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Fracture mechanics-based methods can be applied to the fatigue assessment of mechanical components containing defects or cracks subjected to cyclic loading. The knowledge of the crack growth behavior for short and long cracks enables the determination of both finite and infinite life regimes. Nevertheless, a first problem arises from the empirical determination of the fatigue data, which vary according to the test conditions, e.g., the test method, frequency and the stress ratio R. This is due to crack closure and further environmental phenomena affecting crack propagation behavior. A second issue regards the lack of standard experimental procedures for the determination of the fatigue crack propagation threshold ΔKth dependency on the crack size. This is problematic, since ΔKth increases with the build-up of the closure effects in the short crack regime and its inaccurate experimental determination leads to large deviations in the assessment and, even more critical, to non-conservative predictions. Aiming at providing more reliable and safer assessment procedures, the present work proposes a method based on effective crack growth data obtained at high stress ratios (R ≈ 0.8). A short-crack model based on the cyclic R-curve and CPLR data is employed for describing the development of the closure effects. The predictions are compared to established methods and validated by experimental tests.
The impact of crack closure and environmental effects on the experimental determination of the fatigue crack propagation threshold is a major problem for the assessment of cyclically loaded components, especially at low stress ratios 𝑅. In this work, the influence of the experimental procedure and air humidity on d𝑎∕d𝑁 − 𝛥𝐾 data at different 𝑅 is discussed. Unlike the results at 𝑅 = −1, the threshold values obtained at 𝑅 ≈ 0.8, i.e. under negligible crack closure levels, show a very small scatter band regardless of the variation of the test parameters and environmental conditions.
The correct determination of fatigue crack propagation data is of great importance for the damage tolerance design of engineering components, especially with regard to the calculation of residual lifetime and the establishment of inspection intervals. The fatigue crack propagation threshold Δ𝐾th, in this respect, is a crucial input parameter for simulating crack growth, since it corresponds to the stress intensity factor range at which a non-growing crack starts to propagate. However, the experimental determination of Δ𝐾th, as well as its application, is still confronted with few issues related among others to the load ratio (R) dependency of Δ𝐾th, the testing procedure, and environmental effects. These can lead to large scatter and significant errors in the prediction of component failure. In this context, the use of the intrinsic fatigue crack propagation threshold Δ𝐾th,eff in component assessment is a promising alternative, since it does not depend on a number of factors that affect Δ𝐾th, but only on the elastic properties (𝐸-modulus) and the lattice (Burger’s vector ‖𝑏‖) of the material. The aim of the present work is therefore to investigate different experimental procedures for the determination of Δ𝐾th,eff, namely: (a) conventional load reduction (LR) procedures, (b) the 𝐾max procedure and (c) compression pre-cracking load reduction and constant amplitude (respectively CPLR and CPCA) methods. Furthermore, the determination of Δ𝐾th has been carried out varying some testing parameters, such as test frequency, Δ𝐾 at the beginning of the crack propagation test (Δ𝐾0) and stress ratio (R). The results are statistically analyzed and a discussion about the use of Δ𝐾th and Δ𝐾th,eff for the component fatigue assessment is presented.
The work presents the procedure developed within the German research project IBESS, which allows for the fracture mechanics-based prediction of the fatigue strength of welded joints under constant amplitude loading. Based on the experimental observations of the crucial failure mechanisms, the approach focuses on the short crack propagation, where elastic-plastic fracture mechanics and the build-up of closure effects must be considered as well as the variability of the local geometry at the weld toe and the modelling of multiple crack interaction. Analytical solutions are provided for the approximation of the through-thickness stress profiles at the weld toe and for the determination of the crack driving force in the form of a plasticity-corrected stress intensity factor range ∆K_p. Proposals for the determination of the initial crack size and the crack closure factor are also included.
The approach is validated against a large number of experimental data, which comprises fatigue tests on individual cracks monitored by heat tinting and beach-marking techniques, as well as stress life curves. Three kinds of welded joints, two steels of significant different strengths and three stress ratios are considered. The results show that the procedure provides good estimations of the statistical distribution of the fatigue strength of welded joints both for the finite and infinite life regime. Furthermore, the predictions are compared with available benchmark data for structural steels.
Analytical flaw assessment
(2018)
The paper provides a review on analytical flaw assessment methods with the focus on fracture under monotonic loading and fatigue crack propagation. The first topic comprises linear elastic as well as elastic-plastic fracture mechanics approaches. It essentially follows their historical development. Topics which are separately discussed are reference/Limit loads, the treatment of secondary stresses, strength mismatch, constraint, unstable crack propagation (monotonic R-curve analyses) and statistical aspects. With respect to fatigue crack propagation the analytical treatment of crack closure and constraint and the Determination of the cyclic elastic-plastic crack driving force is discussed. Finally, cyclic Rcurve analyses are briefly addressed.
An analytical fracture mechanics model for predicting the finite life fatigue strength of components is
presented which combines a number of well established and newly developed approaches such as
Murakami's and McEvily's approach for describing the transient behaviour of crack closure of short
cracks, the analytical (long) crack closure function of Newman which became part of the widely used
NASGRO approach, the R6 procedure, a method for improving the ligament yielding correction f(Lr ) of
R6 proposed by the authors of the present paper and other elements. Basic assumption is the preexistence
of initial flaws such that the crack initiation or nucleation stage is small and can be neglected.
The application of the model is demonstrated for small tension plates of aluminium Al 5380 H321 with
artificial initial defects generated by FIB technology, the size of which was fixed on the basis of
fractographic investigations on broken, smooth specimens.
Simulation of fatigue crack growth by crack tip plastic blunting using cohesive zone elements
(2010)
The propagation of a fatigue crack in 2D specimens by crack tip plastic blunting is simulated by using a Finite Element implementation of the strip-yield model. At each cycle, the crack growth is assumed to result from the variation of the Crack Tip Opening Displacement (ΔCTOD). The implementation is based on cohesive elements with an elastic perfectly plastic behavior, which allows to simulate, the crack closure phenomena due to the plastic wake behind the crack tip. A mesh sensitivity analysis provides the minimum number of elements requested in the crack tip plastic zone for a given accuracy. Then, the advance scheme based on crack tip blunting is described. Finally, the variation of the opening load with respect to the load ratio R, and to the maximum load is investigated and compared with analytical results.