Papers by Keyword: Variable Amplitude Loading

Abstract: Loading history makes fatigue crack propagation modelling complex. This article focus on life prediction models which take into consideration the variability of fluctuating loads. In particular it emphases on the comparative studies of prediction models involving the significance of one model’s over another. The paper studies models based on multifarious loadings (constant amplitude load, variable amplitude load, overload/underload etc.). The major parameters of load interaction modelling are plasticity, crack closure, effective stress intensity, effective stress ratio and damage accumulation. For large deformation, elasto-plastic fracture mechanics based models are also included. The complexity of models, their features and focusing on their limitation and strengths are stated with various conditions and also validation of models with experimental data are reported. The paper speculates on the directions the study of crack propagation will take in future.

Abstract: An experimental device was constructed with the aim of testing various cylindrical V-notched specimens until fracture and under variable amplitude torsional loads. The specimens had different notch depths resulting in the same number of values for the stress concentration factor. Strain gages directly bonded at the specimens’ surface and using a slip ring system for their communication with the conditioner, allowed the measurement of the actual applied loads. The well-known rain flow cycle counting procedure was then applied on the scaled signal for identifying the frequency of the 64 classes of stress amplitudes and means. The traditional nominal stress-based approach was then evaluated as the most widely used tool for fatigue lifetime calculations. As the occurrence of stress amplitudes above the endurance fatigue limit tends to lower it, the Miner elementary method was used. The results show damage sum ranges between 0.5 and 6.4 with a mean value of 2.0. Despite the small size of the sample used in the present paper (only 13 tests), these significant deviations are in agreement with previous results reported by different researchers.

Abstract: Generally, mechanical components or structures are subjected to random and a three-dimensional stress state; there are very few field loading paths which can be experimentally fully simulated in laboratory. Loading path parameters such as load sequence, stress level or proportionality/non-proportionality presences are unknown variables with unknown levels under random loading conditions which are impossible to modulate in laboratory because the load spectra is unknown. The load spectrum depends on numerous factors such as environmental, mechanical or user behavior. At design stages the fatigue life estimation is based on typical loading paths or typical loading spectra, however that assumption may be very different from the usage regime. From here it can be concluded that the random multiaxial fatigue issue is of utmost importance to monitoring the in-field damage accumulation. This work presents a proposal to estimate the accumulated damage resulted from multiaxial random loadings based on the SSF equivalent stress and SSF virtual cycle counting concept.

Abstract: There are currently serious doubts about the accuracy of Miners rule for designing welded structures subjected to variable amplitude loading, particularly under spectra cycling down from a constant maximum stress. The deficiency has been attributed primarily to stress interaction effects that cause crack growth acceleration. In this paper the crack growth response of a structural steel and an aluminium alloy to a loading spectrum designed to promote fatigue crack acceleration is studied and the potential mechanisms responsible are evaluated.

Abstract: Due to its specific mechanical properties, tantalum is often used in strength-demanding military applications. High-cycle fatigue (HCF) behaviour of pure tantalum, however, has been rarely reported and the mechanisms at stake to account for deformation under cyclic loadings are still badly understood. This paper aims at better understanding the fatigue behaviour of tantalum and at clarifying the mechanisms of damage formation encountered under such loadings. HCF experiments performed at room temperature on commercially-pure tantalum are presented. Mean stress effects were investigated in the aim of clarifying the interaction between fatigue and creep. Fracture mechanisms were observed to vary from intergranular to transgranular depending on applied stress amplitude and mean stress. Damage mechanisms were investigated under tension and torsion. Results are analyzed in the light of existing fatigue criteria, the limitations of which are discussed. Finally, complex sequential loadings, representative of in-service loadings, were applied to tantalum smooth specimens. The contribution of each loading sequence to the overall damage was quantified and analyzed in terms of linear or non-linear cumulative damage rule.

Abstract: Aluminium alloys are used for a large variety of safety relevant applications for example in the automotive and aviation industries. With the introduction of high strength but possibly less corrosion resistant alloys it is essential to determine if an enhanced sensitisation against corrosion comes into effect under simultaneous mechanical and corrosive loading. Within this work corrosion fatigue tests under constant and variable amplitude loading were carried out on aluminium alloys established for chassis applications such as EN AW-5018 with slightly elevated magnesium content (AlMg3.5Mn), EN AW-6082-T6, EN AW-6110A-T6 and EN AC‑42100-T6 as well as alloys sensitised to corrosion. Sensitisation was obtained by a borderline (17 h at 130 °C) and an excessive (500 h at 130 °C) thermal ageing treatment and elevated copper contents for the forged and cast alloys. Aforementioned alloys and material conditions were assessed concerning the impact of mechanical loading conditions such as load signal type (sinusoidal and square-wave signal), strain rate and load spectra on the damaging process and on corrosion fatigue life. Fatigue tests were complemented by simultaneous determination of electrochemical characteristics as well as the type of corrosion by metallographic investigations.

Abstract: The main focus of this investigation is to clarify the influence of variable amplitude loadings on subsurface crack initiation and crack growth. Therefore, differently reconstructed load sequences on the basis of a standardized load time history called FELIX are investigated with an R-ratio of -1. The major amount of cycles is situated beneath the fatigue strength. A new damage calculation approach considering inclusion sizes is presented. Thus, the stress amplitude in the S-N curve was normalized with a calculated fatigue limit σ_w(area), which is defined by Murakami. Afterward, the fatigue life depending on the inclusion size is calculated using a Palmgren/Miner rule. The largest inclusion in the measurement volume was determined using extreme value statistics. Fatigue lives for each investigated load sequence were calculated taking the scatter of inclusion sizes into account.

Abstract: Fastener holes have a high stress concentration at the edge of the hole and are primary sources of fatigue crack initiation, resulting in widespread fatigue damage leading to fatigue failures in airframe structures. The split-sleeve cold expansion (SsCx) technology is a simple and cost-effective way to improve the fatigue resistance of fastener holes by the introduction of compressive residual stresses around the holes. An investigation was carried out by DSTO to quantify the effectiveness of this technology, in terms of fatigue life improvement factors on a typical airframe aluminium alloy. Open hole (zero load transfer) coupons were tested to failure in non-cold expanded and cold expanded conditions. Coupons were also pre-cracked to specified crack lengths at the open hole, and cold expanded or left non-cold expanded, and tested to failure. This paper will present the results of the initial phases of the experimental program, involving constant amplitude fatigue loading of open hole coupons with and without cracks. The fatigue life improvement achieved by the use of hole cold expansion technology will be presented.

Abstract: This paper investigates the growth behavior of fatigue cracks initiated at corrosion pits in laboratory coupons of LC9 aluminum alloy subjected to a transport aircraft loading spectrum. Corrosion pits were introduced by exposing the coupons to EXCO solution for a variety of periods to produce corrosion damage varying from mild to severe. In general, the presence of corrosion damage reduced the fatigue lives of components to a severe extent. It was found that the depth of the corrosion pit was a suitable parameter for characterizing the corrosion damage and for predicting the fatigue life of the coupons using commercial fatigue crack growth software

Abstract: Fatigue is the most common failure mechanisms in structures of construction machinery with inherent defects under variable amplitude loading, which have to endure for a long term cycle loadings in the actual service. In the present investigation an exponential model is proposed for fatigue life prediction with retardation caused by a single spike under constant amplitude loading. Comparisons between the predicted life and the experimental data are provided to demonstrate the utility and robustness of the proposed model.