Intralaminar Damage Research Articles

Effect of Cohesive Properties on Low-Velocity Impact Simulations of Woven Composite Shells

The effect of the interface stiffness and interface strength on the low-velocity impact response of woven-fabric semicylindrical composite shells is studied using finite element (FE) models generated with continuum shell elements and cohesive surfaces. The intralaminar damage is accounted for using the constitutive model provided within the ABAQUS software, while the interlaminar is addressed utilising cohesive surfaces. The results show that the interface stiffness has a negligible effect on the force and energy histories for values between 101 N/mm3 and 2.43 × 106 N/mm3. However, it has a significant impact on the delamination predictions. It is observed that only the normal interface strength affects the maximum impact force and the delamination predictions. Increasing its value from 15 MPa to 30 MPa resulted in an 8% growth in the maximum force, and a substantial reduction in the delaminated area. The obtained results serve as guidelines for the accurate and efficient computation of delamination. The successful validation of the FE models establishes a solid foundation for further numerical investigations and offers the potential to significantly reduce the time and expenses associated with experimental testing.

Numerical Simulations of the Low-Velocity Impact Response of Semicylindrical Woven Composite Shells.

This paper presents an efficient and reliable approach to study the low-velocity impact response of woven composite shells using 3D finite element models that account for the physical intralaminar and interlaminar progressive damage. The authors' previous work on the experimental assessment of the effect of thickness on the impact response of semicylindrical composite laminated shells served as the basis for this paper. Therefore, the finite element models were put to the test in comparison to the experimental findings. A good agreement was obtained between the numerical predictions and experimental data for the load and energy histories as well as for the maximum impact load, maximum displacement, and contact time. The use of the mass-scaling technique was successfully implemented, reducing considerably the computing cost of the solutions. The maximum load, maximum displacement, and contact time are negligibly affected by the choice of finite element mesh discretization. However, it has an impact on the initiation and progression of interlaminar damage. Therefore, to accurately compute delamination, its correct definition is of upmost importance. The validation of these finite element models opens the possibility for further numerical studies on of woven composite shells and enables shortening the time and expenses associated with the experimental testing.

Mesoscale modelling of progressive damage and failure in single-lap and double-lap thin-ply laminated composite bolted joints

Composite bolted joints are being used extensively in primary load-bearing structures of modern aircrafts. However, simulating progressive damage and failure of the bolted composite structures under high bearing loading is still challenging. In this paper, numerical simulation and experimental verification on the bearing failure of single-lap and double-lap thin-ply laminated composite bolted joints were carried out for the entire loading process. 3D explicit finite element models have been developed using Abaqus/Explicit together with a 3D physically-based intralaminar damage model implemented in a VUMAT subroutine. An interface cohesive-zone model based on the Benzeggagh-Kenane (B-K) law was used to simulate delamination initiation and evolution. For the intralaminar damage, the initiation was determined based on Pinho’s failure criteria while the evolution was regularized with the crack-band approach to alleviate mesh-size dependence. Element deletion and in-situ effect as practical numerical strategy are discussed and analyzed in detail. Element deletion is used to prevent excessive element distortion and capture corresponding post-peak bearing failure behavior. In-situ effect has been proved to have a key influence on the damage initiation and progression of thin-ply composite matrix. The final simulation results are shown to agree well with experimental data and reproduce the mechanical response curves. More importantly, the model can accurately capture the local crushing of bolt hole in the later loading stage. This study shows that the numerical model can be helpful for the design and optimization of composite bolted structures.

The role of intralaminar damages on the delamination evolution in laminated composite structures

Delamination is a major failure mode affecting laminated composite structures. This failure mechanism, if not properly monitored, can lead to uncontrolled cracks growth and premature structural collapse. Thus, predicting delamination propagation is mandatory to determine the structural integrity. At present, delamination has been extensively investigated in laminated composite structures but only a few studies have been performed on how the intralaminar damages influence the interfacial release energy and, consequently, the delamination evolution. In this paper, the well-established SMart-Time XB delamination simulation tool, has been coupled with a Hashin Criteria based User-Material Subroutine (UserMat) to study the role of the intralaminar damages in delamination propagation. A benchmark case based on the mixed-mode I/II Single Leg Bending (SLB) specimen for delamination evolution assessment under quasi-static loading has been investigated. Subsequently, a composite material plate, characterized by an artificial circular delamination, under compression has been considered. This study proved to be valuable by underlining the influence of fibre and matrix breakage on the interlaminar damages evolution. Interestingly, taking into account the effect of intralaminar flaws changes the local energy release rates values on the delamination front and, consequently, the crack shape and evolution.

Experimental Investigation on the Failure Behavior of Single-Stiffened Composite Panels under Compressive Load

Whilst the thin-walled composite structures have been widely used to build the primary components in aircraft, Low-Velocity Impact (LVI) can cause severe intra-laminar and inter-laminar damage in the composite material, reducing the mechanical performance of these components. This paper presents an experimental study on the compressive failure behavior of a Single Hat-Stiffener Panel (SHSP) with two typical flaws, i.e., impact-induced damage and skin/stiffener debonding. LVI tests were performed to introduce the damage around the radius edge of the stiffener and the center of the flat skin panel, respectively, while the skin/stiffener debonding was introduced to the SHSP by artificially embedding a PTFE film at the skin/stiffener interface during autoclave curing. The impact energy to cause Barely Visible Impact Damage (BVID) was determined to be 22 J for the thin SHSP configuration. Whilst the considerable damage area was detected by the ultrasonic scan for the thick SHSP at an energy level of 35 J, the impact-induced indentation was not visually detectable (<0.35 mm), indicating a high impact resistance of this thick SHSP. Compression After Impact (CAI) results showed a significant reduction (59.1%) in the ultimate compressive strength of the SHSP when the impact location was on the stiffener compared to the pristine specimen. However, this reduction is less significant (34.7%) when the damage resulted from the same impact energy in the flat skin panel. In this study, the presence of skin/stiffener debonding does not adversely influence the compressive strength of the thick SHSP, while this debonding flaw appears to cause a minor reduction in the compressive strength of the thin SHSP.

Experimental and numerical studies on the repeated low-velocity impact response and damage accumulation in woven S2-glass fibre/epoxy composites

In this paper, the repeated low-velocity impact response of woven S2-glass/epoxy composites is studied. The impacts were performed with energies from 18.4 to 59.2 J using a drop-tower apparatus, and a post-mortem analysis after each impact was employed to assess the impact response. A damage index was used to describe the changes in impact response due to repeated impacts. Finite element simulations considering both interlaminar and intralaminar failure modes were performed. The results showed that the impact force and bending stiffness decreased with the number of impacts, while impact duration and maximum central displacement increased. The shape of the damaged area was also affected. The numerical results showed that interlaminar damage initiated at most interfaces during the first impact, followed by in-plane propagation in the next impacts. Also, intralaminar damage initiated at the backside of the laminate, and then in-plane and through-thickness propagations followed until penetration occurred.

Low-velocity impact resistance behaviors of bionic double-helicoidal composite laminates

To improve the low-velocity impact resistance of composite laminates, a bionic double-helicoidal composite laminate (BDHCL) emulated from the micro-structure of coelacanth scales is proposed. The micro-structure is a double helicoidal Bouligand structure in which two adjacent collagen layers are arranged into an orthogonal bilayer and then rotating at a small helix angle. Three types of helicoidal configurations with different derivatives are designed, including sine rotation angle (SRA), linear rotation angle (LRA), and exponential rotation angle (ERA). The progressive intra-laminar and inter-laminar damage model is adopted to capture the failure behaviors of material. The impact behaviors of BDHCLs with several helix angles and included angles of the bilayer are studied. The results show that the double-helicoidal Bouligand structure has a higher resistance load and shallower penetration depth. The impact resistance of ERA laminate has the maximum improvement among the above three types of helix angle. A certain included angle of bilayer for the BDHCL will exhibit excellent impact resistance.

Design, Manufacturing, and Numerical Characterization of Hybrid Fiber Reinforced Polymer under Dynamic Loads

In the present work, the response of composite hybrid laminates made of carbon and glass fibers in different stacking configurations was tested under low-velocity impact loads. Experimental drop impact tests were conducted on three different stacking sequences and three rising impact energy levels. The results from the tests were assumed for the development and validation of a numerical impact model reproducing for each stacking sequence and all the impact energy levels, the laminates impact response. The validated model investigated the occurred damage mechanisms, their distribution in the panel thickness and their extension on the plane of the laminate. Depending on the stacking sequence and the impact energy level, the energy absorption capacity was related to the dominant damage mechanism. The percentage contribution of interlaminar and intralaminar damages was presented and conclusions were drawn about the influence of stacking sequences on energy absorption and damage characteristics.

A numerical investigation of the interaction between interlaminar and intralaminar damages in a fatigued composite panel

AbstractThe fatigue behavior of composite materials is still a very challenging issue for the scientific community. So far, several numerical methodologies allow us to faithfully simulate the propagation of delamination due to fatigue. However, the reduction of material properties due to the applied cyclic load should be considered. The main objective of this work is to investigate the role of the material property degradation and the intralaminar damages on the propagation of delamination in composite laminates under fatigue loads. The numerical tool FT‐SMXB, based on the Paris Law and virtual crack closure technique, for the mimic of fatigue‐driven delamination, has been integrated with a user material subroutine, based on the generalized residual material property 7degradation model and the Hashin fatigue failure criteria. Typical aeronautical stiffened panel has been considered as test case. Matrix damages and strength degradation due to fatigue cycles have been found to influence the propagation of delamination.

Modelling the effects of patch-plug configuration on the impact performance of patch-repaired composite laminates

The patch-plug configuration has been widely used to repair composite structures and restore the structural integrity of damaged composites. In the present research, single-sided CFRP patch-repaired panels, with different patch-plug configurations, are prepared. This is where a circular-shaped damaged area has been removed and a CFRP patch has been adhesively-bonded onto the panel. In some cases, a CFRP plug is inserted into the hole, caused by removal of the damaged area, before the patch is applied. Such patch-repaired panels, and the pristine CFRP panel, are subjected to a low-velocity impact at an energy of 7.5 J. These impacted pristine and repaired panels are then examined using ultrasonic C-scan and optical microscopy to inspect the impact-associated permanent indentation, interlaminar and intralaminar damage. A finite element analysis (FEA) model, which significantly extends a previously validated elastic-plastic (E-P) numerical damage model, has been developed to predict the impact behaviour of the pristine CFRP panel and the various designs of patch-repaired CFRP panels. The comparison between the experimental and numerical results for all the studied cases shows the maximum deviations for the loading response and the damage area are 12% and 15%, respectively. The good agreement between the experimentally-measured impact properties and those predicted using the numerical model demonstrates that the model is a useful design tool.

Mechanical properties and damage behavior of tapered unidirectional CFRP laminates

An understanding of damage mechanisms induced by drop-off plies in tapered composite laminates is crucial. This study investigated the mechanical properties and damage behaviors of asymmetric tapered unidirectional carbon fiber-reinforced plastic laminates. Damage observations of laminates with simultaneous tapered and staggered tapered structures under monotonic and cyclic tensile loads were done by an optical microscope and X-ray radiography. Based on the results, matrix cracks in the resin pocket and delamination occurred earliest in the simultaneous tapered specimen due to a greater stress concentration in a single large resin pocket in the structure. Intralaminar damages in the staggered tapered specimen with a shorter step spacing occurred mainly in the lower dropped ply as there was a more significant interaction between the neighboring steps in the specimen. The drastic decrease in the stress concentration after the occurrence of damages in the neighboring step(s) then suppressed the occurrence of intralaminar damages in the upper dropped ply. This is in contrast to the specimen with a longer step spacing where the intralaminar damages occurred in both dropped plies due to the distributed stress distribution. Delamination propagated between the dropped plies and continuous (belt and core) plies for both staggered laminates regardless of the length of the step spacing at higher applied stress levels. The results showed that internal ply-drop configuration and step spacing in staggered tapered structures contribute to significant differences in the mechanical properties and damage behavior in the laminates.

Damage in hybrid corrugated core sandwich structures under high velocity hail ice impact: A numerical study

Potential damage in composite structures caused by hail ice impact is an essential safety threat to the aircraft in flight. In this study, a nonlinear finite element (FE) model is developed to investigate the dynamic response and damage behavior of hybrid corrugated sandwich structures subjected to high velocity hail ice impact. The impact and breaking behavior of hail are described using the FE-smoothed particle hydrodynamics (FE-SPH) method. A rate-dependent progressive damage model is employed to capture the intra-laminar damage response; cohesive element and surface-based cohesive contact are implemented to predict the inter-laminar delamination and sheet/core debonding phenomena respectively. The transient processes of sandwich structure under different hail ice impact conditions are analyzed. Comparative analysis is conducted to address the influences of core shape and impact position on the impact performance of sandwich structures and the corresponding energy absorption characteristics are also revealed.

Numerical-experimental study on the compressive behaviour of repaired composite panels

Barely Visible Impact Damages (BVID) caused by impact phenomena during maintenance and in-service missions can be a critical issue for composite structures. BVIDs may induce delaminations and intra-laminar damages, which can considerably reduce stiffness and strength of composite laminates leading to component replacement or repair.This work is aimed to numerically and experimentally investigate the mechanical behaviour of a repaired aeronautical composite stiffened panel subjected to compressive loading conditions. A numerical activity has been performed by introducing Finite Element models in the ABAQUS FEM environment with the aim to numerically simulate the mechanical behaviour of the investigated stiffened panel configuration. Indeed, a pristine panel configuration and a repaired panel configuration have been numerically modelled. In the repaired configuration a stepped bonded patch has been used to simulate the repair process. Then, an experimental testing activity has been performed on the repaired stiffened panel configuration and experimental data have been compared to results obtained from the numerical studies for model validation purposes. The validated numerical model has been used to perform a sensitivity analysis on the compressive behaviour of repaired stiffened panels with patches to assess the influence of stacking sequence and sizes of the repair patches on the panel's mechanical behaviour. Although other components are involved in the tightness of the repair patch, the work proved that the stacking sequence of the patch play the main role in the effectiveness of the process in recovering the mechanical performances and overall behaviour of the component.

Damage characterization of CFRP laminates using acoustic emission and digital image correlation: Clustering, damage identification and classification

Damage mechanisms in composite laminates are quite complex, and it is necessary to perceive their effects on the degradation of laminate mechanical properties. This work employs acoustic emission (AE) and digital image correlation (DIC) techniques to describe the evolution of intra/inter-laminar damage modes in the CFRP laminates under in-plane/out-of-plane loading conditions. In this study, laminates of stacking sequences [900]8, [450]8, [450/−450]2s, and [00]8 under tensile load are investigated to distinguish the intra-laminar damages like matrix cracking, fiber–matrix debond, and fiber breakage. Double cantilever beam, end notch flexure, and mixed-mode bending specimens are used to characterize delamination failure in the laminate. An unsupervised k-means clustering technique is used to classify the AE data based on peak frequency and amplitude. The surface displacement and strain data are evaluated using the DIC technique to understand the damage evolution in the laminates. Post failure analysis is carried out using a digital microscope, and fractography studies are used to identify and assign the damages to different AE clusters. This investigation yields a taxonomy of damage modes, their sequence of occurrence, and failure strains that can be used for structural health monitoring and progressive damage modeling of composite laminates.

Examining slow crack growth metrics and competing failure modes in IM7/5320-1 carbon fiber reinforced polymer laminates with pre-existing damage states

The growing use of composites in aircraft structures has necessitated a slow growth criteria to assess the life of these materials and an appropriate similitude parameter. Due to the simplified stress state and inherent interactions of intralaminar and interlaminar damage progression in multi-directional ply layups, there have been questions about the applicability of double cantilever beam specimens to aerospace applications. This study examines these interconnected damage growth mechanisms in [±453]s and [±452/902]s layups via compact tension specimen geometries, with and without pre-existing damage, in order to examine the efficacy of proposed similitude parameters in capturing slow crack growth. The change in stored potential strain energy per cycle provided the most unified results across the 18 tests under different loading and pre-existing damage conditions. Via microscopy of the specimens after damage progression, the similitude parameters were connected to overall trends in the damage mechanisms. The fatigue results, coupled with the microscopy of the interactions amongst the complex damage mechanisms, demonstrates the need to analyze proposed similitude parameters across various sample geometries and stress states, which will be more enable an understanding of composite material fatigue performance in aerospace applications.

Modeling Large-Surface Impact-Induced Damage in Iteratively Characterized Filament-Wound Composite Pipes: A Numerical and Experimental Investigation

With the widespread use of fiber reinforced polymer (FRP) composite pipes, their susceptibility to impact damage remains a significant cause of concern. This work investigates the structural response and damage propagation of glass-fiber reinforced epoxy (GRE) pipes under large-surface low-velocity impacts. A series of drop-weight impact tests of varying heights is conducted and compared to numerical finite element (FE) simulations. Then, plies are individually modeled and assigned with properties obtained from the authors’ earlier work. Utilizing composite failure theories and mixed-mode delamination theories, the simulated structural responses including the load–displacement, strain–displacement response and damage propagation are compared and validated with the experimental results. It was found that the structural response is well predicted at higher drop heights and there is a significant change in damage type and propagation with increasing drop heights. The proposed approach builds on the authors’ earlier work and provides a modeling approach for the prediction of structural response, inter- and intra-laminar damage with just pipe level properties.

A progressive damage model for composite laminates based on non-linear continuum mechanics—Static and fatigue loadings

Efficient yet reliable predictive modelling tools for damage tolerance analysis became an aerospace industry requirement as composite materials provided the potential for design performance. As damage can be separated into interlaminar and intralaminar, different approaches were developed over the years. For fracture analysis of intralaminar damage, a physical link to the Linear Elastic Fracture Mechanics (LEFM) provides a reliable framework for Continuum Damage Mechanics (CDM) models. These models, however, may exhibit pathological problems related to mesh dependence, objectivity and convergence issues. Differently from Cohesive Zone Modelling (CZM) for interlaminar damage, most intralaminar models are based on phenomenological approaches for fatigue analysis and life prediction. This work provides a description of some of the current intralaminar CDM models shortcomings, related to large strain analysis and finite element topology. A novel progressive damage model is proposed where damage variables are linked to the deformation gradient. Additionally, a fatigue damage behaviour under the assumption of a Paris law for crack evolution is implemented in the proposed methodology. Both static and fatigue analysis were performed on a Compact Tension (CT) specimen geometry and compared to experimental data available on the scientific literature. Model predictions and experimental data were confronted allowing for conclusions to be drawn.

An integrated modeling of barely visible impact damage imaging of CFRP laminates using pre-modulated waves and experimental validation

Nonlinear ultrasonic techniques have been broadly used for detecting barely visible impact damage (BVID) in carbon fiber reinforced plastic (CFRP) from both numerical and experimental approaches. However, the interaction between the BVID and nonlinear ultrasonic is not considered in the currently available numerical models and BVID needs to be more accurately modeled. This work presented an integrated three-dimensional (3D) finite element (FE) model for imaging BVID using pre-modulated wave (PMW) technique, in which the 3D Hashin damage criterion and energy-based damage evolution are introduced to predict intralaminar and interlaminar damage during the impact process. The non-contact PMW tests based on a laser scanning Doppler vibrometer (LSDV) are adopted to experimentally validated the proposed numerical methodology. The results show that the BVID predicted by the FE model agrees well with the experiments. Then, the damage contour images based on frequency spectra of modulation are illustrated to identify the presence of BVID, and damage severity is quantified based on the proposed maximum response amplitude (MRA). It is found that the location of BVID can be identified by the PMW approach numerically and experimentally and the sum of MRA of sidebands exhibits the best sensitivity of the variation of BVID with different energies.

Effects of contact friction and ply blocking on the crush behaviour of thin-walled composite structures: A numerical study

The present paper presents a three-dimensional composite damage model for predicting the crush response of thin-walled structures, i.e., cylindrical tubes, manufactured employing fibre-reinforced polymer–matrix composites. This computational model is based upon a Continuum Damage Mechanics (CDM) approach and accounts for both the intralaminar and interlaminar damage as well as nonlinear behaviour that occur in the composite materials. Experimental data, obtained from published literature, are employed to validate the proposed composite damage model. A comparison between the experimental and computationally predicted results, including the load response, energy absorption and damage morphology, shows good agreement. Subsequently, the validated computational model is employed to investigate the effects of contact friction and ply blocking on the crush response of thin-walled composite structures. The results reveal that the friction between the tube/platen surfaces has a positive effect on the crushing performance of the composite structures and the ply blocking can somehow inhibit the crushing performance of the investigated composite structures.

Finite element method of a progressive intralaminar and interlaminar damage model for woven fibre laminated composites under low velocity impact

A novel three-dimensional damage model based on continuum damage mechanics (CDM) is established for the analysis of damage and fracture in woven carbon fibre reinforced plastic (CFRP) laminates. The damage model was developed in LS-DYNA, a nonlinear explicit finite element software, and the model was incorporated with the user-defined material. The multiple dimensions damage model was applied to predict the intralaminar damage initiation and propagation. A cohesive zone model was used to simulate the delamination induced by interlaminar cracks. The simulation results of four different impact energies (5 J, 8 J, 10 J, and 15 J) were analysed by impact responses. Moreover, the results of different damage modes and fracture modes of two representative ply orientations ([0]8 and [45]8) are discussed in detail.The impact peak force, energy absorption, and maximum crack length in the results of simulating woven carbon fibre laminated composites with different ply orientations are within 5% of the experimental results. Accordingly, the established model can accurately predict the crack evolution of [0]8 and [45]8, thus demonstrating that the proposed methodology is more suitable for damage simulations of woven CFRP laminates with different ply orientations under low velocity impact than existing methods.