Determination of Fracture Toughness of Bone Cement by Nano-Indentation Test

The nano-indentation test was used to measure fracture toughness of the bone cement. The cement sample was prepared using two different mixing methods i.e., hand mixing and vacuum mixing. For this purpose, some cubic specimens, each of the size 10×10×5?mm³ were produced and then the nano-indentation test was performed on both the hand-mixed and the vacuum-mixed specimens by nano-indenter setup and atomic force microscopy observation. The fracture toughness values obtained from the hand-mixed and vacuum-mixed cements were compared. The results indicate that the vacuum-mixed cement has significantly higher fracture toughness compared with the hand-mixed ones. Since the nano-indentation test method needs less sample material, decreases costs and obtains reliable results, it can be considered as a suitable technique for determination of the mechanical properties of bone cements instead of the macroscale test methods.     

Short-rod elastic-plastic fracture toughness test using miniature specimens

The standard ASTM-E399 plane-strain fracture toughness (K _IC) test requires (1) the test specimen dimensions to be greater than a minimum size and, (2) fatigue precracking of the specimen. These criteria render many materials impractical to test. The short-rod elastic-plastic plane-strain fracture toughness test proposed by Barker offers a method of testing not requiring fatigue precracking and furthermore, it appears that test specimens smaller than that stipulated by ASTM can be used to obtain validK _IC values. In this study, the use of a modified miniature short-rod fracture toughness test specimen was investigated. Our miniature short-rod specimen is approximately 7 mm long and 4 mm diameter. These mini specimens are well suited for the purpose of testing biomaterials. The value of the minimum stress intensity factor coefficient (Y _m ^* ) for the mini short-rod specimens was determined experimentally using specimens machined from extruded acrylic rod stock. An elastic-plastic fracture toughness analysis using the mini specimens gave values ofK _IC for extruded acrylic (nominally PMMA) equal to 0.67 ± 0.06 MPa m^1/2. The problem of testing non-flat crack growth resistance curve materials (such as PMMA) using the short-rod fracture toughness test method is discussed. A modification to the test procedure involving the use of aY ^* value corresponding to a short crack length is suggested as a method of overcoming this difficulty.Nomenclature a crack length - a ₀ initial crack length - a ₁ length of the chevron notch on the mini short-rod specimen - a _m critical crack length — crack length atY _m ^* - C specimen compliance - C dimensionless specimen compliance = CED - D mini short-rod specimen diameter - E Young's modulus - K ₁ stress intensity factor - K _1C plane-strain fracture toughness - K _max fracture toughness calculated usingP _max - P load applied to the test specimen during a short-rod fracture toughness test - P _c load applied to the test specimen atY _m ^* - P _max maximum load applied to the specimen during a short-rod fracture toughness test - p plasticity factor - W mini short-rod specimen width - Y ^* stress intensity factor coefficient - Y _m ^* minimum of the stress intensity factor coefficient - dimensionless crack length =a/W - ₀ dimensionless initial crack length = ₀/W - ₁ dimensionless chevron notch length =a ₁/W - _m dimensionless critical crack length =a _m/W     

Mechanical and thermal behaviour of an acrylic bone cement modified with a triblock copolymer

The basic formulation of an acrylic bone cement has been modified by the addition of a block copolymer, Nanostrength^® (NS), in order to augment the mechanical properties and particularly the fracture toughness of the bone cement. Two grades of NS at different levels of loading, between 1 and 10 wt.%, have been used. Mechanical tests were conducted to study the behaviour of the modified cements; specific tests measured the bend, compression and fracture toughness properties. The failure mode of the fracture test specimens was analysed using scanning electron microscopy (SEM). The effect of NS addition on the thermal properties was also determined, and the polymerisation reaction using differential scanning calorimetry. It was observed that the addition of NS produced an improvement in the fracture toughness and ductility of the cement, which could have a positive contribution by reducing the premature fracture of the cement mantle. The residual monomer content was reduced when the NS was added. However this also produced an increase in the maximum temperature and the heat delivered during the polymerisation of the cement.     

On the magnitude and variability of the fatigue strength of acrylic bone cement

When used for the fixation of orthopaedic implants poly(methyl methacrylate) bone cement is prepared during surgery, and polymerises in situ. The technique for preparation of the bone cement involves mixing the liquid monomer and powder: two common mixing methods are hand mixing and vacuum mixing. Previous studies have shown that porosity depends on mixing technique. In this study, the fatigue strength of hand-mixed and vacuum-mixed cements is measured and correlated with the pore distribution resulting from each mixing technique. S–N curves show that vacuum mixing improves the fatigue strength by an order of magnitude. However, there is greater variability of fatigue strength associated with vacuum-mixed cement. This is correlated with the appearance of an occasional large pore in the vacuum-mixed cement. If the cross-sectional area is corrected to take account of porosity in vacuum-mixed cement, an 8% increase in the association of the data is found. Using a two-parameter Weibull model, it can be shown that the vacuum-mixed cement has a greater Weibull life at the 50% probability-of-survival level. However, if a probability-of-survival close to 100% is required (i.e. high reliability), the hand-mixed cement is found to have superior fatigue behaviour. The S–N curves can be explained by examination of the fracture surface features. The initiation stage of fatigue cracking is notably different for the two different mixing techniques. The lower fatigue strength of the hand-mixed cement can be explained by the interactions of pores on the fracture surface causing stress concentrations, whereas no such pore interactions occur in the vacuum-mixed cement.     

On fatigue lifetimes and fatigue crack growth behavior of bone cement

Bone cement is used to develop a mechanical bond between an artificial joint and the adjacent bone tissue, and any degradation of this bond is of serious concern since it can lead to loosening and eventually malfunction of the artificial joint. In the present study, the fatigue lives and fatigue crack propagation behavior of two bone cements, CMW Type 3 and Zimmer, were investigated, and it was found that the size and distribution of pores played a major role in influencing both the fatigue crack initiation and propagation processes. The fatigue lifetimes of CMW exceeded those of Zimmer because of a lesser density of large pores. When the fatigue lifetimes were plotted as a function of K _limax, the maximum initial stress intensity factor based upon the initiating pore size, the difference in fatigue lifetimes between CMW and Zimmer bone cements was greatly reduced. The fatigue crack growth behavior of both bone cements were similar. This is a further indication that the noted differences in fatigue lifetimes were related to the size of the pore at the crack initiating site.     

Bone fracture analysis on the short rod chevron-notch specimens using the X-ray computer micro-tomography

Mechanical fatigue of bone leads to micro-cracking which is associated with remodeling, establishing a balance in the microcrack population of the living tissue, thus, in the steady-state, the microstructure of bone provides sites of discontinuity acting as stress raisers. Hence fracture toughness plays a decisive role in bone functionality by determining the level to which the material can be stressed in the presence of cracks, or, equivalently, the magnitude of cracking which can be tolerated at a given stress level. Cortical bone, which behaves as a quasi-brittle solid when fractured, was tested as short-rod chevron-notched tension specimens (CNT). The main features of the CNT specimen are its geometry and the V shaped notch. The notch leads to steady-state crack propagation whilst the requested geometry allows a diameter 40% smaller than the thickness of a standard compact tension specimens (CT). These features are essential to distinguish the inhomogeneties in the fracture properties of materials like bone. Bone structure and crack propagation of the CNT specimens were analyzed using X-ray computed micro-tomography (XMT), which is a non-invasive imaging technique. The unique feature of the micro-CT is the high resolution three-dimensional image which consists of multi-sliced tomographs taken in a fine pitch along the rotational axis. Fracture toughness (K _IC) computed according to the peak load was 4.8 MNm^-3/2 while that derived from experimental calibration tests using XMT was 4.9 MNm^-3/2.     

Fracture toughness of composite acrylic bone cements

Composite bone cements incorporating one of four different filler particles (hydroxyapatite powder, graphite flakes or one of two types of rubber-modified acrylic particles) were made and the fracture toughness properties (K _lc) and curing characteristics (peak curing temperature and cement extrudability while in the doughy state) assessed. The results showed that all filler types studied resulted in significant increases in fracture toughness while maintaining acceptable working and curing characteristics of the composite cements. The increase inK _lc was related to the amount of filler incorporation. The observed dependence of the change inK _lc on the wt% filler could be rationalized through the application of proposed mechanisms for toughening of particle-reinforced polymers.     

Dynamic mechanical behavior of PMMA based bone cements in wet environment

The mechanical properties of three wet commercial bone cements, namely Braxel (from Bioland®), Simplex-P (from Howmedica®) and CMW1-G (from DePuy®) are investigated by means of stress relaxation and dynamic mechanical analysis (DMA). The geometry of loading that was used is the three point bending method (ASTM D790); all the tests were performed in a water chamber by means of temperature sweeps between 17 and 57 °C and spanning four frequency decades. The results show that viscoelastic properties are strongly dependent on specimen conditioning (i.e. water uptake and heat treatment). The results also show that all the cements that were analyzed show mechanical properties which are intermediate between the ones of the cancellous bone and of the metals of which prostheses are normally made. As a consequence, the cement is able to reduce the stress concentrations due to the interfacing of materials which have very different stiffnesses. Moreover, the results of the DMA, particularly the ones concerning the damping factor (tan ), indicate that at body temperature the bone cements tested show an increased capacity of dissipation, the higher is the loading frequency, thus displaying shock absorbing properties.     

A comparison of mechanical properties of discontinuous Kevlar 29 fibre reinforced bone and dental cements

A comparative study of the fracture behaviour of Kevlar 29 reinforced bone and dental cements is undertaken using both linear elastic and non-linear elastic fracture mechanics approaches. Results from both approaches reflect improved fracture toughness at very low fibre contents. Flexural modulus is not apparently improved in either system, and flexural strength is only improved in the bone cement system probably because of poor interfacial bonding and the presence of voids in the dental cement. In all cases, however, bone cement is seen to be superior to dental cement. This is interpreted in terms of smaller voids and better fibre distribution due to the lower viscosity of the bone cement material. When compared to carbon-polymethyl methacrylate (PMMA) cements, Kevlar 29 reinforced systems appear to be superior. More work is underway to optimize the properties of these systems with regard to structural parameters.     

Improved mechanical properties of acrylic bone cement with short titanium fiber reinforcement

Acrylic bone cements are widely used in total joint arthroplasties to grout the prosthesis to bone. The changes in the tensile properties and fracture toughness of polymethylmethacrylate (PMMA) bone cements obtained by the addition of control and heat treated short titanium fibers are studied. Heat treatment of titanium fibers is conducted to precipitate titania particles on the fiber surface, which may improve the biocompatibility of the metal. Control (non-heat treated) and heat treated short titanium fibers (250 μm long and 20μm diameter) were used as reinforcements at 3 volume %. X-ray diffraction indicated the presence of a rutile form of titania due to the heat treatments. Results indicate that the tensile and fracture properties of unfilled bone cement were improved by the addition of control and heat-treated fibers. The fracture properties of bone cements reinforced with control titanium fibers were at least 10% higher than those reinforced with heat treated titanium fibers. Therefore, we recommend further studies on the use of non-heat treated titanium fibers to reinforce acrylic bone cement.     

The fracture properties of glass polyalkenoate cements as a function of cement age

The fracture properties of two glass polyalkenoate cements based on a short chain-length and on a long chain-length poly (acrylic acid) have been studied as a function of the cement age. The stress intensity factor, K _I, increases with cement age for both cements. The un-notched fracture strength _f increases with cement age, largely as a result of an increase in the Young's modulus accompanying crosslinking of the polyacrylate chains by metallic ions. The toughness G _I remains approximately constant for the short chain-length cement, but reduces with cement age for the long chain-length cement. Analysis of the toughness data using a chain pull-out model leads to the conclusion that chains distant from the fracture plane are involved in fracture, and that the number of chains that take part in chain pull-out decreases as the crack opening displacement reduces with cement age.     

Experimental and numerical investigation of cracked chevron notched Brazilian disc specimen for fracture toughness testing of rock

The cracked chevron notched Brazilian disc (CCNBD) specimen has been suggested by the International Society for Rock Mechanics to quantify mode I fracture toughness (K_Ic) of rock, and it has also been applied to mode II fracture toughness (K_IIc) testing in some research on the basis of some assumptions about the crack growth process in the specimen. However, the K_Ic value measured using the CCNBD specimen is usually conservative, and the assumptions made in the mode II test are rarely assessed. In this study, both laboratory experiments and numerical modeling are performed to study the modes I and II CCNBD tests, and an acoustic emission technique is used to monitor the fracture processes of the specimens. A large fracture process zone and a length of subcritical crack growth are found to be key factors affecting the K_Ic measurement using the CCNBD specimen. For the mode II CCNBD test, the crack growth process is actually quite different from the assumptions often made for determining the fracture toughness. The experimental and numerical results call for more attention on the realistic crack growth processes in rock fracture toughness specimens.     

Influence of poly(acrylic acid) molar mass on the fracture properties of glass polyalkenoate cements

The failure behaviour of glass polyalkenoate cements was investigated using a linear elastic fracture mechanics (LEFM) approach. Cements were based on four model glasses with varying reactivity and four poly(acrylic acid)s (PAA)s with number average molar masses (Mn) ranging from 3.25 × 104 to 1.08 × 105. Cement properties were studied at time intervals of one, seven and twenty eight days. Compressive strengths (c) of the cements increased with increasing fluorine content of the glass, with increased molar mass of the PAA and with ageing time. The Young's moduli increased with time, but were lower for cements based on the fluorine free glass. Moduli values were independant of PAA molar mass. The un-notched fracture strength (f) of the cement increased with the molar mass of the PAA and with ageing time. Glass composition did not appreciably influence the un-notched fracture strength. The fracture toughness (KIC) increased with the molar mass of the PAA and with ageing time, but reduced with increasing fluorine content of the glass. The toughness (GIC) was dependant on molar mass. The influence of molar mass was not as great as predicted by the reptation chain pull-out model for fracture. The molar mass dependence of toughness was greatest with the lower fluorine content glasses. The plastic zone size at the crack tip increased with the molar mass of the PAA. However the plastic zone size decreased with ageing time for all the cements studied and was smaller for the more reactive higher fluorine content glasses.     

Effect of laser beam welding on fracture toughness of a Ti-6.5Al-2Zr-1Mo-1V alloy sheet

Investigation of fracture toughness on Ti-6.5Al-2Zr-1Mo-1V alloy thin sheet and its laser-welded joints has been carried out. In the test compact tension (CT) specimens and single specimen technology were used. In addition, hardness distribution and microstructure of the welded joints were examined. Fracture test indicates that brittle unstable fracture occurs after slow crack propagation for all the specimens, except that one heat affected zone (HAZ) specimen is brittle crack initiation. It is found that rolling directions have no obvious effect on fracture toughness of base metal. Moreover, fracture toughness of weld metal is obviously decreased in comparison with base metal whatever in as-welded condition or in stress relief condition. Post-weld heat treatment (PWHT) leads to fracture toughness of the welds further decreasing. Fractography observation shows that the fracture mode is predominantly dimpled in base metal. However, there exists intergranular fracture in the weld metal. Thus, the transition of fracture mode from both base metal and HAZ to weld metal may lead to dramatic decrease in fracture toughness. Microstructure examination reveals that the microstructure of weld metal consists of large grains with fine acicular structure. The formation of fine α acicular structure is due to rapid cooling during laser welding. After PWHT, the acicular structure is coarsened.     

Dual setting α-tricalcium phosphate cements

An extension of the application of calcium phosphate cements (CPC) to load-bearing defects, e.g. in vertebroplasty, would require less brittle cements with an increased fracture toughness. Here we report the modification of CPC made of alpha-tricalcium phosphate (α-TCP) with 2-hydroxyethylmethacrylate (HEMA), which is polymerised during setting to obtain a mechanically stable polymer-ceramic composite with interpenetrating organic and inorganic networks. The cement liquid was modified by the addition of 30–70 % HEMA and ammoniumpersulfate/tetramethylethylendiamine as initiator. Modification of α-TCP cement paste with HEMA decreased the setting time from 14 min to 3–8 min depending on the initiator concentration. The 4-point bending strength was increased from 9 MPa to more than 14 MPa when using 50 % HEMA, while the bending modulus decreased from 18 GPa to approx. 4 GPa. The addition of ≥50 % HEMA reduced the brittle fracture behaviour of the cements and resulted in an increase of the work of fracture by more than an order of magnitude. X-ray diffraction analyses revealed that the degree of transformation of α-TCP to calcium deficient hydroxyapatite was lower for polymer modified cements (82 % for polymer free cement and 55 % for 70 % HEMA) after 24 h setting, while the polymerisation of HEMA in the cement liquid was quantitative according to FT-IR spectroscopy. This work demonstrated the feasibility of producing fracture resistant dual-setting calcium phosphate cements by adding water soluble polymerisable monomers to the liquid cement phase, which may be suitable for an application in load-bearing bone defects.     

Mode I fracture toughness of the thermally pretreated red Verona marble by means of the two‐parameter model

The paper aims to analyse the effects of pretreatment thermal cycles on both mechanical and fracture parameters of the red Verona marble, which is a natural stone of sedimentary formation. The effects of the thermal pretreatment, consisting of freeze/thaw cycles and simulating the atmospheric ageing on the material, are evaluated in terms of changes of the aforementioned parameters. Note that a wide variety of both specimen types and methods to determine mode I plain strain fracture toughness of rocks are available in the literature. The two‐parameter model originally proposed for plain concrete is herein adopted. Such a method, based on the experimental data obtained from three‐point bending tests on single edge‐notched specimens, is able to take into account the slow nonlinear crack growth occurring before the peak load, typical of quasibrittle materials, and presents the advantages of easy specimens preparation and simple test configuration.     

Creep behavior comparison of CMW1 and palacos R-40 clinical bone cements

The restrained dynamic creep behaviors of two clinical bone cements, Palacos R-40 and CMW1 have been investigated at room temperature and body temperature. It was found that the two cements demonstrated significantly different creep deformations, with Palacos R-40 bone cement demonstrating higher creep strain than CMW1 bone cement at each loading cycle. For both cements, two stages of creep were identified with a higher creep rate during early cycling followed by a steady-state creep rate. The test temperature had a strong effect on the creep performance of the bone cements with higher creep rate observed at body temperature. The relationship between creep deformation and loading cycles can be expressed by single logarithmic model. The SEM examinations revealed that CMW1 bone cement is more sensitive to defects within the specimen especially to the defects at the edges of the specimen than Palacos R-40 bone cement. However, in the absence of micro-cracks or defects within the inner surface layer, the dynamic loading (at less than 10.6 MPa) is unlikely to produce micro-cracks in the CMW1 bone cement. The different behaviors between the two bone cements may be attributed to differences in chemical compositions and molecular weight distributions.     

Fracture toughness testing of hard metals using compression-compression precracking

Compression-compression precracking of brittle materials has recently been applied to fracture toughness testing. This paper reports the results of an experimental programme of fracture toughness testing of a WC-Co alloy containing 10% cobalt by weight. Tests were performed on specimens precracked by cyclic compression and on specimens in which this compression-compression precrack was subsequently extended by tensile fatigue. Toughness data obtained in this way were compared with the results of short-rod toughness tests. The causes of differences etween these various data are discussed.     

Chevronproben zur näherungsweisen Bestimmung der Bruchzähigkeit

Chevron Specimen for the Estimation of Fracture Toughness Fracture toughness is a material property which is presently used in many industrial areas, either as material selection criteria or as material quality requirement. In some areas, nuclear power plants and aerospace, it is also a design parameter for design against catastrophic failures. Determination of the fracture toughness in accordance with ASTM E 399 is relatively elaborate. Depending on the material concerned, a certain minimum material cross section is required to obtain the necessary size of the specimen. Many semi-finished product forms of the different materials can not be tested for fracture toughness due to the specimen size requirements. For these reasons, alternative test methods were sought of which testing of chevron-notched specimens is one method. In the work to be presented, the test method to determine fracture toughness via chevron-notched specimens is briefly described. The most frequently used chevron-notched specimens are shown together with loading grips to be used in conjunctions with universal testing machines. Certain effects associated with some of the chevronnotched specimens are pointed out which result in a large difference between the fracture toughness determined in accordance with ASTM E 399 and that obtained via chevron-notched specimens. The aim of our research effort is to develop a chevron-notched specimen geometry which furnishes fracture toughness values compatible with K_Ic values without complicating the test method. Such a chevronnotched specimen is presented and the fracture toughness values obtained from these specimens of 7475-T 7351 and different Ti-alloys are compared to the K_Ic values obtained in accordance with ASTM E 399 for the same materials.     

A new method to estimate the plane strain fracture toughness of materials

Although the testing method for fracture toughness K_IC has been implemented for decades, the strict specimen size requirements make it difficult to get the accurate K_IC for the high‐toughness materials. In this study, different specimen sizes of high‐strength steels were adopted in fracture toughness testing. Through the observations on the fracture surfaces of the K_IC specimen, it is shown that the fracture energy can be divided into 2 distinct parts: (1) the energy for flat fracture and (2) the energy for shear fracture. According to the energy criterion, the K_IC values can be acquired by small‐size specimens through derivation. The results reveal that the estimated toughness value is consistent with the experimental data. The new method would be widely applied to predict the fracture toughness of metallic materials with small‐size specimens.