This study comparatively investigates the in-plane compressive properties of 3D braided honeycomb composite core (3D-BHC) and 3D braided honeycomb-foam sandwich composite (3D-BHFSC). The effects of joint wall length on mechanical properties, energy absorption, and failure mechanisms were analyzed using quasi-static compression tests and 3D digital image correlation (3D-DIC). The results show that the maximum load and energy absorption of 3D-BHFSC increase with the number of free wall columns, while the failure displacement is primarily governed by free wall rows number. The addition of foam filling and face sheets to form sandwich structure (3D-BHFSC) significantly enhances structural performance: the maximum load approximately doubles compared with that of 3D-BHC, energy absorption improves by 1.7–1.8 times, and the in-plane compressive modulus rises by about 500 MPa. However, 3D-BHFSC exhibit reduced failure strains and displacements due to progressive damage accumulation. Strain-field analysis reveals shear-dominated failure modes in 3D-BHC, evolving into V-shaped or cross-shaped fractures in 3D-BHFSC. These findings unravel the interplay between honeycomb topology and sandwich performance and provide quantitative guidance for designing lightweight sandwich structures in aerospace, automotive, and defense applications.

An integrated molding composite honeycomb has been proposed, in which a seamless, 3D braided natural fiber cellular fabric serves as the reinforcement, with epoxy resin as the matrix. Three-point bending behaviors of the honeycomb, taking account of the effects of joint wall length and opening angle, were investigated. The fracture mechanisms during bending were monitored using 3D Digital Image Correlation. The validated Finite element model was developed and used to perform a parametric analysis identifying the effect of material Young's modulus and geometric variations on the flexural stiffness. The results reveal that fracture occurs at the junction of the joint wall and the free wall, characterized by shear-type failure and structural geometry parameters significantly affect flexural performance. Decreasing the joint wall length from 55 to 4 mm in 90° honeycombs reduced the maximum load by approximately 26% and the flexural stiffness (P/y) by about 55%, accompanied by an increase in maximum deflection. Conversely, for specimens with a 17 mm joint wall, increasing the opening angle from 60° to 120° decreased the maximum load and P/y by approximately 32% and 55%, respectively, while the flexural deflection gradually increased. The knowledge generated from this study is key in design and performance evaluation of 3D braided composite honeycomb cores for sandwich structures, which is crucial for enhancing the out-of-plane bending resistance of sandwich structures.

To ensure safety in structural design, a method to quantify the damage in thermoplastic ultrasonic single-spot-welded Single-Lap Shear (SLS) joints is needed. This paper investigates whether detailed knowledge regarding the shape of the weld is required when using the global compliance to quantify damage. A finite element model using cohesive zone elements is developed in Abaqus to simulate single-spot SLS specimens with varying weld areas, aspect ratios, and damage growth directions, covering damage levels from 0 to 90% of the initial weld area. For each configuration, the relationship between intact weld area and global compliance is evaluated, and the numerical trends are compared to previously published experimental data from similar joints. The results show that weld size and damage growth direction have negligible influence on the relationship between global compliance and weld area, and that weld shape is also insignificant as long as the aspect ratio remains within a practical range; only very elongated welds with an aspect ratio over 4.4, which are unlikely in production, deviate significantly. Global compliance can be used as a reliable indicator of damage in single-spot ultrasonic welds that is insensitive to weld shape. This enables simplified in situ damage monitoring and reduces the need for detailed geometric characterisation during mechanical testing.

Delamination growth in composite laminates is essentially two-dimensional (2D), indicating a multidirectional spreading of interlaminar damage. However, the evaluation and prediction of delamination growth mainly relies on the quantification of one-dimensional (1D) growth using unidirectional specimens. In this study, the discrepancies and similarities between 1D and 2D delamination behaviours of composite laminates are investigated, both experimentally and numerically. The fracture toughness of mode II delamination, measured experimentally through 1D tests, is compared with the numerically fitted critical Energy Release Rate (ERR) in 2D delamination using Cohesive Zone Modelling (CZM) method. The fracture mechanisms involved in 1D and 2D delamination growth are investigated through fractography at the delamination interfaces. Although similar damage mechanisms are present in 1D and 2D tests, using the fracture toughness measured from 1D tests to predict 2D growth is proven to be insufficient due to distinct extrinsic toughening effects. Variations in local stress states significantly influence delamination growth, necessitating different cohesive constitutive models to accurately describe 1D and 2D delamination processes.

This study investigates the influence of pre-crack conditions (introduced under Mode I and Mode II loading prior to fracture testing) and specimen compliance on the Mode II fracture characterization (G_IIC) of adhesively bonded composite joints. Calibrated End-Loaded Split (CELS) and 3-Point-Bending ENF tests were performed using structural AF163-2 K adhesive. Various data reduction schemes were employed to account for pre-crack morphology and compliance in the development of the R-curve. The data reduction schemes showed significant scatter, ranging from 8.07 ± 0.17 to 17.3 ± 1.19 N/mm, depending on the pre-cracking conditions and compliance effect. Mode I pre-cracked specimens consistently exhibit higher G_IIC values compared to Mode II pre-cracked specimens, a difference governed by the morphology and extent of the fracture process zone (FPZ). Mode I pre-cracking forms a localized FPZ that subsequently transitions into a shear-dominated FPZ for G_IIC evaluation during the subsequent Mode II fracture test. In contrast, Mode II pre-cracked specimens contain an already-developed shear FPZ that is broader and more diffuse, resulting in lower strain-energy release rates and lower G_IIC values. High compliance effects cause significant bending, additionally introducing high derogatory energy deformation from the test fixtures, obscuring the actual crack tip. The apparent crack length methods demonstrated reliable estimates of fracture energy and R-curve behavior by accounting for the effects of large FPZ, thereby capturing both crack-tip and distributed dissipation mechanisms. The experimental findings correlate with computational results, displaying stable cohesive disbond growth in the adhesive layer. This study indicates that pre-cracking and compliance effects significantly influence Mode II fracture characterization and, therefore, need to be properly addressed.

The fracture process zone (FPZ) significantly influences the damage tolerance of adhesively bonded composite joints, governing crack-growth mechanisms and migration. Existing fracture characterization approaches generally evaluate pure-mode behavior independently and extend these results to mixed-mode conditions using a power-criterion, such as the Benzeggagh-Kenane (B–K) criterion. This process assumes that FPZ-dependent mode-mix behavior from a standard mixed-mode test is transferable to another complex loading condition. This assumption remains unchecked for toughened adhesive joints, where FPZ morphology varies with loading conditions. This study addresses this gap through experimental and numerical investigation using digital image correlation (DIC) and cohesive zone modeling (CZM). The pure mode I test displayed localized FPZ ahead of the crack tip, influenced by carrier bridging. Two different pure Mode II tests demonstrated that the apparent crack length method accurately accounts for the large FPZ ahead of the crack tip. The mixed-mode bending (MMB) test linked pure modes through the B–K criterion. The Crack-Lap Shear (CLS) specimens exhibited evolving FPZ and mode II-dominated fracture. The fracture toughness predicted by the B–K criterion deviated from the CLS tests as the loading became more mode II dominant. It was observed that the FPZ morphology during the CLS test differed significantly from that observed during the MMB test, through DIC and CZM. These results highlight that differences in FPZ affect the mixed-mode fracture toughness and demonstrate the limitations of applying a single empirical power-criterion. It underscores for FPZ-sensitive approaches to accurately predict the fracture resistance of toughened adhesive joints under evolving mixed-mode conditions.

This chapter demonstrates how proper similitude utilizing principles from physics improves understanding of fatigue crack growth. After introducing elementary physical principles, a physical theory is outlined based on concepts separating available (driving) energy and material’s intrinsic resistance. Several lessons learned are presented, after which these lessons are extrapolated to formulating the influence plasticity has on fatigue crack growth. It is illustrated how a proper energy balance, potentially can enable prediction of fatigue crack growth without typical Paris curves.

FRP structures are subjected to a combination of environmental and mechanical loads that act in an interactive way, determining service life. This study investigates the isolated and combined effects of in-situ temperature and relative humidity on monotonic and tension-tension fatigue response of two flax/epoxy laminates ([0/90/0]_S and [+45/-45]_2S), benchmarked against equivalent GFRP laminates. Particular emphasis was given to stiffness evolution, strain accumulation, and hysteretic behaviour particularly energy dissipation. Increasing temperature consistently reduced stiffness, strength, and fatigue life for both flax FRP laminates, leading to downward shifts and tilts of the S–N curves. The effect of moisture alone was laminate-dependent: elevated moisture content reduced stiffness, strength and fatigue life in the shear-dominated [+45/-45]_2S laminate, whereas the [0/90/0]_S laminate showed increased fatigue life attributed to enhanced ductility and increased laminate strength. Combined elevated temperature and moisture content lead to reduced monotonic stiffness and strength whilst their effects on fatigue life were cumulative. The largest effect was observed for the [+45/-45]_2S laminate, where fatigue life decreased by approximately three orders of magnitude. Across all hygrothermal conditions, energy dissipation was found to be an indicator of fatigue life with higher hysteretic energy dissipation per cycle correlated with reduced fatigue life. When assessed relative to baseline S–N behaviour, flax FRPs exhibit a proportional sensitivity to combined temperature and humidity comparable to GFRPs, indicating that flax composites are not disproportionately penalised under hot–wet fatigue loading.

Three-phase composites, especially those composed of high performance thermoplastics, have not been properly investigated with respect to their interlaminar fracture toughness. Therefore, this study investigates effect on the interlaminar fracture toughness by adding carbon nanotube buckypaper (BP), tested under cyclic loading in mode I and II. BP weakened the interlaminar fracture toughness in mode I, creating an easy path for crack growth and reducing the strain energy release (SERR) values in the Paris curves. Conversely, under mode II BPs presented no significant influence to the interlaminar fracture toughness and fatigue life; however, a slight improvement was observed due to the bridging effect. The energy balance principle model for opening delamination showed that BP composites require less energy per unit of area to crack growth, resulting in a smoother fracture surface with fewer failure mechanisms. In contrast, BP slightly increased the energy per unit of area for crack growth, leading to a rougher fracture surface with a higher prevalence of failure mechanisms under mode II. This work underscores the importance of examining the individual effects of mode I and II loadings on BP laminates since these interleaves affect the interlaminar toughness and fatigue life differently.

Multidirectional (MD) composite laminates are extensively employed in structural applications owing to their superior mechanical characteristics. Nevertheless, the evaluation of the fracture toughness of composite laminates primarily relies on tests using unidirectional (UD) specimens. This study evaluates the reliability of characterizing mode II delamination behaviour in MD laminates by using UD specimens. The quantification of delamination area through Digital Image Correlation (DIC) analysis is integrated with a physical Energy Release Rate (ERR) method to ascertain the fracture resistance, which is compared with the ERR derived via a modified J-integral method and the standardized compliance methods. Fractographic analysis reveals similar fracture mechanisms in specimens with identical interfaces. The physical ERR increases notably due to large-scale fibre bridging induced by fibre nesting at 0^∘//0^∘ interfaces. Conversely, in 0^∘//90^∘ interfaces, large-area matrix cracking enhances the intrinsic fracture resistance, excluding the extrinsic toughening provided by fibre bridging.

This paper investigates 3-point bending failure of five different types of GLARE laminates (2A, 2B, 3, 4A and 4B). 73 configurations (419 specimens), with different stacking sequences and aluminum layer thicknesses are tested. Failure mechanisms, effect of stacking sequence, effect of aluminum rolling direction, effect of displacement rate and energy absorption are analyzed. Configurations with predominantly 0°glass fiber layers fail with delamination as the major failure mode, while configurations with predominantly 90°glass fiber layers fail with central cracking as the major failure mode. GLARE 3, with 1:1 ratio of 0°and 90°fibers, fail with an equal mix of delamination and central cracking. A semi-analytical framework that can be used to predict the force versus displacement curve for central cracking failure is proposed and validated.

This study investigates the Mode-I fracture toughness of laminates with varying interface angles. A method for identifying crack tip location using grayscale characteristic parameters in DIC is proposed. The findings demonstrate that both initial and steady-state fracture toughness exhibit a bilinear relationship with interface angle. A cohesive constitutive model incorporating the interface angle was developed and integrated into a double cantilever beam finite element model, predicting delamination propagation behavior that was highly consistent with experimental results. Numerical analysis suggests that zigzag cracks may improve fracture toughness before steady-state toughness is achieved, with peak toughness correlating to the length of the zigzag cracks.

Fiber-metal laminates are a well-known and established material concept featuring an enhanced crack propagation resistance when compared to their metal and fiber reinforced plastic (FRP) constituents. In this paper, this approach is transferred to purely carbon fiber reinforced plastic (CFRP) based laminates made from layers having polyetherimide (PEI) and epoxy matrices in an alternating laminate architecture. The laminates are manufactured via hot pressing. Double-cantilever beam (DCB) tests are performed on standard samples for both the hybrid laminates in different configurations as well for the both constituent materials, i.e. carbon fiber reinforced PEI (CFR-PEI) and carbon fiber reinforced epoxy. As the formation of an interphase is already reported in literature for this matrix combination, microstructural investigations have also been carried out in addition to fractography on crack surfaces. It is shown that the hybrid materials outperform both constituents regarding the crack resistance when crack initiation starts in the tougher CFR-PEI layer and the laminate layup is 0/90°. In the other configurations investigated, there is no significant effect. The energy dissipating mechanisms are crack jumping and the formation of several parallel cracks. Consequently, crack resistance in such hybrids might be controlled in future by adjusting the crack resistance of the constituents as well as the laminate architecture.