Procedia Structural Integrity最新文献

This work explores the effects of notching method and element layout on the fracture loading properties of thermoplastic materials processed using fused filament fabrication (FFF). Three common thermoplastic materials were used (acrylonitrile butadiene styrene, polylatide, and polycarbonate). Four different notching methods were used, with printed and machined notches and with and without pre-cracking on ASTM D5045 compact tension specimens (n = 36). It was concluded that the notching method has a statistically significant impact on the sample preparation and that pre-cracking is necessary in all cases. Using this information to prepare specimens, a designed experiment using four different element layout strategies and two different nozzle sizes was completed with a total of 72 tests. The layout pattern was shown to have a very strong effect on the maximum fracture load, with the nozzle size showing a smaller but still statistically significant impact. With the exception of one layout using polycarbonate with likely design-driven printing defects, the results were very consistent through several replications. The results of this study are useful for making design decisions with FFF-processed materials, for better understanding the impact of the process design, and for working toward standardized printing and testing methods for additive manufacturing.

Fused Deposition Modeling (FDM) technique is a subcategory of additive manufacturing processes that works by extruding a fine polymeric filament on the heated bed. The current research paper surveys the influence of nozzle diameter as a manufacturing parameter on the mechanical properties and mode I fracture behavior of the FDM-PLA samples. Four different nozzle diameters of 0.4, 0.6, 0.8, and 1 mm with two raster configurations of 0/90° and 45/-45° were considered for printing the dog-bone and Semi-Circular Bending (SCB) samples. Also, to evaluate the fracture resistance of FDM-PLA pre-cracked samples, the critical value of J-integral (Jc) was used and calculated through a finite element analysis. The obtained results indicated that the raster angle of 45/-45° resulted in higher mechanical properties compared to 0/90° one, also, the 1 mm nozzle diameter presented a better performance from a mechanical property point of view. The SCB sample printed through the 1 mm nozzle diameter and 45/-45° raster orientation had the highest value of Jc (10400 J/m²). Besides, the crack extension paths were monitored and discussed comprehensively.

The study investigates the mechanical properties and fatigue behavior of steering knuckles used on commercial vehicles. The steering knuckle is made of hot forged bainitic steel (18MnCrMoV6-4-8), which is known to demonstrate high levels of fatigue strength, toughness, and hardness. The local strain concept was adopted to assess the durability of the steering knuckle based on the stabilized cyclic material behavior. For this purpose, experimental investigations have been conducted on both the steering knuckle as well as fatigue specimens under constant and variable amplitude loadings. The fatigue specimens were removed from the area next to the crack initiation location, to represent the microstructure in the critical area of the component. Fatigue life estimations were performed under different load ratios using the FKM guideline nonlinear, employing damage parameters PRAM and PRAJ. The assessment enables a fatigue strength assessment for the steering knuckle by considering the local non-linear material behavior. The estimations of the material's fatigue lifetime using the FKM guideline nonlinear approach were unsatisfactory.

Wire Arc Additive Manufacturing is an additive manufacturing process with a high rate of material deposition capable of producing near-net shape parts. This process involves the reduction of production costs (material and lead times) and considers innovative designs. However, the deposition technique induces heterogeneities in the material, in particular the presence of porosity and a degraded surface finish. The process-induced surface asperities have a first-order influence on the fatigue life of as-built parts as they act as stress raisers. Various finishing treatments can be considered to reduce the criticality of the surface finish influence over crack initiation and propagation: conventional ones such as hammer, laser or shot peening and some specially developed for Additive Manufacturing (AM) processes such as in-situ cooling or hot rolling. The multitude of AM parameters and the different finishing surface post-treatments entail many configurations that will modify fatigue properties. For this reason, rapid fatigue evaluation methods are an asset for process evaluation.

Thermo-elastic Stress Analysis (TSA) is a non-contact technique for measuring the distribution of stress at the surface of a component subject to cyclic loading using an infrared camera. The analysis of the thermo-elastic coupling amplitude maps allows the detection of initiation and monitoring of crack propagation. A four-point bending fatigue test protocol is conducted on CuAl9 WAAM specimens take in different direction for the deposition direction. Then failure mode and life duration are compared for the 2 directions.

Crane runway girders are subjected to cyclic loading and must therefore be designed against fatigue failure. For existing structures, however, there are no standards for handling pre-damaged components at the end of their calculated lifetime. The Institute for Material and Building Research of the University of Applied Sciences Munich examines different approaches on how to deal with existing welded steel structures. The research project addresses the following questions:

How can pre-damaged components without visible cracks be strengthened?

How can components with visible cracks be repaired in a fatigue-proof manner?

How can pre-damaged components be reinforced through a low-notch application of steel cover plates?

In order to answer these questions several numerical and experimental investigations are carried out. Different innovative fastening techniques like lockbolts, adhesives and self-tapping screws for attaching reinforcement cover plates are tested on small and large specimens. Within this paper the research project will be presented and previous results on components without visible cracks will be summarized.

One of the simplest and most efficient ways to design lightweight structural components is the combination of welding and aluminum alloys. However, welded joints are extremely sensitive to fatigue failure and making accurate lifetime predictions is still challenging when Variable Amplitude (VA) loading conditions are involved. Among all design criteria available in the literature, the present investigation focuses on the Peak Stress Method (PSM), an engineering finite element (FE)-based approach to rapidly assess the fatigue strength of welded joints. In more detail, the PSM suggests modelling both weld toe and weld root as sharp V-notches having null tip radius and correlates their fatigue strength using the intensity of the local linear elastic asymptotic stress distributions described by the Notch Stress Intensity Factors (NSIFs). The theoretical formulation of the PSM for the fatigue strength assessment of welded joints subjected to VA loadings has been recently proposed by combining its Constant Amplitude (CA) formulation with the Palmgren-Miner's cumulative linear damage rule. Such VA formulation has been successfully validated against a large bulk of experimental fatigue results generated by testing welded joints made of structural steels under uniaxial as well as multiaxial loadings. In the present investigation, the VA formulation of the PSM has been further validated against experimental data relevant to welded joints made of aluminium alloy under VA loadings.

In order to predict fatigue lifetime of cableway installations components, EN 1993-1-9 standard (Eurocode 3) is commonly used. However, EN 1993-1-9 is based on a major hypothesis : stress state has to be uniaxial. But for some components, this uniaxial hypothesis is not verified (for example : fixed grip of chairlift submitted to horizontal tightening stress and vertical load stress, or chairlift structure stressed by gravity and lateral shake when passing a tower). It then appears important to use a fatigue criterion taking into account multiaxial stress state. Our research work proposes to apply for fatigue study the Dang Van criterion, which takes into account multiaxial stress state. Results are then compared to Eurocode SN curves, representing a huge experimental database on unixaxial fatigue, that we propose to use in multiaxial fatigue thanks to an appropriate recalibration. The scientific originality of this work lies in the justification of that Dang Van criterion's recalibration. Therefore, a theorical study ensures that our use of Dang Van's multiaxial criterion is still consistant with the Eurocode SN curve even if these SN curves are experimentally performed under uniaxial stress state. Finally, that hitherto unpublished use of the Dang Van criterion makes it possible to consider the entire database of detail categories defined in NF EN 1993-1-9, by generalising their use for components under multiaxial fatigue.

To ensure reliability of hybrid cylindrical roller bearings each ceramic roller is inspected, and scrapped if a surface imperfection above the critical size is detected. This inspection aims to reduce the potential root cause of Rolling Contact Fatigue (RCF) which can occur during operation, shortening the bearing life. The rejection criterion is based on experimental and theoretical knowledge, which for the last decade was developed in SKF for the material imperfections mainly located on the rolling element raceway. These imperfections are subjected to high contact pressure and therefore are considered as the primary root cause of RCF failure. Regarding rollers, however, imperfections can be present beyond the raceway, i.e. at the roller chamfer where the lower risk of RCF is expected, because the edge imperfections are typically out of the rolling contact zone. Nevertheless, the risk associated with these features should be assessed too, chiefly because the size of edge imperfections can be rather large. In our previous study, the imperfection termed as a Missing Material was studied, combining the semi-analytical tool for the contact mechanics and the Finite Elements (FE) method for the stress analysis. In the current work, another imperfection type is considered, and this is a surface crack located at the chamfer of ceramic roller. The RCF analysis is based on the semi-analytical evaluation of the rolling contact pressure (between a ceramic roller and a steel inner ring), and computational fracture mechanics for the estimation of fatigue crack propagation.

The frequency effect is a commonly encountered challenge in ultrasonic fatigue testing (UFT) of low-carbon, ferritic steels, wherein factors such as the increased strain rate and reduced test duration change the apparent fatigue resistance of the tested material. The usability of UFT for rapid f atigue testing of these materials is therefore limited as the results cannot be directly compared to conventional fatigue results. In this investigation, fatigue curves were evaluated at frequencies of 20Hz and 20kHz for two comparable grades of ferritic structural steels: Q355B and S355JR, using different conventional frequency specimen geometries. Methods to evaluate the frequency sensitivity of the steels based on the finite life regime were adapted from previously proposed models in literature to produce corrected curves and to allow comparison to similar steels in literature. It was found that previously reported results may be overestimating the frequency sensitivity due to the influence of size effects. It was also found that these models are of limited use for producing corrected SN curves based on UFT data.