Luis P. Canal

Traction-Separation Relations in Delamination of Layered Carbon-Epoxy Composites Under Monotonic Loads: Experiments and Modeling

It is well known that large-scale bridging accompanying delamination and fracture in layered composites is among the most important toughening mechanisms. The resulting resistance to fracture, however, is dependent on the specimen geometry rendering its modeling difficult. As a consequence, characterization of the tractions on the wake of the crack, the so-called bridging zone, is very important in the efforts to predict the loading response of composite structures. In this chapter, experimental results and modeling of delamination and fracture in layered composite specimens are discussed. The experimental part consists of displacement-controlled monotonic tests of interlaminar and intralaminar fracture. Selected specimens are equipped with wavelength-multiplexed fiber Bragg grating (FBG) sensors to monitor crack propagation and strains over several millimeters in the wake of the crack. The modeling part involves an iterative scheme to calculate the traction-separation relation, due to bridging, using the strains from the FBG sensors, parametric finite elements, and optimization. The experimental results demonstrate an important effect of specimen thickness in interlaminar and intralaminar fracture: the bridging zone length at steady state linearly increases with specimen thickness, while the maximum bridging stress is independent of thickness in each case. Results of a similar study in cross ply specimens, limited to a selected specimen thickness, show important effects of specimen width on the extent of large-scale bridging. The obtained traction-separation relations for each investigated case are employed in cohesive zone simulations to predict the corresponding load-displacement curves. On the basis of the experimental results, a micromechanics model is used, based on an embedded-cell model, to predict the observed specimen thickness effects on large-scale bridging. The results of the reported studies demonstrate that the so-called bridging law is not a material parameter and the proposed methods of analysis, predict very well the load-displacement response.

On the mechanical characteristics of a self-setting calcium phosphate cement

OBJECTIVE To perform a mechanical characterization of a self-setting calcium phosphate cement in function of the immersion time in Ringer solution. MATERIALS AND METHODS Specimens of self-setting calcium phosphate cement were prepared from pure α-TCP powder. The residual strains developed during hardening stage were monitored using an embedded fiber Bragg grating sensor. Additionally, the evolution of the elastic modulus was obtained for the same time period by conducting low-load indentation tests. Micro-computed tomography as well as microscope-assisted inspections were employed to evaluate the porosity in the specimens. Moreover, diametral compression tests were conducted in wet and dried specimens to characterize the material strength. RESULTS The volume of the estimated porosity and absorbed fluid mass, during the first few minutes of the materials exposure in a wet environment, coincide. The immersion in Ringer solution lead to a noticeable increase in the moduli values. The critical value of stresses obtained from the diametral compression tests were combined with the data from uniaxial compression tests, to suggest a Mohr-Coulomb failure criterion. CONCLUSIONS This study presents different techniques to characterize a self-setting calcium phosphate cement and provides experimental data on porosity, mechanical properties and failure. The investigated material possessed an open porosity at its dried state with negligible residual strains and its Youngs modulus, obtained from micro-indentation tests, increased with hardening time. The failure loci may be described by a Mohr-Coulomb criterion, characteristic of soil and rock materials.