Alejandro Yawny

Fracture Micromechanisms of Fibre-Metal Laminates: In-Situ SEM Observations

The monotonic fracture micromechanisms of an aramid-aluminum laminate were studied using very small single edge bend specimens, SE(B) tested in a small-instrumented testing machine inside a scanning electron microscope. Thefracture process was followed by simultaneous observation of the sample and recording the loading variables. The instability fracture toughness was strongly dependent on the fibre-reinforced epoxy layer and the crack-growth in the external aluminum layers, prior to fracture instability, was influenced by the notch acuity. The load versus loadline displacement behaviourand load instabilities were similar to the ones obtained by conventional testing procedures. The technique implemented proved to be a powerful tool to study the fracture micromechanisms of aramid-aluminum laminates or other fibre-metal laminates and to correlate them with the recorded macro events.

In Situ Fracture Toughness Measurement Using Scanning Electron Microscopy

In situ tests were developed to measure fracture toughness (KIc, JIc, CTOD, R-curves) in small samples, simultaneously observing details of the crack blunting, initiation, and propagation by using scanning electron microscopy (SEM). A small load frame was employed for this purpose and in situ tests were performed in a Philips 515 SEM with a small chamber. A load-load line displacement record could be obtained from the different types of tests in order to calculate any fracture mechanics parameter, determine the initiation and amount of stable crack growth, measure directly the CTOD or Schwalbes δ5 and correlate any of these parameters with the observed micromechanisms. The applicability of the proposed in situ fracture toughness measurement technique has been exemplified making use of a wide set of materials, and its limitations were also evaluated.

Fracture toughness in metal matrix composites

Evaluations of the fracture toughness in metal matrix composites (Duralcan reinforced with 15% of Al203 and SiC) are presented in this work. The application of Elastic Plastic Fracture Mechanics is discussed and the obtained values are compared with the ones obtained by means of Linear Elastic Fracture Mechanics. Results show that JIC derived KJC values are higher than the corresponding values obtained by direct application of the linear elastic methodology. The effect of a heat treatment on the material fracture toughness was also evaluated in which the analyzed approaches showed, not only different toughness values, but also opposite tendencies. A second comparison of the JIC and KJC values obtained in this work with toughness values reported in the literature is presented and discussed.

Thermal Effects in a Mechanical Model for Pseudoelastic Behavior of NiTi Wires

A mechanical model for pseudoelastic behavior of NiTi wires is proposed with the aim to predict the behavior of Shape Memory Alloys(SMA) damping wire elements in model structures. We have considered at first a simple linearwise stress-strain relationship to describe the basic isothermal behavior of the SMA members. Then, this basic model is modified in order to include the effect of the strain rate. The model is based on detailed experimental characterization performed on a Ni rich NiTi superelastic wire which included the study of the localized character of the deformation and the local heat generation associated with the stress induced martensitic transformation occurring in these alloys. Heat conduction along the wire and heat interaction with the surroundings was also considered. In that way, the resulting local temperature field around the transformation front is assessed and its effect on the progression of the transformation is evaluated. It is shown how the simple mechanical model reproduces the global mechanical behavior, including the existence of a maximum in the damping capacity with the transformation rate.

Effect of Variable Amplitude Blocks’ Ordering on the Functional Fatigue of Superelastic NiTi Wires

Accumulation of superelastic cycles in NiTi uniaxial element generates changes on the stress–strain response. Basically, there is an uneven drop of martensitic transformation stress plateaus and an increase of residual strain. This evolution associated with deterioration of superelastic characteristics is referred to as “functional fatigue” and occurs due to irreversible microstructural changes taking place each time a material domain transforms. Unlike complete cycles, for which straining is continued up to elastic loading of martensite, partial cycles result in a differentiated evolution of those material portions affected by the transformation. It is then expected that the global stress–strain response would reflect the previous cycling history of the specimen. In the present work, the consequences of cycling of NiTi wires using blocks of different strain amplitudes interspersed in different sequences are analyzed. The effect of successive increasing, successive decreasing, and interleaved strain amplitudes on the evolution of the superelastic response is characterized. The feasibility of postulating a functional fatigue criterion similar to the Miner’s cumulative damage law used in structural fatigue analysis is discussed. The relation of the observed stress–strain response with the transformational history of the specimen can be rationalized by considering that the stress-induced transformation proceeds via localized propagating fronts.