CERES :: Browsing by Author "Xu, Yigeng"

Browsing by Author "Xu, Yigeng"

Now showing 1 - 20 of 20

Open Access
Adhesive joint geometry variation in non-rigid aircraft structures
(Cranfield University, 2019-11) Coladas Mato, Pablo; Webb, Phil; Xu, Yigeng
Adhesive bonding is a proven alternative to mechanical fasteners for structural assembly, offering lighter and thus more fuel efficient aircraft and cost-effective manufacturing processes. The effective application of bonded structural assemblies is however limited by the tight fit-up requirement, which is with tolerance ranges of hundreds of microns; this can be a challenge for the industry to meet considering the variability of current part manufacturing methods and the conservative nature of the conventional tolerance stack-up analysis method. Such a (perceived) limitation can discourage effective exploitation of bonding technologies, or lead to development of overengineered solutions for assurance. This work addresses such challenge by presenting an enhanced bondline thickness variation analysis accounting for part deflection of a bonded skinstringer assembly representing a typical non-rigid airframe structure. A semianalytical model accounting for unilateral contact and simplified 1D adhesive flow has been developed to predict bondline thickness variation of the assembly given the adherends’ mechanical properties, adhesive rheological properties, and external assembly forces or boundary conditions. A spectral-analysis method for assembly force requirement estimation has also been tested. The bondline dimensions of several representative test articles have been interrogated, including a reconfigurable test assembly designed specifically to test the input conditions that affect bondline geometry variation. It has been demonstrated that the part deflections need to be accounted for regarding the fit-up requirement of bonded non-rigid structural assembly. The semi-analytical model has been found to more reliable and realistic prediction of bondline thickness when compared to a rigid tolerance stack-up. The analysis method presented can be a major technology enabler for faster, more economical development of the aircraft of the future, as well as of any analogue structures with high aspect ratios where weight savings and fatigue performance may be core objectives.
Open Access
An Analytical Model for the Identification of the Threshold of Stress Intensity Factor Range for Crack Growth
(Hindawi, 2017-01-18) Grasso, Marzio; De Iorio, Antonio; Xu, Yigeng; Haritos, George; Mohin, M.; Chen, Yong K.
The value of the stress intensity factor (SIF) range threshold ( ) for fatigue crack growth (FCG) depends highly on its experimental identification. The identification and application of are not well established as its determination depends on various factors including experimental, numerical or analytical techniques used. A new analytical model which can fit the raw FCG experimental data is proposed. The analytical model proposed is suitable to fit with a high accuracy the experimental data and capable to estimate the threshold SIF range. The comparison between the threshold SIF range identified with the model proposed and those found in literature is also discussed. The identified is found to be quite accurate and consistent when compared to the literature with a maximum deviation of 5.61%. The accuracy with which the analytical model is able to fit the raw data is also briefly discussed.
Open Access
Attention mechanism enhanced spatiotemporal-based deep learning approach for classifying barely visible impact damages in CFRP materials
(Elsevier, 2024-03-14) Deng, Kailun; Liu, Haochen; Cao, Jun; Yang, Lichao; Du, Weixiang; Xu, Yigeng; Zhao, Yifan; This work was partially supported by the Royal Academy of Engineering Industrial Fellowship [#grant IF2223B-110], and partially supported by the Science and Technology Department of Gansu Province Science and Technology Project Funding, 22YF7GA072.
Most existing machine learning approaches for analysing thermograms mainly focus on either thermal images or pixel-wise temporal profiles of specimens. To fully leverage useful information in thermograms, this article presents a novel spatiotemporal-based deep learning model incorporating an attention mechanism. Using captured thermal image sequences, the model aims to better characterise barely visible impact damages (BVID) in composite materials caused by different impact energy levels. This model establishes the relationship between patterns of BVID in thermography and their corresponding impact energy levels by learning from spatial and temporal information simultaneously. Validation of the model using 100 composite specimens subjected to five different low-velocity impact forces demonstrates its superior performance with a classification accuracy of over 95%. The proposed approach can contribute to Structural Health Monitoring (SHM) community by enabling cause analysis of impact incidents based on predicting the potential impact energy levels. This enables more targeted predictive maintenance, which is especially significant in the aviation industry, where any impact incidents can have catastrophic consequences.
Open Access
Damage tolerance enhancement of thermoset composites modified with thermoplastic veil interleaves
(Cranfield University, 2019) Ramji, Amit; Xu, Yigeng
A feasible method of overcoming the stringent certification and ecological sustainability objectives within the aerospace sector is to substitute conventional engineering alloys with high-performance lightweight composite materials for primary structures. Although, widespread utilisation of laminated composites are constrained by the significant reduction of the residual compressive strength stemming from a lack of through-thickness reinforcement leading to interface delamination. This work presents the detailed investigations on the characteristics of the fibre-matrix interactions in conjunction with ply interface enhancements; to develop new knowledge for damage tolerance enhancement of thermoset composite laminates modified with thermoplastic veil interleaves. The influence of fracture initiation and propagation is the focus of this work, through the experimental exploration of various failure mechanisms. The failures of toughened carbon-fibre/epoxy laminates with poly-phenylene sulfide (PPS) veil interlayers are considered under; inter-laminar, intra-laminar, fracture migration and low-velocity impact damage. Conservative test standards for the interlaminar fracture resistance present veil interleaves as having a 48% enhancement on the interlaminar fracture parameters for longitudinal propagation of unidirectional (UD) fibre reinforced polymer (FRP) laminates. However, interleaving enhancements are revealed to be over-projected when fracture propagation is transverse to the interfacial fibre orientation with enhancements being <2%. Under mode I loading, intra-laminar fracture plane migration is the dominant damage mechanism for transverse yarn interfaces, which trigger migration away from the toughened interface. In contrast, woven lamina cannot permit through-thickness migration due to yarn interlacing, where 136% for longitudinal bias and 20% for transverse biased enhancements are apparent using 20 gm-2 PPS. Furthermore, reductions of interfacial fibre orientation bias sensitivity were demonstrated for veil interleaved interfaces. Similarly, under mode II loading, transverse biased interfaces exhibited up to 75% improvement in delamination resistance with gradual growth characteristics resulting from the veil and transverse fibre interactions. Interleaved laminates were subsequently explored for low-velocity impact damage, emphasising on veil distributions within the laminate. Interleaving the outside interfaces transformed the damage from inter-laminar to intra-laminar and increased the delamination threshold load (DTL) by 9% or entirely removed DTL occurrences for heavily interleaved layups. The veil damage mechanisms resulting in improved fracture resistance and enhanced damage tolerance are due to the inclusion of lower modulus PPS fibres. These discrete thermoplastic regions within the laminate provide a cushioning effect while increasing the localised ductility resulting in increased fibrematrix interactions at the fracture process zone.
Open Access
Delamination migration in CFRP laminates under mode I loading
(Elsevier, 2020-02-10) Ramji, Amit; Xu, Yigeng; Yasaee, Mehdi; Grasso, Marzio; Webb, Philip
This paper focuses on the effect of interfacial fibre orientation and interleaved veil on the delamination migration of carbon fibre reinforced polymer laminates under Mode I loading. Double cantilever beam specimens with midplane interfacial fibre orientations of 0/0, 90/90, 0/90, 0/45 and 90/45 were tested under two conditions: one with interleaved thermoplastic polyphenylene sulfide veil at the midplane and one without. Results show that, except for the 0/0 configuration, all other orientations exhibit varying levels of migration associated with the interfacial fibre orientation and veil interleaving. The apparent fracture toughness determined with the modified compliance calibration method is closely related to the delamination migration and hence a structural energy dissipation measure dependent on interfacial fibre orientation and the interleaved veil. Distributions of the fibre and matrix materials around the delamination front are found to be closely related to the delamination migration behaviour along its path. The experimental observation and rationalisation presented in this paper provide further knowledge regarding delamination migration and its correlation to the apparent fracture toughness, which is of direct relevance to the damage tolerance design of laminated composite components
Open Access
Effect of electromagnetic treatment on fatigue resistance of 2011 aluminum alloy
(World Scientific Publishing, 2016-08-08) Mohin, M. A.; Toofanny, H.; Babutskyi, A.; Lewis, A.; Xu, Yigeng
Beneficial effects of the electromagnetic treatment on fatigue resistance were reported on several engineering alloys. These could be linked to the dislocation activity and the rearrangement of the crystal structure of the material under the electromagnetic field (EMF), resulting in delayed crack initiation. This paper presents an experimental study on the effect of pulsed electromagnetic treatment on the fatigue resistance of 2011 aluminum alloy. Circular cantilever specimens with loads at their ends were tested on rotating fatigue machine SM1090. Fatigue lives of treated and untreated specimens were analyzed and compared systematically. It has been found that the effect of the pulsed electromagnetic treatment on the fatigue resistance is dependent on the intensity of the pulsed EMF and the number of the treatment applied. Clear beneficial effect of the pulsed electromagnetic treatment on the fatigue resistance of the aluminum alloys has been observed, demonstrating a potential new technique to industries for fatigue life extension.
Open Access
Effect of electropulsing on the fatigue resistance of aluminium alloy 2014-T6
(Elsevier, 2019-11-12) Babutskyi, A.; Mohin, M.; Chrysanthou, A.; Xu, Yigeng; Lewis, A.
The effects of electropulsing on the fatigue resistance of aluminium alloy 2014-T6 were studied in relation to electric current amplitude, pulse duration, and number of repetitions. Utilising the Taguchi method, the present study identified the current amplitude and the duration of the electropulsing as the two critical treatment parameters for improved fatigue resistance. A 97% fatigue life improvement was achieved under the electropulsing conditions that were applied. An increase in microhardness and a decrease in electrical conductivity due to electropulsing were correlated with enhanced fatigue resistance in the alloy. Mechanisms related to the effects of the electropulsing treatment were elucidated based on observations from scanning electron microscopy (SEM) and transmission electron microscopy (TEM) as well as numerical simulation results. The mechanisms identified by observation included dislocation movement and the secondary precipitation of GP-zones. Further explication of these mechanisms was provided by the application of a “magnetic field’’ model.
Open Access
Effect of interfacial fibre orientation and PPS veil density on delamination resistance of 5HS woven CFRP laminates under mode II loading
(Elsevier, 2021-02-19) Ramji, Amit; Xu, Yigeng; Grasso, Marzio; Yasaee, Mehdi; Webb, Philip
This paper presents an experimental study on the effect of interfacial fibre orientation and interleaved thermoplastic veil on Mode II interlaminar fracture toughness of 5-harness satin woven carbon fibre reinforced polymer composite laminates. Three-point End-Notched Flexure tests were carried out to determine delamination resistance, GIIC, of specimens with five fibre orientation biases and two veil densities at the midplane. Results show that delamination resistance of 5-harness satin woven laminates depends on the layup configurations at the midplane with 90/45 fibre orientation bias exhibiting the greatest resistance. The delamination resistance enhancement from polyphenylene sulfide (PPS) veil interleaves is also fibre orientation dependent but a further increase of the veil density from 10 gm−2 to 20 gm−2 offers little extra benefit. Fracture surface morphologies were examined under SEM to understand the failure mechanism and fracture process of the woven laminate under the combined effects of the interfacial fibre orientation and the veil density. Fibre orientation relative to the delamination path, surface texture misfit, and veil density are the three main contributors identified for the variation of delamination resistance of 5HS woven laminates.
Open Access
Effect of plastic deformation on compliance curve based crack closure measurement
(Trans Tech Publications, 2016-09-30) Mohin, M.; Xu, Yigeng; Lewis, A.; Chrysanthou, A.
Fatigue crack growth depends heavily on near tip stress-strain behavior controlled by many micromechanical and microstructural factors. Crack closure is widely used to rationalize crack growth behaviour under complex loading conditions. Reliable crack closure measurement is essential for enhanced damage tolerance design and remains a challenge to the industry. This paper focuses on the effect of plastic deformation ahead of a notch/crack on the non-linearity of compliance curves of 6082-T651 aluminium alloy specimens to highlight a potential issue in the conventional compliance curve based crack closure measurement technique. Experimental and numerical simulation results demonstrate that plastic deformation ahead of the notch will introduce non-linear stress-strain behavior in the absence of crack closure. It is proposed that the effect of crack tip plasticity on the non-linearity of the compliance curve be separated to obtain reliable crack closure measurement.
Open Access
Enhanced bondline thickness analysis for non-rigid airframe structural assemblies
(2019-05-13) Mato, Pablo Coladas; Webb, Philip; Xu, Yigeng; Graham, Daniel; Portsmore, Andrew; Preston, Edward
Adhesive bonding is a proven alternative to mechanical fasteners for structural assembly, offering lighter and thus more fuel efficient aircraft and cost-effective manufacturing processes. The effective application of bonded structural assemblies is however limited by the tight fit-up requirement, which is with sub-mm tolerance and can be a challenge for the industry to meet considering the variability of current part manufacturing methods and the conservative nature of the conventional tolerance stack-up analysis method. Such a challenge can discourage effective exploitation of bonding technologies, or lead to development of overengineered solutions for assurance. This paper addresses this challenge by presenting an enhanced bondline thickness variation analysis accounting for part deflection of a bonded skin-stringer assembly representing a typical non-rigid airframe structure. A semi-analytical model accounting for unilateral contact and simplified 1D adhesive flow has been developed to predict bondline thickness variation of the assembly under two typical curing conditions: namely autoclave curing and out-of-autoclave curing. The effects of component stiffness and manufacturing variations on bondline thickness are investigated by incorporating stringers of different stiffness, as well as shims of different thicknesses in-between the skin and stringer, in the stringer-skin assembly. A small-scale bonding demonstrator has been built and the physical results are in good agreement with the model prediction. It has been demonstrated that the part deflections need to be accounted for regarding fit-up requirement of bonded non-rigid structural assembly. The semi-analytical model offers more reliable and realistic prediction of bondline thickness when compared to a rigid tolerance stack-up. The analysis method presented can be a major technology enabler for faster, more economical development of the aircraft of the future, as well as of any analogue structures with high aspect ratios where weight savings and fatigue performance may be key objectives.
Open Access
Experimental and computational vibration analysis for diagnosing the defects in high performance composite structures using machine learning approach
(MDPI, 2022-11-26) Jakkamputi, Lakshmipathi; Devaraj, Saravanakumar; Marikkannan, Senthilkumar; Gnanasekaran, Sakthivel; Ramasamy, Sivakumar; Rakkiyannan, Jegadeeshwaran; Xu, Yigeng
Delamination in laminated structures is a concern in high-performance structural applications, which challenges the latest non-destructive testing techniques. This study assesses the delamination damage in the glass fiber-reinforced laminated composite structures using structural health monitoring techniques. Glass fiber-reinforced rectangular laminate composite plates with and without delamination were considered to obtain the forced vibration response using an in-house developed finite element model. The damage was diagnosed in the laminated composite using machine learning algorithms through statistical information extracted from the forced vibration response. Using an attribute evaluator, the features that made the greatest contribution were identified from the extracted features. The selected features were further classified using machine learning algorithms, such as decision tree, random forest, naive Bayes, and Bayes net algorithms, to diagnose the damage in the laminated structure. The decision tree method was found to be a computationally effective model in diagnosing the delamination of the composite structure. The effectiveness of the finite element model was further validated with the experimental results, obtained from modal analysis using fabricated laminated and delaminated composite plates. Our proposed model showed 98.5% accuracy in diagnosing the damage in the fabricated composite structure. Hence, this research work motivates the development of online prognostic and health monitoring modules for detecting early damage to prevent catastrophic failures of structures.
Open Access
Exploring advanced functionalities of carbon fiber-graded PEEK composites as bone fixation plates using finite element analysis
(MDPI, 2024-01-14) Zhang, Chenggong; Wen, Pihua; Xu, Yigeng; Fu, Zengxiang; Ren, Guogang
This study aims to address the challenges associated with conventional metallic bone fixation plates in biomechanical applications, such as stainless steel and titanium alloys, including stress shielding, allergic reactions, corrosion resistance, and interference with medical imaging. The use of materials with a low elastic modulus is regarded as an effective approach to overcome these problems. In this study, the impact of different types of chopped carbon fiber-reinforced polyether ether ketone (CCF/PEEK) functionally graded material (FGM) bone plates on stress shielding under static and instantaneous dynamic loading was explored using finite element analysis (FEA). The FGM bone plate models were established using ABAQUS and the user’s subroutine USDFLD and VUSDFLD, and each model was established with an equivalent overall elastic modulus and distinctive distributions. The results revealed that all FGM bone plates exhibited lower stress shielding effects compared to metal bone plates. Particularly, the FGM plate with an elastic modulus gradually increased from the centre to both sides and provided maximum stress stimulation and the most uniform stress distribution within the fractured area. These findings offer crucial insights for designing implantable medical devices that possess enhanced mechanical adaptability.
Open Access
Fatigue life prediction of woven composite laminates with initial delamination
(Wiley, 2020-07-13) Wan, Ao-Shuang; Xiong, Junjiang; Xu, Yigeng
An engineering approach for fatigue life prediction of fibre‐reinforced polymer composite materials is highly desirable for industries due to the complexity in damage mechanisms and their interactions. This paper presents a fatigue‐driven residual strength model considering the effect of initial delamination size and stress ratio. Static and constant amplitude fatigue tests of woven composite specimens with delamination diameters of 0, 4 and 6 mm were carried out to determine the model parameters. Good agreement with experimental results has been achieved when the modified residual strength model has been applied for fatigue life prediction of the woven composite laminate with an initial delamination diameter of 8 mm under constant amplitude load and block fatigue load. It has been demonstrated that the residual strength degradation‐based model can effectively reflect the load sequence effect on fatigue damage and hence provide more accurate fatigue life prediction than the traditional linear damage accumulation models.
Open Access
Finite element modeling and fatigue life prediction of helicopter composite tail structure under multipoint coordinated loading spectrum
(Elsevier, 2020-09-01) Wan, Ao-Shuang; Xu, Yigeng; Xue, Li-Heng; Xu, Ming-Rui; Xiong, Junjiang
This paper presents a numerical study on fatigue life prediction of helicopter composite tail structure under multipoint coordinated loading spectrum. The FE model of a full-scale helicopter composite tail structure was established and then validated with the experimental results of strain and displacement distributions. Good agreement has been achieved between simulation and experiments under two multipoint coordinated static loading conditions representing left yawing and two-point horizontal landing. A progressive damage analysis was performed on the tail structure model under multipoint coordinated spectrum loading. The predicted fatigue life of the helicopter tail structure is 64 repeated applications of the load spectrum. No element failure was predicted on the tail structure after 48 repeated applications of the load spectrum. The impact damage introduced on the left-side wall thereafter propagated after another 6 repeated applications of the load spectrum, which agrees well with experimental observations. The progressive damage analysis has been shown to be a practical engineering tool for life prediction of helicopter composite structures
Open Access
Identification of the key design inputs for the FEM-based preliminary sizing and mass estimation of a civil aircraft wing box structure
(Elsevier, 2021-12-14) You, Chao; Yasaee, Mehdi; He, Shun; Yang, Daqing; Xu, Yigeng; Dayyani, Iman; Ghasemnejad, Hessam; Guo, Shijun; Webb, Phil; Jennings, James; Federico, Giovanni
FEM-based preliminary structural sizing has been successfully carried out for a typical single-aisle wing box structure using MSC Nastran, by considering various load cases representing typical aircraft manoeuvres, engine loads, landing and ground handling conditions. The strength, buckling and fatigue criteria have been applied as the design constraints for sizing. The resultant total mass and the structural (static and modal) behaviour of the sized wing box model have been verified against a validated high-fidelity wing box model. A sensitivity analysis has been performed to evaluate the influence of the number of design fields and the selected design inputs (i.e. load cases and design constraints) on the accuracy of sizing and mass estimation of the wing box. This sensitivity analysis has also been extended to the static and modal behaviour of the wing box structure obtained from sizing. It provides an insight into the significance of considering the buckling and fatigue constraints, aircraft rolling loads, engine loads and landing loads in sizing, in addition to the commonly-applied 2.5 g aircraft pull-up loads under the strength constraint. The findings of this study highlight the trade-off between the sizing efficiency and accuracy of a civil aircraft wing for modelling purposes.
Open Access
Low-velocity impact behaviour of woven laminate plates with fire retardant resin
(Elsevier, 2019-04-27) Grasso, Marzio; Xu, Yigeng; Ramji, Amit; Zhou, Gang; Chrysanthou, Andreas; Haritos, George; Chen, Yong
The understanding of the damage mechanisms for woven laminate plates under low-velocity impact is challenging as the damage mechanisms at the interface of adjacent layers are dominated by the fibre architecture. This work presents an experimental investigation of the behaviour of woven glass and carbon fibre composite laminates in a matrix of fire retardant resin under low-velocity impact. The performance is evaluated in terms of damage mechanisms and force time history curves. Six impact energy levels were used to test standard plates to identify the type of damage observed at various energy levels. Scanning electron microscopy (SEM) along with C-scans were used to characterise the damage. It has been observed that in woven composites, the damage occurs mostly between the fibre bundles and matrix. As the impact energy increases, the failure involves extended matrix cracking and fibre fracture. Moreover, due to the fibre architecture, both the contact forces between bundles of fibres and stretching of the bundles are responsible for the dominant matrix cracking damage mode observed at the low-impact energy level. As the impact energy increases, the damage also increases resulting in fibre fracture. The experimental evidence collected during this investigation shows that for both the carbon fibre and the glass fibre woven laminates the low-velocity impact behaviour is characterised by extended fibre fracture without a noticeable sudden load drop.
Open Access
Mixed mode fatigue crack propagation behaviour of aluminium F357 alloy
(Elsevier, 2018-04-05) Grasso, Marzio; Xu, Yigeng; Russo, Roberto; Rosiello, Vincenzo
Manufacturing defects are often not in the plane perpendicular to the loading direction and will propagate under mixed mode fatigue loading condition. This paper presents a numerical study of mixed mode crack growth behaviour in H-shaped specimens made of aluminium F357 alloy. The size and orientations of the crack are based on the fractographic observation of defects in F357 specimens manufactured by foundry. Equivalent values of the stress intensity factor (SIF) and the maximum circumferential tensile stress criterion have been adopted to simulate growth of cracks at angles of 90°, 60° and 45° to the loading direction, respectively. Mixed mode fatigue crack growth behaviours are analysed in terms of the shape of crack front, SIF variation, and kink angle. The mixity of SIFs of three modes is complex at early stage of growth with the maximum mode III SIF value at the two ends and the maximum mode II SIF value at the middle of the crack front. The crack surface rotates during the mixed mode crack growth, becoming normal to the loading direction regardless of the initial orientation of the crack. The simulated crack front agrees well with the final elliptical shape of the crack front observed in the physical test specimens. The initial crack orientated at 45° to the loading direction has the longest fatigue life compared with other two crack orientations.
Open Access
Notch effect on strength and fatigue life of woven composite laminates
(Elsevier, 2019-06-15) Wan, Ao-Shuang; Xu, Yigeng; Xiong, Junjiang
This paper presents an experimental and numerical study of the notch effect on strength and fatigue life of double edge notched (DEN) composite laminates made of woven glass fibre lamina 3238A/EW250F and woven carbon fibre lamina 3238A/CF3052. Experimental results show that the notch effect is dependent on fibre type, notch depth, load type and load sequence. As-n-R-d-r" role="presentation"> s - n - R -d-r residual strength model was proposed to account for the effects of the notch and the stress ratio, and a progressive damage algorithm was developed to predict damage propagation and residual life of composites under spectrum fatigue load. Good agreement between the experimental results and the numerical predictions has been achieved.
Open Access
Three-dimensional subsurface defect shape reconstruction and visualisation by pulsed thermography
(Elsevier, 2019-12-04) Sirikham, Adisorn; Zhao, Yifan; Liu, Haochen; Xu, Yigeng; Williams, Stewart; Mehnen, Jorn
Defects detected by most thermographic inspection are represented in the form of 2D image, which might limit the understanding of where the defects initiate and how they grow over time. This paper introduces a novel technique to rapidly estimate the defect depth and thickness simultaneously based on one single-side inspection. For the first time, defects are reconstructed and visualised in the form of a 3D image using cost-effective and rapid pulsed thermography technology. The feasibility and effectiveness of the proposed solution is demonstrated through inspecting a composite specimen and a steel specimen with semi-closed airgaps. For the composite specimen, this technique can deliver comparatively low averaged percentage error of the estimated total 3D defect volume of less than 10%.
Open Access
Threshold identification and damage characterization of woven gf/cf composites under low-velocity impact
(MDPI, 2022-10-11) Grasso, Marzio; Xu, Yigeng
The Delamination Threshold Load (DTL) is a key parameter representing damage resistance of a laminate and is normally identified by locating a sudden drop in the impact force-time history for the laminate made of unidirectional layers. For the woven composite, however, their failure mechanisms appear different and the current literature is not providing any clear procedure regarding the identification of the delamination initiation, as well as the evolution of the failure mechanisms associated with it. In this paper, experimental data have been collected using woven glass and carbon fiber composites. The results are analyzed in terms of force-time and force-displacement curves. While delamination and other damages were clearly observed using ultrasonic scans, the analysis of the results does not reveal any trend changes of the curves that can be associated with the incipient nucleation of delamination. A preliminary discussion regarding the nature of the mechanisms through which the delamination propagates in woven composite and a justification for the absence of a sudden change of the stiffness have been presented. It raises a question on the existence of DTL for woven composites under low velocity impact.