Methods and systems for ultrasonic rock bolt condition monitoring
A methods and systems for determining a change in condition of a rock bolt. Some methods may comprise, at a first point in time, propagating shear and longitudinal ultrasonic waves along the rock bolt to measure a first time of flight for each of the shear and longitudinal waves, at a second point in time after the first point in time, propagating shear and longitudinal ultrasonic waves along the rock bolt to measure a second time of flight for each of the shear and longitudinal waves, and using the relative changes of the first and second time of flights, determining the change in condition of the rock bolt section.
1. A method for determining a change in axial stress that a rock bolt is experiencing, the method comprising,
at a first point in time, propagating shear and longitudinal ultrasonic waves along the rock bolt section to measure a first time of flight for each of the shear and longitudinal waves;
at a second point in time after the first point in time, propagating shear and longitudinal ultrasonic waves along the rock bolt section to measure a second time of flight for each of the shear and longitudinal waves;
using the relative changes of the first and second time of flights, determining the change in axial stress,
wherein during the period from the first point and second point in time the rock bolt section is in an elastic deformation regime and determining a change in axial stress in a rock bolt section further comprises, using the first and second time of flights, determining the change in axial stress in the rock bolt section between the first and second points in time according to
〈
σ
e
〉
-
〈
σ
0
e
〉
=
β
T
τ
S
e
-
τ
S
0
e
τ
S
0
e
-
τ
L
e
-
τ
L
0
e
τ
L
0
e
β
T
C
σ
S
e
-
C
σ
L
e
,
where σ e − σ 0 e is the average change in axial stress between the first and second points in time, τ L0 e and τ S0 e are the time of flights of the longitudinal and shear ultrasonic waves, respectively, at the first point in time, τ L and τ S are the time of flights of the longitudinal and shear ultrasonic waves, respectively, at the second points in time, β T is a ratio of a temperature coefficient for the longitudinal ultrasonic waves to a temperature coefficient for the shear ultrasonic waves, and C σS e and C σL e are the stress coefficients for the shear and longitudinal ultrasonic waves, respectively.
2. The method of claim 1 , wherein the rock bolt section consists of the entire rock bolt.
3. The method of claim 1 , further comprising determining an arrival time of a longitudinal wave echo signal based on an arrival time of a corresponding shear ultrasonic wave echo.
4. The method of claim 1 , further comprising simultaneously exciting a first ultrasound transducer configured to emit shear ultrasonic waves at a first frequency and a second ultrasound transducer configured to emit longitudinal ultrasonic waves at a second frequency, wherein the first and second ultrasound transducers are arranged in a side-by-side configuration.
5. The method of claim 4 , wherein a polarization direction of the first ultrasound transducer is aligned with a preferred radial direction of the rock bolt section.
6. The method of claim 4 , wherein the first frequency and the second frequency are distinct enough that corresponding echoes are capable of being singled out using one or more band-pass filters.