A Practical Micro Mechanical Model Based Local Approach Methodology for the Analysis of Ductile Fracture of Welded T-Joints

This thesis concentrates on developing a practical local approach methodology based on micro mechanical models for the analysis of ductile fracture of welded joints. Two major problems involved in the local approach, namely the dilational constitutive relation reflecting the softening behaviour of material, and the failure criterion associated with the constitutive equation, have been studied in detail. Firstly, considerable efforts were made on the numerical integration and computer implementation for the non trivial dilational Gurson Tvergaard model. Considering the weaknesses of the widely used Euler forward integration algorithms, a family of generalized mid point algorithms is proposed for the Gurson Tvergaard model. Correspondingly, based on the decomposition of stresses into hydrostatic and deviatoric parts, an explicit seven parameter expression for the consistent tangent moduli of the algorithms is presented. This explicit formula avoids any matrix inversion during numerical iteration and thus greatly facilitates the computer implementation of the algorithms and increase the efficiency of the code. The accuracy of the proposed algorithms and other conventional algorithms has been assessed in a systematic manner in order to highlight the best algorithm for this study. The accurate and efficient performance of present finite element implementation of the proposed algorithms has been demonstrated by various numerical examples. It has been found that the true mid point algorithm (a = 0.5) is the most accurate one when the deviatoric strain increment is radial to the yield surface and it is very important to use the consistent tangent moduli in the Newton iteration procedure. Secondly, an assessment of the consistency of current local failure criteria for ductile fracture, the critical void growth criterion, the constant critical void volume fraction criterion and Thomason's plastic limit load failure criterion, has been made. Significant differences in the predictions of ductility by the three criteria were found. By assuming the void grows spherically and using the void volume fraction from the Gurson Tvergaard model to calculate the current void matrix geometry, Thomason's failure criterion has been modified and a new failure criterion for the Gurson Tvergaard model is presented. Comparison with Koplik and Needleman's finite element results shows that the new failure criterion is fairly accurate indeed. A novel feature of the new failure criterion is that a mechanism for void coalescence is incorporated into the constitutive model. Hence the material failure is a natural result of the development of macroscopic plastic flow and the microscopic internal necking mechanism. By the new failure criterion, the critical void volume fraction is not a material constant and the initial void volume fraction and/or void nucleation parameters essentially control the material failure. This feature is very desirable and makes the numerical calibration of void nucleation parameters(s) possible and physically sound. Thirdly, a local approach methodology based on the above two major contributions has been built up in ABAQUS via the user material subroutine UMAT and applied to welded T joints. By using the void nucleation parameters calibrated from simple smooth and notched specimens, it was found that the fracture behaviour of the welded T joints can be well predicted using present methodology. This application has shown how the damage parameters of both base material and heat affected zone (HAZ) material can be obtained in a step by step manner and how useful and capable the local approach methodology is in the analysis of fracture behaviour and crack development as well as structural integrity assessment of practical problems where non homogeneous materials are involved. Finally, a procedure for the possible engineering application of the present methodology is suggested and discussed.

Surgically-treated Hip Fracture in older People. With Special Emphasis on Mortality Analysis

Hip fractures are associated with significant morbidity and mortality. Cervical and trochanteric fractures have a different morphometry, surgical treatment, and outcome. Polypharmacy, common in older people, is associated with increased mortality. The risk factors for mortality can be identified based on cause-of-death analysis. In this population-based study, 461 older, surgically in 1999-2000 treated hip fracture patients were enrolled. Incidence, morphometry, medication, mortality, and cause-of-death were analysed. Hip fractures were most commonly sustained by women, occurred mostly indoors, and often in institutions. One in four patients had sustained a previous fracture. Routine clinical radiographs revealed no differences in the hip geometry between hip fracture types. Age-adjusted mortality was higher in men than in women during the follow-up. Chronic lung disease and male sex were predictors of mortality after cervical fracture. In men, potent anticholinergics were associated with excess age-adjusted mortality. Men were more likely to die from circulatory disease and dementia after hip fracture than women. Mortality after hip fracture was 3-fold higher than that of the general population, including every cause-of-death class. Fracture prevention in institutions and homes, indoor safety measures, and treatment of chronic lung diseases should be encouraged. Hip morphometry analyses require more accurate measures than that provided by routine radiographs. Careful use of potent anticholinergics may reduce mortality. Compared to the general population, excess mortality after hip fracture was evident up to 9 years after hip fracture. Cause-of-death analysis indicates that all major comorbidities require optimal treatment after hip fracture surgery.

Uusintamurtumien ehkäisy kaatumisen aiheuttaman yläraajamurtuman kokeneilla yli 50-vuotiailla henkilöillä

Tarkoitus: Tarkoituksena oli selvittää murtumien tehostetun ehkäisyohjelman vaikutuksia yläraajamurtuman saaneiden yli 50-vuotiaiden henkilöiden elämäntapoihin, murtumien riskitekijöihin, kaatumisiin, kaatumisvammoihin ja elämänlaatuun. Aineisto: 219 yli 50-vuotiasta kotona asuvaa, kaatumisen seurauksena yläraajamurtuman saanutta henkilöä satunnaistettiin koe- (n = 105) ja kontrolliryhmiin (n = 114). Menetelmä: Koeohjelma sisälsi yksilöllisen, yhdessä lääkärin, terveydenhoitajan ja tutkittavan kanssa laaditun hoito- ja kuntoutussuunnitelman sekä kutsun murtumien ehkäisyn koulutusohjelmaan. Kontrolliryhmän tutkittavat saivat kehotuksen hakeutua murtumahoitajan ohjaukseen. Seuranta-aika oli 14 kuukautta. Tulokset analysoitiin ryhmissä tapahtuneina muutoksina alkuja loppumittausten välillä sekä ryhmien välisenä erona muutoksissa. Tulokset: Keskimääräinen luun tiheys lannerangassa lisääntyi koeryhmässä (p<0,001) ja kontrolliryhmässä (p = 0,038), ryhmien välinen ero ei ollut tilastollisesti merkitsevä (p = 0,134). Reisiluun kaulassa luun tiheys ei muuttunut kummassakaan ryhmässä. Päivittäinen kalsiumin saanti lisääntyi koeryhmässä keskimäärin 167 mg (p<0,001) ja kontrolliryhmässä 30 mg (p = 0,475), ryhmien välinen ero oli tilastollisesti merkitsevä (p = 0,031). Kala-ateriat ja päivittäinen D-vitamiinilisä sekä kenkien liukuesteiden säännöllinen käyttö liukkaalla lisääntyivät molemmissa ryhmissä, mutta ryhmien välillä ei ollut eroa. Viiteen tuolilta nousukertaan käytetty aika vähentyi koeryhmässä keskimäärin 0,49 sekuntia (p = 0,185) ja lisääntyi kontrolliryhmässä 0,39 (p = 0,475), ero ryhmien välillä oli tilastollisesti merkitsevä (p = 0,02). Kymmenen metrin kävelyssä ei tapahtunut tulosten keskiarvossa muutoksia kummassakaan ryhmässä. Ortostatismi vähentyi kontrolliryhmässä (p = 0,049), ja masentuneisuus vähentyi koeryhmässä (p = 0,041), mutta ryhmien välillä ei näissä ollut merkitsevää eroa. Osteoporoosilääkkeiden käyttö lisääntyi molemmissa ryhmissä, ryhmien välillä ei ollut eroa. Alkoholin käytössä oli vähäisiä mutta ei merkittäviä muutoksia. Muissa testatuissa muuttujissa, kuten liikunnan viikoittainen määrä, tupakointi, näkökyky, tasapaino ja elämänlaatu, ei ollut merkitseviä muutoksia ryhmissä tai ryhmien välillä. Kaatumisten (koeryhmä 33, kontrolliryhmä 35) ja murtumien (koeryhmä 5 ja kontrolliryhmä 3) määrissä ei myöskään ollut merkitseviä eroja ryhmien välillä. Johtopäätökset: Noin neljällä viidestä kaatumisen seurauksena yläraajamurtuman saaneista, yli 50 vuotta täyttäneistä henkilöistä oli alentunut luun tiheys, ja noin joka toisella oli lisääntynyt kaatumisriski. Kaatumisen riskitekijöiden kliiniset mittaukset olivat nopeita toteuttaa ja auttoivat ehkäisytoimenpiteiden suunnittelussa. Murtuman saaneet olivat motivoituneita murtumien riskitekijöiden selvittämiseen ja myös toteuttamaan annettua lääkehoitoa. Elämäntavoissa helpoimmin toteutuivat muutokset ruokailutottumuksissa. Liikuntatottumuksien muuttaminen sen sijaan onnistui melko huonosti. Ryhmien välillä oli merkitsevä ero ainoastaan kalsiumin päivittäisessä käytössä ja tuolilta nousutestissä. Murtumariskien tunnistaminen, niiden syiden selvittäminen ja toimenpiteiden käynnistäminen riskien vähentämiseksi tulisi kuulua jokaisen pienienergisen murtuman saaneen potilaan kokonaisvaltaiseen hyvään hoitoon.

Bone Health, Osteoporosis and Fracture Risk in Neurofibromatosis 1 - An Emphasis on Osteoclasts

Neurofibromatosis 1 (NF1) is an autosomal dominant hereditary syndrome, affecting skin, neural tissues and skeleton. Hallmarks of NF1 include benign cutaneous neurofibroma tumors, pigmentation lesions on the skin and in the iris, learning disabilities and predisposition to selected malignancies. Low bone mineral density (BMD) and osteopenia/osteoporosis are common in NF1. Osteoporosis is a systemic disorder characterized by low bone mineral density and increased fracture risk. Treatment of osteoporosis aims to prevent falls and decrease fracture risk. Osteoporosis is diagnosed in adults by measuring BMD and evaluating clinical risk factors of the patient. Bone turnover is a process of old bone resorbed by osteoclasts and new bone formed by osteoblasts. Multinuclear osteoclasts are derived from osteoclast progenitors, which can be isolated from peripheral blood. Osteoclast progenitors were isolated from 17 NF1 patients and healthy controls, and cultured in vitro to osteoclasts. NF1 osteoclasts are hyperactive, displaying increased differentiation and resorption capacity, abnormal morphology and tolerance to serum deprivation compared to control osteoclasts. These findings expanded the study to evaluate the effects of bisphosphonates, drugs designed to treat osteoporosis, in osteoclasts derived from blood samples of 20 NF1 and control persons. The number of control osteoclasts was expectedly reduced after bisphosphonate treatment. However, NF1 osteoclasts tolerated the apoptotic effect of alendronate, zoledronic acid and clodronate in vitro compared to controls. NF1-related osteoporosis was found in ~20 % of the patients, and selected laboratory parameters were measured. Patients with NF1 have increased levels of serum CTX and PINP, reflecting increased bone turnover in vivo. BMD decreases progressively in NF1 as evaluated in 19 NF1 patients 12 years after their initial BMD measurement. Patients with NF1-related osteopenia often progress to osteoporosis. This was found in patients aged 37-76.

Bioactive Glass in Lumbar Spondylodesis. A Pre-Clinical and Clinical Study

Bioactive glasses (BGs) form a group of synthetic, surface-active, composition-dependent, silica-based biomaterials with osteoconductive, osteopromotive, and even angiogenic, as well as antibacterial, properties. A national interdisciplinary research group, within the Combio Technology Program (2003–2007), developed a porous load-bearing composite for surgical applications made of BG 1–98 and polymer fibers. The pre-clinical part of this thesis focused on the in vitro and in vivo testing of the composite materials in a rabbit femur and spinal posterolateral fusion model. The femur model failed to demonstrate the previously seen positive effect of BG 1–98 on osteogenesis, probably due to the changed resorption properties of BG in the form of fibers. The spine study was terminated early due to adverse events. In vitro cultures showed the growth inhibition of human mesenchymal stems next to BG 1–98 fibers and radical pH changes. A prospective, long-term, follow-up study was conducted on BG–S53P4 and autogenous bone used as bone graft substitutes for instrumented posterolateral spondylodesis in the treatment of degenerative spondylolisthesis (n=17) and unstable burst fractures (n=10) during 1996–1998. The operative outcome was evaluated from X-rays and CT scans, and a clinical examination was also performed. On the BG side, a solid fusion was observed in the CT scans of 12 patients, and a partial fusion was found in 5 patients, the result being a total fusion rate in all fusion sites (n=41) 88% for levels L4/5 and L5/S1 in the spondylolisthesis group. In the spine fracture group, solid fusion was observed in five patients, and partial fusion was found in five resulting in a total fusion rate of 71% of all fusion sites (n=21). The pre-clinical results suggest that under certain conditions the physical form of BG can be more critical than its chemical composition when a clinical application is designed. The first long-term clinical results concerning the use of BG S53P4 as bone graft material in instrumented posterolateral spondylodesis seems to be a safe procedure, associated with a very low complication rate. BG S53P4 used as a stand-alone bone substitute cannot be regarded as being as efficient as AB in promoting solid fusion.

Determinants of Bone Strength and Predictors of Hip Fracture among Finnish Adults. Results from the Health 2000 Survey and the Mini-Finland Health Survey

The objective of this thesis was to identify the determinants of bone strength and predictors of hip fracture in representative samples of Finnish adults. A secondary objective was to construct a simple multifactorial model for hip fracture prediction over a 10-year follow-up period. The study was based on the Health 2000 Survey conducted during 2000 to 2001 (men and women aged 30 years or over, n=6 035) and the Mini-Finland Health Survey conducted during 1978 to 1980 (women aged 45 years or over, n=2 039). Study subjects participated in health interviews and comprehensive health examination. In the Health 2000 Survey, bone strength was assessed by means of calcaneal quantitative ultrasound (QUS). The follow-up information about hip fractures was drawn from the National Hospital Discharge Register. In this study, age, weight, height, serum 25-hydroxyvitamin D (S-25(OH)D), physical activity, smoking and alcohol consumption as well as menopause and eventual HRT in women were found to be associated with calcaneal broadband ultrasound attenuation (BUA) and speed of sound (SOS). Parity was associated with a decreased risk of hip fracture in postmenopausal women. Age, height, weight or waist circumference, quantitative ultrasound index (QUI), S-25(OH)D and fall-related factors, such as maximal walking speed, Parkinson’s disease, and the number of prescribed CNS active medication were significant independent predictors of hip fracture. At the population level, the incremental value of QUS appeared to be minor in hip fracture prediction when the fall-related risk factors were taken into account. A simple multifactorial model for hip fracture prediction presented in this study was based on readily available factors (age, gender, height, waist circumference, and fallrelated factors). Prospective studies are needed to test this model in patient-based study populations.