Impact performance - design using fibre reinforced polymer composites

TWI Bulletin, Mar/Apr 1997

Gareth McGrath is Manager of the Centre for Adhesive Technology. He has been involved with research and development of adhesives and composites for 12 years.

In this paper, he explores the control of constituents within a composite to optimise energy absorption, impact resistance and damage tolerance. He also explains how further improvements can be achieved by combining different composites in hybrid forms and through modifications in structural designs.

Introduction

Unlike conventional metals and alloys, which are characterised by their ductility, homogeneity, and isotropy, fibre reinforced composites are characterised by heterogeneity and anisotropy, and may be brittle. Fibre reinforced composites possess excellent properties along the fibre, but are relatively weak transversely. The fibre orientation is, however, under control of the designer who has the opportunity to tailor specific properties to meet the needs of a particular application.

The degree of progress in the application and acceptance of fibre reinforced composite is based upon availability and maturity of appropriate technology. In a structure, certain requirements of strength, stiffness, durability, damage tolerance and energy absorption must be met. Satisfaction of these conditions provides a degree of assurance for the integrity, economic life and safety of that structure.

Despite some misconceptions that composites cannot withstand impact loads, when correctly used polymer composites can perform better than metals in absorbing energy. However, these materials are susceptible to the effects of accidental impact damage, resulting in significant reductions in residual compressive strength. This currently limits design strain levels to 0.3-0.4%, well below the capability of most fibres. To develop damage tolerant structures, it is necessary to consider the constituents of the composite and their effect upon the damage tolerance.

Performance of composite structures

The energy absorbing properties of composite materials may be described in terms of work of fracture , which arises from the failure mechanisms occurring during impact. The inherent brittleness of composites ensures that they do not undergo the yield processes characteristic of ductile materials but, on application of a load, deform elastically up to the point of fracture. A number of modes of deformation are available, the primary energy absorbing ones being:

• cracking and fracture of fibres
• matrix fracture
• debonding (pull-out) of fibres from matrix
• delamination of the layers making up the structure

A composite disintegrates both macro and microscopically during an impact event.

Impact resistance of materials and structures can be evaluated using instrumented impact testing apparatus[1] . Devices of this type consist of a weighted impactor in free-fall (Fig.1) or are spring assisted. Velocity measurement and a load transducer are interfaced with a computer to measure key parameters and characterise the impact event. Energy required to cause damage, load/time and load/deflection histories and total energy absorbed are typical of the data collected for quantification of damage tolerance.

Development of damage tolerance systems

Designers of composite structures have long been aware of the dangers that relatively small amounts of impact damage can cause, resulting in significant reductions in mechanical properties. The designer must satisfy three criteria with regard to impact damage. The first is damage resistance - the component must withstand an impact strike with a minimum amount of damage incurred on the composite. The second condition is that of damage tolerance - the ability of a material to be damaged and yet still able to function within the defined loading regime. The final consideration is that of energy absorption, which is probably the most important factor since it governs the structural integrity[2] .

A composite is considered damage tolerant if the threshold for damage initiation is high, the tolerance to that damage is high, or a combination of both. Three criteria are important: damage initiation, damage growth and residual compressive strength. When designing for damage tolerance it is necessary to consider minimisation of the damage and maximisation of residual compressive strength. Development of damage tolerance is very dependent on materials in the composite: the fibre, resin and interface. It is helpful to consider the critical properties of the fibre, matrix and interface that affect the various forms of damage. The matrix and interface are considered to be the weak links of the composite, and consequently have significant influence upon damage propagation. The properties of the fibre cannot be ignored, because they significantly affect the type of damage formed.

Damage initiation

In the impact event, the threshold for damage initiation depends on how much elastic strain energy the composite can absorb. The elastic strain energy can be increased by improving the fibre's strength or failure strain, thereby increasing the damage threshold. Use of fibres with different properties should be considered, because of the different effects that the fibres can have upon damage formation. High modulus fibres generally lead to greater stress and strain concentrations in the matrix, and to formation of many types of damage. Lower modulus fibres, with higher failure strains, induce more widespread demands on the matrix and may also lead to damage formation. Therefore, skilled interpretation of performance requirements and material properties is essential when designing the structure.

It is the nature of the damage and absorbed energy associated with it that affects final selection of material. The needs of the final structure define the preferred energy absorption mechanism. For example, the structure may need to have sufficient integrity to carry on after damage, or the component may be a safety device, which has a 'one shot' demand, and is then replaced.

Damage propagation

The effect of the matrix properties on damage propagation in the composite is significant, and composites made from tougher resins are generally less susceptible to damage. The interface between the matrix and the fibre is particularly important in developing a damage tolerant composite. Early composite materials were notoriously brittle, not only because the matrix was inherently brittle, but also because of a lack of understanding abix properties on damage propagation in the composite is significant, and composites made from tougher resins are generally less susceptible to damage. The interface between the matrix and the fibre is particularly important in developing a damage tolerant composite. Early composite materials were notoriously brittle, not only because the matrix was inherently brittle, but also because of a lack of understanding about controlling the interface.

Carbon fibres were, therefore, given an oxidative surface treatment during manufacture to increase bond strength between the fibre and the matrix. Increased interlaminar shear strength is the result of increased oxidative treatment. An optimum value is desirable, because a plateau is reached and notched tensile strength then decreases with increased oxidative treatment, Fig.2.

This shows the effect of carbon fibre surface treatment and hence fibre/matrix bond strength on the interlaminar shear strength of a unidirectional CFRP composite and on the notch sensitivity of a [0 2 + 45

This shows the - 45 2 0 2] s CFRP laminate. Fibre surface treatment reduces intraply splitting, delamination at the notch tip, and the size of damage zone formed. This causes greater residual stress concentrations and reduced residual tensile strength after impact. Under compressive loading, however, increasing the surface oxidative treatment has a beneficial effect, because reduced areas of damage result and delamination growth is restricted.

Damage tolerance of the composite is thus dependent upon interactions between:

• the modulus of the carbon fibre
• toughness of the resin
• selection of an appropriate level of oxidative surface treatment to develop an appropriate interface

Damage tolerance

The majority of work carried out on the improvement of damage tolerance within composite systems has concentrated on either producing higher strength fibres or toughening the resin matrix. The toughening process is usually achieved by creating plastic zones within the matrix and/or mechanisms to blunt and deflect propagating cracks. This is generally achieved by means of thermoplastic second phase additions to the primary resin matrix, which achieves both objectives.

The effect of toughening agents is to increase the work of fracture within the composite. The data collected in impact tend to be comparative rather than quantitative, despite the well-defined test procedures used. The reasons for this are that composite structures are far more complex than the simple laminates evaluated under test, and improvements in toughness change the nature of the failure from a brittle to a 'pseudo-plastic' mode. Since the theory is based on linear elastic behaviour, any increase in toughness will progressively invalidate the mathematical model by which it is described. Qualitative studies have shown that introduction of tougher resin systems has significantly improved damage resistance and tolerance.

Damage tolerance is evaluated by inflicting measured levels of impact upon test coupons. Ultrasonic damage maps produced before and after the impact can be used to map the extent and severity of cracking and delamination with respect to the incident energy. Once impacted, the coupons are mechanically tested in order to evaluate deterioration in properties from which damage tolerance characteristics can be implied. Mechanical testing is generally carried out under compressive loading conditions, since it is in this configuration that composites are most prone to failure [3] .

Advances in computer controlled servo-hydraulic testing equipment allow very sophisticated assessment of the damage tolerance of composite components and sub-assemblies. Impact damage may be simulated during fabrication or deliberately created using falling weight impactors or some form of projectile. Damaged samples may be subsequently tested either statically to destruction, or dynamically for endurance.

Energy absorption

Attainment of high values of specific energy absorption depends on the ability to initiate a controlled disintegration process at loads below those required to cause failure of the structure as a whole. Designing-in of mechanisms capable of triggering such processes may well result in structural weakness and loss of primary strength and stiffness. One possible approach to avoid structural weakness is use of structural forms which are self-triggering. Typical of such a shape would be a tapered tube, this would fail at the narrow end with a crack zone moving progressively rearwards.

Experimentation has shown that the fibre fracture mode of deformation is the most efficient in absorbing energy. Use of high strength fibres, which would improve damage tolerance, tends to lower the energy absorbing efficiency. The reason for this is that high strength fibres tend to induce a delamination failure rather than local fracturing of the component. Once again, performance specification of the component dictates the choice of fibre (and matrix).

Compression after impact

Residual compressive strength after impact is a key design parameter, and is often design limiting. Low impact energies can result in delamination if the properties of the constituents are not optimised which causes a fall in compressive strength due to local ply instabilities [4] . At higher impact energies, fibre damage is also incurred, as well as further reductions in residual compressive strength. Optimum properties are not always available in a single fibre-matrix system, and hybridisation offers significant improvements in damage tolerance, with reduced damage areas and improved residual properties. The combination of ultra high modulus fibres with high strength fibres offers the designer significant benefits for stiff, reduced weight, energy absorbing structures.

Impact Considerations

During the course of its lifetime a composite structure is likely to be subjected to a number of different forms of severe impact loading.

A number of designers have expressed concern as to the suitability of such brittle materials in a dynamic application. However, high stiffness allows impacts to be absorbed by the structure as a whole, rather than being concentrated at the point of impact. Composite materials are able to absorb the energy of impact by a controlled disintegration of the structure. By contrast, the forces generated from the impact of a structure in a ductile metal, such as aluminium, are sufficient to exceed the material's elastic limit which does not have the variety of energy absorbing mechanisms to respond to the impact.

One does not need to delve very far into available literature to realise that the dynamic response of both composite materials and structures is very poorly understood. It is becoming increasingly necessary to include impact resistance in the design of a great many engineering systems. This requirement has been emphasised recently in the UK, following enquiries into engineering disasters [5] , [6] .

The factors which govern the behaviour of composite materials are very much misunderstood. However, one of the most attractive features of composite materials in engineering design is the ability to exploit their anisotropy in tailoring mechanical properties to suit a specific application. It is not unreasonable to presume that the energy absorbing properties may also be tailored to provide an optimum performance. It is very doubtful that the arrangement best suited to mechanical properties will be the same as that designed for impact resistance. The techniques and tests used are a product of evolution and experience rather than any rigid scientific analysis.

Designing With Composites

Composite components are often stiffness critical they are designed to transmit rather than absorb the various loads generated during operation. Carbon fibres exhibit the highest specific stiffness of any widely available engineering materials. Since stiffness is the major design criterion, one might expect materials selection to be a simple matter of choosing fibres with the highest modulus. Unfortunately, producing fibres of increasing modulus involves a corresponding increase in brittleness.

Fig.3 Impacted high modulus carbon fibre composite

The low strength and elongation of this material result in fibre fracture being the primary mode of deformation, with most of the damage confined to the locality of the impact. Figure 4, on the other hand, is a micrograph illustrating damage in a high strength 'intermediate modulus' fibre composite.

High fibre strength and increased strain-to-failure have resulted in delamination and fibre pull-out being the more dominant failure modes. It is interesting to note that the total energy absorbed in these two cases is very similar. This would seem to suggest that, although very susceptible to impact damage, high modulus fibre structures ought to be safe in terms of impact performance.

Conclusions

Designing to provide for energy absorption requires that the engineer knows exactly how he wishes a component to react to different impact conditions. Areas of research that require further investigation are those of temperature effects on impact resistan resistance/tolerance and projectile velocity effects.

Impact performance is a topic which is fraught with misconceptions throughout industry as a whole. Application of static proof loads to determine impact resistance is unrealistic. Since the impact behaviour of composite materials and structures are very 'strain rate dependent', tests of this type are inappropriate other than to define a minimum level of structural integrity. Impact testing must be considered.

Consequently it is very doubtful if impact testing will ever provide more than qualitative or, at best, semi-quantitative data, especially considering our present lack of understanding. Notwithstanding this, the testing carried out to date at TWI has yielded significant improvements in damage resistance and tolerance [7] .

References