J. Perez Ipiña

Effects of hydrogen content and temperature on fracture toughness of Zircaloy-4

The influence of hydrogen content and temperature on the fracture toughness of a Zircaloy-4 commercial alloy was studied in this work. Toughness was measured on CT specimens obtained from a rolled material. The analysis was performed in terms of J-integral resistance curves. The specimens were fatigue pre-cracked and hydrogen charged before testing them at different temperatures in the range of 293–473 K. A negative influence of the H content on material toughness was important even at very small concentrations, being partially restored when the test temperature increased. Except for some specimens with high H concentration tested at room temperature, the macroscopic fracture behaviour was ductile. The role of Zr-hydrides and Zr(Fe,Cr)2 precipitates in the crack growth and the dependence with hydrogen content were analysed by observation of the fracture surfaces and determination of the Zr(Fe,Cr)2 precipitates density on them.

In situ crack growth observation and fracture toughness measurement of hydrogen charged Zircaloy-4

The effect of hydrogen on the fracture behaviour of a Zircaloy-4 alloy was analysed performing simultaneous fracture mechanics tests of small SE(B) specimens and in situ observation of crack initiation and propagation inside the chamber of a scanning electron microscope. Load and displacement were continuously measured and JIC, J–R curves and CTOD determinations were obtained. Detailed images of the zone close to the crack tip were taken and the resistance to crack growth was correlated with hydrogen content and hydride morphology. The size and orientation of hydride precipitates showed an important influence on the fracture process. A good agreement with results obtained using standard CT specimens was met.

Degradation of the mechanical properties of Zircaloy-4 due to hydrogen embrittlement

During nuclear reactor operation, the embrittlement of components made of zirconium-based alloys is observed. The degradation of their mechanical properties is due to the combined effect of hydrogen absorption and the damage caused by neutron irradiation. In this work we studied the influence of hydrogen content on the fracture toughness of a Zircaloy-4 alloy. Compact tension (CT) specimens were obtained from a hot-rolled, annealed and finally cold-rolled material. The observed microstructure consisted of α-Zr rounded grains with diameters of about 15 μm. Selection of the tested material was guided by the need to perform experiments on samples with a texture equivalent to the cladding components of Candu-type nuclear reactors. The specimens were fatigue precracked and hydrogen charged before testing. Two different reactions were performed. Specimens with a final hydrogen content ranging from 10 to 400 ppm were obtained by electrochemical charging and those with a final concentration of up to 2000 ppm were charged by absorption under a gaseous atmosphere. In both cases, an homogeneous distribution of dissolved hydrogen and hydride phases was obtained. The dependence of the toughness on temperature and hydrogen content was measured on CT specimens. The analysis was performed in terms of J-integral and resistance curves.

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.

Mechanical Properties Degradation at Room Temperature in ZRY-4 by Hydrogen Brittleness

A hot rolled Zircaloy-4 alloy, annealed with a final cold rolling, presenting rounded grains, was studied. Hydrogen cathodic charge with a homogenization heat treatment was used to pre-charge the specimens with different hydrogen contents. Hydrogen embrittlement susceptibility analysis was held using J integral and J-R curve results from CT specimens (compact tension specimens) tested at room temperature. As JIC values showed scatter, toughness was evaluated for Da = 1mm. Toughness clearly tended to decrease as hydrogen content increased abruptly for low H contents and gradually for high contents. A few specimens with high hydrogen content failed in brittle mode, or presented instability and posterior crack arrest. Fractographic observations showed that, despite the records had presented no signs of brittle fracture, certain specimens showed cleavage-like zones. More cleavage-like area percentage was present the higher the hydrogen content was.

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.

Determination of CTOD C in Fibre Metal Laminates by ASTM and Schwalbe Methods

Fibre Metal Laminates (FMLs) have arisen as a demand of the aeronautical industry to use thin sheets with high resistance to fatigue crack growth, high damage tolerance, corrosion resistance and high specific strength. Considering these requirements, FMLs are an advantageous choice when compared to metal alloys currently used. In order to employ FMLs in aircraft structures, designers must hold a deep knowledge of a wide set of their properties including fracture toughness. The aim of this work was to evaluate the available methodologies to measure fracture toughness at instability (CTODC) in unidirectional fibre metal laminates reinforced with aramid fibres (ARALL®). To achieve this, tests were performed to obtain traditional and Schwalbe CTODs by using experimental ASTM based techniques, especially adapted to these laminates. Results achieved point out that Schwalbe method is more appropriate and also that there are differences between both CTOD parameters.

Reproducibility of pop-ins in laboratory testing of welded joints

The pop-in phenomenon, quite common in fracture mechanics tests of welded joints, corresponds to a brittle crack initiation grown from a local brittle zone (LBZ) that is arrested in reaching the higher toughness material that surrounds this LBZ. A methodology to obtain a high percentage of pop-in occurrence in laboratory testing is necessary to study the pop-in significance. Such a method is introduced in this work and includes the consumable combination and welding procedures for the SMAW welding process to generate artificial LBZ. In order to find out the influence of the loading state upon the pop-in phenomenon, laboratory CTOD tests were performed using two specimen configurations: some single edge-notched specimens were loaded on a three-point bending (SE(B)) fixture while others were tested in tensile load (SE(T)). A higher frequency of pop-in occurrence was observed in the SE(B) geometry.

On the Applicability of Elastic-plastic Fracture Mechanics to Discontinuously Reinforced Aluminum Composites

Fracture toughness of AA6061/Al2O3/15p and A359/SiC/15p discontinuously reinforced aluminum alloys was evaluated using an elastic-plastic methodology (JIC). KJC values derived from JIC resulted higher than critical K values obtained by the ‘straight’ application of procedures proper to linear elastic methodology. The discrepancy between both procedures is demonstrated by means of analyzing the incidence of an arbitrary annealing heat treatment on the material fracture toughness. The results indicate that elastic-plastic fracture mechanics rather than linear elastic fracture mechanics is the appropriate approach for fracture toughness evaluation of reinforced aluminum alloys.