Smart materials for structural health monitoring.

An extensive study of the alloy chemistry, material processing and transformation properties was therefore carried out, including an analytical model for quantifying the energy associated with martensitic nucleation at a dislocation-disclination level. 13(b): The elastic energy gain LlE versus the normalized embryo size d for FO and different values ​​of grain boundary misorientation for large values ​​of 97.

LIST OF TABLES

INTRODUCTION

Stretch memory alloys, as the name suggests, provide a measure of the strain (deformation) experienced by the material. A measurement of the material's magnetic permeability can therefore be correlated to the amount of stress the material experiences.

CHAPTERl

STRUCTURAL HEALTH MONITORING

Smart materials and their use in structural health monitoring

• Piezoelectricity
• Electrostriction
• Magnetostriction

If an alternating current is applied to the electrodes (as in figure (vi) in Figure 1.1), the physical dimensions of the crystal will change at the same frequency as the alternating current. It is the square root of the ratio of stored electrical energy to the mechanical energy input.

Table 1.2: Summary ofthe various properties of piezoelectric material r2461.

A new philosophy in structural health monitoring

The irreversible transformation from paramagnetic austenite to ferromagnetic martensite means that only successively higher stress readings will be registered within the crystal structure of the material. An indication of peak strain experienced by the material is therefore available to the engineer.

Figure 1. 3: The development of martensite, dependent on strain

STRUCTURAL HEALTH MONITORING IN SOUTH AFRICAN GOLD MINES

• Tunnel conditions
• Support systems currently available
• Continuous mechanical coupled elements
• Determination of support requirements
• Implementing a structural health monitoring solution

Tilting therefore takes place preferably in the side walls, where the size of the slabs depends on the rock type and stress level. These are, as the name suggests, connected to the stone mass along the entire length of the element.

Table 2.1: Summary of environmental conditions within South African mines [81,83,84]

MINING BOLT DESIGN

SQUID

Where H == The component of the magnetic field to be measured, n == Number of turns of the signal winding. Some of the semiconductor materials used are indium antimonide (InSb), indium arsenide (!nAs), and gallium arsenide (GaAs).

Figure 3. 5: The components of a pancake coil induction probe [109]

Loading conditions generally experienced by rock anchors

It is noticeable that the axial stress increases sharply from the nut/exposed end, peaks just before the discontinuity and then decreases rapidly to nothing at the hidden end, indicating that the portion of the bolt closest to the bearing plate is the does most of the work in stopping. rock movement. Unfortunately, the shear stresses were not measured in the test described above because they are more difficult to quantify [79]. The loading/deformation characteristics of grout cables have been the subject of much research within the mining community.

Their rigid nature suggests that they are best suited for conditions where moderate rock movement is expected.

Figure 3.12 shows the test results from a test wherein the bolt was instrumented at various points along its axial length, and the load created by a major discontinuity subjected to shearing action, measured.

Prototype design

SECTION A-A

A check that the coil is intact will be the connection of the sensor readout section, provided there is no open circuit (caused by coil breakage) a nominal reading should be displayed. The re-bar portion of the bolt will be filled and the sensor portion will then be attached. A conceptual schematic is shown below, along with a diagram of the visible instrument face of the bolt.

The other is an LED display section which has only three lights, green, yellow and red, indicating safety, caution and danger levels, obviously predetermined by acceptable levels of rock movement correlated to load levels seen in the sensor section of the bolt.

Figure 3.14: The conceptual schematic of the sensor section ofthe smart mining bolt

ALLOYING AND PROCESSING

Material Requirements

Cost: Although smart bolts are expected to cost more than a regular mining bolt, material, processing and manufacturing costs must be kept as low as possible. Unfortunately, each of the above requirements cannot be met in isolation, many of these requirements influence each other, as seen in Figure 4.1 below, which shows that the sensitivity and accuracy of the component depends on two main factors: the design of the inductive circuit that used to measure the transformation (see chapter 3) and the actual transformation characteristics of the material. The remainder of this chapter is then devoted to an in-depth study of alloy chemistry and material processing.

Figure 4.1: The factors influencing alloy development for the smart mining bolt.

Alloying Chemistry

• Open y field
• Expanded y field
• Contracted y-field

162] When added in sufficient amounts, it improves ductility and formability due to the FCC structure it produces at room temperature. The formation of a martensite in this way should only be a product of the applied strain. The last group of the four belongs to those elements that strongly contract the y-loop, but compound formation is present and has the diagram shown in Figure 4.6 below.

For example, in the case of the smart mining bolt, it is necessary to add chromium for corrosion resistance, as well as some austenitic stabilizers to produce the metastable austenitic phase at room temperature.

Figure 4.2: An example of an open y-field Iron binary phase diagram

Material Processing

• Time Temperature Transformations

In light of the possibility created by the metastable bay of Figure 4.11, machining can be developed around a warm working process. However, another method of strength enhancement has also been proposed by Koppenaal et al [178] to replace the then costly nonmechanical treatment, with a subsequent process said to achieve similar strength levels. The most significant effect of processing is undoubtedly the increase in the strength of the material.

The figure below indicates how the stability of the austenite changes depending on the level of thermomechanical processing.

Figure 4.10: An example ofthe TTT curve for a plain carbon eutectoid steel

Mechanical properties of strain memory alloys

• Stress-strain curves
• Ductility

And larger amounts of PDA appear to be more beneficial for the propagation properties of fatigue cracks than smaller amounts [179]. Fatigue crack propagation is the result of cumulative damage caused by stress cycles before the crack tip. High cyclic plastic strain induces the formation of martensite, which tends to increase the effective rate of strain hardening, which should give rise to higher fatigue crack propagation rates.

In contrast to the beneficial effects experienced by fatigue crack propagation properties, low cycle fatigue properties are negatively affected.

Figure 4. 19: Comparison ofstrength and ductility for various grades ofsteel

Conclusion

CHAPTERS

TRANSFORMATION MECHANICS

Stress-assisted and strain-induced martensite

Several researchers have shown that the martensite morphology depends on the mode of nucleation and growth, which differ in thermally spontaneous martensite, stress-assisted martensite, and strain-induced martensite. Near the Ms temperature, stress-assisted martensite dominates, but near the MD temperature, strain-induced martensite forms [164]. Stress-assisted martensite can form in some alloys in the presence of an applied stress, but it forms by the same nucleation and growth process as found in martensite that forms spontaneously on cooling below the Ms temperature.

Plate martensite is internally twinned and is said to be lenticular, with a midrib as evidenced in the figures below, which show stress-assisted martensite forming in Fe-Ni-C samples having a Ms temperature of - 72°C, in tension.

Figure 5.3(c): Stress-assisted martensite Figure 5.3(d): Stress-assisted martensite

Modelling the transformation

5.4). where E~ is the self-elastic energy of the dipole of 0 -disclinations, E~ is the self-elastic energy of the dipole of OJ-disclinations. E:'1ll is the energy of interaction between the ensemble of virtual dislocations and the dipole of OJ disclinations,. The sum nuclear energy of dislocations modeled by the dipole of Q /2 -disclinations, E~/2' is similar to eq.

The total energy of the initial state defect configurations, El', is thus given by Eq.

Figure 5.10: Dislocation-disclination model of HCP-martensite embryo nucleation at a tilt grain boundary with extrinsic dislocations

Results and Discussion of model verification

When the embryo is small (see Figure 5.11(a), the total free energy gain flW is negative and reaches its minimum value at d = deq, which can be considered an equilibrium size for the martensitic embryo. This energy increase causes the negative values ​​of till at small d as well as the existence of the equilibrium of embryo size deq. This observation can be used to estimate the values ​​of re analytically depending on other parameters of the considered model normalized embryo size d for the temperature T=200K and different values ​​of external shear stress, for small values of d. normalized embryo size J for the temperature T=200K and different values ​​of external shear stress, for large values ​​of

The critical external stress is studied as a function of temperature and characteristic grain boundary parameters where fcc-to-hcp martensite transfonnation.

Figure 5. l1(a): Different terms ofthe total free energyJain ilW vs. normalised embryo size for small values of d

ALLOY FORMULATION AND TESTING

Tensile testing

Before starting the tensile test, a coil was wound around the narrow test section of the specimen as shown in Figure 6.7 below and connected to an inductance meter. Then, graphs of uniaxial tensile strain were plotted against inductance, which varied with changing permittivity of the core material as predicted by Equation 3.5. The greater the percentage change in inductance in the material's testing range, the more sensitive the material is to transformation caused by deformation.

The changing magnetic permeability of the material was measured using a coil and miniaturized electronic circuitry, which provided a reading measured in volts.

Figure 6.7: A coil was wound around the necked down section of the tensile sample

Stress Vs Strain: Fe-er-Ni Alloy 1000

Fe-Cr-Ni alloys

Of the three, only 304 is actually commercially available in South Africa, and was tested, together with a low-carbon version of 301, specially prepared at Columbus Stainless Steel, the composition of which is given in Table 6.8 below, and designated Cl. This transformation is further manifested in microstructures taken from the constricted region of the fractured tensile sample. Some of the most common alloy materials are shown below, with their prices (July 2002) per kg.

If then the alternative would be to develop a new alloy which would be custom made just for mining bolts (and the facilities for such an operation do exist in South Africa as we enjoy the existence of many smaller manufacturing companies more flexible), then the obvious strategy would be to replace some of the expensive nickel with cheaper austenitic.

Figure 6.12:Typical Iuductance Response for varying load - Alloy 304

Fe-Cr-Mn alloys

Based on the mechanical and transformation properties of C2 and the poor machining properties of the BI alloy, the C2 alloy was selected as a smart material for mining screws. The lack of significant change in inductance indicated that the Ms temperature of alloy C2 was too low for large amounts of martensite to form at liquid nitrogen temperatures. The small increase in hardness after cooling was attributed to the small amounts of martensite that did form, as seen in the microstructure shown in Figure 6.21.

The first prototype of this sensor has been received and tested, the results of which are shown in Figure 6.22 below.

Figure 6.15: Typical Inductance Response of alloy C2, varying with applied load.

Conclusion

CONCLUSION

The general corrosion properties were manipulated at alloy formulation stage with the inclusion of sufficient chromium, and therefore there was no need to test specific corrosion response of the material. That is, it has taken into account the fact that industrial smelting facilities need an alloy window to work on. So Analloy was chosen which would be "forgiving" or have a decent operational window. The choice of an inductance method was prompted by the fact that it was most easily adapted to the physical constraints of the rock anchor environment.

Further work is still needed to confirm the mixed load behavior of the boIt, as well as a robustness test that only an actual installation in a mine can do.

GLOSSARY

Tiersten, RF., "Linear Piezoelectric Plate Vibrations - Elements of the Linear Theory of Piezoelectricity and the Vibrations of Piezoelectric Plates", Plenum Press, 1969. Egusa, S., Iwasawa, N., "Piezoelectric paint as one approach to smart structural materials with health monitoring capabilities." Smart Mater. A and Maksimov, A Yu., "Giant Electrostriction of Ferroelectrics with Diffuse Phase Transition: Physics and Application", Ferroelectrics, Volume 134,1993.

0 E Schmid, R D Knutsen, "Reducing the Nickel Content in Metastable Austenitic Stainless Steel", Proceedings of the 1st international Chromium Steel and Alloys Congress, Kaapstad, Vol.2, 1992, pp 151-156.

APPENDIX A