Development of high strength material for smart aircraft bolt.

Elements that create an open y field

This important group of alloys is cheaper than the well-known Fe-Cr-Ni stainless steel; and can also be manipulated to improve the transformation temperature and strain rate insensitivity of the stress induced martensitic transformation [6]. Temperature and strain rate sensitivity are not beneficial for practical use of the strain memory technique and should be eliminated if possible.

Figure 1.2 Schematic illustration of the effect of Ni in shifting Gamma Loop and stabilizing austenite down to low temperatures [97]

Elements that expand the 'Y field

Manganese: is only half as effective as an austenitic stabilizer compared to nickel, but has the added effect of lowering stacking fault energy. Copper: In addition to the austenitic stabilizing effect of copper, it has good corrosion properties [39], but with a small negative effect on ductility [37].

Figure 1.4 Influence of carbon on Ms and Me temperature [31]

Elements that contract the y field

Cr ferrite Improves general corrosion resistance and resistance to oxidizing environments and lowers SFE. Mn austenite Improves hot cracking resistance, increases nitrogen solubility, austenite stabilizer, lowers SFE, austenitization index 1/2.

Table 1.1 General effect of alloying elements on the stability of austenite and the properties of stainless steel

Material Processing

Strain hardening mechanisms

Field-extending alloying elements such as nickel and manganese lower the position of the TTT versus temperature curve. Austenitic stainless steel exhibits a slight degree of ferromagnetism, which depends on the amount of cold working and the chemical composition of the steel.

Figure 1.10 An example of the TTT curve for a plain carbon eutectoid steel

Inhomogeneous transformation

The lower speed resulted in a greater amount of martensite formation at a given strain (strain) than the higher speed (see Figure 1.18). The decrease in martensite with increasing speed is in turn related to the increased temperature of the samples during high-speed testing (see Figure 1.17).

Figure 1.14 Ferromagnetic response curves of Fe- 18Mn-13Cr-ONi sensor alloy at three temperatures

Ductility

Fatigue properties

In order to meet the strict requirements for the use of screws in aircraft, high strength TRIP steel is required. The Fe-C-Cr-Ni family shows promising results after 80% PDA at 450 °C, with a carbon concentration of 0.35% to increase strength. At this carbon concentration, the deformation-induced martensite reaction can be inhibited at room temperature. Ductility will also be reduced due to PDA and high carbon content (brittle material).

A TRIP steel must be used with strengths, both yield and fatigue endurance, that exceed those of conventional high-strength structural material for aircraft bolts. No currently commercially available material meets the requirements. Yield strength equal to or higher than AISI 4340 (HSLA steel) currently used for the aircraft wing bolt;. Low incubation load or stress for strain-induced transformation III temperature range -50°C to 50°C;.

Figure 1.23 High-Cycle Fatigue Properties of TRIP Steel and High Performance Structural Steels [93]

I.requisiremecluuW:al properties

Experimental alloys selection

On the strain-induced transformation side, alloy 1 will have good strain-induced phase transformation (see Figs. 1.9 and 1.25) and 0.35% C and 0.5%. The calculated C.E = 11.3 and N.E= 18.4 dip in the austenite region of the De Long diagram near the boundary of the smart region (see Figs. 1.9 and 1.25) and should therefore have acceptable strain-induced phase transformation. The elemental content of 0.30% C, 2% Mo and 0.5% Si should raise the strength of this alloy to that required for the manufacture of aircraft bolts.

The content of 0.35% C and 0.5% Si contributes to increasing the strength of the alloy to the strength level of aircraft bolts. Tensile test results for the first fusion are presented later in this chapter. The second melt was used to perform not only resistance tests, but also to test the transformation characteristics of the connections. The following notation will be adopted to identify the links. The first number is the alloy number and the fusion (either 1, 2, 3) will be marked /1,/2,/3.

Figure 1.25 De Long diagram showing location of selected strain memory alloys

Melting

This means that decarburization occurred during heat treatment of the cast ingot and during thermo-mechanical processing (hot rolling at 550°C). This first experiment showed promising results, especially in terms of the strength required for the smart plane bolt. Although the magnetic permeability of this material was not monitored during the test, the suppression of the necking phenomenon in sample B1 (see Figure 2.7) is characteristic of TRIP steel.

The use of the extensometer and the magnetic susceptibility meter simultaneously during tensile testing on the Instron generated some problems; The extensometer fixation head touched the gauge. When a rock is brought near the coils, a voltage proportional to the sample's magnetic susceptibility is induced in the receiver coil. A phase-locked amplifier detects this signal and, after rectification, is used to drive the circuit for displaying the magnetic susceptibility readings. The reading is directly calibrated for sensitivity.

Table 2.2 Physical characteristics of as-received plates, 75% warm rolled (second experimental melting).

Compression Test

Note: Magnetic susceptibility measurements (in Table 2.7) were made at different locations and in different orientations (directions). Compression tests show that all the alloys investigated in this study showed strain-induced phase transformation (Table 2.7 and Figs.), further confirming that all of them can be used as strain memory alloys. As the materials were overhardened (42HRC ), in order to be v-notched using a machine available at UKZN, the samples were sent to MET-LAB for v-notching and testing.

The magnetic permeability of the impact samples was measured before (on the sample) and after the impact test on the fractured area of the fractured sample. Martensitic transformation occurred during the impact test in all tested samples at both temperatures, and the results are presented in Chapter 3, Sections 3.4.5 and 3.6.5.

Figure 2.17 Measurement of inductance of compression Specimen

Thermal Martensitic Formation Test

Metallography Analysis

Final cutting of the specimens to metallographic dimensions was performed using a soft silicon carbide grinding wheel with liquid cooling at a slow feed rate. Metal saw cutting was not used due to the work-hardening properties of these alloys (strain-induced martinsitic transformation). Between polishing, the samples were washed with soap and running water, rinsed with alcohol and dried in a blast of hot air. Although this has become known as etching, it does not always refer to the selective chemical dissolution of various structural features.

The above principle is used to monitor the health of a smart wing bolt prototype using an inductance meter during the test. A spindle type device (Figure 2.20) was constructed to assist in winding a coil with accurate dimensions and the maximum possible number of turns to increase the magnetic sensitivity of the coil. Release wax was applied to the surface of the accessory part to ensure that the spool slides off easily when removed from the spindle.

Figure 2.19 Smart aircraft bolt prototype

MATERIALS TESTING RESULTS

Alloy 2 test results

Chemical analysis results
Tensile test results
Metallographic analysis results
Spontaneous martensite formation by cooling

The magnetic susceptibility measured on impact samples before and after impact shows that the magnetic susceptibility increases after impact testing for samples at both temperatures (see Fig. 3.20 and Table 3.7). Magnetic susceptibility for each sample was recorded for different temperatures, and the effect of lowering the temperature was quantified in table 3.7 which presents the magnetic susceptibility data confirming that stress-induced martensite transformation occurred within Alloy 2/2 during the impact test (see Table 3.7). and Fig 3.20). ID Specimen's magnetic susceptibility Specimen's magnetic susceptibility before impact test after impact test on fracture area.

The magnetic susceptibility varied from 0.05 (from the surrounding material under tensile test) to 0.175 units (see Figs. 3.32-3.33 and Table 3.11), and the strain-induced phase transformation started before the material plastically yielded (see Figs. 3.32-3.33) . This was verified by cooling with dry ice (as opposed to alloy 2/2), as was done for alloy 4/2, and results are shown in Fig. 3.44-3.45, where samples A and B show no change in the magnetic susceptibility with the decrease in temperature. There was no measurable change in the magnetic susceptibility down to -80°C for samples A and B (see figures), from which it can be inferred that there was no thermally induced transformation.

Figure 3.10 Stress-strain A and magnetic susceptibility-strain B curves for alloy 2/2 Specimen 1 (tensile test)

SMART BOLT PROTOTYPE DESIGN

Smart Washer .1 Principle of action

Using the principle of an inductor embedded in a washer or nut (the smart washer and smart nut in the picture as a strain sensor, the strain (damage) can be recorded by reading an inductance meter connected to the washer or nut coil located on the approved area of the bolt , which is found to have the greatest voltage concentration. The concept of an inductor is simplified by considering an inductor connected to an AC source. We can then add a constant C\ related to the dimension of the coil core and,.

Paramagnetic core materials such as austenite have permeability higher than or equal to vacuum (41tX1O-7H/m), while ferromagnetic materials (martensite) have permeability much greater than vacuum. The above principle is used to monitor the health of smart wing screws using an inductance meter reading. In Figures 4.5-4.7 you can see the designed smart washers and drawings of the nut, which clearly show the small groove that houses the inductor, which is now fully shielded (see Figure 4.6 (2»).

DETAIL B

Inductor Location

Air Force workshop and inspection personnel report that the failure occurs on the first engaged thread [94]. Finite element analysis for a center-drilled hole bolt shows a high stress region at the first engaged thread. The end screw of the smart aircraft is shown in Figures 4.10 and 4.11 (1) with the ends of the coiled wires to detect the deformation state of the screw.

Two smart bolt prototypes, one with a groove under the bolt head (see Figure 4.13), the other bolt without a groove (see Figure 4.14), were tested for tension and during the test the strain-induced phase transformation was monitored as described in section. The results can be seen in Fig. where it can be seen that a significant change in inductance (magnetic susceptibility) occurs at the first engaged thread where both screws have failed (see Fig. The tension was measured with the cross head of the tensioning machine; no extensometer was used which explain the high exaggerated value obtained during the bolt tensile test.

Figure 4.8 Stress distributions on bolt and nut model with 5mm drilled hole through the centre of the bolt [94]

Conclusion

CHAPTERS

The second concept involves placing a damage concentrator (or groove) under the head of the bolt. And the third involves the location of a coil near the location of the first wire, which is where fatigue failure begins. 10.A Hall James, FF Zackay and R E Parker, "Structural Observation in a Metastable Austenitic Steel", Transactions of the ASM, vol.

G W Form and W M Baldwin,"The Influence of Temperature on the Ductility of Austenitic Stainless Steels", Transactions of the ASM, vol. Composition on tensile properties of metastable austenitic stainless steels, Transactions of the ASM, volume 59, 1966 pp 223-235. E Schmid, R D Knutsen, "Reducing the Nickel content in metastable Austenitic Stainless steel", Proceedings of the 1st International Chromium Steel and Alloys Congress, Cape Town, vol.

APPENDIX A

SECTION A -A

DETAI LB

APPENDIXB