Antti Pramila

Thermal Conductivity of Fiber Reinforced Composites by the FEM

Applicability of the finite element method (FEM) in predicting the effective transverse thermal conductivity of fiber reinforced composites is systematically studied. Four different boundary condition combinations representing the periodicity of the temperature field are employed for ideal composites having perfect bond between fiber and matrix. Both circular and square cross-section fibers are studied. Comparisons of present FEM results with available analytical and experimental results reveal that periodicity realized by prescribed temperatures yields most accurate results up to high fiber volume fractions. In composites with interfacial thermal barrier resistance the effective conductivity varies in a wide range depending on the interfacial conductance between fiber and matrix. Best fit with available experimental results is obtained for both circular and square fibers when the dimensionless interfacial conductance is about 30. By employing the modeling practice found successful in the cases for which analytical andlor experimental results exist, some typical combined effects of partial debonding and matrix cracking, for which no such results exist, are finally considered.

Biocompatibility and strength properties of nitinol shape memory alloy suture in rabbit tendon.

Nitinol (NiTi) is a promising new tendon suture material with good strength, easy handling and good super-elastic properties. NiTi sutures were implanted for biocompatibility testing into the right medial gastrocnemius tendon in 15 rabbits for 2, 6 and 12 weeks. Additional sutures were implanted in subcutaneous tissue for strength measurements in order to determine the effect of implantation on strength properties of NiTi suture material. Braided polyester sutures (Ethibond) of approximately the same diameter were used as control. Encapsulating membrane formation around the sutures was minimal in the case of both materials. The breaking load of NiTi was significantly greater compared to braided polyester. Implantation did not affect the strength properties of either material.

Bone modeling controlled by a nickel-titanium shape memory alloy intramedullary nail

Nitinol (NiTi) shape memory metal alloy makes it possible to prepare functional implants that apply a continuous bending force to the bone. The purpose of this study was to find out if bone modeling can be controlled with a functional intramedullary NiTi nail. Pre-shaped intramedullary NiTi nails (length 26 mm, thickness 1.0-1.4 mm) with a curvature radius of 25-37 mm were implanted in the cooled martensite form in the medullary cavity of the right femur in eight rats, where they restored their austenite form, causing a bending force. After 12 weeks, the operated femurs were compared with their non-operated contralateral counterpairs. Anteroposterior radiographs demonstrated significant bowing, as indicated by the angle between the distal articular surface and the long axis of the femur (p = 0.003). Significant retardation of longitudinal growth and thickening of operated femurs were also seen. Quantitative densitometry showed a significant increase in the average cross-sectional cortical area (p = 0.001) and cortical thickness (p = 0.002), which were most obvious in the mid-diaphyseal area. Cortical bone mineral density increased in the proximal part of the bone and decreased in the distal part. Polarized light microscopy of the histological samples revealed that the new bone induced by the functional intramedullary nail was mainly woven bone. In conclusion, this study showed that bone modeling can be controlled with a functional intramedullary nail made of nickel-titanium shape memory alloy.

Finite Element Analysis of Linear Thermal Expansion Coefficients of Unidirectional Cracked Composites

The linear thermal expansion of coefficients of unidirectional composites are systematically studied by the Finite Element Method (FEM). Thermal expansion coefficients are first determined for composites with perfectly bonded interface between fiber and matrix. Results are compared with available experimental and analytical results. Next cracks caused by debonding along the fiber-matrix interface are studied to investigate the effects of interface cracking on the transverse thermal expansion coefficients. Due to the presence of cracks and shearing along the debonded interface, the principal direction of transverse thermal expansion changes. The obtained results showthat the transverse thermal expansion coefficients change significantly due to the presence of thin interfacial cracks.

A novel treatment of grade III acromioclavicular joint dislocations with a C-hook implant

Introduction: This study evaluates the results of the new surgical treatment of complete acromioclavicular (ac) dislocations using coracoclavicular (cc) fixation with a shape memory metal C-hook implant. Materials and methods: Fifteen patients were prospectively analyzed. They all had a Tossy III ac dislocation due to trauma. The ac ligament was reinserted using a surgical bone anchor, and the position of the joint was restored by fixing it with a C-hook. After 3 months the C-hook was removed. Functional status, symptom severity, X-rays and patient satisfaction were analyzed during clinical control visits. The follow-up time was 1 year. Results: At 12 weeks, full shoulder function had been achieved by 93% of the patients. The final control visit showed full recovery of active ROM in all patients. Two patients had mild pain during certain movements. X-rays showed the precise anatomical position of ac joint with no statistically significant differences compared to the healthy side. Patient contentment was excellent in 14 cases and satisfactory in one case. The average sick-leave was 58 days, including the removal operation. Minor osteolysis of the clavicle was noticed in two patients. Conclusion: The new C-hook implant provides accurate anatomical reduction, conserves the articular surfaces and enables fast functional recovery with excellent patient contentment. Technically, the implant is easy to use. Based on this study, the C-hook presents a reliable novel treatment option in surgical ac repair.

Comparison of the bone modeling effects caused by curved and straight nickel-titanium intramedullary nails.

Nitinol (NiTi) shape memory metal alloy makes it possible to prepare functional implants. A curved intramedullary NiTi nail has been shown to cause bending of the bone, bone thickening, increase in cortical area, and reduction in bone longitudinal growth. The purpose of the present study was to find out whether these changes are caused by the bending force of the curved nail or by the intramedullary nailing itself. Pre-shaped intramedullary NiTi nails were implanted in the cooled martensitic form into the medullary cavity of the right femur in 12 rats, where they started to restore their austenitic form, causing a bending force. Straight nails were used as controls in another 12 rats. After 12 weeks, the operated femurs were compared with their non-operated contralateral counterparts and the differences were compared between the groups. Anteroposterior radiographs demonstrated bone bowing only in the curved nail group. Retardation of longitudinal growth was observed in both groups, showing that the growth effect seems to be due to the intramedullary nailing itself. Increase in bone cross-sectional area and cortical thickness were found in both groups. However, this increase was more evident with the curved nail, indicating that the bending force of the functional nail seems to induce these changes.

Accuracy of composite shell elements in transient analysis involving multiple impacts

The accuracy of composite shell elements in linear transient analysis involving multiple impacts was studied. Results obtained by finite element method and by an analytical solution were compared with each other. The previously developed analytical solution for the impact problem was in closed form only until the end of the first contact. In this work the solution was extended to multiple impacts in closed form. Therefore, it was possible to solve problems with several successive impacts. Comparisons revealed that the composite shell elements of commercial FEM software give very accurate results for specially orthotropic laminates in transient analysis involving flexural and shear wave propagation. The accuracy began to deteriorate in problems where multiple impacts occur. Also, it was noticed that commercially available finite element programs may have serious problems in the analysis of composite structures, for example, in the calculation of stresses.

Extrema and Zeros of Coefficients of Thermal Expansion of a Balanced Symmetric Laminate

Expressions for coefficients of thermal expansion (CTEs) of a [θ/ — θ] s lami nate are recast into a form showing explicity their dependence on lamination angle and thermoelastic constants of laminae used. Using these new expressions, zeros and extrema have been studied. Angles giving extrema for CTEs of a laminate are shown to be indepen dent of CTEs of laminae used. Moreover, limits for G 12 of laminae used are derived lead ing to 2, 1 or no angles giving extrema in addition to the trivial ones at 0° and 90°.

COUPLING TERM MAKES TEMPERATURE FIELDS UNSYMMETRIC IN LAMINATED COMPOSITE PLATES

Abstract The importance of the coupling term k xy in heat conduction analysis of laminated, fibre reinforced plates is considered. A simple proof is given which shows that even with symmetric boundary conditions the temperature field in an off axis ply or in a general laminate cannot be symmetric. Moreover an example problem is presented which indicates that very large errors can occur when the coupling term is neglected.

Onsager's reciprocal relation and effective coefficients of transverse coupled hygrothermal diffusion in fiber reinforced composites

A method for the determination of the effective constant coefficients of the transverse coupled hygrothermal diffusion is presented. Onsager’s reciprocal relation [2] is utilized to reduce the number of the diffusivity coefficients of constitutive materials from four to two. The so called cross coupling diffusivities are expressed in terms of the thermal conductivity and the coefficient of diffusion. The representative volume element of the composite is analyzed by applying a twodimensional finite element model. It is assumed that the composite is undamaged, and there is no resistance in diffusion and heat conduction at the interface of the fiber and the matrix. The method is tested numerically with a set of different cases. The coupling affects most clearly to the heat flux in the tests. The average coupled heat flux is significantly greater than the uncoupled heat flux.