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MECHANICAL CHARACTERIZATION

By Ferhat SONAT

1. INTRODUCTION

The selection of the proper material is a key step in the design process. The enormity of this decision process can be appreciated when it is realized that there are over 40,000 currently useful metallic alloys and probably close to that number of nonmetallic engineering materials. An improperly chosen material can lead not only to failure of the part or a structure but also to unnecessary cost. Selecting the best material for a part involves more than selecting a material that has the properties to provide the necessary service performance; it is also intimately connected with the processing of the material into a finished part. Thus, a poorly chosen material can add to manufacturing cost and unnecessarily increase the cost of the part. Also, the properties of the part may be changed by processing, and that may affect the service performance of the part.

Material properties are the link between the basic structure and composition of the material and the service performance of the part. The performance or functional requirements of a material usually are expressed in terms of physical, mechanical, thermal, electrical, or chemical properties. Each property has a significant effect on material properties however this study is only concerned with mechanical characterization [1].

Mechanical characterization is the classification of materials according to their mechanical properties. It is possible to identify three main aspects of mechanical characterization. First, it is used to provide data for design. The designer can, in many cases, formulate a fairly precise estimate of the loads that a structure will be required to withstand. Once he has devised methods of dealing with these loads, he can define the nature of the stresses that will be imposed on the various components. Determination of the shapes and sizes of these components requires knowledge of permissible stresses in materials considered for use, and this knowledge is derived from mechanical testing experience. Tests are undertaken by well-established methods at the manu­facturing stage, with the object of ensuring that certain properties of the material are satisfactory for the proposed application, by matching the observed properties against some specification. For certain applications where design stresses are of a simple nature, information of this kind may be all that is needed. In other cases, it should preferably be supplemented by more detailed knowledge of the behaviour of the material as affected by the size, shape, method of finishing, etc., of components under actual service condi­tions.

Secondly, mechanical characterization may be employed as part of 'trouble-shooting' procedure, when it becomes necessary to establish the cause of some service failure, to verify that the material was in fact what it purported to be. It may be noted here that only a very small minority of the failures which do occur arise from the use of material not conforming to specification and it is frequently possible to elucidate failures without recourse to testing.

Thirdly, mechanical characterization of various kinds are very widely employed as tools of research scientists such as metallurgists developing new alloys, metal-physicists studying the microscopic and sub-microscopic mechanism of flow or fracture in materials, or engineers studying the macroscopic behaviour of materials in structures or machines [2].

2. BASIC CONCEPTS OF MECHANICAL CHARACTERIZATION

2.1 Strength

All solid materials can be deformed when subjected to external load. There are three basic ways by which a load can be applied; tension, compression, and shear. These loads can affect the material in two manners: elastically and plastically. It is known that up to certain limiting loads a material will recover its original dimensions when the load is removed. The recovery of the original dimensions of a deformed body when the load is removed is known as elastic behaviour. The limiting load beyond which the material no longer behaves elastically is the elastic limit. If the elastic limit is exceeded, the body will experience a permanent set or deformation when the load is removed. A body which is permanently deformed is said to have undergone plastic deformation. If the load is static or changes relatively slowly with time and is applied uniformly over a cross section, the mechanical behaviour may be expressed by a simple stress-strain phenomenon [2].

2.1.1 Elastic Strength

Elastic strength can be defined as a stress level below which no permanent distortion can be observed in a material. Elastic strength can be stated with some parameters. Basic ones are yield strength and elastic (Young’s) modulus. These parameters can be expressed in a plot of stress vs. strain diagrams.

Yield strength is the stress below which no or very small plastic deformation is observed. Also it can be defined as the stress required to produce a very slight amount of plastic strain, a strain offset of 0.002 is commonly used. The unit of the yield strength is MPa.

Another important parameter is the Elastic Modulus or Young’s modulus (E). For most materials that are stressed in tension and at stress levels below yield strength, stress and strain are proportional to each and this relationship is known as Hooke’s law. Modulus of elasticity is the ratio of stress to strain when deformation is totally elastic. It can be calculated from a stress-strain diagram by taking the slope of the linear portion which means the elastic region. The unit of this parameter is GPa.

If the applied load is in the shear mode, the ratio of stress to strain in elastic region is called shear modulus (G). Shear modulus can also be calculated from a stress-strain diagram by taking the slope of the linear portion.

While stating the mechanical properties of a material, it is important to compute the Poisson’s ratio (υ). It is the ratio of the unit deformation at right angles to the load to the unit deformation in the direction of the load within the elastic range of stress. In other words, it can be simply calculated as the negative ratio of lateral and axial strains that result from an applied axial stress in the elastic region. Poisson’s ratio has no unit [3].

2.1.2 Ultimate Strength

Ultimate strength is the maximum stress level that the material can withstand without a failure. The ultimate strength can be classified into three main groups according to the applied stress type. They are ultimate tensile strength (UTS), ultimate compression strength and ultimate shear strength. The common unit of the ultimate strength is MPa.

2.2 Ductility

Ductility is another important mechanical property. It is a measure of the degree of plastic deformation that has been sustained at fracture. A material that experi­ences very little or no plastic deformation upon fracture is termed brittle. Ductility may be expressed quantitatively as either percent elongation or percent area reduction. The percent elongation %EL is the percentage of plastic strain at fracture. The ductility of materials is important for at least two reasons. First, it indicates to a designer the degree to which a structure will deform plasti­cally before fracture. Second, it specifies the degree of allowable deformation during fabrication operations. The opposite mechanical property of ductility is the brittleness. They show nearly no plastic deformation before the failure. Brittle materials are approximately considered to be those having a fracture strain of less than about 5%.

2.3 Resilience

Resilience is the capacity of a material to absorb energy when it is deformed elastically and then, upon unloading, to have this energy recovered. Also resilience can be defined ass property that enables a material to with stand impact without distortion. The associ­ated property is the modulus of resilience, U_r, which is the strain energy per unit volume required to stress a material from an unloaded state up to the point of yielding. It is numerically equal to the area under the elastic portion of a stress-strain diagram for a tensile or compressive test. 

2.4 Toughness

Toughness is a mechanical term that is used in several contexts. It can be generally defined as a measure of the ability of a material to absorb energy up to fracture. Speci­men geometry as well as the manner of load application is important in tough­ness determinations.

For dynamic (high strain rate) loading conditions and when a notch (or point of stress concentration) is present, notch toughness is stated as a parameter of toughness. Also it can be define as a measure of the energy absorbed during the fracture of a specimen of standard dimensions and geometry when subjected to very rapid (impact) loading.

Furthermore, fracture toughness is a property indicative of a material's resistance to fracture when a crack is present. In other words, it can be defined as a critical value of the stress intensity factor below which no crack growth can be observed [3].

For the static (low strain rate) situation, toughness may be ascertained from the results of a tensile stress-strain test. It is the area under the stress-strain curve up to the point of fracture. For a material to be tough, it must display both strength and ductility; and often, ductile materials are tougher than brittle ones.

2.5 Fatigue

Fatigue of materials is the changes in properties which can occur due to repeated application of stresses and usually means an effect which leads to cracking or failure. Fatigue failure can occur for many reasons as a result of ordinary mechanical fatigue, thermomechanical fatigue, creep fatigue, corrosion-fatigue, etc. A common point of these failure processes is that they take place under the influence of cyclic loads whose peak values are considerably smaller than the safe load [4]. The basic method of presenting the fatigue data is to plot a stress vs. cycles to failure. By the fatigue data remaining life time estimations for materials can be done.

An important parameter in fatigue is the endurance limit. It is defined as the minimum stress under which no fatigue behaviour can be observed. In determining the fatigue limit of a material, it should be recognized that each specimen has its own fatigue limit, a stress above which it will fail but below which it will not fail, and that this critical stress varies from specimen to specimen for very obscure reasons. However, it is now recognized that the fatigue limit is really a statistical quantity which requires special techniques for an accurate determination.

2.6 Hardness

Another mechanical property that may be important to consider is hardness, which is a measure of a material's resistance to localized plastic deformation. Also it can be taken as resistance to abrasion, to scratching, or to cutting, or it may denote brittleness, stiffness, resilience, toughness, high strength, or combinations of these properties.

Hardness techniques have been developed over the years in which a small indenter is forced into the surface of a material to be tested, under controlled conditions of load and rate of application. The depth or size of the resulting indentation is measured, which in turn is related to a hardness number; the softer the material, the larger and deeper the indentation, and the lower the hardness value. Measured hardness values are only relative (rather than absolute), and care should be exercised when comparing values determined by different techniques.

Two of the most widely used indexes of hardness are Rockwell, Brinell and Vickers. However, these indexes can be correlated one with another. A noticeable point is that there is a reasonable correlation between ultimate strength and the hardness indexes and also between the endurance limit and the hardness indexes obtained from the hardness tests. For instance, among some materials the tensile strength can be estimated with sufficient accuracy from its Brinell hardness number. In addition to these, they often constitute a convenient non-destructive test to determine whether or not the material is in the physical state which other tests, generally destruc­tive, have shown to be suitable. If a material is of uniform composition and has been produced by a well-controlled process of manufacture, uniform indentation-hardness numbers are nearly always a sufficient guarantee of its uniform quality.

3. BASIC METHODS OF MECHANICAL CHARACTERIZATION

3.1 Tensile testing

Tensile testing has made it possible to deter­mine ulti­mate tensile strength and elongation, yield point, yield strength, modulus of elasticity, resilience, toughness and some other material properties. The mechanical properties from a tensile test can be best illustrated by plotting a stress-strain curve. From this plot, the deformation behaviour of a material can be seen. Thus the material can be characterized by its yield strength, ultimate tensile strength, stiffness, ductility, resilience and toughness. An example of a stress-strain curve is given in Figure 3.1.

Figure 3.1: Typical stress-strain curve

One of the primary aims in conducting ten­sion tests is to determine conformance or non-conformance to specifications. The data may thus serve as an index of the quality of a product in comparison with data obtained from other sources. Such data may also be used to compare a given material with other materials and, in conjunction with other factors, may assist in the replacement or improvement of a material.

Tension testing consists of subject­ing a prepared specimen of specified size and shape, or a full-size specimen, to a grad­ually increasing (static) uniaxial load (stress) until failure occurs. The operation is accom­plished by gripping opposite ends of the specimen and pulling it, which results in elongation of the test specimen in a direction parallel to the applied load. Tension testing machines apply uniaxial loading in a uniform manner and generally are universal in their capabilities and applica­tions, rather than specific to one type of test or material. Conducting load versus elonga­tion (stress versus strain) tests involves con­trol of forces up to 45 000 kgf or more, holding onto a specimen that may be thin sheet metal, machined flat plate, or a cylindrical polished sample, and measuring forces(stress) and deformations(strain) with accuracy and repeatability. The effect of the instrumentation on the test data must be minimized to avoid testing errors. Automatic recording of stress and strain is highly desir­able, and modules to allow specific func­tions, such as automatic shutoff when the specimen breaks, simplify machine use.

A tension tester should have adequate loading capacity to cover the expected testing force range and interchangeable grips to hold various test specimens that can be easily mounted and dismounted. Typically, the test load is applied through multiple drive screws and drive nuts that move a crosshead, which is attached to a grip holding one end of the test specimen. The other end of the specimen is attached by a grip to a fixed base. By driv­ing the screws at various rates, tension tests can be conducted over a wide range of test conditions. A schematic representation of the a machine and apparatus used to conduct tensile stress-strain tests is given Figure 3.2 [4,5].

Figure 3.2: A schematic representation of the a tensile testing machine and apparatus

3.2 Compression Testing

Compressive test give information about a material under a load. From this test the mechanical properties such as yield point, yield strength, modulus of elasticity and ultimate compression strength. Compressive tests are commonly used as a basis for acceptance of ceramic and polymeric materials. They are seldom specified for metals since tensile and, in a few cases, flexural tests supply essential information more easily. In a compressive acceptance test the ultimate strength is usually the only property observed. The specifications devote particular attention to the selection of the samples, the types of specimens, and the technique of testing. Compressive tests, like other tests, are also employed in research to determine mechanical properties and characteristics of materials under load.

Compression testing is the same in many ways as tension testing. The specimen shortens, however, instead of lengthening. Compression testing is the same in many ways as tension testing. The specimen shortens, however, instead of lengthening. The ultimate compressive stress can be found for brittle materials tested in this way but ductile materials often compress indefinitely without fracture. Up to, and through the yield point, the compression stress-strain curve is similar to that for tension; beyond this point, however, the compression curve becomes steeper because of the increased cross-sectional area in compression.

3.3 Torsion Testing

The torsion test has not met with the wide acceptance and the use that have been given the tension test. However, it is useful in many engineering applications and also in theoretical studies of plastic flow. Torsion test can be carried out on most materials to determine mechanical properties such as shear modulus, yield shear strength, ultimate shear strength, modulus of rupture in shear, and ductility. The torsion test can also be used on full-size pans and structures to deter­mine their response to torsional loading like maximum torque. In torsion testing, unlike tension testing and compression testing, large strains can be ap­plied before plastic instability occurs, and complications due to friction between the test specimen and dies do not arise.

Torsion-testing equipment consists of a twisting head, with a chuck for gripping the specimen and for applying the twisting moment to the specimen, and a weighing head, which grips the other end of the specimen and measures the twisting moment, or torque. The deformation of the specimen is measured by a twist-measuring device called a troptometer. Determination is made of the angular displacement of a point near one end of the test section of the specimen with respect to a point on the same longitudinal element at the opposite end. A torsion specimen generally has a circular cross section, since this represents the simplest geometry for the calculation of the stress. Since in the elastic range the shear stress varies linearly from a value of zero at the centre of the bar to a maximum value at the surface, it is frequently desirable to test a thin-walled tubular specimen. This results in a nearly uniform shear stress over the cross section of the specimen. The shear stress versus shear strain curve can be determined from simultaneous measurements of the torque and angle of twist of the test specimen over a predetermined gage length [3, 4].

3.4 Bending Testing

When a piece of material, initially straight, is stressed in bending, the relationships between the applied bending moment and the deformation, on the one hand, and between the deformation and the stress at any point in the material on the other hand, are given by the ordinary elastic formulae of strength of materials

M =   bending moment,

I    =   second moment of area of the section about the neutral plane,

E   =   Young's modulus of elasticity for the material,

R  =  radius of curvature,

f    = the tensile or compressive stress in the material at any distance y from the neutral axis.

In mechanical tests undertaken using bending, whether the tests are static or dynamic, we are usually concerned with beams of simple cross-section, often rectangular or circular in shape, and generally with beams of uniform cross-section which are initially straight. The two formulae given above, together with those expressing the second moment of the area of a section in terms of its dimensions are all that are normally needed to calculate required stresses.

The bending test is used of thermoplastic to determine crossbreaking strength. As well as that for ductile materials, like some metals, bending tests are usually conducted on a qualitative basis, with no attention paid to the initial stages of bending, but bending continued far beyond the limit of elastic behavior. Some common methods are shown in Figure 3.4 [3].

Figure 3.4: Some common methods of bending test.

3.5 Fatigue Tests

As with other mechanical characteristics, the fatigue properties of materials can be determined from laboratory simulation tests. A test apparatus should be de­signed to duplicate as nearly as possible the service stress conditions (stress level, time frequency, stress pattern, etc.). The stress system to which a component is subjected in service may be very complex; equally the manner in which the stress varies with time may differ very much from the simple pattern which the words 'repetition of stress' are likely to conjure up. To perform experiments which will yield quantitative information concerning the ability of a material to withstand a dynamic stress system, some rationalization of the type of stress is desirable, and of the stress-time function, essential. The types of stress system employed in fatigue testing are:

Direct stresses: The simplest stress system. A test piece is subjected to tension or com­pression stress along its length; at any moment, the stress is, for all practical purposes, uniformly distributed across a transverse section.

Plane bending: A test piece is repeatedly bent about a particular neutral plane.

Rotating bending: A test piece is bent about a neutral plane which is continuously rotated with respect to the test piece.

Torsion: A test piece is repeatedly twisted about a fixed axis.

Combined stresses: The term simply implies a combination of types of stress, so that at first sight it might appear possible to combine any or all of the previous four stress systems.

In general, these stresses can be applied in three different fluctuating stress-time modes. First one is a regular and sinusoidal time dependant mode in which the amplitude is symmetrical about a mean zero stress level; this is referred to reversed stress cycle. Another type is termed repeated stress cycle in which the maxima and minima are asymmetrical relative to the zero stress level. Finally, the stress level may vary randomly in amplitude and frequency and this type is called as random stress cycle. All the three types are shown in Figure 3.6 [5].

A series of test are performed by subjecting a specimen to the stress cycling at a relatively large maximum stress amplitude, usually on the order of two third of However some materials do not have a fatigue limit, in that the S-N curve continues its

Figure 3.5: Variation of stress with time that accounts for failure. a) Reversed stress cycle     b) Repeated stress cycle     c) Random stress cycle

downward trend at increasingly greater N values in Figure 3.6. Thus, fatigue will ultimately occur regardless of the magnitude of the stress. For these materials, the fatigue response is specified as fatigue strength, which is defined as the stress level at which failure will occur for some specified number of cycles. The determination of fa­tigue strength is also demonstrated in Figure 3.6 Another important parameter that characterizes a material's fatigue behaviour is fatigue life N_f. It is the number of cycles to cause failure at a specified stress level, as taken from the S-N plot and this cab be seen from Figure 3.6. Unfortunately, there always exists considerable scatter in fatigue data, that is, a variation in the measured N value for a number of specimens tested at the same stress level. This may lead to significant design uncertainties when fatigue life and/or fatigue limit (or strength) is being considered. The scatter in results is a consequence of the fatigue sensitivity to a number of test and material parameters that are impossible to control precisely [5, 6].

Figure 3.6: Stress amplitude (S) versus logarithm of the number of cycles to fatigue failure (N) for a) a material that displays a fatigue limit, and b) a material that does not display a fatigue limit.

3.6 Hardness Tests

As stated before, hardness usually implies the resistance to local deformation. There are three general types of hardness measurements depending on the manner in which the testis conducted. These are scratch hardness, indentation hardness and dynamic hardness. Only indentation hardness is the major engineering concern.

The first widely accepted and standardized indentation-hardness test was pro­posed by J. A. Brinell in 1900. The Brinell hardness test consists in indenting the metal surface with a 10-mm-diameter steel ball at a load of 3,000 kg. For soft metals the load is reduced to 500 kg to avoid too deep an impression, and for very hard metals a tungsten carbide ball is used to minimize distortion of the indenter The load is applied for a standard time, usually 30 s, and the diameter of the indentation is measured with a low-power microscope after removal of the load The average of two readings of the diameter of the impression at right angles should be made. The surface on which the indentation is made should be relatively smooth and free from dirt or scale.

The Vickers hardness test uses a square-base diamond pyramid as the indenter. The included angle between opposite faces of the pyramid is 136°. This angle was chosen because it approximates the most desirable ratio of indentation diameter to ball diameter in the Brinell hardness test. Because of the shape of the indenter, this is frequently called the diamond-pyramid hardness test. The diamond-pyramid hardness number (DPH), or Vickers hardness number (VHN, or VPH), is defined as the load divided by the surface area of the indentation. In practice, this area is calculated from microscopic measurements of the lengths of the diagonals of the impression. The Vickers hardness test has received fairly wide acceptance for research work because it provides a continuous scale of hardness, for a given load, from very soft materials with a DPH of 5 to extremely hard materials with a DPH of 1500. The loads ordinarily used with this test range from 1 to 120 kg, depending on the hardness of the metal to be tested. In spite of these advantages, the Vickers hardness test has not been widely accepted for routine testing because it is slow, requires careful surface preparation of the specimen, and allows greater chance for personal error in the determination of the diagonal length. A perfect indentation made with a perfect diamond-pyramid indenter would be a square.

The most widely used hardness test in the United States is the Rockwell hardness test. Its general acceptance is due to its speed, freedom from personal error, ability to distinguish small hardness differences in hardened steel, and the small size of the indentation, so that finished heat-treated parts can be tested without damage. This test utilizes the depth of indentation, under constant load, as a measure of hardness. A minor load of 10 kg is first applied to seat the specimen. This minimizes the amount of surface preparation needed and reduces the arbitrary hardness numbers. The dial contains 100 divisions, each division rep­resenting a penetration of 0.002 mm. The dial is reversed so that a high hardness, which corresponds to a small penetration, results in a high hardness number. This is in agreement with the other hardness numbers described previously, but unlike the Brinell and Vickers hardness designations, which have units of kilo­grams per square millimetre (kgf mm^-2), the Rockwell hardness numbers are purely arbitrary. One combination of load and indenter will not produce satisfactory results for materials with a wide range of hardness. A 120° diamond cone with a slightly rounded point, called a Brale indenter, and 1.6 and 3.2 mm-diameter steel balls are generally used as indenters. Major loads of 60, 100, and 150 kg are used. Since the Rockwell hardness is dependent on the load and indenter, it is neces­sary to specify the combination which is used. This is done by prefixing the hardness number with a letter indicating the particular combination of load and indenter for the hardness scale employed. A Rockwell hardness number without the letter prefix is meaningless [6].

Many material research problems require the determination of hardness over very small areas. The development of the Knoop indenter by the National Bureau of Standards and the introduction of the Tukon tester for the controlled application of loads down to 25 g have made microhardness testing a routine laboratory procedure. The Knoop indenter is a diamond ground to a pyramidal form that produces a diamond-shaped indentation with the long and short diagonals in the approxi­mate ratio of 7:1 resulting in a state of plane strain in the deformed region. The Knoop hardness number (KHN) is the applied load divided by the unrecovered projected area of the indentation. The special shape of the Knoop indenter makes it possible to place indentations much closer together than with a square Vickers indentation. Its other advantage is that for a given long diagonal length the depth and area of the Knoop indentation are only about 15 percent of what they would be for a Vickers indentation with the same diagonal length. This is particularly useful when measuring the hardness of brittle materials where the tendency for fracture is proportional to the volume of stressed material. The low load used with microhardness tests requires that extreme care be taken in all stages of testing. The surface of the specimen must be carefully prepared. Metallographic polishing is usually required. Work hardening of the surface during polishing can influence the results [6].

3.5 Impact Testing

Impact loads are loads which are applied suddenly or with shock. The essential difference between static and impact loads is that an impact load always produces a peak stress higher than that produced by a load of the same magnitude if it is applied slowly. Impact tests are performed, because it is recognized that the resistance of a material to shock is dependent upon factors other than those which control its resistance to a steady or slowly applied load. Resistance to a slowly applied load may be measured in terms of stress, but resistance to impact involves in addition to the capacity for developing stress, the capacity of the material for being deformed without damage.

There are basic types of impact test according to the applied material, specimen and the application field. One of the basic types of impact test is drop weight test. This form of impact test consists in simply dropping a weight on a specimen from successively increasing heights until the specimen fails. The impact or energy load causing failure is taken as the weight multiplied by the height of final drop. Such a procedure disregards the probable weakening effect of the blows received prior to final failure.

Another basic type is the notched-bar impact tests. Various types of notched-bar impact tests are used to determine the tendency of a material to behave in a brittle manner. This type of test will detect differences between materials which are not observable in a tension test. Two classes of specimens have been standardized¹ for notched-impact testing. Charpy bar specimens are used most commonly in the United States, while the Izod specimen is favoured in Great Britain. The impact resistance of metallic materials is usually measured by the Charpy test or the Izod test. Each of these tests requires a standard specimen having an accurately formed notch. The energy required to break the specimen is provided by releasing a pendulum from a known height. The specimen is supported as a beam at the bottom of the arc described by the pendulum. When the pendulum released, it swings down, breaks the specimen, and rises on the opposite side. The height to which the pendulum rises indicates the residual energy in the pendulum, and from that the energy required to break the specimen may be approximated closely. However, little is known relative to the distribution of the energy through­out the specimen. When tested by this method, specimens of the same material and of similar geometric proportions, but of different sizes, will not give equal values for the modulus of toughness. Therefore, one must recognize that such tests supply only an index, of toughness rather than a measure of toughness as a property of the material. A schematic representation is given in Figure 3.7.

Figure 3.7: Schematic drawing of loading in Charpy and Izod impact tests

4 Some Applications of Mechanical Characterization in Biomechanics

Material improvement and selection is also a key element in biomaterials technology. As the material selection and design have very significant effect on biomaterial applications and research, mechanical characterization is one of the most important step in this subject. In the recent years, with the improvements in electronics and mechanics, the application on mechanical characterization of biomaterials, thus the mechanical testing, has wide and different application area with becoming more important parameter in designs.

In 1990's Schechtman and Bader studied on fatigue damage of human tendons. During the experiments they apply fatigue test to monitor the fatigue damage characterisation which was a part of mechanical characterisation [7]. Another research in mechanical characterisation application in biomaterial science was done by J. Cordey and E. Gautier. They studied on strain gauges used in the mechanical testing of bones. The theoretical and technical aspects specific to the use of strain gauges on bones were presented in this study [8].B. Vázquez et al studied on Radiopaque acrylic cements prepared with a new acrylic derivative of iodo-quinoline. In this study the tensile and compressive properties of cured resins were determined at room temperature using Adamel Lhomargy and Instron electromechanical testing machines with a cell load of 10 kN and at a crosshead speed of 1 mm/min. An extensometer was used to measure displacement [9].

In 2000's Kemal Şerbetci et al studied on thermal and mechanical properties of hydroxyapatite impregnated acrylic bone cements. They applied compression, tensile and fatigue tests in order to determine the mechanical characteristics of the material [10]. Linny Angker et al performed some mechanical tests to understand the mechanical properties of dentine. During these evaluations, they used microintendation tests [11]. Also J.L.Guy et al performed nanointendation tests to make the mechanical characterisation of the tooth enamel [12]. Miguel et studied on mechanical testing of aerosol-gel deposited titania coatings for biocompatible applications. Their main objective has been to evaluate the mechanical properties of the coatings and to prove their in-vitro biocompatibility. For this purpose, the hardness and Young’s modulus of the coatings were measured by nanoindentation with loads in the 6–30 mN range [13]. Lang Yang et al made research on stiffness characteristics and inter-fragmentary displacements with different hybrid external fixators. Their objective was to determine how the different approaches of constructing a hybrid external skeletal fixator affect the mechanical environment at the fracture site. Mechanical testing was performed on four types of hybrid fixators in which the overall stiffness of the fixator as well as the axial, transverse shear and angular displacements at the fracture site were measured [14]. Another important study was done by Nabhani and Bamford on mechanical testing of hip protectors. The purpose of the work was to compare the new combination hip protector designs. A mechanical test rig was designed and built to simulate a person falling with sufficient impact energy to fracture the greater troachanter if unprotected. Essentially the test rig is a drop test machine using weights that can be draped from predetermined height [15]. Eileen Gentleman et al studied on mechanical characterisation of collagen fibers and scaffolds.

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9.    Vázquez, B., "Radiopaque acrylic cements prepared with a new acrylic derivative of iodo-quinoline", Biomaterials, Vol. 20, Issue 21, November 1999, Pages 2047-2053.

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