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Fracture Mechanics and Structural Integrity of Composite Materials
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Fracture Mechanics and Structural Integrity of Composite Materials

A special issue of Applied Sciences (ISSN 2076-3417). This special issue belongs to the section "Mechanical Engineering".

Deadline for manuscript submissions: closed (20 November 2021) | Viewed by 17888

Special Issue Editor

Dr. Ana Martins Amaro

E-Mail Website
Guest Editor
Department of Mechanical Engineering, University of Coimbra, 3030-788 Coimbra, Portugal
Interests: composites structures; nanocomposite; structural integrity; finite element analysis; biomechanics; impact; non-destructive analysis; mechanical properties
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

Composite materials have been increasingly used because of their high specific strength and stiffness, good fatigue performance and corrosion resistance. However, in many cases, there are some problems in their application as consequence of the poor tolerance to damage. Composite materials are very susceptible to degradation/damage which can reduce significantly their structural integrity. For example, when submitted to impact events, like low velocity impact, which can occur in-service or during the maintenance activities. Also, composite structures can be exposed to a range of corrosive environments during their in-service life, which causes degradation in terms of material properties, where irreversible material degradation occurs and some chemical changes can be found. For example, composite pipes are largely used in the chemical industry, building and infrastructures. Fracture mechanics is used to predict and diagnose failure of a component with an existing crack or flaw. In this context, their presence magnifies the stress in the vicinity of the crack and may result in failure prior to that predicted using traditional strength-of-materials methods.

Hence, this Special Issue intends to contribute to the publication of reports containing increase the state-of-the-art related to composite materials, namely in terms of fracture mechanics and structural integrity. For this purpose, experimental and numerical studies are welcome.

It is my pleasure to invite you to publish your original research contributions and reviews in the Special Issue, “Fracture mechanics and structural integrity of composite materials”, of Applied Sciences.

Dr. Ana Paula Betencourt Martins Amaro
Guest Editor

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Keywords

  • Composite materials
  • Fracture mechanics
  • Crack propagation
  • Environmental degradation
  • Structural integrity
  • Finite element method

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Published Papers (7 papers)

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Research

Jump to: Review

12 pages, 3267 KiB  
Article
Seawater Effect on Fatigue Behaviour of Notched Carbon/Epoxy Laminates
by Ricardo Branco, Paulo N. B. Reis, Maria A. Neto, José D. Costa and Ana M. Amaro
Appl. Sci. 2021, 11(24), 11939; https://doi.org/10.3390/app112411939 - 15 Dec 2021
Cited by 10 | Viewed by 1796
Abstract
This paper studies the effect of seawater immersion on the fatigue behavior of notched carbon/epoxy laminates. Rectangular cross-section specimens with a central hole were immersed in natural and artificial seawater for different immersion times (0, 30 and 60 days), being the water absorption [...] Read more.
This paper studies the effect of seawater immersion on the fatigue behavior of notched carbon/epoxy laminates. Rectangular cross-section specimens with a central hole were immersed in natural and artificial seawater for different immersion times (0, 30 and 60 days), being the water absorption rate evaluated over time. After that, fatigue tests were performed under uniaxial cyclic loading using a stress ratio equal to 0.1. After the tests, the optical microscopy technique allowed the examination of the failure micro-mechanisms at the fracture surfaces. The results showed that saturation appeared before 30 days of immersion and that water absorption rates were similar for natural and artificial seawater. The S–N curves showed that the seawater immersion affects the fatigue strength, but there were no relevant effects associated with the type of seawater. Moreover, it was also clear that fatigue life was similar for long lives, close to 1 million cycles, regardless of the immersion time or the type of seawater. On the contrary, for short lives, near 10 thousand cycles, the stress amplitude of dry laminates was 1.2 higher than those immersed in seawater. The failure mechanisms were similar for all conditions, evidencing the fracture of axially aligned fibres and longitudinal delamination between layers. Full article
(This article belongs to the Special Issue Fracture Mechanics and Structural Integrity of Composite Materials)
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Figure 1

Figure 1
<p>Specimen geometry used in the fatigue tests (units: mm).</p> Full article ">Figure 2
<p>Experimental apparatus used in the uniaxial stress-controlled fatigue tests.</p> Full article ">Figure 3
<p>S–N curve for the series: (<b>a</b>) without seawater immersion, (<b>b</b>) 30-day immersion in natural seawater, (<b>c</b>) 30-day immersion in artificial seawater, (<b>d</b>) 60-day immersion in natural seawater, and (<b>e</b>) 60-day immersion in artificial seawater.</p> Full article ">Figure 4
<p>Effect of immersion time on the fatigue life for: (<b>a</b>) natural seawater and (<b>b</b>) artificial seawater.</p> Full article ">Figure 5
<p>Effect of seawater type on fatigue life for: (<b>a</b>) 30-day immersion; (<b>b</b>) 60-day immersion.</p> Full article ">Figure 6
<p>Weight of water absorbed by the composite during seawater immersion ageing.</p> Full article ">Figure 7
<p>E/E<sub>0</sub> versus N/N<sub>f</sub> for tests: (<b>a</b>) with different stress amplitudes using samples not immersed and (<b>b</b>) with different immersion times and types of seawater (natural and artificial seawater).</p> Full article ">Figure 8
<p>Fracture surfaces for: (<b>a</b>) dry sample (control sample), (<b>b</b>) sample immersed 30 days in natural seawater, (<b>c</b>) sample immersed 60 days in natural seawater, and (<b>d</b>) sample immersed 60 days in artificial seawater.</p> Full article ">
18 pages, 4851 KiB  
Article
Impact Response of Composite Sandwich Cylindrical Shells
by Paulo N. B. Reis, Carlos A. C. P. Coelho and Fábio V. P. Navalho
Appl. Sci. 2021, 11(22), 10958; https://doi.org/10.3390/app112210958 - 19 Nov 2021
Cited by 10 | Viewed by 2408
Abstract
Nowadays, due to the complexity and design of many advanced structures, cylindrical shells are starting to have numerous applications. Therefore, the main goal of this work is to study the effect of thickness and the benefits of a carbon composite sandwich cylindrical shell [...] Read more.
Nowadays, due to the complexity and design of many advanced structures, cylindrical shells are starting to have numerous applications. Therefore, the main goal of this work is to study the effect of thickness and the benefits of a carbon composite sandwich cylindrical shell incorporating a cork core, compared to a conventional carbon composite cylindrical shell, in terms of the static and impact performances. For this purpose, static and impact tests were carried out with the samples freely supported on curved edges, while straight edges were bi-supported. A significant effect of the thickness on static properties and impact performance was observed. Compared to thinner shells, the failure load on the static tests increased by 237.9% and stiffness by 217.2% for thicker shells, while the restored energy obtained from the impact tests abruptly increased due to the collapse that occurred for the thinner ones. Regarding the sandwich shells, the incorporation of a cork core proved to be beneficial because it promoted an increase in the restored energy of around 44.8% relative to the conventional composite shell. Finally, when a carbon skin is replaced by a Kevlar one (hybridization effect), an improvement in the restored energy of about 20.8% was found. Therefore, it is possible to conclude that numerous industrial applications can benefit from cylindrical sandwiches incorporating cork, and their hybridization with Kevlar fibres should be especially considered when they are subject to impact loads. This optimized lay-up is suggested because Kevlar fibres fail through a series of small fibril failures, while carbon fibres exhibit a brittle collapse. Full article
(This article belongs to the Special Issue Fracture Mechanics and Structural Integrity of Composite Materials)
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Figure 1

Figure 1
<p>(<b>a</b>) Manufacturing process; (<b>b</b>) geometry and dimensions of the specimens in mm (t = thickness showed in <a href="#applsci-11-10958-t001" class="html-table">Table 1</a>).</p> Full article ">Figure 2
<p>Support used in the experimental tests.</p> Full article ">Figure 3
<p>Effect of thickness on the compressive curves.</p> Full article ">Figure 4
<p>Cork core effect and hybridization effect on the compressive curves.</p> Full article ">Figure 5
<p>Failure mechanisms for shells with: (<b>a</b>) eight layers; (<b>b</b>) six layers: (<b>c</b>) four layers.</p> Full article ">Figure 6
<p>Failure mechanisms for composite sandwich cylindrical shells.</p> Full article ">Figure 7
<p>Load–time and energy–time curves for: (<b>a</b>) composite cylindrical shells with different thicknesses; (<b>b</b>) composite sandwich cylindrical shells.</p> Full article ">Figure 8
<p>Failure mechanisms after impact for shells with: (<b>a</b>) four layers; (<b>b</b>) six layers; (<b>c</b>) eight layers.</p> Full article ">Figure 8 Cont.
<p>Failure mechanisms after impact for shells with: (<b>a</b>) four layers; (<b>b</b>) six layers; (<b>c</b>) eight layers.</p> Full article ">Figure 9
<p>Maximum load versus areal density.</p> Full article ">Figure 10
<p>Failure mechanisms after impact for: (<b>a</b>) carbon sandwich shells; (<b>b</b>) hybrid sandwich shells.</p> Full article ">Figure 10 Cont.
<p>Failure mechanisms after impact for: (<b>a</b>) carbon sandwich shells; (<b>b</b>) hybrid sandwich shells.</p> Full article ">
16 pages, 4577 KiB  
Article
Effect of Harsh Environmental Conditions on the Impact Response of Carbon Composites with Filled Matrix by Cork Powder
by Marco P. Silva, Paulo Santos, João Parente, Sara Valvez and Paulo N. B. Reis
Appl. Sci. 2021, 11(16), 7436; https://doi.org/10.3390/app11167436 - 12 Aug 2021
Cited by 5 | Viewed by 2103
Abstract
Composites are used in a wide range of engineering applications, as a result, exposure to hostile environments is rather common and its mechanical properties degradation is unavoidable. It is necessary to have a complete understanding of the impact of hostile environments on mechanical [...] Read more.
Composites are used in a wide range of engineering applications, as a result, exposure to hostile environments is rather common and its mechanical properties degradation is unavoidable. It is necessary to have a complete understanding of the impact of hostile environments on mechanical performance, namely critical solicitations as low velocity impacts. Therefore, this work intends to analyse the low velocity impact response of a carbon fibre/epoxy composite, and a similar architecture with an epoxy matrix filled with cork, after immersion into different solutions: diesel, H2SO4, HCl, NaOH, distilled water, seawater, and seawater at 60 °C. These solutions significantly affected the impact properties. In this context, the maximum load, maximum displacement, and restored energy behaviour were studied to understand the influence of exposure time. It was possible to conclude that such impact parameters were significantly affected by the solutions, where the exposure time proved to be determinant. The benefits of cork on the perforation threshold were investigated, and this parameter increased when the epoxy matrix was filled with cork. Finally, cork filled epoxy laminates also show less variation in maximum load and recovered energy than carbon/epoxy laminates. Full article
(This article belongs to the Special Issue Fracture Mechanics and Structural Integrity of Composite Materials)
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Figure 1

Figure 1
<p>For an impact energy of 2 J, typical: (<b>a</b>) load versus time curves; (<b>b</b>) energy versus time curves.</p> Full article ">Figure 2
<p>For different impact energies: (<b>a</b>) maximum load, (<b>b</b>) maximum displacement, (<b>c</b>) restored energy.</p> Full article ">Figure 3
<p>Penetration threshold for laminates with neat resin and matrix filled with cork powder.</p> Full article ">Figure 4
<p>Influence of solution type and exposure time on the maximum impact load after immersion into: (<b>a</b>) diesel; (<b>b</b>) H<sub>2</sub>SO<sub>4</sub>, (<b>c</b>) HCl, (<b>d</b>) NaOH, (<b>e</b>) distilled water and seawater; (<b>f</b>) seawater at room temperature and at 60 °C.</p> Full article ">Figure 4 Cont.
<p>Influence of solution type and exposure time on the maximum impact load after immersion into: (<b>a</b>) diesel; (<b>b</b>) H<sub>2</sub>SO<sub>4</sub>, (<b>c</b>) HCl, (<b>d</b>) NaOH, (<b>e</b>) distilled water and seawater; (<b>f</b>) seawater at room temperature and at 60 °C.</p> Full article ">Figure 5
<p>Influence of solution on the displacement after immersion into: (<b>a</b>) diesel; (<b>b</b>) H<sub>2</sub>SO<sub>4</sub>, (<b>c</b>) HCl, (<b>d</b>) NaOH, (<b>e</b>) distilled water and seawater; (<b>f</b>) seawater at room temperature and at 60 °C.</p> Full article ">Figure 5 Cont.
<p>Influence of solution on the displacement after immersion into: (<b>a</b>) diesel; (<b>b</b>) H<sub>2</sub>SO<sub>4</sub>, (<b>c</b>) HCl, (<b>d</b>) NaOH, (<b>e</b>) distilled water and seawater; (<b>f</b>) seawater at room temperature and at 60 °C.</p> Full article ">Figure 6
<p>Influence of solution type and exposure time on the restored energy after immersion into: (<b>a</b>) diesel; (<b>b</b>) H<sub>2</sub>SO<sub>4</sub>, (<b>c</b>) HCl, (<b>d</b>) NaOH, (<b>e</b>) distilled water and seawater; (<b>f</b>) seawater at room temperature and at 60 °C.</p> Full article ">Figure 6 Cont.
<p>Influence of solution type and exposure time on the restored energy after immersion into: (<b>a</b>) diesel; (<b>b</b>) H<sub>2</sub>SO<sub>4</sub>, (<b>c</b>) HCl, (<b>d</b>) NaOH, (<b>e</b>) distilled water and seawater; (<b>f</b>) seawater at room temperature and at 60 °C.</p> Full article ">
18 pages, 2889 KiB  
Article
Micromechanical Analysis in Applications of Active Mono-Slip and Continuum Dislocations in the MDCM
by Temesgen Takele Kasa
Appl. Sci. 2021, 11(7), 3135; https://doi.org/10.3390/app11073135 - 1 Apr 2021
Cited by 1 | Viewed by 1437
Abstract
The key purpose of this paper is to propose a mono-slip-dependent continuum dislocation method for matrix-dominated composite structure (MDCS) analysis. The methodology focuses on dissipation energy theories utilizing a continuum dislocation method (CDM) integrated with small-strain kinematics. The mathematical modeling of the CDM [...] Read more.
The key purpose of this paper is to propose a mono-slip-dependent continuum dislocation method for matrix-dominated composite structure (MDCS) analysis. The methodology focuses on dissipation energy theories utilizing a continuum dislocation method (CDM) integrated with small-strain kinematics. The mathematical modeling of the CDM comprises active mono-slip system formulations, thermodynamic dislocation analysis (TDA), free energy dissipation analysis, and the progression of dislocations. Furthermore, zero and non-zero energy dissipation due to dislocation progression is formulated by using an energy minimization technique with variational calculus. The numerical analysis, performed with Wolfram Mathematica©, is presented using zero and non-zero energy dissipation energy formulations. The outcomes indicate that the formulated approach can be effective for obtaining optimal analysis results for matrix-dominated composite (MDC) materials with a mono-slip system. In sum, this study confirms the feasibility of using the proposed approach to investigate MDCS with inclusions. Full article
(This article belongs to the Special Issue Fracture Mechanics and Structural Integrity of Composite Materials)
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Figure 1

Figure 1
<p>2D simplest representation of the MDCS unit cell.</p> Full article ">Figure 2
<p>Active mono-slip system with a unit cell (UC) of <span class="html-italic">ξ</span> thickness.</p> Full article ">Figure 3
<p>The evolution of plastic distortion (<math display="inline"><semantics> <mi mathvariant="sans-serif">ϒ</mi> </semantics></math>).</p> Full article ">Figure 4
<p>Evolution of plastic distortion (<math display="inline"><semantics> <mi mathvariant="sans-serif">ϒ</mi> </semantics></math>) in the energy dissipation case.</p> Full article ">Figure 5
<p>Unit cell of composite material with a periodic array of elastic particles.</p> Full article ">Figure 6
<p>Comparison of shear stress and shear strain (<math display="inline"><semantics> <mrow> <mi>τ</mi> <mo>−</mo> <mi mathvariant="normal">Γ</mi> </mrow> </semantics></math>) for materials I and II.</p> Full article ">Figure 7
<p>Stress–strain curve for material II.</p> Full article ">Figure 8
<p>Overall dislocation density <math display="inline"><semantics> <mi>ρ</mi> </semantics></math> evolution for zero energy dissipation compared with discrete and nonlocal distortion in material II.</p> Full article ">Figure 9
<p>Shear strain profiles at two values of overall shear strain for a single slip compared with discrete and nonlocal distortion in material II.</p> Full article ">
16 pages, 8340 KiB  
Article
Numerical Study of the Toughness of Complex Metal Matrix Composite Topologies
by Julie Lemesle, Cedric Hubert and Maxence Bigerelle
Appl. Sci. 2020, 10(18), 6250; https://doi.org/10.3390/app10186250 - 9 Sep 2020
Cited by 3 | Viewed by 2549
Abstract
Fracture toughness tests (compact tension) of a dual material composed of a structured Metal Matrix Composite (MMC) (martensitic steel and titanium carbides, named MS-TiC) surrounded by martensitic steel (MS) are simulated with a Discrete Elements Model (DEM) developed with the GranOO Workbench. The [...] Read more.
Fracture toughness tests (compact tension) of a dual material composed of a structured Metal Matrix Composite (MMC) (martensitic steel and titanium carbides, named MS-TiC) surrounded by martensitic steel (MS) are simulated with a Discrete Elements Model (DEM) developed with the GranOO Workbench. The MMC structures are micro-lattices such as gyroid, octet-truss and Face and Body-Centered Cubic with Z-truss (FBCCZ). The volume fraction of these MMC inserts and their cell size are fixed, the influence of the cell orientation is studied. The aim of the study is to determine the configuration of topology (shape and cell orientation) which absorbs the most energy and is the most crack resistant. From experimental tests, the Young’s moduli and the failure stresses of the MMC material and the metal are estimated, and thanks to beam network discretization, a local stiffness and a failure criterion are evaluated to finally obtain a crack propagation path. To verify the suitability of the DEM model, a Compact Tension (CT) experimental test on MMC specimens is performed and a stress intensity factor is computed. A good agreement with an error less than 10% is obtained between experimental and simulated KIc with values respectively equal to 35 and 37 MPam. From DEM simulations based on the CT tests, the FBCCZ cell absorbs the most energy at the crack propagation compared to other structures and the steel. The crack propagation length depends on the shape of the topology. The originality of the study lies in the modeling, with granular properties using DEM, of the mechanical and elastic fracture behavior of these topological structures classically solved by Finite Elements Method (FEM): the microscopic constitutive relations have been validated macroscopically by experimental tests on homogeneous MMC materials. Full article
(This article belongs to the Special Issue Fracture Mechanics and Structural Integrity of Composite Materials)
►▼ Show Figures

Figure 1

Figure 1
<p>Topological cell of reinforcement—gyroid (<b>a</b>), octet-truss (<b>b</b>) and FBCCZ (<b>c</b>).</p> Full article ">Figure 2
<p>Type-I load-displacement curves for the determination of the critical load <math display="inline"><semantics> <msub> <mi>P</mi> <mi>Q</mi> </msub> </semantics></math> then the calculation of the stress intensity factor <math display="inline"><semantics> <msub> <mi>K</mi> <mi>Q</mi> </msub> </semantics></math> [<a href="#B17-applsci-10-06250" class="html-bibr">17</a>].</p> Full article ">Figure 3
<p>Geometry of a CT specimen (<b>a</b>) and crack mouth opening displacement (<b>b</b>).</p> Full article ">Figure 4
<p>Topological inserts cut according to a CT specimen—Z-orientation gyroid (Gyroid_01) (<b>a</b>), X-orientation gyroid (Gyroid_02) (<b>b</b>), octet-truss (<b>c</b>), Z-orientation FBCCZ (FBCCZ_01) (<b>d</b>), X-orientation FBCCZ (FBCCZ_02) (<b>e</b>) and Y-orientation FBCCZ (FBCCZ_03) (<b>f</b>).</p> Full article ">Figure 5
<p>Calibration domain and boundary conditions for the MS (red) and MS-TiC (yellow).</p> Full article ">Figure 6
<p>Views of the DEM domain of the Z-orientation gyroid (Gyroid_01). The gyroid is represented by yellow discrete elements which correspond to the MS-TiC particles, whereas red elements correspond to the MS matrix.</p> Full article ">Figure 7
<p>Distribution of the discrete elements radius in a full MS CT specimen. The discrete elements have an average radius of 640 μm. Rigid pins are in both holes of the CT specimen.</p> Full article ">Figure 8
<p>Load-crack mouth displacement curves used for the determination of <math display="inline"><semantics> <msub> <mi>K</mi> <mi>Q</mi> </msub> </semantics></math>: experimental testing (<b>a</b>) and simulation (<b>b</b>).</p> Full article ">Figure 9
<p>Load-displacement curves obtained for the pre-cracking tests on full MS CT specimen.</p> Full article ">Figure 10
<p>Load (<b>a</b>), broken surface in the CT specimen (<b>b</b>) and total absorbed energy (<b>c</b>), versus displacement.</p> Full article ">Figure 11
<p>Crack propagation in the full MS CT specimen (<b>a</b>) and in the full MS-TiC CT specimen (<b>b</b>).</p> Full article ">Figure 12
<p>Crack paths in the MS-TiC inserts according to the failure stress of the DEM bonds. The blue color of the bonds corresponds to the MS-TiC failure stress. Gyroid_01 (<b>a</b>), Gyroid_02 (<b>b</b>), octet-truss (<b>c</b>), FBCZZ_01 (<b>d</b>), FBCCZ_02 (<b>e</b>) and FBCCZ_03 (<b>f</b>).</p> Full article ">Figure 13
<p>Crack paths in the metal which surrounds the MS-TiC topologies (MS) according to the failure stress of the DEM bonds. The red color of the bonds corresponds to the MS failure stress. Gyroid_01 (<b>a</b>), Gyroid_02 (<b>b</b>), octet-truss (<b>c</b>), FBCZZ_01 (<b>d</b>), FBCCZ_02 (<b>e</b>) and FBCCZ_03 (<b>f</b>).</p> Full article ">Figure 14
<p>Crack path in the MS of a CT specimen containing a MS-TiC cylindrical reinforcement (<b>a</b>) and in the MS-TiC cylindrical insert (<b>b</b>).</p> Full article ">Figure 15
<p>Broken bonds at the MS-TiC insert/MS interface. Gyroid_01 (<b>a</b>), Gyroid_02 (<b>b</b>), octet-truss (<b>c</b>), FBCZZ_01 (<b>d</b>), FBCCZ_02 (<b>e</b>) and FBCCZ_03 (<b>f</b>).</p> Full article ">
12 pages, 2594 KiB  
Article
The High-Velocity Impact Behaviour of Kevlar Composite Laminates Filled with Cork Powder
by Ana Martins Amaro, Paulo Nobre Balbis Reis, Ines Ivañez, Sonia Sánchez-Saez, Shirley Kalamis Garcia-Castillo and Enrique Barbero
Appl. Sci. 2020, 10(17), 6108; https://doi.org/10.3390/app10176108 - 3 Sep 2020
Cited by 14 | Viewed by 2908
Abstract
The literature reports benefits when the cork powder obtained from industrial by-products is used as the filler of composite laminates. For example, while the fatigue life is insensitive to the presence of cork in the resin, significant improvements are achieved in terms of [...] Read more.
The literature reports benefits when the cork powder obtained from industrial by-products is used as the filler of composite laminates. For example, while the fatigue life is insensitive to the presence of cork in the resin, significant improvements are achieved in terms of to low-velocity impact strength. However, in terms of ballistic domain, the literature does not yet report any study about the effect of incorporating powdered cork into resins. Therefore, this study intended to analyse the ballistic behaviour and damage tolerance of Kevlar/epoxy reinforced composites with matrix filled by cork powder. For this purpose, high-velocity impacts were studied on plates of Kevlar bi-directional woven laminates with surfaces of 100 × 100 mm2. It was possible to conclude that the minimum velocity of perforation is 1.6% higher when the cork powder is added to the resin, but considering the dispersion, this small difference can be neglected. In terms of damage areas, they are slightly lower when cork dust is added, especially for velocities below the minimum perforation velocity. Finally, the residual bending strength shows that these composites are less sensitive to impact velocity than the samples with neat resin. In addition to these benefits, cork powder reduces the amount of resin in the composite, making it more environmentally friendly. Full article
(This article belongs to the Special Issue Fracture Mechanics and Structural Integrity of Composite Materials)
►▼ Show Figures

Graphical abstract

Graphical abstract
Full article ">Figure 1
<p>Photo of the equipment used in the high-velocity impact tests.</p> Full article ">Figure 2
<p>Photo of the high-velocity impact test setup.</p> Full article ">Figure 3
<p>Impact energy vs. absorbed velocity.</p> Full article ">Figure 4
<p>Damage area of the back side. (<b>a</b>) Specimen of Kevlar impacted at 231 m/s and (<b>b</b>) specimen of Kevlar + cork powder impacted at 230 m/s.</p> Full article ">Figure 5
<p>Image of damage area of the back side. (<b>a</b>) Specimen of Kevlar impact to 438 m/s and (<b>b</b>) specimen of Kevlar + cork powder impacted to 439 m/s.</p> Full article ">Figure 6
<p>Images of damage areas by C-scan. (<b>a</b>) Specimen of Kevlar impact to 231 m/s and (<b>b</b>) specimen of Kevlar + cork powder impacted to 230 m/s.</p> Full article ">Figure 7
<p>Images of damage areas by C-scan. (<b>a</b>) Specimen of Kevlar impact to 438 m/s and (<b>b</b>) specimen of Kevlar + cork powder impacted to 439 m/s.</p> Full article ">Figure 8
<p>Extension of damage area measured by C-scan.</p> Full article ">Figure 9
<p>Impact velocity vs. ratio of bending stress.</p> Full article ">

Review

Jump to: Research

19 pages, 2339 KiB  
Review
Joining of Fibre-Reinforced Thermoplastic Polymer Composites by Friction Stir Welding—A Review
by Miguel A. R. Pereira, Ivan Galvão, José Domingos Costa, Ana M. Amaro and Rui M. Leal
Appl. Sci. 2022, 12(5), 2744; https://doi.org/10.3390/app12052744 - 7 Mar 2022
Cited by 14 | Viewed by 3510
Abstract
The objective of the current work is to show the potential of the friction stir welding (FSW) and its variants to join fibre-reinforced thermoplastic polymer (FRTP) composites. To accomplish that, the FSW technique and two other important variants, the friction stir spot welding [...] Read more.
The objective of the current work is to show the potential of the friction stir welding (FSW) and its variants to join fibre-reinforced thermoplastic polymer (FRTP) composites. To accomplish that, the FSW technique and two other important variants, the friction stir spot welding (FSSW) and the refill friction stir spot welding (RFSSW), are presented and explained in a brief but complete way. Since the joining of FRTP composites by FSSW has not yet been demonstrated, the literature review will be focused on the FSW and RFSSW techniques. In each review, the welding conditions and parameters studied by the different authors are presented and discussed, as well as the most important conclusions taken from them. About FSW, it can be concluded that the rotational speed and the welding speed have great influence on heat generation, mixture quality, and fibre fragmentation degree, while the tilt angle only has residual influence on the process. The reduction of internal and external defects can be achieved by adjusting axial force and plunge depth. Threaded or grooved conical pins achieved better results than other geometries. Stationary shoulder tools showed better performance than conventional tools. Regarding the RFSSW, it has not yet been possible to deepen conclusions about most of the welding parameters, but its feasibility is demonstrated. Full article
(This article belongs to the Special Issue Fracture Mechanics and Structural Integrity of Composite Materials)
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Figure 1

Figure 1
<p>Schematic illustration of FSW with conventional tool and its main welding parameters: (<b>a</b>) overview of the process and (<b>b</b>) detail on parameters related to tool penetration.</p> Full article ">Figure 2
<p>FSW main steps: (<b>a</b>) plunging stage, (<b>b</b>) dwell stage, (<b>c</b>) welding stage, and (<b>d</b>) end of welding and tool retracting stage.</p> Full article ">Figure 3
<p>Main pin geometries used in FSW: (<b>a</b>) straight cylindrical, (<b>b</b>) cylindrical threaded, (<b>c</b>) cylindrical grooved, (<b>d</b>) straight conical or tapered, (<b>e</b>) straight hexagonal, (<b>f</b>) straight square, and (<b>g</b>) straight triangular.</p> Full article ">Figure 4
<p>FSSW main steps: (<b>a</b>) plunging stage, (<b>b</b>) stirring stage, and (<b>c</b>) retraction stage.</p> Full article ">Figure 5
<p>Schematic illustration of the cross section of a friction stir spot weld.</p> Full article ">Figure 6
<p>RFSSW with sleeve plunge configuration main steps: (<b>a</b>) initiation stage, (<b>b</b>) sleeve penetration stage, (<b>c</b>) weld seam compression stage and (<b>d</b>) tool retracting stage.</p> Full article ">Figure 7
<p>Schematic illustration of the fibre distribution on the cross section of joints produced by: (<b>a</b>) conventional welding methods and (<b>b</b>) FSW (based on Czigány and Kiss [<a href="#B45-applsci-12-02744" class="html-bibr">45</a>]).</p> Full article ">Figure 8
<p>Schematic illustration of the pin geometries most recommended to join FRTP composites by FSW: (<b>a</b>) conical threaded pin and (<b>b</b>) conical grooved pin.</p> Full article ">
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