V. Fontanari

Influence of shot peening on bending tooth fatigue limit of case hardened gears

Abstract The effect of different surface treatments on the fatigue behaviour at the tooth root of spur gears is investigated. Case hardening and case hardening followed by shot peening were considered for 16MnCr5 steel gears. Pulsating tests ( R =0.1) were carried out on gear teeth to determine the fatigue endurance at 6×10 6 cycles. Residual stress profiles were measured at the tooth root by means of the XRD technique and carefully analysed in order to establish the effect of different treatment parameters on the residual stress field and to find a correlation with the measured fatigue properties. The XRD technique was also adopted for measuring the retained austenite content. The effective stress field at the gear tooth during the fatigue test was reproduced by finite element modelling to check—using a multiaxial fatigue criterion—whether the fatigue crack initiation can be considered as the precondition for failure. A satisfactory agreement between experimental and predicted fatigue limits was found for unpeened gears, whereas for peened specimens a significant underestimation of fatigue strength was found. This is discussed by considering the micro-structural improvement and the importance of compressive residual stress peaks in the early stages of fatigue crack propagation.

Weight function for an inclined edge crack in a semiplane

The matrix-like structure of the Weight Function (WF) for determining the Stress Intensity Factors (SIFs) in a nonsymmetric plane body is obtained from the general properties of the elastic field. General asymptotic and symmetric properties of the WF are discussed. By extending a previously proposed methodology, an analytical approximate WF is determined for an edge crack in a semiplane within the range of inclination (-75^–75^). Finite Element evaluations considering the minimum number of loading conditions were performed to this purpose. The accuracy of the SIFs obtained by the WF is found in the order of a few tenths of percent. The solution of a typical problem illustrates the practical usefulness of the WF.

Numerical simulation of residual stress relaxation in shot peened high-strength aluminum alloys under reverse bending fatigue

The mechanism of the residual stress relaxation during the fatigue life of shot peened high-strength aluminum alloys was investigated. Experiments were conducted on specimens subjected to three different shot peening treatments and tested under reverse bending fatigue. x-ray diffraction (XRD) measurements were carried out to determine the initial and stabilized residual stress fields. The residual stress field created by the surface treatments has been introduced into a finite element (FE) model by means of a fictitious temperature distribution. The elastic-plastic response of the superficial layers affected by the shot peening treatments has been derived through reverse strain axial testing combined with microhardness tests and implemented in the FE model. The proposed numerical/experimental approach is able to satisfactorily predict the residual stress field evolution. Notably, relaxation has been correctly simulated in the low-cycle fatigue regime and imputed to plastic flow in compression when the superposition of compressive residual and bending stresses exceeds the local cyclic yield strength of the material. Conversely the residual stress field remains stable at load levels corresponding to the 5 × 10 6 cycles fatigue endurance.

Influence of post weld treatments on the fatigue behaviour of Al-alloy welded joints

In this paper the influence of different post welding treatments, such as ageing or shot peening, on the fatigue behaviour of Al-alloy welded joints was investigated. The analysed joints were candidates for car structural applications. Several four point bending fatigue tests were conducted on GMAW specimens subjected to different post weld treatments. The residual stress field acting on specimens was measured by the X-ray diffraction (XRD) technique. The results of tests were discussed with the aid of a finite element model of the specimen aimed to calculate the actual fatigue cycle, also taking account of residual stresses and of their redistribution during the test. This allowed to characterize the fatigue resistance of the joints, taking account of the effective stress acting in the region of crack initiation.

Fatigue behaviour of induction hardened notched components

Abstract In this paper the results of a numerical and experimental study concerning the effect of induction hardening on the fatigue behaviour of mechanical parts is presented. The study was performed on the UNI50CrV4 steel. Rotating bending fatigue tests were carried out in order to obtain the basic fatigue properties, while three point bending tests were carried out on notched specimens in the “as tempered” and in the “induction hardened” condition. Residual stresses produced by the induction hardening were evaluated by means of a numerical-experimental technique, making use of XRD measurements. A finite element model of the notched specimen was set up for the analysis of the stress field acting during fatigue loading; this allowed a prediction of its fatigue behaviour, which appeared to be in satisfactory agreement with experimental observations.

True and apparent Young’s modulus in ferrous porous alloys

Residual porosity in ferrous powder metallurgical alloys induces the phenomenon of localized yielding, or first yielding, during tensile testing. This gives rise to the existence of a true (E1) and apparent (E2) Young’s modulus. The true Young’s modulus is similar to the dynamic modulus (Ed) determined by the acoustic resonance method, whereas the apparent Young’s modulus is lower than both E1 and Ed. For alloys with hard microstructures the apparent Young’s modulus turned out to be about 6% lower than the true Young’s modulus and a negligible influence of matrix hardness and pore morphology was highlighted. However, for ferritic or ferritic–pearlitic materials this difference was higher, ranging between 14 and 31% and it decreases as pore roundness is increased. For austenitic AISI 316L alloys both E1 and E2 are lower than Ed because of the presence of oxides on the powder surface, which favour early decohesion at the necks during tensile testing.

Stress intensity factors for an inclined edge crack in a semiplane

Abstract Mode I and II Stress Intensity Factors under uniform general biaxial loadings were derived for an inclined edge crack in a semiplane. By interpolating Finite Element results in the angular range [0°÷80°], analytical expressions were obtained for both KI and KII with an accuracy better than 1%. Influence coefficients were defined in the crack reference frame thus highlighting the coupling effects between Modes I and II due to the loss of symmetry when the crack is not normal to the surface.

Non-linear analysis of the low-cycle fracture behaviour of isolated Tee stub connections

Abstract The present work deals with the numerical analysis of the low-cycle fracture behaviour of isolated Tee stub connections with partial fillet welds. The overall objective of the research is to assess the seismic performance of bolted partial strength beam-to-column joints under seismic loading; but first, the complexity of these joints is approached after understanding the behaviour of more simple geometries which govern the response. In fact, the study focuses on the low-cycle fracture behaviour of Tee stub connections that are elemental components of extended end plate connections with partial fillet welds. First, the general experimental program dealing with bolted end plate joints and component parts is presented. The program comprises sets of constant and variable displacement amplitude cyclic tests both on complete specimens and on components. Test results have shown good performance of bolted extended end plate moment joints under cyclic loading as well as that of fillet welds that represent an economic solution for thin and moderate extended end plates. Then, the connection material is characterized from a microstructural and fracture mechanics standpoint. Successively, detailed three-dimensional non-linear finite element analyses are carried out in order to tune model material parameters; whilst two-dimensional inelastic analyses are performed in a monotonic and cyclic loading regime to validate connection finite element models. Lastly, a parametric study is conducted in order to define details able to reduce loading-induced toughness demands. Analyses have inferred that fracture driving force demands, quantified in terms of crack tip opening displacements, are reduced by using fillet welds matching the end plate material, by limiting welding-induced residual stresses and by increasing the yield-to-ultimate strength ratio compatibly with plastic analysis requirements.

A weight function technique for partially closed inclined edge cracks analysis

The Green Functions, giving the Crack Opening Displacement components of an inclined edge crack under general loading conditions, were obtained starting from the matrix-like structure of the Weight Functions developed for determining the Stress Intensity Factors. The mathematical formulation of the problem is presented and the computational efficiency of the method is demonstrated by solving and discussing the non linear problem of a partially closed inclined edge crack under bending. By means of an iterative procedure, the closed portion of the crack is determined and the effects of normal and friction contact forces on the Crack Opening Displacement components and on the Stress Intensity Factors are discussed. The efficiency and the accuracy of this approach are assessed by comparison with Finite Element solutions.

Multiaxial Fatigue Resistance of Shot Peened High-Strength Aluminium Alloys

This paper is aimed at investigating multiaxial fatigue of shot peened Al-7075-T651 alloy. Plain axi-symmetric specimens were subjected to combined in-phase tension and torsion loading, under nominal load ratio R = 0.05 and biaxiality ratio λ = τ a /σ a = 2. The results from multi-axial tests are discussed together with those obtained under pure tension and pure torsion loading. Fatigue crack initiation sites have been investigated through scanning electron microscopy fractography and the role of surface roughness on fatigue resistance has been analyzed. The initial and the stabilized residual stress profiles were used to discuss the improvement in the fatigue response in the hypothesis of crack initiation and early crack propagation as fatigue controlling parameters. For this purpose, several multiaxial fatigue criteria were used to account for the residual stress field.