Loading Rate Effects on Tensile Properties and Fracture Toughness of Steel

Presented at 1999 TAGSI Seminar - 'Fracture, Plastic Flow and Structural Integrity' (dedicated to Sir Alan Cottrell in the year of his Eightieth Birthday), Held at TWI, Cambridge, UK, 29 April 1999

^[1]Note: Mr MacGillivray is chairman of the ESIS TC5 committee, which is concerned with dynamic mechanical testing.

Abstract

Because of inherent complexities of testing at high loading rates, the development of suitable test techniques for measuring both tensile and fracture toughness properties has been a topic of marked investment in the last 40 years or so, but increasing consensus has now been achieved leading to the development of test standards. Generally, the effect of increasing loading rate is to increase strength (positive strain rate dependence) and the strain rate sensitivity increases with temperature, but there are exceptions when dynamic strain ageing effects intervene. The effect of loading rate on the fracture toughness of ferritic steels is dependent on overall material behaviour: for temperatures below the brittle to ductile transition region, toughness decreases with increasing loading rate. At upper shelf temperatures, the ductile initiation toughness and tearing resistance generally increases with loading rate, but exceptions exist which are outlined in the paper. In or near the transition region, increasing loading rates can cause a shift from fully ductile behaviour at static rates to brittle behaviour at high rates of loading. The brittle to ductile transition temperature of ferritic steels increases with increasing loading rates and methods have been proposed to predict this shift.

1. Introduction

For more accurate, fracture mechanics-based, structural integrity evaluations, Charpy results are not sufficient so fracture toughness input and fracture toughness tests using pre-cracked specimens are generally employed. When characterising the fracture toughness of an engineering component, the rate of loading to which the structure may be subjected should be taken into account, and the fracture toughness test should be carried out at the appropriate rate, so as to simulate the structural configuration of interest. Mechanistically, the reduction in cleavage toughness generally observed with increasing loading rate is due to the increase in yield strength associated with high loading rates, which elevates the crack tip stresses such that critical conditions in the crack tip region are reached at lower levels of remote load than under static conditions.

There are many examples where structural behaviour is affected by dynamic loading rates. For example, brittle fracture events in steel frame buildings during recent earthquakes in Japan and North America are likely to have been influenced by dynamic loading rate effects caused by the rapid ground movement during the earthquake event. Postulated high loading rate earthquake events are also worst-case accident scenarios during the design of nuclear pipework and storage tanks in certain locations. Other design scenarios causing high loading rates are accidental drops of heavy lids on nuclear storage containers, and impacts in land, sea and air transport.

Typical examples of loading rates are given in Table 1, which was produced from information given in Ref 1 to 3 . These should be taken as estimates only since the exact values will depend strongly on local geometry, loading configuration and flaw dimensions. Loading rates in Table 1 are expressed as strain rate, ε, and the rate of increase of the linear elastic stress intensity factor, Κ. The latter parameter is a more appropriate measure for loading rates in fracture mechanics specimens and flawed structures as it includes the relevant geometries and flaw dimensions.

Table 1 Typical loading rates in some engineering components

The present paper provides an overview on test techniques for determining tensile and fracture toughness properties under dynamic conditions and examples of effects of dynamic rates on tensile properties and fracture toughness, as well as structural assessment considerations. The emphasis is on ferritic steels with some consideration given to stainless steels.

2. Experimental Methods

2.1 Introductory Remarks

Dynamic tests have been used and standardised to characterise qualitatively material properties for many years. Such tests include Charpy and Pellini drop-weight tests. Although valuable for comparative purposes, they offer little detailed insight into failure mechanisms and quantifiable properties. This has led to instrumented versions of these tests, and to new test methods being developed over the past 20 years or so. A number of test techniques, initially developed for research purposes, are now being refined and compared to establish if they can meet the strict requirements of standardisation. Repeatability, accuracy and ease of use are vital if such methods are to become accepted. There is co-operation within Europe and with the USA to ensure that any methods are truly international. Seminars arranged by ESIS TC5 at Mol, Belgium in April 1994 ^[4] , and by ASTM in Seattle, USA in May 1999, showed that there remains great interest in developing and using dynamic test methods.

Dynamic testing is usually expensive. Special testing machines may have to be used. Furthermore, it is normally necessary to instrument the test machine and/or the specimen in some way. High speed recording equipment will be needed, and the analysis of the test date may be complex. Skilled and experienced personnel will be needed to perform the tests and evaluate the results correctly. Extra safety precautions need to be taken. For all these reasons, it is increasingly realised in Europe and the USA that such tests should be performed and analysed using, as far as possible, agreed international standard methods. This paper will consider dynamic tensile testing and fracture testing in both linear-elastic and elastic-plastic regions.

There are three general regions at which dynamic tests can be performed.

1. Medium rates, when inertial effects are negligible or can be controlled and quasi-static analyses remain applicable.
2. High rates where inertia dominates, and special measurement and analyses techniques must be applied.
3. Very high rates where stress wave loading is dominant.

This paper is limited to the first two regions.

2.2 Fracture Toughness Test Methods and Standards

For Κ exceeding 3000MPam ^0.5s ^-1, an appendix exists in BS 6729 which gives guidance as to how to carry out tests. The main issue with such high loading rates is that conventional instrumentation is no longer appropriate to record the actual conditions experienced by the specimen. For example, Fig.1 (from Ref 30 ) shows load-time plots recorded during static and dynamic fracture tests using both a conventional load cell and a calibrated strain gauge attached directly to the specimen. It can be seen ( Fig.1a) that both give identical results under static conditions, but only the strain gauge is responsive enough to record load data at high loading rates ( Fig.1b), whilst the conventional load cell data are dominated by inertial effects.

with:

For a ratio of dynamic to static loading rate of 10 ⁴, the predicted shifts are as shown in Fig.21. It can be seen that the shifts are a function of yield strength (with lower predicted shifts for higher yield strength, as in Fig.20), but also depend strongly on absolute value of static fracture toughness transition temperature, with markedly lower predicted shifts for higher transition temperatures.

Equations [1] and [2] permit predictions of material property changes to be made when material data cannot be generated. This enables preliminary engineering assessments to be made to study the sensitivity of the integrity of a particular component with respect to the effect of dynamic loading rates.

5. Summary and Concluding Remarks

Because of the inherent complexities of investigating material behaviour under high loading rates, the development of suitable test techniques for measuring both strength/fracture toughness has been a topic of marked investment in the last 40 years or so, but increasing consensus has now been achieved leading to the development of test standards.

There is an important effect of loading rate on tensile properties, especially at very high rates. Generally, the effect of increasing loading rate is to increase strength (positive strain rate dependence), but microstructural influences (such as dynamic strain ageing) can cause negative strain rate dependence). Strain rate sensitivity increases with temperature.

The effect of loading rate on the initiation fracture toughness of ferritic steels is dependent on overall material behaviour. For temperatures below the brittle to ductile transition region (i.e. for cleavage toughness or overall brittle behaviour), toughness decreases with increasing loading rate. At upper shelf temperatures, the ductile initiation toughness and tearing resistance generally increases, but this depends on strain hardening behaviour as decreases in ductile toughness with increasing loading rates have been observed for certain ferritic steels with low yield to tensile strength ratios. In or near the transition region, increasing loading rates can cause a shift from fully ductile behaviour at static rates to brittle behaviour at high rates of loading. The brittle to ductile transition temperature of ferritic steels increases with increasing loading rates and methods have been proposed to predict this shift.

6. References

Copyright by TWI, 1999