an embodiment, the two point mount (121) slides or moves along the guide path of the adjustment block (20.3; 20.4) so that specimens (134) of different thickness can be mounted. The specimen holder (122) holds the specimen (134) geometry along its length. The support points (124) are mounted at varied distances with respect to the specimen (134) length. Once the specimen (134) is placed inside the specimen holder (122), the support points (124) are inserted in any of the three positions along the specimen holder (122) depending on the specimen (134) length and the user preference. By adjusting the two point mount (121) position, varied thicknesses of the specimen (134) can be tested. By adjusting the support point (124) positions along the specimen holder (122), specimens (134) of different length can be tested. Once the specimen (134) position is adjusted and the support pins are mounted (124) on the specimen holder (122), the position locked tightening the bolts (123) on the adjustment blocks (20.3; 20.4), thereby fixing the specimen (134) between three support points (4.6; 124). The figure 14 depicts the direction of the force applied by the support points on a specimen.
In an embodiment, as in figure 11 and figure 12 the cam (131) and the yoke (129) are supported on a radial roller bearing (130) and fastened using the needle roller thrust bearing (132) with lock nut (133). Through said assembly, the position of the cam (131) is controlled and thereby displacement or the load applied to the specimen (134) dynamically.
In another embodiment, the cam (131) geometry ensures the sinusoidal test waveform in deflection and load controls. It allows the algorithms to control the load and displacement in the Sinusoidal pattern as per the user test requirement. The rotary actuator (108) ensures the precise frequency of the test. The rotary actuator (110) controls the cam (131) position along the axis to adjust the amplitude of the load and displacement sinusoidal test waveform. The unique design of the follower allows the cam to have a surface contact at specific points without damaging the cam. For example, the micro stepper driver provides (MdPPR) 40,000 PPR (Pulse Per Revolution); Pitch of Lead screw (P) is 2 mm; Horizontal Length of the cam (31) profile (L) is 25 mm; Peak Amplitude of the sinusoidal waveform (APeak) is 5 mm.
Small change in cam (131) position (?L ) per pulse is calculated as
?L = ?? / ?????????? , ????/pulse;
?L = 2 / 40000 = 0.05 ????/pulse

Small Change in Amplitude of the Sinusoidal profile is given by

???????????? = ( ??L x A ) / L
???????????? = ( 0.05 x 5 x 10^3 ) / ( 25 x 10^3 ) = 0.01 ????/??????????

In the said example, a 10 nm change in sinusoidal wave is observed.

The temperature and the humidity are also recorded continuously, so that when different specimens (134) of the same material are tested it is under the same temperature and humidity parameters or the test can be aborted.
The flow of data for the test control and the data acquisition is depicted in the figure 18. The user inputs the specimen details such as the dimensions, material and test criteria such as stress control or strain control, number of cycles to be tested, and stress or strain level selection within which the test must be performed; to the machine through the software. The DAQ and the control system will process the input information and control the actuators (108 / 110) by sending the control signals to the actuator driver which in turn controls the rotary actuators (108 / 110). The amplitude is controlled by the rotary actuator (108) and the frequency is controlled by the rotary actuator (110). While controlling, the control system takes feedback from the DAQ, which in turn collects the data from the interface and load sensor instrumentation for measuring the load and displacement. The control signals are used to control the actuators (108 / 110) to meet the test criteria. The feedback is collected and stored in the DAQ for further analysis of the test. The figure 18 shows the signal interactions. To mitigate the risk of cam (131) moving out of position which causes the amplitude of the displacement to go wrong, two limit switches are placed in either limit of the cam (131) position. The switches are activated within the control system to hard stop the machine completely and thereby eliminate the risk of damage to the spline shaft (114) and the cam (131).
The test procedure adopting the machine (100) is as follows:
a. The specimen (134) is placed in the specimen holder (122) and the supporting
pins (124) are inserted to fix the specimen (134) position,
b. The initial load on the specimen (134) is taken as zero,
c. The user records the specimen (134) information such as geometry and material name,
d. Test parameters such as test control type -strain or stress and frequency is
considered as the input,
e. Depending on the test parameters the test is controlled by controlling the rotary actuator (108) speed and the cam (131) position through the rotary actuator (110) for frequency and amplitude control respectively. The test control is dependent on the input from the readhead and the load cell for displacement and the load data respectively. The cam (131) actuates the follower (20.8) that in turn applies the required force or displacement to the specimen (134). The live data similar to the graph as shown in figure 16 is recorded, analysed through analysis software and results are displayed through display system (not shown in figure).
In an embodiment, the data is input and analysed through the test setup and the analysis in the software by the following steps given below.
a) Selection of the test frequency in accordance with the requirement and the machine limitations such as the size of the specimen, maximum frequency and maximum load.
b) The error in the displacement measuring instrument i.e the optical encoder shall not exceed +/- 0.5 % of the maximum deflection.
c) The software/microcontroller will plot the load vs time, deflection vs time and the number of cycles.
d) The specimen material dimensions are selected based on the stress or strain proportional limits that are calculated using the formulas mentioned above with respect to the stress or strain proportional limits.
e) The end of the test cycles with maximum programmed cycles, for example 107 cycles.
f) Test continues until any of the below mentioned criteria are met:
i) The specimen material is ruptured or
ii)The specimen material yields in a load controlled test, the material might yield if the material’s maximum deflection is increased by more than 10% during testing, for strain controlled test, the material might yield if the maximum measured load decreases by more than 10% during testing.
g) The test is repeated using untested specimens for at least three times at each of the four stress or strain levels for producing accurate fatigue test data.
h) The test data recorded is plotted and the results are derived. The plots are as follows-
A)S-N curve- the maximum stress level versus the logarithm of the number of cycles to failure is plotted, or
B) e-N curve- the maximum strain level versus the logarithm of the number of cycles to failure is plotted, or
C) If there are many specimens of the same material that have been tested at the given stress or strain levels, probability plots and other statistical analysis shall be used to create a mean S-N or e-N curve as shown in Figure 17.
i) The interpretation of the results from the plots is as follows-
A) If the S-N or the e-N curve is horizontal asymptote to the constant stress or strain, the stress or strain value is interpreted as the endurance limit of the test material.
B) If a horizontal asymptote to the constant stress or strain is not observed on the curve, then additional stress or strain levels are tested for the specimens until the number of cycles reached is greater than that of the material’s expected life. The amplitude of the stress or strain at which the number of cycles is greater than the material’s expected life must be interpreted as the endurance limit of the material.
C)If the tests are optionally terminated by the user before the failure of the material, then the constant stress or the strain value determined from the resulting S-N or e-N curve shall be reported as Estimated Endurance Limit.
j) Finally, the report is generated with all the results obtained from the analysis using the test data.
The material behaviour under the flexural fatigue is also determined using the in-situ mode. In said mode, the machine (100) is adopted with SEM or XRD machines. The specimen under test is put on hold when the material behaviour is reviewed by the user using any of the above machines. An avenue is provided for mounting a microscope with a camera on the machine, the material behaviour under flexural fatigue is observed and recorded in real time. Various behaviours can be observed and studied including crack initiation and crack propagation, damage accumulation and fatigue failure, factors that influence the fatigue life such as surface finish, material composition and the like.
An aluminium specimen selected based on the size and above mentioned professional limits is tested with the machine(100). The figure 15 shows an example of a live plot for the load vs time during the test. In the figure 15 the maximum and minimum amplitude with mean cycle i.e., frequency, is visible. The values are checked by the control system during the test to control the test as per the user input in a load control test. If the user is running a displacement controlled test then it checks with respect to the displacement vs time. The figure 16 shows a typical example of a live plotted graph of the load vs time and displacement vs time. The plots are shown in real time as the data is being recorded when the test is in progress. The graph is used to determine the maximum and minimum amplitude of both load and displacement with respective time to control the test as per the selected stress or strain level.
Also, the graphs are plotted in the analysis section for stress vs number of cycles ( S-N curve ) and strain vs number of cycles ( e-N curve ). The figure 17 shows the graphs of amplitude of stress vs number of cycles i.e., S-N curve.
In figure 17 the S-N curve of both Aluminium 2024 - T6 and Steel 1045 is plotted for comparative results on determining the endurance limits of the materials using the mean S-N curve. It is observed that the steel has a clear endurance limit, whereas in aluminium no distinct endurance limit is observed. In mechanical applications, the number of cycles more than 107 is considered as the endurance limit for metals.
To determine the fatigue properties of different materials due to bending of the specimen are conducted using the equipment. Some of the materials that are tested are pure metals, ferrous and non-ferrous alloys. The test specimen dimension that can be fixed into the present equipment are:
i. Length: 20mm, 40mm and 60mm.
ii. Width: up to 10mm.
iii. Thickness: up to 4mm.

Advantages of machine (100)
The machine (100) is imbibed with the advantage of space, ease of operation and user friendliness, hence can also be adopted as a stand alone or a table top equipment or with an instrument like SEM or XRD or other instruments to perform advanced studies.
Since the centre of the test specimen (134) does not experience any change in vertical position, it is possible to use an external microscope for microstructural studies. By properly housing within a microscope, the crack growth with respect to the number of cycles can be observed by looking at normal to the thickness of the test specimen (134). The crack growth studies can be conducted by providing a notch in the specimen (134).
By changing the adapters, the same machine can be used to conduct 4 point bend tests. By choosing the proper cam (131) geometry, that is the ratio between the length and maximum amplitude of the cam (131) profile, users can achieve better resolutions in terms of load and displacement amplitude applications. The software programming enables conducting the tests in load control and displacement control.
The electro-mechanical setup of the machine is devoid of compressed air or hydraulic setups with hydraulic pumps.
The compactness of the machine and weight aids in being adopted in a vacuum environment or any other instrument in-situ.

,CLAIMS:WE CLAIM
1. A flexural fatigue testing machine(100) for a material; comprising a base plate (101) onto which a rotary actuator (108,110), spline shaft (114), cam (131), load cell (104), optical non-contact based displacement sensor assembly (4.3), material specimen holder (122), linear bearings (115), springs (128), belt (136) and pulley (118, 119) are assembled;
Wherein,
- the rotary actuator (110) comprising an assembly of an integral lead screw (10.1) and follower (20.8) is attached to cam (131) mounted on a spline shaft (114);
- the rotary actuator (108) is attached to gearbox (107) with a rotary actuator mount (106) and secured with clamp (135);
- the pulley (118) is mounted on the gearbox (107) shaft and pulley (119) is mounted on the spline shaft (114);
- the linear bearings(115) support the carriage assembly (120) rested on the four springs (128);
- the belt (136) connects the pulleys (118, 119);
- the load cell (104; 104A) comprises of flexure (4.2), sensor (4.3), housing (4.1), center support pin (4.6), support pin carriage (4.5), and connecting pin (4.4);
and
- the machine (100) is connected to a display system with microcontroller for data acquisition, control and display result of test of the material specimen(134).

2. The flexural fatigue testing machine (100) as claimed in claim 1, wherein in the machine (100) the specimen for testing flexural fatigue is supported on three points (124, 4.6).

3. The flexural fatigue testing machine (100) as claimed in claim 1, wherein the connecting pin (4.4) is inserted between inner end of the flexure (4.2) and the sensor (4.3); and center support pin (4.6) is fixed on the support pin carriage (4.5) fixed on outer end of flexure (4.2).

4. The flexural fatigue testing machine (100) as claimed in claim 1, wherein the spline shaft (114) is supported by two bearing housings (102; 103) for mounting the cam (131).

5. The flexural fatigue testing machine(100) as claimed in claim 1 and 4, wherein the spline shaft (114) comprises left shaft (113) and right shaft (112) mounted on the bearing housings (102, 103); wherein the left shaft (113) is clamped with a spacer (116) and a lock nut (117) to the bearing housing (103) and the right shaft (112) is clamped to the bearing housing (102) with a spacer (116) and the pulley (119).

6. The flexural fatigue testing machine (100) as claimed in claim 1, wherein the optical non-contact based displacement sensor (4.3) assembly comprises an optical encoder (125) and readhead (127).
7. The flexural fatigue testing machine (100) as claimed in claim 1, wherein the linear bearings assembly (115) comprises a guide rail (15.1), a carriage (15.3) and two wipers (15.4).

8. The flexural fatigue testing machine (100) as claimed in claim 1, wherein the specimen holder (122) comprises adjustment blocks (20.3; 20.4) mounted on carriage base (20.1).
9. The flexural fatigue testing machine as claimed in claim 7, wherein the specimen holder (122) accommodates the material specimen (134) of compatible dimensions.

10. A method of testing flexural fatigue of a material employing flexural fatigue testing machine (100) comprising steps of -
a) placing material specimen (134) in the specimen holder (122) and inserting supporting pins (124) to fix the specimen (134) position,
b) considering the initial load on the specimen (134) as zero,
c) recording the specimen (134) information such as geometry and material name,
d) considering and recording test parameters such as test control type-strain or stress, frequency as the input,
e) controlling the rotary actuator (108) speed and the cam (131) position through the rotary actuator (110) for frequency and amplitude control respectively to apply force or displacement to the specimen (134), and
f) ending the test when the specimen (134) is ruptured/ yielded or has reached programmed number of cycles, and
g) acquiring data, recording and analysed through software embedded microcontrollers and displaying the results of the test of the flexural fatigue of the material specimen.
11. The method of testing flexural fatigue of a material as claimed in claim 10, wherein the material is selected from a group comprising metals, ceramics, glass, composites, plastics and surface coatings.