Marino Quaresimin

Damage accumulation under multiaxial fatigue loading

Abstract In this chapter, the fatigue behavior of composite laminates under multiaxial loading is analyzed and discussed. In the scientific literature it is widely shown that the multiaxial stress state as well as the load ratio (minimum to maximum load) are strong factors in determining the cycles spent for the final failure of laminates. Fewer studies report that also damage evolution, consisting of the initiation and propagation of multiple off-axis cracks, depends on the degree of multiaxiality. Understanding and predicting these phenomena, which are governed by matrix and interface properties, is of fundamental importance, mainly for a stiffness-based fatigue design. With the aim to provide a deeper insight into this area an extensive experimental study was carried out by the present authors using glass–epoxy tubular samples under combined tension and torsion. It was found that the life to crack initiation, the crack propagation rate, and the damage mechanisms at the microscopic scale are strongly dependent on the value of the shear stress relative to the transverse stress. In addition, the influence of the load ratio was investigated for three different multiaxial conditions. Finally, it will be shown that the characterization of multiaxial fatigue behavior, in terms of crack initiation and propagation, can be suitably conducted by means of properly designed flat laminates under uniaxial loading, exploiting the local multiaxial stress state arising from the material anisotropy.

A Model for the Energy Absorption Capability of Composite Laminates

A new methodology for the analysis of impact data has been recently proposed [1], based on the definition of two normalised coefficients, the absorption coefficient (ratio of absorbed to penetration energy) and the intensity coefficient (ratio of impact to penetration energy). The new approach allowed the definition of an empirical master curve suitable to summarize the impact data irrespectively of material system, laminate thickness and lay-up. This work is oriented to quantitatively validate the master curve on the basis of experimental impact data previously obtained and other “external” data taken from the open literature, referred to laminates with significant variation of properties in terms of type and architecture of the reinforcing fibers, lay-up and thickness. In spite of the very large number of data collected for the validation, the master curve turned out suitable to describe all the normalized data with a reasonably reduced scatter.Copyright