Massimo Rossetto

Comparison of fatigue data using the maximum likelihood method

The paper describes procedures to compare the fatigue behaviour of two populations corresponding to batches that differ for a factor on the basis of fatigue data of two samples. Fatigue data are elaborated by means of the maximum likelihood method to obtain the SNP curves and their confidence intervals. The likelihood ratio test is used to identify the most adequate distribution and model. To analyse the influence of the factor, two integrated procedures are adopted: the first one is based on the LR test, the second is an empirical procedure, based on the comparison of the confidence intervals of the curves.

Effect of the Strain Rate on the Mechanical Behaviour of Epoxy Adhesives

In this paper the dynamic mechanical behaviour of two epoxy adhesives for structural bonding is investigated. The study has been focused on the influence of the strain rate on the tensile and compressive strength of specimens made of adhesive. The experimental tests have been performed with an hydraulic universal testing machine and a tension-compression Hopkinson bar. The results of the tests show that the σ−ε curve rises significantly increasing the strain rate. The Cowper-Symonds and Johnson-Cook models of strain rate dependence have been used to fit the experimental data with unsatisfactory results, thus a poly-linear fit has been adopted.

Moisture degradation of open-faced single lap joints

To obtain experimental data in short time on the degradation of adhesives exposed to moisture, a valuable technique is represented using the open-face configuration. With this technique, a layer of adhesive is first applied on one adherend and exposed to the humid environment; then, the second adherend is bonded and the joint can now undergo mechanical testing. Apart from the acceleration of moisture uptake which is obtained due to the larger area exposed, a further advantage is the uniformity of degradation. A further acceleration can be obtained by adding a hygroscopic contaminant at the adhesive/adherend interface, which speeds up moisture uptake and accentuates the interfacial nature of the failure. The main aim of this work was to evaluate the decay of the mechanical strength in the absence or presence of a contaminating agent. The specimens studied were single lap joints, tested under static shear loading. Two sets of specimens were considered; in the first set, the adhesive was applied in standard way and in the second set, the adhesive/adherend interface was contaminated with droplets of CaCl2 aqueous solution. Both sets were subjected to humid and warm environment (100% relative humidity, 50 °C). After the desired exposure times in the range 1–5 weeks, groups of specimens were dried and bonding of the second adherend was carried out. Then, mechanical testing was performed; the fractured surfaces were examined by scanning electron microscopy. The results show that before degradation the failure type is cohesive, but it changes to interfacial failure as the degradation proceeds. Uncontaminated specimens exhibit gradual degradation during the exposure time; contaminated specimens achieve almost half of the degradation in less than one week; after that, the process continues at lower speed and at the end of the observed period both methods show similar values of failure loads. Additional tests were carried out to assess the moisture absorption in the adhesive layer and relate it to the exposure time.

Comparison between dog-bone and Gaussian specimens for size effect evaluation in gigacycle fatigue

Gigacycle fatigue properties of materials are strongly affected by the specimen risk volume (volume of material subjected to a stress amplitude larger than the 90% of the maximum stress). Gigacycle fatigue tests, performed with ultrasonic fatigue testing machines, are commonly carried out by using hourglass shaped specimens with a small risk volume. The adoption of traditional dog-bone specimens allows for increasing the risk volume, even if the increment is quite limited. In order to obtain larger risk volumes, a new specimen shape is proposed (Gaussian specimen). The dog-bone and the Gaussian specimens are compared through Finite Element Analyses and the numerical results are validated experimentally by means of strain gages measurements. The range of applicability of the two different specimens in terms of available risk volume and stress concentration effects due to the cross section variation is determined.

Evaluation of the Singular Stresses in Adhesive Joints

This paper deals with the evaluation of the singularity in the stress field of bonded joints, in particular with the determination of the stress intensity factor. After reviewing the key aspects of the problem, the attention is focussed on establishing a procedure to estimate the stress intensity factor without the need for a detailed finite element analysis of the joints, which typically requires mesh refinement of the order of one-millionth of the adhesive thickness. This is accomplished by means of a numerical procedure to solve the equations of the problem, and is not restricted to a specific geometry and is ready to be implemented also in a personal computer environment. The required boundary conditions are found by a classical structural (monodimensional) solution for the stresses in bonded joints, and are easy to implement as well. The approach is tested for single lap joints, considering a combination of different values of joint parameters (adherend and adhesive thicknesses, overlap length). The results obtained and the involved approximations are examined, and this provides also the opportunity to establish the type of stress that should be considered to obtain the most appropriate definition of the stress intensity factor.

Different Inclusion Contents in H13 Steel: Effects on VHCF Response of Gaussian Specimens

The effect of different inclusion contents on the VHCF strength of H13 tool steels is presented. Two different H13 tool steels were investigated: the Uddeholm Orvar® 2 Micronized obtained by conventional casting, and the Uddeholm Orvar® Supreme obtained by electroslag remelting (ESR). Ultrasonic tests were performed on Gaussian specimens (risk volume about 2300 mm3) up to 1010 cycles or up to failure and fracture surfaces were investigated with SEM in order to analyze the inclusions from which VHCF crack nucleated. Experimental results show that the VHCF strength estimated by using the Murakami’s model of the H13 Uddeholm Orvar® Supreme steel is about 15% larger than that of the H13 Uddeholm Orvar® 2 Micronized steel.

Statistical Estimation of Duplex S-N Curves

In recent years, experimental tests investigating properties of materials in gigacycle regime have suggested modifications to well-known statistical fatigue life models. Classical fatigue life models based on a single failure mode and by the presence of the fatigue limit, have been integrated by models that can take into account the occurrence of two failure modes (duplex S-N curve).Duplex S-N models involve a number of unknown parameters that must be statistically estimated from experimental data. The present paper proposes a simplified and automated procedure for statistical parameter estimation. The procedure is applied to experimental datasets taken from the literature. Parameter estimation is carried out by applying the Maximum Likelihood Principle and by taking into account the possible presence of runout specimens with unequal number of cycles. The application of the procedure permits to estimate different key material parameters (e.g., the characteristic parameters of transition stress and fatigue limit), as well as to statistically predict the failure mode of each tested specimen.

Duplex S-N fatigue curves: statistical distribution of the transition fatigue life

In recent years, very-high-cycle fatigue (VHCF) behavior of metallic materials has become a major point of interest for researchers and industries. The needs of specific industrial fields (aerospace, mechanical and energy industry) for structural components with increasingly large fatigue lives, up to 10 10 cycles (gigacycle fatigue), requested for a more detailed investigation on the experimental properties of materials in the VHCF regime. Gigacycle fatigue tests are commonly performed using resonance fatigue testing machines with a loading frequency of 20 kHz (ultrasonic tests). Experimental results showed that failure is due to cracks which nucleate at the specimen surface if the stress amplitude is above the conventional fatigue limit (surface nucleation) and that failure is generally due to cracks which nucleate from inclusions or internal defects (internal nucleation) when specimens are subjected to stress amplitudes below the conventional fatigue limit. Following the experimental evidence, the Authors recently proposed a new statistical model for the complete description of S- N curves both in the high-cycle-fatigue (HCF) and in the VHCF fatigue regions (Duplex S-N curves). The model differentiates between the two failure modes (surface and internal nucleation), according to the estimated distribution of the random transition stress (corresponding to the conventional fatigue limit). No assumption is made about the statistical distribution of the number of cycles at which the transition between surface and internal nucleation occurs (i.e., the transition fatigue life). In the present paper, the statistical distribution of the transition fatigue life is obtained, according to the statistical model proposed. The resulting distribution depends on the distance between the HCF and the VHCF regions and on the distribution of the random transition stress. The estimated distribution can be effectively used to predict, with a specified confidence level, the number of cycles for which an internal nucleation may probabilistically occur in a VHCF test and it is also informative for properly choosing the end of HCF tests in terms of number of cycles. A numerical example, based on experimental datasets taken from the literature, is provided.

Gaussian Specimens for Gigacycle Fatigue Tests: Evaluation of Temperature Increment

Experimental tests investigating very-high-cycle fatigue (VHCF) properties of materials are commonly performed with ultrasonic testing machines, which allow for a significant reduction of testing time. In order to evaluate the effect of tested material volume (size-effect) on VHCF properties, the Authors recently proposed to adopt Gaussian specimens for VHCF tests. Investigation of size-effect with Gaussian specimen induces large mechanical power dissipation and temperature increment that must be taken into account. The present paper proposes an analytical model, which allows to approximately predict the dissipated mechanical power and the temperature increment in Gaussian specimens. The analytical model is also numerically verified through a Finite Element Analysis.

Analytical design of gigacycle fatigue specimens for size effect evaluation

Research activities on gigacycle fatigue properties of materials have recently focused on the influence of specimen size used for ultrasonic fatigue tests. In order to increase the volume of material subjected to the maximum stress (risk volume) a new specimen shape with a Gaussian profile is proposed. An approximation of the Gaussian profile with an arc of circle is presented in the paper, in order to simplify the Gaussian specimen manufacturing process. The effect of the approximation is evaluated analytically and through a finite element analysis.