Issue 45

D. Peng et alii, Frattura ed Integrità Strutturale, 45 (2018) 33-44; DOI: 10.3221/IGF-ESIS.45.03 35 strength of the remaining ligament occurs. The advantage of this approach is that it negates the need to explicitly model cracks, see [8, 14, 17]. A crack of any size can be analyzed using the original (un-cracked) finite element model. As cracks are not modelled explicitly, a coarser mesh can be used to minimize the number of degrees of freedom, thereby reducing the analysis time. Solutions for the stress-intensity factors can then be obtained for a variety of cracks using the original finite element analysis quickly and easily. To illustrate how this approach can be used to compute the growth of cracks that arise due to natural corrosion in bridge steels a simplified analysis of V/Line Bridge 62 in Kilmore East, Victoria, Australia is performed. By comparing the life obtained by i) allowing only for corrosion and ii) by performing a coupled corrosion-fatigue analysis we find that method i) is very un-conservative. We also show that the interaction between fatigue crack growth and the stress increase created by corrosion induced section reduction needs to be considered when assessing the remaining life of an aged steel bridge. Having shown how to predict the effect of surface corrosion on the fatigue life of mild steel attention is then focused on the effect of surface roughness on fatigue cracking in Additively manufactured T-6Al-4V. Whilst additive manufacturing (AM) offers the potential to economically fabricate customized parts with complex geometries, the mechanical behavior of these materials must be better understood before AM can be utilized for critical load bearing applications. This is particularly true for aircraft applications where, as detailed in MIL-STD 1530, the design and certification approval require analytical tools that are capable of capturing crack growth and the role of testing is to validate or correct the damage tolerance analysis. To this end it is first shown that the growth of small cracks in a 350 MPa mild steel is similar to the growth of fatigue cracks in both conventionally manufactured Ti-6Al-4V and in in Additively Manufactured LENS (Laser engineered net surface) Ti-6Al-4V. This suggests that the methodology discussed above may also be applicable to study the effect of surface roughness in Additively Manufactured Ti-6Al-4V. T HE AASHTO CORROSION STANDARD efore we can assess the coupled effect of corrosion and fatigue we first need a knowledge of the rate of corrosion. In this paper we will adopt the American Association of State Highway and Transportation Officials (AASHTO) recommended metal loss model [22, 23] which states that the metal loss versus time curve is bi-linear, see Fig. 1. However, as can be seen in Fig. 1, there is little actual data to support this model and the data shown in Fig. 1 is not particularly convincing. This approach to assessing the “steady state” corrosion rate is consistent with the International Standard Corrosion of Metals and Alloys - Corrosivity of Atmospheres, ISO designation 9224 [24], which specifies guiding values of corrosion rate for metals exposed to the atmosphere consisting of an average corrosion rate during the first 10 years of exposure. A detailed review of the corrosion of bridge steels, the AASHTO and ISO corrosion standards and documented steady state corrosion rates associated with a range of locations and steels is in given in [25]. Figure 1: An example of the bi-linear metal loss versus time curve, from [22]. One problem with aging bridges is that if there is any serious corrosion it is likely to have developed over a reasonable number of years. However, to know its significance we need to know how fast the bridge is corroding, i.e. its corrosion rate, at this moment in time. That said you do not have the luxury to locate corrosion sensors or weight loss samples on a B