Issue 37

V. Anes et alii, Frattura ed Integrità Strutturale, 37 (2016) 124-130; DOI: 10.3221/IGF-ESIS.37.17 124 Focussed on Multiaxial Fatigue and Fracture On the assessment of multiaxial fatigue damage under variable amplitude loading V. Anes, L. Reis, M. de Freitas IDMEC, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal. vitor.anes@tecnico.ulisboa.pt , http://orcid.org/0000-0002-8526-398X luis.g.reis@tecnico.ulisboa.pt , http://orcid.org/0000-0001-9848-9569 manuel.freitas@tecnico.ulisboa.pt, http://orcid.org/0000-0003-3525-9218 A BSTRACT . In this work, the performance of the SSF criterion is evaluated under variable amplitude loading conditions. The main objective was to inspect the validity of the hypothesis in which the SSF damage map remains valid for any high strength steel. In order to achieve that, fatigue life correlation of the 1050QT steel and 304L stainless steel was analyzed under multiaxial loading conditions. The loading block considered in the study comprises 360 proportional loading cycles with different stress amplitude ratios and stress levels. Despite being made of proportional branches, this loading block is a non-proportional loading due to its principal directions variation. This feature allows the evaluation of combined loading effects under variable amplitude loading conditions, which makes this loading block suitable to mimic the loading effects usually found in the field. Results show very good agreements, which reinforces the aforementioned hypothesis. K EYWORDS . Multiaxial fatigue; Variable amplitude loading; Multiaxial cycle counting method; Fatigue life; Damage accumulation. I NTRODUCTION n the field, random loadings are the most common type of load that we can find; typical examples can be found in car suspensions, aircraft wings or wind towers. This type of loadings is complex and difficult to deal with, specially their simulation in the lab. Also, their stochastic behavior increases the variability usually found in fatigue life experiments, which increases the complexity in damage accumulation assessment. They may have several loading effects that are usually studied separately in literature, however their combined effect can be activated simultaneously or sequentially under random loading conditions [1, 2]. Loading effects such as proportionality, non-proportionality, cyclic hardening, cyclic softening, mean stress, sequential and asynchronous loading, among others effects, cause different well known damage mechanisms [3-7], but their combined effect remains more or less unknown. Nowadays, there is a lack of knowledge in this matter and further research is required. This subject becomes extremely important when the assessment of instantaneous damage accumulation is required, such as in structural health monitoring procedures. Currently, the assessment of fatigue damage is not only performed in mechanical design stages, but also in structural integrity evaluation performed in the field. For example, damage accumulation tools can support on-condition maintenance decisions in order to optimize inspections and replacement costs. In the lab, the most common approach to simulate random loadings is to load pre-defined loading blocks in a randomly way. Thus, multiaxial fatigue models must be able to evaluate fatigue damage from variable amplitude loadings such as the ones found in multiaxial loading blocks. In this work, the present authors study the capability of the SSF criterion to capture fatigue damage under variable amplitude loading, which is a I