Application Note: Barkhausen Noise on Landing Gear Piston

The problem: Traditional techniques for grind burn detection like acid etching are not applicable to coated or plated surfaces

During the manufacture and overhaul of many landing gear components, precision grinding of plated steel (i.e. hard chrome or HVOF coatings) is required. The steel substrates of these pins, axles, pistons, among others, are easily susceptible to re-temper or grinding burn during the grinding process. However, traditional techniques for grind burn detection like acid etching are not applicable to coated or plated surfaces, as the critical areas are under the coatings. Part quality must still be verified in these critical components and it has to be done by non-destructive means.

The testing method: Barkhausen noise testing non-destructively measures the ferro-magnetic steel substrate through the coating

Barkhausen noise is created by a ferro-magnetic material under the influence of a magnetic field and is sensitive to the changes in stress and microstructure that can occur during the grinding process. Barkhausen noise testing non-destructively measures the ferro-magnetic steel substrate through the coating. The measurements were carried out in accordance with the SAE document, ARP4462B: Barkhausen Noise Inspection for Detecting Grinding Burns in High Strength Steel Parts, which dictates the test parameters and criteria for rejection. In this case, the testing was performed semi-automatically, with the sensor traversing axially as the part was rotated for 100% coverage.

Landing gear piston — Figure 1: Photo of the measured piston

The results

Data collection software was able to automatically provide a pass/fail determination as well as generate a ‘heat mapping’ of the surface for identifying and locating any points of interest.

Figure 2: 3D heat map showing measured Barkhausen value vs. location on a chrome-plated landing gear piston

Instrument used for this application:

Rollscan 350

Barkhausen noise signal analyzer Rollscan 350 is designed for surface quality control and testing of near-surface defects such as grinding burns, heat treatment defects, as well as changes in stress and microstructure in a wide variety of ferritic steel and other ferromagnetic materials.