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This paper evaluates the impact performance of various materials against single and repeated low-velocity impacts. The objective is to identify a lightweight, thin, and cost-effective bumper that can effectively protect a composite aircraft structure. Drop-weight impact tests were performed on eight candidate materials with similar areal densities, including three 3D woven and three 2D woven composites, an aluminium alloy (Al 7075 T6) and an aluminium foamed sandwich. To enable a comparison of impact performance across materials with different characteristics, a normalisation method was defined for both single and repeated impact loading. The relationship between single and repeated low-velocity impacts was also explored. Finally, the influence of adding a polymeric foam on the amount of energy transmitted to the composite airframe was also discussed. The results indicate that, overall, the 3D woven composites exhibited superior impact performance in all tested scenarios. • The impact-fatigue resistance of 3D woven composites exceeds that of 2D composites. • 3D woven composites spread damage over a wider area than 2D composites and metals. • The impact-fatigue resistance decreases with the severity of the initial impact. • The normalised impact-fatigue curves can predict the number of impacts to failure. • The selected multi-material protection can absorb 80 % of the impact energy. [ABSTRACT FROM AUTHOR]

Abstract: This paper deals with impact damage and permanent indentation modeling. A numerical model has been elaborated in order to simulate the different impact damage types developing during low velocity/low energy impact. The three current damage types: matrix cracking, fiber failure and delamination, are simulated. Inter-laminar damage, i.e. interface delamination, is conventionally simulated using interface elements based on fracture mechanics. Intra-laminar damage, i.e. matrix cracks, is simulated using interface elements based on failure criterion. Fiber failure is simulated using degradation in the volume elements. The originality of this model is to simulate permanent indentation after impact with a “plastic-like” model introduced in the matrix cracking elements. This model type is based on experimental observations showing matrix cracking debris which block crack closure. Lastly, experimental validation is performed, which demonstrates the model’s satisfactory relevance in simulating impact damage. This acceptable match between experiment and modeling confirms the interest of the novel approach proposed in this paper to describe the physics behind permanent indentation. [Copyright &y& Elsevier]

Abstract: Z-fibre® pinning is a new method of through-thickness reinforcement of laminated composites. This paper presents an experimental test and theoretical analysis on how and why these pins can improve the resistance to impact loading and post-impact compression. Specimens were made of carbon/epoxy T300/914C with nominal thicknesses of 2, 4, and 6mm. For the specimens tested in this study, z-pinning reduced impact damage area by 19–64% depending on the specimen thickness and impact energy. Experimental results obtained also indicate that z-pins can significantly increase the compression-after-impact (CAI) strength by about 45%. In this paper all of these observations are discussed in the context of theoretical and numerical models that have been developed previously to predict the critical impact force and z-pinning performance in terms of the laminate in-plane stiffness and mode I delamination fracture toughness with the z-fibre bridging effect. [Copyright &y& Elsevier]

Micro-warpping of sub-scale fibre bundles with thermoplastic fibres has been explored for improved intra-tow hybridisation in comparison to traditional commingling process. This paper investigates matrix toughening of thermoset composites with thermoplastic fibres delivered through textile preforming and hence does not adversely effect resin viscosity during the infusion process. Tow-scale thermoplastic fibres appear to diffuse cracks from inter-laminar to intra-tow due to relatively weak interface, and at the same time protect the reinforcing fibres by acting as elastic foundations during impact loading. In this paper, two hybridisation methods, traditional commingling and novel micro-wrapping, have been compared for assessing the impact damage performance of thermoset composite laminates. It appears that micro-wrapped laminates exhibit better impact and damage tolerance performance than the commingled composite laminates due to the smaller impact induced damage areas and better PP fibre distribution. Improved thermoplastic fibre distribution in micro-wrapped tow architechture has been shown to improve impact damage tolerance through mechanisms of fibre cushioning and intra-tow crack dispersion. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract: Linear elastic fracture mechanics (LEFM) implies that crack-like flaws would intensify stress in brittle materials with stress intensity scaling up with the square root of the crack size. Therefore, the apparent strength of materials tends to be much smaller than the theoretical value. In this paper, we examine the stress state in nacreous composites and find that in such materials the crack-induced stress intensification and its dependence on crack size can be suppressed greatly. This feature of nacreous composites can be attributed to the unique “brick-and-mortar” (B-and-M) structure and a synergistic match of the mechanical properties between “brick” (e.g. minerals) and “mortar” (e.g. proteins) phases. Our findings not only provide a fundamental insight into the origin of the excellent mechanical properties of nacreous composites such as high strength, high toughness and flaw tolerance, but also will be of great value to the design and synthesis of new structural materials for superior mechanical properties. [Copyright &y& Elsevier]

Abstract: In this paper, the damage failure and behaviour of stitched composites under compression after impact (CAI) loading are experimentally investigated. This study focuses on the effect of stitch density and stitch thread thickness on the CAI strength and response of laminated composites reinforced by through-thickness stitching. Experimental findings show that stitched composites have higher CAI failure load and displacement, which corresponds to higher energy absorption during CAI damage, mainly attributed to greater energy consumption by stitch fibre rupture. The coupling relationships between CAI strength, impact energy, stitch density and stitch thread thickness are also revealed. It is understood that the effectiveness of stitching has high dependency on the applied impact energy. At low impact energy range, CAI strength is found to be solely dependent on stitch density, showing no influence of stitch thread thickness. It is however observed that stitch fibre bridging is rendered ineffective in moderately stitched laminates during compressive failure, as local buckling occurs between stitch threads, resulting in unstitched and moderately stitched laminates have similar CAI strength. The CAI strength of densely stitched laminates is much higher due to effective stitch fibre bridging and numerous stitch thread breakages. At high impact energy level, CAI strength is discovered to be intimately related to both stitch density and stitch thread thickness. Since CAI failure initiates from impact-induced delamination area, stitch fibre bridging is considerable for all specimens due to the relatively large delamination area present. Stitch threads effectively bridge the delaminated area, inhibit local buckling and suppress delamination propagation, thus leading to increased CAI strength for laminates stitched with higher stitch density and larger stitch thread thickness. Fracture mechanisms and crack bridging phenomenon, elucidated by X-ray radiography are also presented and discussed. This study reveals novel understanding on the effectiveness of stitch parameters for improving impact tolerance of stitched composites. [Copyright &y& Elsevier]

Abstract: This paper presents an experimental study of low energy impacts on composite plates covered with a protective layer. In service, composite materials are subjected to low energy impacts. Such impacts can generate damage in the material that results in significant reduction in material strength. In order to reduce the damage severity, one solution is to add a mechanical protection on composite structures. The protection layer is made up of a low density energy absorbent material (hollow spheres) of a certain thickness and a thin layer of composite laminate (Kevlar). Energy absorption ability of these protective layers can be deduced from the load/displacement impact curves. First, two configurations of protection are tested on an aluminium plate in order to identify their performance against impact, then the same are tested on composite plates. Test results from force–displacement curves and C-scan control are compared and discussed and finally a comparison of impact on composite plates with and without protection is made for different configurations. [Copyright &y& Elsevier]

Abstract: This paper presents a study of the effects of ply clustering on polymer-based laminated composite plates subjected to a drop-weight impact loading. The tools used to define the impact configurations, as well as the experimental results obtained, are described in detail. These tools are simplified analytical models for the description of the impact behavior and of the damage thresholds that result in a significant reduction on the structure stiffness and strength, caused by delamination. The results obtained demonstrate that the analytical tools are useful to define the impact configurations, to obtain a preliminary understanding of the effects of each parameter that can influence the response, and to interpret the experimental results. It is concluded that ply clustering reduces the damage resistance of the structure. However, the damage tolerance assessed by the compression after impact tests is unaffected by ply clustering. [Copyright &y& Elsevier]

Abstract: This paper presents the experimental and analytical investigation of the effect of variable amplitude load sequences on delamination behavior in Fibre Metal Laminates . Delamination tests were performed and results are compared with linear damage accumulation predictions. Scanning Electronic Microscopy was used to analyse the delaminated surfaces to study the delamination growth rate under VA loading in more detail. The correlation between test results and predictions highlighted the absence of load sequence and interaction effects in delamination growth rate under VA loading. This correlation is supported by the SEM observations. [Copyright &y& Elsevier]

Abstract: This paper addresses the effect of a local quasi-static indentation or a low-velocity impact on the residual strength of foam core sandwich panels subjected to edgewise compression. The damage is characterized by a local zone of crushed core accompanied by a residual dent in the face sheet. Experimental studies show that such damage can significantly alter the compressive strength. Theoretical analysis of the face sheet local bending is performed for two typical damage modes (with or without a face–core debonding). The solutions allow estimation of the onset of (a) an unstable dent growth (local buckling) or (b) a compressive failure in the face sheet. The theoretical results are in agreement with the test data for two considered sandwich configurations. [Copyright &y& Elsevier]

Abstract: In the paper, data of repeated impact tests performed on seven laminates of different lay-up and thickness are used to illustrate how the damage index (DI), a damage variable recently introduced by the authors to monitor the range of the penetration process in thick laminates, can be applied in case of repeated impact tests to get information on the rate of initial steady damage accumulation as well as on the onset of severe damage modes. Curves of the rate of growth of the DI in the steady phase (ΔDI/ΔN) vs. the normalised impact energy (ratio of the impact energy E i and the laminate penetration threshold P n) show no significant damage accumulation besides initial specimen indentation for impact energies below 0.2P n. For intermediate levels of impact energy, repeated impact tests are characterised by an initial region of steady damage accumulation followed by an abrupt change in the rate of damage growth a few impacts before laminate perforation. For higher impact energies (E i/P n >0.4–0.45), no phase of steady damage accumulation is present, suggesting that severe damage mechanisms take place from the very first impacts. Values of the DI at the end of the steady phase (DIunsteady) are shown to be rather peculiar to each laminate regardless of the impact energy used in the tests and therefore may be used to get a first indication of the laminate performance to repeated impacts. The extent of the steady phase may also be used to compute the total energy absorbed by the laminate in the steady damage accumulation phase (E aTOT_steady). [Copyright &y& Elsevier]

Abstract: In the current paper, a series of high velocity impact tests using ϕ50 and ϕ25mm ice spheres and 0.32g granite stones on non-crimp fabric (NCF) composite plates are reported. The impact tests were performed using an air gun and velocities between 100m/s and 199m/s. The impact events were monitored using a high-speed camera, with a 20 million frames per second capacity, as well as by a displacement transducer for out-of-plane displacement measurements of the impacted plates. NCF composite plates of two different thicknesses were impacted. The composites were manufactured from carbon fibre and epoxy resin by vacuum infusion. Engineering type models were employed to predict impact response and impact damage formation. Comparison between predicted and resulting damage for the impact test validates the application of a semi-empirical model for predicting impact velocity thresholds for damage formation. Analytical models relying on the assumption of solid impact bodies cannot be employed for these types of impact. [Copyright &y& Elsevier]

Abstract: This paper presents an experimental investigation concerning low-velocity impact and quasi-static indentation tests on highly oriented laminates used in aeronautical and aerospace applications. The damage observed in such laminates is very particular. Post mortem analysis were carried out which helped to define an impact damage scenario. Microscopic observations led to explain the mechanism of permanent indentation formation which is a fundamental point of damage tolerance justification. Equivalence between static and dynamic is also discussed. [Copyright &y& Elsevier]

Abstract: This paper is the follow-up on the previous work by the authors on the experimental evaluation of the impact damage resistance of laminates with dispersed stacking sequences. The current work focuses on the evaluation of the impact performance of the tested laminates by innovative numerical methods. Constitutive models which take into account the physical progressive failure behaviour of fibres, matrix, and interfaces between plies were implemented in an explicit finite element method and used in the simulation of low-velocity impact events on composite laminates. The computational effort resulted in reliable predictions of the impact dynamics, impact footprint, locus and size of delaminations, matrix cracks and fibre damage, as well as the amount of energy dissipated through delaminations, intraply damage and friction. The accuracy achieved with this method increases the reliability of numerical methods in the simulation of impact loads enabling the reduction of the time and costs associated with mechanical testing. [Copyright &y& Elsevier]

Abstract: The stacking sequence design of composite laminates is often limited to combinations of 0°, 90°, and ±45° fibre angle plies. Furthermore, in order to comply to certain stiffness requirements, clustering of plies becomes unavoidable. Although such laminates might have the desired stiffness properties, they may show poor impact and/or compression-after-impact behaviour. A method to redesign the traditional stacking sequences such that the alternative laminates have improved damage resistance whilst keeping similar in-plane and bending stiffness properties as their original traditional stacking sequences is proposed. This method makes use of optimisation tools based on genetic algorithms. In the alternative laminates, the difference between fibre angles of two consecutive plies is maximised and allowed to vary in the 0–90° fibre angle range at intervals of 5°. Manufacturing of such laminates is practical nowadays as the industry is changing its production techniques into accurate automated fibre-placement and tape-laying technologies. A two-step approach is proposed for the design of laminates. In the first step, the optimal laminate is designed in the traditional fashion to cope with the expected quasi-static loads on the structure. The second step consists of redesigning this laminate to better withstand impact loads by dispersing its stacking sequence while keeping similar stiffness properties as in the first step. A traditional laminate and two dispersed stacking sequence alternative layups were tested under low-velocity impact and compression-after-impact loads in order to compare their impact resistance and damage tolerance characteristics. The evaluation of these laminates will also be carried out by the innovative numerical tools proposed in the follow-up of the present paper. [Copyright &y& Elsevier]

Abstract: Post-stretching is a potential method to change the unfavorable residual stress system in fibre metal laminates (FMLs). During post-stretching of the material, the metal layers will be strained into the plastic region of the stress–strain curve, while the fibre layers remain elastic. After unloading, the residual stress system due to curing will be reduced or even reversed dependent on the amount of stretching. In the past, the classical laminate theory (CLT) has been modified, incorporating the thermal expansion for the application to FMLs. Furthermore, a fatigue crack growth prediction model has been developed based on this extended theory. Here, this classical laminate theory is further extended to calculate the stress redistribution after stretching the uni-directional laminates (ARALL and GLARE). This paper present the method to calculate the residual internal stress distribution in the fibre direction. For the validation of the post-stretching formulations, fatigue crack growth tests on GLARE1-2/1, 3/2, 4/3, 5/4-0.3 have been performed. Good agreement has been obtained. [Copyright &y& Elsevier]

Abstract: A damage variable (damage degree DD) representing the ratio between the absorbed energy and the impact energy was introduced in 1998 by Belingardi and Vadori to assess damage accumulation caused by low velocity impacts. More recently, repeated impact tests carried out on thick laminates pointed out the significance and extent of the penetration process in thick laminates, which the DD is unable to monitor as, by definition, it reaches the value of one at penetration to remain unchanged over the entire penetration process. In the present paper, a new damage variable (damage index DI) is proposed to overcome shortcomings of the DD with respect to thick laminates. By introducing the displacement of the impactor into the definition of the DI, the depth of the penetration process is taken into account. Normalization by the displacement of the quasi-static perforation test allows for a non-dimensional damage variable which is shown to reach the value of one at perforation. Validity of the approach is proven against impact data obtained for different fibre-matrix architectures and laminate thickness. Results show that the DI can effectively differentiate between penetration and perforation thresholds, increasing monotonically within the range of the penetration process. In particular, the DI is shown to increase linearly with the impact energy up to penetration and to rise quite abruptly within the range of the penetration process. Results for repeated impact tests prove that at first the DI increases quite linearly impact after impact, owing to a steady accumulation of damage. A few impacts before penetration a sudden rise in the DI value points out a change in the rate of damage accumulation. [Copyright &y& Elsevier]

Abstract: The aim of this paper is to present an experimental study of impact and compression after impact (CAI) tests performed on composite laminate covered with a cork thermal shield (TS) intended for launchers fairing. Drop weight impact tests have been performed on composite laminate sheets with and without TS in order to study its effect on the impact damage. The results show the TS is a good mechanical protection towards impact as well as a good impact revealing material. Nevertheless, totally different damage morphology is obtained during the impact test with or without TS, and in particular at high impact energy, the delaminated area is larger with TS. Afterwards, CAI tests have been performed in order to evaluate the TS effect on the residual strength. The TS appears to increase the residual strength for a same impact energy, but at the same time, it presents a decrease in residual strength before observing delamination. In fact, during the impact tests with TS, invisible fibres’ breakages appear before delamination damage contrary to the impacts on the unshielded sheets. [Copyright &y& Elsevier]

This paper compares interlaminar shear strength, interlaminar fracture toughness and compression-after-impact strength of epoxy resin matrix composites reinforced with carbon fiber woven fabrics and carbon fiber felts, and investigates the influences of weave pattern and needle-punching density. With regard to the weave pattern of fabric layers in woven laminates and felt/resin composites, plain weave with a smaller radius of curvature of yarns is preferable for obtaining higher damage tolerance of the composites. Felt/resin composites have improved interlaminar and damage tolerant properties as compared with woven laminates. These properties of the felt/resin composites improve with increasing needle-punching density. The compression-after-impact strength of the woven laminates and felt/resin composites can be uniquely related to the mode I interlaminar fracture toughness regardless of the types of composites, and the result has been interpreted by applying the equation derived by Ilic and Williams [Theoret. Appl. Fract. Mech. 6 (2) (1986) 121]. Damage tolerance of the felt-reinforced composites utilizing more brittle carbon matrix has also been investigated. [Copyright &y& Elsevier]

In the quest to improve the compression after impact (CAI) strength of thin laminates, ply-hybrid laminates (where plies of different thicknesses are mixed) have been used in a previous study to mitigate the fibre failure and, consequently, improve the CAI strength. In the same study, hybrid laminates were proposed following qualitative design rules. In this paper, we systematically look for hybrid stacking sequences with improved damage tolerance by virtually testing all the laminates in a defined design space. While the laminates in the design space are made of intermediate and thick ply grades, the baseline laminate has only intermediate grade plies. Using an in-house numerical model, we virtually tested, (impact and CAI at two impact energies), all the candidate stacking sequences. The best hybrid laminates considerably improved the CAI strength over the baseline (31% and 40% improvement for the symmetric and unsymmetrical hybrid laminates, respectively). One of the best hybrid laminates was then manufactured and tested experimentally to validate the approach. Through virtual testing, this study demonstrates the benefits of using ply thickness hybrid laminates and the feasibility of optimizing the stacking sequence for impact damage tolerance. [ABSTRACT FROM AUTHOR]