Key Engineering Materials Vol. 627

Abstract: A theory for mathematical framework is developed with axioms for fatigue damage, quantification and relativity concept, boundary conditions, and compatibility, allowing us to evaluate the validity of candidate damage equations/quantities on S-N plane for prediction of fatigue life. Manifestation points for accumulated damage were defined for boundary conditions by differentiating between damage accumulated before failure, and failure caused by damage at quantised fatigue loading cycles. A selected damage equation leading to a theoretical S-N curve was validated as an example.

Abstract: Iso-XFEM is an evolutionary-based topology optimization method which couples the extended finite element method (X-FEM) with an isoline/isosurface optimization approach, enabling a smooth and accurate representation of the design boundary in a fixed-grid finite element mesh. This paper investigates the application of the Iso-XFEM method to the topology optimization of structures which experience large deformation. The total Lagrangian formulation of the finite element method is employed to model the geometrically non-linear behaviour and equilibrium is found by implementing the Newton-Raphson method in each evolution. A cantilever beam is considered as a test case and the Iso-XFEM solutions obtained from linear and non-linear designs are compared with bi-directional evolutionary structural optimization (BESO) solutions.

Abstract: Selective laser melting (SLM) is an attractive metal additive manufacturing technique that can create functional finished components. The microstructure that originates from SLM, however, differs in many aspects from that obtained from conventional manufacturing. In addition, the microstructure-mechanical properties relationship is not yet fully understood. In this research, the high-cycle fatigue performance of SLM Ti-6Al-4V was studied. The dominant fracture mechanisms were reported and discussed in relation to the microstructure of the specimens.

Abstract: Selective Laser Melting (SLM) is an Additive Manufacturing method that enables greater design freedoms than traditional manufacturing methods in the production of high value, low volume metallic parts. Despite this now being a well-established processing method, there are a number of issues impeding industrial uptake, including the generation of residual stress and part distortion during manufacture. Prediction of residual stress is invaluable for tuning process parameters, and optimising the part geometry and support structures to limit residual stress based distortion during manufacture. This paper establishes a thermal modelling strategy to predict temperature distribution within a 3D SLM part that is a precursor towards a residual stress analysis.

Abstract: The paper contains results of an experimental evaluation of fatigue properties of two steel types used for shaft manufacture, namely rather conventional, medium carbon high quality C45+C steel and quite modern low alloy ETG^®88 steel, recently developed with the aim to reduce overall manufacture cycle costs together with maintenance of high mechanical and fatigue properties. Actual critical fatigue damage modes of shafts, namely at stress concentrators of shaft shoulders, was experimentally modeled by small and quite large notched specimens loaded by rotating bending. ETG^®88 steel was characteristic by somewhat higher fatigue resistance, but higher scatter of results. Differences are discussed considering chemical composition, microstructure and results of simple analyses of fatigue fracture surfaces.

Abstract: V-ROL N is an aircraft steel produced in Czech Republic at Poldi Kladno, used for many years for a manufacture of aircraft landing gears, particularly for a small passenger aircraft L 410. The V-ROL N steel, known by its very good or even excellent fatigue and fracture properties, has not been more available due to recent privatization of the company. Therefore, a substitution of materials previously used became an issue. The paper contains results of a comprehensive experimental programme aimed at comparison of the V-ROL N steel used in the past, still available for research purposes, with another steel of a similar type, VIP P-ROL N steel, manufactured in Czech Republic by another company. Fatigue resistance of both versions can be considered as good. Slightly higher scatter of fatigue results of VIP P-ROL N version can be attributed to an occurrence of impurities, although quite rare, responsible for fatigue crack initiation.

Abstract: As a result of the production process, there are axial and tangential residual stresses present in pressure pipes made of polymer materials such as polyethylene or polypropylene. The residual stress magnitude and distribution have a significant influence on the pipe lifetime. In this contribution the results from experiments focused on determining the tangential residual stress distribution in the walls of polypropylene pipes of different dimensions are compared. The experimental method used involves measuring the deformation of ring shaped specimens that were slit in the axial direction. Measured deformation of the ring specimen is a result of the tangential and axial stress superposition. However, the effect of the axial residual stress depends on the specimen axial dimension and tangential residual stress estimated basing on experimental data should be corrected according to axial dimension of the specimen used. The correction suggested in this article is determined based on three-dimensional FEM simulations of the experiment.

Abstract: The paper is dedicated to experiments on near-threshold fatigue cracks under mixed-mode II+III in the ARMCO iron. The mixed mode crack growth was described using an equivalent factor ΔJeff,eq=(1-α)ΔJeff,eqII+αΔJeff,eqIII. The most appropriate description was found for α = 0.67 indicating that the mode III component should be more efficient than that of the mode II which is in contradiction of a ΔK_eq–based analysis. This result shows that, unlike in the austenitic steel, the difference in the efficiency of modes II and III in the ARMCO iron is very small.

Abstract: This paper deals with fatigue crack propagation under 4 bar hydrogen atmosphere in low alloyed steel (3.5Ni-1.5Cr-0.5Mo-V) used for turbine generator of nuclear plant. The tests are conducted in the same way in ambient air and high vacuum on CT specimens and the fatigue crack growth rate specially investigated in the near threshold range is plotted with respect to the applied stress intensity factor. It is shown that the propagation under hydrogen atmosphere is similar to that obtained in air up to a K_max value of 16,5 MPam^1/2 with increased growth rate compared to that in high vacuum leading to a threshold value lower that in vacuum, this effect being related to residual water vapor. For K_max higher than 16,5 MPam^1/2, much faster growth rates under hydrogen atmosphere becomes are associated to an intergranular propagation mechanism induced by an hydrogen effect. The results are discussed on the basis of available models.

Abstract: Fatigue crack propagation is studied in thin wires of about 1 mm in diameter of an ultrahigh strength steel (σ_max> 2400MPa) in ambient air and in air with controlled residual humidity. A specific equipment is developed based on an electro-dynamic testing machine equipped with an environmental chamber for air humidity control. Threshold tests are run using a load shedding procedure specifically adapted to the specimen size. The relation between load ratio and crack closure is evaluated from constant K_max tests. The results are discussed on the basis of fracture surface observations and of existing modelling for environmentally assisted fatigue crack propagation.