One of the most critical elements of the rolling stock equipment (any rail car or locomotive) is an axle of the truck (or bogie). The axle performs a function of holding the wheels together, supporting disk brake surface (depending on brake configuration) and supporting the rest of the structure sitting on the wheels (such as a passenger/freight train car or an entire locomotive). It an absolutely fascinating part and has a lot of interesting elements which we will cover in the future articles. For now lets just keep in mind that it is a super important part and its failure can lead to catastrophic rail/transportation accidents. Hence the need to validate the “health” of the axle through Ultrasonic Testing (UT).
What is Ultrasound Testing (UT)?
It is a type of non-destructive testing used to find defects in a material. It utilizes a probe, which integrates an (ultrasonic) source and a detector/receiver, which emits ultrasonic waves and then reads the corresponding wave deflections in the material via detector/receiver. Depending on the frequency of the ultrasonic source and the angle with which probe is applied on different parts of the sample (in our case axle) can be scanned. In addition to detecting cracks in the axle’s surface, another goal is to detect any and all surface defects which might lead to formation of the cracks during in-service operation. In some cases these surface defects can be addressed and the axle returned to surface.
Why use UT verses other approaches?
Due to the sensitivity of properly configured test equipment, very minor defects can be detected, which may not be found during the visual inspection of the axle. This is critical because axles can go through substantial mileage between inspection intervals and a failure during the operation can be extremely dire (and costly). Another benefit of UT is that it can be performed with wheelset still fully assembled and “see” portions of the axle inaccessible to the visual inspection. It is also cleaner as compared to the die-penetrate testing, which utilizes a die to penetrate surface and a developing agent. Use of die also requires surface accessibility and proper application of both the die and the developing agent.
When is UT typically performed?
It is always performed when the axle is initially manufactured (see AAR standard M-101) and during periodic maintenance intervals specified by the end user (which are typically based around wheelset overhauls and other truck maintenance events). There is a trade-off between testing too frequently (with the corresponding expenses) and not testing frequently enough (and risking failure).
What are types of UT equipment utilized?
There are typically two types of UT scanning test equipment. The first is a handheld version with a probe which can be applied to the desired inspection surface to check for defects. It is typically utilized with a couplant, a substance which improves transmission of sound energy between the probe and the material being tested. The main benefit of this approach is portability of testing, in terms of location and setup. The main drawback is possible inconsistency in setup and hence testing results. Proper training is very important in addressing this issue. Second type is a stand alone automated equipment which immerses entire axle in a “bath” and performs a complete scan automatically. The main drawback is need for additional equipment space and the need to disassemble the wheelset. The main benefit is smaller reliance on quality of the operator and high repeatability (because actual scan is done by a machine).
Testing methodology
UT testing is usually performed in axial and radial directions. Combination of the two ensures that as much of the axle’s exposed surface is inspected as possible, especially cross-sectional center line of the axle which sees a lot of cyclical loading. In addition the use of angled probes on the ends of the axle allow scanning of the high risk areas such as wheel seats and brake disk mounting surface (if disks are mounted directly on the axle).
End result of UT testing
After UT testing there are typically two outcomes for an axle, it is either scrapped or re-machined to eliminate the surface defect. The exact action is determined by the size of the defect and the design parameters of the axle. Machining can be performed if post machining it can still meet initial design dimensional tolerances.
Relevant railroad accident
An accident involving derailment of BNSF train helps to illustrate importance of UT testing of the railroad axles. In this unfortunate event, the axle broke in two and caused a seriously damaging derailment. One of the suspected culprit of this accident was internal void in the material of the axle which created conditions for the crack propagation and internal failure. NTSB Derailment investigation
Relevant Standards for further reference
ISO 5948 – Railway rolling stock material – Ultrasonic acceptance testing.
AAR M101 – Manual of Standards and Recommended Practices Wheels and Axles. Axles, Carbon steel, Heat-Thread.
EN 13261 – Railway applications – Wheelsets and bogies – Axle – Product requirements