JEC COMPOSITES MAGAZINE - Issue #76 - October/November 2012 - 84

CFRC

- epoxy fracture, - epoxy-fabric decohesion, - rupture of the individual fibres, - catastrophic rupture of the remaining fibres when the load is too high.

Plate impact test
A plate impact test was performed using a TITAN single-stage gas gun [ 5 ]. Metrology consists in measuring impact velocity with laser beams and material velocity at the rear of the specimen with a photonic Doppler velocimeter (see [ 6 ] and [ 7 ] for further details on the PDV system). The experiment was designed to identify the dynamic behaviour of the composite under shock wave conditions, i.e. very high strain rate conditions. An experimental configuration was chosen to observe the elastic-plastic behaviour and rupture. The experimental setting (Fig. 4) uses a 5.0 mm AU4G thin disk (impactor) to impact a 10.2 mm composite disk (the target) at 412 m/s. These thicknesses were calculated to generate a tensile area right at the centre of the target. The tensile condition level in the target depends on the impact velocity level.

Fig. 2: Tensile test curves recorded with the CFRC material for the six specimens (#1-3: 0°, #4-6: 45°)

The experimental diagram obtained during the plate impact is shown in Fig. 5. It consists in a first compressive wave generated by the aluminium impactor followed by the release wave. The blue arrow shows the spalling that appears prior to the second compressive wave (a macroscopic picture of the fracture is also presented in the graph). An analysis of the postmortem specimen with an ultrasonic C-scan imaging system is also planned in order to obtain precise details about the delaminated area (depth, shape and size) [ 8 ].

Fig. 3: Tensile test postmortem specimens (top: #2 - 0° orientation; bottom: #5 - 45° orientation)

As-received properties
The results of the material characterization in quasi-static tensile regime are presented in Fig. 2, while postmortem specimens with 0° and 45° orientations are presented in Fig. 3. As expected, the tests in the direction of the fibres give the highest strength level with a mean ultimate tensile strength of 927.3 MPa. At 45°, the ultimate tensile strength is 5.7 times lower than the 0/90° specimens with a mean value of 162 MPa. All the curves have a bilinear shape (whatever the orientation) but the slopes are higher in the fibre direction. The first part of the curve is assumed to be the elastic portion and the second part the portion where multistep degradation of the composite occurs. This point needs to be clarified because all tensile tests were performed up to sample rupture. This transition (red arrows in Fig. 2) will be further investigated by performing tensile tests with controlled load and release steps. The mechanical behaviour of the specimen is directly governed by the multistep degradation of the composite:

Fig. 6: CFRC plate target before impact (left); projectile captured in the CFRC plate after an impact at 289 m/s (right)

Fig. 7: Examples of high-speed camera pictures before impact (left), rebound (middle) and petal ejection (right)

Ballistic tests
Fig. 4: Plate impact experiment

Two ballistic tests were performed on 450 x 323 x 10.2 mm CFRC plates (Fig. 6 left). The projectile was a 10.2 g, 13.2 mm diameter copper sphere impacting the composite plates at 277 m/s and 289 m/s (391 and 486 J impact energy). A 10 kframe/s high-speed camera was used to record the impact event and facilitate the analysis of the target efficiency (Fig. 7).

Fig. 5: Velocity diagram obtained on the CFRC back face during shot #TI0310

Postmortem targets show that the rear sides are damaged and delami-

84 jec composites magazine / No76 October - November 2012

JEC COMPOSITES MAGAZINE - Issue #76 - October/November 2012

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #76 - October/November 2012