In materials science, fatigue is the progressive and localized structural
damage that occurs when a material is subjected to cyclic loading. Fatigue occurs when a material
is subjected to repeated loading and unloading. If the loads are above a certain threshold,
micro-cracks will begin to form and grow. Ply is weakened due to matrix cracking and will failure
under fatigue loading due to delamination and fiber failure. The shape of the structure will
significantly affect the fatigue life; square holes or sharp corners will lead to elevated local
stresses where fatigue cracks can initiate. Round holes and smooth transitions or fillets are
therefore important to increase the fatigue strength of the structure. Several types of fatigue
loading must be accounted for, some are listed here: low cycle fatigue, high cycle fatigue and
random fatigue using spectrum loading. High cycle fatigue usually deals with situations that
require more than 10,000 cycles to failure. In some applications, fatigue loading includes
changes in stress ratio, frequency, or temperature between several stages of the loading process.
This is referred to as multi-stage fatigue.
|Validation of Fatigue Crack Growth of Aircraft Panel Under Tension Fatigue Loading; Panel is made from CentrAl Fiber Metal Laminate |
Main challenges with composite fatigue are the determination of:
Constituents fatigue properties (strength cycle S-N curve)
Effect of defects and environment on fatigue properties of the lamina
da/dN curve for crack growth under fatigue loading. Other
challenges are associated with the accumulation of damage under multi-stage fatigue loading
and with dealing with the probabilistic nature of composite.
Relying on test alone to qualify material or structure under fatigue loading is
cost prohibitive. Limited tests can be used to guide the fatigue life assessment of composite
strictures at various levels of the building block (coupon, element, sub-component, and component).
Not accounting for the probabilistic nature of composite and manufacturing defects, can result
in reduced durability and reliability, and in increased risk to the structure.
How we address it
By use of multi-scale based progressive failure analysis integrated with finite
element solution. Traditional finite element analysis (FEA) solvers assess damage at the lamina
level whereby damage initiates at lower scale in the fiber and matrix.
GENOA Progressive Failure Analysis augments finite element analysis with multi-scale composite
mechanics, and damage tracking and fracture to determine damage evolution under single or multi-stage
fatigue loading condition. It identifies the number of cycles where damage or delamination initiates
and grow to failure. It also identifies location and types of damage/failure modes. Damage
is accumulated for each fatigue cycle loading. Damage history and damage indices are extracted
for use in subsequent analysis. For fatigue crack growth, AlphaSTAR developed an approach that
enable the generation of da/dN curve starting from fracture toughness properties. It also established
a process for reverse engineering of fatigue constituent properties starting from lamina fatigue
properties. Technology is applicable to polymer composites, hybrid material (fiber metal laminate),
and sandwich materials and structures.
Reverse engineering of constituent fiber and matrix fatigue properties
Calculation of da/dN curve using mode I and II fracture toughness properties
Capture damage and fracture evolution process under all types of fatigue loading
Determine how failure occurs and how to fix it.
Reduce number of tests by relying on accurate analysis and simulation methods.
Can be used to guide testing of critical structures under fatigue loading (example, what is
the maximum number of cycles to test without compromising the structure itself?)
Eliminate unnecessary increase in weight by inclusion of effects of scatter in properties and manufacturing
Increased durability and reliability
AlphaSTAR designed critical aerospace and automotive structures for low/high/random/multi-stage
fatigue applications including civilian and military applications. Some references from open
literature are listed below:
| M. Garg, F. Abdi, and G. Abumeri, and M. Heinimann, "Fatigue Life Prediction of Center Cracked CentrAl Stiffened Panel Subject to Spectrum Loading", AIAA-SDM 2011 conference, Denver Colorado.|