Experimental and numerical investigations about the combined effect of interference fit and bolt clamping on the fatigue behavior of Al 2024-T3 double shear lap joints

In this paper the fatigue behavior of double shear lap joints treated by different combinations of interference fit and bolt clamping have been investigated both experimentally and numerically. To do so, specimens made from aerospace structural material of aluminum alloy 2024-T3 plates were interference fitted at the sizes of 1.5% and 4.7% and torque tightened with 2 and 4 N m to be prepared for fatigue tests. Consequently, the joints were subjected to cyclic load at different levels to obtain fatigue life. Finite element (FE) analysis was also performed to find the stress and strain distributions and the results were used to help explain the trends observed in the experimentally obtained S–N data. The experimental tests showed that during the interference fit process a protruded region is created at around the hole in the exit plane due to directional material plastic flow as a consequence of the oversized bolt force fitting. This protruded region has a bigger height for the bigger interference fit size. The finite element results showed that the protruded region generally localizes the compressive effect of bolt clamping and reduces its capability in fatigue life enhancement, by relaxing the clamping force. The fatigue test results showed that a better fatigue life improvement was achieved by employing the combination of a smaller interference fit size and bigger clamping force.     

Experimental investigation on low cycle fatigue of DZ125 with film cooling holes in different processes of laser drilling

Due to the different low cycle fatigue (LCF) properties and fatigue fracture behavior around film cooling holes on DZ125, the LCF tests are carried out using tension cycling under stress control conditions (stress ratio R = 0.1) at 900 °C. The specimens were designed as thin-wall plate with single hole and multi holes under picosecond and nanosecond laser drilling processes. Comparative analyses of the differences between fatigue life and microscopic fracture morphology are conducted. It is shown that under the same stress condition, the relationship between fatigue life is as follows: picosecond laser single-holed specimen > nanosecond laser single-holed specimens > picosecond laser multi-holed specimens > nanosecond laser multi-holed specimens. Scanning electron microscope (SEM) analyses of the fracture revealed that the crack initiates from the film cooling holes where fatigue source zone, fatigue crack propagation zone and fatigue fracture zone can be found. However, the different processes lead to slightly different fracture morphology: radial-type ridge centering on the fatigue source zone is more apparent and uniform in picosecond laser drilling specimens than in the nanosecond laser drilling ones. On the other hand, the radial-type ridge is biased toward large-aperture side with nanosecond laser drilling.     

Fatigue behavior and modeling of short fiber reinforced polymer composites including anisotropy and temperature effects

Effects of anisotropy and temperature on cyclic deformation and fatigue behavior of two short glass fiber reinforced polymer composites were investigated. Fatigue tests were conducted under fully-reversed (R = −1) and positive stress ratios (R = 0.1 and 0.3) with specimens of different thicknesses, different fiber orientations, and at temperatures of −40 °C, 23 °C, and 125 °C. In samples with 90° fiber orientation angle, considerable effect of thickness on fatigue strength was observed. Effect of mold flow direction was significant at all temperatures and stress ratios and the Tsai–Hill criterion was used to predict off-axis fatigue strengths. Temperature also greatly influenced fatigue strength and a shift factor of Arrhenius type was developed to correlate fatigue data at various temperatures, independent of the mold flow direction and stress ratio. Micromechanisms of fatigue failure at different temperatures were also investigated. Good correlations between fatigue strength and tensile strength were obtained and a method for obtaining strain–life curves from load-controlled fatigue test data is presented. A fatigue life estimation model is also presented which correlates data for different temperatures, fiber orientations, and stress ratios.     

Fatigue failure investigation on anti-vibration springs

Fatigue failure investigation on anti-vibration springs, involving both metal and rubber materials, is presented. Rubber-to-metal bonded springs are widely used in industry as anti-vibration components giving many years of service. Recently a need to improve time and cost efficiencies has caused an unexpected early fatigue failure of the component with no immediate explanation. The required total fatigue life was 1.25 million cycles but only 0.7 million cycles achieved. There was an urgent need to investigate the causes of the fatigue failure and to modify the component design accordingly to meet the customer requirement and the supply schedule.The investigation, based on the actual fatigue loads, is carried out on these failed and modified products using a method of continuum mechanics. To simplify the simulation, a non-linear quasi-static analysis is carried out and then the residual stresses are superimposed to obtain the effective stress range to predict the metal crack initiation. For the rubber parts a three-dimensional effective stress criterion is employed to predict the fatigue crack initiation. The fatigue failure is taken as visual crack observation (normally 1–2 mm).The fatigue crack initiation for the metal parts of the failed component is predicted at 225 K cycles under specified fatigue load against total metal broken at 700 K cycles from the test. For the modified part the minimum total fatigue life for the metal parts of the component, estimated conservatively, is 2.1 million cycles against 1.75 million cycles from the test without any crack observed. The rubber fatigue crack initiation is predicted at 90 K cycles against crack onset around 79 K cycles and crack length 40 mm at 145 K cycles from the test. From design point of view it is important to optimize the rubber profile under this very tight allowable space to provide the maximum support of the metal interleaves and at the same time to meet the minimum requirements of the manufacture process. It is shown that this approach can be employed at a design stage for both metal and rubber fatigue evaluations on anti-vibration springs.     

Experimental and finite element analysis of fatigue performance of copper power conductors

Fatigue performance of a 95 mm² stranded copper conductor was investigated. Individual copper wires were tested in tension–tension loading with a stress ratio R = 0.1. The specimens were taken from the core wire and from the inner and outer layer of the conductor. Due to the compacting process that was applied during manufacturing, geometrical irregularities were observed on the wires from the outer and inner layers. Finite element (FE) analyses were performed to investigate the combined effects of these irregularities and of material plasticity on the fatigue performance. The FE models were validated by convergence studies. Full cross section conductors were tested in a specially designed rig providing constant tension and variable (reversed) curvature simulating bending inside a bellmouth. In this test the fatigue failures were found to be governed by local bending effects in individual wires. The data from the single wire tests are presented on S–N format and applied in a model for prediction of fatigue strength of full section copper conductors.     

An experimental investigation of the bolt clamping force and friction effect on the fatigue behavior of aluminum alloy 2024-T3 double shear lap joint

In this article, the effect of bolt clamping force on the fatigue life of bolted double shear lap joints was investigated. To do so, fatigue tests were carried out on the bolt clamped double shear lap joint specimens made of aluminum alloy 2024-T3. These fatigue tests were conducted with applied torques of 0.25, 2 and 4 N m at different cyclic longitudinal load levels in un-lubricated and lubricated states. From these tests the stress–life (S–N) data for different clamping forces for un-lubricated and lubricated states were obtained. The results show that clamping force increases fatigue life compared to clearance fit specimens. In general, at higher tightening torque higher fatigue lives were achieved, however, below a certain load level the life improvement was discontinued because of fretting phenomenon. Also lubricating the parts of the specimens reduces the advantage of clamping force or torque tightening.     

Improved mechanical properties of an epoxy glass–fiber composite reinforced with surface organomodified nanoclays

An organomodified surface nanoclay reinforced epoxy glass-fiber composite is evaluated for properties of mechanical strength, stiffness, ductility and fatigue life, and compared with the pristine or epoxy glass-fiber composite material not reinforced with nanoclays. The results from monotonic tensile tests of the nanoclay reinforced composite material at 60 °C in air showed an average 11.7% improvement in the ultimate tensile strength, 10.6% improvement in tensile modulus, and 10.5% improvement in tensile ductility vs. these mechanical properties obtained for the pristine material. From tension–tension fatigue tests at a stress-ratio = +0.9 and at 60 °C in air, the nanoclay reinforced composite had a 7.9% greater fatigue strength and a fatigue life over a decade longer or 1000% greater than the pristine composite when extrapolated to 10⁹ cycles or a simulated 10-year cyclic life. Electron microscopy and Raman spectroscopy of the fracture and failure modes of the test specimens were used to support the results and conclusions. This nanocomposite could be used as a new and improved material for repair or rehabilitation of external surface wall corrosion or physical damage on piping and vessels found in petrochemical process plants and facilities to extend their operational life.     

Enhanced fatigue behavior of a glass fiber reinforced hybrid particles modified epoxy nanocomposite under WISPERX spectrum load sequence

Two types of glass fiber reinforced plastic (GFRP) composites were fabricated viz., GFRP with neat epoxy matrix (GFRP-neat) and GFRP with hybrid modified epoxy matrix (GFRP-hybrid) containing 9 wt.% of rubber microparticles and 10 wt.% of silica nanoparticles. Fatigue tests were conducted on both the composites under WISPERX load sequence. The fatigue life of the GFRP-hybrid composite was about 4–5 times higher than that of GFRP-neat composite. The underlying mechanisms for improved fatigue performance are discussed. A reasonably good correlation was observed between the experimental fatigue life and the fatigue life predicted under spectrum loads.     

Tensile and fatigue behavior of superelastic shape memory rods

The tensile and fatigue behavior of superelastic shape memory alloy (SMA) bars heat-treated at three different temperatures were examined. Low cycle fatigue tests at variable load rates were carried out to determine the effect of stress and frequency on residual strain and energy dissipation in a fatigue cycle. The mechanism of energy dissipation was studied by monitoring the temperature changes in the fatigued samples as a function of applied stress and frequency of testing. Results from the tensile tests revealed that the stress for the Austenite to Martensite transformation decreased from 408 MPa to 204 MPa with an increase in temperature of heat treatment from 300 to 450 °C. The ultimate strength of the SMA increased from 952 MPa to 1115 MPa when the heat treatment temperature was increased from 300 to 450 °C. Fatigue testing prior to conducting the tensile test decreased the ultimate strength of the SMA and also reduced the failure strain. The energy dissipation in fatigue tests was found to decrease as test frequency increased from 0.025 Hz to 0.25 Hz and the change in sample temperature during the test at the lower test frequency was found to be considerably higher than at the higher frequency.     

Influence of hold times on the elevated-temperature fatigue behavior of an oxide–oxide ceramic composite in air and in steam environment

The effect of hold times at maximum stress on fatigue behavior of an oxide–oxide ceramic composite was investigated at 1200 °C in laboratory air and in steam environments. The composite consists of a porous alumina matrix reinforced with woven mullite/alumina (Nextel™720) fibers, has no interface between the fiber and matrix, and relies on the porous matrix for flaw tolerance. Tension–tension fatigue tests with a ratio R (minimum to maximum stress) of 0.05, and hold times of 10 and 100 s were performed for fatigue stresses of 125 and 154 MPa in laboratory air, and for fatigue stresses of 100 and 125 MPa in steam environment. Block loading tests incorporating periods of cyclic and sustained loading were carried out to assess the effects of loading history on material behavior and environmental durability. In laboratory air, lives produced in fatigue tests with hold times exceeded those produced in creep but were shorter than those obtained in fatigue. Prior fatigue resulted in an order of magnitude improvement in creep life. Prior creep had no effect on subsequent fatigue life. Presence of steam significantly degraded the material performance. In steam, lives produced in fatigue tests with hold times were close to those obtained in creep. Prior fatigue reduced the creep resistance, and prior creep degraded the subsequent fatigue life. Composite microstructure, as well as damage and failure mechanisms were investigated. A qualitative spectral analysis showed evidence of silicon species migration from fiber to matrix, especially in steam.     

J-integral-based approach to fatigue assessment of laser stake-welded T-joints

The aim of this paper is to investigate the influence of the plate thickness on the fatigue strength of laser stake-welded T-joints under the tension loading condition. Fatigue tests were conducted on specimens with plate thicknesses below 5 mm subjected to tension loading with the load ratio R = 0. The statistical analysis of the weld geometry showed a normal distribution of the each parameter that was measured. In addition, the parameters had similar proportions in comparison to the specimens with plate thicknesses above 5 mm. FE analysis was performed with the aim of determining the stress state in the joint along with the J-integral. If the square root of the J-integral, √ΔJ, is used as the fatigue strength assessment parameter, the fatigue strength obtained at five million cycles is similar as in the case of other steel welded joint types. The investigation concluded that the stress state changes with the reduction of the plate thicknesses and the contribution of fracture mode II becomes significant. However, using √ΔJ as a fatigue strength assessment parameter ensures that the complex state of the mixed fracture mode loading is accurately accounted for. This fact further enables the fatigue strength of laser stake-welded T-joints of any plate thicknesses to be described by means of a narrower scatterband than the one obtained by the nominal stress approach.     

Influence of reinforcing bar type on autogenous shrinkage stress and bond behavior of ultra high performance fiber reinforced concrete

This study investigated the effects of reinforcing bar type and reinforcement ratio on the restrained shrinkage behaviors of ultra high performance fiber reinforced concrete (UHPFRC), including autogenous shrinkage stress, degree of restraint, and cracking potential. In addition, the influence of the type and embedment length of reinforcing bars on the bond behavior of UHPFRC was evaluated by performing pullout test. Three different reinforcing bars (deformed steel bar, round steel bar, and GFRP bar) were investigated in the restrained shrinkage and pullout tests. The GFRP bar exhibited the best performance in relation to the autogenous shrinkage stress, degree of restraint, and cracking potential because of its low stiffness. The highest bond strength was obtained for the deformed steel bar, and the bar yielding was observed when the bar embedment length of l_b = 2d_b was used. The round steel bar exhibited the poorest behaviors for both of the restrained shrinkage and pullout.     

Prediction of the fatigue life of natural rubber composites by artificial neural network approaches

A back-propagation artificial neural network (BP-ANN) model was established to predict fatigue property of natural rubber (NR) composites. The mechanical properties (stress at 100%, tensile strength, elongation at break) and viscoelasticity property (tan δ at 7% strain) of natural rubber composites were utilized as the input vectors while fatigue property (tensile fatigue life) as the output vector of the BP-ANN. The average prediction accuracy of the established ANN was 97.3%. Moreover, the sensitivity matrixes of the input vectors were calculated to analyze the varied affecting degrees of mechanical properties and viscoelasticity on fatigue property. Sensitivity analysis indicated that stress at 100% is the most important factor, and tan δ at 7% strain, elongation at break almost the same affecting degree on fatigue life, while tensile strength contributes least.     

On the strengthening effect of increasing cycling frequency on fatigue behavior of some polymers and their composites: Experiments and modeling

Effect of cycling frequency on fatigue behavior of neat, talc filled, and short glass fiber reinforced injection molded polymer composites was investigated by conducting load-controlled fatigue tests at several stress ratios (R = −1, 0.1, and 0.3) and at several temperatures (T = 23, 85 and 120 °C). A beneficial or strengthening effect of increasing frequency was observed for some of the studied materials, before self-heating became dominant at higher frequencies. A reduction in loss tangent (viscoelastic damping factor), width of hysteresis loop, and displacement amplitude, measured in load-controlled fatigue tests, was observed by increasing frequency for frequency sensitive materials. Reduction in loss tangent was also observed for frequency sensitive materials in DMA tests. It was concluded that the fatigue behavior is also time-dependent for frequency sensitive materials. A Larson–Miller type parameter was used to correlate experimental fatigue data and relate stress amplitude, frequency, cycles to failure, and temperature together. An analytical fatigue life estimation model was also used to consider the strengthening effect of frequency in addition to mean stress, fiber orientation, and temperature effects on fatigue life.     

Improving low-cycle fatigue properties of rolled AZ31 magnesium alloy by pre-compression deformation

The effect of pre-compression deformation on the low-cycle fatigue properties and cyclic deformation behavior of as-rolled AZ31 alloy was investigated by performing the stress-controlled low-cycle fatigue tests at room temperature. Fatigue properties and cyclic damage process should be closely related to the twins. The present work aimed to investigate the deformation mechanism and fatigue life caused by the introduced {1 0−1 2} twinning–detwinning from the viewpoint of stress amplitude. The results reveal that the twins contribute to the fatigue properties and cyclic damage process of AZ31 alloy. There were noticeable changes in hysteresis loops, microstructures and fatigue lives when the stress amplitude increased from 120 to 150 MPa. The fatigue life of pre-compressed samples was more superior to that of the as-rolled sample under different stress amplitudes, especially under the stress amplitude close to the tensile yield strength of the as-rolled sample.     

The role of oxidation damage in fatigue crack initiation of an advanced Ni-based superalloy

The effects of prior oxidation on the room temperature fatigue life of coarse-grained Ni-based superalloy, RR1000, have been investigated. High cycle fatigue tests were conducted, on both machined and pre-oxidised testpieces, at room temperature at an R ratio of 0.1. The oxidation damage was produced by pre-exposures at 700 °C for either 100 or 2000 h. Pre-oxidised testpieces tended to fail with shorter fatigue lives than those obtained from the as-machined testpieces although they were also observed to outperform the as-machined test pieces at peak stress levels around 900 MPa. The chromia scale and intergranular alumina intrusions formed during pre-oxidation are prone to crack under fatigue loading leading to early crack nucleation and an associated reduction in fatigue life. This has been confirmed to be the case both below and above a peak stress level of ∼900 MPa. The better fatigue performance of the pre-oxidised specimens around this stress level is attributed to plastic yielding of the weaker γ′ denuded zone, which effectively eases the stress concentration introduced by the cracking of the chromia scale and intergranular internal oxides. This γ′ denuded zone is also a product of pre-oxidation and develops as a result of the selective oxidation of Al and Ti. Over a limited stress range, its presence confers a beneficial effect of oxidation on fatigue life.     

Evaluation of residual stress relaxation and its effect on fatigue strength of AISI 316L stainless steel ground surfaces: Experimental and numerical approaches

This paper is aimed at evaluating the residual stress relaxation and its effect on the fatigue strength of AISI 316L steel ground surfaces in comparison to electro-polished surfaces. An experimental evaluation was performed using 3-point and 4-point bending fatigue tests at R_σ = 0.1 on two sets of notched specimens finished by electro-polishing and grinding. The residual stress fields were measured at the notch root of specimens, before and after fatigue tests, by means of the X-ray diffraction technique. It was found a degradation of about −35% for the 4-point bending fatigue limit at 2 × 10⁶ cycles of the ground specimens in comparison to the electro-polished ones. This degradation is associated with a slight relaxation of the grinding residual stresses which remain significant tensile stresses at the stabilized state. While under the 3-point bending test, these residual stresses relax completely and provoke a noticeable increase of the fatigue limit estimated at about 50% in comparison to the 4-point bending fatigue test. The numerical evaluation of residual stress relaxation was carried out by FE analyses of the cyclic hardening behaviour of the ground layer. The isotropic and nonlinear kinematic model proposed by Chaboche was used and calibrated for the base material and the ground layer. The results show that residual stresses relax to a stabilized state characterized by elastic-shakedown response. This stabilization is occurred after the first cycle of the 4-point bending test corresponding to the higher stress concentration (K_t-4p = 1.66), while it requires many cycles under the 3-point bending test corresponding to the lower stress concentration (K_t-3p = 1.54). The incorporation of stabilized residual stress values into the Dang Van’s criterion has permitted to predict with an acceptable accuracy the fatigue limits under both bending modes.     

Fatigue properties and fracture analysis of a SiC fiber-reinforced titanium matrix composite

Tension–tension fatigue properties of SiC fiber reinforced Ti–6Al–4V matrix composite (SiC_f/Ti–6Al–4V) at room temperature were investigated. Fatigue tests were conducted under a load-controlled mode with a stress ratio 0.1 and a frequency 10 Hz under a maximum applied stress ranging from 600 to 1200 MPa. The relationship between the applied stress and fatigue life was determined and fracture surfaces were examined to study the fatigue damage and fracture failure mechanisms using SEM. The results show that, the fatigue life of the SiC_f/Ti–6Al–4V composite decreases substantially in proportion to the increase in maximum applied stress. Moreover, in the medium and high life range, the relationship between the maximum applied stress and cycles to failure in the semi-logarithmic system could be fitted as a linear equation: S_max/μ = 1.381 − 0.152 × lgN_f. Fractographic analysis revealed that fatigue fracture surfaces consist of a fatigued region and a fast fracture region. The fraction of the fatigued region with respect to the total fracture surface decreases with the increase of the applied maximum stresses.     

Experiments under pure shear and rolling contact fatigue conditions: Competition between tensile and shear mode crack growth

In the present paper, the mechanism of shear crack growth under both pure torsion and mixed mode loadings, simulating rolling contact fatigue testing conditions, has been investigated for a bearing steel and the role of the superimposed compressive stress in subsurface RCF has been clarified both numerically and experimentally. In particular a previous data set of fatigue tests on micro-notched specimens subjected to torsion and out-of-phase loads with |σ_min|/τ_max ≅ 3.5 (LP1) has been complemented with the new tests onto micro-notched specimens loads with |σ_min|/τ_max ≅ 0.7 (LP2) and a test under pure compression. The same tests have been also simulated numerically with a non-linear FE analysis of crack advance. The numerical analyses have been conducted with the aim of demonstrating that the compressive stress fully suppresses the tendency to tensile mode growth as the crack extends.Eventually, the competition between tensile and shear mode growth during a fatigue cycle has been investigated theoretically in terms of local branch SIFs. In particular, the conditions for the branch crack growth have been examined on the basis of the effective SIFs: the crack tip shielding effects due to the crack surface interference (both the mode I contribution caused by the asperity mismatch and the shear attenuation produced by the frictional stresses) have been quantified by employing a model for crack sliding interaction under pure mode III and mixed mode I + III loadings.