Multiaxial fatigue and failure analysis of helical compression springs

Multiaxial fatigue criteria are applied to the analysis of helical compression springs. The critical plane approaches, Fatemi–Socie and Wang–Brown, and the Coffin–Manson method based on shear deformation, were used to predict fatigue lives of the springs under constant amplitude loading. Experimental fatigue lives are compared with the multiaxial fatigue criteria predictions. The stress analysis was carried out in the finite element code ANSYS, and the multiaxial fatigue study was performed using the fatigue software nCode. A failure analysis was conducted in order to determine the fatigue crack initiation point and a comparison of that location with the most damaged zone predicted by the numerical analysis is made. The Fatemi–Socie critical plane approach gives a good prediction of fatigue life. While the Wang–Brown criterion overestimates spring fatigue life, the Coffin–Mason model gives conservative results.     

Fatigue damage accumulation of cold expanded hole in aluminum alloys subjected to block loading

Fatigue damage accumulation of cold expanded hole in aluminum alloys used in land transportation components was investigated. Tests were carried out using pre-cracked SENT specimens and inserting an expanded hole at the crack tip. The degree of the cold expansion was chosen equal to 4.3%. Tests were performed in two and four block loading under constant amplitude. Two sequences were compared.The increasing and the decreasing magnitude were compared. The experimental results were compared to the damage calculated by the Miner's rule and a new simple fatigue damage indicator. This comparison shows that the ‘model of the damage stress’, which take into account of the loading history, yields a good estimation of the experimental results. Moreover, the error is minimized in comparison to the Miner's model.     

Fatigue life estimation after crack repair in 6005 A-T6 aluminium alloy using the cold expansion hole technique

In this paper, the hole drilling (HD) and the cold expansion (CE) processes, which were used as a technique for crack repair, were investigated in order to estimate the beneficial effects on fatigue crack initiation (FCI). The FCI life is defined as the number of cycles to initiate a new crack of 0.2 mm on the surface of the specimen. Three hole radii and three degrees of cold expansion (DCE%) values were tested after a crack propagation period. Crack retardation after the CE process was observed. This phenomenon is due to two mechanisms: retardation owing to both geometric and mechanical effects, which is produced by the stress concentration at the drilled hole, and the large strain‐induced compressive residual stresses around the hole. In this report, the influence of the loading conditions was studied. For high values of the stress intensity factor range ΔK_ρ around the hole (based on the pseudo crack length a + ρ), the number of cycles corresponding to crack initiation N_i is low. At the edge of the hole, the maximum stress range can be approximated by the following formula: Δσ_max = 2ΔK_ρ /√πρ , where ρ is the hole radius and ΔK_ρ is the related stress intensity factor range.The FCI life extension, defined by the number of cycles corresponding to crack re‐initiation N_i , is related to the relative maximum stress range ratio R_σ = [(Δσ_max )/(Δσ_max )_th ] where (Δσ_max )_th is the value of the threshold maximum stress range obtained when N_i = 2 × 10⁶ cycles. The relationship between N_i and R_σ may be written as a power function.     

Analytical method of calculating preheating temperature in order to avoid cold cracking

Summary

The IIW method is shown to be one of the most precise and that it may be programmed to advantage.     

Fatigue life prediction under variable loading based on a new damage model

To examine the performance of nonlinear models proposed in the estimation of fatigue damage and fatigue life of components under random loading, a batch of specimens made of 6082 T 6 aluminium alloy has been studied and some of the results are reported in the present paper. The paper describes an algorithm and suggests a fatigue cumulative damage model, especially when random loading is considered. This paper contains the results of mono-axial random load fatigue tests with different mean and amplitude values performed on 6082 T 6 aluminium alloy specimens. Cycles were counted with rainflow algorithm and damage was cumulated with a new model proposed in this paper and with the Palmgren–Miner model. The proposed model has been formulated to take into account the damage evolution at different load levels and it allows the effect of the loading sequence to be included by means of a recurrence formula derived for multilevel loading, considering complex load sequences. It is concluded that a ‘damaged stress interaction damage rule’ proposed here allows a better fatigue damage prediction than the widely used Palmgren–Miner rule, and a formula derived in random fatigue could be used to predict the fatigue damage and fatigue lifetime very easily. The results obtained by the model are compared with the experimental results and those calculated by the most fatigue damage model used in fatigue (Miner’s model). The comparison shows that the proposed model, presents a good estimation of the experimental results. Moreover, the error is minimized in comparison to the Miner’s model.     

Experimental and numerical investigation on single specimen methods of determination of J in rubber materials

In this paper, we have verified the validity of some formulations allowing the determination of the fracture surface energy in the case of rubber-like materials. The J-integral is chosen as a fracture characterizing parameter which is experimentally determined by considering a multiplying form that; numerically evaluated using a finite element method. The numerical results are compared to the experimental data and a good agreement has been pointed out for the deeply cracked specimen (a/w0.5). Below this limit, a significant divergence is observed which is attributed to a lack of accuracy of the experimental data processing.     

On the formation of double zones of necking during hot tensile tests

The phenomenon of double necking during hot tensile tests is discussed in this work. The investigation was carried out on high-carbon steel using a tensile testing machine equipped with an integral heating system. Tensile tests were performed in a large temperature range to obtain the mechanical characteristic of the material. Microstructure evolution was observed by light microscopy and hardness measurements to determine the position of double zones of necking. A significant increase in flow stress of as-transformed austenite was found to be at the origin of the double necking behaviour.     

Fatigue behavior of a 7075-T6 aluminum alloy coated with an electroless Ni–P deposit

An investigation has been carried out in order to study the fatigue and corrosion–fatigue behavior of a 7075-T6 aluminum alloy coated with an electroless Ni–P (EN) deposit, in the as-plated condition, of approximately 38–40 μm in thickness and a high P content, of approximately 18 wt%. The results obtained, show that the EN coating can give rise to a significant improvement in the fatigue and corrosion–fatigue performance of the substrate, depending on the testing conditions. When the coated system is tested in air, it is observed that the increase in fatigue properties decreases as the alternating stress applied to the material increases. At stresses of the order of 0.4 σ_0.2% the increase in fatigue life is more than about 100%. However, as the stress increases to values in the range of 0.7 σ_0.2%, no improvement in the fatigue performance of the system is observed and the behavior is similar to that of the uncoated substrate. Under corrosion–fatigue conditions, the fatigue life is observed to increase between approximately 60% and 70%, depending on the stress applied. It is shown that fatigue cracks are associated with nodular-like defects present on the surface of the coated samples. The deleterious effect of such defects seems to be more pronounced as the alternating stress applied to the material increases. A crude estimate of the yield strength of the EN coating from tensile measurements indicates that such a parameter is in the range of 3.8 GPa, in agreement with the computation of the absolute hardness of the deposit, of about 4 GPa, by means of Meyer’s law. It is also shown that the EN deposit has a very good adhesion to the substrate even when the system is subjected to tensile stresses greater than the yield strength. Such characteristics as well as the higher mechanical properties of the EN coating in comparison with the aluminum alloy substrate and the preservation of its integrity during fatigue testing contribute to the better fatigue performance of the coated system.     

CHARACTERIZATION OF THERMO-MECHANICAL FATIGUE PROPERTIES FOR PARTICULATE REINFORCED COMPOSITES

A Voronoi cell element, formulated with creep, thermal and plastic strain was applied for investigation of thermo-mechanical fatigue behavior for particulate reinforced composites. Under the in-phase fatigue loading, the maximum of tensile deformation at the maximum given loading are larger than that at the same maximum under the out-phase fatigue. The stiffness decreases nonlinearly with the increasing of the phase angle, which results in increasing of the area of fatigue loop curve and the decrease in fatigue life. The spatially centralizing of inclusions results in decreasing of the plastic strain amplitude and the area of fatigue loop curve, which will also reduce the consumption of single-circle plastic strain energy and prolong the fatigue life.     

Evaluation of the energy parameter J on rubber-like materials: Comparison between experimental and numerical results

The general purpose of our study is the determination of the energy parameter J when dealing with fracture of rubber-like materials. The energy parameter J is expressed in a multiplicative form in which a calibration factor is introduced in order to take into account the finite dimensions of the specimen. The parameter J, issued from fracture tests performed on S.E.N.T specimen of an E.P.D.M rubber is compared with the the J integral which is computed using a finite element procedure for the non-linear elastic materials with large deformation. The numerical results are in good agreement with the experimental data when considering the deeply cracked specimen (a/w ≥ 0.5). Below this limit, a divergence is pointed out which is attributed to a lack of accuracy of the identification procedure used to determine the experimental calibration factor. When this one is determined using the numerical J integral results, a better concordance is obtained.