Influence of Fibre Architecture on Impact Damage Tolerance in 3D Woven Composites
3D woven composites, due to the presence of through-thickness fibre-bridging, have the potential to improve damage tolerance and at the same time to reduce the manufacturing costs. However, ability to withstand damage depends on weave topology as well as geometry of individual tows. There is an extensive literature on damage tolerance of 2D prepreg laminates but limited work is reported on the damage tolerance of 3D weaves. In view of the recent interest in 3D woven composites from aerospace as well as non-aerospace sectors, this paper aims to provide an understanding of the impact damage resistance as well as damage tolerance of 3D woven composites. Four different 3D woven architectures, orthogonal, angle interlocked, layer-to-layer and modified layer-to-layer structures, have been produced under identical weaving conditions. Two additional structures, Unidirectional (UD) cross-ply and 2D plain weave, have been developed for comparison with 3D weaves. All the four 3D woven laminates have similar order of magnitude of damage area and damage width, but significantly lower than UD and 2D woven laminates. Damage Resistance, calculated as impact energy per unit damage area, has been shown to be significantly higher for 3D woven laminates. Rate of change of CAI strength with impact energy appears to be similar for all four 3D woven laminates as well as UD laminate; 2D woven laminate has higher rate of degradation with respect to impact energy. Undamaged compression strength has been shown to be a function of average tow waviness angle. Additionally, 3D weaves exhibit a critical damage size; below this size there is no appreciable reduction in compression strength. 3D woven laminates have also exhibited a degree of plasticity during compression whereas UD laminates fail instantly. The experimental work reported in this paper forms a foundation for systematic development of computational models for 3D woven architectures for damage tolerance.
Keywords3D weaves Impact Damage tolerance Compression after impact
Advanced composites have seen a rapid growth in recent years driven by civil aircraft programs such as Boeing 787, Airbus A350XWB and Bombardier C series. Significant volumes of production are coming from wind turbine blades and automotive chassis for battery powered cars such as BMW Megacity. Traditional method of composites manufacturing based on manual prepreg lay-up and autoclave curing is a big bottleneck for the projected increase in volumes. In addition, prepregs are too expensive for high-volume applications. While automated tape laying and fibre placement processes are addressing throughput issues, composites industry is seriously looking at dry fibre preforms in conjunction with resin infusion techniques. 3D woven preforms are particularly attractive because of the reduced part count, their ability to create near-net shapes as well as the presence of through-thickness reinforcement. In addition to manufacturing costs and production rates, damage tolerance has become a major issue for the composites industry. Various resin toughening schemes developed over the years for prepreg systems are not suitable for resin infusion processes due to their high viscosity. As a result, there is an urgent need to develop alternate toughening schemes for dry preforms. Amongst various toughening schemes, inter-laminar fiber bridging is considered to be the most effective method in improving the damage tolerance. This paper investigates 3D woven structures for their ability to resist damage in comparison to 2D laminar structures.
The need for damage tolerant composites transcends all parts of the composites industry common to structures made from carbon or glass fibres for applications in aerospace to marine and wind turbine applications. Perhaps the most high profile and historically most critical applications have been carbon fiber composites in primary aerospace structures. Here the problem of damage is compounded by the fact that damage is commonly sub-surface, undetectable by eye, but can have a considerable detrimental effect on key laminate properties such as compression after impact . The importance of damage tolerance in the aerospace sector is increasing given the trend to manufacture more primary civil aircraft structures from CFRP, including the fuselage which is susceptible to significant airfield abuse (baggage handling, vehicles etc) and from large hail stones. However damage tolerance in general is also of prime concern to the wind turbine industry as it is simply not economic to replace damaged blades in service, especially if the turbines are deployed off-shore .
The industry has made considerable progress for a 30 year time period in improving the damage tolerance of all classes of composites. This is exemplified by the improvements in carbon fiber composites intended for service as primary structures in aerospace where the damage tolerance of the composite laminate is usually quantified by measuring the compression after impact strength according to some recognized standard . Low energy impact is a significant in-service problem which results in difficult to detect damage, primarily consisting of delaminations, which can seriously reduce the compression strength of a composite laminate.
The compression after impact properties have increased when measured using the Boeing standard test method from a level of about 170 MPa to almost 400 MPa in this period. The improvements have been largely the result of increases in resin toughness leading to inter-laminar shear strength. The developments have included the introduction of formulated epoxies, followed by rubber toughened systems, thermoplastic toughened systems, through to thermoplastic, phase inverted thermoplastic-epoxies, monolithic thermoplastics and interleaf toughened epoxies.
Mechanisms for damage tolerance for various material systems have been studied by number of authors over the years. Challenger  conducted a key workshop on damage tolerance in 1986 funded by Office of Naval Research. This workshop identified two important areas for further study: i) understanding of micro-mechanisms for damage formation and growth ii)development of analytical/computational models. Hull et al  conducted a detailed study of the damage morphology and concluded that BVID (Barely Visible Damage) in UD laminates is triggered by intraply cracking and subsequent delamination propagation. Shyr et al  reported the importance of number of layers on the mode of failure—fibre fracture dominated a thirteen layer laminate where as delamination dominated a seven layer laminate during an impact event. Edgren et al  studied damage tolerance analysis of NCF sandwich composite panels for both un-notched and notched specimens. Naik and Logan  studied damage resistance of 3D woven architectures. Damage resistance is investigated by impacting test panels with predetermined impact energy levels and measuring the resulting damage using non-destructive or destructive methods. Damage tolerance, which is the ability of a structure to retain its load carrying capacity, is evaluated using in-plane tensile, compressive and shear tests . Of these tests, compression after impact (CAI) is the most important test method. The magnitude of this damage has been characterized by different groups according to either the area of the damage zone or the width of the damage zone . Delaminations propagate typically perpendicular to the loading direction; this has prompted the use of damage width as being the key damage parameter .
2 3D Weaving
3D woven fabrics have the potential to reduce preforming cycle time and at the same time provide through-thickness reinforcement. In this work, we developed a range of 3D woven architectures on a rapier loom equipped with an electronic Jacquard shedding.
Conventional rapier weaving machine is utilized to produce all the 3D structures and 2D plain fabric. Considering machine set up time, design for all the structures is done such that warp density remain more or less same and weft density can be varied according to the requirement. Warp yarns are coming through creel arrangements which maintain the individual warp tension, even though they follow different path. 2D plain fabric density is kept such that 3 layers of plain fabric approximately equal density of other 3D architectures. 0/90 Unidirectional fabric is produced on robotic tow placement machine at the University of Manchester . A pin board has been employed in order to place the fibres around the pins and then tufting is done on edges in order to handle the fabric conveniently for infusion.
2.1 Sample Preparation
3D woven as well as 2D laminates were prepared using vacuum infusion process. Standard un-toughened epoxy resin system has been chosen, as the main objective of this study is to understand the influence of fibre bridging provided in 3D woven architectures. Additionally glass fibres rather carbon fibres were used in this study in order to improve the visibility of damage area as well as tow visibility in x-ray tomography images.
Layer to layer
Modified layer to layer
Plain 2D (3 layers)
Yarn Count (Tex)
No.of warp layers
No. of weft layers
Warp Crimp % Binder/stuffer
Weft Crimp %
Plain 2D (3 layers)
binder weave angle (o)
stuffer weave angle (o)
Fibre Volume Fraction %
2.2 Geometry of Weaves
In layer-to-layer weaving, all the warp yarns are crimped and the individual warp yarns connect two adjacent layers (Fig. 2a).
Binder yarns in an orthogonal weave have large waviness and as a result weft yarns do not neatly stack-up (Fig. 2b). This arrangement leads to relatively low fibre volume fractions. Orthogonal woven laminate has the lowest fibre volume fraction (35%). Stuffer yarns are slightly crimped by the nesting of the weft yarns (Fig. 2c).
Modified layer-to-layer construction has better fiber volume fraction(47%) due to the presence of additional warp yarns closer to the surface (Fig. 2d)
Angle interlocked weave has relatively straight yarn segments inclined at an angle to the warp direction. The weft yarn packing is more orderly. The fiber volume fraction is 40.3%.
3 Impact Testing
4 Compression After Impact Testing
UD cross-ply has the highest undamaged strength followed by angle interlock weave; layer-to-layer weaves have the lowest compression strength due to tow waviness; orthogonal weave has median compression strength due to the presence of combination of stuffer and binder tows. Compression strength of a 2D woven laminate is closer to orthogonal weave. Rate of change of compression strength with respect to impact energy is somewhat similar for all four 3D weaves, and comparable to the UD cross-ply. 2D woven laminate appears to have steeper reduction in compression strength with respect to impact energy.
4.1 Influence of Weave Angle on the Compression Strength
It may be observed from the Fig. 8 that compression after impact strength of composite laminates is primarily a function of undamaged compression strength. Undamaged compression strength is in turn a function of tow waviness in the loading directions. There have been several studies on the influence of off-axis angle on compression strength [13, 14, 15]. Emehel et al  presented a tow collapse model for 3D woven and braided composites, based on Budiansky’s model.
4.2 CAI as a Function of Damage Width
For damage width in the range of 8–10 mm, ratio of net-section strength to undamaged compression strength is close to unity (Fig. 11). At lower damage levels, this ratio is higher than 1, indicating that the damage area has some residual compression strength.
4.3 Nature of Compression Failure in 3D Laminates
Damage area increases approximately linearly with the impact energy; all 3D woven laminates have similar order of magnitude of damage area. Individual differences may be accounted for by the fibre volume fractions (FVF). Amongst 3D weaves, Orthogonal laminate had the lowest FVF and the highest damage area; modified layer-to-layer laminate had the highest FVF and hence the lowest damage area. UD cross-ply laminate exhibited highest damage area with 2D woven laminate having damage area half way between UD and 3D laminates.
In UD laminates damage appears to spread along the fibre directions and the damage is predominantly intra-ply. In 3D woven laminates, damage appears to be highly localized and spread in the thickness direction (damage area smaller than the impactor area).
Damage resistance, energy required for damaging unit area, is lowest for UD cross-ply laminate. 3D woven laminates exhibit significantly higher damage resistance.
Undamaged compression strength is primarily a function of average tow waviness angle and less influenced by the topology of interlacement.
Rate of change of CAI with respect to impact energy is similar for all 3D weaves; UD laminate also exhibited similar gradient. CAI strength of 2D woven laminates appears to drop at steeper rate compared to other weaves.
Angle interlocked laminates exhibited highest compression strength values in both the undamaged and damaged samples. In this weave, the warp yarns appear to be un-crimped but inclined at an angle to the loading direction. Orthogonal and layer-to-layer weaves exhibit high crimp and hence have lower compression strengths.
3D woven laminates have a critical damage width below which there is no apparent degradation of compression strength. UD and 2D woven laminates do not exhibit similar behavior.
In 3D woven laminates, rate of change of CAI with damage width (after the critical damage width) appears to be steepest (Fig. 10). However, this reduction in strength is also a function of the sample width, as the net-section analysis shows that CAI strength is a function of undamaged width. For larger panels, rate of change of CAI will be less steep.
3D woven laminates exhibit a degree of plasticity during a compression test and form a ‘plastic hinge’ at the mid-section. UD laminates exhibit a sharp failure where as 2D woven laminates exhibit a degree of plasticity before a sharp failure.
It was demonstrated that 3D weaves can significantly reduce the damage area. By improving the fibre architecture if one can reduce tow waviness in 3D weaves then CAI values can be improved at the same time.
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