Tensile Strength, Impact Strength, Hardness and Corrosion Resistance

During manufacture and assembly of products, there is a wide range of testing and inspection carried out to ensure the materials and items satisfy their specifications or are fit for the required purpose. Routine testing is often in-house and it is vital that the limits of the test methods are known and the meaning of the results are understood.

The same understanding is needed when interpreting specifications for quotations or when assessing test results supplied by others. This section details the materials property tests most frequently required. Related Australian Standard designations are provided where relevant.

Mechanical Properties

The ductility and strength (measured by a tensile test), related hardness properties and fracture toughness (or impact resistance) are the three most frequently required materials properties.

A secondary batch include properties related to torsion, shear, bending and fatigue (although usually on components rather than raw materials) with additional measurements of creep behaviour required for elevated temperature service.

Service behaviour such as wear resistance is usually inferred from other tests while intrinsic properties such as density and damping capacity are generally only considered by the designer.

Tensile Testing

Tensile tests are usually carded out on wire, strip or machined samples with either circular or rectangular cross section. Test pieces are screwed into or gripped in jaws and stretched by moving the grips apart at a constant rate while measuring the load and the grip separation.

This data is plotted as load vs extension and then converted to engineering stress (load/original area) vs engineering strain (fractional change in length over the test section assuming the deformation is uniform).

In special circumstances, the actual stress and strain may be calculated if the true cross section is measured during the test. AS1391 sets out the requirements for sample size, test methods and equipment and includes examples of the typical shapes of stress vs strain plots which may be expected when tensile tests are performed.

The uniform section gauge length (where the deformation is presumed to be contained) can be between 25 and 100mm tong. The orientation of the sample relative to rolling or solidification directions will obviously affect the results obtained.

Normal parameters measured are the yield stress at 0.2% deformation (estimated by using a rule parallel to the initial linear portion of the load/elongation plot and off setting the measurement by 0.2% displacement), the maximum stress, Rm, or the ultimate tensile stress (UTS), i.e. the maximum applied stress and the ductility which is measured by percent reduction in area of the fracture face or the percentage change in gauge length.

If the sample necks significantly, the (high strain) final part of the curve will dip below the UTS. Brittle materials will only deform by a small amount before fracture. The slope of the linear portion approximates the elastic modulus (or Young’s modulus) while the area under the entire, non-linear portion of the curve gives the energy absorbed during deformation, and is thus an indication of toughness.

Impact Strength

Impact strength is measured by allowing a pendulum to strike a grooved machined test piece and measuring the energy absorbed in the break (AS1544).

The Izod test is at ambient temperature while the temperature controlled Charpy test (AS1544.2) uses typically 10x10mm, rectangular cross section samples cut at specified orientations to the material axes. The absorbed energy decreases at lower temperatures. Absorbed energies >27J are generally considered satisfactory.

In ferrous materials a low ductile to brittle transition temperature is important for structures such as LPG tanks so that they will not suffer catastrophic brittle failure when chilled. After fracture, the percentage of brittle fracture area is estimated (AS1544.5).

Hardness

Hardness is not an intrinsic property of a material. The values ascribed are due to a complex combination of deformation and elastic behaviour.

Conversion between hardness scales and tensile strength is carried out, but the conversions are empirical because the measurements are of a combination of material properties.

The common methods are:

Vickers (AS1817) which uses a microscope to measure the depression caused on a polished surface by a diamond indenter with a load of kilograms.

Brinell (AS1816) which uses large loads (up to 3000kg) on a rough polished surface and gives impressions from 2 to 6mm.

Rockwell (AS1815) which forces a pointed probe into the surface and measures the increase in penetration when the load is increased from one level (minor load) to another (major load). The penetration is in tens of micrometres and if the sample deforms or moves, significant errors may arise.

Rebound (AS2731) where the bounce of a ball indicates the resistance to surface deformation (i.e. hardness).

Electronic rebound (ASTM A956) which uses the ratio of spring driven impact velocity to rebound velocity to give an LD value which is converted into conventional hardness numbers, and

Microhardness where loads less then 1kg are used for Vickers or Knoop indenters. Knoop indenters are common in the USA where they are used for thin sheet. There is good evidence that loads of less than 100gm give significant inaccuracies in results.

Scratch (Mohs) or file tests are fairly qualitative and imprecise.

All of these hardness techniques deform the surface and if the surface is non uniform or there are variations in hardness through the material or an indent is too close to an edge or another impression, then inaccuracies occur.

A practical check on all but the Rockwell and rebound methods, is that the impression on the surface is clear and symmetrical.

When hardness values are used to estimate ultimate strength, errors will occur if the material is cold worked or, in the case of steel, austenitic. The conversion does not give an estimate of yield strength.

Corrosion Resistance

Corrosion tests are not usually required unless the material is to be used for the carriage of dangerous goods or it is a corrosion resistant alloy (usually a stainless steel) in which case tests may be carried out on as-supplied and as-finished material.

Intergranular corrosion tests for austenitic stainless steels are specified in ASTM A262 and some of these tests are reproduced in AS2038. Pitting and crevice corrosion susceptibility is assessed using ASTM G48 and measuring weight loss and/or pit depths after exposure to a ferric chloride solution. Both these standards are also used for duplex stainless steels.

Increasingly, electrochemical tests are specified with requirements for minimum critical pitting temperatures (CPT). The NACE (USA) test manual provides test methods for corrosion assessment on metals and other materials largely related to the oil industry. It includes the multiple time exposure test used on specific alloys in the dangerous goods corrosion test.

Other corrosion tests include the various salt spray exposures of which the most frequent is the neutral salt spray (AS2331.3.1). This test is nominally equivalent to ASTM B117 although as written, the AS test is less controlled and more severe.

With the possible exception of wet/dry cycling tests, there is not a scaling factor between life in a salt spray test and life in a field exposure. One of the few non-ferrous alloy tests commonly specified is the dezincification test for brass. In contrast to other standards A52345 permits acceptance of specific compositions without corrosion testing.

 

This information has been sourced, reviewed and adapted from materials provided by Lloyd Instruments Ltd.

For more information on this source, please visit Lloyd Instruments Ltd.

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