Modelling of damage and failure of glass/epoxy composite plates subject to impact fatigue

An investigation has been carried out to study the impact fatigue damage of glass/epoxy laminated composites. Accumulation of damage, such as matrix cracking, delamination and fibre breakage, with repeated impact of the composite material may reduce the overall stiffness. These damage modes have been combined in a very complicated way to describe damage growth and fracture. A model is proposed for characterising the damage as a function of the normalised impact number. The scalar variable D, which characterises the material damage, is written as a function of the life duration β, using a modified form of the Mankowsky empirical law [Int J Solids Struct 32(11) (1995) 1607]. The macroscopic failure mode and the internal damage in laminated specimens of glass/epoxy as a consequence of impact fatigue are analysed at different levels of incident impact energy. The impact fatigue tests have been conducted on an apparatus built in our laboratory.     

Design of composite materials with improved impact properties

Composites have been widely used in applications where there is a risk of impact, due to the excellent properties these materials display for absorbing impact energy. However, composites during impact situations typically generate an enormous number of small pieces, due to the energy absorption mechanism of these materials, a mechanism which does not include plastic deformation. This can prove dangerous in sports competitions, where the small fragments of the original structure may harm competitors.This study was designed to explore the possibility of incorporating a material which, whilst maintaining a high level of energy absorption without any plastic deformation mechanism, was able to maintain its original form, or at least significantly reduce the number of pieces generated after impact.The addition of a polyamide layer, NOMEX^®, to a monolithic fabric laminate was investigated in this paper. The process of fabrication is described and the different properties of the material under consideration: interlaminar fracture toughness energy (G_IC), indentation (id) and delamination after impact (A_i) and compression after impact (σ_CAI), were measured and compared with those of the original monolithic fabric.     

Finite element simulation of low velocity impact on shape memory alloy composite plates

Delamination of composite materials due to low velocity impacts is one of the major failure types of aerospace composite structures. The low velocity impact may not immediately induce any visible damage on the surface of structures whilst the stiffness and compressive strength of the structures can decrease dramatically.

Shape memory alloy (SMA) materials possess unique mechanical and thermal properties compared with conventional materials. Many studies have shown that shape memory alloy wires can absorb a lot of the energy during the impact due to their superelastic and hysteretic behaviour. The superelastic effect is due to reversible stress induced transformation from austenite to martensite. If a stress is applied to the alloy in the austenitic state, large deformation strains can be obtained and stress induced martensite is formed. Upon removal of the stress, the martensite reverts to its austenitic parent phase and the SMA undergoes a large hysteresis loop and a large recoverable strain is obtained. This large strain energy absorption capability can be used to improve the impact tolerance of composites. By embedding superelastic shape memory alloys into a composite structure, impact damage can be reduced quite significantly.

This article investigates the impact damage behaviour of carbon fiber/epoxy composite plates embedded with superelastic shape memory alloys wires. The results show that for low velocity impact, embedding SMA wires into composites increase the damage resistance of the composites when compared to conventional composites structures.     

Effect of preload on the impact response of curved composite panels

While in many structures, such as aircraft wings, curved laminates are widely utilized, researchers have mostly focused on flat ones. On the other hand, during their service these panels are under pre-stress which can affect their performance. This study therefore investigated the effect of preloading on the impact response of curved laminates. Low velocity impact tests were conducted on GFRP for three different initial impact energies and pre-loads in two different boundary conditions. The results showed that the damage mode is considerably different in these situations. For example, applying the compressive load on curved laminates, the upper and lower surfaces are under tension and compression stress. The compression stress reduces the propagation of cracks, while in a tension stress field cracks propagate easily. The influence on other impact parameters like maximum force and maximum displacement was also significant.     

Experimental study of low-velocity impacts on glass-epoxy laminated composite plates

In this investigation, glass-epoxy laminated plates were subjected to crush experimental tests in a SHIMATSU universal traction machine and low-velocity crash impact tests in a drop test IMATEK machine. The results are shown and the dimensions of the damage are evaluated. The characterization of the damage is done in relation to the type of test, the ply stacking sequence, the plate dimensions and the maximum force achieved in the impact. In order to verify if low-velocity impacts can be modelled by crush tests on laminated composites, the results are compared and several relevant ideas are developed.     

The low velocity impact response of non-woven hemp fibre reinforced unsaturated polyester composites

Hemp fibre reinforced unsaturated polyester composites (HFRUPE) were subjected to low velocity impact tests in order to study the effects of non-woven hemp fibre reinforcement on their impact properties. HFRUPE composites specimens containing 0, 0.06, 0.10, 0.15, 0.21 and 0.26 fibre volume fractions (V_f) were prepared and their impact response compared with samples containing an equivalent fibre volume fraction of chopped strand mat E-glass fibre reinforcement. Post-impact damage was assessed using scanning electron microscopy (SEM). A significant improvement in load bearing capability and impact energy absorption was found following the introduction hemp fibre as reinforcement. The results indicate a clear correlation between fibre volume fractions, stiffness of the composite laminate, impact load and total absorbed energy. Unreinforced unsaturated polyester control specimens exhibited brittle fracture behaviour with a lower peak load, lower impact energy and less time to fail than hemp reinforced unsaturated polyester composites. The impact test results show that the total energy absorbed by 0.21 fibre volume fraction (four layers) of hemp reinforced specimens is comparable to the energy absorbed by the equivalent fibre volume fraction of chopped strand mat E-glass fibre reinforced unsaturated polyester composite specimens.     

The effect of temperature on the capability of industrial safety helmets to absorb impact energy

The fundamental document specifying the requirements and testing methods applicable to industrial safety helmets in European Union member states is the standard EN 397:2012. According to that standard, one of the most important parameters of a helmet is shock absorption, determined for an impact of a striker with a kinetic energy of 49 J. The shock-absorbing performance of a safety helmet involves absorbing the energy of a striking object associated with a deformation of the shell and cradle, as well as an increase in the force transferred to the user’s head. The paper presents a study conducted with the aim to estimate the actual amount of energy absorbable by various helmet types without exceeding the threshold value of the force acting on the user’s head. A method of testing helmet deformation and the force acting on the helmet during an impact exerted by a falling object is presented. The effect of the temperature used for conditioning various helmet types on their capability to absorb impact energy was determined. The causes of deterioration of that capability due to temperature changes are analyzed for various designs of helmets made of different materials, and possible solutions to that problem are offered.     

The role of temperature on impact properties of Kevlar/fiberglass composite laminates

This paper demonstrates results of an experimental study on Kevlar/fiberglass composite laminates subjected to impact loading at variable temperatures. The effect of temperature on maximum energy, elastic energy, maximum deflection, maximum impact force, ductility, and compression after impact was studied at several low velocity impact energy levels (8, 15 and 25 J). The temperatures considered were in the range of −50 to 120 °C. Results indicated that impact performance of these composites was affected over the range of temperature considered. Testing at ambient temperature is not fully sufficient and therefore additional testing must be performed for full understanding of composite laminate properties.     

Efficient modelling and optimisation of hybrid multilayered plates subject to ballistic impact

Among the key challenges present in the modelling and optimisation of composite structures against impact is the computational expense involved in setting up accurate simulations of the impact event and then performing the iterations required to optimise the designs. It is of more interest to find good designs given the limitations of the resources and time available rather than the best possible design. In this paper, low cost but sufficiently accurate finite element (FE) models were generated in LS Dyna for several experimentally characterised materials by semi-automating the modelling process and using existing material models. These models were then used by an optimisation algorithm to generate new hybrid offspring, leading to minimum weight and/or cost designs from a selection of isotropic metals, polymers and orthotropic fibre-reinforced laminates that countered a specified impact threat. Experimental validation of the optimal designs thus identified was then successfully carried out using a single stage gas gun. With sufficient computational hardware, the techniques developed in this pilot study can further utilise fine meshes, equations of state and sophisticated material models, so that optimal hybrid systems can be identified from a wide range of materials, designs and threats.     

Modelling low-speed drop-weight impact on composite laminates

The impact responses of typical laminates are investigated numerically in this research. Delamination responses among plies and fibre and/or matrix damage responses within plies are simulated to understand the behaviours of laminates under different impaction conditions. Damage resistance of a laminate is highly dependent upon several factors including geometry, thickness, stiffness, mass, and impact energies (impact velocities), which are here considered by the finite element (FE) method. Three groups of composite laminates are simulated and the numerical results in general are in good agreement with corresponding experiments. Models containing different stacking sequences and impact energies are built to study their influence on impact responses and demonstrate that clustered (or nearly clustered) plies in the laminate can effectively reduce the degree of interface damage. Models containing different indenters and plate shapes are also built to systematically study their influence on the low-speed drop-weight behaviour of composite laminates. Suggestions are proposed for designing impact tests for particular purposes.     

The energy-absorbing characteristics of composite tube-reinforced foam structures

This paper reports the findings of a research study investigating the energy-absorbing characteristics of polymer foams reinforced with small carbon fibre reinforced epoxy tubes. Initial attention focuses on establishing the influence of tube diameter on the specific energy absorption (SEA) characteristics of the chamfered CFRP tubes. Here, it is shown that the SEA of the tubes increases rapidly with decreasing diameter/thickness ratio, with the highest values being close to 93 kJ/kg. Similar tests were conducted at dynamic rates of strain, where it was observed that the measured values of SEA were lower than the corresponding quasi-static data, possibly due to rate-sensitive effects in the delamination resistance of the composite material. In the next stage of the investigation, the composite tubes were embedded in a range of polymer foams in order to establish the influence of both tube arrangement and foam density on the crush behaviour of these lightweight structures. In addition, a limited number of blast tests have been undertaken on structures based on these core materials. Here, extensive crushing of the composite tubes was again observed, suggesting that these structures should be capable of absorbing significant energy when subjected to this severe loading condition. Finally, the results of these tests are compared with previously-published data from studies on a range of different cores materials. Here, it has been shown that the energy-absorbing characteristics of these systems exceed values associated with other core materials, such as aluminium honeycombs, polymer honeycombs and metal foams.     

Monotonic and cyclic behaviour of isolated FRP anchors loaded in shear

Using anchors made of fibre reinforced polymers (FRP) is an increasingly accepted method to delay the delamination of FRP sheets from the concrete surface and to enhance the capacity of FRP strengthened concrete structures. For many applications, FRP anchors are primarily loaded in shear. When used for seismic retrofitting schemes, the anchors are subjected to cyclic loads which may lead to premature fatigue failure. To date, however, shear strength of FRP anchors has experienced much less attention than their tension resistance. This paper documents tests on isolated FRP anchors which were conducted to determine the seismic shear capacity of FRP anchors and to propose design rules. To this end, a test setup was developed which allows direct and reverse loading of FRP anchors.     

Experimental investigation of impact on composite laminates with protective layers

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.     

Effects of mean stress and fibre volume content on the fatigue-induced damage mechanisms in CFRP

The effect of the load type (tension and compression) in quasi-static and of the applied mean stress in fatigue tests on the mechanical behaviour and on the damage mechanisms in unidirectional (UD) carbon/epoxy laminates has been studied in combination with the influence of fibre volume content. Results show that the fibre volume content increases the mechanical properties in tension–tension fatigue tests for all tested angles 0°, 45° and 90°. The tensile damage mechanisms of off-axis specimens depend on the fibre volume content and change from matrix cracking and matrix–fibre debonding to fibre-pull out with an increasing amount of fibres as investigated in detail in a previous work. In tension–compression tests, higher fibre volume contents are only beneficial in fatigue tests at angles of 0° and 45°. Fatigue strengths of UD 90° specimens in tension–compression tests are not significantly improved by the fibre volume content which can be ascribed to breakage of entire fibre bundles and crushed fibres on the fracture surfaces.     

Finite-element simulation for modeling composite plates subjected to soft-body,high-velocity impact for application to bird-strike problem of composite fan blades

We presented a numerical simulation to address the impact-induced deformation and damage of composite plates subjected to soft-body, high-velocity impacts for application to the bird-strike problem of composite fan blades. A new stabilized contact algorithm was developed based on the Lagrange multiplier method to predict appropriate impact forces applied to the plate, in order to solve soft-body impact at high velocity without causing severe numerical instabilities. The bird-strike impact on composite fan blade was simply modeled by discussing the damage characteristics of a unidirectional composite plate. Combining the model of a soft-body impactor with an appropriate contact algorithm, we could capture the transition from the global bending mode at low velocity to the local deformation mode at high velocity, enabling a discussion of the ballistic limit using the damage analysis of the laminate. As the impact velocity increased, the damage in the composite changed from bending-induced matrix-cracking to an intensive fiber-breakage mode causing local shear perforation. The damage mode transition allows us to detect the transition velocity as a ballistic limit, which is one of the critical factors for discussing the bird-strike resistance of composite fan blades.     

The effects of repeated transverse impact load on the burst pressure of composite pressure vessel

The present experimental study deals with the repeated transverse impact effect on the burst pressure of composite pressure vessels. Filament winding method is used to produce the vessels. Glass fiber reinforced (GFR) vessels are manufactured by using E-glass and epoxy resin. Composite pressure vessel was manufactured from fibers oriented [+55°/−55°/+55°/−55]_2s and the impact energies were chosen as 10, 15, 20, 25, 30 J for empty vessel during the impact tests. In addition, 10, 15, 20, 25 J for water filled conditions at 25 and 70 °C. The transverse impact load was applied in single and three times repeated form. The results show that when the impact load and water temperature increases, the burst pressure decreases.     

Charpy impact damage behaviour of single and multi-delaminated hybrid composite beam structures

The ply delamination which is known as a principle mode of failure of layered composites due to separation along the interfaces of the layers is one of the main concerns in designing of composite material structures. In this regard, the effect of hybrid laminate lay-up with different delamination positions in composite beam was investigated. The Charpy impact test was chosen to study the energy absorbing capability of delaminated composite beam. Hybrid composite beams were fabricated from combination of glass/epoxy and carbon/epoxy composites. It was shown that composite beams with closer position of delamination to impacted surface are able to absorb more energy in comparison with other delamination positions in hybrid and non-hybrid ones. The Charpy impact test of delaminated composite beams was also simulated by finite element software LS-DYNA and the results were verified with the relevant experimental results.     

Behaviour of composite plates with drilled and moulded hole under tensile load

Drilling of composite material causes damages of different natures. To avoid machining in composite material, it is proposed to include holes while moulding. In the literature, there is few works concern the behaviour analysis of specimens with moulded holes. All the literature works concern a specimens with moulded holes made of woven fabric. Moreover no literature is available on local behaviour of specimens around the hole. In this paper, several mechanical tests are carried out on specimens with quasi-isotropic stacking sequence made from UD prepreg with drilled holes and with moulded holes. To analyse the behaviour of the specimens around the hole, the strain field is measured using three-dimensional Digital Image Correlation technique (DIC). The pictures analysed given by the CCD camera reveal that, the damage mechanisms are different between the plates with drilled holes and those with moulded holes. The SEM observation have shown that, the fibre content and porosity content near the hole is around ≈ 8% higher compared to the values obtained far the hole. Moreover the tensile strength of the plates with moulded holes is 28% higher than with drilled holes.