Residual Strength and Fatigue Lifetime of ... - Solid Mechanics

These peculiar damage modes often result in considering higher safety factors during a design
process compared to similar metallic structures, which makes a detailed study of failure modes
of sandwich structures essential if maximum structural efficiency is to be reached. Face/core
debonding and its propagation are among the most critical damages a sandwich structures may
experience, as structural integrity is closely linked to the adequacy of the bonding in the
face/core interface in these structures. Debonds may emerge due to production defects, in-service
overloading and local loads like impact, see Figure 1.1. A debond may be initiated directly in the
face/core interface, in the core just below the resin-rich cells or in the face sheet (for composite
face sheets). If a debond emerges in any of these locations, depending on the loading at the crack
tip and toughness of other neighbouring layers, it may continue propagating in the original
debond position or kink into the core, interface or face sheet, see Figure 1.2. Studies have shown
that face/core debonding considerably reduces the load carrying capacity of sandwich structures
(Nøkkentved et al., 2005, and Berggreen et al., 2005). A question that arises for debond
damaged sandwich structures is that of damage tolerance: how is the structural performance
influenced by the presence of debonding? The question of damage tolerance does not only apply
to the design and optimisation of sandwich strcutures, but is also relevant to the residual strength
and lifetime of already in service structures with minor or major damages. In recent years, a
number of analytical and numerical studies have been conducted to predict the initiation and
propagation of debonds in sandwich structures exposed to static loading, e.g. Kardomateas and
Huang (2003), Sankar and Narayan (2001), Chen and Bai (2002) and Avilés and Carlsson
(2007). Furthermore, experiments have been performed to determine the residual strength and
identify the failure mechanisms of debonded sandwich structures, e.g. Avery and Sankar (2000),
Vadakke and Carlsson (2004) and Xie and Vizzini (2005).
Linear Elastic Fracture Mechanics (LEFM) has been extensively used to model debond initiation
and propagation where the energy dissipation zone (fracture process zone) is relatively small
compared to specimen dimensions, (Hutchinson and Suo, 1992). Furthermore, several studies
have dealt with the determination of the fracture toughness of face/core interface in sandwich
structures, e.g. Cantwell and Davies (1996), Li and Carlsson (1999) and Østergaard et al. (2007).
In sandwich structures with composite face sheets the fracture process zone becomes large due to
kinking of the crack into the composite face sheet and consequent fibre bridging which violates
LEFM assumptions, see Figure 1.2. As an alternative to LEFM, cohesive zone modelling has
been used in the literature, e.g. Lundsgaard-Larsen et al. (2008, 2010) and Østergaard et al.
(2008) to model face/core debonding in the presence of fibre bridging.
In few studies experiments have been conducted to some extent in order to examine the accuracy
of the developed analysis methods in debonded sandwich structures e.g. see Berggreen et al.
(2005), Jolma et al. (2007) and Aviles et al. (2006). However, despite all the proposed numerical
and analytical methods, a comprehensive study of debond damaged sandwich structures,
addressing systematically issues like debond propagation, characterisation of the fracture
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