Untitled

Funder

Organizational Unit

Publications

Fatigue damage assessment under random and variable amplitude multiaxial loading conditions in structural steels

Publication . Anes, Vitor; Caxias, J.; Freitas, M.; Reis, L.

Fatigue damage assessment of multiaxial random loadings is a complex issue and a subject of actual interest in mechanical design. In this work, the performance of the stress scale factor (SSF) criterion is evaluated under variable amplitude loading conditions, and damage accumulation approaches. This evaluation is performed by taking into account two types of loading spectra, namely the loading block spectra (where the loading pattern is well identified and repeated until rupture), and the random loading spectra (where the stochastic behaviour of the axial and shear loading components do not allow a direct identification of the loading pattern). Moreover, the validity of the hypothesis in which the SSF damage map remains valid for any high strength steel under variable amplitude loading conditions is also inspected by analysing fatigue life correlation of the 1050 QT steel and the 304L stainless steel under a multiaxial loading block.

2017-07Journal article

Restricted access

A new risk prioritization model for failure mode and effects analysis

Publication . Anes, Vitor; Henriques, Elsa; de Freitas, M.; Reis, Luis

Failure modes and effects analysis is a framework that has been widely used to improve reliability by prioritizing failures modes using the so‐called risk priority number. However, the risk priority number has some problems frequently pointed out in literature, namely its non‐injectivity, non‐surjectivity, and the impossibility to give weights to risk variables. Despite these disadvantages, the risk priority number continues to be widely used due to its higher simplicity when compared with other alternatives found in literature. In this paper, we propose a novel risk prioritization model to overcome the major drawbacks of the risk priority number. The model contains 2 functions, the risk isosurface function that prioritizes 3 risk variables considering their order of importance in a given risk scenario, and the risk prioritization index function which prioritizes 3 risk variables considering their weights. The novelty of the proposed model is its injectivity, surjectivity, and ease of use in failure modes prioritization. The performance of the proposed model was analyzed using some examples typically used to discuss the conventional risk priority number shortcomings. The model was applied to a case study and its performance correlated with other risk prioritization models. Results show that the failure modes prioritization reached with the proposed model agrees with the expectations made for the risk scenario.

2018-06Journal article

Restricted access

The damage scale concept and the critical plane approach

Publication . Anes, Vitor; de Freitas, M.; Reis, L.

Critical plane criteria seek the plane in which their damage parameter has its maximum value. On this plane, the fatigue damage of both normal and shear stresses is updated to the same damage scale by a constant, typically based on material normal/shear fatigue limits. This paper focuses on the damage scale concept under a stress-based critical plane standpoint, and the main objective is to evaluate its performance. To perform this study, data from the high strength steel 42CrMo4 were considered. Five proportional loading paths with different stress amplitude ratios and stress levels were analysed using 2 different multiaxial fatigue criteria, namely, the Findley critical plane model and the stress scale factor equivalent shear stress. Results show that the damage scale between normal and shear stress components varies accordingly to the loading plane direction considered.

2017-08Conference object

Restricted access

Multi-variable prioritization model for risk evaluation scenarios in mechanical design

Publication . Anes, Vitor; Henriques, Elsa; Freitas, Sofia de; Reis, Luís

Risk prioritization is an important procedure in FMEA methodology where failure modes are prioritized using the so-called risk priority number (RPN), which prioritizes failure modes based in only three risk variables usually, severity, occurrence, and detectability. However, in some risk scenarios, more than three risk variables can improve risk prioritization, but an ease risk prioritization model like RPN for more than three risk variables cannot be found in literature. We developed a risk prioritization model to prioritize failure modes evaluated with more than three risk variables. The model has the same ease of use found in the traditional risk prioritization number model, but it overcomes its drawbacks. Results show that the risk prioritization made with the proposed model agrees with the expectations made for the risk scenario. Moreover, it was shown that the proposed model is injective, surjective and can prioritize failure modes based on a given order of importance defined by the user. These features show that the major shortcoming found in the conventional risk prioritization were overcome on the proposed model. Our model opens the possibility of traditional FMEA users to use more than three risk variables without the need of prior knowledge, fact that has been the main reason of traditional risk priority number popularity, despite its strong shortcomings.

2018-07Conference object

Restricted access

Bonded joints of dissimilar adherends at very low temperatures - An adhesive selection approach

Publication . Anes, Vitor; Pedro, R.; Henriques, E.; Freitas, M.; Reis, L.

Maintenance, repair and overhaul companies have been reporting corrosion failure events in the Airbus A320 CFM56-5b intakes. These intakes are attached to the power plant frame by a dissimilar material bonded joint, where liquid shim adhesive is used to avoid the dielectric formation between dissimilar materials. In previous works, the authors reported that the A320 intakes corrosion is a result of the adhesive inability to avoid the dielectric formation, which is a result of micro-cracks formation within the adhesive layer. The main reason that lead to these cracks is the adhesive aging and thermal cycling at very low temperatures, which quite often reach values lower than -50ºC. This paper studies the effect of negative thermal loading on dissimilar materials bonded joints. Two epoxy adhesives are studied and compared, namely the Hysol EA-934, which is the adhesive currently used in the A320 Airbus intakes, and the Hysol EA-9394, a second generation adhesive candidate to replace the actual adhesive. A numerical study was performed in order to simulate the adhesive joint using a finite element analysis commercial package, where several hypotheses were explored. These hypotheses where focused on the effects of several factors on the adhesive layer stress distribution. Factors such as temperature range, boundary conditions, variation of the coefficient of thermal expansion with temperature, and interfacial cracks between the adhesive layer and dissimilar adherend materials were analyzed. Results show that very low temperatures have a negative impact on the adhesives strength and micro-cracks formation may result from thermal loads below zero degrees Celsius, even for adhesives without any aging. Moreover, low temperatures in dissimilar materials bonded joints may create stress states that surpass the adhesive lap shear strength. Some conclusions are drawn regarding adhesive selection for dissimilar materials bonded joints in order to overcome these issues.

2016-10Conference object

Restricted access