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Open research questions in Mechanical Behavior of Composites

27 unresolved questions extracted from the limitations and future-work sections of 538 Mechanical Behavior of Composites papers in our library. Each links back to the study that raised it.

What the literature leaves open

  • second contour in the surface integral becomes negligible. Next, a user subroutine in the post-processing can extract the J values by ignoring the first and second contours during the surface integral which may remove the decaying s23 in mode II and mode III at the free surface. (cid:15) The importance of the collapsed elements (the element type whereby the nodes at one of the sides of the element coinciding the crack tip have similar coordinates) at the crack tip/front is explicitly discussed in LEFM. It is shown that despite providing good result for the Stress Intensity Factor by the non-collapsed elements, they can only produce singularity on the element boundary and are not recommended for the LEFM analyses. Furthermore, there can be only four non-collapsed elements at the crack tip/front which makes the stress calculation for different angles around the crack difficult for these elements. (cid:15) In LEFM, the size of the K-dominated zone in the DCB-type specimens is very small (less than 0.013% of the specimen height) and by moving from the center to the edges, its size shrinks. (cid:15) Based on the obtained results, in LEFM, there is always a coupling mode II, IIc, under anti-plane loading which induces the mode III. Therefore, the pure mode III, in the form of what usually is expected, is not achievable and of course is not limited only to case studies in the present project. For Small Fracture Process Zone, the coupling mode II is dominant in the vicinity of the crack. Further away from the crack, for the Large Fracture Process Zone, i.e, Large-Scale Bridging condition; the specimen acts like a beam under anti-plane loading. (cid:15) The DCB specimens with low thickness are not suitable for mode III delamination toughness testing in LEFM. By reducing the thickness, the singularity field inside the specimen vanishes. (cid:15) Among several LEFM case studies for the suitable cross-section for mode III delamination toughness testing, the horizontal beams on the side of the applied rotation with the width five times larger than the DCB specimen is capable of generating mode III although the coupled mode II cannot be vanished. The cross-section is illustrated in figure 10.1. Figure 10.1: The optimum cross-section for mode III delamination toughness testing. x3x2Elastic beamDCB specimenCrackMyMy 10.1 Recommendations for Further Studies 127 (cid:15) Unlike the mode I where the crack opens from the center to the edges, in mode II and III the crack extends from the edges to the center. The result from the Cohesive Zone Modeling also confirms this observation for mode III. (cid:15) The bilinear cohesive law under the assumption of neglecting mode II can be utilized for the Large-Scale Bridging problems and the crack evolution for the DCB specimen with the isotropic material. For Large Fracture Process Zone, far from the evolved crack front is predominated by the mode III. The DCB specimen exhibits a stable crack growth.

    FEM analysis of a Mode III fracture mechanics test specimens for cohesive law determination of fibre composite · 2026 · DOI

  • The Double Cantilever Beam (DCB) specimen under mode I, mode II and mode III is analyzed by FEM and compared with the theory. For the FEM, ABAQUS as a reliable Finite Element package is utilized; therefore, all the conclusions are based on the results obtained from this commercial code. (cid:15) The two-dimensional mode I and II results from the LEFM Finite Element model agree with the theory for both the plane stress and plane strain conditions; however, the pri- In the three-dimensional mary weakness appears in the three-dimensional analysis. mode I, the Finite Element model is unable to provide enough number of points for the normal stress, s22, along the crack ligament at the free surface. The insufficient number of points originates from inadequate number of elements in the specimen longitudinal direction due to exceeding a million number of elements for the whole model and limitation in computational time. The smaller specimens for the mode II and III allow a finer mesh at the free surface; however, the 1= r-singularity is not still accomplished. Furthermore, a noticeable difference between the singularity exponent, l, is observed at the free surface and the plane very close to the free surface. The results are in agreement with the obtained from Pook et al. (2014) and Pook et al. (2015) who had an element size virtually ten times smaller than the elements used in this study. p (cid:15) In LEFM, the effect of the mid-node distortion in the elements at the crack tip should be investigated thoroughly since the stress is not calculated correctly in these elements. With the improvement in the computational power, it might be unnecessary to move the mid-nodes to the quarter point since there is a possibility of influencing the stress calculation at the free surface by such elements. Overall, it is believed that a Finite Element model with a very fine mesh at the free surface is needed before commenting on whether the 1= r-singularity at the free surface exists or the distorted elements at the crack tip may affect the stresses at the free surface. p (cid:15) The three-dimensional LEFM analysis of mode II reveals a coupling mode III, due to the presence of shear stress, s23, at the crack tip and the first ring of the elements where the mid-nodes are shifted to the quarter point. ABAQUS uses the surface integral to calculate the J integral and K is determined from the J values.

    FEM analysis of a Mode III fracture mechanics test specimens for cohesive law determination of fibre composite · 2026 · DOI

  • Recommendations for Further Studies.. 10.2 Limitations.... 125..................... 127............................. 127 A LEFM stress and displacement fields for an isotropic material 129 B Cohesive traction determination from J integral 131 B.1 J integral for the CZM.............................. 131 B.2 Cohesive law derivation from the J integral...................

    FEM analysis of a Mode III fracture mechanics test specimens for cohesive law determination of fibre composite · 2026 · DOI

  • The Double Cantilever Beam (DCB) specimen under mode I, mode II and mode III is analyzed by FEM and compared with the theory. For the FEM, ABAQUS as a reliable Finite Element package is utilized; therefore, all the conclusions are based on the results obtained from this commercial code. (cid:15) The two-dimensional mode I and II results from the LEFM Finite Element model agree with the theory for both the plane stress and plane strain conditions; however, the pri- In the three-dimensional mary weakness appears in the three-dimensional analysis. mode I, the Finite Element model is unable to provide enough number of points for the normal stress, s22, along the crack ligament at the free surface. The insufficient number of points originates from inadequate number of elements in the specimen longitudinal direction due to exceeding a million number of elements for the whole model and limitation in computational time. The smaller specimens for the mode II and III allow a finer mesh at the free surface; however, the 1= r-singularity is not still accomplished. Furthermore, a noticeable difference between the singularity exponent, l, is observed at the free surface and the plane very close to the free surface. The results are in agreement with the obtained from Pook et al. (2014) and Pook et al. (2015) who had an element size virtually ten times smaller than the elements used in this study. p (cid:15) In LEFM, the effect of the mid-node distortion in the elements at the crack tip should be investigated thoroughly since the stress is not calculated correctly in these elements. With the improvement in the computational power, it might be unnecessary to move the mid-nodes to the quarter point since there is a possibility of influencing the stress calculation at the free surface by such elements. Overall, it is believed that a Finite Element model with a very fine mesh at the free surface is needed before commenting on whether the 1= r-singularity at the free surface exists or the distorted elements at the crack tip may affect the stresses at the free surface. p (cid:15) The three-dimensional LEFM analysis of mode II reveals a coupling mode III, due to the presence of shear stress, s23, at the crack tip and the first ring of the elements where the mid-nodes are shifted to the quarter point. ABAQUS uses the surface integral to calculate the J integral and K is determined from the J values.

    FEM analysis of a Mode III fracture mechanics test specimens for cohesive law determination of fibre composite · 2026 · DOI

  • should be acknowledged. First, the present study used electrical resistance as an indicator of interfacial electrical isolation rather than directly measuring corrosion current, corrosion rate, or electrochemical impedance. Therefore, the results should be interpreted as evidence of improved electrical resistance stability and potential reduction of galvanic coupling, not as complete validation of long-term corrosion resistance. Second, the experiments were con- ducted under ambient laboratory conditions, and controlled humidity, salt-fog exposure, or environmental aging tests were not performed. Since galvanic corrosion is strongly moisture-dependent, future studies should evaluate the nanocomposite sealants under controlled humidity, salt- fog, temperature cycling, ultraviolet exposure, and chemi- cal aging conditions [18]. Third, detailed microscopic verification of nanoparticle dispersion and coating-thick- ness uniformity was not performed in the present work. Future work should include cross-sectional SEM, optical microscopy, and elemental mapping to evaluate nanopar- ticle possible agglomeration within the cured epoxy sealant layer. distribution, uniformity, coating and should also be Future research should also investigate additional elec- trically insulating nanoparticles, such as boron nitride, silica, alumina, mica, zirconia, and other selected ceramic or metal-oxide fillers. These materials may offer different combinations of barrier performance, mechanical rein- forcement, insulation. thermal stability, and electrical Advanced dispersion techniques, such as ultrasonic mixing, high-shear mixing, and chemical functionalization of explored to improve nanoparticles, nanoparticle distribution within the epoxy matrix [63]. In addition, controlled bolt-preload conditions and computa- tional stress modeling should be incorporated to better understand the relationship between bolt geometry, contact pressure, local stress concentration, sealant deformation, electrical resistance, and damage evolution. Transitioning from laboratory-scale testing to scalable manufacturing implementation. By will also be essential for practical addressing these areas, future research can further advance nanocomposite sealing strategies for durable, electrically stable, and corrosion-resistant CFRC–metal assemblies in demanding engineering applications.

    Enhancing Electrical and Mechanical Stabilities of Carbon Fiber-Reinforced Composite Holes via Nanocomposite Sealing under Fatigue Loadings · 2026 · DOI

  • The effect of seawater ageing (SWA) on various mechanical properties of glass fibre/acrylic matrix composites has been reported by some researchers; however, changes in Mode I fracture toughness and compressive performance of these composites under SWA conditions have not been reported.

    Mode I fracture toughness and compressive properties of glass fibre reinforced acrylic matrix composites before and after seawater ageing · 2026

  • Sandwich composite structures are widely employed in aerospace applications because of their high specific stiffness and low mass; however, their susceptibility to impact damage remains a major limitation, particularly under repeated loading and in service environments where residual integrity is critical.

    Experimental characterisation of self-healing composite structures for low and high-velocity impact applications · 2026 · DOI

  • Future research will focus on validating these compensation models and exploring auto- mated excitation systems to further enhance the precision of determining the structural integrity of complex sandwich composites.

    The influence of temperature and delamination characteristics on the modal properties of composite sandwich beams · 2026 · DOI

  • Regarding future research, the following directions are proposed:   Fabrication of butt joints cured at different temperatures (40, 80, 100, and 120 °C), in order to analyse the effect of curing temperature on the performance and dimensional stability of the tool developed for joint fabrication. Production of butt joints using substrates with other geometries and dimensions, since the current tool has only been validated for substrates of 12.7 mm in diameter and 40 mm in length. This would allow evaluation https://doi.org/10.53941/jmem.2026.100018 16 of 19 Pedroso et al. J. Mech. Eng. Manuf 2026 of the tool’s adaptability and assessment of the potential influence of geometric parameters on joint mechanical performance.    Manufacture of butt joints using alternative adhesives and varying adhesive layer thicknesses, as the current tooling configuration has been validated only for adhesive layers with a thickness of 0.2 mm. This would enable analysis of thickness-dependent stress distribution and failure behaviour. Execution of additional floating roller peel tests using alternative adhesives and substrate materials, to broaden the comparative database and assess the versatility of the developed tool across different material– adhesive systems. Performance of shear-dominated mechanical tests (e.g., lap shear configurations) in cases where tensile testing demonstrates cohesive adhesive failure, to investigate potential relationships between ultimate shear strength and normal tensile strength. Such investigation may enable assessment of whether failure criteria based on equivalent stress approaches (e.g., von Mises or Tresca) can describe adhesive behaviour within restricted regimes (e.g., same adhesive family and controlled stress states). Conduct peel tests under quasi-constant loading or controlled steady-state peeling conditions, combined with tensile and shear characterisation, to explore whether restricted correlations between peel resistance and intrinsic adhesive strength can be established under fully cohesive failure and well-defined stress states.

    A Comprehensive Evaluation of Peel and Tensile Strength in Adhesive Bonding · 2026 · DOI

  • The numerical framework does not appear to incorporate theory-guided machine learning approaches (referenced in Zobeiry et al., 2020) for characterizing damage evolution in carbon/epoxy laminates under combined hygrothermal and mechanical loading, limiting predictive capability for complex environmental scenarios.

    A High-Fidelity Numerical Study of Trans-Laminar Fracture in Carbon/Epoxy Laminates Under Varying Environmental Conditions · 2026 · DOI

  • The paper addresses trans-laminar fracture in carbon/epoxy laminates, but the applicability of the numerical model to composite laminates manufactured via automated fiber placement (AFP) with gaps and overlaps, as discussed in reference 24, under environmental conditioning has not been explored for fracture toughness prediction.

    A High-Fidelity Numerical Study of Trans-Laminar Fracture in Carbon/Epoxy Laminates Under Varying Environmental Conditions · 2026 · DOI

  • The reference to discrete ply models in compact tension testing (Rutar et al., 2025) indicates that R-curve prediction for CFRP under hygrothermal aging using discrete ply simulation remains unvalidated in the context of varying environmental conditions affecting translaminar fracture behavior.

    A High-Fidelity Numerical Study of Trans-Laminar Fracture in Carbon/Epoxy Laminates Under Varying Environmental Conditions · 2026 · DOI

  • While the paper employs numerical simulation of trans-laminar fracture in carbon/epoxy laminates, the application of phase field fracture mechanics (referenced via Tan & Martínez-Pañeda, 2022) for predicting microscopic bridging behavior under environmental degradation has not been integrated into the high-fidelity model to validate composite damage evolution.

    A High-Fidelity Numerical Study of Trans-Laminar Fracture in Carbon/Epoxy Laminates Under Varying Environmental Conditions · 2026 · DOI

  • The numerical study focuses on carbon/epoxy laminates under varying environmental conditions, but the reference to hygrothermal effects (Gong et al., 2024) on translaminar fracture toughness across different stacking sequences suggests that the interaction between moisture absorption, temperature cycling, and stacking sequence optimization for trans-laminar fracture resistance in carbon/epoxy composites has not been comprehensively characterized in the high-fidelity model.

    A High-Fidelity Numerical Study of Trans-Laminar Fracture in Carbon/Epoxy Laminates Under Varying Environmental Conditions · 2026 · DOI

  • The study focuses exclusively on BFRP (Basalt Fiber Reinforced Polymer) materials; the applicability to other composite materials or fiber types remains unexplored.

    The response of the composite circular diaphragms under thermo-mechanical stresses via bulge testing · 2026 · DOI

  • Despite their widespread use, the fundamental differences in how distinct glass fiber types interact with polymer matrices under variable temperatures remain underexplored.

    Temperature dependent structural integrity of S2-glass and E-glass fiber reinforced epoxy laminates under indentation and flexural loading · 2026 · DOI

  • The analysis of recent studies and publications shows that the degradation of mechanical properties of anisotropic materials under elastoplastic loading of engineering structures is insufficiently studied and is the subject of an extensive body of scientific work.

    Damage evolution during the service life of structures made of anisotropic materials · 2025 · DOI

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27 open questions have been extracted from the limitations and future-work passages of 538 Mechanical Behavior of Composites papers in our library. Each one below links back to the study that raised it, so you can read the original claim in context.

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