Amusement Park Non-destructive Testing: Conventional and Emerging Non-destructive Testing Techniques

Introduction

Non-destructive testing (NDT) of amusement park infrastructure is critical to the safe operation of the broad spectrum of rides. The majority of amusement park infrastructure is fabricated from structural steel to American Welding Society (DWS) Structural Welding Code D1.1 [1] or comparable ride manufacturer specifications. Amusement park infrastructure includes everything from massive roller coaster infrastructure to smaller lap bars and pins. Many components are coated very thick coating and foams which prevent direct visual inspection. Additional, many components like gears and pins are coated with grease inhibiting non-destructive testing. Consequently, nondestructive testing of amusement park components requires coating and/or foam removal and degreasing prior to performing visual inspection (VT), magnetic particle inspection (MT), and liquid penetrant testing (PT).

Some advanced NDT like eddy current testing (ET), eddy current array (ECA) testing and alternating current field measurement (ACFM) show potential for inspection with limited surface preparation. Non-destructive testing by qualified non-destructive testing personnel are routinely performed on chasses, axles, bogies, and track. The inspection objectives are in-service cracks and metal loss due to wear or corrosion. Non-destructive testing professionals will accurately locate and quantify the extent the material defect.

Visual Non-destructive Testing

Visual testing is the first line of defense for all amusement park infrastructure including welds, small redundant parts, and large complex components. Based on TechKnowServ Corporation’s 15+ years of amusement park NDT experience, upwards of 80% of all indications can be isolated using a thorough visual inspection. By using adequate viewing angles, lighting, and magnification, an NDT inspector can identify the majority the damage mechanisms encountered on the multitude of different components found in the amusement industry. Visual testing is a great starting point, but poor technique, inadequate lighting, and dirty components tend to mask indications from the naked eye. Non-destructive testing is performed in accordance with American Society for Boiler and Pressure Vessel Code (ASME BVPC), AWS D1.1, and ride manufacturer guidelines.

Figure 1: In-service crack on a welded repair

Wet Fluorescent Magnetic Particle Testing

The most common supplement to visual testing in the amusement industry is magnetic particle testing (MT). Both dry MT and wet MT are common with the latter used more since the method is more efficient [1]. This technique is made possible by exploiting the predictable nature of magnetic fields. Magnetic fields are introduced into the test part using standard yokes and coils. Magnetic flux leakage field are generated around surface cracks. This leakage field draws the small iron particles to the crack, narrow weld undercuts, and lack of fusions. In contrast to the dry technique, the fluorescent technique highlights the area of concern when viewed under adequate ultraviolet lighting. Wet fluorescent magnetic particle testing (WFMT) is the preferred technique to dry particle due in part due to comparably higher sensitivity to smaller flaws. The higher sensitivity is driven by higher MT particle mobility and the peak sensitivity of the human eye to the green-yellow emitted by the wet MT bath. While the probability of detection level is high, depth quantification of indications is not possible and subsurface indications are not detectable MT.

Figure 2: In-service crack on critical member of a carousel style ride using WFMT

Eddy Current Testing - Alternating Current Field Measurement

ET is an electromagnetic testing technique that is digitized and quantified through a simple bridge circuit and a set of pre- and post-signal amplifying processes. Alternating Current Field Measurement (ACFM) is a derivation of ET that was developed in the oil and gas industry to inspect underwater infrastructure welds. The main advantage of ACFM is its ability to detect and depth size cracks underneath coatings as thick as 0.10” . In ET and ACFM, when the signal is “balanced” the electrical output is registered as zero causing no onscreen impedance plane deflection.

ACFM differs from conventional ET as it deploys a dual coil configuration and monitors for direction specific variation in magnetic fields generated by surface cracks. The ACFM array is scanned parallel to the weld toe and measures two orthogonal magnetic field components generated by surface breaking cracks. The two components measured are the Bx corresponding to the crack length and Bz which provided depth information. Changes in the Bx and Bz are monitored on the ACFM butterfly plot shown below.

Figure 3: ACFM butterfly plot

Figure 3: ACFM fatigue crack

Eddy Current Testing

Standard eddy current testing can be performed to validate visual and magnetic particle testing but is not commonly used as a primary crack detection method. NDT personnel use ET to verify and help quantify depth of in-service surface breaking cracks. Since eddy current testing is such a sensitive testing method many factors can skew the interpretation of the inspector. It is nearly impossible to determine the type of damage mechanism viewed on the display without the use of magnified visual inspection when viewing very small indications. In addition, other geometric and metallurgical variances can influence results and interpretation testing results. The most common cause of dot drift is called edge effect [3]. This is where the magnetic field produced by the coil meets the edge of the part causing the lines of flux to scrunch up, much like a slinky. This process affects the primary and secondary magnetic field in the part which in turn effects the voltage (the eddy current) produced by the conflicting magnetic fields causing the dot to drift on the screen. Another large drawback to ECT is the speed and depth of inspection. This testing method requires a painfully slow scanning speed to ensure proper probe angle and part contact to obtain true and usable deflection readings. The eddy current depth of penetration, or skin depth, is influenced by material properties and frequency range of the probe. and type of material.

Figure 3: ET calibration setup on carbon steel test coupons

Eddy Current Array

Eddy current array (ECA) is another advanced nondestructive testing technique that shows promise in the amusement park testing market. The basic working principles of ECA are the same as conventional ET except there are multiple coils contained in the array probe. ECAs are constructed using a multi-layer printed circuit board coil, a multi-layer configuration dedicated to a cross-wound array on the lower layer and reflection coil probe on the upper layer. The eddy current reflection probe provides dynamic lift-off compensation for rough surfaces and weld reinforcements. In this configuration, eddy-current signal polarity can provide direction correlation to crack orientation relative to coil axis.

The primary use of ECA is the evaluation of ID/OD corrosion on tubing and piping. This is applicable for a multitude of structures found in amusement parks. From square/round track tubing, to pitting on axles, ECT provides a quantifiable evaluation of corrosion, erosion, and service wear experienced on these structures. ECA is a sensitive method that experiences the same issues as conventional ECT, the main issue being the method is susceptible to permeability changes. For this reason, weld inspection with ECA is difficult due to the varying heat input changes during the heat treatment and welding process. The varying temperatures throughout the weld material and heat effected zones typically decrease magnetic permeability (how susceptible a material is to an external magnetic field). The major advantage to ECA is the ability to maximize the depth of penetration of the useable eddy current field.

180-lag between stainless steel surface crack and lift-off using a cross-wound coil

Figure 4: 180-lag between stainless steel surface crack and lift-off using a cross-wound coil [10].

References

1. AWS D1.1/D1.1M:2020, Structural Steel Welding Code

2. ASTM F1193-18a: Standard Practice for Quality, Manufacture, and Construction of Amusement Rides and Devices

3. ASTM E1444/E1444M Standard Practice for Magnetic Particle Testing

4. Standard Practice for Examination of Welds Using the Alternating Current Field Measurement Technique

5. ASTM E3052-21 Standard Practice for Examination of Carbon Steel Welds Using An Eddy Current Array