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SM(AG27): Fractographic aspects of fatigue
failure in complex composite Contents Completion of milestones
· Monitoring Responsable: Prof. P. Curtis (Dstl) · Chairman: Mr. M. Hiley (QinetiQ) The key aim of SM(AG27) is to develop a sound understanding of the
fracture processes occurring in composite materials manufactured from complex
materials such as woven and non-crimped fabrics (NCF’s). The
understanding gained should assist in the failure analysis of prototype and
'in-service' components. The work builds on studies within SM(AG20) which
used fractography to understand the failure processes associated with
interlaminar fracture in unidirectional composites tested under static and
fatigue loading. The primary activities of SM(AG27) are focussing on the
following objectives: 1.
Establish the extent to which the findings of earlier
fatigue studies (SM(AG20)) can be applied to the analysis of failure in
multi-directional and woven/NCF laminates; 2.
Identify the macroscopical and microscopical features
associated with static and fatigue damage growth in multi-directional and
woven/NCF laminates; 3.
Establish the relationship between fractographic features
in multi-directional and woven/NCF laminates and the direction of crack
delamination; 4.
Establish microscopical mechanisms by which the features
mentioned at 2. occur under different loading modes; 5.
Establish the material dependency, in particular the
effect of brittle and tough matrices, on the fractographic features mentioned
at 2.; 6. Establish the read across between the fatigue failure processes occurring in coupons and those observed in structural components.
Main achievements The AG activities are based on the ‘blind’ fractographic
assessment of model fracture surfaces generated under controlled conditions.
A series of four round robin (RR) exercises are being performed in which
fractured specimens manufactured by one group of SM(AG27) members are
examined by other members of the group. Round robins 1 and 2 have been
completed. RR1 involved the assessment of mode I and mode II fatigue
fractures in the carbon/epoxy T800/8552, generated between a 0°/45° interface. RR2 was split into two parts, the first of which was to investigate
the effect of R-ratio and G-level on the fractographic features observed in
specimens similar to those used in RR1. This study has put on hold due to the
unavailability of material at the time. The second part of RR2 which has also
been completed involved the assessment of static interlaminar fractures in
woven laminates 0°/90° (weft/weft). Two weave
architectures were used (carbon/8552 5 HS weave and carbon/920 2:2 twill
weave). Based on this analysis, a set of ground rules has been developed
allowing the global modes and direction of fracture to be established. Samples for RR3 and RR4 are currently being distributed but progress has been very slow due to difficulties in obtaining the small quantities of the material required for this study. In RR3, static fractures in woven laminates made from the same materials as those of RR2 but with ±45° interfaces are to be examined. One NCF material will also be studied. RR4 will look at fatigue fractures in woven laminates with a 0°/90° interface at the centre. To assess read across between coupon failures and structural failures, sub-elements tested in fatigue will be examined where available. Resources
Completion of milestones
Benefits Complex laminates, such as woven and non-crimped fabrics (NCF), are
being used with increased frequency for the manufacture of components with
complex geometries. Within the study in SM(AG27), failure in such laminates
will be better understood. The group draws on the experience of more than ten failure experts
from industry and research institutions throughout The work performed SM(AG27) will enhance the
understanding of failure in complex laminates, such as woven and non-crimped
fabrics (NCF), which are being used with increased frequency for the
manufacture of components with complex geometries. Although the group will
concentrate predominantly on materials used for aerospace application, the
results will equally be applicable to composites materials used in the
manufacture of naval and land based vehicles. Management Issues The AG activities are currently
running over 20 months behind schedule. Delays have having been incurred due
the loss/retirement of several experienced members of the group, difficulties
with testing some woven specimens (some of which failed in flexure rather
than in an interlaminar mode) and material supply problems; the latter having
by far the biggest impact on progress. The Chairman had put in place a plan of action to try to recover some of the time lost, which will effectively involve the amalgamation of RR3 and RR4 and cutting out one biannual meeting. By running these activities in parallel, it was hoped that the stop/start approach associated with each round robin exercise could be minimised. Unfortunately, difficulties in obtaining small quantities of the 920 material continued to be a problem throughout 2004. The material could not be purchased, as the cost of buying the minimum order quantity was too high. The project has therefore been relying on ‘spare’ material becoming available from other projects on which members are working. A switch to another material was not considered appropriate since data from RR1 and RR2 could not then be easily read across to the following round robin exercises. A supply of 920 material became available in November 2004 and the manufacture, testing and evaluation of samples are now underway. AG membership
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Last
Update : 22 June 2005 |
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