Special mechanical testing of aluminum
The standard mechanical test for metals is the tensile test.. In this case, the following mechanical properties of the metal are obtained: tensile strength, yield strength and elongation.
Special mechanical tests include:
- Compression
- Bearing
- Shear
- Hardness
- Ductility
Fig. 1 – The Test Methods of Compression Strength, Bearing and Shear [1]
Compressive strength
The strength test of aluminum alloys under compressive load is little used in practice.. This is because, that when loading structural elements in compression, warping often occurs, and the actual compressive strength of the metal is rarely achieved. For most engineering purposes, it is common to use the same design stress for compressive work., as for tensile work.
In the testing machine, aluminum alloy will show higher compressive strength, than tensile. This also happens because, that the cross-sectional area of the samples increases in this case. Cylindrical specimens made from softer aluminum alloys can be compressed into thick discs without cracking.. Harder alloys tend to crack.
Bearing capacity (collapse strength)
aluminum crushing strength is also difficult to determine, testing and communicate with conventional strength properties, as for the other metals. Wrinkling often an important criterion for structures using compounds rivets and bolts and therefore "crushing strength" is a widely recognized feature. Collapse strength is quite arbitrarily defined as pressure (per unit effective collapse area), a pin attached in a circular hole. This hole for distributing pre 2 % from the original diameter (figure 1). This strength for most aluminum alloys is 1,8 from tensile strength (ultimate strength) (figure 2).
Fig. 2 – Compression, Shear, Bearing Strength and Hardness for Different Aluminium Alloys [1]
Shear strength
loading scheme when testing shear shown in Figure 1. For deformable aluminum alloys ratio shear strength to the tensile strength varies depending on the chemical composition and manufacturing method from 0,5 to 0,75 (cm. Figure 2). In the absence of data on the shear strength it usually takes 0,55 of tensile strength.
Rivets made of aluminum grades and Al-Mn alloys (3xxx series) are produced by cold deformation methods with shear strength reaching up to 200 MPa. Rivets of thermally hardenable alloys are produced in the annealed condition, then immediately subjected to hardening and natural aging to achieve shear strength up to 260 MPa.
Hardness
Resistance local plastic deformation of the material, arising from the introduction of a more solid body into it - an indenter – It is a rough indicator of the state of the alloy, and therefore widely used in the production control. Brinell methods are used for aluminum alloys (steel ball), Vickers (diamond pyramid) and Shor (falling diamond cone). Brinell hardness varies from 20 units of pure aluminum to 175 units for thermally hardened alloy 7075 (cm. Figure 2). Hardness readings, usually, not calculated their tensile strength, as is usually done for steels, since for aluminum alloys the ratio of these two characteristics is far from constant.
formability
Elongation of the sample in a tensile test is useful information, but it is not enough to fully understand the properties of the plastic alloy. Therefore, for different types of products depending on the destination using various additional technological tests.
To assess the ability of the metal to the second finishing is often used simple test Bend. The strip of articles is bent through 90 ° or 180 ° on the mandrels predetermined diameter. Applying successively decreasing diameters of the mandrels can receive a minimum bend radius, where no cracks occur. For pipes criterion may be the degree of flattening.
To evaluate ductility sheets, for example, deep drawn, frequently used test Erichsen, wherein hemispheric punch is pressed into predetermined dimensions sample sheet, mounted in a special matrix, with the formation of a bowl-shaped hole (figure 4). The depth of the hole obtained (before the formation of a crack) is determined by the indications of the corresponding scales of the testing device. This depth is indicative of the suitability of metal, for example, to the deep stamping.
Fig. 3 – Erichsen Cupping Test: Effect of Sheet Thickness [1]
The value of this test is, it is capable of detecting defects such as a coarse metal structure and excessive anisotropic properties. When it obtained coarse grains severely roughened surface of the wells or early degradation due to local necking. The anisotropy of the properties affects the fracture shape in the well - in the absence of the anisotropy it extends circumferentially.
Source:
- TALAT 1501
