Ultrasonic Testing as Applied to the Rail Industry. By Troy Elbert, Herzog Services, Inc.

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1 Ultrasonic Testing as Applied to the Rail Industry By Troy Elbert, Herzog Services, Inc. Rail is a leading asset of the rail industry as it represents a big portion of capital investments. Millions of dollars per year are spent to maintain this asset through various programs. Ultrasonic testing is one such program that inspects the steel for internal flaws and fissures caused by mill defects and material fatigue. Without routine testing, these flaws and fissures can lead to interruptions in service and costly derailments. Derailments and service failures impact the rail companies profit margins and can potentially impact the environment and safety of surrounding communities. Ultrasonic sound waves are a mechanical energy. The waves cause the molecules of the subject material to vibrate as they propagate through the material. The physical properties of the material, such as density and elasticity, determine wave velocity. The denser or less elastic the material, the faster the wave will travel through it. Below is a table of common materials and their relative velocities. Material Acoustical Properties Of Common Materials Ultrasonic Velocity Longitudinal Transverse (Shear) Impedance in / us mm / us in / us mm / us Z METALS Aluminum Aluminum 2024-T Aluminum 6061-T Beryllium Brass (70% Cu - 30% Zn Bronze (Phosphor 5%) Copper (CP) Gold Hastelloy C Hastelloy X Inconel (Wrought) Iron (Cast), Various Alloys Lead (94Pb-6Sb) Magnesium, Various Alloys Monel Nickel (CP) Silver (0.99 Fine) Steel Steel Steel, CRES 300 Series Steel, CRES 400 Series Titanium, 6AI-4V Zircaloy Zirconium POLYMERS Acrylics

2 Cellulose Acetate No Shear Component 3.19 Nylon No Shear Component Phenolic No Shear Component 1.90 Polycarbonate No Shear Component 2.71 Polyethylene No Shear Component 2.94 Polystyrene Rubber (Natural) No Shear Component 1.74 Rubber (Carbon Filter) No Shear Component Rubber (Silicone) No Shear Component 1.40 Teflon MISCELLANEOUS SOLIDS Alumina (Al2O3) No Shear Component 43.1 Concrete Glass (Plate) No Shear Component 14.5 Granite Ice ( -16C) No Shear Component 3.60 Quartz, Natural Quartz, Fused Sapphire Tungsten Carbide No Shear Component 67.6 COMPOSITE MATERIALS Fiberglass (50 v/o) Graphite/Epoxy (60 v/o) Boron/Epoxy (50v/o) LIQUIDS Ethylene Glycol No Shear Component 1.80 Glycerin No Shear Component 2.42 Oil (SAE 20) No Shear Component 1.51 Water (20C) No Shear Component 1.48 Gases Air (20ºC) No Shear Component Nitrogen (20ºC) No Shear Component Oxygen (20ºC) No Shear Component Table 1 Acoustic Properties To perform an ultrasonic test on any material, the sound beam must be introduced and propagated through the test material by a search unit. Detection occurs when the sound beam encounters a barrier where the velocity of the material or a bordering substance is dramatically different. Such is the case where an internal flaw or fissure is present and the material velocity on the opposing side is higher due to a presence of a gas, such as air, or vacuum which will not allow sound to pass. At this barrier, the sound wave will reflect at an angle that is a product of the sound beam angle and the flaw or fissure facet. If this reflection returns to the detection unit, the testing hardware and software then translates and displays this reflected energy for the inspector. The most common inspection angles used for rail testing are 0, 45 and 70. Each unique beam angle targets different surfaces of the rail and specific categories of flaws.

3 The 0 beam will search the rail from the running surface to the base directly below the web. The sound beam used to perform this test is a longitudinal wave. This sound wave is transmitted into the material at an angle that is perpendicular to the surface and will travel in the same direction it was introduced. Plexiglas wedge Steel When the sound beam encounters the bottom of the rail, it will reflect back toward the running surface. There, the search unit will receive the sound energy and the software presents it to the operator as a positive indication on the instrumentation. A loss of this signal could be indicative of a defect present in the rail, such as a split web or head/web separation, since it may interfere with the wave propagation to the base. If a defect lies parallel to the running surface, then it too, can produce a positive indication. This positive indication will appear on the screen in a region designated to be between the head and base of the rail. This will give the operator estimation to the depth of the defect from the running surface. The 45 will enter the rail at the center of the running surface and travel through the web at an angle that permits it to scan the perimeter of the bolt holes for any cracks. These can be caused from stress risers due to dull drill bits and holes that were not chamfered properly, leaving sharp edges and nicks. Cracks emanating from the bolt hole areas can also be caused by pumping joints. This search angle can also detect split webs and other longitudinal defects that have some deviation from the horizontal plane. Defects that occur in the head near the transverse plane are detected using a 70 inspection angle. This angle permits sound propagation through the rail head and is designed to optimize the reflection from defects such as detail fractures and transverse fissures. The 45 and 70 beams utilize an ultrasonic shear wave. Shear waves are produced in the test material when the wave enters at any non zero incident, or oblique angle. A phenomenon called refraction occurs in this situation and is dependent on the incident angle and relative velocities of the materials. Snell s law describes this relationship: Where θ₁ is the angle of entry, v₁ is the velocity of the first material, θ₂ is the refracted (resulting) angle and V₂ is the velocity of the second material. When inspecting rail using manual methods, a Plexiglas

4 wedge, or shoe, is used to induce shear waves into the steel. Looking at the table 1 above, the values for the material velocities of Plexiglas (longitudinal) and steel (shear) are.109 in./µs and.128 in./µs respectively. By inserting these values into Snell s law, and provided that an entry angle of 53 to the transverse plane it can be found that a shear wave will propagate through the rail at approximately 70 to the transverse plane Plexiglas wedge Steel 70. In addition to longitudinal and shear waves, ultrasonic energy also propagates as a surface wave in the test material. As the inspection angle becomes shallower, a higher percentage of overall energy is converted into surface waves. Longitudinal Plexiglas wedge Surface Steel Shear

5 As a result, surface conditions have the greatest impact on the 70 test, and are one of the leading nuisances in rail testing. Surface conditions come in many forms such as grinder marks, head checking, shelling and center ball spall. These surface irregularities along with a host of others can be problematic to the accuracy of the test being performed. Rail testing is typically conducted with a Rolling Search Unit (RSU) or multiple RSUs. The search units are mounted on a carriage that rolls along the running surface of the rail. Here, the probes are encased in a fluid filled urethane membrane that acts as a tire with the tread in contact with the running surface of the rail. This is referred to as a modified immersion technique. The chemical composition of the fluid and urethane is very important in that it possesses both the elastic and density properties that allow the sound path to propagate at a calculated angle and velocity. When calculating the incident angle in an immersion or modified immersion method, the water path and relative sound velocities have to be considered. By using Snell s law, the angle of the sound beam produced by the transducer will be refracted, but not mode converted by the water path. Hence, the longitudinal velocity of water is used. Where is the block angle, is the resulting angle out of the block, V₁ is the velocity of the block material and V₂ is the velocity of the water or water/glycol mixture which is common in ultrasonic testing. By using an originating angle of 40.2 in the block, a velocity of.088 in/µs for the block material and a velocity of.058 in/µs for the water/glycol mixture and solving for, the incident angle into the steel can be calculated..425 By using this as the incident angle into the steel and using the appropriate shear velocity for steel, the resulting angle into the rail can be calculated for the immersion method..939 The surface waves generated during the 70 mode conversion will travel along the head of the rail just under the running surface. These surface waves will reflect the sound beam back to the inspection probe when it encounters an obstruction, such as marks left by grinding stones. This wave travels along

6 the surface of the rail at approximately 90% of the speed of a shear wave, which makes it difficult to accurately disregard these signals through hardware or software settings. When performing a hand inspection with a wedge or shoe, it is possible to discern a surface wave by dampening the signal. This is done by pressing your finger on the surface of the rail in front of the probe, causing the signal amplitude to drop and confirming the indication is from a surface wave. In an application where a RSU is used to perform the test, the operator is remotely located within the vehicle and surface waves cannot be dampened as in the hand inspection method. As a result, surface waves can create false indications on the test screen. When testing in areas that are heavily populated with these events, a true transverse fracture can be hidden by the multitude of indications coming from these surface events. Grease, which is necessary to prolong track life in curves, etc., can also play havoc on an ultrasonic test. When using a RSU along sections of track that are heavily greased, or grade crossings that have a fair amount of dirt on the running surface, the foreign materials can collect on the tread of the search unit and block the sound from entering the steel. This forces the operator to stop the test and clean the search units before proceeding. Many times, repeated passes over the area are required to clear the material from the running surface and allow the sound to penetrate the steel for a continuous test. Snow and ice can cause similar problems, but may require more intense removal processes. False indications from surface events and signal degradation from contaminates can dramatically hinder average test speeds and reduce overall performance and accuracy numbers. Another factor that can dramatically affect the performance of an ultrasonic test system is the shape and related geometry of the rail head. As the sound path enters the steel, it goes through a mode conversion that yields the final sound path angle. To set up an inspection angle, it must be assumed that the exit point of the search unit and the entry point of the material to be tested lie on parallel planes. In this case, the exit plane of the search unit and the running surface of the rail are assumed to be parallel. From there, using Snell s law, it can be calculated at what angle to introduce the sound path through the different mediums to obtain the correct inspection angle. If the planes are not parallel, then a skewing effect comes into play and can redirect the sound path in the rail. In curves where the gauge side of the head can be dramatically worn down resulting in gauge face loss and vertical head losses at the same time, all previous assumptions are no longer valid. When the running surface begins to slope, it begins to skew the resulting sound path(s) toward the gauge side. As the deviation from horizontal progresses with wear, the skewed angle increases and approaches the second critical angle of an ultrasonic sound beam. The second critical angle is where, during mode conversion, all or most of the sound energy is converted into surface waves. It can be calculated for given materials by using Snell s law and setting the resulting angle to 90.

7 .851 From this calculation, the second critical angle for the sound beam going from water into steel is this is only a 1.7 deviation from the calculated entry angle for a 70 inspection. If a progressive comparison is made that shows the sound path in a section of rail with no wear to a profile that yields the second critical angle, the skewing of the sound beam to the gauge side is apparent. It is possible to predict the amount of impact the running surface deviation will have on the resulting angle. This is done by combining the theoretical incident angle and the amount of deviation, in degrees, of the running surface. The following table was calculated by combining the calculated incident angle with the head tilt to find the amount of material loss to produce the second critical angle. Head Tilt New incident angle New Refracted Angle ( ) Block Speed longitudinal wave Glycol Speed longitudinal wave Steel Speed shear wave Block Angle Glycol Angle

8 Notice that at approximately 10 of deflection on the running surface, the second critical angle of the 70 inspection angle is reached. Using a software package to model the sound path emanated from an element and the resultant refracted beams, a visual representation can be shown of the effects of gauge face wear. Typically, a 70 inspection angle is introduced to the field, center and gauge regions of the rail head. For simplicity, only the gauge side 70 will be modeled since it is the most dramatically affected. To model gauge wear as a product of both gauge face loss and head loss, the wear was set up as a deviation of the running surface of the rail from horizontal. The vertical measurement was calculated from the normal full ball gauge corner to the theoretical gauge corner of the worn rail. It is represented as an angle of deflection. Vertical measurement to describe amount of head loss in gauge worn rail. When on full ball rail, the gauge side element will scan approximately 1/3 of the rail head on the gauge side. It is designed to detect any fissures or anomalies that may be occurring from stress risers, defective field welds or any other event that disturbs the continuity of the material.

9 1 Gauge side 70 Full Ball Rail As the head wears, the sound path begins to bend toward the gauge corner. The following model is an approximation of 2 of deviation from the running surface. 2 Gauge Side Loss The beam starts to bend upward toward the surface of the rail and outward to the gauge side. At this point, reflection from a detail fracture is still probable. At 5 deviation, the sound beam approaches the extreme gauge corner and surface of the rail.

10 3 Gauge Side 70.25" Loss As the sound beam bends toward the gauge side of the rail, a typical defect that is lying in the transverse plane has a lesser chance of being detected. The sound beam, as it bends away, has a lesser chance of encountering the defect facet at a perpendicular or near perpendicular angle. The sound energy will be deflected away from the transducer array, reducing the amount of energy that is received. When modeled with 7 deviation, the sound path seems to disperse as more energy is converted into surface waves. 4 Gauge Side " Loss At 10 deviation, the model shows that there is no sound penetration into the steel from the gauge corner. This is the calculated point where all sound introduced to the steel is either reflected away or

11 mode converted into a surface wave. Note in the simulation below there are no blue lines representing mode converted sound waves in the rail material. Other standard inspection angles are affected as well, but not as dramatically since they are not as near the second critical angle in this material combination. The 0 sound path, which is used to monitor the bottom of the rail and also detect horizontally inclined defects, will begin to bend toward the gauge side. This can redirect the sound path away from a perpendicular line through the web toward the base and interrupt the expected return signal. This negative return can also be misinterpreted as a defective section of rail. While there is no evident solution to eliminating the production of surface wave components, there are some measures that can be taken to help control the amount of false indications on any given track. Surface contaminates are a difficult problem to remedy as there are not many controls that can be put in place for dirt on crossings, etc. However, turning off or reducing the output of the greasers a few days before and during the test would eliminate the grease buildup on the tread of the RSU. This would benefit the speed and reliability of the test through these areas. Grinding programs that are designed to re shape the head into a standard conformity is probably the single most useful preventative measure that can be taken. Besides the inherent rail life prolonging benefits, it reduces the likelihood of stress fractures caused by improper loading. Grinding also helps to eliminate many of the surface borne irregularities caused by rolling contact that can reflect the surface waves generated by mode conversion. However, grinding also produces grinding marks, which stated previously, will cause false indications. To alleviate this as a potential hindrance to an ultrasonic test, allowing a fair amount of traffic over the area will smooth out the marks and make them less of an obstruction to an ultrasonic inspection. In today s market, accuracy and speed of the test is of utmost importance. Limited work windows and restricted budgets demand high performance from not only the testing fleet, but also the follow up procedures to repair detected flaws. Grinding to the correct head profile helps keep the resultant

12 inspection angles at or near the calculated values. This is important when performing a non destructive test in that defect identification is greatly based on mathematical functions. If the rail is properly maintained with good running surface conditions and correct head geometry, the internal inspection process will also be improved. Production will increase with the absence of false indications, which require an interruption in the testing process to inspect each indication. Accuracy will also improve since the geometry closely matches a predetermined template, which all calculations are based. Both of these factors, based on rail maintenance, will yield a more productive and valuable internal inspection. References: 1.