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Abstract

While prosthetic devices have been extensively used to treat a wide range of human diseases and injuries, failure of these devices due to fatigue under cyclic loading has been recognized as a primary concern on therapeutic longevity. Experimental testing has long been a dominant approach to characterizing the fatigue behavior of prosthetic devices. However, experimental methods could be of multiple shortcomings such as their restrictive nature in-vivo in medical studies and limitations of extrapolating the testing results. This study develops a numerical approach for modeling fatigue failure in some commonly-used osteofixation devices that are implanted to support various major bone defects/trauma and fractures. The eXtended Finite Element Method (XFEM) is employed herein to model fatigue crack formation and propagation as per level set functions to suppress the need for re-meshing. For validation purpose, a benchmark problem involving a modified compact tension structure is first carried out, in which the modeling results are compared with the relevant experimental data to demonstrate the effectiveness of the proposed XFEM approach. Further, two representative orthopedic examples are studied for characterizing the fatigue behavior of a femoral osteofixation plate and a mandibular reconstruction mini-plate, respectively. The results reveal that healing/remodeling of grafted bone as well as tissue ingrowth to the scaffold have significant bearing on fatigue life of fixation plates. This study showcases a valuable approach for predicting fatigue failure of prosthetic devices in-silico, thereby providing an effective tool for design optimization of patient-specific prosthetic devices to ensure their longevity.

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... XFEM has been used to assess the structural integrity by considering the cracked composite specimens and different orientations of the fibers reinforced in them (Abdullah et al. 2019). It has also been used for predicting the damage initiation when the polymer prosthetics are subjected to fatigue (Wan et al. 2021). However, these techniques were found to be deficient considering the effect of anisotropy and heterogeneity of the resulting composite, and also, these methods are computationally very expensive (Hashin 1970(Hashin , 1979. ...
Chapter
Geometrical features like size and shape of the particles which are used to reinforce the composites affect the mechanical behavior of the resulting particulate polymer composites to a great extent. The aspect ratio of the reinforcing filler is of great importance specially when such composites are subjected to impact loading. Usually, an increase in the aspect ratio results in a significant increase in the energy-absorbing ability which ultimately improves the fracture toughness of the resulting composite. However, the experimental procedure followed for determining the fracture toughness of polymer composites reinforced with particles of varying aspect ratio is very complex and time-consuming. In this view, this chapter investigates the applicability of a machine learning algorithm known as K-nearest neighbor (KNN) for determining the dynamic fracture toughness of glass-filled polymer composites. The proposed methodology aims to predict the fracture toughness in terms of stress intensity factor with limited experimentation and maximum accuracy. The current framework of machine learning utilizes time, dynamic elastic modulus, aspect ratio, and volume fraction of the glass particles as the independent model parameters. The proposed KNN model predicts the fracture behavior of these composites with an accuracy of ~96%.
... Bone plates are subjected to relatively high stress in human body applications that may lead to the exceeding of the ultimate strength of the material or, more likely, to fatigue failure [1]. The failure of a bone plate may result in additional injury and further complications for the patient and may lead to significant supplementary costs due to the need for reoperation [2]. ...
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The aim of this study was to investigate the fracture behaviour of fissural dental enamel under simulated occlusal load in relation to various interacting factors including fissure morphology, cuspal angle and the underlying material properties of enamel. Extended finite element method (XFEM) was adopted here to analyse the fracture load and crack length in tooth models with different cusp angles (ranging from 50° to 70° in 2.5° intervals), fissural morphologies (namely U shape, V shape, IK shape, I shape and Inverted-Y shape) and enamel material properties (constant versus graded). The analysis results showed that fissures with larger curved morphology, such as U shape and IK shape, exhibit higher resistance to fracture under simulated occlusal load irrespective of cusp angle and enamel properties. Increased cusp angle (i.e. lower cusp steepness), also significantly enhanced the fracture resistance of fissural enamel, particularly for the IK and Inverted-Y shape fissures. Overall, the outcomes of this study explain how the interplay of compositional and structural features of enamel in the fissural area contribute to the resistance of the human tooth against masticatory forces. These findings may provide significant indicators for clinicians and technicians in designing/fabricating extra-coronal dental restorations and correcting the cuspal inclinations and contacts during clinical occlusal adjustment.
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Fatigue is a major issue for critical structures, and it can be very significant for structural systems composed of metal plate-like components. Finite Element (FE) analysis has been proved to be an efficient and reliable simulation tool for damage assessment of plate structures under fatigue. However, this approach is still quite challenging and some issues still need to be fully addressed. The FE models are often extremely complex as well as the required computational costs are frequently high. This study presents the numerical simulation of the fatigue fracture in Nickel steel compact tension (CT) samples by means of FE analysis in ABAQUS. The eXtended Finite Element Method (XFEM) is coupled to the direct cyclic Low-Cycle Fatigue (LCF) approach to address the issues related to common modelling of fracture. The fatigue response is implemented by using the well-known Paris law. The model is easy to implement and the analysis does not require high computational time. The numerical crack propagation curves fit the experimental results better than the analytical solution. The numerical assessment of the fatigue life and fracture toughness also agrees with the experimental data.
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Bone plates have been widely used for the treatment of bone defects and trauma. These fixation plates can stabilize or replace bone tissue to restore appropriate load-bearing functionality. Nevertheless, the use of bone plates may lead to the stress shielding, thereby weakening prosthetic bone substitutes (e.g. bone graft or scaffolds) due to significant change in the biomechanical environment after implantation. To address this issue, we propose a time-dependent topology optimization procedure for the design of bone plates by taking into account bone remodeling. A solid isotropic material penalization (SIMP) model is used to interpolate design variables. The objective is to maximize total bone density within a reconstruction area at the final stage of bone remodeling, subject to a volume constraint of the bone plate and maximum allowable compliance of the prosthetic system. The sensitivity of bone density at the final stage is derived with respect to the topological variables of the plate in a step-wise manner. To facilitate sensitivity analysis, a bone remodeling rule is formulated in two different ways to accommodate a C1 continuity. A jaw reconstruction problem is exemplified in this study to demonstrate the effectiveness of the proposed approach. Through this specific case, the non-differentiality issue due to the lazy zone of a remodeling rule is smoothed; and the proposed approach is also compared with that of a time-independent design. The effects of volume fraction and compliance constraints are also investigated to gain further insights into the design of prosthetic substitutes. The proposed time-dependent topology optimization procedure is expected to form a useful tool for the design of implantable devices ensuring favourable long-term treatment outcomes.
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Damage tolerance assessment requires the accurate prediction of fatigue crack growth lives. Numerical simulation techniques using mixed mode Paris' law and various equivalent stress intensity factor models (ΔKeq) are widely employed for fatigue life prediction. In the present work, mixed mode (I/II) fatigue crack growth experiments are performed using the compact tension shear specimens made of AISI 316 austenitic stainless steel for various loading angles. Finite element fatigue crack growth simulations are carried out, and the effect of ΔKeq model in fatigue life prediction is studied. To achieve this, a three parameter double exponential type best fit is proposed for fitting the experimental mixed mode crack length vs fatigue life. The performance and capability of various selected models are assessed by comparing the predicted life with the experimental results. Fractographic studies at different stages of crack propagation for different loading angles are also presented to aid the above assessments. Based on the overall consistent performance, Irwin's and Tanaka models are predicting life close to the experimental data and Richard's and Yan's models provide conservative solutions.
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Mechanical failure of zirconia-based full-arch implant-supported fixed dental prostheses (FAFDPs) remains a critical issue in prosthetic dentistry. The option of full-arch implant treatment and the biomechanical behaviour within a sophisticated screw-retained prosthetic structure have stimulated considerable interest in fundamental and clinical research. This study aimed to analyse the biomechanical responses of zirconia-based FAFDPs with different implant configurations (numbers and distributions), thereby predicting the possible failure sites and the optimum configuration in biomechanical aspect by using finite element method (FEM). Five 3D finite element (FE) models were constructed with patient-specific heterogeneous material properties of mandibular bone. The results were reported using volume-averaged von-Mises stresses to eliminate numerical singularities. It was found that wider placement of multi-unit copings was preferred as it reduces the cantilever effect on denture. Within the limited areas of implant insertion, the adoption of angled multi-unit abutments allowed the insertion of oblique implants in the bone and wider distribution of the multi-unit copings in the prosthesis, leading to lower stress concentration on both mandibular bone and prosthetic components. Increasing the number of supporting implants in a FAFDPs reduced loading on each implant, although it may necessarily not reduce the stress concentration in the most posterior locations significantly. Overall, the 6-implant configuration was a preferable configuration as it provided the most balanced mechanical performance in this patient-specific case.
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Purpose: A cohort review was performed to compare the effect of a number of variables on mandible reconstruction plate (R-plate) survival and to identify the potential risk factors for plate fracture. We also reported our preliminary results of 3-dimensional (3D) printed reconstruction plates. Patients and methods: The data from patients who had undergone mandibular reconstruction using reconstruction plates were evaluated for age, gender, mandibular resection indication, defect site and length, remaining occluded teeth, reconstruction plate type, simultaneous soft or bone tissue reconstruction, and radiotherapy. The plate survival rate was estimated using the Kaplan-Meier curve, and the variables were compared using the log-rank (Mantel-Cox) test. Multifactorial risk correlation was determined using logistic regression analysis. Results: The study included 159 patients who had been followed for 97 ± 5.4 months. Of the 159 patients, 22 had experienced plate fracture that had occurred within 20 months. Most of the plate fractures had occurred near the mandibular bone stump, passing through the shoulder of the plate hole or the bridge between the subsequent plate holes. The overall survival was 86.2%. Patients with few occluded teeth (type I) had a significantly greater R-plate survival rate compared with those with many occluded teeth (P = .045). Laterocentral "LC" defects had a significantly lower survival rate (44.4%) compared with lateral "L" defects (84.5%; P = .00). The survival rates with soft tissue (88.7%) or bone tissue reconstruction (100%) were significantly different compared with that for R-plate alone (40%; P = .000 and P = .004, respectively). Four patients received 3D printed R-plates and were followed for 2 to 8 months (mean, 4 months) with no complications. Conclusions: Patients with many remaining occluded teeth, LC defect, and the absence of simultaneous soft or bone tissue reconstruction were associated with a lower plate survival rate. Bending of the plate increased the incidence of plate fracture, and the use of 3D printed customized R-plates seems a valuable alternative.
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In this paper, the recently proposed UniGrow model modification is further improved and a modeling framework of the proposed model is developed to account for both the short and long crack growth behavior. The modeling approach incorporates the integration of short and long propagation into the UniGrow crack growth model to address the short and long crack growth in a unified manner. A systematic study is performed to further validate the proposed modeling approach and assess its prediction capabilities and shortcomings to predict crack growth behavior of short and long cracks. The proposed crack growth model is assessed by first comparing the predicted fatigue crack growth results with long crack growth data sets of 7010-T7, 2024-T3, 2324-T3, 7050-T7 and 7075-T6 aluminum alloys at four different R ratios; The results are further assessed by comparison of short and long crack growth predictions to both short and long crack growth data sets of 2090-T8E41, LC9cs aluminum alloys and Ti-6Al-4V titanium alloys at two different R-ratios. The model showed good correlation with long crack data sets of 7010-T7, 2024-T3, 2324-T3, 7050-T7 and 7075-T6 and predicted crack growth results matched well with the nonlinearity of four material data sets at four different R-ratios. The results revealed that even though the proposed model was in good agreement with long crack data sets of 2090-T8E41, LC9cs aluminum alloys and Ti-6Al-4V titanium alloys at two different R-ratios, the model did not correlate well with the short crack data sets of these three materials and to accurately account for the variability of short crack growth.
Article
This paper presents a 3D crack growth simulation of an aircraft engine high pressure compressor blade which fractured in service. Using FRANC3D, an initial crack is inserted and grown using the max tensile stress theory. A numerical method was developed to capture the stress field under low cycle fatigue (LCF) and high cycle fatigue (HCF) loading by superimposing test measured dynamic loading with steady stresses. In doing so, the predicted crack trajectory, aspect ratio, and shape more closely agreed with the fractured airfoils compared to performing the simulation under LCF loads alone. This supports findings from fractographic investigation that the crack exhibits HCF rather than LCF characteristics.
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The human masticatory system has received significant attention in the areas of biomechanics due to its sophisticated co-activation of a group of masticatory muscles which contribute to the fundamental oral functions. However, determination of each muscular force remains fairly challenging in vivo; the conventional data available may be inapplicable to patients who experience major oral interventions such as maxillofacial reconstruction, in which the resultant unsymmetrical anatomical structure invokes a more complex stomatognathic functioning system. Therefore, this study aimed to (1)establish an inverse identification procedure by incorporating the sequential Kriging optimization (SKO)algorithm, coupled with the patient-specific finite element analysis (FEA)in silico and occlusal force measurements at different time points over a course of rehabilitation in vivo; and (2)evaluate muscular functionality for a patient with mandibular reconstruction using a fibula free flap (FFF)procedure. The results from this study proved the hypothesis that the proposed method is of certain statistical advantage of utilizing occlusal force measurements, compared to the traditionally adopted optimality criteria approaches that are basically driven by minimizing the energy consumption of muscle systems engaged. Therefore, it is speculated that mastication may not be optimally controlled, in particular for maxillofacially reconstructed patients. For the abnormal muscular system in the patient with orofacial reconstruction, the study shows that in general, the magnitude of muscle forces fluctuates over the 28-month rehabilitation period regardless of the decreasing trend of the maximum muscular capacity. Such finding implies that the reduction of the masticatory muscle activities on the resection side might lead to non-physiological oral biomechanical responses, which can change the muscular activities for stabilizing the reconstructed mandible.
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Achieving adequate healing in large or load‐bearing bone defects is highly challenging even with surgical intervention. The clinical standard of repairing bone defects using autografts or allografts has many drawbacks. A bioactive ceramic scaffold, strontium‐hardystonite‐gahnite or “Sr‐HT‐Gahnite” (a multi‐component, calcium silicate‐based ceramic) is developed, which when 3D‐printed combines high strength with outstanding bone regeneration ability. In this study, the performance of purely synthetic, 3D‐printed Sr‐HT‐Gahnite scaffolds is assessed in repairing large and load‐bearing bone defects. The scaffolds are implanted into critical‐sized segmental defects in sheep tibia for 3 and 12 months, with bone autografts used for comparison. The scaffolds induce substantial bone formation and defect bridging after 12 months, as indicated by X‐ray, micro‐computed tomography, and histological and biomechanical analyses. Detailed analysis of the bone‐scaffold interface using focused ion beam scanning electron microscopy and multiphoton microscopy shows scaffold degradation and maturation of the newly formed bone. In silico modeling of strain energy distribution in the scaffolds reveal the importance of surgical fixation and mechanical loading on long‐term bone regeneration. The clinical application of 3D‐printed Sr‐HT‐Gahnite scaffolds as a synthetic bone substitute can potentially improve the repair of challenging bone defects and overcome the limitations of bone graft transplantation. Strontium–hardystonite–gahnite scaffolds show strong ability to repair large and load‐bearing defects in the long bones of sheep over 1 year, without the addition of cells or growth factors. These 3D printed bioactive ceramic implants may be useful as purely synthetic bone substitutes to augment the clinical treatment of challenging bone defects.
Article
Objectives: This study aimed to develop a simple and efficient numerical modeling approach for characterizing strain and total strain energy in bone scaffolds implanted in patient-specific anatomical sites. Materials and methods: A simplified homogenization technique was developed to substitute a detailed scaffold model with the same size and equivalent orthotropic material properties. The effectiveness of the proposed modeling approach was compared with two other common homogenization methods based on periodic boundary conditions and the Hills-energy theorem. Moreover, experimental digital image correlation (DIC) measurements of full-field surface strain were conducted to validate the numerical results. Results: The newly proposed simplified homogenization approach allowed for fairly accurate prediction of strain and total strain energy in tissue scaffolds implanted in a large femur mid-shaft bone defect subjected to a simulated in-vivo loading condition. The maximum discrepancy between the total strain energy obtained from the simplified homogenization approach and the one obtained from detailed porous scaffolds was 8.8%. Moreover, the proposed modeling technique could significantly reduce the computational cost (by about 300 times) required for simulating an in-vivo bone scaffolding scenario as the required degrees of freedom (DoF) was reduced from about 26 million for a detailed porous scaffold to only 90,000 for the homogenized solid counterpart in the analysis. Conclusions: The simplified homogenization approach has been validated by correlation with the experimental DIC measurements. It is fairly efficient and comparable with some other common homogenization techniques in terms of accuracy. The proposed method is implicating to different clinical applications, such as the optimal selection of patient-specific fixation plates and screw system.
Article
Introduction: As the popularity of volar locked plate fixation for distal radius fractures has increased, so have the number and variety of implants, including variations in plate design, the size and angle of the screws, the locking screw mechanism, and the material of the plates. Hypothesis: carbon-fiber reinforced polyetheretherketone (CFR-PEEK) plate features similar biomechanical properties to metallic plates, representing, therefore, an optimal alternative for the treatment of distal radius fractures. Materials and methods: three different materials-composed plates were evaluated: stainless steel volar lateral column (Zimmer); titanium DVR (Hand Innovations); CFR-PEEK DiPHOS-RM (Lima Corporate). Six plates for each type were implanted in sawbones and an extra-articular rectangular osteotomy was created. Three plates for each material were tested for load to failure and bending stiffness in axial compression. Moreover, 3 constructs for each plate were evaluated after dynamically loading for 6000 cycles of fatigue. Results: the mean bending stiffness pre-fatigue was significantly higher for the stainless steel plate. The titanium plate yielded the higher load to failure both pre and post fatigue. After cyclic loading, the bending stiffness increased by a mean of 24% for the stainless steel plate; 33% for the titanium; and 17% for the CFR-PEEK plate. The mean load to failure post-fatigue increased by a mean of 10% for the stainless steel and 14% for CFR-PEEK plates, whereas it decreased (-16%) for the titanium plate. Statistical analysis between groups reported significant values (p <.001) for all comparisons except for Hand Innovations vs. Zimmer bending stiffness post fatigue (p = .197). Discussion: the significant higher load to failure of the titanium plate, makes it indicated for patients with higher functional requirements or at higher risk of trauma in the post-operative period. The CFR-PEEK plate showed material-specific disadvantages, represented by little tolerance to plastic deformation, and lower load to failure. Level of evidence: N/A.
Article
In the present paper, a mesh independent computational algorithm is developed and incorporated into a commercial finite element software (Abaqus) for automated fatigue crack growth analysis under mixed mode variable amplitude loading conditions. The algorithm calculates the stress intensity factor (SIF) at predetermined small crack growth increments in the finite element software by using Extended Finite Element Method (XFEM) and predict the fatigue crack growth (FCG) path through local symmetry (KII = 0) criterion. The aforementioned algorithm also computes the FCG life by means of cycle-by-cycle integration method through Nasgro equation based on the equivalent SIF range. Load history effects are also taken into account by using appropriate retardation models according to nature of the loading. For verification purpose, experimental crack path trajectories and fatigue life data available in the open literature are compared with computational results. Quite good agreements are obtained between the computed and the experimental results.
Article
Purpose To evaluate the biomechanical performance of a commercially available bridging plate (2.4) as well as screws and bone simulating the reconstruction of hemimandibular defects and to indicate alternatives of reinforcement to prevent plate fractures either by strength or fatigue. Material and Methods Two common hemimandibular defects are investigated using computed finite element analysis (FEA) approach. Simplified and refined computational models are developed for the geometry of the screw. Conditions of non-locking and locking plate-screw interfaces are considered. Static loads of 120N are applied. Von Mises stresses and fatigue are calculated. As reinforcement, a second complete or partial plate is placed onto the original plate. Results Results demonstrate that reconstruction plates are often subjected to excessive stress that may lead to fracture either by strength or by fatigue. An attached complete or partial second plate is able to reduce stress in the plate, in screws and bone so that stress remains below the allowable limit of the materials. Conclusion A simplified technique of attaching a whole or sectioned second plate onto the original plate can reduce the stress calculated and may reduce the frequency of plate fractures for the patient’s comfort, security and financial savings.
Article
Purpose: This study develops a novel hybrid (NH) reconstruction plate that can provide load-bearing strength, secure the bone transplant at the prosthesis favored position, and also maintain the facial contour in a mandibular segmental defect. A new patient-match bending technique which uses a three-dimensional printing (3DP) stamping process is developed to increase the interfacial fit between the reconstruction plate and mandibular bone. Materials and methods: The NH reconstruction plate was designed to produce a continuous profile with non-uniform thickness and triangular cross-screw patterns with a locking-screw feature at the plate base. Two mandible segmental defect finite element models including the NH reconstruction plate to secure a bone flap for occlusal requirement and the commercial straight (CS) reconstruction plate to secure a bone flap along the lower mandible border were generated for biomechanical fatigue testing. Results: The simulated results showed that the maximum von Mises stresses of the reconstruction plate for CS secured model are about 4.5 times more than the NH secured model. The bone strains around the fixation screws showed that the CS secured model was meaningfully higher than that of the NH secured model and exceeded the bone limit value. No fracture of any component was found in any sample in the fatigue testing. Conclusion: In conclusion, the newly developed NH reconstruction plate can secure the transplant position in accordance to the individual occlusal requirements without sacrificing the maintenance of facial contour. Finite element-based biomechanical evaluation demonstrates superior mechanical strength compared to commercial standard plates.
Article
Bone fracture pattern prediction is still a challenge and an active field of research. The main goal of this article is to present a combined methodology (experimental and numerical) for femur fracture onset analysis. Experimental work includes the characterization of the mechanical prop- erties and fracture testing on a bone simulant. The numerical work focuses on the development of a model whose material properties are provided by the characterization tests. The fracture location and the early stages of the crack propaga- tion are modelled using the extended finite element method and the model is validated by fracture tests developed in the experimental work. It is shown that the accuracy of the numerical results strongly depends on a proper bone behaviour characterization. Keywords—Bone
Article
This work presents the numerical simulation and validation of a fatigue propagation test of a semi-elliptical crack located at the side of the rectangular section of a beam subjected to four-point bending. For most common fatigue test configurations there are equations that allow calculating the stress intensity factors (SIFs). However, no solution is provided if the crack is located on any of the lateral sides of the rectangular section, since one part of the crack is located in the tractive zone while the other is at the compressive zone. In these cases, it is necessary to use alternative methods. The extended finite element method (XFEM) provides a new alternative for the calculation of SIFs, and to simulate crack propagation, by using special interpolation functions. Furthermore, XFEM-based LEFM approach offers the advantage of performing crack growth analysis without the need for updating the mesh (re-meshing). The experimental tests have been carried out in an Instron 8874 biaxial testing machine. Crack growth was controlled by optical microscopy and by progressive crack surface heat tinting. For the numerical simulations, the Extended Finite Element Method (XFEM) implemented in the Abaqus® 2017 software has been used. The comparison between the experimental and numerical results shows very good correlation regarding crack shape and number of cycles to failure. The capabilities of the XFEM-based LEFM approach to simulate fatigue crack growth in complex crack fronts are validated.
Article
Fatigue is a major reason for the failure of components subjected to cyclic loadings, and the fatigue failure of components can be divided into two phases: crack initiation and crack propagation. In this study, continuum damage mechanics (CDM) combined with the extended finite element method (XFEM) is proposed to predict the total fatigue life of components, that is, the crack initiation life and propagation life. First, the damage-coupled elastic-plastic constitutive equations and fatigue damage evolution equations are derived to calculate the fatigue damage and to predict the crack initiation life of a material under cyclic loads. Second, according to the distribution of the damage field and trend of fatigue damage evolution, the criterion to judge the formation of crack initiation is proposed. Third, based on linear elastic fracture mechanics (LEFM) with XFEM, the crack propagation life is predicted. Then, fatigue crack initiation and propagation analysis for a specimen with a preset pit and for a fuselage structure with opening are conducted using the method described above. Finally, fatigue experiments are conducted to verify the proposed method, and the predicted total fatigue life and crack propagation path are in accordance with the experimental results.
Article
Medial opening wedge high tibial osteotomy (MOWHTO) is a surgical procedure to treat knee osteoarthritis associated with varus deformity. However, the ideal final alignment of the Hip-Knee-Ankle (HKA) angle in the frontal plane, that maximizes procedural success and post-operative knee function, remains controversial. Therefore, the purpose of this study was to introduce a subject-specific modeling procedure in determining the biomechanical effects of MOWHTO alignment on tibiofemoral cartilage stress distribution. A 3D finite element knee model derived from magnetic resonance imaging of a healthy participant was manipulated in-silico to simulate a range of final HKA angles (i.e. 0.2°, 2.7°, 3.9° and 6.6° valgus). Loading and boundary conditions were assigned based on subject-specific kinematic and kinetic data from gait analysis. Multiobjective optimization was used to identify the final alignment that balanced compressive and shear forces between medial and lateral knee compartments. Peak stresses decreased in the medial and increased in the lateral compartment as the HKA was shifted into valgus, with balanced loading occurring at angles of 4.3° and 2.9° valgus for the femoral and tibial cartilage respectively. The concept introduced here provides a platform for non-invasive, patient-specific preoperative planning of the osteotomy for medial compartment knee osteoarthritis.
Article
OBJECT This study was undertaken to quantify the in vitro range of motion (ROM) of oblique as compared with anterior lumbar interbody devices, pullout resistance, and subsidence in fatigue. METHODS Anterior and oblique cages with integrated plate fixation (IPF) were tested using lumbar motion segments. Flexibility tests were conducted on the intact segments, cage, cage + IPF, and cage + IPF + pedicle screws (6 anterior, 7 oblique). Pullout tests were then performed on the cage + IPF. Fatigue testing was conducted on the cage + IPF specimens for 30,000 cycles. RESULTS No ROM differences were observed in any test group between anterior and oblique cage constructs. The greatest reduction in ROM was with supplemental pedicle screw fixation. Peak pullout forces were 637 ± 192 N and 651 ± 127 N for the anterior and oblique implants, respectively. The median cage subsidence was 0.8 mm and 1.4 mm for the anterior and oblique cages, respectively. CONCLUSIONS Anterior and oblique cages similarly reduced ROM in flexibility testing, and the integrated fixation prevented device displacement. Subsidence was minimal during fatigue testing, most of which occurred in the first 2500 cycles.
Article
A concurrent simulation and experimental validation for curvilinear fatigue crack growth (FCG) analysis under both constant amplitude and overload spectrums is proposed in this paper. The simulation methodology is based on a small time-scale fatigue crack growth model and the extended finite element method (XFEM) to calculate the stress intensity factor solution of an arbitrary curvilinear crack. Parametric studies are used to determine the algorithm parameters in the numerical fatigue crack growth simulation. Following this, experimental testing on modified compact specimens is performed under both constant amplitude and overload loadings for model validation. Experimentally measured crack growth orientations and lengths are compared with numerical simulations. Both the experimental and simulation results show the overload retardation behavior for curvilinear cracks under overload loadings. The investigated periodic overload loading has no significant impact on the crack growth orientations. Several conclusions and areas of future work are identified based on the proposed numerical and experimental investigations.
Article
A mesoscale model of fatigue crack formation and stress–strain behavior in crystalline alloys entitled Sistaninia–Niffenegger Fatigue (SNF) model is applied to AISI 316L austenitic stainless steel. An inelastic hysteresis energy criterion in conjunction with continuum damage modeling provides a strong tool for studying the behavior of the austenitic steel under cyclic loading. The model predictions are validated against fatigue experimental data. The results show that this microstructural-based modeling approach is capable for predicting the behavior of the steel even under complex loading conditions. It can reproduce and help to understand well known fatigue experimental facts, e.g. the effect of grain size and initial defects, by considering the anisotropic behavior of crystalline materials at the level of the microstructure.
Article
This paper presents numerical analyses of fatigue behaviors of three-dimensional (3-D) 4-step rectangular braided composite material under three-point low-cyclic bending. A microstructure model of the 3-D braided composite was established to calculate the bending fatigue deformation and failure with finite element method (FEM). The stiffness degradation and failure morphologies were obtained from the FEM results and compared with those from experimental. The stress distributions, stress hysteresis and failures of fiber tows and resins at different parts of the 3-D braided composite material have been collected from the FEM calculations to analyze the fatigue failure mechanisms. The influences of the braided preform microstructure on the fatigue damage were discussed. It is found that the surface yarns share more loads than the yarns of the inner part. The stress concentration appeared at the regions with larger changes of fiber tow orientation angles. The fatigue damage evolutions were also used to explain the mechanical behaviors degradation. The crack generation and fatigue damages development of the braided composite appeared at early stages and followed by crack propagation afterwards. A series of damage evolution at the different loading cycles were obtained to unveil the fatigue damage mechanisms. From the investigation, the fatigue resistance of 3-D braided composite could be optimized from improving the mechanical behaviors of surface fiber tows and decreasing the change of fiber tows orientation angles.
Article
A new finite element-based mesoscale model is developed to simulate the localization of deformation and the growth of microstructurally short fatigue cracks in crystalline materials by considering the anisotropic behavior of the individual grains. The inelastic hysteresis energy is used as a criterion to predict the fatigue crack initiation and propagation. This criterion in conjunction with continuum damage modeling provides a strong tool for studying the behavior of materials under cyclic loading at the level of the microstructure. The model predictions are validated against an austenitic stainless steel alloy experimental data. The results show that a combined microstructural and continuum damage modeling approach is able to express the overall fatigue behavior of crystalline materials at the macroscale based on the microstructural features. It correctly predicts the crack initiation on slip bands and at inclusions in low-cycle and high-cycle fatigue, respectively, in agreement with experimental observations reported in the literature.
Article
This study is the last in a series detailing an investigation into the all-ceramic, inlay supported fixed partial denture, the major concern of which has been the examination of the stress responses of the bridge via the use of finite element analysis (FEA) and its validation. The progression from a classic FEA to the current extended or enriched FEA (XFEA) will be described and the validation performed. XFEA modelling was compared and validated against the experimental model analysis (EMA) as described in a previous study. The two EMA load case fracture strengths of 160 N and 313 N compared favourably with the best two fracture predictions from the XFEA of 185 N and 213 N (maximum principal stress criterion) respectively, with the origin of fracture and overall trajectory and pattern of crack propagation agreeing very well. XFEA load prediction is within 15% of the EMA in the best case. The sensitivity of the bridges to loading position variations was accurately predicted by the XFEA, together with the change in fracture origin from the molar to premolar embrasures. With this, the authors believe that they have provided a convincing validation, both qualitatively and quantitatively, of an anatomically realistic dental bridge.
Article
Hip fracture remains a major health problem for the elderly. Clinical studies have assessed fracture risk based on bone quality in the aging population and cadaveric testing has quantified bone strength and fracture loads. Prior modeling has primarily focused on quantifying the strain distribution in bone as an indicator of fracture risk. Recent advances in the extended finite element method (XFEM) enable prediction of the initiation and propagation of cracks without requiring a priori knowledge of the crack path. Accordingly, the objectives of this study were to predict femoral fracture in specimen-specific models using the XFEM approach, to perform one-to-one comparisons of predicted and in vitro fracture patterns, and to develop a framework to assess the mechanics and load transfer in the fractured femur when it is repaired with an osteosynthesis implant. Five specimen-specific femur models were developed from in vitro experiments under a simulated stance loading condition. Predicted fracture patterns closely matched the in vitro patterns; however, predictions of fracture load differed by approximately 50% due to sensitivity to local material properties. Specimen-specific intertrochanteric fractures were induced by subjecting the femur models to a sideways fall and repaired with a contemporary implant. Under a post-surgical stance loading, model-predicted load sharing between the implant and bone across the fracture surface varied from 59%:41% to 89%:11%, underscoring the importance of considering anatomic and fracture variability in the evaluation of implants. XFEM modeling shows potential as a macro-level analysis enabling fracture investigations of clinical cohorts, including at-risk groups, and the design of robust implants.
Article
The fatigue life estimation of orthotropic steel bridge decks using the finite element method is most frequently associated with the application of the structural hot spot stress approach or the effective notch stress approach, rather than the traditional nominal stress approach. The application of these approaches to a welded joint with cut-out holes in orthotropic bridge decks, where it is not easy to distinguish the non-linear stress caused by the notch at the weld toe from the stress concentration effect emanating from the hole in the detail, was investigated. The results of the finite element calculations were compared with the results of the fatigue tests which were carried out on full-scale specimens. The results of the finite element analyses revealed that the structural hot spot stresses obtained from the shell element models were unrealistically high when the welds were omitted. Moreover, the way in which the welds were represented had a substantial influence on the magnitude of the hot spot stress. The results of the analysis when using the effective notch stress approach showed that the agreement between the estimated fatigue life using this approach and the fatigue life obtained from the fatigue tests was good.
Article
In the present work, the fatigue failure of a locking compression plate (LCP) fixed across a transverse fracture (8-mm gap) at the midshaft of femur was experimentally evaluated. The complete fracture of LCP occurred after 42,000 cycles of loading, i.e. equivalence to about 8 days of walking. The fatigue failure of LCP was possible before the adequate healing of fracture, and the full load of walking should not be allowed for the patient with the present fracture condition. The fatigue crack firstly initiated from a subsurface inclusion embedded under the surface of compression hole. After some cycles of loading, another fatigue crack also initiated from the surface of locking hole, and then both cracks propagated inside the LCP. As an evidence of the propagation of fatigue crack, the striations were observed on the fracture surface of the LCP. The striation spacing was long when observed far from the crack initiation site, and became shorter when observed around the crack initiation site. Based on the striation spacing, the number of cycles for the propagation of fatigue crack from the initiation site to the bottom part of LCP was estimated to be approximately 5000 cycles.
Article
Effective and reliable clinical uses of dental ceramics necessitate an insightful analysis of the fracture behavior under critical conditions. To better understand failure characteristics of porcelain veneered to zirconia core ceramic structures, thermal induced cracking during the cooling phase of fabrication is studied here by using the eXtended Finite Element Method (XFEM). In this study, a transient thermal analysis of cooling is conducted first to determine the temperature distributions. The time-dependent temperature field is then imported to the XFEM model for viscoelastic thermomechanical analysis, which predicts thermal induced damage and cracking at different time steps. Temperature dependent material properties are used in the both transient thermal and thermomechanical analyses. Three typical ceramic structures are considered in this paper, namely bi-layered spheres, squat cylinders and dental crowns with thickness ratios of 1:2 and 1:1, respectively. The XFEM fracture patterns exhibit good agreement with clinical observation and the in-vitro experimental results obtained from Scanning Electron Microscopy (SEM) characterization. The study reveals that fast cooling can lead to thermal fracture of these different bi-layered ceramic structures and cooling rate (in terms of heat transfer coefficient) plays a critical role in crack initiation and propagation. By exploring different cooling rates, the heat transfer coefficient thresholds of fracture are determined for different structures, which are of clear clinical implication.
Article
Objective: The aim of this study was to evaluate strain distribution in peri-implant bone, stress in the abutments and denture stability of mandibular overdentures anchored by different numbers of implants under different loading conditions, through three-dimensional finite element analysis (3D FEA). Methods: Four 3D finite element models of mandibular overdentures were established, using between one and four Straumann implants with Locator attachments. Three types of load were applied to the overdenture in each model: 100N vertical and inclined loads on the left first molar and a 100N vertical load on the lower incisors. The biomechanical behaviours of peri-implant bone, implants, abutments and overdentures were recorded. Results: Under vertical load on the lower incisors, the single-implant overdenture rotated over the implant from side to side, and no obvious increase of strain was found in peri-implant bone. Under the same loading conditions, the two-implant-retained overdenture showed more apparent rotation around the fulcrum line passing through the two implants, and the maximum equivalent stress in the abutments was higher than in the other models. In the three-implant-supported overdenture, no strain concentration was found in cortical bone around the middle implant under three loading conditions. Conclusions and clinical significance: Single-implant-retained mandibular overdentures do not show damaging strain concentration in the bone around the only implant and may be a cost-effective treatment option for edentulous patients. A third implant can be placed between the original two when patients rehabilitated by two-implant overdentures report constant and obvious denture rotation around the fulcrum line.
Article
The cyclic hysteresis energy of four kinds of steel was analysed using fully reversed constant-strain-controlled fatigue tests. Mathematical formulae for the cyclic hysteresis energy of the materials, which have different cyclic properties, are proposed. Because of the experimental and theoretical relations of cyclic hysteresis energy and the total absorbed energy to failure versus fatigue failure life, it is considered that part of the hysteresis energy is absorbed through heating, vibration and non-propagating defects during the process of fatigue life. The total absorbed energy to failure is associated with the variation of cyclic hysteresis energy.
Article
ABSTRACT The purpose of this paper is to present a unified analysis to both high and low cycle fatigue based on shakedown theories and dissipated energy. The discussion starts with a presentation of the fatigue phenomena at different scales (microscopic, mesoscopic and macroscopic) and of the main shakedown theorems. A review of the Dang Van high cycle fatigue criterion shows that this criterion is essentially based on the hypothesis of elastic shakedown and can therefore be expressed as a bounded cumulated dissipated energy. In the low cycle fatigue regime, recent results by Skelton and Charkaluk et al. show that we can speak of a plastic shakedown at both mesoscopic and macroscopic scale and of a cumulated energy bounded by the failure energy. The ideas are also justified by infrared thermography tests permitting a direct determination of the fatigue limit.