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Executive summaries of Action Groups of Group of Responsables in Structures and Materials

 

  • SM(AG27) ‘Fractographic aspects of fatigue failure in complex composite laminates and structures’

 

GARTEUR SM(AG27) aimed at examining the fractographic aspects of static and fatigue failures in laminates with complex lay-ups and fibre architectures. Laminates chosen for evaluation included those fabricated with multidirectional lay-ups and from woven and and non-crimped fabrics (NCFs). The work performed in AG27 has built on previous studies within AG20 which used fractography to understand the failure processes associated with interlaminar fracture in unidirectional composites. Features identified in this earlier study included striations within the resin and fibre imprints, as well as matrix rollers. In performing the study within AG27 the objectives below were addressed:

1)    Establish how the findings of the earlier studies within AG20 can be applied to the analysis of failure in multidirectional and woven/NCF laminates;

2)    Identify the macroscopical and microscopical features associated with static and fatigue damage growth in multidirectional and woven/NCF laminates;

3)    Establish the relationship between fractographic features in multidirectional and woven/NCF laminates and the direction of crack delamination;

4)    Establish microscopical mechanisms by which the features in (2) occur under different loading modes;

5)    Establish the material dependency, in particular the effect of brittle and tough matrices, on the fractographic features in (2);

6)    Establish the read across between the fatigue failure processes occurring in coupons with those observed in structural components.

The group’s activities, which involved participants from QinetiQ, BAe Systems, CETIM, CSM Materialteknik, EADS (Germany and France), INTA, NLR, FOI and SICOMP, were based around the ‘blind’ fractographic assessment of model fracture surfaces generated under controlled conditions. Efforts were directed primarily at the interpretation of interlaminar fractures generated under mode I, mode II and mixed-mode (I+II) loading, since these loads are often the main drivers of damage in composite structures. Fractographic investigations were undertaken by exchanging specimens between members within a series of round robin exercises, where fractured specimens manufactured by one group of AG27 members were examined by other members of the group. The first round robin focussed on the assessment of mulitidirectional laminates, made from unidirectional carbon fibre prepreg, fractured in mode I and mode II fatigue. Fractures were generated between a 0°/45° interface. The remaining three round robin exercises focussed on the assessment of static and fatigue fractures in laminates made from woven fabric. Two weave architectures were investigated (carbon/8552-5HS weave and carbon/920-2:2 twill weave). Due to problems with procuring materials, difficulties with the mechanical testing of some laminates (resulting in invalid failures) and the loss of key AG members, only approximately 60 % of the intended programme was completed.

The fractographic assessment of multidirectional laminates showed that the orientation of the plies and the test specimen were important in controlling delamination growth. Studies on 0°/45° ply interfaces showed that delamination tests could be conducted using conventional mode II and mixed-mode bending specimens (as used to test unidirectional laminates), provided the 0° ply was positioned on the upper (compressive) half of the laminate during testing. This was because shear stresses present in the laminates tended to drive microcracks, forming ahead of the crack tip, upwards to the 0° ply, through which they could not migrate any further. When the specimens were reversed, so that the angle ply was on the compressive surface, the delamination quickly migrated through the plies to other ply interfaces, thereby changing the mode and mechanisms of fracture. Fractographic assessment of the fracture surfaces revealed the presence of striations within matrix and fibre imprints, similar to those observed previously in AG20.

The fractographic assessment of interlaminar fractures in laminates made from woven fabrics was performed and features, enabling the modes and direction of crack propagation to be determined, were identified. Mode I fractures in the two materials appear to be identifiable by the presence of river lines and parabolic cusps. Both sets of features can also be used to give some indication as to the global direction of crack propagation. Static mode II and mixed-mode fractures could also be differentiated by the presence of ‘shear cusps’, the inclination of the cusps giving an indication of the mixed-mode ratio. Where shearing loads were present (due to specimen bending) it was noted that one side of the fractured woven laminate tended to show fibre imprints in the resin, whilst the other surface contained mainly fibres (see Figure 1). This had important implications when determining crack growth directions. Tools for assessing the global directions of crack propagation under these failure modes were identified, but careful assessment of the fractures is required if failure in the warp and weft fibres are to be interpreted correctly. The examination of fatigue fractures in the woven specimen revealed striations and roller features similar to those identified in AG20, however, these showed some modification depending on the resin and weave style used. The need to make use of the electron microscope to interpret fractures accurately will mean that future failure investigations are likely to be more time-consuming and costly compared with the analysis of failures in metallic components or those made from conventional unidirectional prepreg.

weft
 

warp
 
The work performed in this study has demonstrated that failures in the latest generation of materials can now be identified through the expert examination of composite fracture surfaces.

 

 

 

 

 

 

 

 

 


      Top surface shows fibres and tows (x10)        Bottom surface shows imprints (x10)

 

Figure 1  Micrographs of mode II fracture surfaces in AS4/8552 material showing fibre rich and resin (imprint) rich surfaces

 

 

References

SM(AG27) - Fractographic Aspects of Fatigue Failure in Complex Composite Laminates and Structures. GARTEUR/TP-151, April 2006, compiled by M.J.Hiley

 

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