What Is The Relationship Between Yield Strength, Tensile Strength, Elongation, And Impact Energy Of Steel?
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What are yield strength and tensile strength?
To clarify these ideas, we must begin by examining the mechanisms of material failure. When a substance is exposed to rising, steady, or fluctuating external loads, it will ultimately surpass a specific threshold and break down. Various forms of stress can lead to material failure, including tension, compression, shear, and torsion. Yield strength and tensile strength are particularly associated with tensile stresses. These properties are established via tensile testing procedures. A tensile testing apparatus-often a universal testing machine that handles tension, compression, and bending-applies a set constant loading rate (the rise in tensile force per unit time) to steadily elongate the material until it breaks or reaches a predefined failure condition (for instance, certain butt weld strength assessments might not necessitate full rupture). The force that results in the material's final failure is its ultimate tensile load. This load is quantified as a force in Newtons (N). Because Newtons are a relatively modest unit, kilonewtons (kN) are more frequently employed in real-world applications. Due to the differing dimensions of various materials, the ultimate tensile load by itself is inadequate for assessing material strength. Hence, by dividing the ultimate tensile load by the test specimen's cross-sectional area, we obtain the ultimate tensile load per unit area. This force per unit area signifies strength, expressed in Pascals (Pa).
Likewise, since Pascals are quite small, megapascals (MPa) are widely used. Consequently, the quotient of the ultimate tensile load and the test specimen's cross-sectional area defines the tensile strength. Tensile strength indicates the highest external force a material can endure per unit area. Surpassing this threshold leads to material fracture. So, what exactly is yield strength? Yield strength is relevant only to elastic materials; non-elastic substances do not possess yield strength. Examples encompass various metals, plastics, rubber, and other elastic materials, all of which exhibit yield strengths. Materials such as glass, ceramics, and stone typically lack elasticity. Even if they display some elasticity, it is minimal, so the notion of yield strength does not pertain to them. When an elastic material is subjected to a constant, progressively increasing external force until it fractures, what transformations take place? Initially, the material experiences elastic deformation under the external force, adhering to Hooke's Law. What does elastic deformation entail? It implies that upon removal of the external force, the material reverts to its original size and shape. As the external force continues to grow beyond a certain limit, the material transitions into the plastic deformation stage. Once plastic deformation happens, the material's initial dimensions and shape cannot be recovered! The strength at this pivotal juncture that distinguishes the two deformation types is the material's yield strength! In the context of tensile force, the tensile value at this critical moment is termed the yield point. From a crystalline standpoint, only when tensile force exceeds the yield point do the material's crystalline bonds start to fracture! Material failure actually commences at the yield point, not at the moment of fracture! Grasping the origins of these two strength measures, it is accurate to state that materials with elevated yield strength can endure greater destructive forces.
Nevertheless, it is crucial to stress that relying solely on a single strength metric-be it yield strength or tensile strength-cannot adequately assess a material's safety or durability. Consider steel as our case in point. In addition, there exists another factor related to yield strength and tensile strength that is not widely recognized, and even fewer grasp its importance. This factor is the yield-to-tensile strength ratio! It denotes the proportion of yield strength relative to tensile strength. Its value lies within the 0 to 1 range. The yield-to-tensile ratio acts as a measure of steel's brittleness. A greater ratio suggests a narrower gap between yield strength and tensile strength, pointing to reduced plasticity and heightened brittleness in the steel! Why does this occur? At this juncture, we introduce a new measure-elongation. In simple terms, this quantifies how much a steel bar extends after being pulled apart in comparison to its initial length. It serves as a key gauge of a steel's plasticity. A higher figure denotes superior ductility. As previously noted, once steel is stretched past its yield point, it cannot revert to its original size. It keeps elongating until it fractures. A higher yield-to-tensile strength ratio implies a smaller disparity between yield strength and tensile strength. With consistent loading rates, this leads to briefer elongation periods and thus lower elongation percentages. Enough digression! Let's focus on the core issue. In line with the principle of energy conservation, energy can only be transformed or transferred.
When steel is subjected to tension, it essentially involves the transformation and absorption of energy. Prior to the yield point, the steel experiences elastic deformation. The external tensile force is nearly entirely balanced by elastic force (transformed into elastic potential energy). Minimal external energy is absorbed or converted, with only a slight portion turning into heat. After surpassing the yield point, some of the external force is offset by elastic force (converted into elastic potential energy), while another portion is transformed into heat. The energy imparted by the external force to the steel is mainly absorbed during the plastic deformation phase! As mentioned earlier, material failure initiates at the yield point. A lower yield-to-tensile strength ratio extends the timeframe from initial failure to fracture, whereas a higher ratio reduces this interval. Between the yield point and the fracture point, a significant amount of energy is converted into heat.
Therefore, simply stating that a material has high yield strength or high tensile strength does not necessarily mean it is superior or safer. Only steel with high yield strength and a low yield-to-tensile strength ratio offers greater safety! Unfortunately, such steel is prohibitively expensive and unlikely to be used in civilian vehicles. Beyond strength, another crucial property of steel is toughness! To date, I haven't seen a single automaker describe the toughness of the steel they use! They mostly hype the strength of the steel! Ironically, in most cases, increasing steel strength often reduces its toughness! Reducing toughness means increasing brittleness! Steel toughness is a critical safety metric. One property that automakers may have intentionally or unintentionally overlooked is impact toughness or impact energy. If the same force pushes you versus strikes you violently, which causes greater harm? The answer is obvious! A steel's ability to withstand impact is the crucial safety factor. Have you ever seen a car accident where the force gradually built up until the vehicle snapped? Collisions are always instantaneous! If you can't withstand an instantaneous force, what good is high tensile strength? Looking at current steel grades, most materials exceeding 1000 MPa in strength primarily achieve this through tensile strength. Reaching a yield strength above 800 MPa is also relatively straightforward. Take 40Cr, a common quenching and tempering processes, its yield strength can approach 800 MPa, while tensile strength exceeds 900 MPa. However, achieving all three simultaneously-high yield strength, high elongation, and excellent impact resistance-is quite challenging!
Nearly all steels face the same trade-off: increasing strength inevitably reduces impact resistance. Take Grade 10.9 high-strength bolts: qualified tensile strength ranges from 1040–1240 MPa, yield strength exceeds 940 MPa, elongation surpasses 10%, and impact toughness reaches 59 J/cm2. In contrast, Grade 8.8 high-strength bolts (one grade lower) require a tensile strength of 830–1030 MPa, yield strength exceeding 660 MPa, elongation greater than 12%, and impact toughness of 78 J/cm2. Thus, for most metallic materials, enhancing certain technical performance indicators inevitably comes at the expense of others. It is impossible to achieve both simultaneously. The steel industry stands as one of humanity's most mature industrial technologies, holding few secrets. The technical specifications of steel materials are not inherently better when higher or lower; rather, they must be adjusted within a balanced range according to specific requirements. For those in our industry, aside from structural defects (referring to product flaws), there is no inherent superiority or inferiority among steel's technical specifications-their value depends entirely on their application. There is such a thing as using the wrong place, but never using the wrong material.
