Influence of stress concentracion and crack initiation on remaining life of biomaterials for reconstructive plates

Biomaterijali namenjeni za izradu ortopedskih rekonstruktivnih pločica su boljih mehaničkih karakteristika u odnosu na kosti, a ipak u velikom broju slučajeva dolazi do otkaza pločica usled zamornog oštećenja. Jasno je da otkazi pločica za fiksaciju preloma dovode do problema u procesu lečenja i potrebe za ponovnom operacijom pacijenata. Pored materijala i geometrije, iskustvo i znanje ortopedskog hirurga su jedan od najbitnijih faktora koji doprinose učestalosti zamornih otkaza pločica. U slučaju neadekvatne ugradnje pločice će biti podvrgnute preopterećenju od samog početka eksploatacije, što pogoduje inicijaciji prsline. Jedan od najčešće korišćenih biokompatibilnih materijala za unutrašnju fiksaciju preloma kostiju je dvofazna alfa-beta legura titana Ti-6Al-4V. Cilj istraživanja doktorske disertacije je da se utvrdi ponašanje rekonstruktivnih biomedicinskih pločica u prisustvu prslina na mestima koncentracije napona i da se utvrdi brzina rasta prslina za različite geometrije, na osnovu čega im je moguće proceniti integritet i preostali radni vek. U tu svrhu, eksperimentalno su određene zatezne karakteristike i parametri mehanike loma legure Ti-6Al-4V u cilju korišćenja podataka u numeričkim simulacijama, dok su udarne karakteristike ispitane na instrumentiranom klatnu u cilju boljeg razumevanja ponašanja samog materijala. Umesto komplikovanih eksperimentalnih ispitivanja, primenjeni su postupci numeričkih simulacija ponašanja ortopedskih pločica od Ti-6Al-4V legure titana u prisustvu zamornih prslina, opterećenih na savijanje u četiri tačke, prvenstveno zbog nemogućnosti primene mernih folija na geometrijama pločica. Takođe, numeričke simulacije predstavljaju bržu i jeftiniju alternativu eksperimentalnim ispitivanjima. Geometrija i modeli pločica rađeni su u programu CATIA, a simulacija rasta prsline proširenom metodom konačnih elemenata (PMKE) u programu ANSYS. U simulacijama je korišćeno opterećenje (moment savijanja) koje se javlja na čovekovoj potkolenici tokom hoda, a uzete su u obzir i različite telesne mase pacijenta: 60, 90 i 120 kg. Analiza je rađena za 5 različitih geometrija pločica. Analiza je rađena isključivo sa mehaničkog aspekta, dok uticaj korozivne sredine čovekovog organizma i biokompatibilnost materijala nisu uzeti u razmatranje. Postoji više metoda za tretiranje materijala u cilju poboljšanja njegovih određenih karakteristika. Jedan od načina za poboljšanje površinskih karakteristika materijala je lasersko ojačavanje materijala (LSP). Razlog za primenu laserskog ojačavanja na leguri titana je istraživanje mogućnosti lokalnog poboljšanja mehaničkih karakteristika i mikrogeometrijskih karakteristika površine, kao i unošenja pritisnih zaostalih napona koji bi doprineli otpornosti na iniciranje prslina, naročito na mestima koncentracije napona. Ispitivanja su rađena na tri grupe uzoraka i pod tri različita tretmana pikosekundnim laserom Nd:YAG. Analiza je rađena sa aspekta mikrostrukture, površinske hrapavosti i mikrotvrdoće.Biomaterials intended for orthopaedic plates manufacturing are of much higher mechanical properties relative to the bone itself and still there are many cases where those plates fracture in service with fatigue as the main failure mode. This causes the problem with healing process and requires that the patients undergo another surgery. Apart from materials and geometry, experience and knowledge of the orthopaedic surgeon is one of the important factors contributing to the frequency of fatigue failures. If incorrectly implanted, plates will be subjected to overloading from the start, which is convenient for crack initiation. One of the most commonly used biocompatible materials for internal bone fixation is two phase alpha-beta titanium alloy Ti-6Al-4V. Focus of this doctoral dissertation is to determine the behaviour of reconstructive biomedical plates in the presence of cracks at the stress concentration locations and to obtain crack growth rates for different plate geometries which are to be used as a basis for structural integrity and life assessment. Experimental investigations of tensile and fracture mechanics parameters of Ti-6Al-4V alloy were conducted to be used in numerical simulations, while impact characteristics, tested on instrumented Charpy pendulum, were investigated for the purpose of better understanding of the material. Instead of complicated experimental four point bend testing of orthopaedic plates in the presence of cracks, numerical simulations were employed, mainly due to inability of measurement foils to be used on given plate geometries. Also, numerical simulations are time and cost efficient when compared to experimental testing. Geometries and models of plates are defined in CATIA software, while extended finite element method (XFEM) crack growth simulations were made in ANSYS. Loads (bending moments) correspond to the ones occurring in human tibia during gait cycle for different body weights: 60, 90 and 120 kg. Analysis was conducted on 5 different plate geometries. Analysis was done only from the mechanical aspect and the influence of corrosive environment of the human body was not taken into consideration. There are many methods for treatment of materials for the purpose of improving its certain characteristics. One of the methods for surface characteristics improvement is laser shock peening. Reason for utilising the laser shock peening on titanium alloy is to research the possibilities of local improvement of mechanical and micro-geometrical properties, as well as inducing the compressive residual stresses which should contribute to the crack initiation resistance, especially at the stress concentration locations. Testing was conducted on three types of samples under three different treatments with picosecond laser Nd:YAG. Analysis was made from the aspect of microstructure, surface roughness and microhardness

Digital image correlation method in experimental analysis of fracture mechanics parameters

U okviru rada su predstavljeni principi i primeri savremenih eksperimentalnih metoda za određivanje parametara mehanike loma. Metodologija analize i određivanja parametara mehanike loma obuhvatila je analizu ponašanja metalnih materijala u odnosu na lom primenom modifikovanih epruveta mehanike loma, sa inicijalnom prslinom i zateznim opterećenjem, i imala je za cilj pre svega određivanje karaktera tih procesa u slučaju problema tankih pločica, uz korišćenje osnovnih postulata mehanike loma. Metodologija uključuje primenu eksperimentalnih postupaka mehanike loma definisanih standardima, uz primenu metode za trodimenzionalno stereoskopsko merenje mehaničkog ponašanja materijala. Pomoću sistema za korelaciju digitalnih slika (Digital Image Correlation, DIC) za ispitivanje deformacija i pomeranja u materijalu ispitani su parametri mehanike loma metalnih materijala, pre svega epruveta od 316L nerđajućeg čelika i titan legure Ti-6Al-4V. 3D optički sistem GOM i softver Aramis su korišćeni za izvođenje eksperimentalne analize na pripremljenim epruvetama. Pošto se ovaj sistem koristi za merenje deformacija i parametra pomeranja otvaranja vrha prsline CTOD na modifikovanim kompaktnim epruvetama za zatezanje C(T) i epruvetama sa zarezom, dat je i kratak pregled mernih procedura i procesiranja rezultata, kao i moguće primene ovog sistema. Prikazani rezultati daju pregled polja deformacija i pomeranja tokom otvaranja vrha prsline, rasta prsline i u trenutku loma epruvete, što nije moguće putem tradicionalnih metoda merenja. Analizom rezultata pokazano je da je sa velikom preciznošću moguće izmeriti pomeranja tokom otvaranja vrha prsline i dobiti CTOD parametar. Rezultati pokazuju da odabrana metoda pruža dobre rezultate u analizi mehaničkog ponašanja i određivanju parametara mehanike loma metalnih materijala.The principles and examples of state-of-the-art experimental methods for measuring the fracture mechanics parameters are presented in this paper. The methodology of experimental analysis of the fracture mechanics parameters includes investigation of fracture behaviour of metallic materials using modified specimens with initial crack under tensile load, with the primary goal of determining the characteristics of fracture processes for the case of thin plates, using basic fracture mechanics postulates. The methodology also includes the application of experimental fracture mechanics procedures as defined by standards, using three-dimensional stereo-metric mechanical behaviour measurement methods. Fracture behaviour of metallic materials, mainly 316L stainless steel and titanium alloy Ti-6Al-4V specimens, is analyzed by using a digital image correlation (DIC) system for measuring strain and displacement in the material. GOM three-dimensional optical system and Aramis software are used to perform experimental analysis of selected specimens. As this system is used to measure strain and crack tip opening displacement (CTOD) parameter on the modified compact tension specimen C(T) and notch specimens, a basic review of measuring procedures and result processing is given, alongside other possible applications for this system. The presented results show strain and displacement fields during crack tip opening, crack growth, and the moment of fracture of specimens, which are not possible using traditional measurement methods. The analysis of results shows that it is possible to measure displacements during crack tip opening with a great precision, and thus obtain the CTOD parameter. The results show that the selected measuring method obtains good results in the analysis of mechanical behaviour and fracture mechanics parameters of metallic materials

Digital image correlation method in experimental analysis of fracture mechanics parameters

U okviru rada su predstavljeni principi i primeri savremenih eksperimentalnih metoda za određivanje parametara mehanike loma. Metodologija analize i određivanja parametara mehanike loma obuhvatila je analizu ponašanja metalnih materijala u odnosu na lom primenom modifikovanih epruveta mehanike loma, sa inicijalnom prslinom i zateznim opterećenjem, i imala je za cilj pre svega određivanje karaktera tih procesa u slučaju problema tankih pločica, uz korišćenje osnovnih postulata mehanike loma. Metodologija uključuje primenu eksperimentalnih postupaka mehanike loma definisanih standardima, uz primenu metode za trodimenzionalno stereoskopsko merenje mehaničkog ponašanja materijala. Pomoću sistema za korelaciju digitalnih slika (Digital Image Correlation, DIC) za ispitivanje deformacija i pomeranja u materijalu ispitani su parametri mehanike loma metalnih materijala, pre svega epruveta od 316L nerđajućeg čelika i titan legure Ti-6Al-4V. 3D optički sistem GOM i softver Aramis su korišćeni za izvođenje eksperimentalne analize na pripremljenim epruvetama. Pošto se ovaj sistem koristi za merenje deformacija i parametra pomeranja otvaranja vrha prsline CTOD na modifikovanim kompaktnim epruvetama za zatezanje C(T) i epruvetama sa zarezom, dat je i kratak pregled mernih procedura i procesiranja rezultata, kao i moguće primene ovog sistema. Prikazani rezultati daju pregled polja deformacija i pomeranja tokom otvaranja vrha prsline, rasta prsline i u trenutku loma epruvete, što nije moguće putem tradicionalnih metoda merenja. Analizom rezultata pokazano je da je sa velikom preciznošću moguće izmeriti pomeranja tokom otvaranja vrha prsline i dobiti CTOD parametar. Rezultati pokazuju da odabrana metoda pruža dobre rezultate u analizi mehaničkog ponašanja i određivanju parametara mehanike loma metalnih materijala.The principles and examples of state-of-the-art experimental methods for measuring the fracture mechanics parameters are presented in this paper. The methodology of experimental analysis of the fracture mechanics parameters includes investigation of fracture behaviour of metallic materials using modified specimens with initial crack under tensile load, with the primary goal of determining the characteristics of fracture processes for the case of thin plates, using basic fracture mechanics postulates. The methodology also includes the application of experimental fracture mechanics procedures as defined by standards, using three-dimensional stereo-metric mechanical behaviour measurement methods. Fracture behaviour of metallic materials, mainly 316L stainless steel and titanium alloy Ti-6Al-4V specimens, is analyzed by using a digital image correlation (DIC) system for measuring strain and displacement in the material. GOM three-dimensional optical system and Aramis software are used to perform experimental analysis of selected specimens. As this system is used to measure strain and crack tip opening displacement (CTOD) parameter on the modified compact tension specimen C(T) and notch specimens, a basic review of measuring procedures and result processing is given, alongside other possible applications for this system. The presented results show strain and displacement fields during crack tip opening, crack growth, and the moment of fracture of specimens, which are not possible using traditional measurement methods. The analysis of results shows that it is possible to measure displacements during crack tip opening with a great precision, and thus obtain the CTOD parameter. The results show that the selected measuring method obtains good results in the analysis of mechanical behaviour and fracture mechanics parameters of metallic materials

Extended FEM analysis of fatigue crack growth in Ti-6Al-4V orthopaedic plates

The extended finite element method (xFEM) was used to analyse fatigue crack growth in orthopaedic locking compression plates (LCP), made of Titanium alloy, Ti-6Al-4V, loaded in four-point bending. The optimal geometry was defined previously in respect to the remaining life of LCP used for patients with different body weights (BW - 60, 90 and 120 Kg). The plate with optimal geometry is analysed in more details here to assess the effect of BW and get better insight into fatigue crack growth path

Extended FEM analysis of fatigue crack growth in Ti-6Al-4V orthopaedic plates

The extended finite element method (xFEM) was used to analyse fatigue crack growth in orthopaedic locking compression plates (LCP), made of Titanium alloy, Ti-6Al-4V, loaded in four-point bending. The optimal geometry was defined previously in respect to the remaining life of LCP used for patients with different body weights (BW - 60, 90 and 120 Kg). The plate with optimal geometry is analysed in more details here to assess the effect of BW and get better insight into fatigue crack growth path

Failure analysis and numerical simulation of slab carrying clamps

The goal of this study was to determine the reasons for failure of slab carrying clamps which had occurred in one of the clamp support levers. Detailed fractography analysis revealed that there was a welded joint at the location where failure occurred, which was not expected, since the second lever was not welded. Once the presence of the welded joint was confirmed by hardness measuring and the analysis of chemical composition, attempts were made to determine which materials were used for the welding, in order to obtain a more detailed insight into the nature of the failure. The first step of this extensive analysis involved the development of a simple, approximated numerical model, with the goal of determining the location of stress concentration in the support lever, so that it could be compared to the real failure location. Further investigations will involve more complex models, including the ones that will simulate crack growth, once its location is numerically verified

Failure analysis and numerical simulation of slab carrying clamps

The goal of this study was to determine the reasons for failure of slab carrying clamps which had occurred in one of the clamp support levers. Detailed fractography analysis revealed that there was a welded joint at the location where failure occurred, which was not expected, since the second lever was not welded. Once the presence of the welded joint was confirmed by hardness measuring and the analysis of chemical composition, attempts were made to determine which materials were used for the welding, in order to obtain a more detailed insight into the nature of the failure. The first step of this extensive analysis involved the development of a simple, approximated numerical model, with the goal of determining the location of stress concentration in the support lever, so that it could be compared to the real failure location. Further investigations will involve more complex models, including the ones that will simulate crack growth, once its location is numerically verified

Comparative Exemination of the Strengthened and Non-Strengthened NIMONIC Specimens with Laser Shot Peening Method

Laser shot peening (LSP) of material strengthening is nowdays widely used method in various branches of industry. In this paper are presented comparative exeminations of specimen made of NIMONIC and strengthened specimen on which laser shot peening method was performed. Specimens were made as thin plates with holes. Macrostructural surface tests were performed around the specimens holes with different magnifications for both specimens as well as certain damages on the specimens. 3D images of specimens damages provide insights into the its dimensions. In addition, the roughness of non-strengthed and strengthed specimens was also performed. Hardness tests using the Rockwell C method of both specimens show a difference in the hardness of both samples and the main characteristics of the laser shot peening method. Also, the analysis on how the damage of samples could represent the location of initial cracks that could cause failure of the specimens or generally machine part is given as well

Numerical Simulation of Fatigue Crack Growth in Titanium Alloy Orthopaedic Plates

Biomaterials intended for orthopaedic plates manufacturing have much higher mechanical properties relative to the bone itself and still there are many cases where those plates fracture in service, with fatigue as the main failure mode. This causes problem with the healing process and requires that the patient undergoes another surgery. Experience and knowledge of the orthopaedic surgeon is one of the most important factors contributing to the frequency of fatigue failures. If incorrectly implanted, plates will be subjected to overloading from the start, which is convenient for crack initiation. One of the most commonly used biocompatible materials for internal bone fixation is α + β titanium alloy Ti-6Al-4V. Focus of this study was to simulate the behaviour of titanium alloy orthopaedic plates in the presence of cracks under four-point bending. The extended finite element method (XFEM) in ANSYS was employed for this purpose. Loads correspond to the ones occurring in human tibia during gait cycle for different body weights. Experimental investigation of tensile and fracture mechanics parameters of Ti-6Al-4V alloy was conducted on tensile testing machine and fractomate. Numerical simulation established the optimal geometry from remaining life point of view, indicating large differences between different geometries. Results also have shown that the remaining life of orthopaedic plates is strongly dependant on patient\u27s body weight (load) and that the relative differences in remaining life between compared plate geometries stay the same under different loads. Influence of corrosive environment of the human body was not taken into consideration

A review on tribological properties of microcomposites with ZA-27 alloy matrix

Manufacturing and testing of novel composite materials are one of the topics that had caught attention of many researchers. There are a vast number of possibilities for improving many characteristics of the base (matrix) material significantly with addition of relatively small amounts of different material to the matrix. This paper should give an overview on tribological properties of microcomposites with ZA-27 alloy as a matrix material. Following characteristics were taken into consideration in the process of making this overview: type, amount and size of reinforcement; manufacturing process; apparatus used for testing; and test parameters and conditions