Papers by Keyword: Fatigue Behaviour

Abstract: A study of the mechanical properties of hybrid additive manufactured IN718 components is presented, optimising mechanical properties due to an in-situ high-speed milling and different heat treatment processes. At first, the impact of different heat treatment processes is investigated, as the changes in microstructure during the process lead to different mechanical properties. Static and dynamic mechanical load behaviour is tested, quantifying microstructural changes by means of the Ultimate Tensile Strength (UTS) and the endurance limit. Furthermore, sole PBF-LB/M- and hybrid built components are compared, investigating the effect of a surface finish on the static and dynamic load behaviour, as superficial cracks and melting errors diminish the UTS and the endurance limit of PBF-LB/M-built components. Within these experiments, a change of fatigue behaviour for the heat-treatedstates can be observed, compared to the as-built state of the PBF-LB/M, as the development of different phases during heat treatment leads to an improvement of the endurance limit for, e.g., solution and ageing treated components. Additionally, the improvement of the surface quality to Ra = 2 µm leads to a significant increase of the dynamic mechanical load behaviour of hybrid-built components, as superficial cracks and surface defects are reduced.

Abstract: We report on a comprehensive study of tensile strength and fatigue behaviour depending on process parameters and ambient temperature. For this, Polyamide-12 components are fabricated using Selective Laser Sintering. Firstly, different process parameters like, e.g, scan-speed, laser power, and applied energy density are varied. Secondly, the ambient temperature during testing is varied, evaluating the impact of decreased, respectively increased test temperatures on the characteristics of the Polyamide-12 components. For all components, the static and dynamic mechanical load behaviour is investigated, quantifying the changes in the Ultimate Tensile Strength (UTS) and the endurance limit. As the applied energy density transpires as a decisive parameter, a variation leads to significant changes in UTS and endurance limit, whereas an adjustment of scan-speed and laser power at a constant energy density do not affect the mechanical properties. Finally, the ambient temperature during testing is evaluated, demonstrating different ambient application conditions. The impact of an active cooling of the component as well as increased temperatures on the mechanical behaviour is tested, providing fundamental findings on the operating life of Polyamide-12 components.

Abstract: Oxidation is one of the main degradation mechanisms that affects most industrial components and occurs as a result of a chemical reaction by which an oxygen atom is added to the molecule of an organic substance or compound. Light, in particular the ultraviolet (UV) component, activates the process of degradation of organic materials, favouring a series of oxidation reactions that occur when the component is in contact with oxygen in the air or water (especially salt water).Given that a number of components work in variable mechanical stress conditions (vibrations caused by the technological process itself or generated by repeated starts / shutdowns of equipment), the protective coatings aim to improve the physico-chemical properties of the surfaces to reduce the degradation, as well as to improve the fatigue behaviour, knowing that the fatigue cracks are initiated from local stress concentrators caused by the surface imperfections of the components. Paper presents the experimental results on fatigue behaviour of the deposited layers, after exposure to UV radiation. In the experimental program, a 2 mm thick AlMg3 sheet (EN AW-5754) was used as substrate. The Aluminium oxide layer naturally created on the metal surface, prevents the effect of oxygen and atmospheric pollutants, but this layer is unstable in corrosive environments. Thus, for a qualitative protection, protective organic layers are used. In experiments, the polyurethane paint PURMAL S 70, RAL 9010 (PUR) and the same paint but aditivate with graphene oxide (PUR + GO) were compared. Prior to fatigue testing, the painted specimens were subjected to accelerated artificial UV-aging process with ultraviolet radiation for 72 hours. Axial fatigue tests were performed with a frequency of 40 Hz, in pulsating mode, with R = 0.053 and with decreasing loads, starting from 0.85% of the tensile strength (R_m), up to 0.5% R_m. Analysis of S‑N curves revealed that, in the field of oligocyclic fatigue there are no significant differences between the fatigue results obtained on PUR-protected specimens, compared to those protected with PUR + GO, instead in the case of polycyclic fatigue, the paint with addition of 1% graphene oxide has improved fatigue resistance.

Abstract: The fatigue behaviour of short fibre reinforced thermoplastics is highly dependent on the morphological conditions. The capability of simulating fatigue behaviour and damage mechanisms of fibre reinforced polypropylene increasingly interests industrial partners of the presented study. For the morphological analysis both destructive and non-destructive methods such as computed Xray tomography and a combination of polishing and microscopic methods were applied. The determination of the composite morphology (fibre distribution, orientation tensor, fibre length distribution) was assisted by several methods of specific automatic data evaluation concepts. Fatigue testing was done on a servohydraulic dynamic testing machine using a testing frequency of 10 Hz, which is in the range of the real loading in application of the components. The analysis included thermal investigations (IR camera) and local strain measurements.

Abstract: Friction stir welding (FSW) is a rapidly emerging joining technology due to significant advancements in tooling and process development. Latest literature relating to finite element analysis (FEA) of mechanical behaviour of FSW joints is reviewed in this paper. The recent development in FEA of mechanical behaviour of FSW joints is described with particular reference to two major factors that influence the performance of FSW joints: static behaviour and fatigue behaviour. The main FE methods used in FSW performance are discussed and illustrated with brief case studies from the literature.

Abstract: Component surfaces can be modified by micro-structuring processes like micro-milling or laser structuring for functionality reasons. This modification induces small notches, whose dimensions are in the same order as the grain size. This could have an influence on the mechanical properties. This paper presents the results of tension-compression fatigue tests with structured and – for comparison – with unstructured micro-tensile specimens of cp-titanium grade 2. Longitudinal metallographic microsections illustrate the grain size of the microstructure and the geometry of the notches. The results of the fatigue tests show the influence of the notches on lifetime and endurance limit. With a Scanning Electron Microscope (SEM) the fracture surfaces, the crack initiation sites, and the crack propagation areas of all samples were analyzed. With these analyses we want to determine which notch structure dimensions relative to the grain size are tolerable.

Abstract: The investigation on fatigue behavior and fracture surfaces of fatigued specimens of as-extruded AZ31 magnesium alloy can provide a reliable theoretical foundation for both fatigue resistant design and reasonable application of magnesium alloys. Through total-strain-amplitude controlled fatigue tests and analysis on fracture surfaces of fatigued specimens, the behavior of cyclic stress response and fatigue life as well as fracture mechanism were identified for as-extruded AZ31 magnesium alloy. The experimental results show that the extruded AZ31 alloy exhibits significant cyclic strain hardening, the relation between elastic strain amplitude, plastic strain amplitude and reversals to failure can be described by Basquin and Coffin-Manson equations respectively. In addition, it has been found that fatigue cracks initiate and propagate in a transgranular mode.

Abstract: Fatigue damage becomes an emerging problem in lots of concrete structures which will subject to cyclic loadings during their working life. This paper presents a study on interfacial shear fatigue performance of a high-strength concrete structure strengthened by carbon fiber-reinforced plastic (CFRP) plate, which has been established as an effective method for rehabilitation and strengthening of concrete structures. Based on the static test, a new experimental investigation of the shear fatigue performance along the concrete-plate interface under the low cycle fatigue load in the condition of R=0.1 is presented. The main variable is the concrete strength. Compared with the static ultimate strength, fatigue strength decreases. Therefore, a safety factor of the fatigue strength at the interface of CFRP and concrete should be applied in design.

Abstract: Fatigue tests were carried out at different stress levels on cruciform welded joints made from mastensitic stainless steel. The purpose of the present paper was to verify the validity of the thermographic method and to extend its capability on welded structural evaluation, considering the real operating situations. Due to limitations of the traditional fatigue test, the infrared thermographic technique was developed to predict and assess the fatigue limit and the entire S-N (Stress-Life) curve of cruciform welded joints. Through the comparison, the predictions of the fatigue behavior by the thermographic method (TM) manifested good agreement with the traditional method. The present research paper concludes that the TM as a promising method enables us to rapidly obtain reliable fatigue characteristics of welded structural components.

Abstract: The NST 37-2 steel represents about 75% volume of Nigerian-produced steel which is yet to be fully characterized for its fatigue behavior. Thus, its suitability for many applications is questionable. This paper presents a framework based on the theory of elasto-plasticity in order to make appropriate recommendations in this regard. Experimentally, tensile tests were carried out on test specimens to establish the baseline material properties of the steel in annealed, as-rolled, normalized and hardened/tempered conditions. Fatigue tests were then conducted at 60% Su; 70% Su and 80% Su of the test material and fractographic examinations on the test specimens were subsequently carried out. The frequency harmonic fatigue analysis was implemented in the ANSYS software environment for the numerical study. The elastic-plastic material property was described by the von Mises yield criterion, the flow rule of Prandtl-Reuss, and the kinematic hardening rule of Prager. The numerical results indicate with respect to rate-dependence fatigue behavior that the annealed test specimen is most resilient under cyclic deformation as compared with the normalized, hardened/tempered and as-rolled specimens respectively. The experimental and numerical results were found to be in close agreement and based on the general performance, the steel material is recommended for use in low cycle, quasi-static fatigue applications.