Introduction
The overarching motivation of the work reported in this paper is to
improve fatigue life prediction for aerospace components. Traditionally,
fatigue assessment is heavily dependent on testing and statistical
analysis. With the introduction of new materials and methods of
manufacture, such as composites and Additive Manufacturing, the material
property and component geometry variation places significantly greater
demands on the numbers of tests required to obtain satisfactory
statistical confidence in the test results. In the engineering sector,
there is an increasing trend to reduce testing, and thus testing costs,
by relying more on validated computational modelling. In the context of
fatigue assessment, this presents a problem, because fatigue
understanding is largely based on experimental results and empirical
models, such as the NASGRO crack growth equation. Empirical models can
be used in a computational context, but still require compelling data
evidence to validate their applicability.
A more desirable objective is to create a modelling capability that is
based on well-established physical principles, and to link the fatigue
assessment to measurable attributes of the material properties and
component geometry. That is the objective for this present paper, which
is entirely based on computational modelling. Two types of component
geometry feature are modelled: surface roughness and porosity. The
material is modelled as a simple piece-wise linear elastoplastic
material, and the computational specimen geometries are subjected to
cyclic loading, at load levels that would be below the yield threshold
for specimen without roughness or porosity. We assess the results of
these computations, and compare with the empirical and pragmatic methods
of fatigue assessment. Although it is difficult to arrive at a firm
conclusion at this stage, we believe that the computational models
presented here enhance our understanding and could form part of a future
fatigue assessment methodology. The reader might be critical of the
simplifications and approximations we have made, but, as Einstein is
frequently paraphrased as saying, “Everything should be kept as simple
as possible, but not simpler” – a paraphrasing of Occam’s Razor. The
investigation of further model enhancement and complexity will be the
subject of future work.
The structure of this paper is as follows. In the present section, we
provide the context and motivation for this work. In Section 2, we
review existing understanding and the prior work on which the present
work is predicated. In Section 3 we discuss the methodology which
underpins this present work, using probing questions, based on Bacon’s
[1] inductive principles. Section 4 explains the construction of the
computational models used. Section 5 presents qualitative results to
illustrate the interactive effects of surface roughness and subsurface
porosity to create a pattern of stress and equivalent plastic strain
(PEEQ) in the subsurface layer. In Section 6, we analyse this data and
present results in answer to the probing questions set out in Section 3.
Section 7 provides a detailed discussion of the methodology, the
modelling methods used, the results obtained and weaknesses in our
approach that could be addressed in future work, and proposes a
potential route map for an analytical approach to fatigue life
assessment for Additive Manufactured aerospace components. The
conclusions to this paper are set out in Section 8.