K12
Fatigue 1

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13:20
conference time (CEST, Berlin)
New Method for Evaluation of Adhesive Joint Failure Under Cyclic Loads
27/10/2021 13:20 conference time (CEST, Berlin)
Room: K
M. Frank, K. Hofwimmer (Magna Powertrain Engineering Center Steyr GmbH & Co KG, AUT); C. Lay (Porsche AG, DEU)
M. Frank, K. Hofwimmer (Magna Powertrain Engineering Center Steyr GmbH & Co KG, AUT); C. Lay (Porsche AG, DEU)
In vehicle structures, combinations of metallic and non-metallic materials are becoming increasingly common. Bonding has established itself in automotive engineering for joining these materials. A generally valid method for calculating the fatigue life of the adhesive joints does not yet exist. This is due to the time- and temperature-dependent material behavior, inhomogeneous stress conditions and complex damage mechanisms. Due to the high computational effort, neither transient fatigue analyses nor fine finite element meshes can be realized in industrial applications. Therefore, methods have been developed which intelligently combine different modelling and calculation approaches. The system is initially simplified, which enables a fatigue analysis to determine the cohesive technical crack initiation in all adhesive joints of an entire vehicle structure. In S/N tests, which were carried out on specimens with stress concentrations at the joint corners, a high cohesive fracture toughness was observed in the epoxy adhesive layers. The pronounced crack propagation phases lead to massive fatigue life differences between crack initiation and total failure. In order to take this finding into account, the failure criterion was extended from the crack initiation to a local cracked zone and so-called zone-S/N curves were determined. A routine for localizing and evaluating the cracked adhesive layer areas in the finite element model was developed. With the zone-S/N curves of different specimen tests it was possible to calibrate the developed evaluation method and to calculate the degree of utilization of the adhesive layer. A major advantage of this approach is that the previous fatigue analysis can be performed in the time and frequency domain. The assessment method and the correlation to tests is presented. In addition, the application on finite element models of complex vehicle structures is shown. Here it is demonstrated which advantages result from the area-wise consideration and how the number of adhesive layer areas, which must be examined in detail, can be limited.
adhesive, joint, evaluation, cracked, zone, assessment, fatigue, fracture, automation, calibratable, approach
13:40
conference time (CEST, Berlin)
Critical Plane Analysis in the Frequency Domain (with Additional Considerations for Rubbers)
27/10/2021 13:40 conference time (CEST, Berlin)
Room: K
N. Bishop (Hexagon, USA); S. Datta (Stellantis, USA)
N. Bishop (Hexagon, USA); S. Datta (Stellantis, USA)
Fatigue calculations require complex stress states to be transformed into an equivalent variable, like von-Mises stress, Principal stress, or some other stress variable. von-Mises is often proposed because it has the ability to account for stress biaxiality (or triaxiality) within the material, but it cannot account for rotation of stress tensor directions in time (or frequency). Principal stresses can be used as long as it is acceptable to “follow” the movement of the principal stress direction, thus ignoring the movement with time. If one is working in the frequency domain, the calculation of Principal stress can only be done if the phase variation for each frequency component is removed by making the assumption that the worst phase angle will be used. In other words, we pick the highest stress value as a function of phase, for each frequency. Von-Mises is also a complicated calculation in the frequency domain because the classic algorithm (for von-Mises) was developed for non-complex stresses. For complex stress tensors a method proposed by Segalman has become widely adopted. All of these methods are well established in both time and frequency domain and acceptable as long as stresses are not rotating too much in space. One modern alternative for highly fluctuating stress vector conditions, which indirectly creates an equivalent normal stress, is the so-called Critical Plane Approach. It is based on scanning all the layers (slices) within the material, transforming the stress state to obtain the normal stress and damage on that layer, rotating the plane (in 3 dimensions) and then picking the slice (plane) that gives the highest damage. This can become a laborious computational process when implemented in the time domain and until recently the time domain was the only option. However, modern software’s such as CAEfatigue have made this a practical option for the time domain. Further, the approach has now been extended to work in the frequency domain and this paper will provide examples of its use for frequency domain fatigue analysis on automotive components. There are extensions of the Critical Plane Approach that have been proposed for elastomers (rubbers) and this paper will also examine these approaches.
Fatigue, Durability, Frequency Domain, Random, Elastomers
14:00
conference time (CEST, Berlin)
Frequency Domain Fatigue Methodology Applied to Boat Design
27/10/2021 14:00 conference time (CEST, Berlin)
Room: K
S. Kerr (Hexagon, USA); L. Macfarlane, L. Walker (Nauti-Craft, AUS)
S. Kerr (Hexagon, USA); L. Macfarlane, L. Walker (Nauti-Craft, AUS)
This year Nauti-Craft will be launching the world’s first recreational production vessel incorporating its patented whole of vessel suspension. This whole of vessel suspension system will be incorporated as a standalone “chassis” structure which will be integrated into the designs of its upcoming NS7 and NR7 vessels. This paper examines a portion of the design process, and the means by which such a unique system can be comprehensively engineered via simulation and analysis. Boat / ship design in general, has been part of our history for thousands of years. However, the push for innovation and innovative design tools is a more recent activity. The team at Nauti-Craft is now partnering with MSC Software (part of Hexagon) to introduce the innovative Frequency Domain fatigue analysis approach into their design process. The potential performance advantages of working in the frequency domain are: • The frequency domain approach requires one frequency response function (FRF) per structural configuration, compared with one for each event in a time domain modal transient analysis. Hence, the solver analysis is decoupled from the fatigue analysis allowing for multiple fatigue evaluations without rerunning the solver analysis. A significant time saver. • With careful planning, very large models can be easily handled in the frequency domain. • The approach can be easily used to generate diagnostic information such as damage per event, frequency at the peak response, contribution diagnostics of the loading channels, etc. • Response statistics like displacement and acceleration can be obtained and used for correlation purposes and as laboratory test data or used in the evaluation of relative response between parts to check on component gap development or part collisions under loading. • Analysis times are extremely fast which provides for design optimisation, even for larger models. This paper will show the required workflow for this innovative approach which includes, (i) gathering the response time histories from an MSC Adams analysis, (ii) converting the time histories to Power Spectral Density (PSD) matrix files and (iii) running a full random response and fatigue analysis as demonstrated on part of the NS7 design. The advantages of the Frequency Domain approach for creating useful diagnostic data will also be discussed.
fatigue, boat, suspension, frequency, frequency-domain, MSC, finite-element, marine, Nauti-Craft, Adams, CAEfatigue
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