THE RELEVANCE OF THE YIELD STRESS/ TENSILE STRESS …

THE RELEVANCE OF THE YIELD STRESS/ TENSILE STRESS RATIO IN MODERN MECHANICAL TESTING REQUIREMENTS

BY MICHAEL WRIGHT * ROBERT GLODOWSKI**

SYNOPSIS As more varieties of high strength steels are being considered in the design of new structures, a question has arisen over the relevance of the yield-tensile ratio on the advanced behaviour of steels. The yield-tensile ratio has been a standard requirement in many Global standards for many years now, and has served its purpose well in traditionally made lower strength level steels. However, it may not be the optimum parameter required to define the behaviour of today's steels in the inelastic zone of the tensile load-elongation curve.

The shape of the engineering stress-strain (load-elongation) curve varies for different steel types. High strengths steel microstructures in the market today can have continuous yield curves, or with extended yield plateaus, or a combination of both of these curvatures. The region of the engineering stress strain curve between the elastic limit and the ultimate tensile strength can vary considerably in shape, height and length, and yet can all appear the same if evaluated simply by using the yield-tensile ratio.

The behaviour of the steels inside the plastic deformation zone is becoming more important for the design of structures to withstand the impact of unexpectedly high forces that may occur in an irregular manner, such as seismic events or explosive forces. The yield-tensile ratio may be too simple to assist in understanding and defining the required properties of the steel in this region of the stress-strain curve. A more sophisticated material property measurement may be required.

Keywords:

* Bachelor Applied Science (Materials), University of Technology, Sydney, Australia Managing Director- Modern Metal Solutions Pte Ltd, Singapore. Michael@.sg

** Bachelor of Science in Metallurgical Engineering, South Dakota School of Mines and Technology, Principal ? RJG Metallurgical, LLC., Pittsburgh, PA, USA. bobglodowski@

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Introduction:

The mechanical properties calculated parameter Yield to Tensile Ratio (YTR) was introduced to Standards in the 1960's (1) and is appearing more frequently in materials specifications, Standards and codes. It is readily determined from certified Standard Tensile testing results, although it is a calculated figure and not a direct measurement, and due to this it can be applied in integrity measurement situations.

But what is the "Yield to Tensile Ratio" in reality? The term "Yield Point" defines the level of Engineering Stress (load/original cross sectional area) that needs to be applied to a steel sample so that it ceases to behave in an elastic manner. The term "Tensile" refers to the tensile strength which is defined by the maximum amount of Engineering Stress that can be applied to a steel before the load starts to drop and failure occurs. A ratio is "the quantitative relation between two amounts showing the number of times one value contains or is contained within the other".

This calculated parameter was implemented in the 1960's to set a minimum level on how many multiples of the yield point it was necessary to be able to stress a metal beyond its yield without it failing for it to be considered a "ductile" material. The concept of the YTR itself is simple and easy to determine. Establishing a required value for pass or fail, and defining why this particular test value has relevance to the material performance may be less easy to understand. For example, Table 1 shows requirements for 6 different steels, all increasing in yield strength, but with the same YTR requirement. Because this is a ratio, the minimum UTS increases further as Yield increases, and so the "separation" between the Yield and the UTS is increasing. This is actually requiring the steel producer to be able to metallurgically manipulate the steel's UTS independently of the Yield of the material, which is not an easy thing to achieve.

Table 1: Different Steel Strengths

It needs to be remembered that this parameter was just a "convenient" way to ensure a level of separation between the yield point and the ultimate tensile strength of a conventionally produced moderate strength level steel in the 1960's (2). In this day of high strength, technically advanced steels, it may not necessarily be the "best" way. There are other characteristics that need to be considered to give an accurate representation of the behaviour of a steel under tensile stress after the yield point has been exceeded.

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Basic Knowledge:

Going back to basics, a unique stress-strain curve exists for each material, and is found by recording the amount of strain (deformation) experienced under tensile loading of a known cross sectional area of the material (stress). An example is shown below, with critical points highlighted on the graphic:

Fig 1: Stress-strain Curve Graphic for a steel (3) O-A: is called the "Region of Proportionality" and is a straight line which indicates that in this region, stress is linearly proportional to strain (Hooke's Law) and the body behaves like a perfectly elastic body. The gradient of the line will be equal to the Elastic Modulus of the material. Within this zone, if the applied stress is removed, the material will return to its original shape. A-B: is the region where the elastic limit occurs, starting the transition from elastic to plastic deformation behaviour. The elastic limit is difficult to determine in a routine test. Point B: is called the Yield Strength (or Yield Point if non-uniform elongation occurs). As you increase the elongation beyond B, the resulting strain will become non-linear to the stress. That is, it ceases acting in an elastic manner. The elastic limit is therefore the lowest stress at which permanent deformation can be measured. Point C: When a material is deformed beyond its elastic limit by increased stress, the strain increases in a non-linear fashion as the material begins to behave in a plastic manner. So even if the deforming load is removed, the material will not recover its original length. The material would follow the dotted line (C-D) on the graph on gradual reduction of load. The remaining strain at zero stress is known as "permanent set". Point E: After the yield point elongation (if any) uniform elongation begins and continues until the maximum load (Tensile Strength) is achieved.

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Point F: The maximum engineering stress endured by the material is called the ultimate tensile strength. Beyond Point F, localized strain (necking) occurs with a continuing reduction of load. The material will eventually fracture. Generally speaking, if a large deformation takes place between the Yield point (B) and Ultimate Tensile Strength (F), the material is called "ductile". If it fails soon after the elastic limit is crossed, it is called "brittle".

Fig 2: Stress-strain curves (4). Curve A on Fig. 2 shows a brittle material. It has broken at the yield point, so the yield point and ultimate tensile strength will be the same value. It has only elastic behaviour there is no plastic behaviour. Curve B on Fig. 2 shows a high strength material, with only a minimal amount of plastic behaviour. There is some small separation between the yield point and ultimate tensile strength. Curve C on Fig. 2 shows a typical steel curve, with both elastic and plastic behaviour. Curve D on Fig. 2 shows a fully plastic material, with a very limited elastic response followed by plastic behaviour as the strain increases continuously with a low application of stress. The point of conjecture here is that current Standards & Codes specify how large this separation between Yield point and Ultimate Tensile Strength needs to be for a material to be deemed "ductile"? How do you put a figure, or a percentage, onto this? And is this a true reflection of the material's ductility??

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The term "ductility" refers to the ability of a material to undergo large deformations without rupturing. Ductile materials can exceed their elastic limit, and permanently deform under applied stresses but they do not fail. This prevents a total structural collapse. Brittle materials will fail suddenly with very little warning, changing loading within a structure and may lead to progressive total collapse. Ductility includes the ability to survive large deformations and a capacity to absorb energy by hysteretic behaviour. In conventional structural design, the working stress is usually a proportion of the yield stress (typically 60-80% depending upon the level of loading). This is still well inside the elastic limit of the material. It is only in cases where the yield point has been exceeded and plastic behaviour is required to prevent catastrophic failure that the YTR becomes significant. In the past, the YTR has been identified as a readily measurable and convenient way to demonstrate a material's ability to withstand increased stress beyond the point of plastic deformation. It describes a measure of the material's capacity to strain harden. In Figure 3, all 4 of these curves have the same YTR, but they will all exhibit different behaviour between the elastic limit and the Ultimate Tensile Stress. As defined by the YTR alone, they would be identical.

Fig 3: Different Stress-strain curves.

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