Figure 2.1 above shows the typical stress and strain diagram. The stress σ is plotted on the Y axis and the resulting strain ϵ is plotted on the X axis.
As the metal under test is stressed, the strain on the X axis increases. To a point this increase is proportional to the increasing stress, up to this point the load is proportionally balanced by the strain, if the stress is removed at or below this point the metal will show no permanent stress - damage. Further loading deviates from the proportionality however the metal remains elastic to the point where elastic stain is reached, this means that further from this point the metal is not elastic, and it will not revert back to its original form or shape – a permanent damage has taken place. The stress below this limit allows the metal to revert back to its original shape and size if the stress is removed. This is the point where the limit of metal’s elasticity is reached.
2.1.3.7 Impact Strength
Impact properties are related to the toughness of the material. It establishes the material’s inherent ability to resist fracture on application of sudden and high impact. In subsequent paragraphs more details is discussed on this very important structure sensitive property of metals.
Impact properties are determined by tests. These tests are conducted to assess metals ductility and toughness. These two properties in combination define metal’s ability to resist fracture on sudden impact.
2.1.3.7.1 Types of Impact Tests
Recognition of the influence of temperature, strain rate and distribution of stresses on the toughness of metals has led to development of several test methods. The initial testing methods were carried out at room temperature, however in present day the lowering of temperature is possible with especial baths.
Most of the later developments are named after the originators of the test methods like Charpy, Izod and Mesnager. The Charpy and Izod methods are industrially more acceptable tests, and they use notched specimen.
2.1.3.7.2 IZOD Method
IZOD method uses notched specimen of round or squire cross section. The specimen is held as a cantilever beam in the gripping anvil of a pendulum machine the specimen is broken by a single overload of the swinging pendulum. A stop pointer moved by the pendulum records the energy absorbed. Izod specimen can be un-notched bar or it may have a 45o V-Notch in the face struck by the pendulum.
2.1.3.7.3 Charpy Impact Method
The Charpy teste specimen design has the choice on the type of notches to use. There are three options of the notch design to choose from, 45o V-Notch, Labeled keyhole, or U-Notch. By far the most common notch used for steel testing is 45o V-notch. The specimen is cooled to the required temperature and placed as a simple beam in horizontal position on the anvil and is struck in the middle by the edge affixed to the swinging pendulum which strikes the specimen on the opposite face of the notch. A single over load breaks or tries to break, the specimen and the absorbed energy is recorded on the stop pointer.
2.1.3.8 Energy Absorption in Impact Testing
The energy absorption is a different way to evaluate toughness, and gain unanimity in decisions regarding the acceptance of the values. The results are analyzed in a number of ways. The minimum energy absorption is often specified but it must be noted that the typical values differ significantly. When “leak-before-break” is the criteria of design that is in most cases including the pipeline design, the Cv must be set to a higher values at the given design temperature. Generally, the test temperature is also changed to about 5oC below the least anticipated service temperature (LAST), this practice is common in design of deep-sea risers and other structures. However, for pipeline the reduction in temperature is further reduced.
2.1.3.9 Transition Temperature for Energy Absorption
The transition temperature provides somewhat similar criteria for analyzing Cv test results. This method requires Cv test over a range of temperatures from a relatively high temperature where the metal exhibits its best toughness down to a low temperature at which cleavage can initiate. The obtained energy levels are then plotted against temperature. Metal with bcc crystalline structures undergo a precipitous drop in energy over a relatively narrow mid-range span of temperature. The drop in energy coincides with the occurrence of cleavage during the fracture.
2.1.3.10 Transition Temperature for Lateral Expansion
The extent of plastic deformation that occurs in the Cv specimen’s cross section during testing also is a quantifiable value, and this feature undergoes a marked transition in the bcc metals with the lowering of test temperature. When a Cv specimen is broken, a small amount of lateral contraction ordinarily occurs across the width, close to and parallel with the root of the notch, conversely expansion should occur across the width opposite the notch. Both changes in dimension from original 10 mm (0.394 inch) width of the specimen are easily measured, and both dimensional changes are indicators of ductility in the presence of a notch. The extent of lateral expansion opposite the notch is the value presently favored for appraising the capacity of metal flow plastically during fracture under impact load.
2.1.3.11 Drop-Weight Tear Test (DWTT)
Drop-Weight Tear Test is another way of determining fracture behavior of steel. The method owes its origins to Naval Research Laboratories tests for determining the NDT of steel, in 1950s.
The process involves use of a 4” wide full section steel plate with a sharp notch in the middle. The notch is made with a sharp chisel that has small cutting tip ground to a radius. The DWTT specimen is broken via three-point impact loading using a drop-weight or pendulum hammer that has a velocity of no less than 5 meters/Second (16 ft/s). The specimen is cooled in the bath to required temperature. The specimen is broken in one single blow.
In this method no attempt is made to measure the absorbed energy. Evaluation of broken surface is based on the texture of the fractured ends. The appraisal is made on the percentage of ductile shear facture.
2.1.3.12 Fracture Toughness
In a very unique way, the ductility is related to another metal property that is metal’s toughness. Most of the metals that show ductility through the stress and strain diagram, often fail in a very brittle manner in very different type of test. Thus, the only conclusion about the ductility that can be made from the bend tests and elongation from the tensile test is that the metal is not likely to behave in a ductile fashion in any other type of mechanical test carried to its failure. In such tests the metal show very little to no plastic deformation, and fracture in brittle manner.
Such fractures with lack of deformation, indicate that the metal did not resist the fracture, and that the metal failed at very little energy. This observation lead metallurgists to talk of another property “Toughness” of the metal.
Toughness is the ability of the metal to deform plastically and absorb energy in the process before fracturing. This mechanical and structure sensitive property is the indicator of how the given metal would fail at the application of stress beyond the capacity of the metal, and will that failure be ductile or brittle. Despite tremendous
