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Nano-Indentation Assessment of Property Changes in Common Engineering Plastics after Exposure to Microwave Energy Adekunle Oloyede and Ayodele Olofinjana School of Mechanical, Manufacturing and Medical Engineering Queensland University of Technology, GPO. Box 2434, Brisbane Queensland, Australia. Email: k.Oloyede@qut.edu.au. Contact

ABSTRACT

The volumetric heating properties of microwaves make them an extremely useful tool in industrial applications for processing materials. One particular material that is experiencing an increase in microwave processing is plastic. Microwaves are being used increasingly more often for high temperature joining of thermoplastic materials. Unfortunately this method of welding creates a heat-affected zone (HAZ) due to the difficulties found in directing the microwave energy solely to the joint interface. Furthermore the material properties exhibited by this HAZ might differ greatly to those of the parent material in its original state.

A non-destructive technique, performed on the Ultra-micro indentation system, is carried out to evaluate the properties of the HAZ. From this test it was possible to obtain results that can be used to analyse the hardness of both heated and unheated material. These results were in turn compared with other destructive methods of analysis such as impact testing and tensile loading, and with another non-destructive test, photo-elastic analysis, to determine the accuracy and validity of the UMIS non-destructive test.

Keywords: microwave heating, nano-indentation, microwave joining, polymer welding, fracture toughness

INTRODUCTION

Microwaves have in the past been utilized to a limited extent as a tool for non-destructive testing of materials defects. They have been used to measure thickness, flaw detection and pin pointing and material parameters variations. Analysis of materials by microwaves is a relatively specialized field. This is due in part to NDT engineers' and technicians' limited knowledge of microwave theory and techniques [1] [2].

Microwaves are currently experiencing increased usage in many differing industrial heating applications. Microwave heating offers advantages over conventional heating in many different areas such as cost and time reductions and some control benefits (industrial microwaves). Despite these obvious advantages there are also some unfortunate disadvantages associated with microwave heating. The fact that microwaves are inherently difficult to direct, they can only be used to heat materials that absorb microwave energy and the added complications of thermal runaway (industrial microwaves) all detract from the usability and versatility of microwaves in heating applications. The limited utilization of microwave heating is due mainly to these factors and, as with microwaves in NDT, the lack of general knowledge needed to find solutions to the associated problems.

The purpose of this paper is to help overcome these hurdles by investigating the effect of microwave heating on materials properties. Throughout the execution of this investigation, a non-destructive way of testing materials response to microwave heating will be developed. In this paper epoxy resin, a plastic composite, is selected to be analysed. This is a material which becomes optically anisotropic when stress is applied, was chosen to be used as the test material. Different tones of colour, which indicates different stress levels were would be observed when placed between crossed polarizers of a polariscope. In order not to damage or induce any changes in these fringe patterns, the hardness value of the specimen was measured using the ultra-micro indentation system (UMIS). Destructive impact and tensile testing was carried out on specimens of the same configuration and values of fracture toughness were acquired to compare against the results of the non-destructive test. Fracture specimens have been chosen for measuring changes in properties in this study because it is much easier to see fringe patterns around crack roots and because the fracture resistance of a material is an important parameter of design.

Microwave heating principles

Microwave is a term used to describe the group of electromagnetic waves falling in the frequency range of 1 to 300 GHz. They are currently used in the telecommunications and materials processing industries. Some examples of materials processing uses are drying timbers and ceramics [3].

Microwave heating is a much more efficient form of heating compared to convection heating, offering saving in the order of 10-100 fold in energy and 10-200 fold in time [4].

Only some materials are suitable for heating by microwaves. Non-conducting materials are transparent to the microwaves and highly conductive materials reflect them. Other materials that absorb the microwave energy do so to different degrees according to their dielectric loss factor or loss tangent. The dielectric loss factor is the measure (the fraction) of how much of the energy produced by the materials interaction with the electric field is released as heat. In short, the dielectric loss of a material is a measure of the amount heating it experiences when it is polarized in an alternating field [5].

In a uniform field at the required frequency, energy distribution is uniform across the interface perpendicular to the propagation of the electro-magnetic waves. The power dissipated throughout the material is explained by the following equation: -

P = 1/2w . e 0. e k.E2.V.e-2a z

(1)

Where:
E = electric field through the surface
V = volume
w = 2*p *frequency
z = distance into specimen
a = attenuation constant


This equation indicates that there is very close to even dissipation of energy throughout the volume of the specimen being heated [6]. Despite this a thermal gradient across the volume of the material is caused by the heat flow being from inside the material towards the cooler surrounding environment. The thermal gradient set up is therefore greater in the middle of the body being heated with the temperature decreasing at an ever-increasing rate towards the surface of the material. This is the opposite situation to that which is found in conventional heating, where the heat energy is conducted from the outer surfaces of the material towards the center. This is why microwave heating of materials is often characterized as heating the material from the inside out.

A further evaluation of the power dissipation equation highlights that the energy is only evenly distributed if the materials properties (permeativity, loss factor and attenuation constant) are consistent throughout the material. Yet in most substances the loss factor or loss tangent properties change as the temperature of that substance changes. Some materials experience a decrease in loss factor values as they increase in temperature. This helps in limiting the amount of heating experienced by those materials within a microwave field and encourages even temperatures throughout the material.

Unfortunately a large amount of materials experience an increase in their dielectric loss factors as a result of increased temperature. As a result of this, the hotter parts of the material absorb more heat from the microwaves. The rate of heating of certain areas within the material is accelerated due to a combination of limited conductive and radioactive heat losses from the material and the heat generated from the polarization process. Due to this effect, certain areas of the sample being heated experience increasingly greater rates of heating while other areas experience increasingly low rates of heating. This phenomenon is known as thermal runaway [12].

Ultra Micro Indentation System (UMIS)

The UMIS is a software driven electro-mechanical instrument, used for determining properties of high technology surface coating, polymers. It was developed in the mid 1980's at NML (National Measurement Laboratory) of CSIRO in Australia. Working on the same principle as conventional indentation hardness testers, an extremely small diamond tip indenter with either pointed or spherical geometry carries out indentations on the surface of the material being tested. The indenter tip is loaded onto specimen with a specific force, with force and depth (or penetration) being measured independently at each step of process. The indenter load is increased in equal steps (increments) of amounts specified prior to testing. When maximum force is reached, the indenter unloads by the same number of increments until a zero load is again reached [7].

Using this load / unload curve, it is possible to determine hardness as a function of depth, elastic modulus as a function of depth, strain index, creep and contact stress-strain. Due to the nano-indentation properties of this machine, limited damage is incurred by the surface of the material being tested. Therefore the UMIS machine is classed as being a non-destructive testing tool. The focus of this paper is to explore the possibilities of using this tool as a non-destructive technique for assessing the properties of engineering polymers in their processed state.

Fracture mechanics

Materials contain defects that are micro structural in origin or introduced during the manufacturing process. These include porosity, shrinkage cavities, quench cracks, grinding and stamping marks (such as gouges, burns, tears, scratches, cracks), seams and weld related cracks. Other micro constituents, such as inclusions, brittle second-phase particles, and grain-boundary films, can lead to crack formation if the applied stress level exceeds some critical Level [8].

These inherent material micro defects have an effect on the mechanical properties of the material. Of these properties the most affected by these micro-defects is the fracture toughness of the material. The fracture toughness value that is quoted for the material is a measure of that materials resistance to fracture. Absence of toughness is denoted by the term brittle and when a material can be induced to fracture with the expenditure of little effort it is so described.

The standard quoted value for materials fracture toughness is the stress intensity factor, K_Ic. In the case of a sharp crack, the stress field established due to any type of loading would influence fracture or crack propagation rate. It is therefore of interest to investigate, if microwave heating creates a stress field at the crack tip and how such stress field affects the mechanical properties especially the toughness of the material.

There are ever increasing examples of electromagnetic waves being used to join plastic polymers in the industry. Also an increasing research has been done on the optimization of these joining processes through the characterization and modeling of the microwave heating processes. Due to increasing concern in this area some experimental work has to be carried out on the physical ramifications of this particular method of heating using empirical results. This particular paper addresses the issue of how the plastics properties of fracture toughness are effected when heated in a microwave joining environment.

Fracture Toughness is the main property of concern in this paper. The focus on fracture mechanics within this paper is for two reasons. Firstly, fracture mechanics is an important field of knowledge for understanding a majority of failures in engineering polymers under working conditions and the fracture toughness is dependant on the stress states within the Material [9]. Secondly, the heating of polymers with microwaves more often than not corresponds to a welding or joining process, and these processes are linked to both uneven heating of the material and the creation of cracks and flaws within the material.

Experiments and Methodology

Impact test specimens were heated by electro-magnetic waves to assess the effect that microwave heating has on the fracture properties of the material. An industrial strength magnetron and a purpose built wave-guide were used for this end. This ensured that there was controlled and uniform heating energy applied to the specimen. The specimen configuration was such that the dimensions complied with those dimension specified in the standard for impact testing of plastics. The specimens had the smallest thickness (3mm) available as specified within the standard [10] [11].This also ensured that there was limited thermal gradient across the thickness of the test specimens and as such the problem of thermal stresses was one limited to a 2-dimensional problem.

In this paper epoxy resin is selected to be the material used to analyse the effects of microwave heating. This epoxy resin is a thermoplastic composite that has a relatively large dielectric loss factor, which means that the material was easy to heat using microwave energy. This material is optically clear also an optically anisotropic material. This means that when stress is applied, different tones of colour, which indicate different stress levels within the body of the material, can be observed when placed between the crossed polarized lenses of a polariscope.

The epoxy resin that was chosen for manufacture of the specimens was not fully characterized in terms of its dielectric properties. Its dielectric constant and dielectric loss factor had not been previously determined for different microwave wavelengths nor was the testing required to determine these values completed during the experimental process carried out for the purposes of this paper. The material was found to heat extremely efficiently at the testing wavelengths employed for heating in this paper, twice as quickly as a known high dielectric loss material (PVC), as discovered from preliminary experiments. The knowledge of the fact that the material heats up over a range of time comparable to that of other polymers commonly heated for processing with electro magnetic waves was deemed as sufficient for this paper.

A process of trial and error was used to arrive at suitable heating times and microwave power settings. Specimens were heated in small increments of time, with the photo-elastic stress pattern checked between each incremental step. The lowest value heating time for a thermal stress to be frozen into the material was then nominated as the first duration length for experimental heating. The time till the melting point of the material was reached was ascertained. A period slightly lower than the melting point heating time was then nominated as another experimental heating duration. The third heating duration used in the experiments was then taken as the time half way between the two other times.

A number of the epoxy resin samples were heated in the microwave for the pre-specified times at the two different power settings. The effect of this heating on the fracture toughness of the specimen was then analysed by destructively testing both non-heated and microwave heated specimens in the instrumented charpy pendulum impact test machine. Energy for fracture and peak load at fracture was measured through the results obtained from the instrumented impact tests. The fracture energy values were used to calculate the fracture toughness for the materials. These values for each different specimen heating time and power were then compared and contrasted to determine the relationship between microwave heating variation and the fracture properties of the material. The findings from this process were backed up with another destructive method of analysis. Specimens of the same configuration as the charpy specimens were stressed to fracture under a steady state tensile load on a Hounsfield machine. A stress strain curve for each different heating duration and rate is acquired from this test and compared with the data from the charpy tests. The destructive analysis of the materials fracture properties within this paper was limited to these two methods.

The analysis of the specimen of the same configuration as the above-mentioned destructive test is also carried out, using Ultra Micro Indentation System (UMIS) a Non-destructive technique. By doing this we were able to obtain results to correlate both destructive testing and non destructive thus proving that traditional methods of destructive test can be replaced by the non-destructive test which is simple, neat and does not damage the specimen. The properties of the specimen allow a photoelastic technique to be utilized to identify the different stress fringe patterns developed within it while under different tensile loading conditions. These stress fringe pattern can then be directly compared to the tensile destructive test on the Hounsfield machine. Hence an accurate correlation can be made between all the different testing techniques employed in this paper.

This photoelastic analysis technique, although extremely useful in analysing the stress patterns within the material, does not lend itself to use on most engineering polymers due to the non-photoelastic nature of these polymers. The UMIS hardness technique of analysis is a much more versatile test that can be carried out on almost all materials without damage incurred on the material being tested. Therefore the main focus of this paper will be to establish a correlation between the UMIS test and the other techniques of analysis of the microwaved and non-micowaved properties of the materials.

Results and Discussion

The values of the energy for fracture were all higher for the microwaved specimens than for the non-microwaved specimens. Both the hounsfield and charpy test results showed no curvature before fracture in their load deformation curves. This indicated that the fracture event was totally elastic with no plastic work being done on the specimen. A good correlation with respect to the behaviour as heating duration increased between the hounsfield machine and the instrumented charpy impact machine was observed for both microwave power settings. This can be seen in the similar graph shapes between the two.

As the duration of microwave heating increased, the values of hardness acquired from the UMIS analysis decreased proportionately. There was a smaller variation in load at fracture for heating duration at the 250W power setting. This translates into a smaller variation in fracture energy for different heating durations at the 250W power setting.

The variation in both Fracture energy and Breaking force with respect to heating duration can be seen to be more linear for heating at 250W than at 500W.

The results from the polariscope tensile test also shows that the breaking tensile load for the microwaved specimen is higher as compared to the non-microwaved specimen for both power level.

The stress strain plot of the hounsfield tensile test shows that the slope is almost identical for both the microwaved and non microwaved specimen since. The only difference in the stress strain plot for the microwaved specimens as apposed to the non-microwaved specimens is the maximum tensile stress at fracture attained in the test. The fracture loads of the heated specimens did not seem to reflect a simple relationship with the heating duration experienced by the specimens.

K_Ic (MPaÖm) values calculated from Charpy impact energy
K_Ic for non-heated specimens = 0.309 MPaÖm

Duration	40sec	50sec	60sec
250W	0.332 MPaÖm (7.5% increase)	0.336 MPaÖm (8.7% increase)	0.339 MPaÖm (9.7% increase)
Duration	20sec	25sec	30sec
500W	0.487 MPaÖm (57.6% increase)	0.427 MPaÖm (38.2% increase)	0.327 MPaÖm (5.8% increase)

The values for K_Ic calculated from the charpy impact energy correlate well with the known values of similar materials such as PMMA, which is slightly more pliable and has a value of K_Ic in the range of .8-2.3 MPaÖm [6].

These K_Ic values are on average greater for the microwaved specimens than those calculated for the non-microwaved material however, this change is relatively insignificant. It is worth noting that for the specimens heated at 500 W the fracture toughness decreased with time, thereby suggesting a considerable modification of the material, since fracture toughness is a material property which should have remained constant. The UMIS values were on average lower for the microwaved specimens than for those specimens that were not microwaved. Although there was not a direct correlation between the UMIS values and the calculated K_Ic values for different power settings and time durations, the results point towards the possibility of UMIS results being used to determine the extent of the change of fracture toughness of the processed material relative to its original properties.

By utilization of the photoelastic properties of the material tested, the application of microwave heat to the specimens can be seen to have left a residual frozen stress within the material. The difference in the stress patterns at the notch tip for a variety of heating rates and durations can be seen in the first row of photos. This residual stress was most concentrated at the notch tip. The frozen stress is not only affecting the material in the non-loaded state but can be seen to be working while the external load is applied. An inspection of figure2 and figure3 compared to figure1 shows the change of characteristics of the stress pattern at the notch tip in the loaded condition.

0N
0W 0s	250W 40s	250W 50s	250W 60s	500W 20s	500W 25s	500W 30s
Fig 1: Photoelastic stress pattern for a variety of specimen heating power and duration at no external load 150N.

0W 0s	250W 40s	250W 50s	250W 60s	500W 20s	500W 25s	500W 30s
Fig 2: Photoelastic stress pattern for a variety of specimen heating power and duration at 150N tensile load 250N.

0W 0s	250W 40s	250W 50s	250W 60s	500W 20s	500W 25s	500W 30s
Fig 3: Photoelastic stress pattern for a variety of specimen heating power and duration at 250N tensile load.

Fig 1: Charpy Test Result for 250W.

Fig 2: Hounsfield Test Result for 250W.

Fig 3: Charpy Test Result for 500W.

Fig 4: Hounsfield Test Result for 500W.

Fig 5: Charpy Test Result for 250W.

Fig 6: UMIS Test Result for 250W.

Fig 7: Charpy Test Result for 500W.

Fig 8: UMIS Test Result for 500W.

Fig 9: Hounsfield Result for 250W.

Fig 10: Charpy plot.

Conclusion

Our results demonstrate that measurements using the nano-indentation equipment, namely UMIS, compare favourably with the data obtained from destructive tensile and impact testing of our thermoplastic resin. With the hardness measured using the UMIS, fracture stress and fracture toughness obtained with traditional destructive test exhibiting the same trend.

REFERENCES

1. D.Birchon, Non-Destructive Testing, Oxford University Press, (1975).
2. Alfred J.Bahr, Gordon and Breach, Microwave nondestructive testing methods, Science Publishers, Inc., (1982).
3. E. Siores, D.D. Rego, Joining materials with microwaveenergy, Aust. Weld. J. (Second-quarter) 39 (2) (1994) 18-21.
4. J. Binner, The potential processing of ceramics, Material World, J. Inst. Mater. (March) 9 (3) (1993) 152-155.
5. A.C. Metaxas & R.J. Meredith, Industrial Microwave Heating, Peter Peregrinus Ltd, (1983).
6. Prasad K.D.V. Yarlagadda, Tan Chuan Chai, An investigation into welding of engineering thermoplastics using forward microwave energy. Journal of Materials Processing Technology 74 (1998) 199-212.
7. http://www.anutech.com.au/asi/UmisMain.html
8. Richard W. Hertzberg, Deformation and Fracture Mechanics of Engineering Materials, John Wiley & Sons, INC., (1996).
9. FAA Crane and JA Charles, Selection and use of Engineering Materials, Butterworths, (1984).
10. AS 1146.2 - 1990 Method for impact tests on plastics Part 2: Charpy impact resistance.
11. ASTM D256-93a Determining the pendulum impact resistance of notched specimens of plastic.
12. Committee of microwave processing of materials, Microwave processing of materials, National Research Council, National Academy Press, Washington DC,(1994).

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