Method and apparatus for fatigue crack detection and propagation analysis

Claim 2

A fatigue monitoring system in which a test specimen is subjected to repeated stresses comprising: means to support the test specimen to be monitored at one end region of said test specimen with the test specimen extending outward from the support means; means to apply a repeated stress to a region of said test specimen remote from said support means; said repeated stress being a rotational stress applied about an axis transverse to the axis of said specimen extending outward from said support means through said test specimen; and means to detect variations in the characteristics of said test specimen resulting from said stress.

Claim 3

The fatigue monitoring system of claim 1 wherein said means to detect variations in the characteristics of said test specimen includes the measurement of electrical resistance between 2 or more separated positions on said test specimen.

Claim 4

The fatigue monitoring system of claim 2 wherein the measurement of electrical resistance includes connecting the portion of said test specimen between said 2 or more separated positions on said test specimens in circuit with a bridge circuit.

Claim 5

The fatigue monitoring system of claim 3 wherein said test specimen includes a rounded section area intermediate of said 2 or more positions.

Claim 6

The fatigue monitoring system of claim 1 wherein said stress selectively includes an offset from the zero position assumed by said test specimen in the absence of said stress

Claim 7

The fatigue monitoring system of claim 5 wherein said stress is applied to said test specimen in a direction selected from the group consisting of one side of said zero position, the other side of said zero position, and both sides of said zero position

Claim 8

The fatigue monitoring system of claim 6 wherein the type of said repeated stress may be selected from the group consisting of oscillatory, random, impulse, and one derived from actual service, to most closely subject said test specimen to the stresses to be encountered in a particular application.

Claim 9

The fatigue monitoring system of claim I wherein the type of said repeated stress may be selected from the group consisting of oscillatory, random, impulse and one derived from actual service.

Claim 10

The fatigue monitoring system of claim 8 wherein said means to apply said repeated stress to said test specimen includes a direct current servo motor.

Claim 11

The fatigue monitoring system of claim 9 wherein said servo motor is driven by a direct current servo amplifier which provides one or more signals to provide the selected offset and the selected type of stress.

Claim 12

The fatigue monitoring system of claim 10 wherein said servo motor is connected in circuit with said servo amplifier without the intermediary of brushes and a commutator in said servo motor.

Claim 15

The method of fatigue analysis of claim 12 including the additional step of connecting a segment of said free end of said test specimen in a circuit to monitor changes in the resistance of said segment as an indication of fatigue in said test specimen.

Claim 16

The method of fatigue analysis of claim 13 including the additional step of providing a rounded indentation in an intermediate region of said segment of said free end of said test specimen.

Claim 17

The method of fatigue analysis of claim 12 including the additional step of coupling a servo motor to said segment to provide said rotational torque to said test specimen.

Claim 18

The method of fatigue analysis of claim 15 wherein said servo motor is a direct current servo motor, and including the additional step of providing power to said direct current servo motor through a servo amplifier, and providing one or more input signals to said servo amplifier including a selective offset to offset the application of said rotational torque.

Claim 19

The method of fatigue analysis of claim 16 including the additional step of providing as one of said one or more input signals a signal selected from the group consisting of oscillatory, random and impulse signals, and a signal derived from actual service.

Claim 20

The method of fatigue analysis of claim 12 including the additional step of determining the number of cycles of said torque at which the value of said electrical characteristic for repeated torque applications significantly increases as indicative of crack initiation in said test sample.

Claim 21

The method of fatigue analysis of claim 12 including the additional step of measuring the amplitude of the stress applied to said test sample, wherein a significant decrease in the stress amplitude is indicative of crack initiation in said test sample.

Claim 22

A fatigue monitoring system in which a specimen is subjected to repeated stresses comprising a holder to support a test specimen to be monitored at one end region of said specimen with the test specimen extending outward from said holder; means to apply a repeated stress to a region of said specimen remote from said holder; said repeated stress being a rotational stress applied about an axis transverse to the axis extending outward from said holder through said test specimen; a motor coupled to apply said stress to said test specimen; a servo amplifier driving said motor in response to one or more input signals; a bridge circuit to detect variations in the electrical characteristics of said test specimen resulting from said stress; and data acquisition and analysis circuitry to determine the initiation of cracks in said test specimen and to monitor the propagation of said cracks based on said variations in the electrical characteristics of said test specimen.

Claim 23

A fatigue monitoring system in which a specimen is subjected to repeated stresses comprising a holder to support a test specimen to be monitored at one end region of said specimen with the test specimen extending outward from said holder; means to apply a repeated stress to a region of said specimen remote from said holder; said repeated stress being a rotational stress applied about an axis transverse to the axis extending outward from said holder through said test specimen; a motor coupled to apply said stress to said test specimen; a servo amplifier driving said motor in response to one or more input signals; a bridge circuit to detect variations in the electrical characteristics of said test specimen resulting from said stress; and data acquisition and analysis circuitry to determine the initiation of cracks in said test specimen and to monitor the propagation of said cracks based on said variations in the electrical characteristics of said test specimen.

Claim 24

The fatigue monitoring system of claim 20 wherein a signal conditioning circuit provides said input signals including a selective offset and a signal selected from the group consisting of oscillatory, random, and impulse signals, and a signal derived from actual service.

Claim 25

The fatigue monitoring system of claim 21 wherein a transducer is connected between said motor and said signal conditioning circuit to provide a closed loop system and closed loop operation.

Claim 26

The fatigue monitoring system of claim 22 wherein a stress amplitude signal is provided to said data acquisition and analysis circuitry to indicate the formation and propagation of a fracture in said test specimen resulting from said stress.

Claim 27

The fatigue monitoring system of claim 20 wherein said motor is a direct current low inertia servo motor in which the field is wired to said servo amplifier without the intermediary of brushes and a commutator in said servo motor.

Claim 30

The method of fatigue analysis of claim 25 wherein said rotational torque is applied about 2 axes both of which are perpendicular to said axis of said test specimen.

Claim 31

The method of fatigue analysis of claim 26 wherein said rotational torques both stress the region of said test specimen between the axes of the rotational torques in the same direction and then in the opposite direction, said directions being substantially orthagonal to said 2 axes and to said axis of said test specimen.

Claim 32

The method of fatigue analysis of claim 26 including the additional step of connecting a segment of said region of said test specimen between the axes of said rotational torques in a circuit to monitor changes in the resistance of said segment as an indication of fatigue in said test specimen.

Claim 33

The method of fatigue analysis of claim 28 including the additional step of providing a rounded indentation in an intermediate region of said segment of said test specimen.

Claim 34

The method of fatigue analysis of claim 28 including the additional step of connecting a servo motor to provide each of said rotational torques to said test specimen.

Claim 35

The method of fatigue analysis of claim 30 wherein said servo motor is a direct current low inertia servo motor, and including the additional step of providing power to said direct current servo motor through a servo amplifier, and providing one or more input signals to said servo amplifier including a selective offset to offset the application of said rotational torque

Claim 36

The method of fatigue analysis of claim 31 including the additional step of providing as one of said one or more input signals a signal selected from the group consisting of oscillatory, random and impulse signals and a signal derived from actual service.

Claim 37

The method of fatigue analysis of claim 31 including the additional steps of generating a signal response to the stress applied to a member in actual service, and providing the generated signal as one of said one or more input signals to said servo amplifier.

Claim 38

The method of fatigue analysis of claim 33 including the additional steps of providing a transducer to sense the stress to which said member in actual service is subjected to, and generating an input signal responsive to the output of said transducer.

Claim 39

A fatigue monitoring system in which a specimen is subjected to repeated stresses comprising at least 2 couplers to support a specimen to be monitored with the specimen extending between said couplers; at least 2 servo motors connected through said couplers to apply repeated stress to said specimen; said repeated stress being rotational stresses applied about separated axes transverse to the axis of said specimen extending between said couplers; at least one servo amplifier driving said servo motors in response to at least one input signal; means to detect variations in the electrical characteristics of said specimen resulting from said repeated stress; and data circuitry to indicate the formation of cracks in said specimen and to monitor the propagation of said cracks based on said variations in the electrical characteristics of said specimen.

Claim 40

A fatigue monitoring system in which a specimen is subjected to repeated stresses comprising at least 2 couplers to support a specimen to be monitored with the specimen extending between said couplers; at least 2 servo motors connected through said couplers to apply repeated stress to said specimen; said repeated stress being rotational stresses applied about separated axes transverse to the axis of said specimen extending between said couplers; at least one servo amplifier driving said servo motors in response to at least one input signal; means to detect variations in the electrical characteristics of said specimen resulting from said repeated stress; and data circuitry to indicate the formation of cracks in said specimen and to monitor the propagation of said cracks based on said variations in the electrical characteristics of said specimen.

Claim 41

A fatigue monitoring circuit of claim 35 wherein one of said one or more input signals is a signal responsive to the stress applied to a member in actual service in a particular application.

Claim 42

A fatigue monitoring circuit of claim 35 wherein said input signals are in the frequency range of up to 350 Hertz.

Claim 43

A fatigue monitoring circuit of claim 37 wherein said input signals are in the order of 150 Hertz.

Claim 44

A fatigue monitoring circuit of claim 35 wherein said servo motors are synchronized to provide a uniform bending moment to said specimen first in one direction, and then in the opposite direction.

Claim 45

A fatigue monitoring circuit of claim 35 wherein said servo motors do not provide a uniform bending moment to said specimen.