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Analysis of intralaminar cracking in 90-plies of GF/EP laminates with distributed ply strength
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.ORCID iD: 0000-0002-7524-0661
RISE SICOMP, Sweden; Department of Management and Engineering, Linköping University, Linköping University, Sweden.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Laboratory of Experimental Mechanics of Materials, Riga Technical University, Riga Technical University, Latvia.ORCID iD: 0000-0001-9649-8621
2021 (English)In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 55, no 26, p. 3925-3942Article in journal (Refereed) Published
Abstract [en]

Intralaminar cracking in relatively thick 90-plies of [(Formula presented.)]s laminates is analyzed using experimental data for two Glass fiber/Epoxy (GF/EP) material systems. Weibull parameters for transverse failure stress of the 90-ply are obtained from experimental intralaminar crack density versus applied strain data, showing that a reliable analysis requires sufficient amount of data in so called noninteractive crack density region. Monte Carlo simulations of cracking were performed using stress distribution between two cracks calculated using two models: Hashin’s model and a novel model that ensures that the average stress is exactly the same as in FEM solution. Due to its features, the Hashin’s model predicts too low intralaminar crack density (it predicts too strong interaction between cracks). The results emphasize the importance of having a proper stress distribution model when performing Monte Carlo simulations. Simulations were used not only to simulate intralaminar cracking in high and very low crack density regions but also for improving the procedure of Weibull parameter determination.

Place, publisher, year, edition, pages
Sage Publications, 2021. Vol. 55, no 26, p. 3925-3942
Keywords [en]
initiation strength, intralaminar cracking, Monte Carlo simulations, Transverse cracks, Weibull distribution
National Category
Composite Science and Engineering
Research subject
Polymeric Composite Materials
Identifiers
URN: urn:nbn:se:ltu:diva-86463DOI: 10.1177/00219983211027346ISI: 000682123000001Scopus ID: 2-s2.0-85109421846OAI: oai:DiVA.org:ltu-86463DiVA, id: diva2:1581894
Funder
Vinnova
Note

Validerad;2021;Nivå 2;2021-11-09 (beamah);

Ytterligare forskningsfinansiär: GKN Aerospace Engines Sweden

Available from: 2021-07-27 Created: 2021-07-27 Last updated: 2024-08-15Bibliographically approved
In thesis
1. Analysis of transverse cracking in cross-ply laminates: Weibull distribution based approach
Open this publication in new window or tab >>Analysis of transverse cracking in cross-ply laminates: Weibull distribution based approach
2022 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Fiber reinforced polymer composite laminates make up more than 50% of modern aircrafts. Such composite laminates are exposed to various environmental and in-service thermo-mechanical load conditions. Transverse/intralaminar cracking is usually the first form of damage appears in a composite laminate and they tend to increase in number during the service life. The growth in number of these cracks significantly degrades the thermo-elastic properties of the composite laminate and eventually leads to final failure. Thus, it is important to predict the crack density (number of cracks per unit length) growth in both non-interactive crack density region and interactive crack density region and its effect in thermo-elastic properties degradation. Non-interactive crack density region is the region where the cracks are far apart and stress perturbation between cracks do not overlap. Interactive crack density region is where the cracks are close to each other and stress perturbation between cracks overlaps and affects the formation of new cracks. In this study, transverse cracks in thick Glass Fiber Epoxy (GF/EP) cross-ply composite laminates under quasi-static tensile loading and tension-tension fatigue loading have been analyzed and predicted.

In the first paper attached here, increase in number of transverse cracks in GF/EP cross-ply laminates under quasi-static tensile loading at room temperature (RT) are analyzed using 2 material systems. The failure stress distribution in 90° plies of the laminates is defined by Weibull distribution and the Weibull parameters are determined from crack density versus applied thermo-mechanical transverse stress in 90° layer (σTCLT) data points within the non-interactive crack density region. The crack density growth is then predicted versus the σTCLT and applied mechanical strain in the laminate from the determined Weibull parameters using Monte Carlo method and the stress distribution models between adjacent cracks. The predicted results using the novel stress distribution model introduced here were in good agreement with the non-interactive and interactive crack density regions of test results. The importance of using the Monte Carlo method and novel stress distribution model to predict the whole crack density region have been emphasized in the article, in addition to that it also redefined the interval of non-interactive crack density region. 

The second paper expands the concept from the first paper, to address the tension-tension fatigue loading at RT. It deals with the crack density analysis and prediction in [0/90]s GF/EP laminate under fatigue loading at RT. The fatigue tests were performed at 3 maximum stress levels. Here the Weibull parameters were determined from the data points within the non-interactive crack density region in quasi-static and fatigue loading. From the determined Weibull parameters of each stress level and using Monte Carlo method and the novel stress distribution model, the crack density versus the number of fatigue cycles were predicted and in good agreement with the fatigue test results at the respective stress level. The intention here was to use Weibull parameters of one stress level to predict crack density at arbitrary stress levels. Based on it, the predicted results were not sufficiently good and suggested to revisit the Weibull parameter determination by performing fatigue tests at two stress levels. 

In the attached paper 3, new methodology on crack density growth simulation and Weibull parameter determination in tension-tension fatigue loading has been developed. In the newly developed methodology, in detailed fatigue tests are performed at one maximum stress level to obtain all data points and at higher stress level to obtain one data point that is a crack density data point at certain number of cycles to determine Weibull parameters. Using the determined Weibull parameters from non-interactive crack density region, the whole crack density region was successfully predicted for other stress levels.

Place, publisher, year, edition, pages
Luleå University of Technology, 2022
Series
Licentiate thesis / Luleå University of Technology, ISSN 1402-1757
Keywords
Intralmainar cracking, Weibull distribution, Failure stress, Monte Carlo simulation, Fatigue loading, Quasi-static loading
National Category
Composite Science and Engineering
Research subject
Polymeric Composite Materials
Identifiers
urn:nbn:se:ltu:diva-90140 (URN)978-91-8048-059-8 (ISBN)978-91-8048-060-4 (ISBN)
Presentation
2022-06-13, E246, E Building (TVM), Luleå University of Technology, Luleå, 10:00 (English)
Opponent
Supervisors
Available from: 2022-04-11 Created: 2022-04-11 Last updated: 2023-09-06Bibliographically approved
2. Transverse cracking in cross-ply composites during static and fatigue loading at different temperatures
Open this publication in new window or tab >>Transverse cracking in cross-ply composites during static and fatigue loading at different temperatures
2024 (English)Doctoral thesis, comprehensive summary (Other academic)
Alternative title[sv]
Transversella sprickor i korslaminat under statisk och utmattningsbelastning vid olika temperaturer
Abstract [en]

Polymer composite laminates are preferred in many load bearing applications for its tailorable mechanical properties while offering light-weight solution, corrosive resistance etc. Hence, polymer composites are attractive material choice for aircraft manufacturers to reduce weight and emissions. However, one of the challenges existing in composite laminates is accumulation of damage before final failure, that reduces mechanical properties of the composite laminates during service life. Hence it is crucial to develop a reliable model to predict damage and consequently mechanical properties degradation. The thesis focuses on transverse/intralaminar cracks, that are the first form of damage to appear in off-axis layers of composite laminates when subjected to tensile load and they increase in number with increase in load. Transverse crack growth in numbers was analyzed in terms of transverse crack density (= number of cracks / observed length) growth. Appended papers present methodologies developed using statistical transverse failure stress distribution approach to predict the transverse crack density growth when composite laminates subjected to quasi-static tensile and tension-tension fatigue loading at different temperatures. For that purpose, continuous fibers reinforced polymer composite cross-ply laminates containing different material systems were manufactured, and damage growth was studied in 90-layer in coupon scale specimens. In static tests, the crack density growth in specimens were analyzed against the thermo-mechanical transverse stress in the 90-layer. Distribution of transverse failure stress to initiate a crack along the transverse direction of the layer has been defined using 2 parameter Weibull distribution model. Paper 1 presents, methodology to predict crack density growth, using probability of failure stress distribution (based on Weibull model) in Monte Carlo simulation along with the developed stress distribution model between cracks, in specimens tested at room temperature (RT). The crack density was well predicted in both non-interactive and interactive crack density region using improved Weibull parameter determination routine. The presented Weibull model was extended to address the effect of iso-thermal heat treatment and test temperature in Paper 4. It was observed that both heat treatment and elevated test temperatures, in general, resulted in reduction of transverse cracking resistance. The effect of heat treatment and test temperature on transverse cracking was modelled as Weibull scale parameter dependency using polynomial expression. The developed model was validated against laminates with same material system but with different layups and fiber content.Fatigue tests were performed at different maximum stress levels and at RT and 150℃. Crack density growth was analyzed against number of fatigue cycles. The observed decrease in resistance to transverse cracking with every cycle of load was interpreted as monotonic decrease of Weibull scale parameter. Simple power function with respect to number of cycles was proposed to decrease the scale parameter. Paper 2 presents, fatigue test results at RT and methodology to predict crack density growth in different fatigue stress levels. The methodology, using maximum local transverse stress in a fatigue cycle in Weibull model and the Weibull parameters determined at a reference fatigue stress level, was limited in ability to predict the crack density growth at other stress levels. It was then found that the crack density growth not only depends on maximum local stress in a fatigue cycle, but also on the local stress ratio in the 90-layer, presented in Paper 3. Wherein, an equivalent stress was introduced to replace maximum local stress in Weibull model by addressing the combined effect of maximum local stress in a cycle and also the local stress ratio. Equivalent stress model was validated across different layups and fiber content with same material system. Paper 5 presents fatigue test results at different stress levels and temperatures. It was found that in fatigue tests at 150℃, in spite of lower thermal stress, crack density growth was more rapid than for RT fatigue tests. Methodology to predict crack density growth in 150℃ fatigue tests by combining the analytical model with the equivalent stress and with enhanced test temperature effect has been presented. 

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2024
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
Keywords
Polymer composite laminate, Static loading, Fatigue loading, Elevated temperature effect, Weibull distribution model
National Category
Composite Science and Engineering
Research subject
Polymeric Composite Materials
Identifiers
urn:nbn:se:ltu:diva-104373 (URN)978-91-8048-490-9 (ISBN)978-91-8048-491-6 (ISBN)
Public defence
2024-05-03, E632, Luleå University of Technology, Luleå, 10:00 (English)
Opponent
Supervisors
Available from: 2024-02-23 Created: 2024-02-23 Last updated: 2024-04-12Bibliographically approved

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