|NDT.net June 2006 Vol. 11 No.6|
Inspection of Aircraft Landing Gear Components by Barkhausen Noise Measurementby Ms Lim Mei Ting, A/P Ng Heong Wah* and A/P B. Stephen Wong**
School of Mechanical and Aeronautical Engineering
*Division of Engineering Mechanics
** Division of Aerospace Engineering
Nanyang Technological University
Messier Services Asia Pte. Ltd.
Paper presented at NCATMC 2006, RSAF, Singapore
ABSTRACTThe conventional method to check for the compressive stress and hardness induced during shot peening and also the residual stress built-up due to grinding is the X-ray diffraction, which is a well-established technique. However, the measurement depth for the X-ray diffraction is relatively low, about 5 - 10 µm. On the other hand, Barkhausen Noise has a measurement depth of up to 0.2mm, hence could be a possibly better technique to be implemented for the measurement of the shot peened parts and the ground parts.
The Barkhausen Noise technique is a Non-Destructive Testing technique which made used of the acoustic emission method by magnetising the test material and then having a coil which detects the response signals generated from the domain wall movements.
Three common landing gear materials, namely, CRES 17-4 PH, the AISI 4130 and the AISI 4340M were evaluated. The Barkhausen Noise behaviour of the test specimens which have undergone shot peening, chrome plating and grinding and stress relief baking were measured.
Shot peening gave relatively good results which followed closely to the theory of the Barkhausen Noise. The work-hardening and work-softening in the materials of different hardness was also observed. Chrome grinding and abusive grinding also showed that at points where a burn was induced, the Barkhausen Noise reading was high. However, when tested for stress relief effects, the results were without a clear trend.
1. IntroductionThe Barkhausen Noise is a non- destructive testing technique that uses an alternating current to magnetise a core which in turns magnetise the test material resulting in the domain wall movements. These domain walls movements within each grain are due to the change in size of the adjacent domains. These domain wall movements induce an electric pulse in a conducting coil placed near the test material, and the sum of all the electric pulses induced by the overall domain wall movements represents a noise-like signal known as the Barkhausen Noise.
The Barkhausen Noise is a function of several functions such as the stress, hardness and microstructure. The function of stress and hardness are represented as shown below.
2. Effects of Barkhausen Noise due to Shot PeeningShot peening is a cold-working process in which a metal part is bombarded by tiny balls of steel shots to work-harden the metal and to increase its fatigue strength. The intensities are set by exposing Almen Strips to the shot stream and plotting the arc-height versus exposure time. The intensity of 0.010-0.015 would mean that the arc-height value is between 0.010 inch and 0.015 inch.
A residual compressive stress is being induced in the metal after shot peening thereby increasing the fatigue strength and increase the resistance to stress-corrosion cracking. This compressive stress is conventionally measured using the X-ray diffraction technique. However, the X-ray diffraction technique has a limitation as the penetration depth is only 5-10 µm while the shallow surface layer of residual compressive stress is approximately 0.1-0.5 mm in depth. On the other hand, BN is a technique which can be used to measure up to 0.2mm for surface hardened material, depending on the permeability of the material.
In addition to the compressive residual stress induced in the material, shot peening also changes the roughness or topology and the hardness of the surface by cold working. Both of which will affect the fatigue behaviour of the metal. In shot peening hardness can increase or decrease depending on the combination of the hardness of the material before shot peening and on the intensity of the shot peening. When the hardness of the steel is low, the surface will mainly work harden in shot peening however when the hardness is high it will mainly work soften. In the intermediate hardness range the shot peened surface both work soften and work harden.
The fact that as hardness increase, shot peening will induce a lower residual compressive stress in the metal due to the compressive state of the shot peened surface which decreases the indentation size. Therefore for a harder material, the dimples caused by shot peening will be small in size. A small size dimple can also be observed in lower intensity than higher intensity.
Suominen and Tiitto  observe that an increase in compressive stress increases the hardness of the metal, and that the change from work hardening to work softening occurs when the hardness of the material exceeds 50 HRC.
Iida   observed that work softening generally occurs at the depth of 0.2-0.6 mm and that the highest compressive stress induced depth-wise is in the range of 0.05-0.15mm. He also mentioned that with the increase in kinetic energy of the shot, the stress increases with depth; however, the surface compressive stress decreases.
3. Barkhausen Noise due to GrindingGrinding is used to manufacture components of high accuracy as it is able to achieve the required tolerance; however grinding requires a very large energy input per unit volume of material removed. A large portion of this energy is converted to heat energy which is concentrated in the surface layers of the material after grind, resulting in a rapid rise in the localised temperature, hence causing high residual stress and some burn defects. The actual rise in temperature is dependent on several factors, such as the type of coolant, the type of grinding wheel and the speed and depth of cut of the wheel.
The residual stress distribution generated by grinding under ideal conditions, results in a residual compressive stress layer, which known to have higher fatigue strength and wear resistance; however, grinding often reduces the compressive stress and even causes tensile stress, thus representing a risk for early material failure. During grinding of hardened and tempered steel samples, the stock removal rate is limited by the increasing risk of thermal damage to the component. The severity of such damage, also known as grinding burn, will depend on the temperature that metal was heated to.
Quoting Siiriainen , soft steels have relatively high MP amplitude and the amplitude decreases with the increase in hardness. Surface hardened materials typically have compressive stresses which also give low MP values, while tensile stresses give high MP values. The MP amplitude in hard materials is dominated by the hardness hence low BN due to compressive stress will aid in detecting small changes in surface hardness.
The conventional method of detecting grinding burn is by using nital etch; however this method has several drawbacks such as not effective to stress, time consuming and difficult to automate. BN on the other hand is responsive to stress and can be automated, hence it is introduced to check work pieces after grind for grinding burns due to abusive grind.
Barkhausen Noise technique is mainly used in the automotive industry, to detect grinding burns parts such as bearings, gears and also cylinders and pins of aircrafts.
4. Materials Examined
5. Equipment used and Production of Calibration ChartThe equipment used to measure the Barkhausen Noise in this paper was the Stresscan 500C manufactured by Stresstech Oy, Finland.
The procedure of use of this Stresscan 500C to measure the Barkhausen Noise is first to obtain the optimised magnetisation value of the individual material type followed by plotting the calibration/correlation chart for each individual material, which can be used to correlate the unit of measurement, the magneto-elastic parameter, MP, to the actual value of stress in MPa. The trends are shown as below in Figure 6 with reducing pressure and hence stress producing lower Barkhausen noise.
6. Experiments and ResultsThe experiment was conducted for 3 different test phases namely shot peening, the chrome and abusive grinding and the stress relief bake. The locations of the points of measurements are as shown in the below figure.
Theoretically, shot peening increases the compressive stress of the material near the surface. And the compressive stress should increase with increase in shotpeen intensity. Also, the hardness of the material will increase with the increase in shotpeen intensity due to work hardening. According to the BN theory, the BN MP value will drop when the compressive stress increase and when the hardness of the material increases. This can be observed in Figure 8.
Figure 9 above is another shot peen chart which showed the opposite trend. It is known that if a material is hard, the hardness will dominate instead of the stress during BN testing. Hence for this material type of hardness 53HRC, the hardness dominates. As mentioned earlier in the BN theory, the MP value decreases with increase in hardness. This chart showed that as the shot peen intensity increases, the MP value also increases, which is to mean that the increase in shotpeen intensity results in a lower hardness in the material. The reason for this trend is due to the worksoftening effect due to the hardness of the material.
AISI 4130 does not show much change with shot peen intensity, indicating work hardening and softening effects (Figure 10).
Over in this chart, it can be observed that abuse grind give a higher MP value than that of the chrome grind. This is due to the tensile stress built-up in the material during grinding. As the abuse grind is conducted by plunging the grinding wheel into the material, there might be locations of stress concentrations which can be explained by the peaking in the abuse grind area.
Figure 12 shows MP values before grinding and after grinding with a stress relief bake in between measurements as shown in figure 13.
It is observed that before and after bake, the MP values are relatively the same. Hence there is no conclusion for whether stress relief bake is an effective method of relieving stress. More studies will have to be done.
Also observed is the cyclic trend of the material type CRES 17-4PH. This should be also looked into. Probably the microstructure of the material has an effect on the trends observed.
9. AcknowledgementsThe work for this paper was conducted by Ms Lim Mei Ting during her final year project, 2005, in the School of Mechanical and Aeronautical Engineering, Nanyang Technological University, Singapore. The work was conducted at Messier Services Asia Pte. Ltd., Singapore and their contribution is gratefully acknowledged.