GLARE Laminates Research Articles

Failure analysis and formability improvement of GLARE laminates in warm active hydroforming technology: Experimental and numerical approach

Warm hydroforming shows considerable promise as a forming technology for Fiber Metal Laminates (FMLs) parts due to its capacity to enhance the formability of materials. However, a detailed investigation into the failure behavior during hydroforming is still lacking. In this paper, a systematic experiment was performed to analyze the possible failure modes for GLARE in warm active hydroforming. Five main defect modes have been identified, including fiber crack, combined Al & fiber crack, wrinkle, low resin, and voids defects. The related formation mechanisms have been analyzed based on both experiments and numerical methods. Additionally, the effects of critical process parameters, including the blank holder force (BHF), liquid pressure rate (PR), and temperature (T) on failure behavior are thoroughly investigated and quantified. The use of step-loading BHF was recommended to enhance formability, which caused a nearly 36% decrease in the prepreg tensile damage criteria coefficient. Conversely, decreasing PR or increasing T significantly improved the formability of GLARE but resulted in low resin content and significant void defects. Consequently, a feasibility map that quantified the BHF and liquid pressure effects on various defects was developed, leading to optimized parameters for manufacturing a box-shaped part. This study not only explores the failure model of FMLs but also offers valuable insights for process parameter improvement and how to optimize key parameters in engineering processes.

Mode I interlaminar fracture toughness of glare laminates by single cantilever beam

Mode I interlaminar fracture toughness of glare laminates by single cantilever beam

Bending behavior and damage evolution of GLARE laminate using in‐situ AE technology and FEM

AbstractGLARE laminate experiences multiple damage modes during bending failure, including fiber fracture, matrix cracking, interlayer delamination, and fracture of aluminum alloy. In most cases, two or more failure modes occur together, which makes the damage process complex and unpredictable. This paper proposes a combination of experimental testing and numerical simulation to investigate the influence of fiber lay‐up direction and center open‐hole on the bending failure behavior of GLARE laminate. Simultaneously, Acoustic Emission (AE) technology was used to monitor the bending process of GLARE laminates in real‐time, and a numerical simulation model for the three‐point bending of GLARE laminates was established. The experimental results showed that the bending strength of intact specimens decreased significantly with the increase of the fiber off‐axis angle, whereas the strength retention of open‐hole specimens increased significantly with the increase of the fiber off‐axis angle. Based on the K‐means++ clustering algorithm, the peak frequencies corresponding to different damage modes of GLARE laminates were identified as follows: aluminum alloy damage [1–70 kHz], matrix cracking [93–210 kHz], interlaminar delamination [211–304 kHz], and fiber breakage [304–516 kHz]. The simulation results agreed well with the experimental results, showing that the damage in intact specimens initiated at the laminate edges and propagated sequentially along the thickness direction, while the damage in open‐hole specimens initiated at the hole edges and then propagated towards the laminate edges.Highlights The present study investigated the effects of fiber lay‐up direction and central open‐hole on the bending performance and failure behavior of GLARE laminate. In‐situ monitoring of the GLARE laminate during three‐point bending was conducted using AE technology. A model for simulating the progressive damage of GLARE laminate was established and its accuracy was verified.

Effect of boundary conditions on the low‐velocity impact damage mechanism of fiber metal laminate

AbstractThe boundary condition is a very important factor when characterizing the impact performance of a material or a structure. However, its effect on fiber metal laminate (FML) has been rarely mentioned. In the present investigation, three levels of low‐velocity impact tests were conducted on the glass fiber reinforced aluminum laminate (Glare) under two different boundary conditions, one was that defined in ASTM D7136 and another one was self‐designed that has the same constraint area but different constraint degree. One basis of the experiments, finite element analyses were performed to gain a deeper understanding of the different damage mechanisms. With which another three boundary conditions were investigated for further considering the effect of constraint areas on both the impact and non‐impact sides, including that defined in another commonly used standard ASTM D5628 and two self‐designed cases. It was indicated that the boundary conditions influenced less the force and energy responses of Glare laminate, regardless of the impact damage levels, and hardly on the damage behavior as well when perforation was encountered. However, influences were obtained under lower and intermediate damage levels, and it was the constraint area on the rear side and constraint degree that played the dominant roles, respectively. The results achieved can provide suggestions for reasonably evaluating the low‐velocity impact performance of FML.Highlights Comparisons were made between the low‐velocity impact behavior of Glare laminates under five different boundary conditions. Boundary conditions influenced hardly on maximum impact force and energy absorbed ratio, but evidently on damage behavior. Constraint area and constraint degree governed damage mechanism under lower and intermediate impact energies, respectively. Influence of boundary conditions on damage behavior disappeared when the impact energy was enough to perforate Glare laminate.

GLARE laminate subjected to multiple oblique low velocity impacts considering frictional tangential compliance

The impacts occurring in real scenarios can be arbitrary, simultaneously impacting at different trajectories, giving rise to complexity in analysing the impact phenomena. This necessitates the implementation of a robust numerical finite element (FE) model capable of modelling such multiple impacts. The present paper investigates the multiple oblique low velocity impacts (OLVIs) on a clamped GLARE plate by spherical impactors exploring the influence of obliquity and their relative trajectories as well as the time delay between the OLVIs on the impact response and the extent of interfacial delamination. A 3D FE code has been developed considering the Hertzian contact model for modelling the normal component of contact and the theory of Mindlin and Deresiewicz is used for modelling tangential component considering friction. Results show that in addition to the obliquity of the impactors and the coefficient of friction, their relative trajectories, sizes and instant of their occurrences significantly influence the contact force histories, contact durations and the delamination at the interfaces.

Characterizing damage patterns and evolution in Multi-Hole GLARE laminates under tensile load via integrated AE and DIC techniques

In this study, a combined numerical simulation and experimental approach was employed to investigate the mechanical behavior and damage mechanisms of multi-hole GLARE laminates with varying off-axis angles under static tensile loading. Digital image correlation techniques were used to monitor changes in the strain field near the open hole region. The acoustic emission parameters were used to recognize damage patterns and evolution. The progressive damage evolution processes of various GLARE components were investigated using a 3D finite element model (FEM) based on the Hashin strain criterion and cohesive zone modeling. The results indicate that GLARE experiences damage modes such as aluminum alloy breakage, matrix cracking, fiber/matrix debonding, delamination, and fiber breakage. The frequency ranges associated with the five damage modes are as follows: [0–50 kHz], [100–175 kHz], [175–220 kHz], [220–300 kHz], and [300–400 kHz]. The dominant damage mode throughout the tensile phase is matrix cracking, but the ultimate failure of the specimen is determined by aluminum alloy breakage and fiber breakage. In addition, increasing the off-axis angle of the multi-hole leads to a shift in the final failure mode from tensile-dominated to tensile shear-dominated, while also slightly reducing the overall initial stiffness of the specimen.

Assessment of tensile damage mechanism of open‐hole GLARE laminates based on acoustic emission and digital image correlation techniques

AbstractGLARE laminates have emerged as a favored option for the construction of fuselage and wing skins in large airliners, owing to their exceptional mechanical characteristics. Nevertheless, the incorporation of open‐hole designs poses a challenge as it disrupts the continuity of the laminates, resulting in stress concentration and subsequent damage. To investigate the impact of various layup orientations and hole sizes on the tensile properties of open‐hole GLARE laminates, this study conducted axial tensile tests. Additionally, the tensile damage process was monitored using DIC and AE techniques, enabling the identification of damage patterns and the analysis of their evolution. The results demonstrate a noteworthy decline in the ultimate strength and failure strain as the size of the opening increases. Moreover, the retention rate of failure strain displays a marked sensitivity to the layup orientation. In conjunction with observations made through DIC and SEM, the k‐means++ algorithm successfully clustered peak frequencies, thereby revealing distinct damage patterns and their corresponding frequency ranges as aluminum alloy damage [0–90 kHz], matrix cracking [104–174 kHz], fiber/matrix debonding [175–224 kHz], interlaminar delamination [234–300 kHz], and fiber fracture [304–469 kHz]. AE cumulative counts were utilized to evaluate the progression of individual damage modes. The results emphasize that matrix cracking demonstrates the most substantial cumulative counts, whereas damage to the fibers and aluminum alloy noticeably affects the load‐carrying capability of the laminate. Furthermore, the fibers/matrix debonding and interlaminar delamination, exhibit heightened susceptibility to layup orientation and hole size.Highlights The tensile performance evaluation of GLARE laminates with open‐hole varying layup orientations and hole sizes was investigated. A combination of DIC and AE techniques was used to monitor the tensile damage process. Damage pattern was identified based on the observations of DIC and SEM. An approach based on Pearson's correlation coefficient and k‐means++ clustering algorithm was used for damage pattern recognition in GLARE laminates. AE cumulative counts were employed to assess the evolution of each damage mode.

A novel active hydroforming & curing process to manufacture GLARE laminates: Numerical and experimental investigations

Fiber Metal Laminates (FMLs) are preferable thin-walled structures in several fields for their exceptional mechanical properties. However, how to manufacture thin-walled FMLs structures with complex shapes and guarantee the high mechanical properties is still a challenge. To address these, this paper proposes an innovative hybrid forming method, referred to as active warm hydroforming and curing, to enhance the formability and performance of FMLs in GLARE form by combining the forming and curing process into a single step. The hybrid method was verified on a box-shaped part as a proof-of-concept to assess the feasibility and parameter influence. Online curing was utilized to enhance efficiency and quality. The outcomes revealed that the new method increases the ultimate tensile stress by 12.1 % and reduces curing deformation from 3.1 mm to 1.3 mm compared to the vacuum bagging method. Furthermore, the critical parameters in forming of thin-walled structures, including blank holder force (BHF), pressure rate (PR), maximum pressure (MP), and forming temperature (FT), were identified using experimental, mathematical, and numerical approaches. Four defects in the formed thin-walled structures were observed and successfully eliminated through the control of these parameters. The method and results presented in this paper also provide direct guidance for optimizing the process parameters of FMLs warm hydroforming.

Dynamic impact behavior and notched fatigue life of GLARE laminates

As a member of the fiber metal laminates (FMLs) family, GLARE laminates have been widely used in aircraft skin structure due to the excellent resistance to dynamic impact and fatigue loadings. The low-velocity impact of GLARE laminates are conducted to explore the mechanical response on the impact energy and punch shape. The impact indentations are characterized by the damage morphology and quantitative dimension analysis. The mechanism of impact damage resistance is illuminated by the contact force-time curve, intra-laminar and inter-laminar damage modes. The lagrangian SPH modelling is employed to numerically analyze the bird-strike behavior of GLARE laminates, revealing the variation of bird particle flow and impact energy with different impact attitudes. The tension-tension fatigue of GLARE laminates is experimentally performed subjected to different stress levels. The influence of the circular open-hole on the fatigue residual life of GLARE laminates is studied. Considering the uncertainty of S-N curves, the probabilistic fatigue life P-S-N curves are developed under different survival probabilities. These would be more positive for the fatigue life estimation and reliability design for GLARE laminates in aircraft engineering application.

DIC-based constant amplitude and two-block loading fatigue life prediction of open hole GLARE laminate

The constant amplitude fatigue life is predicted using the normalized fatigue stress with the goodness of fit above 0.98. The crack initiation life was estimated based on the stress concentration factor and the surface axial strain measured by DIC. The high-low amplitude sequence two-block loading fatigue life was predicted using an iterative calculation method based on the corrected crack tip effective stress intensity factor range, almost all results are in a 1.3-scatter band. The previous nonlinear fatigue damage cumulative model was further modified for two-block loading, the life prediction results are within a 1.2-scatter band.

Quasi‐static fracture characterization of fiber metal laminate with crack resistance curve based on δ5

AbstractFiber metal laminate (FML) is a hybrid composite typical in aircraft industry. It is necessary to effectively characterize the quasi‐static fracture performance of FML, considering the importance of damage tolerance design of aircraft structures and the complicated fracture mechanism of FML. In present investigation, glass fiber reinforced aluminum laminates (Glare) with middle cracks were tested under quasi‐static tensile load. The experimental crack resistance curve (R curve) based on the special crack opening displacement at original crack tip (δ5) was used for revealing the superior crack resistance of Glare laminate to aluminum sheet, which expressed in crack extension aspect was the extended critical crack length. This was thought to be benefit from the contributions of glass fiber layers and fiber/metal delamination, by performing the progressive damage analysis. Since δ5 can embody the elastic–plastic response of the crack but without need to interactively correct for crack tip plasticity. The δ5‐R curve‐based macro and meso fracture models were proposed to accurately predict the residual strength and stable crack extension of Glare laminate, which gave further evidence for the effectiveness of δ5‐R curve in characterizing the quasi‐static fracture performance of FML.

Machining GLARE fibre metal laminates: a comparative study on drilling effect between conventional and ultrasonic-assisted drilling

GLARE laminates are multi-layered metal-composite materials created from bonding sheets of metallic alloys with carbon or glass fibre layers. The application of hybrid-conventional machining processes such as ultrasonic-assisted drilling (UAD) is becoming of great interest to the aerospace industry due to its capability in reducing the cutting forces and tool wear which are directly responsible for drilling-induced delamination. There is rich literature on the conventional drilling (CD) of GLARE, but no work reported using UAD process. This study will fill this gap and investigate the UAD of GLARE laminates using an indigenously developed UAD system. The influence of spindle speed and feed rate on thrust force and surface roughness metrics (Ra and Rz) were investigated under CD and UAD regimes. The quality of the borehole and damage mechanisms in the laminate constituents was examined using scanning electron microscopy (SEM). The contribution of the drilling parameters on the measured outputs was further evaluated using the analysis of variance (ANOVA) statistical analysis. It was found that UAD resulted in a significant reduction in thrust force by up to 65% while surface roughness metrics Ra and Rz were unaffected by the type of drilling process used. SEM analysis showed irregular and fuzzier surfaces in glass fibre layers in holes machined using UAD due to the longitudinal vibration of the tool.

Low-velocity impact (LVI) and post-impact fatigue properties of GLARE laminates with holes

The impact peak force and the post-impact maximum permanent displacement of GLARE laminates with double holes are predicted with the goodness of fit above 0.98 within the impact energy range of 5–30J. The normalized post-impact residual fatigue life decreases exponentially with the increase of impact energy and the post-impact maximum permanent displacement. DIC analysis shows that delamination and local deformation promote crack initiation between two holes and simultaneous initiation of multi cracks. The compressive plastic deformation in the dent center hinders the fatigue crack propagation. A multiple fatigue source failure mode is observed with the increase of impact energy.

Delamination in GLARE laminates subjected to oblique low velocity impact considering friction

Study of oblique low velocity impact and the subsurface delamination caused in fibre metal laminates is not well reported in literatures. The present paper numerically investigates the oblique low velocity impact behaviour of clamped GLARE plates by a spherical impactor to study the influence of obliquity and coefficient of friction on the impact mechanism and on the extent of interfacial delamination. A three dimensional finite element formulation employing Newmark-β method has been developed for analyzing the oblique contact impact response of GLARE plate where Hertzian contact model is used for the normal component of contact and Mindlin and Deresiewicz theory is used for the tangential component. Results show that besides impactor velocity, obliquity of incidence and the coefficient of friction significantly influence the contact force history, contact duration and the delamination at the interfaces. In addition, the oblique angle also decides the instant and duration of reversal of tangential contact force where increasing obliquity leads to delayed instant and reduced duration of such reversal.

The effect of surface treatments on the interlaminar shear failure of GLARE laminate included AA6061-T6 layers by comparing failure characteristics

ABSTRACT The effect of surface treatments on hot-pressed GLARE 3/2 laminates included AA6061-T6 is evaluated in this paper. Sandpapering, degreasing and both degreasing and sandpapering are applied on AA6061-T6 sheets in order to contribute the improvement of the interlaminar shear strength (ILSS). The three-point bending tests are carried out by using two different span length-to-specimen thickness ratios (S/h) to examine also the effect of shear and bending on deformation and failure characteristics. Strength values and failure modes are obtained for both S/h ratios and all surface treatment types. Load-displacement and stress-displacement curves are evaluated. The results show that the maximum shear strength values for surface-treated laminates are increased by between 29% and 37% against untreated one. It is observed that the strength values and failure modes of all surface-treated GLARE laminates are influenced by an increase in the S/h ratio from 4.0 to 10.0 because of the increase in bending effect against shear.

Numerical Shear Buckling Investigation of GLAREs with Initial Delamination

GLAREs as hybrid materials are widely employed members of fibre metal laminates (FMLs) in aerospace structures. Delamination is one of the most critical failure modes that happen during the manufacturing or service life, decreasing the stability and reliability of the GLARE laminates. Accordingly, this study presents an investigation of the shear buckling phenomenon, a very common load condition in aerospace structures, of different GLAREs, differentiating in the laminate layout, containing embedded delamination by the FEM ABAQUS software. The cohesive zone interface elements of FM94 are considered to evaluate the delamination growth. This parametric study displays the effect of the initial delamination dimension and its position as well as the thickness of the metal layer on the stability of GLARE laminates. The model predicts the stability reduction of GLARE laminates as a function of the delamination radius and its position. The results show that aluminium layer reinforcement increases the overall stiffness and shear buckling capacity and the specimen’s resistance to delamination growth. In addition, the produced results in this study can encourage designers to select appropriate GLARE grades based on the intended application, such as different shear loading conditions and available raw materials.

Comparative Study on the Influence of Hybrid Nano Fillers Modified Epoxy in the Mechanical Behavior of GLARE Laminates

Comparative Study on the Influence of Hybrid Nano Fillers Modified Epoxy in the Mechanical Behavior of GLARE Laminates

Evolution Behaviors and Reduction Mechanism of Curing Residual Stresses in GLARE Laminates under a Hot-Pressing Condition.

Nowadays, variable preparation, forming and processing methods of fiber metal laminates are constantly developing to meet the requirements of different application fields, hence the characteristics and evolution of residual stresses under different manufacturing conditions deserve more attention. In this work, the evolution behaviors of curing residual stresses in GLARE under a hot-pressing condition were studied, and the residual stress reduction mechanism was also explained. Results suggested the FE prediction models of the entire cure process, verified by the fiber Bragg grating (FBG) sensors, were more precise than the traditional elastic model. Moreover, the stress evolution during the cure process mainly occurred in the cooling stage, in which the different coefficient of thermal expansion (CTE) of aluminum and GFRP played a major role. Meanwhile, curing shrinkage stress in the GFRP layer during the holding stage at curing temperature obviously influenced the final stress level. The residual stresses in GFRP layers differed by 9.6 MPa under a hot-pressing and autoclave condition, in which the convection heat transfer condition played a major role as it caused lower thermal stress in the holding stage and a smaller temperature gradient in the cooling stage. Considering this, a lower cooling rate could be a feasible way to obtain GLARE with lower residual stress under a hot-pressing condition.

Comparative Study on the Experimental Results on Low-Velocity Impact Characteristics of GLARE Laminates with Simulation Results from LS Dyna

Comparative Study on the Experimental Results on Low-Velocity Impact Characteristics of GLARE Laminates with Simulation Results from LS Dyna

Drilling of glare laminates: effect of cutting parameters on process forces and temperatures

Drilling of glare laminates: effect of cutting parameters on process forces and temperatures