TWI Knowledge Summary

Mechanical testing of metals and metallic joints

by Christoph Wiesner

Introduction

Mechanical tests are employed to determine relevant material properties of metals and weldments. They may be divided into two groups:

Large-scale, structurally representative tests
Small-scale materials characterisation tests which determine material properties or give results which are known to correlate with structural behaviour

Structurally representative tests

Large-scale or full-scale tests are carried out to simulate the geometry and the loading of an actual structure as closely as possible. If the test set-up is such that the conditions of the actual structure can be modelled, the experiment is described as 'structurally' representative. Well-designed, structurally representative tests were used extensively in the past to develop appropriate materials properties criteria in design standards. An example is the series of wide plate tests carried out by Woodley, Burdekin and Wells (1964) which were the foundation for the fracture avoidance design rules in the British Pressure Vessel and Storage Tank design codes.

More recently, research into mechanical behaviour and properties has developed to encompass new materials and concepts. The predictive capability of computer modelling for mechanical and structural behaviour has improved markedly such that it may be considered as a future alternative to routine testing. However, for new materials, novel joining processes and design fracture to be accepted with confidence, validation testing using structurally representative test set-ups remains a crucial part of the qualification process (Wiesner, 1999).

The following provides an overview of test types which can be considered structurally representative:

Full-scale mechanical tests on actual pressure vessel, pipeline and tubular structures, bridges, aircrafts and vehicles or any other complete components. The idea is to model accurately the situation arising during operation such that the experiments give information on the behaviour and likely failure modes of the structures being tested.
This allows the designer or operator to take preventative action in design or during maintenance prior to any serious failure events occurring during operation. Whilst this way of mechanical testing clearly reflects the structure of concern accurately, the cost of full-scale tests is very significant and alternative testing methods to ascertain structural integrity have been, and continue to be, sought.
Wide plate-testing is one possibility for assessing mechanical behaviour using medium-scale tests. The tests are designed to simulate the structural configuration and failure mode of interest and are of sufficient size that the complex effects of geometry (i.e. material thickness and specimen dimensions) on mechanical behaviour are included in the test set-up. Wide plate tests have been used extensively to determine collapse, fracture and fatigue properties.

Material characterisation tests

Structurally representative testing, albeit the most realistic way of assessing mechanical behaviour is, however, neither economic nor practical in many circumstances. Small-scale material characterisation tests have therefore been developed. Because of their smaller size, not all small-scale tests give results which can be considered a material property. Some obviously do (such as tensile tests) but the predictive capability of others (e.g. Charpy tests) relies frequently on empirical correlations with full-scale behaviour, based on past experience (although recent theoretical work is making process to improve the transferability of small-scale results to structural behaviour). Such correlations exhibit a degree of inherent uncertainty due to the following factors (Fearnehough, 1973):

small-scale specimens do not have the same constraint as the structure due to differences in dimensions and stress state; this can cause, for instance, general yielding of small-scale specimens at stresses when structural behaviour is still elastic
the loading and strain rates in small-scale tests cannot always simulate structural conditions
the combination of small-scale specimen compliance and testing machine stiffness is not normally representative of the structure

This means that developed correlations have to be re-established for new materials. Nevertheless, there exists now a large number of material characterisation tests aimed at many different properties, materials and failure modes.

Most of the tests are standardised in relevant international and/or national test specifications such as ISO, CEN, BSI, ASTM, DIN and many more (see references for ASTM and BSI sources).

The list below gives a brief overview of the various mechanical tests used for metals and metallic joints:

- Strength properties (e.g. Metals Handbook, Vol. 8)
- Hardness tests
- Tensile tests (slow or high rates of loading
- Bend testing
- Compression tests
- Shear and torsion tests
- Formability/workability tests
- Fracture properties (e.g. Garwood et al 1994)
- Charpy and instrumented Charpy tests
- Drop-weight ('Pellini') tests - see also FAQ: What is the drop-weight (or 'Pellini') test
- Drop-weight tear tests
- Fracture mechanics tests
  - Static fracture initiation tests (many variations)
  - Static resistance curve tests
  - Dynamic initiation or resistance curve tests
  - Crack arrest tests
- Fatigue properties (e.g. Garwood et al 1994)
- Fatigue endurance tests (many variations for different sectors)
- Fatigue crack growth tests
- Corrosion properties (e.g. Metals Handbook Vol. 8)
- Stress corrosion tests
- Hydrogen embrittlement tests
- High temperature properties
- Strength, fracture and fatigue tests as above at elevated temperatures (i.e. test temperature greater than about 30% of the metal melting temperature)
- Creep tests
- Creep crack growth tests
- Low cycle fatigue tests (creep/fatigue interaction)
- Miniature tests
- Miniature Charpy tests
- Punched disk tests
- Indentation tests

References

American Society for Testing and Materials: Annual Books of ASTM Standards, Section 3, Metal Test Method and Analytical Procedures, Vol. 03.01 Metals - Mechanical Testing; elevated and low temperature tests, metallography.

British Standards Institution: Products and Services Catalogues, International Classification for Standards (ICS) Correspondence Index, 77.040.10: 'Mechanical testing of metals, BSI, London.

G D Fearnehough: 'The small-scale test and its application to fracture propagation problems'. Proc. Conf. Dynamic Crack Propagation, Noordhoff, 1973, pp77-102.

S J Garwood, H G Pisarski, S J Maddox and M G Dawes: 'Fracture and fatigue testing standardisation in Europe and practical applications to welded engineering structures'. Proc. Conf. Mechanical Testing of Materials, Institute of Metals and Materials Australasia Ltd, Melbourne, Australia, 1994, pp1-28.

Metals Handbook, Vol.8, 'Mechanical Testing', American Society for Metals, Metals Park, Ohio, 1985.

H G Pisarski: 'Philosophy of welded wide plate testing for brittle fracture assessment'. Proc. Conf. 'The Fracture Mechanics of Welds'. EGF Publication Z (Eds., J Blauel, K Schwalbe), MEP, London, 9 ^th Edition, 1987, pp191-208.

C S Wiesner: 'Validation through specialised mechanical testing - The Abington solution'. TWI Bulletin, Vol. 40, No. 6, Nov./Dec. 1999, pp83-89. (Only available to Industrial Members of TWI).

C C Woodley, F M Burdekin and A A Wells; Brit. Welding Journal, March 1964, pp123-136.

Further information

Also, you can use the Weldasearch literature database to supplement what you find in JoinIT.