Key Engineering Materials Vols. 525-526

Abstract: In orthotropic bridge decks, the rib to floorbeam connection is a major source of fatigue problems. Commonly, the trapezoidal ribs cross the floorbeam continuously necessitating clearance holes in its web, and frequently additional web cutouts are foreseen to relieve the ribs lower edges. This solution is favorable for rib cracking but will generate stress concentrations in the web itself. The shape of the additional cutout has a major influence on the sizes of the concentrations rendering differences of a factor 2 or 3 for corresponding overall geometries and loading schemes. Various authors have studied cutout shapes through full scale testing or by computation by finite element modeling. This paper presents such a study, but focuses on the influence of the finite element modeling itself. It is shown that the mesh density, the element type, the choice between shell and volume elements. This is an important finding and should not be overlooked when comparing finite element based results to coded values or measured results. In order to do so, the results of the numerical work are compared to strain results from full scale tests.

Abstract: This paper presents a finite element analysis regarding the stress distribution in a cemented Austin Moore type hip prosthesis. The 3-D model was obtained using a Roland PICZA 3-D laser scanner. The applied loads simulate the normal gait cycle. The prosthesis is made from stainless steel with a femoral head of 45mm diameter. The numerical analysis was performed using the ABAQUS code. The results showed that the stress level in the cement is sensitive to the femoral neck angle. Starting with a standard, 125° angle, and increasing the angle with up to 5°, the resulting stress can be reduced with more than 10%. The proposed angle increase produces a more uniform stress distribution in the cemented section, increasing the durability of the arthoplasty.

Abstract: In this paper the failure behaviour of natural stitched composite materials in the skin-stiffener of wind turbine blade structures are investigated. For this study, the laminated composite beams were stitched using Flax yarns before curing process. Two stiffener structures of T-beam and Box-beam are studied in this paper. These specimens were tested under quasi-static loading condition to compare the failure resistance of adhesive and stitched bonding methods. Furthermore, the cohesive zone modelling (CZM) which is known as a variation in the cohesive stresses with the interfacial opening displacement along the localised fracture process zone is used to predict bonding failure in the skin-stiffener of wind turbine blade structures.

Abstract: In this study, interfacial shear strength of carbon fiber reinforced polypropylene were investigated. Two kinds of reinforcements are used. One is non-treated carbon fiber, another is acetone-treated carbon fiber. And two kinds of matrices are used. One is non-treated polypropylene, another is maleic anhydride-polypropylene. Three point flexural tests and micro debonding tests are conducted. As a result, following conclusions are obtained. Acetone treatment and maleic anhydride are effective to the adhesives on the surface between fiber and matrix. But simultaneous treatments are not effective. The shear strength is not dependent on fiber embedded length. The contact angle and fracture load are dependent on fiber embedded length. The interfacial strength is dependent on the contact angle. As the contact angle increases, the interfacial strength increases.

Abstract: In this study, effect of surface treatment on creep property of green composite under environmental temperature was investigated. Jute fiber was used as reinforcement. PLA (polylactic acid) was used as matrix. Surface treatments were conducted using 5 % solution of silane coupling agent and PVA (polyvinyl alcohol). The flexural test was conducted under 25^°C environment. The flexural creep test was conducted for 50 hours at 25, 40 and 50^°C environment. As a result, the flexural property of composite increased by the surface treatments. And the surface treatments affected the adhesion of fiber/resin interface. Creep strains of surface-treated jute fiber/resin composites were lower than that of virgin composite. The creep strain was decreased by the treatments. The effects are confirmed under various temperature conditions.

Abstract: t is increasingly a requirement to be able to determine the mechanical properties of materials: (i) at the micro-scale, (ii) that are in the form of surface coatings and (iii) that have nanoscale microstructures. As a consequence micro-scale testing is an important tool that has been developed to aid the evaluation of the mechanical properties of such materials. In this work cantilever beam specimens (typically 2μm by 2μm by 10μm in size) have been prepared by gallium ion milling and then deformed in-situ within a FEI Helios Dual Beam workstation. The latter is achieved using a force probe with a geometry suitable for loading the micro-scale test specimens. Thus force and displacement can be measured together with observing the deformation and fracture of the individual specimens. This paper considers the evaluation of the mechanical properties in particular elastic modulus, yield strength and fracture strength of materials that result in relatively large deflections to the micro-scale cantilever beams. Two materials are considered the first is linear elastic single crystal silicon and the other elastic-plastic nanocrystalline (nc) nickel. The results are discussed with respect to the reproducibility of this method of mechanical testing and the evaluated properties are compared with those derived by alternative procedures.

Abstract: Connection between steel and concrete parts is frequently required in constructions where the steel-concrete joints are often realized by welded shear studs. In order to avoid stress concentrations, corrosion proneness, and other negative consequences of the welding process, steel-concrete connection without welded mechanical shear connectors is sought nowadays. Connection can be realized via an epoxy adhesive layer and gritted with granules. In the paper, the assessment of the push-out test configuration was performed from the generalized fracture mechanics point of view. The numerical-analytical modelling of a steel-concrete connection is performed without and with the epoxy interlayer, while 2D and 3D modelling is used. Thus conditions of crack initiation can be predicted from knowledge of the standard mechanical and fracture-mechanics properties of particular materials. The model of a bi-material notch with various geometry, and material properties is used to simulate various singular stress concentrators that can be responsible for failure initiation. Various manners of preparation of the epoxy interlayer are tested experimentally. Results of the fracture-mechanics studies are compared with each other and with experimental results. On the basis of the comparison, the 2D simulation of the steel-concrete connection without the epoxy interlayer is shown to be suitable for the estimation of failure conditions.

Abstract: This paper reports the microscopic fracture behavior of natural fiber-reinforced green composites. The acoustic emission (AE) method of nondestructive and real-time testing was applied to detect small-scale energy release phenomena during tensile deformation of the green composites. The unidirectional abaca fiber was embedded in a starch-based biodegradable resin matrix. Two kinds of pre-damaged abaca fibers as well as as-received (i.e. undamaged) fiber were used to examine the effect of the pre-damaged abaca fiber on the overall fracture behavior of the unidirectional green composites. In the case of the green composites reinforced with as-received abaca fiber, both of the tensile strength and fracture strain were relatively high. In the case of the green composites reinforced with pre-damaged abaca fiber, however, showed relatively smaller tensile strength and fracture strain. In addition, a wide range of amplitude AE events were measured during the tensile deformation. This tendency was enhanced in the composites reinforced with heavily damaged abaca fiber. The experimental results showed that the AE activity in the early deformation stage was associated with such the microscopic fracture of pre-damaged abaca fibers.

Abstract: For modern high strength steels, instead of metal instability, ductile damage triggered by the formation of microvoids or microcracks resulting from the complex material microstructure, has become the key factor responsible for the final failure in the forming process of such steels. The target of this study is to describe the initiation and evolution of damage in a dual-phase (DP) steel (DP600). By applying a newly proposed approach that is able to indicate the onset of damage in an engineering sense and quantify the subsequent damage evolution, to predict the forming limits for DP600 are predicted by simulating Nakajima test. Accordingly, two forming limit curves (FLC) are numerically computed to characterize two moments: when damage becomes pronounced and when the final failure is triggered by the accumulation of damage. Comparing with the conventional experimentally calibrated FLC at necking, the limit at crack initiation predicted by modeling gives a lower but defect-free forming boundary. The forming limit at final fracture is well captured by allowing the subsequent damage evolution to a critical value.

Abstract: Textile composites have increasingly been used as a structural material because of their balanced properties, higher impact resistance, and easier handling and fabrication compared with unidirectional composites. However, the complex architecture of textile composites leads to difficulties in predicting the response in spite of the fact that there is the need to determine mechanical properties in product design. Micromechanical analysis, using the Finite Element Method, was conducted in order to evaluate the effective mechanical properties of plain woven and 3D woven composites. In this study, numerical models of unit cells were used and it is shown that the predicted values of homogenized mechanical properties using the developed procedure were in good agreement with experimental results.