# Remedial Modelling of Steel Bridges through Application of Analytical Hierarchy Process (AHP)
## 2 Deterioration Mechanism
### Fatigue  
In steel bridges, fatigue is the most common deterioration mechanism, reported by the American Society of Civil Engineers (ASCE), contributes to approximately 90% of all metallic failures, and is often a limiting factor in the load carrying capacity of steel bridges [9].  
The cost involved in maintaining or repairing fatigue related deterioration, can in some instances exceed the initial investment. Similarly, imposed load restrictions due to the presence of fatigue cracks, can result in major disruptions for commuters [10].  
Fatigue is a form of fracture failure that results from crack initiation and propagation to final failure of metallic elements subjected to cyclic loading [9]. Failure may occur at stress levels below that of the materials tensile yield stress and is dependent on the application of external loads, geometric features, material properties, and the presence of stress concentrations and/or aggressive environments [9].  
Figure 1: The Proposed Decision Model for Remediation Planning.  
Mechanical failure occurs in the presence of stress concentrations that permit fatigue crack propagation under cyclic loading [11]. Common sources of stress concentration include: surface anomalies (roughness, inhomogeneous microstructure, and dents); geometrical variations (sharp edges, notches and threads); and localised forms of corrosion [12].  
Crack initiation can be explained with reference to the slip band mechanism at a microscopic level. Upon the application of a cyclic load, displacement of microscopic grains due to sliding crystallographic planes will cause plastic deformation [10]. Dependent on the metallic properties, the exposed slip step may immediately be covered in an oxide layer. Similarly, strain hardening will develop from an increase in the applied load, thus, preventing a reversal of this effect, and introducing microscopic intrusions and extrusions that promote fatigue crack propagation [10].  
The transition from crack initiation to propagation refers to growth that is no longer dependent on surface conditions, rather the materials ability to prevent further crack growth [13]. Crack propagation will generally proceed after each cyclic load, in a direction perpendicular to the applied stress. This growth can be represented by benchmarks and striations on the fracture surface that radiate outwards from the point of crack initiation in a concentric pattern [12].  
Strations correspond to the growth of fatigue cracks during each cyclic load, and will vary in width dependent on the magnitude of applied stress, whilst benchmarks are indicative of interruptions caused by changes in the applied stress, temperature variations, or the presence of corrosive mechanisms during the propagation stage [10].  
Strations (microscopic) and benchmarks (macroscopic) on the fracture surface provide reliable evidence of fatigue failure. The propagation of fatigue cracks will continue until a critical size is reached [10; 12].  
Fatigue failure will generally occur suddenly and without prior warning in a brittle like manner, once propagation reaches a critical size [9].