G16
Comp Struct Mechanics

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10:40
conference time (CEST, Berlin)
A Finite Element Study of Elastic Stress Singularities and Stress Concentrations in the Vicinity of Inclusions in Forged ANSI 4330 Steel
28/10/2021 10:40 conference time (CEST, Berlin)
Room: G
J. Wood, G. Barnard, (University of Strathclyde, GBR)
J. Wood, G. Barnard, (University of Strathclyde, GBR)
Inclusions found in 4330 high strength low alloy steel, play an important role in fatigue failure of components in corrosive environments. This paper uses FEA-derived SCF’s and theoretical elastic stress singularities at surface-breaking non-metallic inclusions in 4330 steel, to explain a possible contributory mechanism for the development of pitting and fatigue crack growth. Previous work on the nature of theoretical elastic stress singularities[1] demonstrated that not all singularities are the same and can be usefully characterized by their “strength”. Different stress components will generally have different singularity strengths (akin to SCF’s being a function of stress component, local geometry, and how the stress “flow” is disturbed). The previous work[1] also demonstrated that singularities are generally not realised in practice (i.e. they are theoretical in nature, for various reasons). In the case of dissimilar joints e.g., there is likely to be a “non-abrupt” transition from one material to another due to melting, alloying, diffusion etc. Nevertheless, in the case of inclusions, the transition region will be narrow. While infinite stresses won’t be realized, a large increase in local stress will arise, which could cause failure at the interface. Propagation of interface cracks could then result in inclusion detachment, leaving a pit. It is argued that stronger elastic stress singularities will give rise to earlier damage. It is proposed herein how the theoretical strength of singularities and associated SCF’s can provide insight to the formation of early corrosion pits and their characteristic shapes. These stresses will no doubt interact with variables such as residual stresses, fatigue, corrosion, erosion, dissolution etc. The size, number, density, physical and chemical properties of inclusions, will clearly influence the pitting process (including corrosion & dissolution). While it has been observed that inclusions in 4330 are often globular, for the purposes of this study, they were considered as 2D plane strain circular. The techniques developed previously[1], for determining singularity strength, involves using a very fine finite element mesh in the vicinity of the singularity and fitting a Power Law to the stress distribution (with an R-squared fit parameter close to unity). The Power Law variable is indicative of singularity strength, e.g -0.5 being the singularity strength for a mode 1 closed crack in fracture mechanics. For a free-surface dissimilar material singularity, the material parameter that influences singularity strength is the moduli ratio. To determine the influence of this, various FEA sensitivity studies were carried out. It was assumed that the elastic modulus of homogeneous inclusions varied over a wide range for different depths of surface-breaking inclusions in a block under tension. The inclusions had different levels of truncation due to forming & machining of the free surface. The singularities and SCF’s for this case were determined, both with and without inclusions. Cases with empty pits, provide an indication of where on the pit wall, fatigue crack development is most likely. A study of SCF versus moduli ratio was also carried out for a block with a centrally located subsurface inclusion (no singularities in this case). The FEA results are qualitatively compared with results from corrosion fatigue tests carried out using 4330 steel specimens. A novel corrosion attachment was produced for RR Moore machines, using layered manufacture. The design allows specimens to experience an interaction of cyclic stressing and corrosion. [1] Wood J. et al, Theoretical elastic stress singularities … much maligned and misunderstood, Proceedings of NWC, San Diego, 2015.
Non-metallic inclusions; 4330 steel; stress singularities & concentrations; finite element analysis, mechanism for early corrosion pitting and cracking; experimental corrosion fatigue testing.
11:00
conference time (CEST, Berlin)
Electrofusion Couplers for Thermoplastic Composite Pipes
28/10/2021 11:00 conference time (CEST, Berlin)
Room: G
M. Gierulski (The University of Sheffield, GBR)
M. Gierulski (The University of Sheffield, GBR)
Most types of pipelines used for gas, water or sewage distribution, are currently made from thermoplastic pipes. The majority of them are connected with electrofusion couplers; the technology is mature, simple, cheap and reliable and the joints are maintenance-free for up to 50 years. Meanwhile, due to the much higher mechanical properties required in the oil and gas sector, it is still mainly steel pipes that are used. They are strong and can contain high pressures, but are prone to corrosion and are problematic in installation and maintenance. The recent development of thermoplastic composite pipes (TCPs) seems to offer a viable alternative; the mechanical properties of TCPs can be as good or even better than those of steel pipes, and they are more resistant to corrosion. Currently, these pipes are still connected with mechanical, steel joints, but efforts are being made to create fully thermoplastic solutions using electrofusion joints that will outperform steel pipelines. This paper describes the development of a finite element model for simulating two types of mechanical test, a hydrostatic pressure test and a whole pipe tensile test, in order to optimise the design of the electrofusion fitting. These initial simulations in Abaqus involve a joint between a standard commercially available electrofusion coupler and TCPs and will be validated with experimental mechanical tests, utilising strain gauges and digital image correlation methods. The extra challenge in creating the model is finding suitable input data on the mechanical performance of the different layers in the TCP; these properties are not readily available in the literature and hence have to be obtained by experiment. The TCP material is produced in the form of a pipe by tape winding, it does not exist as flat plates, which makestesting difficult, as most standard tests require flat sample. Therefore, a non-standard set of tests has been developed in order to obtain the mechanical properties needed for the FEA.
Electrofusion welding, thermoplastic composite pipes, reinforced thermoplastic pipes, stress simulation, electrofusion joint
11:20
conference time (CEST, Berlin)
Simulation of Power Springs
28/10/2021 11:20 conference time (CEST, Berlin)
Room: G
G. Hannig, C. Beck (Scherdel Siment GmbH, DEU)
G. Hannig, C. Beck (Scherdel Siment GmbH, DEU)
In practice, the analytical calculation of power springs often delivers only unsatisfactory results. The reason for this is the observed strong dependence on the manufacturing process as well as pronounced nonlinearities, such as the coil contact, the anisotropic elastic-plastic material behavior and the strong influence of friction. These manifold influencing variables and their feedback leads to a large number of prototype loops with correspondingly high development costs. In this paper, a methodology for the numerical calculation of power springs is presented in order to finally obtain a characteristic curve for the functional design and the corresponding load stresses for the fatigue assessment, which can be carried out using the FKM guideline "Springs and spring elements". Due to the prestressed installation situation, it is necessary to calculate the coiling process with subsequent installation in the housing. The plastic deformation and subsequent springback lead to a superposition of residual stresses and load stresses, which varies over the number of spring coils. As a result, the spring properties are significantly influenced by the underlying manufacturing process. The influence of the friction between the coils is shown in the paper as well as a direct comparison between measured characteristic curves and those predicted by the methodology shown in this paper. Only the integration of the manufacturing simulation into the design process makes it possible to realize an accurate prediction of the fatigue strength for power springs, since only in this way manufacturing residual stresses and load stresses for individual applications can be determined in advance of prototypes and then serve as input data for the evaluation of the degree of utilization according to the FKM guideline. The virtual spring must successfully meet all requirements in order to be manufactured in reality. This agile and, from the author's point of view, forward-looking development process makes it possible to face the challenges mentioned at the beginning and allows a detailed insight into variables that were previously difficult to access.
power spring, process simulation, load simulation
11:40
conference time (CEST, Berlin)
Fast and Accurate Contact Dynamics With a Detailed FE Mesh on the Base of Contact and Stress Modes
28/10/2021 11:40 conference time (CEST, Berlin)
Room: G
W. Witteveen (FH OÖ Forschungs- und Entwicklungs GmbH, AUT)
W. Witteveen (FH OÖ Forschungs- und Entwicklungs GmbH, AUT)
Contact mechanics plays a crucial role in many numerical simulations. Examples are sliding bearings, bolted and differently jointed areas, gear wheels or the contact between two rollers. Here, the contact forces depend on the one hand on the global deformations of the whole body, but also on the local deformations due to the local contact forces. These problems are therefore very challenging, so that usually the nonlinear Finite Element method is used. In order to keep the exploding computation times at least within tolerable limits, various simplifications, such as coarse meshes or quasi-static instead of dynamic simulations are common. In many cases, however, these simplifications lead to a reduced expressiveness of the result. This may lead to additional computational effort like stress reconstruction with a detailed FE mesh. In this presentation, we show an alternative approach that does not have all these drawbacks. The crucial point is, that instead of nodal degrees of freedom special contact modes are used. This is called model reduction. Those contact modes can be computed a priori, without a costly simulation of the unreduced system. If, in addition, stress modes are used for the reduction of the contact force computation, then it is called hyper-reduction. The above mentioned approaches have been implemented into multibody dynamics and successfully applied in academic and complex industrial examples like elastic crank drive simulations. With dramatically shorter computation times, the result accuracy of Finite Element method is obtained. At the beginning of the presentation, the basic ideas are explained. In the second part, case studies are shown that underline the strength of the approach. Those case studies are from the field of elastohydrodynamics and dry contact. The impressive reduction in computational times by orders of magnitude allows strategies to be applied to such problems that were not previously possible. One example is the consideration of nonlinear contact in optimization processes.
contact mechanics; multibody system, model reduction,
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