Fracture analysis of an aeroengine filter tube bushing

2020-11-10 8:29:25 No Comments Epower Metals News / Trade News

What is a tube bushing?

As an important part of aero-engine, tube bushing is used for the installation of gears and bearings in the shell. The materials of pipe sleeves are carbon steel and titanium alloy. In order to reduce the weight of the engine, titanium alloy bushing is the majority. The material of tubing bushing in this paper is TC4 titanium alloy, which is processed by bar, and its inner and outer surfaces are anodized. TC4 alloy is a kind of medium strength α + β type two-phase titanium alloy, which has excellent comprehensive properties. It has been widely used in the manufacture of aero-engine fan and compressor disk, blade and fastener. The fuel pipe in the Bush is made of stainless steel, and a rubber pipe is set between the fuel pipe and the bush. During disassembly, it was found that the oil pipe bushing was broken, the rubber tube was worn, and the stainless steel oil pipe was in direct contact with the bushing.

In this paper, macro and micro observation, metallographic examination, hardness test, energy spectrum analysis and residual stress test were carried out to determine the fracture property of the tubing bushing and analyze the fracture reason.

What is TC4?

Ti-6Al-4V (UNS designation R56400), also sometimes called TC4, Ti64, or ASTM Grade 5 Titanium – it is one of the most popular alloys in the titanium industry and accounts for almost half of the titanium used in the world today. Commonly referred to as Ti-6AL-4V (or Ti 6-4), this designation refers to its chemical composition of almost 90% titanium, 6% aluminum, 4% vanadium, 0.25% (max) iron and 0.2% (max) oxygen. It features excellent strength, low modulus of elasticity, high corrosion resistance, good weldability and it is heat treatable. The addition of aluminum and vanadium increases the hardness of the material in the alloy matrix, improving its physical and mechanical properties.
The most common market for 6AL-4V is aerospace. Lightweight grade 5 titanium is well suited for applications such as compressor blades, discs, and rings for jet engines; airframe components; pressure vessels; rocket engine cases; helicopter rotor hubs and critical forgings requiring high strength-to-weight ratios. Ti-6AL-4V titanium round bar stock is also age hardenable by heat treatment to achieve even higher strengths. This biocompatible material is also well suited for medical implants. Its mechanical and physical properties allow good capacity for titanium to join with bones and other tissue
Comments: Tooling should consist of tungsten carbide designations C1-C4 or cobalt type high speed tools. Generally, machining characteristics are similar to those of austenitic stainless steels. Ti 6AL-4v Grade 5 Titanium bar stock can be machined using slow speeds, high feed rates, rigid tooling, and flooding the workpiece with non-chlorinated cutting fluid.

Chemical Properties of TC4/6AL-4V Grade 5 Titanium

Element		Percent by Weight
Ti	Titanium	90.00%
Al	Aluminum	6%
V	Vanadium	4.00%
C	Carbon	< 0.10%
O	Oxygen	< 0.20%
N	Nitrogen	< 0.05%
H	Hydrogen	< 0.0125%
Fe	Iron	< 0.3%

Physical and Mechanical Properties of TC4/6AL-4V Grade 5 Titanium

	Density, g/cm³	Young’s Modulus, GPa	Shear Modulus, GPa	Bulk Modulus,GPa	Poisson’s Ratio	Yield Stress, MPa (Tensile)	Ultimate Stress, MPa (Tensile)	Hardness, Rockwell C	Uniform Elongation, %
Min	4.429	104	40	96.8	0.31	880	900	36 (Typical)	5
Max	4.512	113	45	153	0.37	920	950	—	18

Heat treatment of Ti-6Al-4V, Grade 5

Annealing at 1,700-1,900°F (927 – 1,038°C) is done where high hardness, tensile and fatigue strength are desired. Ti-6Al-4V, Grade 5 Alloy can be heat treated in several ways.

1. Anneal: 1,275 -1,400°F; (691 – 760°C), ½ to 2 hours, air or furnace cool;
2. Stress Relief Anneal: 1,000 -1,200°F; (538 – 649°C), 1 to 8 hours, air or furnace cool;
3. Solution Heat Treatment: 1,675 -1,750°F; (913 – 954°C), 1 hour, water quench;
4. Aging Treatment: 975 -1,025°F; (524 – 552°C), 4 to 8 hours – air cool.

The very best of properties in the solution treated and aged condition are obtained in small cross sections that are rapidlyquenched. Larger sections sizes and/or a quench delay may cause properties to be lower than the optimum values.

Appearance inspection

The appearance of the bushing tube assembly for inspection is shown in Figure 1. The appearance of the broken oil pipe bushing is shown in Figure 2. The upper end of the bushing is similar to spline shape, and the surface layer is knurled technology; the lower end is threaded, the surface of the middle connection is smooth, and there is a small step in the inner circle of the spline end. The fracture position is located at the step near the spline end, and the fracture direction is divided into three sections along the circumferential direction of the bushing. The fracture length of the left and right sections is close to the half arc, which are respectively marked as 1# and 2# respectively; the middle section of the fracture is located in the middle of the two R corners of the spline, which has been lost when collecting the fracture.

The results show that there are different degrees of extrusion wear marks on the inner surface of the bushing, and the circumferential wear marks are obvious. The wear marks on the side of spline end close to the missing fracture are deeper than those on the opposite side, and the wear marks at the thread end opposite to the missing fracture are deeper.

Fig.1 appearance of conduit assembly

(a) Overall appearance of broken tubing bushing

(b) Fracture appearance of 1#, 2#

Fig.2 appearance of broken oil pipe bushing

Macroscopic and microscopic observation and energy spectrum analysis

Macroscopic observation

The fracture was observed under stereomicroscope. Fig. 3 shows the enlarged observation of fracture. The fracture surface of nut dropping is black gray, and the extended edge line diverging from angle R can be seen at the cross section of R angle of two fracture openings, and is accompanied with fatigue arc characteristics, as shown in Fig. 3A; the radius of R angle is small (see Fig. 3b), and the machining marks of transition zone are obvious; clear wear marks can be seen on the step surface of the inner surface of the two breaks towards the thread end (see Fig. 3C), and the wear marks are more obvious on the side near the missing fracture. The macro morphology of the fracture surface of 2# and 1# fracture is basically consistent.

(a) Section at angle R

(b) R angle side

(c) Wear marks of steps on inner surface
Fig.3 1# stereoscopic observation of fracture

Microscopic observation and energy spectrum analysis

The 1# fracture was ultrasonically cleaned with acetone and placed under a scanning electron microscope for microscopic observation. Figure 4 shows the microscopic morphology of the 1# fracture. The crack originated from the R angle and appeared as a line source. It can be seen that the expanding ridge extends from the source area to the inner surface of the bushing at about 45°; the secondary source area is the knurled sharp corner of the outer surface. Where, the ridge line extends from the outer surface to the inner surface (see Figure 4a). The enlarged morphology of the main source area is shown in Figure 4b. There are attachments on the surface of the fracture. The energy spectrum test mainly contains C and O elements. A large number of clear small facets can be seen on the cross section, showing quasi-cleavage characteristics. The overall observation of the fracture shows that there are clear ridges at all the knurled corners of the outer surface, which extend from the outer surface to the inner surface, as shown in Figures 4c and 4d. The extension zone is also a feature of quasi-cleavage fracture. The micro morphology of 2# fracture is consistent with the characteristics of 1# fracture.

A sample of the knurled surface near the fracture was manually interrupted along the circumferential direction. The micro morphology of the artificial fracture is shown in Figure 5, which is a typical dimple feature.

(A) Low-magnification morphology of the source area

(B) High magnification morphology of the main source area

(C) Low magnification morphology of fatigue expansion zone

(D) The morphology of fatigue strips in the fatigue expansion zone
Figure.4 1# Fracture microstructure

Figure.5 Microscopic morphology of artificially interrupted fracture

Residual stress test

The residual stress test was performed on the knurled surface of the bushing and the smooth surface of the knurled and threaded connection. The results are shown in Table 1. It can be seen that the circumferential residual stress is small.
Table 1 Residual stress test data (N·mm-2)

Position	Knurled surface position 1	Knurled surface position 2	Smooth surface
Axis direction	3.4±16.4	6.1±6.3	-10.3±13.2
Axis direction	112.1±7.6	96.0±11.4	81.5±8.1

Metallographic inspection

Cross-section and longitudinal section metallographic samples were taken perpendicular to the axial and circumferential directions of the bushing near the fracture. The microstructures are shown in Figures 6a and 6b. The metallographic inspection of the tubing bushing is a two-state structure, the knurled surface can see obvious plastic deformation, and the metallographic structure is an equiaxed α+β two-phase structure. Refer to the GJB1538 “TC4 Titanium Alloy Bar Specification for Aircraft Structural Parts” standard rating. The organization state conforms to Figure A1-A2. There is no abnormality in the metallographic structure of the nut.

(A) Cross section (500×)

(B) Longitudinal section (500×)
Figure.6 Metallographic inspection

Hardness check

Vickers hardness test was performed on the cross-section and longitudinal section samples taken from the metallographic inspection of the tubing bushing. The test results are shown in Table 2, which conform to the normal TC4 titanium alloy hardness range.
Table 2 Hardness test results of tubing bushing (HV_0.5)

Position		1	2	3	Average value
Cross section	Near the knurled place	309.2	320.9	317.1	315.7
Cross section	Hypokeimenon	314.2	292.6	294.5	300.4
A longitudinal section	Hypokeimenon	304.6	303.2	337.7	315.2

Analysis and discussion

Fracture analysis of tubing bushing

The macro and micro observation results of the fracture show that the tubing bushing cracks first linearly originate from the spline end, one groove and two R angles. The fatigue arc characteristics can be seen macroscopically, and the obvious expansion ridges can be seen macroscopically near the source area, and then the fracture is in the entire circumferential direction. Expanded from the outer surface and the inner surface, clear extended ridges can be seen at all the knurled corners of the outer surface, which is a multi-point secondary fatigue source, and the extended area is a quasi-cleavage feature, which is different from the typical dimple features of artificially broken fractures. From the above characteristics, it can be judged that the fracture nature of the tubing bushing is fatigue fracture.

Cause analysis of the fracture of tubing bushing

The metallographic structure of the tubing bushing is normal, no obvious metallurgical defects are seen, and the hardness is normal, indicating that the bushing cracking has nothing to do with the material.
Analyze the force of the tubing bushing: first, clear circumferential wear marks can be seen on the inner surface of the tubing bushing. The wear marks on the side where the crack originated from the spline end are deeper than the opposite side, and the thread end is worn on the side opposite to the crack origin. The trace is deep (see Figure 7). This wear trace should be caused by the relative movement of the stainless steel tubing in the bushing. The coaxiality of the butt joint between the two steel pipes in the bushing is poor, which is easily caused by vibration during engine operation. Uneven wear of the bushing. In the process of dismantling the failed tubing bushing, the rubber ring between the tubing and the bushing was completely worn out, which proved the serious wear between the tubing and the bushing. In addition, there are also signs of wear on the step surface facing the thread inside the spline end (see Figure 3c and Figure 4c), indicating that the tubing step and the bushing step are in contact and there is an interaction force. In addition, the R angle of the groove on the spline end is relatively sharp, and there is stress concentration, which promotes the origin of fatigue cracks.

Figure 7 Schematic diagram of bushing and tubing cooperation
In summary, the fracture nature of the tubing bushing is fatigue fracture. The main reason for the fracture is the uneven vibration and additional stress caused by the extrusion caused by the poor fit between the bushing and the tubing. At the same time, the stress concentration on the structure promotes the initiation and propagation of fatigue cracks, and finally leads to the fatigue fracture of the bushing.

Conclusion

(1) The fracture nature of the tubing bushing is fatigue fracture, and no defects are seen in the fatigue source area.
(2) The main cause of tubing bushing fracture is the uneven vibration and additional stress caused by extrusion caused by the poor fit between the bushing and tubing. The stress concentration on the structure promotes the initiation of fatigue cracks.
(3) The R angle of the groove on the spline end is sharp and there are obvious processing marks. It is recommended to round the R angle.

Author: Bai Hao, Liu Jing, Zhang Yindong

Source: China Tube Bushing Manufacturer – Yaang Pipe Industry (www.epowermetals.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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