Iron and Steel testing
The metals normally encountered in construction are aluminium, copper and iron in various forms. With a few specialist exceptions the primary structural metal is iron. Since the development of commercially viable smelting, at Coalbrookdale in the late 18th Century, structural designers were able to use cast iron.
Modern construction uses iron almost exclusively as various grades of steel. Modern steels were developed in the late 19th century after Bessemer patented his furnace in 1855.
The principal differences between the materials for identification purposes are chemical analysis, primarily carbon content, and fracture pattern:
||2.0 - 4.5%
||0.02 - 0.05%
||0.2 - 1.0% *
* usually less than 0.5%
Frequently during the refurbishment or appraisal of existing buildings, it is necessary to identify both the existing dimensions and the properties of structural steelwork. (For convenience steelwork is deemed to include cast iron, wrought iron and steel). This information may be required to assess the suitability of particular members for inclusion in a new scheme and the load bearing capacity of individual elements of a structure. A number of test methods are frequently used.
The tensile strength of structural steel is of prime importance in the design of a structure or in the appraisal of the residual strength of a specific element.
The tensile test is carried out on a specimen bar of uniform cross-section in a testing machine which grips and pulls opposite ends of the specimen whilst applying an axial load. The machine records the extension of a measured length of the specimen (the gauge length) against increasing load. The applied load is increased gradually and initially the elongation, and hence the strain, is proportional to the load (and hence the stress). This relationship (Hooke's Law), is valid up to a stress value known as the limit of proportionality.
Slightly beyond the limit of proportionality the material may still behave elastically (i.e. if the load were removed the strain would return to zero). The point beyond which the strain is not recoverable and plastic deformation occurs is known as the elastic limit.
At a certain point beyond the elastic limit (the yield point) the metal starts to show a large increase in strain sometimes even without further increase in load. In the case of mild steel two distinct points can be identified on the stress/strain curve, known as the upper and lower yield points.
After yielding has occurred the stress/strain curve continues to rise to a maximum known as the ultimate tensile stress. This is calculated as the maximum load divided by the original cross-sectional area.
Some materials such as alloy steels exhibit no definite yield and a proof stress is adopted to determine the onset of plastic strain. This is usually the stress at which a residual set would remain after removal of the load. Proof stress is normally quoted as a value of 0.1% or 0.2% (viz. 0.2% proof stress).
Hardness is a measure of the resistance of a material to indentation. For testing samples in the laboratory, two principal methods are in general use:
The Brinell Method where a hardened steel ball is pressed into the surface under a specified load for a fixed time and then released. The permanent deformation of the surface is measured and the ‘Brinell Number’ is defined as the ratio of the applied load in kilograms to the diameter of the indentation.
The Brinell method is only suitable for materials with hardness numbers below 500, predominantly cast-iron and wrought iron.
The Vickers Pyramid Diamond Method is similar to the Brinell method except that by employing a 136° pyramid diamond it can be used over the full range of material hardnesses. Calculation of the ‘Vickers Pyramid Number’ (V.P.N) is based on the ratio of load to impressed area by measuring the length of a diagonal of the square impression at the suface of the test specimen.
Where it is impracticable to remove a sample from a structure, the Shore Scleroscope Method can be used in-situ. This comprises a small hammer fitted with a diamond tip or steel bar which is dropped from a height of 250mm onto the surface under test. The height of rebound can be used as a measure of the relative hardness of different materials. However, there is no direct relation between the Shore hardness and the Brinell and V.P.N. hardnesses.
Hardness can be related to tensile strength by the relationship:
Ultimate tensile strength = k x hardness number x 15.4 N/mm2
(where k = (approx.) 0.23 for mild steel and 0.21 for alloy steel)
In order to identify the composition and hence type and quality of a particular metal, a small coupon specimen is first sectioned, mounted in thermosetting resin and polished to a one micron finish. The specimen is then etched in a 2% nital solution and examined at various magnifications and evaluated in accordance with ASTM A247 (Ref. A1.7.5). By examining the microstructure of the material the method will readily identify the difference between steel, steel alloys, cast iron and wrought iron. Typical micrographs for different materials are given in ASTM A247.
Additional information regarding the type and quality of particular metal samples may be obtained by chemical analysis to determine the proportions of different elements present.
In particular the mechanical properties of plain carbon steels vary considerably depending primarily on the carbon content. As the carbon content increases there is an increase in ultimate tensile strength, hardness value and elastic limit (and hence yield point). However at the same time ductility rapidly declines and the material becomes more brittle, with increased carbon content.
Nickel-chrome alloy steels are widely used for high tensile strength combined with some ductility. Additional heat treatment is often used to produce a better combination of mechanical properties. Typically hardening from 850°C by quenching in oil or cooling in air followed by tempering at about 180°C, although slightly reducing ultimate tensile strength, has the effect of raising the yield point, increasing ductility and maximising impact value.
A number of different methods may be adopted to analyse the proportions of the different elements present in an alloy. Nowadays spectrographic techniques by optical emission, instrumental techniques such as a Leco sulphur/carbon analyser and atomic absorption are generally employed.
Identification of steel sections/steelwork
An accurate visual inspection and measurement on site can obtain the shape and dimensions of various structural members. This in itself may give some clues as to the origin of a particular member. However for design purposes, it is also necessary to determine the following information:
(a) Whether the section is cast iron, wrought iron or steel
(b) Properties and strength of the individual members
(c) Design loading and stresses appropriate to the particular member
Items (a) and (b) may be obtained by the methods discussed above. A considerable amount of research work has been carried out to determine the properties of historical steelwork in Ref. A1.7.7.
Table A1.1 below summarises significant dates in the development of steelwork and Table A1.2 suggests typical design stresses:
Table A1.1 Significant dates in the development of steelwork
||Changes in Use
||Reduced use from 1900 onwards. Columns still being made for limited use in early 1930’s
||Began to be replaced by steel from 1850 onwards. Very little used after 1890 although some sections produced as late as 1910
(in limited sizes only)
|Increase in size and quality after 1880. By 1887 Dorman Long produced a range of 99 beams and a vast range of channels and sections
n.b. British Standards first came into being in 1900
Table A1.2 Typical design stresses
||Ultimate strength in tonnes/m2 (N/mm2)
||Allowable stresses for safety factor 5 in tonnes/m2 (N/mm2)
|Cast iron beams
(based on 1879 figure)
||pbt or pt
|Wrought iron beams
||pbt or pt
||28.32 (432.4 to 494.2)
||pbt or pt
pbt = allowable bending stress in tension
pt = allowable axial stress in tension
pbc = allowable bending stress in compression
pq = allowable shear stress
BS 4360: 1990 (withdrawn) Specification for weldable structural steels. (Replaced by BS 7613: 1994 BS 7668: 1994)
BS EN 10029: 1991, Parts 1 to 3 of BS EN 10113: 1993
BS EN 10155: 1993 and BS EN 10210-1: 1994
BS 7613: 1994 (withdrawn) Specification for hot rolled, quenched and tempered weldable structural steel plates.
BS EN 10002-1: 1990 Tensile testing of metallic materials. Method of test at ambient temperatures.
BS EN 10002-2: 1992 Tensile testing of metallic materials. Verification of the force measuring system of the tensile testing machine.
ASTM A247-67 Standard test method for evaluating the microstructure (reapproved 1998) of graphite in iron castings.
BS 240: 1986 (withdrawn) Method for Brinell hardness test and for verification of Brinell hardness testing machines. (Replaced by BS EN 10003-1: 1995.
BS EN 10003-2: 1995 and BS EN 10003-3: 1995.
BCSA 1987 Historical structural steelwork handbook.