Macroscopic Examination

Macroscopic examination is carried out with unaided eye or a simple handheld magnifier, or a stereobinocular microscope with a magnification generally below 100 diameters. In the stereobi- nocular microscope, reasonably large specimens can be handled, and the microscope can easily be adopted for examination of com- ponents in the field or accident site. The amount of information one can obtain by macroscopic examination is remarkable. The features revealed during macroscopic examination are:

● Type of fracture

● Origin of fracture

● Presence of secondary cracks

● Presence of external debris or corrosion products

● Discoloration

● Presence of wear marks in the vicinity of fracture

● Plastic deformation preceding fracture

● Dimensional changes in the component

● Evidence of any overheating

● Post-fracture damage such as rub marks

The aim of macroscopic examination is to have a general ap- preciation of the features of the failed component and, in particular, the fracture surface. When a component fractures, certain distinct features can be observed on the fracture surface. These features may be termed the signatures of fracture. These signatures are characteristic of the type of loading and the relative ductility or

brittleness of the component. The science of studying the fracture surface is termed fractography. Thus, depending on the level of examination, one can have macrofractography and microfractog- raphy.

4.1.1 Macrofractography

The signatures of fractures on the fracture surfaces can easily be lost or obliterated by subsequent damages caused by oxidation (as in the case of fire or thermal exposures), corrosion, abrasion due to improper handling, and so forth, and hence, the fracture surfaces should be carefully handled and preserved. Oil and grease on the fracture surface can be removed with organic solvents. The corrosion and oxidation products often hide the true fracture sur- face beneath. Fine fissures on the surface of these corrosion prod- ucts may sometimes mislead because they do not represent the real features of the original fracture surface. Mild chemicals such as acetic acid, phosphoric acid, sulfamic acid, ammonium oxalate, ammonium citrate, and NaOH, in dilute solutions, and 6N solution of HCl inhibited with 2g/l hexamethylene tetramine, are available for the removal of these layers. However, the best method of clean- ing the fracture surface is by a blast of air followed by repeated cleaning with replicating tape until the tape comes out clean and free from adhering debris, as described later.

One of the features often revealed by macrofractography is the origin of fracture. In many instances, the origin is apparent even to the naked eye. Once a crack is initiated, then during the prop- agation of the crack, other features such as radial lines, chevron marks, and beach marks develop and are left behind.

A ductile tensile fracture in a component of circular cross sec- tion consists of three distinct zones (Fig. 4.1) (Ref 3). The inner flat zone with a fibrous appearance is where the fracture starts and grows slowly. The fracture propagates fast along the intermediate radial zone. The radial lines extended backward point to the frac- ture origin. Sometimes the radial lines start from the origin itself. The fracture finally terminates at the shear lip zone that is the annular region near the periphery of the fracture surface. The shear lip zone is at an angle of 45⬚ to the tensile stress direction.

Fast, unstable crack propagation may sometimes be manifested as a series of chevron marks or herringbone pattern on the fracture surface (Fig. 4.2). The apexes of these chevrons point to the origin of fracture.

In fatigue failures, the origin is indicated by the presence of beach marks or clam shell marks, which also signify the direction of advancement of the crack (Fig. 4.3). Fatigue failure is also char-

Failure Analysis of Engineering Structures: Methodology and Case Histories V. Ramachandran, A.C. Raghuram, R.V. Krishnan, S.K. Bhaumik, p25-30 DOI:10.1361/faes2005p025

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Fig. 4.3 Beach marks characteristic of fatigue fracture. Source: Ref 3

Fig. 4.4 Ratchet marks around the perimeter of a shaft that failed in fatigue indicate multiple origins of fracture.

1 mm

a

c d

b

Fig. 4.5 Polished and etched longitudinal section of a bolt-nut assembly. The bolt and nut are held together by very few threads.

Fig. 4.2 Chevron marks indicating fast fracture

Shear-lip zone Fibrous zone Radial zone (a) (b)

Fig. 4.1 The three distinct zones on the fracture surface of a ductile material failed under tensile load. Source: Ref 3

acterized by the presence of a smooth region on the fracture surface denoting the crack propagation zone and a rough region indicating the final rapid overload failure. Because of longer exposure to the environment, the smooth fatigue crack propagation region appears slightly tarnished, whereas the final fracture appears relatively

bright. When there are multiple fatigue crack origins, for example, in a shaft, the approaching cracks meet and form a step. This re- sults in a series of small steps or ratchet marks around the periph- ery of the fracture surface (Fig. 4.4).

Ductile fractures are characterized by significant plastic defor- mation prior to fracture. Unlike in fatigue failures, the origin is not so well defined in ductile fractures. Dimensional changes in the failed component also denote effects such as wear, erosion, and corrosion. In some materials, thermal excursions experienced by the component can be discerned by the oxidation colors on the surface. Blueing of steels is an example.

4.1.2 Other Macro Features

Macroscopic examination after some sectioning and surface preparation can provide additional information about the malfunc- tioning or failure of certain components. Two such cases are illus-

Chapter 4: Examination Methods / 27

Fig. 4.8 TEM fractograph of a shear fracture, characterized by open-ended dimples. Source: Ref 5. With kind permission of Metals and Ceram- ics Information Center

Fig. 4.7 TEM fractograph showing equiaxed dimples caused by tensile overload. Source: Ref 5. With kind permission of Metals and Ce- ramics Information Center

trated here. It must be noted that sectioning is a destructive opera- tion and should be conducted only if necessary and after completing the microfractography described later in this chapter.

In a turbogenerator, the turbine was coupled to the gear box by a set of bolts and nuts. In addition, the nuts were also welded to the bolts. One of the nuts failed and the turbine was separated from the gear box, resulting in a catastrophe. Macroscopic examination after sectioning the bolt-nut assembly in the longitudinal direction and polishing clearly indicated that only a few threads were hold- ing the nut to the bolt (Fig. 4.5). Further, the welding operation had altered the microstructure of the bolt in the heat affected zone, rendering it brittle.

In metal forming operations, the metal flows in specified direc- tions, depending on the tooling and the forces employed. The di- rection of metal flow can be delineated by revealing the flow-line pattern. This is done by sectioning the component, grinding, pol-

ishing, and deep etching. The flow-line pattern can be discerned by macroscopic examination. Discontinuities in the flow-line pat- tern are sources of material weakness and can eventually lead to service failures. These are common in forged components having disrupted flow-line patterns (Fig. 4.6). This technique has been useful in the analysis of failures of such components as crane hooks, crankshafts, and so forth.