Page 79 - Research Report 2021 - Institut für Leichtbau und Kunststofftechnik

Doctorate Dr.-Ing. Richard Protz

On the influence of defects on the strain rate-dependent

material behaviour of fibre-reinforced plastics

Supervising Professor: Prof. Dr.-Ing. habil. Maik Gude

Fibre-reinforced plastic (FRP) composites are predestined
for crash-loaded lightweight structures due to their ad-
justable energy absorption capacity. However, in FRP can
occur defects that develop during manufacture (voids)
and in operation (fatigue) (Fig. 02). The influence of such
defects, especially on the strain rate-dependent material
behaviour, is currently still insufficiently researched and
can represent a safety risk. Therefore, in this successfully
completed doctorate (Fig. 01), the formation and charac-
teristics of defects were analysed in detail and a damage Fig. 02 FRP with productional (left) and operational (right) de-
model was developed for their mathematical description. fects.

The analysis of the influence of defects on the strain
rate-dependent material behaviour under tensile load
at different strain rates showed an increasing material
degradation with increasing pore volume contents and
higher pre-fatigue. Pre-damage due to interacting pores
and fatigue impair additionally the strain rate-dependent
material behaviour to a significant amount.

Based on the experimental work, a phenomenological
Adjoint Damage Model (ADM) was developed. The ADM
Fig. 01 PhD Committee Attendees (from left to right): Prof. can described mathematically the strain rate-dependent
Ullrich, Prof. Gude, Dr. Protz (doctorate), Prof. Kyosev. stress-strain behaviour of nominally defect-free and de-
fect-affected FRP. The interaction of both defect types
Through a fundamental description of the formation is formulated in logic terms as an adjunctive OR oper-
mechanisms of voids and of fatigue-related defects, man- ation of the defect parameters. With increasing defect
ufacturing and testing methodologies for defined defect sizes and the associated increase in defect interactions,
initiation were developed. These allow the investigation the strain rate-dependent deformation behaviour can be
of the influence of separately and jointly occurring man- precisely predicted. The result is a flexible and practical
ufacturing- and operation-related defects on the strain model approach for describing the strain rate-depend-
rate-dependent material behaviour for the first time. The ent material behaviour. Furthermore the transferability
characterisation of the introduced defects by means of of the ADM to another FRP has already been successfully
In-situ computed tomography under tensile load showed demonstrated.
that voids with a low void volume content have no influ-
ence on the crack progression. At higher pore volume The knowledge gained makes a significant contribution to
contents, voids act both as crack stoppers and as source an improved understanding of materials, which will en-
of cracks. When voids and fatigue-related cracks are pres- able the future defect-tolerant design of FRP structural
ent in the material at the same time, they partially ap- components.
pear independently of each other, but can also overlay.

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