Embedding eddy current sensors into LPBF components for structural health monitoring

If a coil inductor is powered with alternating current (AC), an alternating magnetic field is also generated. When the coil approaches a conductive material, EC are generated within the material due to the law of induction. These EC lead to the formation of a magnetic field that is counteracting the primary magnetic field of the coil, therefore affecting its equivalent impedance. The path and intensity of these circulating currents are conditioned by the material properties and the presence of defects such as cracks or porosity; a representation of the EC is schematically visualized in Fig. 2a. As in typical NDT usage, the impedance is expressed with real and imaginary part in normalized units and represented on the complex plane (see Fig. 2b). The characteristic curve depends on the material properties (conductivity σ, permeability µ) as well as on the sensor parameters (coil radius a, angular frequency ω). In Fig. 2b, it is also shown that the detection of a crack leads to a significantly different trajectory than the detection of a liftoff between sensor and material to be tested, which enables to distinguish among the interest signal features. A deeper insight to the functionality of EC sensors technology for NDT is given in [12].

For the experimental study, only the EC sensors are embedded in the LPBF parts, whereas the electronics for data acquisition and signal processing are positioned outside. Nevertheless, the integration of the electronics into the LPBF part could also be possible, as the necessary elements would fit in an extremely small volume, comparable to the size of the sensor itself. All these components were provided by Sensima inspection Sarl [14]. The instrument provides a direct readout of the real and imaginary parts related to the variation of the coil impedance, expressed in arbitrary units (a.u.), as typically used by NDT inspectors.

A Concept Laser m2-machine equipped with a Nd:YAG laser with 1064 nm wavelength and maximum power of 400 W operating in cw-mode was used for the manufacturing of the test specimens. The parts were manufactured from SS 316L powder with a standard process parameter set (P_Laser = 180 W, scan speed v_scan = 1350 mm/s, hatch distance h = 75 μm, layer thickness t = 30 μm, with O₂ atmosphere and without base plate heating) leading to relative part densities > 99.0% compared to the reference of 7.95 g/cm³ given in the certification of the supplier, Carpenter Powder Products. SS 316L is a beneficial material for the measurement of embedded cracks with EC, since the low magnetic permeability (µ_rel = 1.25 [15]) allows a larger penetration depth of the induced currents compared to materials with higher permeability. The embedded sensor configuration offers a sensitivity for crack detection up to 3.5 mm.

In Fig. 3, the cavity design for sensor embedding is schematically visualized. Figure 3a shows the CAD model used for the manufacturing of the component, while Fig. 3b and c schematically represent the demonstrator and in particular the direction of crack growth with respect to the position of the sensor. The EC sensor with a diameter of 3.3 mm is placed inside a cylindrical cavity with a diameter of 5.0 mm, i.e. there is a circular clearance of 0.85 mm between sensor and cavity to account for the roughness of the LPBF surfaces. The cavity is closed gently with an overhanging surface (see Fig. 3). Such structure has no influence on the EC measurements since the sensor is directional, i.e. the major region of sensitivity is oriented toward the location of crack initiation during the experiments. To achieve a proper positioning, the EC sensor is pressed to the bottom of the cavity and subsequently held in place by filling the entire cavity with a quick hardening resin (Plus 2-K-Epoxidkleber from UHU). While this process does not require any special attention, the outlet of the copper wires from the LPBF component needs to be designed properly. The wires forming the coils are only 60 µm in diameter, including an epoxy insulation to prevent a short circuit. Due to these small dimensions, for the use phase of the part the wires have to be protected from the sharp edges of the LPBF component. Hence, a heat shrink tube is inserted into the horizontally orientated cable channel preventing any contact between the wires and the LPBF part. In Fig. 4, the sensor integration process is shown. The epoxy insulation of the copper wires also limits the maximum temperature the sensor system can withstand, as at temperatures above 250 °C the insulation would melt, resulting in a short circuit between the wires and the LPBF component. The continuation of the LPBF process does not require any special precautions. It is expected that there is an impact of the interruption of the process on the structural integrity of the component. However, this field of research was not in the scope of this work focusing on the process of sensor embedding and sensor survivability.

After the LPBF process, the fine copper wires are soldered to shielded twisted pair cables suitable for the interaction with the data processing unit. In addition, to prevent a rupture of either the thin copper wires or the soldering region, all is encapsulated and protected by a layer of hot-melt adhesive, thus ensuring a good mechanical fixation also against any vibration. Figure 4c shows an LPBF test part with an embedded EC sensor ready to be tested.

In this setup, the crack that is propagating towards the embedded sensor and is detected by the sensor is generated by a wire EDM cut. The machine for the tests is an Agie Charmilles Technologies type Robofil 4030SI-TW with a wire of 250 µm diameter. Although this dimension deviates significantly from a typical crack width that the embedded EC sensors should detect, it perfectly fulfils the requirements for the experiment. Generally, the width of the crack to be detected is not that important since it is more than an order of magnitude smaller than the diameter of the coil. Although the wire-cut EDM machine provides information on the current position of the wire, the data acquisition has to be done in variable discrete steps from 50 to 100 µm during the pausing of the cutting. Otherwise, the very strong currents of the wire-EDM process significantly disturb the EC sensor signal. The specimens were cut until the wire was reaching the sensor surface, which cannot be cut as it is from insulating material. This condition, verified visually by the operator, was used to determine the effective crack length. Unfortunately, this procedure suffers from uncertainty, estimated to ± 0.25 mm, due to the difficulties in ensuring the parallelism of the specimen surface with respect to the cutting axis.

In the second experiment, EC sensors were embedded in CT specimens usually used for crack propagation analysis in material characterization. Hence, this setup is distinctly closer to real crack growth behavior. The general design and dimensions of a CT specimen according to ASTM E1681 [16] are depicted in Fig. 5a. Z indicates the LPBF build-up direction and the sensor within the cavity is also shown. With LPBF, merely blocks were manufactured, while the final geometry was realized by wire-EDM. In the test cycle, the CT specimens were cyclically loaded to initiate and propagate a crack. The machine used for the tests is a Rumul Testronic 100 kN from Russenberger Prüfmaschinen AG. During the measurements, it operated at a maximum frequency of 55.6 Hz and a maximum load of 6.11 kN. In Fig. 5b, the propagating crack is clearly visible. As for the wire-EDM process, the data points were measured discretely, regularly stopping the machine. The step size was defined by a frequency drop of 1.0 Hz, which is a machine-specific value and which was upfront correlated to a crack propagation of ≈1 mm. The evaluation of the effective crack length at the discrete stops was done visually by beach mark analysis after the testing, which was controlled by the application of a constant ΔK.

In both experiments (Sects. 2.3.1,2.3.2, EC data are acquired. As previously introduced, the instrument provides a normalized readout of the real and imaginary parts of the coil impedance. In Fig. 7, the raw data collected during an experiment are visualized on the complex plane, showing the characteristic variation of the impedance signal of an EC sensor with respect to the crack growth. The modulus of the impedance variation since the beginning of the experiment until the full crack growth is the quantity used for all the subsequent evaluations. This quantity also referred to as the “EC signal” in the graphs, represents the sensor response to the embedded crack growth. An example of this curve can be observed in Fig. 6. It should be noted that the sensitivity is a function of the distance to the sensor and it increases when the defect gets closer to the sensor, according to the distribution of the induced currents. It follows, that the inverse function of this curve can be used for a direct estimation of the crack to sensor distance, provided in input the EC signal, as could be more desirable for a final application. Some further data processing can be applied without a doubt to this purpose, but this is not in the scope of the paper at hand and is left to future works.