Turbine blade inspection method

Abstract

The present application relates to a turbine blade inspection method for inspecting a thermal barrier coating applied to a narrow section of a turbine blade. This turbine blade inspection method comprises determining an inspection position in the narrow section, emitting terahertz waves to the narrow section using a measurement head installed at the inspection position, and detecting reflected waves of the terahertz waves. The film thickness of the thermal barrier coating is calculated on the basis of the detected reflected waves.

Description

Turbine Blade Inspection Method

[0001] The present disclosure relates to a turbine blade inspection method. This application claims priority based on Japanese Patent Application No. 2024-190203 filed with the Japan Patent Office on October 30, 2024, and incorporates its content herein.

[0002] In a gas turbine, a turbine blade provided on a rotor receives high-temperature and high-pressure combustion gas as a working fluid, and the rotor is rotationally driven thereby. At this time, since the turbine blade is exposed to a harsh environment of high temperature and high pressure, in order to protect the turbine blade, for example, a thermal barrier coating (TBC: Thermal Barrier Coating) may be applied to the surface of the turbine blade.

[0003] For a turbine blade with a thermal barrier coating applied to its surface as described above, an inspection for measuring the film thickness of the thermal barrier coating may be performed for quality control. For example, Patent Document 1 discloses a technique for measuring the film thickness of a thermal barrier coating using an eddy current testing method (ECT: Eddy Current Testing).

[0004] Japanese Patent Application Laid-Open No. 2024-17286

[0005] In the eddy current testing method as in Patent Document 1 above, by energizing a coil of a probe, an eddy current is generated on the surface of the turbine blade to be inspected by electromagnetic induction, and the film thickness of the thermal barrier coating can be measured by converting the voltage change due to the eddy current that changes according to the distance between the coil and the base material into the film thickness. Such an eddy current testing method is suitable when the surface shape of the base material is relatively flat. However, for example, when measuring a portion (narrow part) having a narrow shape such as an R shape or a chamfered C surface, the eddy current generated by the coil is affected by the shape of the base material, and the measurement accuracy decreases.

[0006] At least one embodiment of the present disclosure has been made in view of the above circumstances, and an object thereof is to provide a turbine blade inspection method capable of accurately measuring the film thickness of a thermal barrier coating applied to a narrow part of a turbine blade.

[0007] A turbine blade inspection method according to at least one embodiment of the present disclosure is a turbine blade inspection method for inspecting a heat-shielding coating applied to a narrow portion of a turbine blade in order to solve the above problems, comprising: an inspection position determination step for determining an inspection position in the narrow portion; an emission step for emitting terahertz waves to the narrow portion using a measuring head installed at the inspection position; a reflected wave detection step for detecting reflected waves of the terahertz waves using the measuring head; and a film thickness calculation step for calculating the film thickness of the heat-shielding coating based on the reflected waves.

[0008] According to at least one embodiment of this disclosure, a turbine blade inspection method is provided that can accurately measure the film thickness of a heat-shielding coating applied to a narrow portion of a turbine blade.

[0009] This is a perspective view showing a turbine blade according to one embodiment. This figure schematically shows how the gas turbine inspection method according to at least one embodiment of this disclosure is performed on a narrow portion of the turbine blade shown in Figure 1. This is an enlarged view of area A in Figure 1. This is an enlarged view of area B in Figure 1. This is an enlarged view of area C in Figure 1. This is a schematic diagram showing the principle of measuring the thickness of the heat-shielding coating by the measuring head in Figure 2. This is a flowchart of the turbine blade inspection method according to one embodiment. This is a schematic diagram showing how the inspection position is marked on the narrow portion in step S2 of Figure 5. This is a schematic diagram showing the auxiliary tool that has been temporarily positioned in step S3 of Figure 5. This is a schematic diagram showing how the tip of the measuring head is inserted in the cross section along line D-D in Figure 7A. This is a schematic diagram showing the state in which the measuring head is in the first position in step S4 of Figure 5. This is a diagram showing the measurement results of the first and second reflected waves detected by the measuring head in the state of Figure 8A. This is a schematic diagram showing the state in which the measuring head is in the second position in step S4 of Figure 5. This is a diagram showing the measurement results of the first and second reflected waves detected by the measuring head in the state of Figure 9A. This is a schematic diagram showing how the installation angle of the measuring head is changed in step S5 of Figure 5.

[0010] Hereinafter, several embodiments of the present invention will be described with reference to the attached drawings. However, the configurations described as embodiments or shown in the drawings are not intended to limit the scope of the present invention, but are merely illustrative examples.

[0011] First, with reference to Figure 1, the turbine blade 1, which is the object of inspection for the turbine blade inspection method according to at least one embodiment of this disclosure, will be described. Figure 1 is a perspective view showing the turbine blade 1 according to one embodiment, and Figure 2 is a schematic diagram showing how the gas turbine inspection method according to at least one embodiment of this disclosure is performed on the narrow portion 5 of the turbine blade 1 in Figure 1.

[0012] The turbine blade 1 is a moving or stationary blade mounted radially upright on a rotor (not shown) that extends along the axial direction of a gas turbine, for example. The turbine blade 1 has a blade length direction that extends from the blade root 2 to the blade tip 3 along the radial direction of the rotor. The turbine blade 1 is also configured such that the blade thickness decreases as it approaches the edge along the blade width direction.

[0013] As shown in Figure 2, the turbine blade 1 uses a metal material as the base material 6, and a thermal barrier coating 4 (TBC: Thermal Barrier Coating) is applied to its surface to a predetermined thickness. The thermal barrier coating 4 is, for example, made by applying an MCrAlY (where M is Co, Ni, CoNi, etc.) alloy, which has excellent oxidation resistance, as a bond coat, and then covering it with a ZrO2-based ceramic with low thermal conductivity as a top coat.

[0014] A turbine blade 1 having such a configuration has a constricted portion 5. The constricted portion 5 is any part of the turbine blade 1 that includes a non-flat characteristic shape, and is the part where the eddy currents generated by the energized coil are affected by the constricted shape when eddy current testing is applied to measure the thickness of the heat shield coating 4. Quantitatively speaking, the constricted portion 5 is the part of the turbine blade 1 surface that includes, for example, a range of 10 mm from the point of maximum curvature.

[0015] Figure 2 shows an example of a narrow section 5, which includes two corners 6a and 6b as points of maximum curvature. The inventors of this application have found that if eddy current testing is used to measure the film thickness of the heat-shielding coating 4 in such a narrow section 5, the eddy currents generated by the coil are affected by the narrow shape of the base material 6, resulting in a large measurement error. This problem can be suitably solved by the turbine blade inspection method described later, which utilizes terahertz waves (THW). In this specification, terahertz waves refer to electromagnetic waves in the frequency range of 0.1 to 10 THz (wavelength range of 3 mm to 30 μm).

[0016] Figures 3A to 3C show other configuration examples of the narrow portion 5, and the turbine blade inspection method according to at least one embodiment of this disclosure is also applicable to these narrow portions 5. Figure 3A is an enlarged view of area A in Figure 1, Figure 3B is an enlarged view of area B in Figure 1, and Figure 3C is an enlarged view of area C in Figure 1.

[0017] For example, Figure 3A shows the tip C surface on the ventral or dorsal side of the turbine blade 1. The tip C surface has two corners 6c and 6d as points of maximum curvature, and the narrowed portion 5 extends 10 mm from each of these corners. Figure 3B shows the PF C surface as another example of a narrowed portion 5. The PF C surface has two corners 6e and 6f as points of maximum curvature, and the narrowed portion 5 extends 10 mm from each of these corners. Figure 3C shows the T / E section as yet another example of a narrowed portion 5. The T / E section has an R section as a point of maximum curvature, and the narrowed portion 5 extends 10 mm from the intersection point 6g with the camber line.

[0018] In the turbine blade inspection method according to at least one embodiment of the present disclosure, as shown in Figure 2, a measuring head 10 is used to emit a terahertz wave THW towards the narrow portion 5, and a reflected wave RW from the narrow portion 5 is detected.

[0019] Here, Figure 4 is a schematic diagram illustrating the principle of measuring the film thickness of the heat-shielding coating 4 using the measuring head 10 shown in Figure 2. Terahertz waves THW emitted from the measuring head 10 toward the narrow section 5 are reflected by the surface of the heat-shielding coating 4 or the base material 6 in the narrow section 5, and the reflected waves RW are detected by the measuring head 10. These reflected waves RW include a first reflected wave RW1 reflected by the surface of the base material 6, and a second reflected wave RW2 from the surface of the heat-shielding coating 4.

[0020] The measuring head 10 identifies the arrival times of the first reflected wave RW1 and the second reflected wave RW2 from the detected reflected wave RW, and calculates the film thickness of the heat-shielding coating 4 based on the difference in their arrival times Δt. This conversion calculation from the arrival time difference Δt to the film thickness of the heat-shielding coating 4 may be performed by a calculation device (not shown) to which the measuring head 10 is connected.

[0021] Next, a turbine blade inspection method according to at least one embodiment of this disclosure will be described in more detail. Figure 5 is a flowchart of the turbine blade inspection method according to one embodiment.

[0022] First, the turbine blade 1 to be inspected is installed (step S1). In step S1, for example as shown in Figure 1, the turbine blade 1 is installed in a stable position by clamping the blade root 2 with the blade root 2 facing downwards.

[0023] Next, the inspection position Pk is marked on the narrow section 5 of the turbine blade 1 installed in step S1 (step S2). The inspection position Pk is a position within the narrow section 5 that is predetermined to be used for measuring the thickness of the heat-shielding coating 4. More specifically, it is the position where the auxiliary device 20 for holding the measuring head 10 is temporarily installed in the temporary positioning step (step S3) described later.

[0024] Figure 6 is a schematic diagram showing how inspection positions Pk are marked on the narrow section 5 in step S2 of Figure 5. In step S2, a template TP having a shape corresponding to the narrow section 5 to be inspected is prepared. Markers M1 to M3 corresponding to inspection positions Pk are formed in advance on this template TP, and with the template TP positioned to correspond to the narrow section 5 (Figure 6 illustrates a state where the long axis of the template TP is aligned with the longitudinal direction of the narrow section 5), inspection positions Pk1 to Pk3 corresponding to each marker M1 to M3 are drawn as ruled lines near the narrow section 5.

[0025] In step S2, instead of directly marking the inspection positions Pk corresponding to the markers M1 to M3 on the narrow section 5, a template TP with the markers M1 to M3 formed on a transparent substrate may be prepared, for example, and the template TP may be attached to the corresponding position near the narrow section 5. In this case, it becomes unnecessary to mark the inspection positions Pk around the narrow section 5 to be inspected, thus preventing contamination of the inspected object.

[0026] Next, using the inspection position Pk marked in step S2 as a reference, the auxiliary tool 20 for holding the measuring head 10 is temporarily positioned relative to the narrow section 5 (step S3). Here, Figure 7A is a schematic diagram showing the auxiliary tool 20 temporarily positioned in step S3 of Figure 5, and Figure 7B is a schematic diagram showing the tip 12 of the measuring head 10 inserted in the cross section along line D-D in Figure 7A.

[0027] The auxiliary device 20 has an opening 22 that penetrates along the thickness direction (vertical direction in Figure 7B) when installed in the narrow section 5. The opening 22 has a substantially constant diameter and is formed to be slightly larger than the tip 12 of the measuring head 10. When the tip 12 of the measuring head 10 is inserted into the opening 22, the orientation of the measuring head 10 can be rotated and adjusted with respect to the tip 12 (see Figure 10).

[0028] On the outward-facing surface (shown in Figure 7A) of the auxiliary device 20 installed in the narrow section 5, a first line L1 and a second line L2 are formed, which are perpendicular to each other. In step S3, the first line L1 is provisionally positioned to correspond to the inspection position Pk marked in step S2, and the second line L2 is provisionally positioned to correspond to the virtual center line C of the narrow section 5 (Figure 7A illustrates the case where the device is provisionally positioned at inspection position Pk2 among several inspection positions Pk).

[0029] Next, with the tip 12 of the measuring head 10 inserted into the opening 22 of the auxiliary tool 20 that was temporarily positioned in step S3, a preliminary inspection is performed to determine the inspection position while moving the auxiliary tool 20 along the surface of the narrow section 5. During the preliminary inspection, while moving the auxiliary tool 20 in this manner, the first reflected wave RW1 from the base material 6 is monitored when a terahertz wave THW is emitted using the measuring head 10. As a result, the inspection position is determined as the range in which the peak value of the first reflected wave RW1 from the base material 6 is greater than or equal to the reference value (step S4).

[0030] Figure 8A is a schematic diagram showing the state in step S4 of Figure 5 where the measuring head 10 is in the first position, and Figure 8B is a diagram showing the measurement results of the first reflected wave RW1 and the second reflected wave RW2 detected by the measuring head 10 in the state shown in Figure 8A.

[0031] In the first position shown in Figure 8A, if the terahertz wave THW emitted from the measuring head 10 hits the inclined surface of the narrow section 5, the first reflected wave RW1 will be directed in a direction different from the incident direction of the terahertz wave THW, and the peak value of the first reflected wave RW1 detected by the measuring head 10 will be less than the reference value. In this case, as shown in Figure 8B, the reflected wave RW detected by the measuring head 10 does not contain a balanced amount of the first reflected wave RW1 and the second reflected wave RW2 necessary for calculating the thickness of the heat-shielding coating 4 (because the arrival time difference Δt cannot be accurately determined due to the first reflected wave RW1 being in an unmeasurable range), and the accuracy of measuring the thickness of the heat-shielding coating 4 decreases.

[0032] On the other hand, Figure 9A is a schematic diagram showing the state in which the measuring head 10 is in the second position in step S4 of Figure 5, and Figure 9B is a diagram showing the measurement results of the first reflected wave RW1 and the second reflected wave RW2 detected by the measuring head 10 in the state of Figure 9A.

[0033] In the second position shown in Figure 9A, when the terahertz wave THW emitted from the measuring head 10 strikes the flat surface of the narrow section 5, the first reflected wave RW1 is directed back towards the measuring head 10, and the peak value of the first reflected wave RW1 detected by the measuring head 10 is greater than or equal to the reference value. In this case, as shown in Figure 9B, the reflected wave RW detected by the measuring head 10 contains a balanced amount of the first reflected wave RW1 and the second reflected wave RW2 necessary for calculating the film thickness of the heat-shielding coating 4, enabling accurate measurement of the film thickness of the heat-shielding coating 4.

[0034] In the turbine blade 1, the heat-shielding coating 4 applied to the base material 6 may have its surface polished or otherwise treated, resulting in the surface of the base material 6 and the surface of the heat-shielding coating 4 not being perfectly parallel to each other, and the thickness of the heat-shielding coating 4 may vary depending on the inspection position. In step S4, by determining the inspection position to be within a range where the peak value of the first reflected wave RW1 from the base material 6 is relatively large, the accuracy of the thickness inspection can be suitably improved.

[0035] Next, the installation angle of the measuring head 10 is determined at the inspection position determined in step S4 (step S5). In step S5, with the position of the auxiliary device 20 relative to the narrow section 5 fixed at the inspection position determined in step S4, a preliminary inspection is performed while changing the angle of the measuring head 10 to find an installation angle of the measuring head 10 suitable for the inspection.

[0036] Here, Figure 10 is a schematic diagram showing how the installation angle of the measuring head 10 is changed in step S5 of Figure 5. As mentioned above, the opening 22 provided in the auxiliary device 20 is formed to be slightly larger than the tip 12 of the measuring head 10, so that, as shown in Figure 10, the measuring head 10 can rotate with the tip 12 as the reference point while the tip 12 is inserted into the opening 22. In step S5, while changing the installation angle of the measuring head 10 in this way, the reflected wave RW generated by the terahertz wave THW emitted from the measuring head 10 is detected, and the installation angle in which the peak value of the first reflected wave RW1 included in the reflected wave RW is maximized is identified.

[0037] As mentioned above, the heat-shielding coating 4 applied to the turbine blade 1 may have its surface polished or otherwise treated, resulting in the surface of the base material 6 and the surface of the heat-shielding coating 4 not being perfectly parallel to each other, and the thickness of the heat-shielding coating 4 may vary depending on the inspection position. Therefore, by changing the installation angle of the measuring head 10 and identifying the angle at which the peak value of the first reflected wave RW1 is maximized, the angle at which the thickness of the heat-shielding coating 4 is minimized at that inspection position can be identified.

[0038] Next, at the inspection position determined in step S4, terahertz waves are irradiated using the measuring head 10 with the installation angle determined in step S5 (step S6), and the reflected wave RW is detected (step S7). Based on the reflected wave RW detected in step S7, the film thickness of the heat-shielding coating 4 in the narrow section 5 is calculated (step S8). Specifically, the film thickness of the heat-shielding coating 4 is calculated based on the arrival time difference Δt between the first reflected wave RW1 and the second reflected wave RW2 included in the reflected wave RW detected in step S7.

[0039] As described above, according to the above embodiment, a terahertz wave THW is emitted from the narrow portion 5 of the turbine blade 1 using the measuring head 10, and its reflected wave RW is detected. Based on the reflected wave RW of the terahertz wave THW detected in this way, the thickness of the heat-shielding coating 4 applied to the narrow portion 5 of the turbine blade 1 can be measured with high accuracy.

[0040] In addition, within the scope not departing from the gist of the present disclosure, it is possible to appropriately replace the components in the above-described embodiments with well-known components, and the above-described embodiments may be appropriately combined.

[0041] The content described in each of the above embodiments is understood as follows, for example.

[0042] (1) A turbine blade inspection method according to one aspect is a turbine blade inspection method for inspecting a heat insulation coating applied to a narrow part of a turbine blade, including an inspection position determination step for determining an inspection position in the narrow part, an emission step of emitting terahertz waves to the narrow part using a measurement head installed at the inspection position, a reflected wave detection step of detecting a reflected wave of the terahertz waves using the measurement head, and a film thickness calculation step of calculating the film thickness of the heat insulation coating based on the reflected wave. [[ID=:7]]

[0043] According to the aspect of (1) above, for the narrow part of the turbine blade, terahertz waves are emitted using a measurement head, and the reflected wave thereof is detected. Based on the reflected wave of the terahertz waves detected in this way, the film thickness of the heat insulation coating applied to the narrow part of the turbine blade can be accurately measured.

[0044] (2) In another aspect, in the aspect of (1) above, the reflected wave includes a first reflected wave from the surface of the base material of the turbine blade and a second reflected wave from the surface of the heat insulation coating, and in the film thickness measurement step, the film thickness is calculated based on the arrival time difference between the first reflected wave and the second reflected wave at the measurement head.

[0045] According to the aspect of (2) above, based on the arrival time difference between the first reflected wave and the second reflected wave at the measurement head, which are included in the reflected wave detected when terahertz waves are emitted to the turbine blade, the film thickness of the heat insulation coating in the narrow part can be accurately measured.

[0046] (3) In another aspect, in the aspect of (2) above, in the inspection position determination step, when the measurement head is moved along the surface of the narrow portion and the terahertz wave is emitted using the measurement head, the inspection position is determined as the range in which the peak value of the first reflected wave detected by the measurement head is equal to or greater than a reference value.

[0047] According to the aspect of (3) above, the inspection position where the measurement head for emitting the terahertz wave and detecting the reflected wave is installed is specified as the range in which the peak value of the first reflected wave from the base material is equal to or greater than the reference value when the terahertz wave is emitted while moving the measurement head along the surface of the narrow portion. The heat insulation coating applied to the turbine blade may have a surface treatment such as polishing, so that the surface of the base material and the surface of the heat insulation coating are not necessarily parallel to each other, and the film thickness of the heat insulation coating may vary depending on the position. In this aspect, by determining the range where the peak value of the first reflected wave from the base material is relatively large as the inspection position, the reflected wave from the narrow portion with respect to the terahertz wave emitted from the measurement head can be preferably detected, and the accuracy of the film thickness measurement of the heat insulation coating can be preferably improved.

[0048] (4) In another aspect, in the aspect of (3) above, in the inspection position determination step, an auxiliary tool is installed on the surface of the narrow portion, and the measurement head is moved along the surface of the narrow portion with the tip of the measurement head inserted into the opening of the auxiliary tool.

[0049] According to the aspect of (4) above, the movement of the measurement head when determining the inspection position is performed with the tip of the measurement head inserted into the opening of the auxiliary tool installed on the surface of the narrow portion. Thereby, the inspection position can be determined in a state where the posture of the measurement head with respect to the narrow portion to be inspected is stabilized.

[0050] (5) In another aspect, in any one of the aspects (2) to (4) above, in the inspection position determination step, when the incident angle of the terahertz wave with respect to the surface of the narrow portion is changed, the installation angle of the measurement head at the inspection position is determined so that the peak value of the first reflected wave detected by the measurement head becomes maximum.

[0051] According to the embodiment of (5) above, the installation angle of the measuring head at the inspection position is determined such that the peak value of the first reflected wave from the base material is maximized when the incident angle of the terahertz wave is changed. As mentioned above, the heat-shielding coating applied to the turbine blade may have surfaces that are not parallel to each other due to polishing or other treatments performed on its surface. In this embodiment, by determining the installation angle of the measuring head such that the peak value of the first reflected wave from the base material is maximized, it is possible to identify the orientation of the measuring head that allows for the appropriate detection of reflected waves necessary for measuring the thickness of the heat-shielding coating in a confined space.

[0052] (6) In another embodiment, in the embodiment of (5) above, in the inspection position determination step, an auxiliary device is installed on the surface of the narrow portion, and the incident angle is changed by changing the installation angle of the measuring head while the tip of the measuring head is inserted into the opening of the auxiliary device.

[0053] According to the embodiment of (6) above, by inserting the tip of the measuring head into the opening of an auxiliary device installed on the surface of the narrow part and changing the installation angle of the measuring head, the installation position of the measuring head relative to the narrow part can be stably maintained while suitably searching for the incident angle of the terahertz wave that maximizes the peak value of the first reflected wave from the base material.

[0054] (7) In other embodiments, in any one of the embodiments of (1) to (6) above, in the inspection position determination step, when an auxiliary tool having an opening into which the tip of the measuring head can be inserted is placed on the surface of the narrow portion, a provisional position is made such that a first line formed on the surface of the auxiliary tool corresponds to a marker formed in advance on the narrow portion, and a second line formed on the surface of the auxiliary tool and perpendicular to the first line corresponds to a virtual center line of the narrow portion.

[0055] According to the embodiment of (7) above, a measuring head used for measuring the thickness of the heat-shielding coating can be temporarily positioned in a simple and accurate manner relative to the narrow portion of the turbine blade using an auxiliary tool having mutually orthogonal first and second lines formed thereon.

[0056] (8) In other embodiments, in any one embodiment of (1) to (7) above, the narrow portion includes a range of 10 mm from the point of maximum curvature on the surface of the turbine blade.

[0057] When eddy current testing is used on the turbine blade surface, the measurement accuracy decreases in the 10 mm range from the point of maximum curvature because the eddy current used for measurement is affected by the shape of the base material in the narrow area. In the embodiment described in (8) above, the thickness of the heat-shielding coating can be measured with good accuracy in the narrow area where the measurement accuracy decreases with eddy current testing.

[0058] 1 Turbine blade 2 Blade root 3 Blade tip 4 Heat shield coating 5 Narrow section 6 Base material 10 Measuring head 12 Tip 20 Auxiliary tool 22 Opening C Virtual centerline L1 First line L2 Second line M1-M3 Marker Pk Inspection position THW Terahertz wave RW Reflected wave RW1 First reflected wave RW2 Second reflected wave TP Template

Claims

1. A turbine blade inspection method for inspecting a heat-shielding coating applied to a narrow portion of a turbine blade, comprising: an inspection position determination step for determining an inspection position in the narrow portion; an emission step for emitting terahertz waves into the narrow portion using a measuring head installed at the inspection position; a reflected wave detection step for detecting reflected waves of the terahertz waves using the measuring head; and a film thickness calculation step for calculating the film thickness of the heat-shielding coating based on the reflected waves.

2. The turbine blade inspection method according to claim 1, wherein the reflected wave includes a first reflected wave from the base material surface of the turbine blade and a second reflected wave from the surface of the heat-shielding coating, and in the film thickness measurement step, the film thickness is calculated based on the difference in arrival times of the first reflected wave and the second reflected wave at the measuring head.

3. The turbine blade inspection method according to claim 2, wherein in the inspection position determination step, the inspection position is determined as a range in which the peak value of the first reflected wave detected by the measuring head is equal to or greater than a reference value when the measuring head is moved along the surface of the narrow portion and the terahertz wave is emitted using the measuring head.

4. The turbine blade inspection method according to claim 3, wherein in the inspection position determination step, an auxiliary device is installed on the surface of the narrow portion, and the tip of the measuring head is inserted into the opening of the auxiliary device, and the measuring head is moved along the surface of the narrow portion.

5. The turbine blade inspection method according to claim 2, wherein in the inspection position determination step, the installation angle of the measuring head at the inspection position is determined such that the peak value of the first reflected wave detected by the measuring head is maximized when the incident angle of the terahertz wave to the surface of the narrow portion is changed.

6. The turbine blade inspection method according to claim 5, wherein in the inspection position determination step, an auxiliary device is installed on the surface of the narrow portion, and the incident angle is changed by changing the installation angle of the measuring head while the tip of the measuring head is inserted into the opening of the auxiliary device.

7. The turbine blade inspection method according to claim 1 or 2, wherein in the inspection position determination step, when an auxiliary tool having an opening into which the tip of the measuring head can be inserted is placed on the surface of the narrow portion, a provisional position is performed such that a first line formed on the surface of the auxiliary tool corresponds to a marker previously formed on the narrow portion, and a second line formed on the surface of the auxiliary tool and perpendicular to the first line corresponds to a virtual center line of the narrow portion.

8. The turbine blade inspection method according to claim 1 or 2, wherein the narrow portion includes a range of 10 mm from the point of maximum curvature on the surface of the turbine blade.

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