FRACTURE TESTING

FRACTURE TESTINGSUBMITTED TO:Mr. Mukesh Kumar

SUBMITTED BY:Prabhat Kumar(2013ppe5149)

The Fundamentals

Fracture = separation of body into two or more pieces due to application of static stress. Tensile,Compressive Shear or torsional.

Modes of fracture :

DUCTILEBRITTLE

WHY FRACTURE TESTINGTo compare and to select from candidate materials the toughest (and most economic) one forgiven service conditions.to compare a given material's fracture characteristics against a specified standardTo be able to predict the effects of service conditions (e.g., corrosion, fatigue, stress corrosion, etc., on the material toughness)To study the effects of metallurgical changes on material toughness.

Two broad categories of fracture tests:Qualitative

Quantitative

The Charpy impact test exemplifies the former and the plane-strain fracture toughness (KIc) test illustrates the latter

IMPACT TESTINGStress concentrations, like cracks and notches, are sites where failure of a material starts. It has been long appreciated that the failure of a given material in the presence of a notch is controlled by its fracture toughness.A number of tests have been developed and standardized to measure this "notch toughness" of a material.In order to simulate the most service conditions, almost all of these tests involve a notched sample to be broken by impact over a range of temperatures

CHARPY IMPACT TESTThe Charpy V -notch impact test is an ASTM standard.The notch is located in the center of test sample. The test sample, supported horizontally at two points, receives an impact from a pendulum of a specific weight on the side opposite that of the notch. The specimen fails in fiexure under impact.In the region around the notch in the test piece, there exists a triaxial stress state due to plastic yielding constraint there. This triaxial stress state and the high strain rates used propitiate the tendency for a brittle failure.

Charpy impact testing machine.(b) Charpy impact test specimen (c) Izod impact test specimen.

An indication of the tenacity of the material can be obtained by an examination of the fracture surface. Ductile materials show a fibrous aspect, whereas brittle material s show a flat fracture.A Charpy test at only one temperature is not sufficient, however, because the energy absorbed in fracture drops with decreasing test temperature.Figure shows this variation of energy absorbed as a function of temperature for a steel in the annealed and in the quenched and tempered state. The temperature at which there occurs a change from a high-energy fracture to a low-energy one is called the ductile-brittle transition temperature (DBTI).

However, as in practice there does not occur a sharp change in energy but instead there occurs a transition zone, it becomes difficult to obtain this DBTI with precision.

The morphology of the fracture surface changes in the transition region.The greater the fraction of fibrous fracture , the greater the energy that is absorbed by the specimen.The brittle fracture has a typical c1eavage appearance and does not require as much energy as the fibrous fracture. BCC and HCP metals or alloys show a ductile-brittle transition, whereas FCC structures do not . Thus, generally, a series of tests at different temperatures is conducted which permits us to determine a transition temperature.

As this transition temperature is, generally,not very well defined, there exist a number of empirical ways of determining it, based on a certain absorbed energy (e.g., 15 J), change in the fracture aspect (e.g., the temperature corresponding to 50% fibrous fracture), or lateral contraction (e.g., 1 %) that occurs at the notch root.The transition temperature depends on the chemical, composition, heat treatment, processing, and microstructure of the material. Among these variables, grain refinement is the only method that results in an increase in strength of the material in accordance with the Hall-Petch re1ation and at the same time reduction in transition temperature.

DROP-WEIGHT TESTThis test is used to determine a reproducible and well-defined ductile-brittle transition in steels.The specimen consists of the steel plate containing a brittle weld on one surface. A saw cut is made in the weld to localize the fracture. The specimen is treated as "simple edge-supported beam" with a stop placed below the center to limit the deformation to a small amount (3%) and prevent general yielding in different steels.The load is applied by means of a freely falling weight striking the specimen side opposite to the crack starter. Tests are conducted at 5-K intervals and a break/no break temperature, called the nil ductility transition (NDT) temperature, is determined.

NDT temperature is thus the temperature below which a fast unstable fracture (i.e., brittle fracture) is highly probable. Above this temperature, the toughness increases rapidly with temperature. This transition temperature is more precise than one of the Charpy-based transition temperatures.The drop-weight test uses a sharp crack that moves rapidly from a notch in a brittle weld material, and thus the NDT temperature correlates better with the information from a K1c test. This test provides a useful link between the qualitative "transition temperature approach and the quantitative "K1c" approach to fracture.

The drop-weight test provides a simple means of quality control through the NDT temperature. It (the NDT temperature) can be used to group and classify various steels. For some steels, identification of the NDT temperature can be used to indicate safe minimum operating temperatures for a given stress. That this drop-weight NDT test is more reliable than a Charpy V -notch value of transition temperature.

The drop-weight test is applicable primarily to steels in the thickness range 18 to 50 mm. NDT temperature is unaffected by section sizes above about 12 mm; beca use of the small notch and the limited deformation due to brittle weld bead material, sufficient notch-tip restraint is ensured.

INSTRUMENTED CHARPY IMPACT TESTThe common Charpy test basicaliy furnishes information of only a comparative character. The transition temperature, for example, depends on the specimen thickness (hence, the need to use standard samples); that is, this transition temperature can be used to compare, say, two steels, but it is not an absolute material property.Besides, the common Charpy test measures the total energy absorbed (ET), which is the sum of energies spent in initiation (E) and in propagation (Ep) of crack (i.e., ET = E + Ep). In view of this problem, a test has been developed called the instrumented Charpy impact test.

This instrumented impact test furnishes, besides the absorbed energy, the variation of applied load with time.

The instrumentation involves the recording of the signal from a load cell on the pendulum by means of an oscilloscope in the form of a load time curve of the test sample. This type of curve can provide information about the load at general yield, maximum load, load at fracture, and so on.

From the load-time curve, one can obtain the energy of fracture if the pendulum velocity is known. Assuming this velocity to be constant during the test, we can write the energy of fracture as:

where E' is the total fracture energy based on the constant pendulum velocity, Vo the initial pendulum velocity, P the instantaneous load, and t the time.

PLANE-STRAIN FRACTURE TOUGHNESS TESTThe fracture toughness Kc may be determined according to the following standards: ASTM E399/79 or BS 5447/77).The essential steps in the fracture toughness tests involve measurement of crack extension and load at the sudden failure of sample. As it is difficult to measure crack extension directly, one measures the relative displacement of two points on the opposite sides of the crack plane. This displacement can be calibrated and related to real crack front extension.

The relation between the applied load and the crack opening displacement depends on the size of the crack and thickness of the sample in relation to the extent of plastic zones.

When the crack length and the sample thickness are very large in relation to the quantity, the load displacement curve is of the type shown in Fig.(a). The load at the brittle fracture that corresponds to Kc is then well defined.

When the specimen is of reduced thickness, a step called "pop-in" occurs in the curve, indicating an increase in the crack opening displacement without an increase in the load Fig.(b). This phenomenon is attributed to the fact that the crack front advances only in the center of the plate thickness, where the material is constrained under plane-strain condition.

When the test piece becomes even thinner, the plane-stress condition prevails and the load displacement curve becomes as shown in Fig.(c).

In the fracture toughness tests, the crack is preferably introduced by fatigue from a starter notch in the sample. The fatigue crack length should be long enough to avoid interference in the crack-tip stress field by the shape of notch.Under an app1ied load, the crack opening disp1acement can be measured between two points on the notch surfaces by various types of transducers. Calibration curves are used for converting disp1acement measurements and resistance measurements into crack extension.

The load-displacement curves generally show a gradual deviation fram linearity and the"pop-in" step is very small (Fig). The procedure used in the analysis of load~displacement records of this type can be explained by using the Fig. Let us designate the linear slope part as OA. A secant line, OPs, is then drawn at a slope 5% less than that of line OA. The point of intersection of the secant with the load~displacement record is called Ps.

Define the load PQ, for computing a conditional value of K1c, called KQ, as follows: If the load on every point of curve before Ps is less than Ps, then Ps = PQ (case). If there is a load more than Ps and before Ps, this load is considered to be PQ (cases 11 and 111 in Fig). In these cases if P max/ PQ > 1.1, the test is not a valid one;KQ does not represent the K1c value and a new test needs to be done. After determining the point PQ, KQ is calculated according to the known equation for the geometry of the test piece used.

CRACK OPENING DISPLACEMENT TESTINGFor crack opening displacement (COD) testing, there exists a British Standards lnstitution BS 5762. The proposed BSI method is very similar to the ASTM E399 method for Kc.

A clip gage is used to obtain the crack opening displacement. During the test, one obtains a continuous record of load, p, versus opening displacement, D..

In the case of a smooth P-D.. curve, the critical value, D..c, is the total value (elastic + plastic) corresponding to the load maximum [Fig.(a)]. In case the P-D.. curve shows a region of increase in displacement at a constant or decreasing load, followed by an increase in load before fracture, one needs to make auxiliary measurements to determine that this is associated with crack propagation. Should this be so, D..c will correspond to the first instability in the curve.

If the P-D.. curve shows a maximum and D.. increases with a reduction in P, then either a stable crack propagation is occurring or a "plastic hinge" is being formed. The "D..c" in this case [Fig.(b)], according to the British Standards Institution, is the value corresponding to the point at which a certain specified crack growth has started.

If it is not possible to determine this onset of crack propagation, one cannot measure the COD at the start of crack propagation. However, we can measure, for comparative purposes, an opening displacement om, computed from the clip gage output D..m, corresponding to the first load maximum. The results in this case will depend on the specimen geometry.

REFERENCES[1] W.J. Langford, Can. Met. Quart., 19 (1980) 13.[2] Standard Methods and Definitions for Mechanical Testing of Steel Products, ASTM Standard Method A370, ASTM Annual Standards, Part 10, ASTM, Philadelphia. [3] J.c. Miguez Suarez and K.K. Chawla, Metalurgia-ABM, 34 (1978) 825.[4] J. Heslop and NJ. Petch, Phil. Mag., 3 (1958) 1128.[5] Insrrumented Impact Testing, ASTM STP 563, ASTM, Philadelphia, 1974.[6] K.K. Chawla and M.R. Krishnadev, unpublished research.[7] B. Augland, Brit. Weld. J., 9 (1962) 434.