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23 pages, 8185 KiB

Open AccessArticle

Elevated Strain Rate Characterization of Compression Molded Direct/In-Line Compounded Carbon Fibre/Polyamide 66 Long Fibre Thermoplastic

by Matthew Bondy, Pouya Mohammadkhani, John Magliaro and William Altenhof

Materials 2022, 15(21), 7667; https://doi.org/10.3390/ma15217667 - 31 Oct 2022

Cited by 4 | Viewed by 1921

Abstract

Compression molded direct compounded carbon fibre D-LFT was evaluated at quasi-static strain rates through uniaxial tension tests (including a specimen size study) and a variation of the ISO 6603-2 puncture test. No significant size effects were observed for the modulus or strength obtained [...] Read more.

Compression molded direct compounded carbon fibre D-LFT was evaluated at quasi-static strain rates through uniaxial tension tests (including a specimen size study) and a variation of the ISO 6603-2 puncture test. No significant size effects were observed for the modulus or strength obtained from tensile specimens with four gauge lengths (6.25 mm to 57 mm). Failure strain decreased by 27.5%/29.9%, respectively, across the gauge length range for the 0°/90° directions. Intermediate strain rate (10 s⁻¹ to 200 s⁻¹) characterization was completed through uniaxial tension tests on a novel apparatus and ISO 6603-2 puncture tests. Intermediate rate tensile tests showed minimal rate sensitivity for the 0°/90° directions. Initial stiffness was 50% higher for ISO 6603-2 impact tests compared to quasi-static tests. Displacement at the onset of fracture was 95% lower for impact tests compared to quasi-static loading. The peak force/displacement at peak force were reduced for impact tests (21% and 20%, respectively) compared to quasi-static testing. Full article

(This article belongs to the Special Issue Mechanical Characterization of FRP Composite Materials)

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10 pages, 1253 KiB

Open AccessArticle

Influence of Test Specimen Geometry on Probability of Failure of Composites Based on Weibull Weakest Link Theory

by Rajnish Kumar, Bo Madsen, Hans Lilholt and Lars P. Mikkelsen

Materials 2022, 15(11), 3911; https://doi.org/10.3390/ma15113911 - 31 May 2022

Cited by 4 | Viewed by 1563

Abstract

This paper presents an analytical model that quantifies the stress ratio between two test specimens for the same probability of failure based on the Weibull weakest link theory. The model takes into account the test specimen geometry, i.e., its shape and volume, and [...] Read more.

This paper presents an analytical model that quantifies the stress ratio between two test specimens for the same probability of failure based on the Weibull weakest link theory. The model takes into account the test specimen geometry, i.e., its shape and volume, and the related non-constant stress state along the specimen. The proposed model is a valuable tool for quantifying the effect of a change of specimen geometry on the probability of failure. This is essential to distinguish size scaling from the actual improvement in measured strength when specimen geometry is optimized, aiming for failure in the gauge section. For unidirectional carbon fibre composites with Weibull modulus m in the range 10–40, it can be calculated by the model that strength measured with a straight-sided specimen will be 1–2% lower than the strength measured with a specific waisted butterfly-shaped specimen solely due to the difference in test specimen shape and volume. Full article

(This article belongs to the Special Issue Mechanical Characterization of FRP Composite Materials)

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17 pages, 27942 KiB

Open AccessArticle

Identification of Multiple Mechanical Properties of Laminates from a Single Compressive Test

by Bo Gao, Huai Yan, Boyi Wang, Qiang Yang, Songhe Meng and Yanyan Huo

Materials 2022, 15(8), 2950; https://doi.org/10.3390/ma15082950 - 18 Apr 2022

Cited by 1 | Viewed by 1951

Abstract

In-plane elastic and interlaminar properties of composite laminates are commonly obtained through separate experiments. In this paper, a simultaneous identification method for both properties using a single experiment is proposed. The mechanical properties of laminates were treated as uncertainties and Bayesian inference was [...] Read more.

In-plane elastic and interlaminar properties of composite laminates are commonly obtained through separate experiments. In this paper, a simultaneous identification method for both properties using a single experiment is proposed. The mechanical properties of laminates were treated as uncertainties and Bayesian inference was employed with measured strain-load curves in compression tests of laminates with embedded delamination. The strain–load curves were separated into two stages: the pre-delamination stage and the post-delamination stage. Sensitivity analysis was carried out to determine the critical properties at different stages, in order to alleviate the ill-posed problem in inference. Results showed that the in-plane Young’s modulus and shear modulus in elastic properties are dominant in the pre-delamination stage, and the interlaminar strength and type I fracture toughness in interlaminar properties are dominant in the post-delamination stage. Five times of property identification were carried out; the maximum coefficient of variation of identified properties was less than 1.11%, and the maximum error between the mean values of the identified properties and the ones from standard experiments was less than 5.44%. The proposed method can reduce time and cost in obtaining multiple mechanical properties of laminates. Full article

(This article belongs to the Special Issue Mechanical Characterization of FRP Composite Materials)

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15 pages, 1420 KiB

Open AccessArticle

A New Failure Theory and Importance Measurement Analysis for Multidirectional Fiber-Reinforced Composite Laminates with Holes

by Shu Li and Fei Han

Materials 2022, 15(6), 2227; https://doi.org/10.3390/ma15062227 - 17 Mar 2022

Cited by 5 | Viewed by 1748

Abstract

In this paper, a failure theory for the multidirectional fiber-reinforced composite laminate with a circular hole is developed. In this theory, the finite fracture mechanics method is combined with the improved Puck’s failure theory including the in situ strength effect. It can predict [...] Read more.

In this paper, a failure theory for the multidirectional fiber-reinforced composite laminate with a circular hole is developed. In this theory, the finite fracture mechanics method is combined with the improved Puck’s failure theory including the in situ strength effect. It can predict the notched strength by only basic material properties of unidirectional laminas, geometries and stacking sequence of the laminate. In advance mechanical properties of the laminate are unnecessary. The notched laminates with different material types and stacking sequences are taken as examples to verify this failure theory, and predicted results are in good agreement with experiments. Based on the developed failure theory, importance measurement of uncertain material properties to the notched strength is analysed. Results show that notched strength increases with increasing longitudinal tensile strength and in-plane shear modulus for the laminate with an arbitrary hole diameter. However, it decreases with increasing transverse modulus. Full article

(This article belongs to the Special Issue Mechanical Characterization of FRP Composite Materials)

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18 pages, 7778 KiB

Open AccessArticle

Effect of Strain Rate on the Transverse Tension and Compression Behavior of a Unidirectional Non-Crimp Fabric Carbon Fiber/Snap-Cure Epoxy Composite

by Khizar Rouf, Aaditya Suratkar, Jose Imbert-Boyd, Jeffrey Wood, Michael Worswick and John Montesano

Materials 2021, 14(23), 7314; https://doi.org/10.3390/ma14237314 - 29 Nov 2021

Cited by 4 | Viewed by 2046

Abstract

The strain rate-dependent behavior of a unidirectional non-crimp fabric (UD-NCF) carbon fiber/snap-cure epoxy composite loaded along the transverse direction under quasi-static and dynamic conditions was characterized. Transverse tension and compression tests at quasi-static and intermediate strain rates were performed using hydraulic testing machines, [...] Read more.

The strain rate-dependent behavior of a unidirectional non-crimp fabric (UD-NCF) carbon fiber/snap-cure epoxy composite loaded along the transverse direction under quasi-static and dynamic conditions was characterized. Transverse tension and compression tests at quasi-static and intermediate strain rates were performed using hydraulic testing machines, while a split Hopkinson pressure bar (SHPB) apparatus was used for transverse compression tests at high strain rates. A pulse shaper was used on the SHPB apparatus to ensure dynamic equilibrium was achieved and that the test specimens deformed homogenously with a nearly constant strain rate. The transverse tensile strength at a strain rate of 16 s⁻¹ increased by 16% when compared to that at quasi-static strain rates, while distinct localized fracture surface morphology was observed for specimens tested at different strain rates. The transverse compressive yield stress and strength at a strain rate of 325 s⁻¹ increased by 94% and 96%, respectively, when compared to those at quasi-static strain rates. The initial fracture plane orientation for the transverse compression tests was captured with high-speed cameras and found to increase with increasing strain rate. The study provides an important data set for the strain rate-dependent response of a UD-NCF composite material, while the qualitative fracture surface observations provide a deeper understanding of the failure characteristics. Full article

(This article belongs to the Special Issue Mechanical Characterization of FRP Composite Materials)

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23 pages, 94128 KiB

Open AccessArticle

Experimental and Numerical Comparison of Impact Behavior between Thermoplastic and Thermoset Composite for Wind Turbine Blades

by Thiago Henrique Lara Pinto, Waseem Gul, Libardo Andrés González Torres, Carlos Alberto Cimini, Jr. and Sung Kyu Ha

Materials 2021, 14(21), 6377; https://doi.org/10.3390/ma14216377 - 25 Oct 2021

Cited by 13 | Viewed by 3076

Abstract

Damage generated due to low velocity impact in composite plates was evaluated focusing on the design and structural integrity of wind turbine blades. Impact properties of composite plates manufactured with thermoplastic and thermoset resins for different energy levels were measured and compared. Specimens [...] Read more.

Damage generated due to low velocity impact in composite plates was evaluated focusing on the design and structural integrity of wind turbine blades. Impact properties of composite plates manufactured with thermoplastic and thermoset resins for different energy levels were measured and compared. Specimens were fabricated using VARTM (vacuum assisted resin transfer molding), using both matrix systems in conjunction with carbon, glass and carbon/glass hybrid fibers in the NCF (non-crimp fabric) architecture. Resin systems used were ELIUM 188O (thermoplastic) from Arkema Co., Ltd. and a standard epoxy reference, EPR-L20 from Hexion Co., Ltd. (thermoset). Auxiliary numerical finite element analyses were performed to better understand the tests physics. These models were then compared with the experimental results to verify their predictive capacity, given the intrinsic limitations due to their simplicity. Based in the presented results, it is possible to observe that ELIUM is capable to replace a conventional thermoset matrix. The thermoplastic panels presented similar results compared to its thermoset counterparts, with even a trend of less impact damage. Additionally, for both thermoplastic and thermoset resin systems, glass layups showed the lowest levels of damage while carbon panels presented the highest damage levels. Hybrid laminates can be applied as a compromise solution. Full article

(This article belongs to the Special Issue Mechanical Characterization of FRP Composite Materials)

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9 pages, 3888 KiB

Open AccessArticle

Strain Acquisition Framework and Novel Bending Evaluation Formulation for Compression-Loaded Composites Using Digital Image Correlation

by Jonas J. A. D’haen, Michael May, Octavian Knoll, Stefan Kerscher and Stefan Hiermaier

Materials 2021, 14(20), 5931; https://doi.org/10.3390/ma14205931 - 9 Oct 2021

Cited by 3 | Viewed by 1659

Abstract

Consistent and reproducible data are key for material characterization. This work presents digital image correlation (DIC) strain acquisition guidelines for compression-loaded carbon fiber composites. Additionally, a novel bending criterion is formulated which builds up on the DIC strain data so that it is [...] Read more.

Consistent and reproducible data are key for material characterization. This work presents digital image correlation (DIC) strain acquisition guidelines for compression-loaded carbon fiber composites. Additionally, a novel bending criterion is formulated which builds up on the DIC strain data so that it is able to completely replace state-of-the-art tactile strain devices. These guidelines are derived from a custom test setup that simultaneously investigates the front and side view of the specimen. They reflect both an observation and post-processing standpoint. It is found that the DIC-based strain progress matches closely with state-of-the-art strain gauges up to failure initiation. The new bending evaluation criterion allows the bending state—and therefore, the validity of the compression test—to be monitored analogously to the methodology defined in the standards. Furthermore, the new bending criterion eliminates a specific bending mode, caused by an offset of clamps, which cannot be detected by the traditional strain gauge-based monitoring approach. Full article

(This article belongs to the Special Issue Mechanical Characterization of FRP Composite Materials)

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20 pages, 4317 KiB

Open AccessArticle

Experimental Method for Tensile Testing of Unidirectional Carbon Fibre Composites Using Improved Specimen Type and Data Analysis

by Rajnish Kumar, Lars P. Mikkelsen, Hans Lilholt and Bo Madsen

Materials 2021, 14(14), 3939; https://doi.org/10.3390/ma14143939 - 14 Jul 2021

Cited by 17 | Viewed by 4555

Abstract

This paper presents an experimental method for tensile testing of unidirectional carbon fibre composites. It uses a novel combination of a new specimen geometry, protective layer, and a robust data analysis method. The experiments were designed to test and analyze unprotected (with conventional [...] Read more.

This paper presents an experimental method for tensile testing of unidirectional carbon fibre composites. It uses a novel combination of a new specimen geometry, protective layer, and a robust data analysis method. The experiments were designed to test and analyze unprotected (with conventional end-tabs) and protected (with continuous end-tabs) carbon fibre composite specimens with three different specimen geometries (straight-sided, butterfly, and X-butterfly). Initial stiffness and strain to failure were determined from second-order polynomial fitted stress–strain curves. A good agreement between back-calculated and measured stress–strain curves is found, on both composite and fibre level. For unprotected carbon composites, the effect of changing specimen geometry from straight-sided to X-butterfly was an increase in strain to failure from 1.31 to 1.44%. The effect of protection on X-butterfly specimens was an increase in strain to failure from 1.44 to 1.53%. For protected X-butterfly specimens, the combined effect of geometry and protection led to a significant improvement in strain to failure of 17% compared to unprotected straight-sided specimens. The observed increasing trend in the measured strain to failure, by changing specimen geometry and protection, suggests that the actual strain to failure of unidirectional carbon composites is getting closer to be realized. Full article

(This article belongs to the Special Issue Mechanical Characterization of FRP Composite Materials)

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Graphical abstract

15 pages, 10928 KiB

Open AccessArticle

Permanent Deformation and Stiffness Degradation of Open Hole Glass/PA6 UD Thermoplastic Composite in Tension and Compression

by Ruben Dirk Sevenois, Xiaoyu Yang, Erik Verboven, Mathias Kersemans and Wim Van Paepegem

Materials 2021, 14(10), 2646; https://doi.org/10.3390/ma14102646 - 18 May 2021

Cited by 1 | Viewed by 2037

Abstract

UD glass/PA6 coupons with an open hole are subjected to tensile and compressive loading. Three layups: [0/90]_5s, [+45/−45]_5s and [+45/0/−45/90]_3s with a shape based on ASTM D5766 were tested. Both monotonic loading as well as loading–unloading–reloading tests were executed. [...] Read more.

UD glass/PA6 coupons with an open hole are subjected to tensile and compressive loading. Three layups: [0/90]_5s, [+45/−45]_5s and [+45/0/−45/90]_3s with a shape based on ASTM D5766 were tested. Both monotonic loading as well as loading–unloading–reloading tests were executed. The strain field on the sample surface was measured with digital image correlation. This allowed identifying the distribution of the strain field during loading, permanent deformation and the evolution of the sample elastic modulus. This information is not frequently measured. Yet, it is vital for the development and validation of advanced failure models. The results indicate that the thermoplastic matrix allows large plastic deformation under tensile loading for the specimens with layup [+45/−45]_5s. In addition, the specimen elastic modulus reduces by about 70%. The other layups show minor permanent deformation, while the elastic modulus reduces by up to 15%. Furthermore, the quasi-isotropic laminate shows a significant post-failure load-bearing capacity under compression loading. The results are complemented with post-mortem damage and fracture observations using optical microscopy and ultrasound inspection. Full article

(This article belongs to the Special Issue Mechanical Characterization of FRP Composite Materials)

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15 pages, 48698 KiB

Open AccessArticle

Vibro-Thermal Wave Radar: Application of Barker Coded Amplitude Modulation for Enhanced Low-Power Vibrothermographic Inspection of Composites

by Saeid Hedayatrasa, Joost Segers, Gaétan Poelman, Wim Van Paepegem and Mathias Kersemans

Materials 2021, 14(9), 2436; https://doi.org/10.3390/ma14092436 - 7 May 2021

Cited by 13 | Viewed by 2071

Abstract

This paper proposes an efficient non-destructive testing technique for composite materials. The proposed vibro-thermal wave radar (VTWR) technique couples the thermal wave radar imaging approach to low-power vibrothermography. The VTWR is implemented by means of a binary phase modulation of the vibrational excitation, [...] Read more.

This paper proposes an efficient non-destructive testing technique for composite materials. The proposed vibro-thermal wave radar (VTWR) technique couples the thermal wave radar imaging approach to low-power vibrothermography. The VTWR is implemented by means of a binary phase modulation of the vibrational excitation, using a 5 bit Barker coded waveform, followed by matched filtering of the thermal response. A 1D analytical formulation framework demonstrates the high depth resolvability and increased sensitivity of the VTWR. The obtained results reveal that the proposed VTWR technique outperforms the widely used classical lock-in vibrothermography. Furthermore, the VTWR technique is experimentally demonstrated on a 5.5 mm thick carbon fiber reinforced polymer coupon with barely visible impact damage. A local defect resonance frequency of a backside delamination is selected as the vibrational carrier frequency. This allows for implementing VTWR in the low-power regime (input power < 1 W). It is experimentally shown that the Barker coded amplitude modulation and the resultant pulse compression efficiency lead to an increased probing depth, and can fully resolve the deep backside delamination. Full article

(This article belongs to the Special Issue Mechanical Characterization of FRP Composite Materials)

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17 pages, 5585 KiB

Open AccessArticle

Creep Testing of Thermoplastic Fiber-Reinforced Polymer Composite Tubular Coupons

by Hai Giang Minh Doan and Pierre Mertiny

Materials 2020, 13(20), 4637; https://doi.org/10.3390/ma13204637 - 17 Oct 2020

Cited by 12 | Viewed by 4750

Abstract

Thermoplastic fiber-reinforced polymer composites (TP-FRPC) are gaining popularity in industry owing to characteristics such as fast part fabrication, ductile material properties and high resistance to environmental degradation. However, TP-FRPC are prone to time-dependent deformation effects like creep under sustained loading, which can lead [...] Read more.

Thermoplastic fiber-reinforced polymer composites (TP-FRPC) are gaining popularity in industry owing to characteristics such as fast part fabrication, ductile material properties and high resistance to environmental degradation. However, TP-FRPC are prone to time-dependent deformation effects like creep under sustained loading, which can lead to significant dimensional changes and affect the safe operation of structures. Previous research in this context has focused, mainly, on testing of flat coupons. In this study, a creep testing method for TP-FRPC tubular coupons was developed. Specimens were fabricated using tape winding and subjected to well-defined loading conditions, i.e., pure hoop tensile and pure axial compressive stress. Strain gauges and digital image correlation were both employed for strain measurements and were found to be in good agreement. The evolution of strain rate, Poisson’s ratio and creep compliance were investigated. The prediction of experimental data by the Burgers model and the Findley’s power law model were explored. The research findings suggest that the developed experimental and analysis approach provides valuable information for the design of material systems and structures. Full article

(This article belongs to the Special Issue Mechanical Characterization of FRP Composite Materials)

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Review

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23 pages, 2798 KiB

Open AccessReview

Residual Properties in Damaged Laminated Composites through Nondestructive Testing: A Review

by Carlo Boursier Niutta, Andrea Tridello, Davide S. Paolino and Giovanni Belingardi

Materials 2021, 14(16), 4513; https://doi.org/10.3390/ma14164513 - 11 Aug 2021

Cited by 8 | Viewed by 1954

Abstract

The development of damage tolerance strategies in the design of composite structures constitutes a major challenge for the widespread application of composite materials. Damage tolerance approaches require a proper combination of material behavior description and nondestructive techniques. In contrast to metals, strength degradation [...] Read more.

The development of damage tolerance strategies in the design of composite structures constitutes a major challenge for the widespread application of composite materials. Damage tolerance approaches require a proper combination of material behavior description and nondestructive techniques. In contrast to metals, strength degradation approaches, i.e., the residual strength in presence of cracks, are not straightforwardly enforceable in composites. The nonhomogeneous nature of such materials gives rise to several failure mechanisms and, therefore, the definition of an ultimate load carrying capacity is ambiguous. Nondestructive techniques are thus increasingly required, where the damage severity is quantified not only in terms of damage extension, but also in terms of material response of the damaged region. Based on different approaches, many nondestructive techniques have been proposed in the literature, which are able to provide a quantitative description of the material state. In the present paper, a review of such nondestructive techniques for laminated composites is presented. The main objective is to analyze the damage indexes related to each method and to point out their significance with respect to the residual mechanical performances, as a result of the working principle of each retained technique. A possible guide for future research on this subject is thus outlined. Full article

(This article belongs to the Special Issue Mechanical Characterization of FRP Composite Materials)

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