SK1252023A3 - Method of performing rotating bending fatigue test - Google Patents

Abstract

Method of performing rotating bending fatigue test based on the principle of computer- controlled cyclic loading and software evaluation of measured values, characterized in that into the jaws (2, 2'), which are on the frame (1), at least one test specimen (3) is clamped to having an exemplary circular cross-section, where the tangent of its axis at the clamping point is coaxial with the axis of the clamping jaw (2, 2'), and the mutual angle of the vectors of the jaws (2, 2') is 0-270°, where in the test specimen (3) is loaded by bending stress, and furthermore, electric motors (4, 4') are started, which start the jaws (2, 2'), the movement of which is synchronized with each other, while the test specimen (3) begins to rotate about its axis in the number of cycles 103 to 109, when using a force sensor (5) load data are recorded.

Description

The field of technology

The invention relates to a method of performing a fatigue test by bending under rotation for test bodies of circular cross-section, during which the test body is repeatedly loaded, especially wires, which simulates the behavior of the tested material during normal use in a given time interval. The purpose of the test is to obtain information about the mechanical properties of the material during loading, thanks to which it is possible to evaluate the service life of the tested material in machine components.

Current state of the art

Material fatigue manifests itself if the number of so-called load cycles reach tens of thousands, millions and more. The stress during cyclic loading of the sample is less than the yield strength of the material, because material fatigue occurs in components that are subjected to repeated loads lower than the yield strength of the material but higher than the fatigue strength. In the case of components loaded in this way, a crack may appear, which slowly spreads with the increasing number of load cycles, until the so-called fatigue fracture of the component. This fracture is characterized by the fact that it is not preceded by plastic deformation. Cracks typically occur in places of stress concentration, that is, in places of notches, that is, for example, in places where there is a hole, a thread, a large surface roughness, a weld, a crack, material damage, therefore fatigue tests are often carried out on test samples with notches.

From the current state of the art, methods and devices are known for carrying out the bending fatigue test under rotation. A device adapted to perform a rotational bending fatigue test typically consists of a motor, a drive spindle, and a support bearing. The sample is placed between the drive spindle, to which it is clamped, and the support bearing. The sample typically has a notch, hole, etc., which serve as stress concentrators. However, these devices are only adapted to examine straight samples (samples with a straight longitudinal axis). In the case of testing, for example, wire, which is typically produced in coils with a certain radius of rounding of the longitudinal axis of the wire, it is necessary to straighten the wire before clamping. However, this leads to plastic deformation, and thus to a fundamental change in the mechanical properties even before the actual testing of the sample.

From the patent file CZ 304633 "Device for bending fatigue tests for testing test specimens with a notch, a device for performing a bending fatigue test is known, where the device contains a guide sleeve for clamping the test sample, which is placed in a cage and a clamping sleeve is placed in the guide sleeve to clamp the test specimen. A horizontal caliper in one axis and a vertical caliper in the other axis are used to monitor the progress of the load on the test sample. The said document provides a completely different method of testing than the claimed invention.

Furthermore, there is a known solution from utility model application 2014-30046 "Jaw for clamping miniature round samples in testing machines for testing fatigue life, which is adapted for clamping miniature round samples, which includes a fixed sample holder equipped with two holes and a clamping stone on which it has a recess in the shape of a cylindrical surface with indentations. The jaw further includes a movable specimen holder, which is also equipped with two holes and a clamping stone. The connection of the fixed and movable sample holder is realized with the help of pins, which are inserted into the corresponding holes and connected by a pressure element. The submitted file also relates to a different field than the claimed invention.

From the patent file CN111337338A "Fatigue test device for repeatedly winding and unwinding winding optical cable" is a known device adapted to the fatigue test of optical cables during their repeated winding and unwinding.

From the patent file CN101221108A "Rotating and bending corrosion fatigue testing device" is a known device for performing a fatigue test by rotating samples that are subject to corrosion. When testing the sample, the examined sample is rotated and, at the same time, a solution causing corrosion is applied using a pump and a nozzle.

The methods of fatigue testing cannot be performed on test bodies with a certain radius on the mentioned devices without equalizing them, or the mentioned documents represent a completely different technical solution.

Furthermore, there are devices that have a drive spindle and a support bearing located on pivot joints. The tested sample is placed between the drive spindle and the support bearing. Then the sample is clamped to the drive spindle and subsequently the spindle and the bearing are deflected by a defined angle so that the test sample is bent. Cyclic loading of the sample is then achieved by rotating the sample. This method of loading is called in the literature the so-called "Nakamura test. A fundamental disadvantage of this method of testing is the effect of friction in the support bearing on the measurement result.

Friction in the support bearing creates a torque causing an additional twisting load along the entire length

SK 125-2023 A3 of the test specimen. As the diameter of the test specimen decreases, the ratio between bending stress (desired, defined) and torsional stress (undesired, undefined) decreases. Therefore, when loading thin samples (e.g. wires), the resistance of the non-driven jaw (e.g. support bearing) significantly distorts the measurement result.

The essence of the invention

The above-mentioned disadvantages are eliminated by the method of carrying out the fatigue test by bending while rotating, based on the principle of computer-controlled cyclic loading and software evaluation of the measured values, according to this invention, the essence of which consists in the fact that the jaws, which are fixed on the frame, are clamped at both ends at least one test body that has a circular cross-section, where the tangent of its longitudinal axis at the point of clamping is co-axial with the axis of the clamping jaw, and the mutual angle of the vectors of the axes of the jaws is freely selectable in the range of 0 - 270°, whereby the test body is loaded with bending stress. Then the electric motors are started, which start the jaws, the movement of which is mutually synchronized, and the test body starts to rotate around its axis in the number of cycles from 10 ³ to 10 ⁹ . The load data is recorded using the force sensor and the fracture is detected using the fracture detection sensor.

It is preferable that the mutual angle of the vectors of the axes of the jaws is 30-180°, and it is also advantageous that the axes of the jaws, together with the clamped test body, point down, thereby eliminating the influence of the obtained test results by unwanted lateral bending due to the weight of the sample.

It is expedient that, after detection of a fracture, the rotation of the test body is quickly stopped by means of a braking mechanism.

Overview of images on drawings

Figure 1 shows schematically the device for carrying out the fatigue test method by bending while rotating.

Figure 2 shows clamping jaws, where the tangent to the longitudinal axis of the sample at the clamping point is perpendicular to the face of the clamping jaw, and Figure 3 schematically shows a sample with a clamping angle of, for example, 90°.

Figure 4 shows the wire geometry with the clamping jaws at 90°, the thick black solid line shows the clamped loaded test specimen, the solid gray line shows the unloaded specimen, the dot-dashed line shows the collet axes, and the dashed line shows the arc guides . Figure 5 shows a graph representing the stress according to the rotation of the sample, where the angle of rotation of the sample is plotted on the horizontal axis, the tensile stress on the surface of the sample is plotted on the vertical axis, and the magnitude of the force is plotted on the vertical axis on the right of the graph, where the thick black line indicates the stress on the internal side of the sample, the thick black dashed line indicates the stress on the outside of the sample, the gray solid and dashed line shows the stress on the sides, and the gray dotted line shows the magnitude of the force on the sensor. Figure 6 shows a graph where the values of tension, forces and distances are shown on the vertical axis, the size of the test radius on the horizontal axis, where the thick black line indicates the maximum stress, the thick black dashed line indicates the maximum force between the collets, the thin solid black line indicates the test length of the specimen, the thin dashed line shows the collet spacing, and the dotted line indicates the camber of the test length of the specimen.

Figure 7 schematically shows a sample with a clamping angle of, for example, 120° and Figure 8 with a clamping angle of 180°, Figure 9 with a clamping angle of 270°.

Figure 10 shows the geometry of the wire, with the clamping jaws occupying an angle of 120°, the thick black solid line shows the clamped loaded test specimen, the solid gray line indicates the unloaded specimen, the dot-dashed line indicates the collet axes, and the dashed line indicates the arc guides . Figure 11 shows a graph representing the stress according to the rotation of the sample, where the angle of rotation of the sample is plotted on the horizontal axis, the tensile stress on the surface of the sample is plotted on the vertical axis, and the magnitude of the force between the jaws is plotted on the vertical axis on the right of the graph, where the thick black line indicates the strain on the inside of the specimen, the thick black dashed line indicates the strain on the outside of the specimen, the gray solid and dashed line shows the strain on the sides, and the gray dotted line shows the magnitude of the force on the sensor.

SK 125-2023 A3

Examples of implementation of the invention

The method of carrying out the rotational bending fatigue test based on the principle of computer-controlled cyclic loading, according to this invention, consists in the fact that a test body 3, which has, for example, a circular cross-section, is attached to the jaws 2,2', which are on the frame 1 o test bodies made of wire. The tangent of the longitudinal axis of the test body 3 at the clamping point is parallel to the axis of the clamping jaw 2, 2' and perpendicular to its face. The mutual angle of the axes of the jaws 2, 2' is 0 - 270°.

After setting the test body 3, the electric motors 4, 4' of the jaws are started, the rotation of which is mutually synchronized to minimize the stress on the test body 3 by twisting. During the rotation of the test body 3, the local values of this voltage then change according to the sinusoidal cycle with each revolution.

At one revolution, the test body 3 is stressed on the surface from maximum tension to maximum pressure and is thus caused by tested cycles of the order of 10 ³ to 10 ⁹ .

Using the force sensor 5, which is connected to the computer, data on the actual values of the stress on the surface of the test body 3 are recorded.

Using the fracture detection sensor 6, which is connected to the computer, the moment of destructive failure of the test body 3 is recorded, and the electric motors 4, 4' ensuring the rotation drive are automatically stopped.

In an advantageous embodiment, after the destruction of the test body 3, the jaws 2, 2' are actively braked by means of the braking mechanism of the electric motor in order to prevent their rotation due to inertia.

In an advantageous embodiment, when the jaws 2, 2' are motorized and adjustable, it is possible to change the value of the bending stress during the test just by changing the position of the jaws and their mutual angle without interrupting the testing process.

Example 1

The method of carrying out the rotational bending fatigue test, according to the present invention, is performed on the test body 3, which is defined by its size of the cross-sectional diameter (diameter of the wire), the production radius of the longitudinal axis (curvature of the wire) and the modulus of elasticity, for example on the test body, which is wire with a diameter of 0.55 mm, a production radius of 200 mm and a modulus of elasticity of 210 GPa. The production radius of the wire is the radius of the wire in a free, unloaded state.

From these parameters and the required test stress, the test radius is calculated, and then, according to the required test length, the appropriate angle of the axes of the jaws 2, 2' is determined, and then the exact test length of the sample 3 and the distance between the jaws 2, 2' ensuring an even distribution of the bending stress after the entire length of the sample 3.

The specified bending stress for testing the wire is 790.9 MPa.

The voltage level is chosen depending on the expected fatigue life.

According to the equation R = E l _x / W _o / (σ — (E l _x / R _o / W _o )), the calculated test radius of the wire is R = 115 mm.

E modulus of elasticity E = 210,000 MPa l _x moment of inertia l _x = 0.004 mm ⁴

W _o cross-sectional modulus W„ = 0.016 mm ³

Ro production radius of the wire Ro = 200 mm o bending stress o = 790.9 MPa

For the above-selected test stress of 790.9 MPa (corresponding to a test radius of 115 mm), the following clamping geometries can be chosen as an example to achieve different test lengths:

- angle 90°, jaw distance 162.6 mm, test length 180.6 mm (fig. 3),

- angle 120°, jaw distance 199.2 mm, test length 240.9 mm (fig. 7), — angle 180°, jaw distance 230 mm, test length 361.3 mm (fig. 8),

- angle 270°, jaw distance 162.6 mm, test length 541.9 mm (fig. 9).

The machine is then started and cycling begins, which is of the order of 10 ³ to 10 ⁷ cycles, until destruction of the test specimen occurs.

Table 1 shows the test parameters for a clamping angle of 90°, a jaw distance of 162.6 mm, and a test length of 180.6 mm.

SK 125-2023 A3

Table 1

Geometry of the sample Cross-section diameter d [mm] 0.550 Cross-sectional area A [mm ² ] 0.238 Sectional module S [mm ³ ] 0.016 Moment of inertia 1 [mm ⁴ ] 0.004 Test radius R [mm] 115.0 The angle of the arc section of the test length and[°] 90.0 Trial length L [mm] 180.6 The distance between the ends of the test length (collets) B [mm] 162.6 Arching of the test length C [mm] 33.7 Production radius Ro [mm] 200.0 Cutting angle shr. length of the free sample ao [°] 51.8 Spacing of the free ends of the test length Bo [mm] 174.6 Bending moments in the plane Ro Modulus of elasticity E [MPa] 210 000 Moment for straightening Ro Mi [Nmm] 4,716 Moment for bending from a straight line to R M ₂ [Nmm] 8,202 Clamping moment M _min [Nmm] 3,486 Moment when turning 180° M _max [Nmm] 12,919 Surface tensile stresses on the inner fiber Ro Voltage during straightening R _o Oíi [MPa] 288.8 Bending stress from a straight line to R o _i2 [MPa] -502.2 Clamping tension OiMin [MPa] -213.4 Voltage when rotated 180° OiMax [MPa] 790.9 Surface tensile stresses on the outer fiber R _o Voltage during straightening R _o □oi [MPa] -288.8 Bending stress from a straight line to R Oo2 [MPa] 502.2 Clamping tension OoMax [MPa] 213.4 Voltage when rotated 180° □oMin [MPa] -790.9 Power to the sensor Maximum power Fmax [N] 0.3835 Minimum force Fmin [N] 0.1035

Example 2

In another example, for a wire with a diameter of 0.55 mm, a modulus of elasticity of 210 GPa and a production radius of 230 mm, a test stress of 636.1 MPa corresponding to a test radius of 150 mm is selected. A jaw angle of 120° is selected and the calculated test length is 314.2 mm and the jaw distance is 259.8 mm, as shown in Figure 10.11.

The test is carried out with the parameters listed in Table 2.

SK 125-2023 A3

Table 2

Geometry of the sample Cross-section diameter d [mm] 0.550 Cross-sectional area A [mm ² ] 0.238 Sectional module S [mm ³ ] 0.016 Moment of inertia 1 [mm ⁴ ] 0.004 Test radius R [mm] 150.0 The angle of the arc section of the test length and ['] 120.0 Trial length L [mm] 314.2 The distance between the ends of the test length (collets) B [mm] 259.8 Arching of the test length C [mm] 75.0 Production radius Ro [mm] 230.0 Cutting angle shr. length of the free sample oh] 78.3 Spacing of the free ends of the test length Bo [mm] 290.3 Bending moments in the plane Ro Modulus of elasticity E [MPa] 210,000 Moment for straightening Ro Mi [Nmm] 4,101 Moment for bending from a straight line to R M ₂ [Nmm] 6,289 Clamping moment M _min [Nmm] 2,187 Moment when turning 180° M _max [Nmm] 10,390 Surface tensile stresses on the inner fiber R _o Voltage during straightening R _o Oíi [MPa] 251.1 Bending stress from a straight line to R O _i2 [MPa] -385.0 Clamping tension OiMin [MPa] -133.9 Voltage when rotated 180° □iMax [MPa] 636.1 Surface tensile stresses on the outer fiber Ro Voltage during straightening R _o Ooi [MPa] -251.1 Bending stress from a straight line to R □o2 [MPa] 385.0 Clamping tension OoMax [MPa] 133.9 Voltage when rotated 180° OoMin [MPa] -636.1 Power to the sensor Maximum power Fmax [N] 0.1385 Minimum force Fmin [N] 0.0292

Industrial applicability

The method of carrying out the bending and rotation fatigue test according to the present invention, based on the principle of computer-controlled cyclic loading and software evaluation of measured values, can be used in testing laboratories and scientific workplaces performing destructive testing 10 to obtain information about the mechanical properties of materials.

SK 125-2023 A3

List of relationship tags

1 2, 2' 5 3 4, 4' 5 6 machine frame jaw test body electric motor force sensor fracture detection sensor

Claims (5)

1. The method of carrying out the fatigue test by bending while rotating, based on the principle of computer-controlled cyclic loading and software evaluation of the measured values, characterized by the fact that the jaws (2,2 ¹ ), which are fixed on the frame (l), are on both at the ends, at least one test body (3) is clamped, which has a circular cross-section, where the tangent of its axis at the point of clamping is co-axial with the axis of the clamping jaw (2, 2 ¹ ), and the mutual angle of the vectors of the axes of the jaws (2, 2 ¹ ) is 0 - 270°, while the test body (3) is loaded with bending stress, and then the electric motors (4, 4') are started, which start the jaws (2, 2 ¹ ), whose movement is mutually synchronized, while the test body (3) it starts to rotate around its axis in the number of cycles from 10 ³ to 10 ⁹ , when the load data is recorded using the force sensor (5). 2. The method of carrying out the fatigue test by bending under rotation according to claim 1, characterized in that the fracture detection of the test body (3) is performed using a sensor (6) for fracture detection. 3. The method of carrying out the fatigue test by bending while rotating according to claim 2, characterized in that after the fracture is detected, the rotation of the test body (3) is stopped at an accelerated rate by active braking of the electric motors (4, 4'). 4. The method of carrying out the fatigue test by bending under rotation according to one of claims 1 to 3, characterized in that the mutual angle of the vectors of the axes of the jaws (2, 2 ¹ ) is 30 - 180°. 5. The method of carrying out the fatigue test by bending under rotation according to one of the claims 1 to 4, characterized in that the axes of the jaws (2,2 ¹ ) together with the longitudinal axis of the clamped test body (3) lie in a vertical plane.