numero25

P. Lorenzino et alii, Frattura ed Integrità Strutturale, 25 (2013) 138-144; DOI: 10.3221/IGF-ESIS.25.20 139 approaching the grain boundary until a new slip band is initiated in the neighbouring grain, along which the crack will propagate next. This nucleation process of slip bands has to be repeated afresh in each grain. The study of the fatigue limit of notched components is another case of interest. Here the problem is one of short crack growth, with the additional difficulty that the stress field through which the crack is growing has usually a very steep gradient. It has been found that in sharp notches it may be possible for a crack to grow through a few grains and then to become non-propagating. This suggests that the critical event in notch fatigue may not be the initiation of the crack itself, but rather the relative capacity of this crack to overcome successive microstructural barriers when the stresses that are driving it diminish rapidly. Typical non-propagating cracks found in sharp notched specimens of carbon steel have lengths of the order of a few tens of microns. We have devised [1] a simple experimental technique whereby all this can be studied with an unprecedented level of ease and detail. The innovative aspect of this technique is the use of specially developed test coupons with grain sizes of a few millimeters - or even centimeters - and the use of low magnification USB cameras by means of which the crack growth process and the interactions with the microstructure can easily be registered and examined. Digital image correlation techniques have also been employed to enhance the technique. E XPERIMENTAL PROCEDURE Thermo-mechanical treatment tudies carried out by the research group of the University of Seville [2], have shown that it is possible to produce a substantial increase in the grain size of commercially pure aluminium sheets by means of a combination of two thermal treatments and an intermediate moderate cold working. The process is simple enough and easy to control, and it provides highly repeatable results. Aluminium 1050 Puraltok 99.5-H24 by Alu-Stock is used in sheets of 4.0 mm thick. Chemical composition: 99.56; Cu: 0.08; Fe: 0.2; Si: 0.1. The aluminum sheets are cut into pieces of 45x300 mm, in parallel to the lamination direction. A tubular furnace is used for the thermal treatments (Carbolite model 215GHA12). The aim of the first thermal treatment is to obtain a deformation-free equiaxial structure; different treatments were tested by changing the recrystallization temperature, the heating rate and the interval at constant temperature. After observing the resulting microstructures, a heating rate of 2.6ºC/min was chosen, from ambient temperature to 550ºC. The constant temperature has to be kept during 5 hours, followed by air cooling. After 5 hours, the surface grains become larger compared to the inner grains, and the larger size along the specimen creates a non uniform deformation along the specimen thickness at the following stage (mechanical treatment). Next, cold working is performed in a MTS 810. The deformation level applied at this stage will determine the size of the grain after second recrystallization. The following deformation percentages were chosen: 0−8−11−14−18%. This treatment is carried out by a MTS 810 controlling each deformation. 0% corresponds to the material undergoing only the first recrystallization. The third stage is again a temperature ramp from T amb to T=550 ºC at a rate of 2.6 ºC/min; then, the temperature is to be kept constant during 15 hours and that it is increased again to 575 ºC and kept at that temperature during 1 hour. Finally, it is air cooled. At this stage new crystals growing at the expense of the old ones are formed. The final size obtained depends largely on the level of plastic deformation applied. Figure 1 shows an example of the possible microstructures obtained and the corresponding level of total deformation applied, the upper image corresponds to a specimen which has undergone 8 % of total strain during the mechanical treatment and the bottom image corresponds to a specimen subjected to 14% of total strain. Figure 1 : Microstructure obtained depending on the degree of applied strain. S