On the applicability of infrared thermography for investigating the fatigue behaviour under variable amplitude loading on the example of HFMI-treated longitudinal stiffeners

Fatigue crack growth of fibre reinforced metal laminates (FRMLs) under constant and variable amplitude loading was studied through analysis and experiments. The distribution of the bridging stress along the crackline in centre‐cracked tension (CCT) specimen of FRMLs was modelled numerically, and the main factors affecting the bridging stress were identified. A test method for determining the delamination growth rates in a modified double cracked lap shear (DCLS) specimen was presented. Two models, one being fatigue‐mechanism‐based and the other phenomenological, were developed for predicting the fatigue life under constant amplitude loading. The fatigue behaviour, including crack growth and delamination growth, of glass fibre reinforced aluminium laminates (GLARE) under constant amplitude loading following a single overload was investigated experimentally, and the mechanisms for the effect of a single overload on the crack growth rates and the delamination growth rates were identified. An equivalent closure model for predicting crack‐growth in FRMLs under variable amplitude loading and spectrum loading was presented. All the models presented in this paper were verified by applying to GLARE under constant amplitude loading and Mini‐transport aircraft wing structures (TWIST) load sequence. The predicted crack growth rates are in good agreement with test results.

Influence of edge cutting process on fatigue behaviour of spectrum loaded aluminum sheets

The initiation and growth behavior of transverse crack in rail steel was experimentally investigated using the notched keyhole specimen under constant amplitude and variable amplitude loads. Fatigue limits of smooth and keyhole specimen in rail steel at R=0 were about 198 and 59 MPa, respectively. The fatigue crack initiation length at notch root was about 1.3 mm. The fatigue lives under variable amplitude load predicted by introducing the modified equivalent stress (σ eq ') agreed closer to the experimental results than those predicted by using the conventional equivalent stress (σ eq ). As the characteristic stress intensity factor range (ΔK rms ) increases, the fatigue crack propagation rate (da/dN) under variable amplitude load derived from the actual load history is faster than that under constant amplitude load. Through the examination of fracture surfaces in each load condition, it was found that this behavior results from the transition of fracture appearance.

A progress update for the pressure controlled atomization process Tierney, J.C. and Glovan, R.J. Proc. Conf. 1994 Thermal Spray Industrial Applications, Boston, MA, USA, 20?24 June 1994, pp. 257?268

An experimental study of the effect of a flaw at a notch root on the fatigue life of cast Al 319

A nonlinear fatigue damage accumulation model under variable amplitude loading considering the loading sequence effect

Lightweight design with welded high-frequency mechanical impact (HFMI) treated high-strength steel joints from S700 under constant and variable amplitude loadings

Fatigue strength of welded and high frequency mechanical impact (HFMI) post-treated steel joints under constant and variable amplitude loading

PurposeWelded components are often subjected to variable amplitude service loads, increasing the uncertainty of fatigue life due to material strength, notch geometries, defect content and residual stresses. In the case of friction stir welding (FSW) of aluminium alloys no data were found available concerning fatigue behaviour under variable amplitude loading. The purpose of this paper is to determine the fatigue strength of friction stir welds in AA6082‐T6 under constant and variable amplitude loading and analyse the validity of Miner's rule for these specific welding conditions.Design/methodology/approachFatigue tests were carried out in a servo‐hydraulic testing machine using a stress ratio of R=0. Typified Gassner amplitude spectra were considered, using four shape exponent values. Microhardness tests were performed to characterize the Vickers hardness profile in the vicinity of the weld area. Relatively to the base material (BM), the FSW process leads to a decrease of the static mechanical properties.FindingsDetailed examination revealed a hardness decrease in the thermo‐mechanically affected zone and the nugget zone average hardness was found to be lower than the base alloy hardness. The comparison with data collected from the literature shows that FSW specimens present higher fatigue resistance than specimens welded by metal inert gas and tungsten inert gas processes. However, they still have lower fatigue lives than the BM. Using the equivalent stress calculated by Miner's rule, a good agreement was observed between constant and variable fatigue loading results. The characteristic curve obtained for friction stir welds is higher than the International Institute of Welding (IIW) fatigue class for fusion welds with full‐penetration both‐sided butt joints.Originality/valueNo data are available concerning fatigue behaviour under variable amplitude loading for friction stir welds of aluminium alloys. Furthermore, this paper analyses the fatigue strength of friction stir welds in AA6082‐T6 under constant and variable amplitude loading in order to verify the validity of Miner's rule for this specific welding process. A comparison between characteristic fatigue curves, using IIW fatigue classes (FAT), is also performed.

A strain‐controlled low‐cycle nonproportional multiaxial loading test, including both constant amplitude and variable amplitude loading, was conducted on AISI 304 stainless steel to investigate the load sequence effect on fatigue behavior. Under constant amplitude loading, the cyclic stress response exhibited initial hardening, with additional hardening induced by nonproportional loading. Under variable amplitude loading, the load sequence effect did not alter the hardening behavior but had contrasting effects on fatigue life depending on the loading characteristics. In a high‐low loading, the stable stress amplitude during the low‐strain stage increased significantly, leading to a reduction in fatigue life. In contrast, in a low‐high loading, both the stress response and fatigue life remained unchanged. The load sequence effect under different loading conditions was analyzed in detail based on variations in stress amplitude and nonlinear damage accumulation. The fatigue life evaluation capabilities of various nonlinear damage accumulation models were compared with the linear damage model.

High cycle fatigue behavior of 316L stainless steel

Lightweight Potential of Welded High-strength Steel Joints from S700 Under Constant and Variable Amplitude Loading by High-frequency Mechanical Impact (HFMI) Treatment

In this study, fatigue behavior of polypropylene fiber reinforced concrete has been studied under constant and variable amplitude loading. Crack length was measured during flexural fatigue test under three loads. Accordingly, damage curves were determined as function of stress levels. The results ascertained that the presence of 1 wt% polypropylene fibers considerably increases resistance against fatigue crack growth under constant amplitude loading. Under variable amplitude loading, it was found that the damage curve approach predicts the fatigue life with 13–15.8% estimation error, whereas linear model predicts the fatigue life with 36.8–56.5% estimation error.

Effects of natural aging and variable loading on very high cycle fatigue behavior of a bearing steel GCr15

The constant/variable amplitude loading fatigue test with interior inclusion‐fine granular area‐fisheye induced failure under R=0 were carried out on carburized Cr−Ni steel. The results showed that the fatigue life under variable amplitude loading is longer than that under constant amplitude loading in very‐high‐cycle fatigue regime under same maximum stress level, and the surface morphology of fine granular area under variable amplitude loading is coarser than that under constant amplitude loading under same order of magnitude of fatigue life. Simultaneously, it can be determined that the formation micro‐mechanism of fine granular area is caused by the continuous deboning due to stress concentration around interior micro‐defects. Furthermore, the life prediction model based on double nonlinear fatigue damage, which considers the coupling effect of local equivalent stress (surface residual stress and local stress concentration effect), loading sequence, failure mechanism and nonlinear characteristics of fatigue damage under constant/variable amplitude loading is established, and predicted life has good accuracy within the factor‐of‐three lines.