Impact behaviour of composite materials is a sudden application of an impulsive force, to a specific part of material or part of the structure. Although the results of an impact in a structure are known, it is difficult to analysis the effects that result in the material properties to have an outcome of a particular event. In an event of impact, it could be largely elastic, with some energy dissipated as heat or sound. On the other hand, it may cause permanent damage, complete penetration of the body structs or fragmentation of the impacting bogy.
Failure modes composite materials
Once the elastic deformation of a composite materials has been consumed, appears four basic mehcanical failure modes. These are:
• Fibre failure: fracture.
• Resin: crazing and microcracking.
•Debonding: between the fibre and matrix.
• Delamination: of adjacent plies in a laminate.
For more information about composite materials failure, this posts explains them in more detail.
Why composite materials have damage under an event of impact?
For fibre reinforced polymers the permanent damage, penetration and fragmentation are the area of interest. Due to the lack of plastic deformation in composite materials, once a certain level of deformation is exceeded, permanent damage occurs.
As an example, a blow with an energy of ~1 J or less at ~2 ms-1 can cause irreversible damage in a composite laminate. The reasons for a low impact strength are:
• Low transverse and interlaminar shear strength.
• Laminar construction, which is required if the reinforcing fibres are to be used efficiently and anisotropy reduced.
• Lack of plastic deformation.
How can be estimated the stresses acting in an impact?
It is very difficult to obtain the exact stress in an event of impact. Simplified methods are used to get an approximate estimation of the stress generated. In practice, compressive loading could cause localised bending and transverse failure in suitably oriented plies and generate interlaminar shear stresses. The compressive stress will continue to act until the impacting body rebounds. The stress wave will propagate into the impacted material, with a decreasing amplitude, and be reflected at the back surface as a tension wave
Test to analyse impact behaviour of composite materials
Due to the difficultu to analyse impact on composite materials, standard test have been created in order to have some data to analyse and improve the design of the composote materials to strength them from impact. The Izod and Charpy tests are some examples of impact tests that have been used for many years in metals particularly with respect to the brittle/ductile transition temperature and notch sensitivity.
In the case of the Izod test is still widely used for polymers. In the Izod test a rectangular or square cross section bar of specified dimensions is clamped at one end and struck towards the top of the test piece with a pendulum.
For the Charpy test a beam is rested freely against two anvils and struck in the centre by a pendulum. In the former case, if the specimen is notched, the notch is at the top of the clamped section and usually faces the direction of strike. Charpy specimens may be machined with U or V notches in the centre of the beam opposite the direction of strike.
However, in either test, and with any material, the impact energy may be overestimated because energy is:
• Stored elastically in the specimen prior to failure.
• Energy is dissipated acoustically, thermally, in the kinetic energy of the failed parts, etc.
Despite the somewhat arbitrary nature of the tests, these tests are used today because the difficulties of crosscorrelating information and of scaling impact energies to bodies with different cross sections.
Examples of impact in composite materials
For example, a spacecraft is susceptible to micrometeroid impact. Although they are small particles, they impact at velocities up to 60 km/s. Damage caused by the impact reduces the strength of the composite structures which consists in cratering, penetration and spallation.
Another type of impact are the ones carried out by Formula One teams. Each year they have to homologate their monocoque through impact tests. These tests are done to verify they are secure and that they fulfil the FIA rules. In the Figure 1 it is show the monocoque of the Red Bull Racing F1 team being test for a front impact.
Summary
To conclude, as explained in other posts, nature fibre-reinforced polymer matrix composites are anisotropic, which means that they have different mechanical properties depending on the direction. If the impact is applied in the principal fibre directions, the modulus and strength are excellent, However, in other directions, transverse to the fibres or in shear, the properties are those of the interface or resin matrix, which are much lower.
In the case of composite materials, they work as wood, once they have exceeded the elastic limit, they cannot be reformed by application of heat and force like metals. The interlaminar (or between ply) direction is the one in which fibre composites are particularly weak and prone to splitting. This type of deformation, known as mode 1, is regarded as limiting and much of the work on impact, fracture and failure concentrates on measuring and analysing this behaviour. Models to account for impact behaviour are based on the bulk strain energy which can be stored prior to failure (proportional to the ratio of the square of the strength to the modulus, in the appropriate direction), micromechanical dissipation mechanisms in which resin cracking, debonding and delamination are particularly prominent and fracture mechanics.
