Why Thin Titanium Plate Needs Guided-Wave Inspection
Conventional straight-beam ultrasonic examination becomes unreliable as plate thickness falls. At 4 mm thickness and a 5 MHz probe, the wavelength in titanium is roughly 1.2 mm, and the near field, the interface echo and the back-wall echo start to overlap. Lamb waves, the guided plate waves that form when the plate thickness is comparable to the wavelength, solve part of this problem because they propagate along the plate and interact with through-thickness defects along their whole path.
That advantage comes with a condition: guided waves are dispersive, meaning velocity depends on frequency and thickness, and they exist in several modes simultaneously. Selecting the right mode and the right excitation angle is therefore the core of the technique rather than a detail of it.
Dispersion, Modes and How They Are Excited
Lamb waves split into symmetric (S) and antisymmetric (A) families, with S0 and A0 as the fundamental modes and higher-order modes appearing above cut-off frequencies. Because the group velocity of each mode varies with the product of frequency and thickness, a group velocity dispersion curve is the starting point for any inspection design.
The mode generated is controlled by the angle at which the beam enters the plate through a wedge, following the phase velocity matching condition.
Changing the excitation angle changes both the mode and the vibration morphology of the plate, so the response to a fixed defect is not constant across angles.
Mode conversion, reflection and superposition mean that amplitude does not always decay monotonically with distance, and longitudinal and transverse components can interfere.
Because several modes can travel at once, a single measurement may contain more than one arrival, so the receiver must be able to separate them.
Phased array probes make the sweep practical. By delaying the excitation of successive elements, the array steers the beam electronically, so a single probe can generate different modes at a sequence of incidence angles without changing the wedge.
Designing the Angle Sweep for 4 mm Titanium Plate
A representative procedure uses a 2 mm diameter through-hole as the reference defect, positioned so that the echo received is the same from any inspection direction. The incidence angle is then treated as the only variable.
The amplitude of the echo from a fixed reference path, for example 50 mm from the plate end, is recorded first. Incidence angle is then stepped upward in 5 degree increments across the 10 to 75 degree range, and the through-hole is scanned at each step.
| Angle band | Observed behaviour on 4 mm Ti plate | Interpretation |
|---|---|---|
| 10 ° - 20 ° | Low defect resolution | Mode poorly matched to thickness |
| 25 ° - 35 ° | Lower instrument gain needed at equal wave height | Useful signal-to-noise ratio |
| 40 ° and 55 ° | Largest identification distance for the through-hole | Best defect detectability |
| 40 ° - 50 ° | Amplitude change with distance follows acoustic propagation law | Reliable quantitative behaviour |
| 60 ° - 75 ° | Irregular amplitude trend, multiple peaks | Mixed-mode interference, not reliable |
Two conclusions follow. First, an excitation angle close to 40 degrees, corresponding to a K1-type angle probe, gives the best compromise between defect resolution and signal-to-noise ratio, and is the recommended starting point for thin titanium alloy plate. Second, the fact that a low gain was needed at 25, 35 and 40 degrees indicates that the echo amplitude from the reference defect was comparatively strong in that band, which is what makes defect detection reliable at those angles.
Setting Up and Verifying the Phased Array System
Phased array results are only comparable between shifts if the equipment is characterised and the setup is documented. The relevant practices are ASTM E2491 for evaluating phased array instrument and array performance, ASTM E2700 for contact phased array examination, ASTM E1065 for search unit characterisation, and ASTM E317 for instrument performance. ASME Boiler and Pressure Vessel Code Section V Article 4 covers ultrasonic examination generally and Article 5 covers ultrasonic examination of welds where guided waves are used for weld-adjacent scanning.
Practical setup controls include confirming element activity and delay laws, recording the wedge angle and element pitch, verifying beam steering accuracy against a reference reflector, and establishing the DAC or TCG curve from the same reference geometry used in production. Probe frequency and element size are chosen so that the desired mode dominates: too high a frequency raises attenuation and mode complexity in thin plate, while too low a frequency degrades lateral resolution.
Reporting, Acceptance and Quality Control
Examination of wrought titanium plate is generally performed to ASTM E2375 for ultrasonic examination of wrought products, with acceptance classes agreed between buyer and supplier because the standard offers more than one severity level. Personnel should be qualified to ISO 9712 or an equivalent scheme at the relevant level.
Reporting for a guided-wave examination should state the mode used, the incidence angle, the probe characteristics, the reference reflector and its size, the scan plan, and the equipment calibration status. Because guided-wave signals can arise from non-relevant features such as edge reflections, mode conversions and thickness changes, any indication reported as a defect should be confirmed by a second method, typically contact pulse-echo or radiography, before disposition.
For production plate, the method is best applied as a screening tool on thin gauges where conventional examination struggles, followed by targeted confirmation. Its value lies in covering long lengths quickly from a limited number of access points, not in replacing conventional volumetric examination where thickness allows it.
Frequently Asked Questions
Q: What are Lamb waves in the context of plate inspection?
A: They are guided ultrasonic plate waves that form when the wavelength is comparable to the plate thickness. They travel along the plate rather than through it, so they can detect through-thickness defects over long distances from a single probe position.
Q: Which incidence angle should be used for thin titanium plate?
A: About 40 degrees, equivalent to a K1 angle probe, gave the best combination of defect identification distance and signal-to-noise ratio on 4 mm titanium alloy plate with a 2 mm through-hole reference defect.
Q: Why do results vary so much with angle?
A: Because the excitation angle determines which Lamb mode is generated and with what energy. Different modes interact differently with the same defect, and mode conversion plus interference between longitudinal and transverse components means amplitude does not decay in a simple way.
Q: Why use a phased array probe instead of a fixed angle wedge?
A: A phased array steers the beam electronically by delaying element excitation, so one probe can generate several modes and sweep a range of incidence angles without mechanical changes. This makes the angle sweep that selects the best mode practical.
Q: Which standards apply to phased array examination?
A: ASTM E2491 for instrument and array characterisation, ASTM E2700 for contact phased array examination, ASTM E1065 for probe characterisation, and ASTM E2375 for ultrasonic examination of wrought titanium products.
Q: Can Lamb waves replace conventional ultrasonic testing on plate?
A: Not normally. They are best used as a fast screening method on thin gauges where conventional straight-beam examination is difficult, with any significant indication confirmed by pulse-echo or radiographic examination.
