Analysis of cold expanded fastener holes subjected to short time creep: Finite element modelling and fatigue tests

The beneficial effects of cold expansion have been well documented in previous studies, yet the performance of cold expanded plates exposed to elevated temperatures is an area of technical interest. In this research, finite element (FE) simulations along with experimental fatigue tests have been carried out to investigate the effect of exposure to elevated temperature on residual stress distribution and subsequent fatigue life of cold expanded fastener holes. According to the obtained results, creep stress relaxation occurs due to exposure to 120 °C for 50 h. FE results demonstrate a non-uniform residual stress relaxation regime through the plate thickness around the cold expanded hole and the fatigue test results show that the subsequent fatigue lives have significantly decreased.     

An FE analysis for assessing the effect of short-term exposure to elevated temperature on residual stresses around cold expanded fastener holes in aluminum alloy 7075-T6

A previous experimental study revealed fatigue life reduction in Al 7075-T6 cold expanded fastener holes exposed to 120 °C for 1 h. The obtained experimental evidence indicated a residual stress reduction associated with material softening at elevated temperatures, termed as thermo-mechanical stress relaxation. In order to identify and characterize the potential features of this phenomenon, FE analysis is carried out in this study and a detailed body of evidence is provided for occurrence of a time-independent thermo-mechanical residual stress relaxation around cold expanded fastener holes due to exposure to elevated temperature. The results of FE simulation demonstrate a good agreement with experimental results obtained earlier.     

Effect of defects on fatigue life of plates with fastener holes

The behavior of defects (inclusions and cavities) on fatigue life of plates with fastener holes under tensile loading has been analyzed. Special attentions have been paid on the influence of the size and location of defects on fatigue life of plates with fastener holes. Thirty‐five different finite element models of plates with different size and location of defects are established and the nominal stress method is used to estimate the fatigue lives of this models. The results show that there is a region whose center is the maximum stress point of the hole without defects. When the defect is located in this region, the influence of defects on fatigue life of plates with fastener holes is obvious. When the defects are far away form this region' center, the defects hardly influence the fatigue life of plates with fastener hole. The larger the size of the defect is, the bigger this region is. In this region, the larger the size of the defect and the shorter the distance between the defect and the region's center, the shorter the fatigue life of plates with fastener hole is.     

An experimental investigation on fatigue crack growth of AL6XN stainless steel

The crack growth behavior of AL6XN stainless steel was experimentally investigated using round compact tension (CT) specimens. The influences of the R-ratio (the ratio of the minimum load over the maximum applied load in a cycle), the tensile and compressive overloads, and the loading sequence on crack growth were studied in detail. The results from the constant-amplitude experiments show a sensitivity of the crack growth rate to the R-ratio. The application of a tensile overload has a profound effect on crack growth, resulting in a significant retardation in the crack propagation rate. A compressive overload (underload) leads to a short-lived acceleration in crack growth. Results from the two-step high-low loading reveal a period of crack growth retardation at the beginning of the lower amplitude step, an effect similar to that of a single overload. A crack driving force parameter together with a modified Wheeler model is found to correlate the crack growth experiments well.     

Fatigue life of cold‐expanded fastener holes with interference‐fit fasteners at short edge margins

The fatigue life of 7075‐T6 aluminium specimens with countersunk fastener holes with cold expansion and interference‐fit fasteners with short edge margins was studied. The study was performed experimentally and through finite element analysis. The experiments measured the total fatigue life and crack growth. The results from the finite element analysis consisted of tangential residual stress profiles, which were combined with applied cyclic stresses for fatigue analysis. The experiments showed that the fatigue life improved with interference‐fit fasteners and cold expansion at all edge margins. The fatigue life also increased with increasing edge margin. The finite element results were used to make fatigue life predictions that corresponded reasonably well with the experimental results.     

On the need to decompose fatigue strain signals associated to fatigue life assessment of the AISI 1045 carbon steel

The evaluation of a new statistical-based analysis is discussed in this paper, leading to the fatigue life assessment at three different applied stresses during the cyclic testing procedure. Fatigue tests were performed following the standard ASTM: E466-07. These tests involve a strain gauge being attached to the specimen. For this test, AISI 1045 carbon steel was used as material due to its wide application in automotive and machinery industries. Fatigue tests were performed at three different stress levels of 305 MPa, 325 MPa, and 345 MPa with the testing frequency of 8 Hz, and the strain signals were collected accordingly. The Integrated Kurtosis-based Algorithm for Z-filter (I-kaz) approach, which provides its coefficient and three dimensional graphics, was utilised to define a statistically-based fatigue behaviour pattern. The I-kaz technique was used to extract strain signal patterns at the respective low, medium, and high frequency ranges for each signal at specific applied testing stress level. Results showed that highest strain amplitude occur at the low frequency range, suggesting the capability of I-kaz to identify fatigue damage pattern of metallic materials using statistical representation.     

The effect of hold-time on fatigue crack growth behaviors of WASPALOY alloy at elevated temperature

The effect of hold-time on fatigue crack growth behaviors of WASPALOY alloy was investigated. It was found that the role of hold-time depends on the competition between the harmful environmental effect and the beneficial effect of creep. If temperature is not higher than 705 °C, fatigue crack growth rate of WASPALOY alloy increases with hold-time. On the contrary, hold-time plays a beneficial role on steady state fatigue crack growth of WASPALOY alloy at 760 °C and lower stress intensity factor. The beneficial effect of hold-time was attributed to the creep caused stress relaxation during the hold-time. However, accumulated creep damages cause to cavity nucleation and growth at the grain boundaries, and then accelerate fatigue crack growth. Hold-time plays a harmful role during the final stage of fatigue crack growth.     

Analytical and experimental investigation of fatigue crack propagation for polyethylene methacrylate

The fatigue crack growth of most polymers is sensitive to temperature. In this paper, tests on fatigue crack growth of polyethylene methacrylate were carried out and the fatigue crack growth rate was obtained at temperature range −50 to 90 °C and frequency 1 Hz. The fatigue crack propagation (FCP) properties of polyethylene methacrylate and metals were studied comparatively and a new modified formula for FCP rate was deduced to describe the polyethylene methacrylate FCP rates. The formula includes four parameters: the FCP threshold, Young's modulus, fracture toughness and stress ratio. The predicted curve based on this modified formula corresponds very well with the test data of polyethylene methacrylate at different temperatures. Therefore, the modified formula can be used to describe the FCP process.     

The effect of hole shape on the extent of fatigue life improvement by cold expansions

The cold expansion of circular holes is known to improve resistance to fatigue. In this study the effect of the cold expansion of a circular hole on fatigue life by means of a quasi-elliptical pin was investigated. Additional evaluations were conducted, including determinations of the effects of crack propagation from the hole. The major life extension was obtained through slower crack growth in the short-crack stage. The decrease in fatigue crack growth in cold-expanded specimens was related to higher crack-opening stresses which are a consequence of the presence of compressive residual stresses arising from cold expansion. In this study, an experimental investigation was carried out to quantify the effect of the cold expansion on the initiation and the propagation of the fatigue crack and was discussed. Fatigue life improvement of the cold-worked hole specimen was explained by determining the hardness results around the cold-worked hole. The results indicate that significant life improvements can be obtained through cold expansion applied with a quasi-elliptical pin in this work with the optimum results being obtained when the pin diameter is 4% larger than the diameter of the specimen hole. Also, a brief examination of the effect of the rivet shape on the fatigue life of a riveted specimen was carried out. To lengthen the fatigue life of a riveted plate which uses countersunk head rivets, the shape of the countersink and the rivet head were improved. The experimental results showed that the fatigue life of the riveted plate was improved where the improved rivet was used.     

Effect of expansion technique and plate thickness on near-hole residual stresses and fatigue life of cold expanded holes

Cold expansion of fastener holes is a common way of improving fatigue performance of airframes. Among the several techniques applicable, the split-sleeve method is the most accepted in creating beneficial compressive residual stresses around expanded holes. In the present work, residual stresses at expanded holes in several types of aluminium plates produced by two different techniques, split-sleeve and roller burnishing, have been evaluated by the novel destructive Sachs method and then compared. It was found that stress distribution particularly at the vicinity of the hole was sensitive to the method of expansion and plate thickness, due to differing characteristics of the plastic material flow. Thus, secondary reverse yielding after cold expansion found to reduce residual hoop stresses at the edge of the hole, and excessive expansion above a limit, was thought to increase reverse yielding. S–N data revealed that no benefit was gained from expanding beyond this limit. It was suggested that the reduction in the number of cycles to crack initiation or more often to crack growth was due to increased reverse yielding at the vicinity of the expanded hole.     

Finite element method and experimental investigation on the residual stress fields and fatigue performance of cold expansion hole

Cold expansion is an efficient way to improve the fatigue life of an open hole. The residual stress fields of cold expansion holes are vital for key components designing, manufacturing and fatigue properties assessment. In this paper, three finite element models have been established to study the residual stress fields of cold expansion hole, experiments were carried out to measure the residual stress of cold expansion hole and verify simulation results. Three groups of specimens with different cold expansion levels are examined by fatigue test. The fracture surfaces of specimens are observed by scanning electron microscope. The finite element method (FEM) results show, with interference values develop, the maximum values of circumferential residual compressive/tensile stresses increase in “infinite” and “finite” domain, and a higher positive stress values are obtained at the boundary of “finite” domain. The effects of the friction between the mandrel and the hole’s surface and two cold expansion techniques on the distribution of residual stress is local, which only affects the radial residual stress around the maximum value and the circumferential residual stress near the hole’s edge. Crack always initiates near entrance face and the crack propagation speed along transverse direction is faster than that along axial direction.     

The effect of electroless Ni–P coatings on the fatigue life of Al 7075-T6 fastener holes with symmetrical slits

In this paper, the fatigue behaviour of Al 7075-T6 fastener holes with symmetrical through slits was studied. The holes were coated with electroless nickel (EN) plating with a high phosphorous content of 10–13 wt% and a thickness of 40 μm. Uncoated open-hole, EN coated open-hole, uncoated bolted hole and EN coated bolted hole specimens were fatigue tested. Bolted samples were clamped with a high tightening torque of 7 Nm. The established S–N curves showed 282–1348% improvements in the fatigue life due to the combined effect of EN coating and bolt clamping, depending on the level of maximum alternating stress. Excellent adhesion was observed between the coating and the aluminium substrate along the crack path. Tensile tests results showed a considerable reduction of 54% in the ductility of the coated material while both the yield and ultimate strengths were found to slightly increase by approximately 6% in comparison with the uncoated aluminium alloy.     

An experimental investigation of fatigue behavior and deformation of double spot friction welded joints

Fatigue behavior of double spot friction welded joints in aluminum alloy 7075-T6 plates is investigated by conducting monotonic tensile and fatigue tests. The spot friction welding procedures are carried out by a milling machine with a designed fixture at the best preliminary welding parameter set. The fatigue tests are performed in a constant amplitude load control servo-hydraulic fatigue testing machine with a load ratio of (R = P_min/P_max) 0.1 at room temperature. It is observed that the failure mode in cyclic loading (low-cycle and high-cycle) resembles that of the quasi-static loading conditions i.e. pure shearing. Primary fatigue crack is initiated in the vicinity of the original notch tip and then propagated along the circumference of the weld’s nugget.     

An experimental approach to estimate damage and remaining life of metals under uniaxial fatigue loading

An experimental procedure to estimate damage evolution and remaining fatigue life of metals associated with fatigue loading is presented. Experimental phase involves uniaxial tension–compression fatigue tests performed with solid API 5L X52 and tubular carbon steel 1018 specimens subjected to both constant and variable amplitude loading. A correlation between the so-called damage parameter and the thermal response of a material at different damage levels is proposed. Results demonstrate that the correlation can estimate damage evolution with reasonable accuracy in both constant and variable amplitude fatigue processes. It is shown that under the conditions tested the evolution of damage parameter with respect to the normalized fatigue life is independent of the load amplitude, load ratio, loading sequence, material properties, and specimen geometry. The proposed correlation and the relationship between the damage parameter and the normalized fatigue life are employed to develop a non-destructive method to predict the remaining fatigue life of metallic specimens with prior fatigue damage. The method is applied to both constant and variable amplitude loading and the predicted results are found to be in good agreement with those obtained from the experiments.     

Effect of stress ratio on fatigue crack growth in TiAl intermetallics at room and elevated temperatures

The fatigue crack growth behavior of γ-based titanium aluminides (TiAl) with a fine duplex structure and lamellar structure has been investigated by scanning electron microscope (SEM) in situ observation in vacuum at 750°C and room temperature. For the duplex structured material the fatigue crack growth rates are dominated by the maximum stress intensity, particularly at 750°C. The threshold stress intensity range for fatigue crack growth at 750°C is lower than that at room temperature for any corresponding stress ratio. The fatigue crack growth rate at 750°C is affected by creep deformation in front of the crack tip. The severe crack blunting occurs when the stress ratio is 0.5. For the lamellar structured material the scatter of fatigue crack growth data is very large. Small cracks propagate at the stress intensity range below the threshold for long fatigue crack growth. The effects of microstructure on fatigue crack growth are discussed.     

The effect of cold expansion on fatigue crack growth from open holes at room and high temperature

In this paper a series of residual stress measurements and fatigue crack growth tests have been carried out using aluminium alloy 2650 specimens containing cold expanded and non cold expanded holes. Residual stress measurements have been done after cold expansion and after various loading and temperature conditions. In order to measure an angular variation of residual stresses, X-ray and a new technique called the Garcia–Sachs method have been employed. Results revealed that residual stress relaxation occurred as a result of exposure at 150°C. The magnitude of relaxation was shown to be dependent on the level and the sign of externally applied load. Fatigue crack growth tests have been carried out at 20°C and 150°C for both cold expanded and non-cold expanded conditions. Fatigue crack growth rates in specimens containing cold expanded fastener holes were affected significantly by elevated temperature exposure. Depending on the exposure time and loading conditions the fatigue life improvement was found to be between one and greater than 10 for tests at 20°C.     

Effect of substrate surface roughening and cold spray coating on the fatigue life of AA2024 specimens

The effects of cold spray coating and substrate surface preparation on crack initiation under cyclic loading have been studied on Al2024 alloy specimens. Commercially pure (CP) aluminum feedstock powder has been deposited on Al2024-T351 samples using a cold-spray coating technique known as high velocity particle consolidation. Substrate specimens were prepared by surface grit blasting or shot peening prior to coating. The fatigue behavior of both coated and uncoated specimens was then tested under rotating bend conditions at two stress levels, 180 MPa and 210 MPa. Scanning electron microscopy was used to analyze failure surfaces and identify failure mechanisms. The results indicate that the fatigue strength was significantly improved on average, up to 50% at 180 MPa and up to 38% at 210 MPa, by the deposition of the cold-sprayed CP-Al coatings. Coated specimens first prepared by glass bead grit blasting experienced the largest average increase in fatigue life over bare specimens. The results display a strong dependency of the fatigue strength on the surface preparation and cold spray parameters.     

Investigation of low temperature effects on fatigue behavior of PVDF

Fatigue behavior of polyvinylidiene fluoride (PVDF) pipes is investigated under low temperatures to characterize the temperature effects. The analysis included experimental evaluation of fatigue life for test samples taken directly from the manufactured pipes used for service as opposed to compression molded compact tension samples used in previous works. In this test, short sections from an extruded pipe are used to better represent the material service conditions. A compact test chamber was designed to control the test temperature. The samples were loaded into the test rig and allowed to cool for 30 min ensuring a constant and even temperature distribution. Cooling was done in a sealed test chamber using carbon dioxide gas. Two test temperatures of −20 °C and −10 °C were chosen since they represent typical temperature during which failure occurs during actual pipe service in cold environments. Fractured surfaces were inspected and fatigue data were analyzed using a standard procedure for calculation of fatigue life with a semi-elliptical surface crack assumption was performed; from which parameters of the Paris law for fatigue fracture were obtained. Comparing the results with previous works it is found that they capture the trend of the PVDF material behavior for high temperature.     

The effect of short time post-weld heat treatment on the fatigue crack growth of 2205 duplex stainless steel welds

The influence of γ content and its morphology on the impact and fatigue crack growth behavior of 2205 duplex stainless steel (DSS) welds were studied in this work. Short time post-heating was able to effectively raise the γ content and the impact toughness of the weld. The variation in microstructures showed less influence on the fatigue crack growth rate (FCGR) of the steel plate and weld except in the low ΔK regime. In contrast, residual welding stresses played a more significant affection on the FCGR of the DSS weld than microstructural factors did. Plastic deformation induced martensitic transformation within a definitely thin layer was responsible for the difference in crack growth behavior between specimens in the low ΔK range. Coarse columnar structure was more likely to have tortuous crack path in comparison with the steel plate.     

Experimental investigation of random loading sequence effect on fatigue crack growth

An experimental study is proposed to investigate the effect of random loading sequence effect on the fatigue crack growth behavior of Al 7075-T6. The testing matrix includes different overload cycle percentage, overload ratios, and deterministic and random loading sequences in the current investigation. Multiple specimen tests and statistical data analysis are performed to show the effect of random loading sequence on the median and scatter behavior of fatigue crack growth. The proposed experimental study suggests that extreme value distribution is a good approximation of fatigue life distribution. It is observed that the effect of uncertain loading is different under different loading spectrums. For high overload cycle percentage spectrums, the random loading sequence has no major impact on the probabilistic crack growth behavior compared to the deterministic loading sequence with identical load cycle distributions. For low overload cycle percentage spectrums, the random loading sequence has huge impact on the probabilistic crack growth behavior compared to the deterministic loading sequence with identical load cycle distributions, for both the median and the scatter of the fatigue crack length curves. Finally, all experimental observations are reported in table format in Appendix A for future numerical model development and validation for interested readers.