A methodology and criterion for acrylic bone cement fatigue tests

Acrylic bone cement is used as a fixing device in total hip arthroplasty and it is based on polymethyl-methacrylate. Fatigue failure of the cement is the primary cause of loosening of cemented arthroplasties. Pores form in the acrylic material during mixing and curing, and an analysis of the fatigue life of the cement requires the elimination of the critical macropores, defined as having a diameter > 1 mm, which may bias the outcome of tests. Previous workers have rejected fatigue specimens either on a qualitative basis or at a specified pore size level. However various different thresholds have been considered but currently there is no quantitative criterion to define them. This investigation proposes a quantitative criterion for establishing a critical macropore size rejection threshold for fatigue specimens, and discusses the effectiveness of this criterion based on fatigue tests of radiopaque cement specimens.     

On fatigue life variability in bone cement

Fatigue life of Simplex P bone cement was tested at three different stress amplitudes by using specimens produced by two different mixers. Fatigue life data showed high variability in all instances. Statistical analysis showed that fatigue life was not affected by the type of mixer. Analysis of fracture surfaces showed that fatigue life variability could be attributed to the presence of defects, such as bubbles and mixing defects. Both Weibull and Tiryakio?lu distributions provided excellent fits to the fatigue life data. Moreover, the Gumbel parameters for fatigue initiator size data estimated in the Tiryakio?lu distribution agreed closely with fractographic measurements.     

Influence of different types of water on strength,porosity and hydric parameters of metakaolin admixtured cement

The present research deals with strength, porosity and hydric behavior of metakaolin cement admixtured with different types of water. The hydration of ordinary Portland cement in the presence of 0%, 10%, 20% and 30% metakaolin treated with distilled, ground and sea water with the water to cement ratio of 0.4 was studied. The experimental results on setting time, strength, porosity and hydric parameters are reported. The results show that, metakaolin percentage increases in strength with a decrease in porosity. The observed results are discussed with SEM micrographs. Further, sea water accelerates the cement hydration at the early stages but retards it in the latter stages of hydration.     

A simulation-based investigation of the staircase method for fatigue strength testing

A critical evaluation of the statistics of the fatigue strength distribution as determined by the staircase (or up-and-down) method is presented. The effects of test parameters (namely, step size and sample size) were analyzed using numerical simulation to determine the accuracy of fatigue strength standard deviation calculations using traditional staircase statistics, resulting in a quantification of standard deviation bias as a function of step size and sample size. A non-linear correction was formulated to mitigate this standard deviation bias inherent in small-sample tests. In addition, the simulation was used to investigate the effectiveness of a bootstrapping algorithm on standard deviation estimates. The bootstrapping algorithm was found to significantly reduce the potential of large standard deviation errors in small-sample tests. Together, the use of the non-linear correction factor and the bootstrapping algorithm may allow an improved method to estimate the statistics of a material’s fatigue strength distribution using a small-sample staircase test strategy.     

The effects of multiple overloads and absorbed hydrogen on the fatigue strength of notched steel specimens

It is well known that earthquakes can damage structures and machinery. After an earthquake, those components, which have been obviously damaged are scrapped and replaced, and most of the components which have not been obviously damaged will continue to be used even after earthquakes. However, as will be shown, the earthquake may have severely impaired the fatigue strength of such components by introducing unfavourable residual stresses and short cracks at stress raisers. In addition, if such components should contain hydrogen, an increasingly possible scenario for the hydrogen economy in the future, then it is shown that the loss of fatigue strength can be even greater. This paper explores the extent of fatigue degradation due to overloads and to absorbed hydrogen. It was shown that generation of small crack and tensile residual stress imposed by overloads caused substantial decrease of residual fatigue strength compared with that in the initial state. It was also shown that hydrogen enhanced more reduction. Hydrogen enhanced reduction in two ways. The crack generated by overloads grew deeper in hydrogen charged material. In addition to this, the reduction of ΔK_th also occurred in hydrogen charged material. These two factors worked together to reduce the residual fatigue strength after multiple overloads.     

Residual fatigue strength assessment of concrete considering tension softening behavior

In this study, the residual strength of plain concrete beams under fatigue loading is assessed. The quasi-brittle nature of the material is considered by including the effect of tension-softening taking place in the fracture process zone. A two step approach is followed. In the first step, the effective critical crack length for unstable fracture to occur is determined by using two different methods, namely a modified LEFM based fatigue crack propagation law and the crack resistance method. In the second step, the moment carrying capacity as a function of increasing effective crack length is obtained in order to assess the residual strength of the member. A parametric study is performed by considering three different softening laws: linear, bilinear and power laws. It is seen that the bilinear softening law matches close to the experimental predictions of other investigators.     

On the assessment of fatigue life of marine diesel engine crankshafts

The fatigue strength and its correct assessment play an important role in design and maintenance of marine crankshafts to obtain operational safety and reliability. Crankshafts are under alternating bending on crankpins and rotating bending combined with torsion on main journals, which mostly are responsible for fatigue failure. The commercial management success substantially depends on the main engine in service and of its design crankshaft, in particular. The crankshaft design strictly follows the rules of classification societies. The present study provides an overview on the assessment of fatigue life of marine engine crankshafts and its maintenance taking into account the design improving in the last decades, considering that accurate estimation of fatigue life is very important to ensure safety of components and its reliability. An example of a semi-built crankshaft failure is also presented and the probable root case of damage, and at the end some final remarks are presented.     

The nature of fatigue damage in bone

Bone is unusual among structural materials as it is alive and capable of self-repair. Fatigue-induced microdamage is repaired by bone remodelling, but if damage accumulates too quickly, or remodelling is deficient, fatigue failure may result. Fatigue is thought to contribute to both stress and fragility fractures which are of major clinical importance. Despite this, we do not fully understand the nature of fatigue damage in bone. Human rib sections, containing microcracks stained with basic fuchsin, were serially sectioned and microcracks identified and reconstructed in three dimensions using computer software. Microcracks were elliptical in shape, 400 μm long and 100 μm wide, typical of a transversely isotropic material. Chelating agents which bind Ca²⁺ were found to label microcracks in rib, as well as mineralising bone surfaces and resorption sites, suggesting that microcracks are Ca²⁺ ion-lined discontinuities in the hydroxyapatite matrix. Ca²⁺ ions were exposed by scratching the surface of bovine bone specimens and labelled with chelating agents in sequence. The optimal four agent sequence was: alizarin, xylenol orange, calcein and calcein blue. Two dye sequences were used to differentiate between pre-existing and test-induced microdamage in bovine samples fatigue tested in compression and longer sequences labelled microcrack growth. Microcrack dimensions can be used to calculate stress intensity values and, together with fatigue test data, can aid theoretical models to predict fatigue failure in bone.     

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.     

No-slip crack model for damaged bone/cement interface

Cemented hip and knee prostheses are glued to cancellous bone with polymethylmethacrylate (PMMA) adhesive. The PMMA fills the cavities of the porous bone leading to a rough interface. It has been postulated that flaws at the bone/PMMA interface lead to loosening of the prosthesis. Several researchers have modeled these flaws, but unfortunately they have not calculated the stress intensity factor (SIF) for all values of applied external loads. We redefine the model for the pressure acting on the crack so that this may be possible and calculate the effect on the SIF due to the rough nature of the bone/PMMA interface.     

A residual strength model for the fatigue strengthening behavior of 2D needled CMCs

An important result of our investigations of the fatigue response of Ceramic Matrix Composites (CMCs) is the modeling of the residual strength after fatigue loading. According to the experimental results, the residual tensile strength of 2D needled CMCs firstly increases and then decreases as the number of fatigue cycles increases. Based on the microstructure observations and the wear-in/wear-out mechanisms presented by previous researchers, a model is developed here to characterize the residual strength vs. the number of fatigue cycles for the composites. After parameter identification, the presented model describes the change of the residual strength quite well.     

Influence of the water to cement ratio W/C on the compressive strength of concrete — An application of the cement hydration equation to concrete

The cement hydration equation was used to determine the influence of the water to cement ratio, W/C, on the compressive strength of concrete up to the end of the hydration age. Using the hydration equation, the equations of the compressive strength of concrete were derived for five values of the ratio W/C equal to 0.50, 0.55, 0.60, 0.65, and 0.70. The curves of variation of the compressive strength versus the W/C ratio for the hydration age of 7, 28, 90 and 5475 days were drawn. Based on these curves a thorough study of the influence of the W/C ratio on the compressive strength of concrete was undertaken.     

水化硅酸钙晶种的制备及对水泥强度的影响

为了研究水泥晶种的制备技术及对水泥强度的影响,在蒸压条件下,选取天然硅质原材料硅藻土和钙质原材料熟石灰,利用水热合成法制备C-S-H凝胶作为水泥晶种,并研究了3种钙硅材料的配制比例和不同热工制度制备的晶种对水泥强度的影响.研究结果确定了钙硅比为1.2,合成压力在0.1 MPa,合成时间1～3 h为水泥晶种的最佳合成工艺制度.试验结果表明:掺晶种后的C3S悬浮液液相中的钙离子析晶时间提前,缩短了凝结时间,加速C3S的早期水化;掺入晶种可有效地提高水泥的早期强度,其原因在于加晶种可以降低产物成核所需克服的势垒、促进成核、并有效抑制大晶粒的生长.     

Industrial experiences of bending fatigue strength in table liner for cement mill

A table liner for the vertical roller mill has been used to grind natural limestone. Unexpected fatigue failure accidents have occurred during portland cement manufacturing process. The design life of a table liner is 4 × 10⁷ cycles, but the actual fatigue life of a table liner is 2 × 10⁶ to 8 × 10⁶ cycles. The fatigue crack of a table liner initiates from the outside edge of the grinding path of the limestone. When such a crack occurs, the table liner has to be replaced, and this requires 30% of the total maintenance cost of the vertical roller mill. Therefore, this study examines the fatigue failure of a table liner by plane‐bending fatigue test, stress measurement test, finite element analysis and fatigue fracture analysis.     

Evaluation of nominal and local stress based approaches for the fatigue assessment of seam welds

In the context of the German joint research project “Applicability of fatigue analysis methods for seam welded components”, fatigue tests were performed by five universities and institutes on welded components, welded parts of larger structures as well as component-like samples of weld details. The sheet thickness t was in the range 1 mm ? t ? 20 mm. The welding parameters for all test coupons and structures tested were chosen according to the industrial production process. Based on the data acquired, nominal, structural and notch stress approaches were analysed with regard to applicability and quality of assessment. The actual weld geometry except the real notch radii was taken into account within the notch stress approach. For the notch radii various values, the reference radii 0.05, 0.3 and 1 mm, were applied.Experimental and numerical results for welded steel components are presented.Approximately equivalent scatter ranges were obtained when applying the various approaches based on the current state of the art. It should be noted that both the nominal and the structural stress approaches are limited in their application compared to the notch stress approach. A comparison of the scatter bands obtained for the various approaches is subject to limitations because it was necessary, in each case, to use different test series as the basis for determining the scatter bands.     

Preliminary evaluation of the in vitro cytotoxicity of PMMA-co-EHA bone cement

This work reports a preliminary in vitro cytotoxicity assessment of new poly (methyl methacrylate)-co-ethyl hexylacrylate (PMMA-co-EHA) bone cement by evaluating the effect of its leachables on the viability of human osteoblast-like cells (MG63 line) and their progression through the cell cycle. MG63 cells were exposed to 72 h-extract dilutions of PMMA-co-EHA and their viability was tested using the 3-(4,5-dimethylthiazolyl-2)-2,5-diphenyltetrazolium bromide (MTT) colorimetric assay. Also, putative changes in the progression of cells through the cell cycle were monitored using flow cytometry. For that the relative nuclear DNA content and the ratio of cells at G₁:S:G₂ stages of the cell cycle were measured after three exposure periods (24, 48 and 72 h). The obtained results revealed a dose-dependent influence of the cement extract in MG63 cell metabolism when compared to cells cultivated in a culture medium only. The MTT assay showed that a moderate number of cells died after exposure to the most concentrated extract. The cell cycle analysis revealed that leachables of PMMA-co-EHA led to significant changes in cellular proliferation, with cells exposed for 48 h to the most concentrated extract being arrested in the S phase of the cell cycle. However, despite the initial period of cytotoxicity, the obtained results suggest that after 72 h of exposure, the surviving cells are able to recover from this arresting condition and continue to proliferate. Therefore, this preliminary study indicates that, at the biological level, PMMA-co-EHA may have potential of being used as a bone cement matrix. However, a more detailed research work is needed to fully understand the factors responsible for the initial cytotoxicity observed.     

The strength of acrylic bone cements and acrylic cement-stainless steel interfaces

The strength and fracture toughness of a surgical acrylic bone cement have been evaluated in air, saline solution, and a blood serum, and little effect of environment was observed. Second phase dispersions of either barium sulphate or glass spheres were found to have significant effects upon the strength and fracture surface energy. In the first composite, a decrease in strength and fracture surface energy may be explained in terms of the formation of voids around the barium sulphate particles and tearing of material between them; in the second composite, an increase in strength and fracture surface energy occurred through a crack front-glass sphere interaction effect. The occurrence of subcritical crack growth in the acrylic bone cement composites was investigated, and crack velocity-stress intensity factor diagrams were constructed for the purposes of fracturesafe design.     

Fatigue strength dependence on the ultimate tensile strength and hardness in magnesium alloys

The relationships between fatigue strength and ultimate tensile strength as well as hardness have been studied in high-strength NZK alloys (Mg–yNd–zZn–xZr) and other magnesium alloys. In the absence of casting defects, clear linear relationships have been found between the fatigue strength and the ultimate tensile strength and the hardness values in these magnesium alloys in both T4 (solutionized) and T6 (solutionized and aged) conditions. The fatigue strength models developed in this work alloys can be directly applied to other defect-free magnesium alloys.