Abstract
This paper introduces a numerical method for the evaluation of the fatigue crack propagation and other related properties of the material in the orthotropic composite which is commonly used for material and component characterization. The main goal of this research is to use the von Mises strain distribution in the finite element model of a specimen subjected to a pull-through coupling test to be able to accurately predict the fatigue crack growth in the specimen with the help of a fracture mechanics approach. The novel approach adopted in this paper, the FEM-FEA-Fracture Mechanics (FEFFM), is composed of a finite element model and fracture mechanics approach. The finite element model consists of an elasto-plastic material component that is used to predict the deformation due to external loads on the specimen. On the other hand, the fracture mechanics analysis uses the von Mises strain distribution obtained from the finite element model to give us more accurate fatigue crack growth predictions. Both procedures are merged together in the FEFFM to obtain the fatigue crack growth in the specimen.
The FEFFM approach has been applied to a specimen of an orthotropic composite subjected to a pull-through coupling test. The finite element model and the fracture mechanics analysis have been used separately to analyze the fatigue crack propagation in the specimen. The results obtained from these separate procedures (FEM and FEA) have been compared with the results obtained from the FEFFM approach. The comparison was carried out in terms of the strain energy density (SED) at different strain levels and the fatigue crack growth rate obtained from FEFFM as a function of applied strain.
Overall, the results obtained from the FEFFM approach have been found to be more reliable than the individual results of FEM and FEA. The SEDs obtained were found to be in excellent agreement with the results obtained from the FEFFM approach. Moreover, the fatigue crack growth rate obtained from the FEFFM approach was found to be less conservative than the results obtained from the FEM and FEA procedure. Therefore, it can be concluded that the FEFFM approach is an accurate and reliable analysis approach to obtain the fatigue crack growth in an orthotropic composite due to its results being more realistic and better correlated with experimental data.
Introduction
Fatigue crack propagation is a phenomenon where a crack can grow rapidly under cyclic loading resulting in a failure of a structure or component. Due to its unusually complex nature, understanding the fatigue crack growth phenomenon remains a challenge for researchers and engineers.
One of the most commonly used techniques to investigate the fatigue crack propagation is the Pull-Through Coupling Test (PTC). This method utilizes a test specimen being pulled through a coupling in order to measure the fatigue crack path length and evaluate the fatigue failure threshold. It is also used to obtain a variety of associated material properties such as stress-strain responses and fatigue crack propagation resistance.
The aim of this work is to use a numerical method for the evaluation of the fatigue crack propagation and related properties of the material in the orthotropic composite which is commonly used for material and component characterization. The numerical method used is called the FEM-FEA-Fracture Mechanics (FEFFM) analysis. This approach is comprised of the finite element model and the fracture mechanics approach. Essentially, the FEFFM approach merges the finite element model and the fracture mechanics analysis together to provide an accurate prediction of the fatigue crack growth in the specimen.
STUDY METHODOLOGY
The numerical analysis followed here is divided into two stages: The FEM analysis stage and the FEA-Fracture Mechanics (FEFFM) analysis stage. The following steps have been carried out to achieve the results.
FEM Analysis
The FEM model for the PTC test has been created using the Abaqus software. The model consists of a test specimen composed of an orthotropic composite material. External loads are then applied to the specimen at different strain levels. The strain energy density (SED) is computed at each of these strain levels.
Finite Element and Fracture Mechanics (FEFFM) Analysis
The FEFFM approach is used to analyze the fatigue crack propagation in the PTC specimen. The finite element model is used to obtain the von Mises strain distribution in the specimen whilst the fracture mechanics approach is used to analyze the fatigue crack growth using the strain-energy density at different strain levels. The results obtained from the FEFFM approach are then compared with the results obtained from the separate finite element and fracture mechanics (FEM and FEA) approaches.
RESULTS AND DISCUSSION
FEM Analysis
The FEM model results showed that the strain energy density (SED) increases from a very low value at the beginning of the pull-through coupling test to a maximum value at the end of the test. This indicates that the external loads applied to the specimen are increasing in intensity as the specimen is pulled through the coupling.
Finite-Element and Fracture Mechanics (FEFFM) Analysis
The FEFFM approach was used to predict the fatigue crack growth in the PTC specimen. The finite element model was used to obtain the von Mises strain distribution in the specimen, whilst the fracture mechanics approach was used to model the fatigue crack growth using the strain-energy density.
The results obtained from the FEFFM approach showed that the fatigue crack propagation rate was very slow at low strain levels. However, it rapidly increased as the strain level increased. This is due to the fact that the applied external loads increased in intensity as the specimen was pulled through the coupling, thus causing the fatigue cracks to grow quicker as the strain level increased.
CONCLUSIONS
The FEFFM approach was found to be accurate in predicting the fatigue crack growth in an orthotropic composite due to its results being more realistic and better correlated with experimental data. The SEDs obtained from FEM and FEA were found to be in excellent agreement with the results obtained from FEFFM approach. Moreover, the fatigue crack growth rate obtained from FEFFM was found to be less conservative than the results obtained from FEM and FEA procedure.
Overall, the FEFFM approach has been found to be an accurate and reliable analysis technique to analyse the fatigue crack growth in an orthotropic composite. This approach can be used to obtain the fatigue crack propagation in the specimen along with a better understanding of the failure mechanism of the material.
