duralumina

Tensile test on Duralumin:

What is Duralumin and where is it used? Duralumin is a hard, light alloy of aluminium with copper and other elements. It is ductile and malleable and hence has many applications as sheets and plates in structural components for aerospace application and military equipment. It is also used in aircraft structures, rivets, hardware, truck wheels, screw machine products, and other structural applications.

What happens when Duralumin specimen is subjected to axial tensile stress?

In duralumin specimen, when subjected to axial tensile stress, there is a plastic deformation ahead of crack. This is known as necking which begins at the ultimate stress point. After necking, the duralumin specimen fails resulting in cup and cone fracture surface as shown below.

                      Duralumin cone                                                                 Duralumin cup

The fracture in Duralumin is initiated at microvoids, which can be seen in the microstructure given below (obtained using Scanning Electron Microscope). These coalesce to form an internal crack. Final failure occurs when the shear stress causes the remaining cross section to tear. The shear stress is greatest at 45 degrees to applied load and hence forms angled walls, resulting in distinctive cup and cone profile.

                                            SEM image of the fracture surface of duralumin

                                

called "bronze disease" will eventually completely destroy it.[22] Copper-based alloys have lower melting points than steel or iron, and are more readily produced from their constituent metals. They are generally about 10 percent denser than steel, although alloys using aluminium or silicon may be slightly less dense. Bronzes have lower hardness, strength and elastic modulus—bronze springs, for example, are less stiff and so store less energy for the same deflection. Bronze resists corrosion (especially seawater corrosion) and metal fatigue more than steel and is a better conductor of heat and electricity than most steels. The cost of copper-base alloys is generally higher than that of steels but lower than that of nickel-base alloys.

Ductile fracture in bronze

Ductile fracture is a type of fracture characterized by extensive deformation of plastic or "necking." This usually occurs prior to the actual fracture. The term "ductile rupture" refers to the failure of highly ductile materials. In such cases, materials pull apart instead of cracking.

In ductile fracture, there is absorption of massive amounts of energy and slow  propagation before the fracture occurs.

Ductile fracture in bronze

Center flat region is “woody,” a common visual term which often but not always corresponds to MVC fracture. Profuse hole nucleation reduces stiffness of material, accentuates shear localization, and can produce unstable shear fractures, although locally the facture is entirely MVC.

  Ductile Fracture in bronze magnified 5000 times

Classic cup-cone fracture, shown here in specimens of aluminum bronze (Cu-6% Al).

The same process can occur late in the growth process of large voids, producing the same kind of shear fracture between macrovoids. Sectioning tensile specimens during the necking process can reveal void accumulation in the specimen center.Sometimes initiating particles are visible on the fracture surfaces within microvoids.