M. Ricotta

Crack propagation analysis in composite bonded joints under mixed-mode (I+II) static and fatigue loading: a damage-based model

In this paper, a first step towards the definition of a unifying model based on the process zone concept has been attempted. The aim of the proposed model is to summarize the fatigue behaviour of composite bonded joints under mixed-mode (I + II) fatigue loading conditions focusing as close as possible on the actual damage mechanisms observed during the fatigue tests, which were found strongly dependent on the mode mixity. The proposed model has been verified on the experimental results obtained from double cantilever beam (DCB), end notch flexure (ENF) and mixed-mode bending (MMB) tests as well as from single-lap bonded joints. Finally, the model is successfully applied to the description of joint behaviour under static mixed-mode loading.

Crack propagation analysis in composite bonded joints under mixed-mode (I+II) static and fatigue loading: experimental investigation and phenomenological modelling

This paper illustrates the results of an extensive experimental investigation on composite bonded joints under mixed-mode (I+II) static and cyclic loading conditions oriented to understand the influence of the mode mixity condition on the crack propagation resistance at the bondline. The double cantilever beam (DCB), end notch flexure (ENF) and mixed-mode bending (MMB) tests were conducted on pre-cracked samples and both fracture toughness and crack propagation resistance were seen to increase, both for static and fatigue loading, respectively, as the mode II contribution increases. The crack propagation and damage evolution were carefully investigated and documented, and a strong dependence of the propagation mode on the mode mixity was found. Fatigue data under the different loading conditions are then described by a phenomenological model based on the strain energy release rate contributions, which represents a useful engineering tool for preliminary design. After that a damage-based model, developed on the basis of the actual damage mechanisms, is presented in a companion paper.

Experimental evaluation of fatigue damage in two-stage loading tests based on the energy dissipation

Heat energy dissipation is a manifestation of damage accumulation in fatigue-loaded components. Once recognized that some mechanical energy has to be expended to fatigue a material, energy partition into heat and stored energy is thought of as a material property in the present testing conditions. However, most of the mechanical input energy is dissipated as heat; therefore, the stored energy is difficult to estimate as difference between the expended and the dissipated energy. In this article heat energy is assumed as an index of fatigue damage. Since it reflects the material response to external loading, heat energy was seen to reduce the scatter of constant amplitude fatigue test results with respect to the use of the stress amplitude. Moreover, two-level fatigue test results could be interpreted with a higher level of accuracy when Miner’s rule was applied in terms of energy rather than stress amplitude.

A Synthesis of the Fatigue Behavior of Stainless Steel Bars under Fully Reversed Axial or Torsion Loading by Using the Specific Heat Loss

In previous papers, the energy dissipated to the surroundings as heat in a unit volume of material per cycle, Q, has been successfully applied to correlate experimental data generated from push-pull, stress- or strain-controlled fatigue tests on AISI 304 L stainless steel plain and notched specimens. In this paper the fatigue behaviour of AISI 304 L un-notched bars under fully-reversed axial or torsional loading was investigated. By using the Q parameter it was found that the experimental data collapse into the same energy-based scatter band previously determined with the push-pull tests. The results found in the present contribution are meant to be specific for the material investigated.

Fatigue Behaviour of a Stainless Steel Based on Energy Measurements

In this paper the low cycle fatigue behaviour of an AISI 304L stainless steel is analysed on the basis of energy concepts. In particular during the fatigue tests different forms of energy in a unit volume of material per cycle involved in the fatigue process were measured: the mechanical energy expended was evaluated from the area of the hysteresis loops, while the energy released as heat by the specimen to the surroundings was estimated from surface temperature measurements by means of an infrared camera. By subtracting the mechanical input energy and that released as heat, the energy stored in a unit volume of material at fracture was calculated for each tested specimen. The mean value obtained from different specimens is in agreement with the energy absorbed by the material in a static test.

Fatigue and Notch Mechanics

Linear Elastic Notch Mechanics (LENM) extends the concepts of the well known Linear Elastic Fracture Mechanics (LEFM) to notches having root radius different from zero and arbitrary notch opening angle. LENM is based on fundamental analytical results and definitions introduced by Williams [43] and Gross and Mendelson [17]. From the experimental point of view, it has been applied for the first time by Haibach [18] on pure phenomenological basis to analyse the fatigue strength of welded joints using the strain gauge technique. Subsequently, LENM was developed thanks to the progressively increasing use of the Finite Element Method (FEM). Nowadays, NM has been formalised and applied to structural strength assessment of components. Different application techniques exist, but the theoretical frame remains unchanged [6, 8, 12, 20, 21, 23, 24, 28, 31, 37, 38, 42]. The present paper, after recalling the classic notch fatigue design criterion and the LEFM, aims at illustrating the link between Notch Mechanics and those classic approaches. In particular the aim is two-fold: on one side the use of Notch Mechanics in notch fatigue design will be illustrated, on the other side it will be shown how it can be used to better analyse and explain in deep the fundamentals of the classic approaches mentioned previously.

A Three Dimensional Graphical Aid for Fatigue Data Analysis

The fatigue behaviour of materials is usually synthesised in terms of stress-life (S-N) curve or in terms of strain-life (e-N) curve, the latter being described by the so-called Manson-Coffin equation. It is known that the assumption of equality of the plastic and elastic components between the Manson-Coffin and the stabilised stress-strain curves leads to the so-called compatibility conditions which connect the equations theoretically. The material constants of the Manson-Coffin and of the stabilised stress-strain curve are commonly determined by best fitting separately the experimental data obtained from strain-controlled fatigue tests. As a consequence the compatibility conditions may not be fulfilled. In this paper a method for fatigue data analysis that ensures the compatibility conditions is proposed and validated against experimental data.

Life prediction for bonded joints in composite material based on actual fatigue damage

After presenting an overview of fatigue behaviour and damage mechanics in single lap composite bonded joints, this chapter illustrates a model which describes the fatigue life of the joint as the sum of initiation and propagation up to failure of an average crack. Life to crack initiation is calculated by using a stress intensity factor approach and life spent in the propagation phase by the integration of a crack growth power law. Details of the analytical and numerical tools required by the model, the procedure for its application and the validation against experimental results are discussed.