Strength retention of self-reinforced poly-L-lactide screws. A comparison of compression moulded and machine cut screws

The effect of the manufacturing method on the strength retention of self-reinforced poly-L-lactide (SR-PLLA) screws was studied in vitro and in vivo from 3 up to at least 15 weeks. SR-PLLA screws were manufactured from axially oriented SR-PLLA billets by the conventional compression moulding process and an in-house developed machine cutting technique. New machined SR-PLLA screws (thread diameters 4.5 mm and 3.5 mm) were significantly stronger than older compression moulded SR-PLLA screws (4.5 mm and 3.5 mm) in bending and torque strength tests but significantly weaker in shear strength tests. In pull out tests there were not significant strength differences between the screws. Mechanical analysis and molecular weight measurements confirmed earlier observations that SR-PLLA degrades faster in vivo than in vitro. These results suggest that the new screws could be suitable for clinical use.     

Shear strength of cancellous bone after osteotomy fixed with absorbable self-reinforced polyglycolic acid and poly-L-lactic acid rods

The right distal femur of 42 adult rabbits was osteotomized and fixed with two 1.5 mm in diameter metallic Kirschner wires, self-reinforced polyglycolic acid (SR-PGA, Biofix®), or self-reinforced poly-L-lactic acid (SR-PLLA) rods. Follow-up times were 6 and 12 weeks. The shear strength of 30 pairs of femora was investigated. The strength of osteotomized bones was compared with the non-osteotomized controls. After six weeks the operated femora had reached 70, 69 and 71% shear strength in metallic, SR-PGA and SR-PLLA groups, respectively. After 12 weeks the shear strength values were 75, 79 and 73%, respectively. There was no statistically significant difference in shear strength values between the groups. Twelve pairs of femora were studied with microradiographic, oxytetracyclic fluorescence, and histologic methods. Normal bone healing was seen in these samples. Metallic and SR-PLLA implants caused slight foreign-body reaction with giant cells and SR-PGA rods led to slight infiltration of macrophages and foam cells.     

Strength retention of self-reinforced poly-L-lactide screws and plates: anin vivo andin vitro study

Self-reinforced poly-L-lactide (SR-PLLA) screws and multilayer plates were studied for their initial mechanical shear and/or tensile strength and strength retentionin vivo andin vitro up to 48 weeks. The plates were studiedin vitro up to 68 weeks. The loss of strength was fasterin vivo thanin vitro. The screws retained more than half of their initial shear strength up to 12 weeksin vivo and over 24 weeksin vitro. By 48 weeks they had lost nearly all of their mechanical strength in both groups. The plates retained over 54% of their initial tensile and 71% of shear strength up to 24 weeksin vivo. The shear strength did not particularly diminish up to 52 weeks of follow-upin vitro whereas the tensile strength was slightly decreased after 36 weeks. After this follow-up time the loss of strength was more rapid and by 68 weeks the shear strength had decreased to 17% and the tensile strength to 0%.     

Dynamic shear strength of adhesive joints made of metallic and composite adherents

The use of adhesive joints has gained good acceptance in the automotive and aerospace industries in recent years, particularly for joining glass fiber reinforced plastics (GFRP) to metals. Such joints will be subjected to short duration dynamic loads in service. The present study focuses on the evaluation of the shear strength of adhesive joints prepared using four different commercial adhesives at loading rates in the range of 0.6–1.2 MPa/μs. The adhesives used were Araldite 2014, Araldite 2011, Epibond 1590 and A/B Loctite 324. Joints were prepared with two different adherent combinations; aluminum–aluminum and aluminum–GFRP. The results of the study indicated that, depending on the adhesive and adherent combination, the dynamic strength was 2–4 times the static strength. Among the four adhesives, Epibond 1590 exhibited the highest rate sensitivity whereas Loctite 324 exhibited the least. Further it was also observed that the dynamic strength was not sensitive to the adherent combination whenever the failure was predominantly within the adhesive layer.     

Propagation of magnetoelastic shear waves in an irregular self-reinforced layer

The propagation of horizontally polarised shear waves in an internal irregular magnetoelastic self-reinforced stratum which is sandwiched between two semi-infinite magnetoelastic self-reinforced media is studied. Two shapes of irregularities on the interface of layer and lower semi-infinite media are considered, namely rectangular and parabolic. The dispersion equation is obtained in closed form. The combined effects of reinforcement, magnetic field and irregularity are also studied. Some important features of the results are highlighted. It is also observed that the dispersion equation is in agreement with the classical Love-type wave equation for an isotropic layer sandwiched between two isotropic half-spaces in the absence of reinforcement, magnetic field and irregularity.     

Comparison of shear and tensile fracture in high strength aluminum alloys

A comparison was made between tensile (mode I) and shear (mode II) fracture characteristics in high strength aluminium alloys (7075-T6 and 6061-T651) using a relatively new mode II fracture specimen to evaluate the critical stress intensity factor. The enlarged plastic zone during mode II fracture required that an increased specimen thickness be used for determining K _Hc under a purely plane strain condition. Plane stress conditions prevailed in the mode II fracture of 7075-T6 with a specimen thickness less than 10 mm, while plane strain controlled mode II fracture at a thickness of 10 mm or greater. Fractographic analysis revealed a distinctive difference in the micromechanisms responsible for crack extension. Small dimples were observed only on the mode II fracture surfaces, resulting from a microvoid nucleation fracture mechanism. The mode I fracture surfaces showed a mixed distribution of dimple sizes resulting from a void growth fracture mechanism. Comparing the critical stress intensity factors, the shear mode of failure exhibited a substantially higher value than the tensile mode, resulting from the effect of the sign and magnitude of the hydrostatic stress state on the microvoid nucleation event. Zero hydrostatic tension in the mode II loading configuration helps delay microvoid nucleation, increasing the apparent toughness. The high hydrostatic tension resulting from a mode I loading configuration enhances microvoid nucleation which promotes crack propagation at relatively lower stress intensity factors.     

Dynamic vs. quasi-static shear failure of high strength metallic alloys: Experimental issues

Ductile fracture of metals by void nucleation, growth and coalescence under positive stress triaxiality is well admitted. This is not the case when metals are submitted to negative stress triaxiality. The present work aims at contributing to a better understanding of the competition between micro-mechanisms at the origin of failure of metals when submitted to shear-pressure loading at low and high strain rates. With this aim in view, experiments were carried out on Ti–6Al–4V shear-compression samples involving a stress triaxiality range comprised between −0.2 and −0.5. Results show that the material failure is the consequence of a void growth induced process. At high strain rate, due to the localization of the deformation within adiabatic shear bands, the failure of the material occurs earlier, leading to maximum shear strain smaller at high strain rate than at low strain rate. Impact tests were also carried out on Kalthoff and Winkler type double notched plates. They showed that the interaction between tension and shear waves leads to a complex Mode I–Mode II crack propagation.     

Comparison of shear strength tests on AV119 epoxy-joined ceramics

The results of an experimental investigation on epoxy-joined CVD SiC and alumina tested in shear and apparent shear mode by four different configurations are presented. Ceramics have been joined by an epoxy adhesive (AV119), which is not to be considered as the final joining material for high temperature applications, but just as a model brittle joining material chosen to obtain several joined samples in a reasonable time. Advantages and disadvantages of each configuration are discussed and compared to results obtained with the same epoxy-joined carbon/carbon composites, tested by the same shear tests.     

Self-reinforced poly-l-lactide screws in the fixation of cortical bone osteotomies in rabbits

This study was carried out to test absorbable self-reinforced poly-l-lactide (PLLA) screws as lag screws in the fixation of cortical bone osteotomies. A right tibial osteotomy was performed in 72 rabbits, of which 36 were fixed with 4.5 mm in diameter metallic cortical screws and 36 with self-reinforced PLLA screws manufactured by a sintering method. Follow-up times were 6, 12, and 24 weeks. After killing the rabbits all operated and the control tibias were examined macroscopically and radiographed. Sixty pairs of tibias were tested for shear strength; the rest were evaluated histologically, microradiographically and by oxytetracycline fluorescence. Macroscopically, 97% of the metallic and 44% of the PLLA screw-fixed osteotomies healed well. Radiographically, the metallic group healed significantly better and there were less malpositions than in the PLLA group. The shear strengths of the operated tibias were divided by those of the controls to give comparative shear strengths. The mean comparative shear strengths were 52% at 6, 61% at 12 and 76% at 24 weeks in the metallic group and 38, 49 and 61% in the PLLA group, respectively. No statistically significant difference in shear strength was observed between the groups. A mild foreign-body reaction was found in the histological evaluation in both groups, and normal bone healing was noticed in the microradiography and in the fluorescence studies. In conclusion, the sintered self-reinforced microradiography and in the fluorescence studies. In conclusion, the sintered self-reinforced PLLA screws were insufficiently strong to be used as a single lag screws in the fixation of tibial cortical bone osteotomies in rabbits.     

Effect of extrusion drawing and twist-orientation on mechanical properties of self-reinforced poly(lactic acid) screws

In order to improve torsional strength of a bioabsorbable poly(lactic acid) (PLA) screw, twist-orientation method was developed in this study. PLA screws were prepared through a series of routes including compression molding, extrusion method, twist-orientation, and forging. Shear and torsional strength were measured as mechanical properties of screw. As a result, twist-orientation improves torsional strength of PLA screw without the decrease in shear strength. The maximum torsional strength was obtained at helix angle of 45° and extrusion ratio of 8. This is because fibrous crystals were oriented close to the direction of principal tensile stress which occurred during torsion test. Since this principal stress occurs in the 45° direction on the surface of specimen, the maximum torsional strength is achieved by twist-orientation at this helix angle.     

Shear-load carrying capacities of the distal rat femora after osteotomy fixed with self-reinforced polyglycolic acid and poly-L-lactic acid pins

Distal femora of 40 rats were osteotomized and fixed with self-reinforced polyglycolide (SR-PGA) and self-reinforced polylactide (SR-PLLA) pin 2.0 mm in diameter and 15 mm in length. The shear-load carrying capacities of the osteotomized bones were compared with each other and with the intact control rat distal femora of the same age of 20 pairs. The follow-up times were 1, 3, 6, 12, 24, 36, 48, and 52 weeks. After killing all operated and control femora were examined macroscopically and radiographically. The shear-load carrying capacities reached their highest values at 24 weeks in the SR-PGA-fixed specimens, after that decreasing to the level where they remained. In the SR-PLLA-fixed specimens the strength values of the pins increased after three weeks, but there was a decrease at 24 weeks. After that the shear-load carrying capacities started to raise because of the influence of the healed osteotomy. In the control bones the shear-load carrying capacities were weaker than in the SR-PGA-and SR-PLLA-fixed specimens except at three weeks, as the osteotomies had not yet healed. During the whole follow-up period the mean shear-load carrying capacity of the SR-PGA-fixed specimens was 199.1 N, in the SR-PLLA-fixed specimens 214.6 N, the corresponding value of the control specimens being 148.2 N.     

Effect of drawing condition on mechanical properties and molecular orientation of self-reinforced poly(lactic acid) screws

Effects of extrusion ratio (ER) during extruding on mechanical properties of poly(lactic acid) (PLA) screws were investigated in terms of shear and twist strength. Shear strength increased by drawing, while twist strength decreased. This might be due to orientation of molecular chains during drawing. In order to evaluate orientation of molecular chains, orientation function in extruded billets was measured as a function of ER. Since orientation function increased with ER, drawing is effective method to improve shear strength of screw, where fibrous structure which was formed during extruding resisted shear stress in cross-section perpendicular to screw axis. Although orientation functions for billet extruded at ER 1.3–4 were equivalent, both strengths changed for the screw extruded at the range of this ER. Infrared spectra suggested transforming from α to β crystal for the billet extruded at the range of this ER. This result suggested that mechanical properties of screw also depended on the crystal forms.     

Strength retention of drawn self-reinforced polyglycolide rods and fixation properties of the distal femoral osteotomies with these rods. An experimental study on rats

Drawn self-reinforced polyglycolide (SR-PGA) rods, Ø 2 mm and 26 mm long, were implanted in the dorsal subcutaneus tissue of 16 rats. Osteotomies of the distal femur were fixed with SR-PGA rods (2 mm by 15 mm) in another 38 rats. The follow-up times varied from one week to one year. After sacrifice, three-point bending and shear tests were performed for subcutaneously placed rods. Radiological, histological, histomorphometrical, microradiographic, and oxytetracycline-fluorescence studies of osteotomized and intact control femora were also performed. At three weeks the flexural strength of the rods was 50% of the initial value, and the flexural modulus was 46% of the initial value. Five osteotomy specimens had to be excluded due to dislocation or non-union. One of the 33 evaluated osteotomy specimens showed signs of postoperative infection. Thirty-two osteotomies healed uneventfully. No gross signs of inflammatory or foreign-body reaction were observed. The amount of osteoid surface and active osteoid formation surface reached their highest value in the histomorphometrical analysis at 24 weeks. The present investigation demonstrated that the mechanical strength and fixation properties of the drawn SR-PGA rods are suitable for fixation of cancellous bone osteotomies in rats. The present article is the first report on the successful application of drawn SR-PGA rods for fixation of cancellous bone osteotomies.     

Annealing of shear bands in metallic glasses

Shear bands introduced into the as-quenched metallic glass alloy, Metglas^R 2826 (Fe₄₀Ni₄₀P₁₄B₆), were etched by immersion in a solution of CUS04 and HCI. Elimination of the etchability property through isothermal annealing had an apparent activation enthalpy of 250 kJ mol^–1, similar to that of stress relaxation. Direct microscopic observations indicated that the etching of shear bands may be a result of stress corrosion cracking. The electrochemical etchability of shear bands in the nickel-based alloy, BNi₂ (Ni_68.8Cr_6.6Fe_2.6B_14.1Si_7.9), polarized in a solution of perchloric acid and acetic acid at 12° C, could also be eliminated by thermal annealing. The loss of etchability had an apparent activation enthalpy of 580 kJ mol^–1, a value indicative of more complex atomic rearrangements taking place within the deformed material. Finally, shear bands introduced into the as-received BNi₂ material retained the ability to reverse shear after heat treatment at temperatures in the range of 192 to 375° C even if the electrolytic etchability was apparently diminished.     

Comparison of shear strength tests on AV119 epoxy-joined carbon/carbon composites

The results of an experimental investigation on carbon/carbon composites (C/C) bonded joints tested in shear at room temperature, under seven different configurations, are presented. The samples have been joined by an epoxy adhesive (AV119): this adhesive is not to be considered as a suitable joining material for C/C for high temperature applications, but just as a model joining material chosen to obtain several bonded samples in a short time. Advantages and disadvantages of each configuration are discussed from an experimental point of view. A finite element analysis is also performed to compare the stress distribution obtained within the joint for the different testing geometries. It is shown that the measured values of the apparent shear strength decrease with the maximum opening stress estimated within the middle of the joint.     

The shear strength of pretensioned I-beams with debonded tendons

The paper describes a few tests on the shear failure of unitormly prestressed pretensioned concrete I-beams with unreinforced webs and with debonded tendons in the end zones. All the beams were tested under two-point loading. To predict the behaviour of these beams in shear, empirical expressions developed from the test results for conventional beams by others have been used. Most of these formulae are found to overestimate the bearing capacity by nearly 100 per cent. The results show that the bearing capacity of these beams in shear is not completely independent of the number of debonded tendons and their location on the end face of the beam.

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Résumé Des essais ont été effectués afin d’étudier l’effet de la suppression de l’adhérence des cables sur la capacité portante en cisaillement des poutres en béton précontraint à cables adhérents. L’ame des poutres essayées n’était pas armée. Les essais ont été effectués avec une machine d’essai universelle OLSEN 1-MN, les poutres étant chargées en deux points avec une portée d’effort tranchant de 500 mm. La longueur sur laquelle l’adhérence a été supprimée et la longueur de transmission des cables de précontrainte étaient d’environ 600 mm. Un enregistreur automatique a fourni les courbes de fléchissement au milieu des poutres. Ces poutres avaient toutes une longueur de 2,90 m et les essais ont été effectués sur une portée de 2.75 m. On a fait varier le nombre de cables à adhérence supprimée et leur localisation dans la zone d’about des poutres. Toutes les poutres ont été rompues en cisaillement oblique. Pour prédire le comportement en cisaillement de ces poutres, on s’est servi d’expressions empiriques élaborées à partir des essais effectués par d’autres sur des poutres conventionnelles. Les résultats d’essai montrent que, lorsque les cables utiles localisés dans la membrure inférieure gardent leur adhérence, la capacité portante de ces poutres sous effort tranchant augmente de 15%. La suppression de l’adhérence de 50% des cables de la membrure inférieure a déterminé une réduction d’environ 20% de la capacité portante en cisaillement, par rapport à une poutre semblable dont les cables étaient restés totalement adhérents. Les résultats d’essai montrent donc que la capacité portante en cisaillement des poutres en béton précontraint à cables adhérents n’est pas entièrement indépendante du nombre et de la localisation des cables à adhérence supprimée dans la zone d’about de la poutre.

     

Mechanical properties of high strength concrete reinforced with metallic and non-metallic fibres

This paper focuses on the experimental investigation carried out on high strength concrete reinforced with hybrid fibres (combination of hooked steel and a non-metallic fibre) up to a volume fraction of 0.5%. The mechanical properties, namely, compressive strength, split tensile strength, flexural strength and flexural toughness were studied for concrete prepared using different hybrid fibre combinations – steel–polypropylene, steel–polyester and steel–glass. The flexural properties were studied using four point bending tests on beam specimens as per Japanese Concrete Institute (JCI) recommendations. Fibre addition was seen to enhance the pre-peak as well as post-peak region of the load–deflection curve, causing an increase in flexural strength and toughness, respectively. Addition of steel fibres generally contributed towards the energy absorbing mechanism (bridging action) whereas, the non-metallic fibres resulted in delaying the formation of micro-cracks. Compared to other hybrid fibre reinforced concretes, the flexural toughness of steel–polypropylene hybrid fibre concretes was comparable to steel fibre concrete. Increased fibre availability in the hybrid fibre systems (due to the lower densities of non-metallic fibres), in addition to the ability of non-metallic fibres to bridge smaller micro cracks, are suggested as the reasons for the enhancement in mechanical properties.     

Shear strength of reinforced aerated concrete beams with shear reinforcement

The paper reports on the analysis of shear strength of reinforced beams made of autoclaved aerated concrete with shear reinforcement. The test data are taken from three different investigations from three countries, in Europe and Japan, and include 61 tests. The analysis of the test data results in regression expressions, suitably modified from a formula used for ordinary concrete members, and shows good agreement with test values. Appropriate expressions are suggested for design.

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Résumé Ce rapport présente l'analyse de la résistance au cisaillement de poutres en béton cellulaire armées (AAC), avec armatures d'effort tranchant. Les résultats d'essais sont pris parmi trois recherches différentes dans trois pays, en Europe et au Japon, et ils regroupent 61 essais. La charge dans la majorité des essais consistait en deux forces symétriques (voir Fig. 1). Dans quelques cas, il y avait une charge en ligne à mi-portée; dans d'autres cas, une charge en ligne était appliquée près d'un appui. Aucun essai n'a été exécuté avec une charge uniformément répartie. Une formule du type de l'équation 1 prenant en compte, les trois variables principales a été utilisée depuis longtemps dans le cas du béton armé sans armature d'effort tranchant. A partir de son développement théorique, cette formule a été adaptée par le passé au béton cellulaire d'ou l'équation 2. Pour les poutres avec armatures d'effort tranchant, l'équation 3 représente l'addition de la résistance au cisaillement due au béton à celle due aux armatures d'effort tranchant. Les résultats d'essais sont présentés dans le Tableau 2, y compris les résistances au cisaillement prédites à partir de l'équation 3 avec les coefficients de régression calculés en utilisant le programme SYSTAT sur un ordinateur personnel. Les résultats des régressions sont donnés dans le Tableau 4 et l'expression finale dans l'équation 4. Dans l'optique du dimensionnement, des valeurs minimales de la résistance ultime au cisaillement avec un seuil de confiance de 90% sont présentées dans l'équation 5. L'analyse rapportée dans cet article fournit des expressions fiables pour la prédiction de la résistance au cisaillement de poutres en béton cellulaire armées avec armatures d'effort tranchant.

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