Brittle fracture of aluminum

Benefits of low modulus of elasticity

It is known, that low aluminum alloys modulus is an advantage, for example, to steel, when the structure is subjected to shock loads. The structural member of aluminum alloy will absorb three times more elastic energy, than the steel member with the same inertia and tensile strength.

brittle failure

By brittle fracture materials exhibit a tendency to realize rapid crack propagation without significant plastic deformation. Information about this type of fracture is vital for the design of structures, that operate under considerable stress and are subject to a large amount of elastic energy, when the rapid destruction can be catastrophic.

Shock aluminum Charpy and Izod

Data on energy absorption of impact test specimens notched Charpy or Izod, as for the other metals, can not be directly used in the construction of. Charpy impact test and Izod are widely used to determine the temperature of steel brittle transition, particularly for ferritic steels. Brittle transition temperature - a temperature, whereby alloys begin to exhibit brittle fracture characteristics. However, these tests are not usually suitable for aluminum and its alloys, as, Unlike steel, they do not exhibit brittle transition. Moreover, notched impact test specimens of aluminum alloy are almost independent of temperature in the temperature range from room temperature to -270 ° C. Little of, most wrought alloys such matings, that the test pieces do not break. Therefore, from these tests it is difficult to get some useful information for a particular design.

Viscosity aluminum fracture

Known test fracture toughness determination method stress intensity factor binds brittle fracture strength of the material with the size of the defect or crack. Fracture toughness is regarded as a resistance material unstable crack growth under elastic tension or any type of non-ductile failure. fracture toughness test requires the sample to initiate a crack or a predetermined length at its cultivation fatigue loading. The sample and its loading scheme when determining the fracture toughness is shown in figure 1.

Fig. 1 – Fracture Toughness test Piece [1]

The relationship between the stress intensity factor K, uniform stress σ_a and crack length 2a It is defined as the ratio K = Ϭ_a (2Pia)^1/2. The stress intensity factor K (at the beginning of unstable crack growth) decreases with increasing sample thickness and reaches a minimum value, which referred to the_Ic- critical elastic stress intensity factor or fracture toughness plane strain. The value of K_IcIt is an analog of the yield stress, as it is - the minimum stress intensity, at which brittle fracture may start at a given temperature and at a sufficient thickness of the sample or article for plane strain state conditions. However, this approach is not suitable for alloys with a high ductility, since they do not show a rapid crack propagation in elastic conditions. Therefore, this approach is usually limited thermally hardened high strength alloys.

Tensile Test on Kanu

In the international practice to evaluate the effect on viscosity characteristics of aluminum alloys of the chemical composition, manufacturing technology, state, etc.. The Kahn tensile test is widely used. The advantage of this method is, that with its help directly measure the amount of energy, which is necessary for crack propagation, even for the most viscous of aluminum alloys. Moreover, This method does not require such "thick" samples, as a method of determining the fracture toughness K_Ic , and it is therefore suitable for more kinds of products.

In this test, the energy, which is required for the nucleation and growth of a crack in the test piece of special shape, is calculated from the corresponding areas under the tension curve (figures 2 and 3). Energy, which is required for the growth of the crack, divided into a working cross-sectional area of ​​the sample and is called "energy density crack growth". It is a measure of resistance to fracture and crack indirectly - a measure of fracture toughness.

Fig. 2 – Tear Test Piece [1]

Fig. 3 – Tear-Test Load Deformation Curves [1]

The specific energy of the crack growth can be directly related to the strain energy release rate, which coincides with the fracture mechanics approach and therefore provides a realistic measure of rapid crack growth resistance. established, that the specific energy of the crack propagation satisfactorily correlated with the values ​​of K_I and K_Ic.

The ability to resist crack growth is still high for most aluminum alloys, even at very low temperatures, and in the case alloy 6061 even increases significantly (figure 4). For most aluminum alloys the ability to deform plastically, and resist crack growth is so great, that unstable crack growth in an elastically strained material, ie, brittle fracture, simply impossible.

Fig. 4 – Unit Propagation Energies of Aluminium Alloys at Various Temperatures [1]

The viscosity of the notch

A convenient way to represent the toughness of an alloy is to calculate the so-called "notch toughness": the ratio of the tensile strength of a notched specimen to the yield strength of an unnotched specimen. Viscosity incision most aluminum alloys remains constant up to cryogenic temperatures, except high 7xxx series alloys, as shown in Figure 5 alloy 7075.

Fig. 5 – Toughness Properties of Aluminium Alloys at Low Temperatures [1]

Source:

1. TALAT 1501