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

Executive Summary
Sandwich composites have been widely used in recent years for weight critical structures such as
airplanes, wind turbine blades and high speed vessels because of superior stiffness/weight ratio
compared to conventional metallic structures. Sandwich composites, composed of different
materials with very different stiffness properties, are prone to different and peculiar damages.
Face/core debonding is one of the most common damages sandwich composites can experience.
A face/core debond may initiate due to different reasons such as problems during the
manufacturing process or due to impact loading. Face/core debonding can be very critical for the
structural performance as the basic sandwich principle is compromised due to absence of
connection between the face and the core resulting in a lack of structural carrying capacity and
integrity.
A question that arises with all applications of sandwich composites is that of damage tolerance:
how is the structural performance influenced by the presence of production defects or in-service
damages? The aim of this thesis is to develop methodologies to answer this question.
Traditionally costly and extensive experiments have been conducted for the assessment of
damaged structure especially when they are exposed to cyclic loading. In this thesis as an
alternative approach to reduce the cost and amount of experimental work, the main focus has
been directed towards the development of numerical schemes replacing costly experiments.
However to examine the accuracy and efficiency of the developed numerical schemes, they are
all validated against experiments.
The thesis is divided into two main parts. In the first part debonded sandwich columns and
panels exposed to static loads are analyzed based on a fracture mechanics based numerical
scheme. To validate the developed scheme, compression tests are conducted on debond damaged
sandwich columns and panels. Furthermore, the face/core interface fracture toughness of the
tested columns and panels are determined and applied in the finite element models to estimate
failure loads. A good accuracy achieved in failure load estimations illustrates the efficiency of
the developed scheme. However in some cases the simulations of the debonded sandwich panels
show around 46% deviation in the determination of the failure loads compared to the
experiments, indicating that the developed scheme should be used carefully.
In the second part of the thesis fatigue lifetime of debond damaged sandwich composites is
studied. To make the finite element simulation of fatigue crack growth practical, a cycle jump
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