J6
Composites - Failure

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10:40
conference time (CEST, Berlin)
Finite Element Analysis Assisted Fatigue Life Estimation Method for Continuous Fiber Reinforced Plastics
26/10/2021 10:40 conference time (CEST, Berlin)
Room: J
L. Kovacs, L. Takacs, T. Olajos (eCon Engineering Kft., HUN)
L. Kovacs, L. Takacs, T. Olajos (eCon Engineering Kft., HUN)
The ever-growing need in industry to improve efficiency and performance of products by preserving or even reducing the overall cost has led to a continuous development of materials with better strength to weight ratio than their conventional counterparts. The most relevant representatives of a new material generation nowadays are the continuous fiber reinforced plastics (FRPs). Contrary to metals, thermoset matrix based FRPs are proven to be less sensitive to cyclic loading resulting in much shallower lifing curves. Thus, the endurance limit as a simple fatigue limit stress is a reasonable choice to design fatigue tolerant composite structures. However, for a better understanding of the cyclic load bearing capacity numerous theoretical models have been developed to describe the progressive damage mechanism initiating from ply level up to the final delamination failure of the entire laminate. The importance to introduce these approaches into standard composite engineering processes is getting higher as thermoplastic matrix based CFRPs spread across different industries. These materials exhibit more sensitivity to fatigue load including time-dependent effects. The work presented here introduces a new, in-house developed fatigue life evaluation method for composite laminates with arbitrary geometry and ply layup sequence. The method implements the residual strength and stiffness theories combined with mean stress correction and multiaxial failure criterion coupled with FE stress analysis to correctly compute the ply stresses over the cyclic degradation process. The algorithm has been coded in a user subroutine collaborating with ANSYS FE solver. The tool was also validated and calibrated with fatigue tests carried out in fiber direction. The test results are presented and compared with the simulation results evaluated with the newly developed method. [1] [1] L. Takacs, L. Kovacs, and T. Olajos, "Numerical tool with mean-stress correction for fatigue life estimation of composite plates," Engineering Failure Analysis, vol. 111, p. 104456, 2020/04/01/ 2020.
composites, fatigue, degradation, residual strength, finite element
11:00
conference time (CEST, Berlin)
Analysis and Simulation Methodology for Strength Prediction of Open Hole Tension and 4-point Bending Plates Made of 2D Woven Fabrics
26/10/2021 11:00 conference time (CEST, Berlin)
Room: J
M. Bruyneel, F. Strepenne, A Rajaneesh (GDTech, BEL); F. Ravailler, N. Preud’homme (SAFRAN Nacelles, FRA)
M. Bruyneel, F. Strepenne, A Rajaneesh (GDTech, BEL); F. Ravailler, N. Preud’homme (SAFRAN Nacelles, FRA)
Open hole tension (OHT) and 4-point bending (4PB) tests are some of the standard qualification tests at coupon level in the aerospace industry. To minimize the number of tests and associated time and cost, it is essential to predict the failure strength using finite element models. In this context, current paper discusses the analysis and simulation methodology that was used to study OHT and 4PB specimens made of carbon/epoxy 2D woven plies. The approach is based on specific meso-models available in the literature (Ladeveze type models) for modeling intra and inter-laminar behaviors of woven plies laminates. The intra-laminar model can represent fiber failure, progressive damage in the matrix (cracks and de-cohesion between fibers and matrix) as well as permanent deformation. The inter-laminar model is used to represent progressive delamination between the plies of the laminate. Numerical simulations are conducted with the SAMCEF (Siemens) finite element software. The parameters of the material models are identified based on specific physical testing at the coupon level. When applied to OHT, a so-called non local approach must be used to handle in a proper way the stress concentration that occurs at the border of the hole, and avoid localisation of damage issues. The characteristic length associated to the non local approach is an additional parameter that must be determined based on test results for specific OHT configurations, and then, once identified, used in the finite element models for the other studied OHT configurations. Comparisons between physical and virtual testing demonstrate the necessity to take the non local effect into account, and the capability of the simulation methodology to predict failure strengths of the considered OHT specimens. The approach is then validated on 4PB cases, with different stacking sequences and numer of plies; here again, simulation is compared to physical test results. [1] Hochard C., Aubourg P.A., Charles J.P. (2001). Modelling of the mechanical behavior of woven fabric CFRP laminates up to failure. Composite Science & Technology, 61, pp. 221-230. [2] Allix O., Ladevèze P. (1992). Inter-laminar interface modelling for the prediction of delamination. Composite Structures, 22, pp. 235-242. [3] Jetteur P., Bruyneel M., Craveur J.C. (2019). Structures en matériaux composites – calcul par éléments finis. Dunod. [4] Lahellec N., Bordreuil C., Hochard C. (2005). Etude de la rupture fragile d’un stratifié quasi-isotrope à plis tissés : mise en évidence d’une longueur interne. C.R. Mécanique, 333, pp. 293-298. [5] SAMCEF. Siemens PLM software.
composite, damage
11:20
conference time (CEST, Berlin)
Inelastic Damage Attributes of Hyperelastic Fiber-reinforced Composites: Matrix-fiber Interface Debonding
26/10/2021 11:20 conference time (CEST, Berlin)
Room: J
M. Mansouri, P. Fuchs (Polymer Competence Center Leoben GmbH, AUT)
M. Mansouri, P. Fuchs (Polymer Competence Center Leoben GmbH, AUT)
It is apparent from the literature that the matrix-fiber mechanical interaction, as a result of adhesive bonds at interface, has a significant contribution to the constitutive modeling of hyperelastic fiber-reinforced materials. The present study represents an initial attempt to model matrix-fiber interface debonding in the context of pseudo-elasticity and, moreover, to characterize and computationally evaluate it. For this, inelastic phenomena such as discontinuous Mullins-type softening and permanent set as a result of the matrix damage, the fiber rupture, and the matrix-fiber interface debonding are modeled. The pseudo-elastic model is based on hyperelastic strain energy functions with two damage variables for each of the matrix, the fibers, and the matrix-fiber mechanical interactions. Each of the material and damage parameters are characterized independently through performing a comprehensive set of monotonic loading and cyclic tensile tests, respectively. The results of the cyclic tensile tests on the pure matrix, fibers, and composites imply that the underlying mechanism producing the Mullins-type softening and permanent deformations can be attributed to the matrix-fiber interface debonding neither matrix nor fibers. It is supported by the matrix-fiber interface debonding observed using the micrographs captured during in situ tensile tests and is reflected in the residual deformations recorded through the stretch maps via digital image correlation after unloading. The in situ optical micrographs are indicative of two different microstructural evolutions: A continuous matrix-fiber debonding along the fibers, which is barely visible after unloading; and a discontinuous, pointwise matrix-fiber debonding, starting from the top vicinity of the fibers and continued to the matrix. Furthermore, an FE-implementation using a user-defined subroutine is presented and compared against experimental data. The results show that the model is capable of reproducing the inelastic behaviors as a result of the matrix-fiber debonding for composites showing significant degradation in their mechanical properties. This work bridges the degradation of the mechanical properties to the microscopically visible matrix-fiber interface debonding for composites undergoing cyclic deformations.
Damage mechanics, Microstructural evolution, Mullins-type softening, Permanent deformation, Matrix-fiber debonding
11:40
conference time (CEST, Berlin)
Data Requirements for Detecting Collision Positions on Fiber Composite Plates Using Artificial Intelligence
26/10/2021 11:40 conference time (CEST, Berlin)
Room: J
A. Raichle (Universität Stuttgart IFB, DEU); A. Damm (Bosch Sensortec GmbH, DEU); Prof. Dr. P. Middendorf (University of Stuttgart, Institute of Aircraft Design, DEU)
A. Raichle (Universität Stuttgart IFB, DEU); A. Damm (Bosch Sensortec GmbH, DEU); Prof. Dr. P. Middendorf (University of Stuttgart, Institute of Aircraft Design, DEU)
With fiber composite components, even relatively weak impacts can lead to delaminations. These are often barely visible and have to be detected at great expense. To avoid such damage from being detected only during a maintenance cycle, a fiber composite component can be monitored by means of structural health monitoring and the live data can be evaluated in real time with the aid of artificial intelligence. This can be done by monitoring local accelerations using integrated sensors. If the sensors detect an event that exceeds a predefined acceleration value, the signal curve around this maximum event can be evaluated. For this purpose, the signal course can be converted into a pixel-poor spectrogram, which can then be trained into a convolutional neural network. Previous work has investigated three piezo elements or three MEMS sensors for live monitoring of the acceleration values. Although the different sensor types have a large difference in maximum sampling rate, both were able to achieve very high accuracies in position determination. In this work, the required data quality regarding the sampling rate of the accelerometer and the required recorded time interval around the impact event is investigated and evaluated. It is shown that a sampling rate of less than 1000 acceleration values per second is already sufficient to be able to reliably determine the impact position for the present use case. In addition, it is shown that mainly the first oscillation of the acceleration impact is important for the position determination. The next oscillations have little effect on the quality of the algorism. With these results, both, the amount of data needed for training and the amount of data needed for field use can be defined and thus greatly reduced. The authors wish to acknowledge the funding provided by the Federal Ministry of Education and Research Germany within the Research campus ARENA2036 – Digitaler Fingerabdruck.
impact detection, structural health monitoring, artificial intelligence, convolutional neural network, fiber-reinforced composites
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