Fracture mechanics CTOD Crack Tip Opening Displacement

Fracture mechanics CTOD Crack Tip Opening Displacement

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  Fracture Mechanics Davalsab.M.L M.Tech (MachineDesign), Reva university [email protected] 8904567449 India
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  CTOD testing Whilst theCrack Tip Opening Displacement (CTOD) test was developed for the characterisation of metals it has also been used to determine the toughness of non-metallics such as weldable plastics. The CTOD test is one such fracture toughness test that is used when some plastic deformation can occur prior to failure - this allows the tip of a crack to stretch and open, hence 'tip opening displacement There are two basic forms - a square or a rectangular cross section specimen. If the specimen thickness is defined as 'B', the depth (W) will be either B or 2B with a standard length of 4.6W. A notch is machined at the centre and then extended by generating a fatigue crack so that the total 'defect' length is half the depth of the test piece- see Fig.1. A test on a 100mm thick weld will therefore require a specimen measuring 100mm thick, 200mm wide and 920mm long - an expensive operation, the validity of which can only be determined once the test has been completed.
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  The test isperformed by placing the specimen into three point bending and measuring the amount of crack opening. This is done by means of a strain gauge attached to a clip placed between two accurately positioned knife edges at the mouth of the machined notch As bending proceeds, the crack tip plastically deforms until a critical point is reached when the crack has opened sufficiently to initiate a cleavage crack. This may lead to either partial or complete failure of the specimen. The test may be performed at some minimum temperature eg the minimum design temperature or, more rarely, at a range of temperatures. The values that are required for the calculation of toughness are firstly the load at which fracture occurs and secondly the amount by which the crack has opened at the point of crack propagation
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  Since the lengthof the crack and the opening at the mouth of the notch are known it is a simple matter to calculate the crack tip opening by simple geometry. Whilst the test is in progress the results are recorded automatically on a load/displacement chart. This illustrates the various shapes of curve that may be produced – (a) It is a test where the test piece has fractured in a brittle manner with little or no plastic deformation. (b) exhibits a 'pop-in' where the brittle crack initiates but only propagates a short distance before it is arrested in tougher material - this may occur several times giving the curve a saw tooth appearance or after this one pop-in deformation may continue in a ductile manner as in (c) which shows completely plastic behaviour.
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  J-test ASTM E 1820has two alternative methods for J tests: the basic procedure and the resistance curve procedure. The basic procedure entails monotonically loading the specimen to failure or to a particular displacement, depending on the material behavior. The resistance curve procedure requires that the crack growth be monitored during the test. The J integral is calculated incrementally in the resistance curve procedure. The basic procedure can be used to measure J at fracture instability or near the onset of ductile crack extension. Because crack growth is not monitored as a part of the basic test procedure, a multiple-specimen technique is normally required to obtain a J-R curve. In such cases, a series of nominally identical specimens are loaded to various levels and then unloaded. Different amounts of crack growth occur in the various specimens. The crack growth in each sample is marked by heat tinting or fatigue cracking after the test. Each specimen is then broken open and the crack extension is measured.
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  Fracture testing ofWeldments
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  Weldment’s test consideration Welded joints,however, have decidedly heterogeneous microstructures and, in many cases, irregular shapes. Weldments also contain complex residual stress distributions. Most existing fracture toughness testing standards do not address the special problems associated with weldment testing Number of factors need special consideration. • Specimen design and fabrication are more difficult because of the irregular shapes and curved surfaces associated with some welded joints. • The heterogeneous microstructure of typical weldments requires special attention to the location of the notch in the test specimen. • Residual stresses make fatigue precracking of weldment specimens more difficult. • After the test, a weldment may be sectioned and examined metallographically to determine whether or not the fatigue crack sampled the intended microstructure.
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  Specimen Design and Fabrication Theunderlying philosophy of the British Standards test procedure on specimen design and fabrication is that the specimen thickness should be as close to the section thickness as possible. Larger specimens tend to produce more crack-tip constraint, and hence lower toughness Achieving nearly full-thickness weldment often requires sacrifices in other areas. For example, if a specimen is to be extracted from a curved section such as a pipe, one can either produce a sub-size rectangular specimen that meets the tolerances of the existing ASTM standards, or a full- thickness specimen that is curved. If curvature or distortion of a weldment is excessive, the specimen can be straightened by bending on either side of the notch to produce a ‘‘gull wing’’ configuration, which is illustrated in Figure 7.34. The bending must be performed so that the three loading points (in an SE(B) specimen) are aligned
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  Notch location and orientation Weldmentshave a highly heterogeneous microstructure. Fracture toughness can vary considerably over relatively short distances. Thus, it is important to take great care in locating the fatigue crack in the correct region. If the fracture toughness test is designed to simulate an actual structural flaw, the fatigue crack must sample the same microstructure as the flaw Once the microstructure of interest is identified, a notch orientation must be selected. The two most common alternatives are a through- thickness notch and a surface notch, which are illustrated in Figure 7.35. Since full-thickness specimens are desired, the surface-notched specimen should be a square section (B × B), while the through- thickness notch will usually be in a rectangular (B × 2B) specimen. For weld metal testing, the through-thickness orientation is usually preferable because a variety of regions in the weld are sampled. However, there may be cases where the surface-notched specimen is the most suitable for testing the weld metal. For example, a surface notch can sample a particular region of the weld metal, such as the root or cap, or the notch can be located in a particular microstructure, such as unrefined weld metal. In typical C–Mn structural steels, low toughness is usually associated with the coarse-grained heat-affected zone (HAZ) and the intercritically reheated HAZ. A microhardness survey can help identify low toughness regions because high hardness is often coincident with brittle behavior.
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  Heat affected zoneNotch location in the HAZ often depends on the type of weldment. If welds are produced solely for mechanical testing, for example, as part of a weld procedure qualification or a research program, the welded joint can be designed to facilitate HAZ testing. Figure 7.36 illustrates the K and half- K preparations, which simulate double-V and single-V welds, respectively. The plates should be tilted when these weldments are made, to have the same angle of attack for the electrode as in an actual single- or double- V joint. For fracture toughness testing, a through-thickness notch is placed in the straight side of the K or half-K HAZ. In many instances, fracture toughness testing must be performed on an actual production weldment, where the joint geometry is governed by the structural design. In such cases, a surface notch is often necessary for the crack to sample sufficient HAZ material. The measured toughness is sensitive to the volume of HAZ material sampled by the crack tip because of the weakest-link nature of cleavage fracture Another application of the surface-notched orientation is the simulation of structural flaws. Figure 7.37 illustrates HAZ flaws in a structural weld and a surface-notched fracture toughness specimen that models one of the flaws. Figure 7.37 demonstrates the advantages of allowing a range of a/W ratios in surface- notched specimens. A shallow notch is often required to locate a crack in the desired region, but most existing ASTM standards do not allow a/W ratios less than 0.45. Shallow-notched fracture toughness specimens tend to have lower constraint than deeply cracked specimens, as
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  Posttest Analysis Correct placementof a fatigue crack in weld metal is usually not difficult because this region is relatively homogeneous. The microstructure in the HAZ, however, can change dramatically over very small distances. Correct placement of a fatigue crack in the HAZ is often accomplished by trial and error. Because fatigue cracks are usually slightly bowed, the precise location of the crack tip in the center of a specimen cannot be inferred from observations on the surface of the specimen. Thus, HAZ fracture toughness specimens should be examined metallographically after the test to determine the microstructure that initiated fracture. In certain cases, a posttest examination may be required in weld metal specimens.
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  Thank you