Brittle fracture - Stress States

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Metals and alloys generally have adequate gross ductility. However,

under certain conditions of stress-state strain rate and temperature,

brittle fracture can occur without a significant plastic flow prior to

fracture. If the part contains a flaw or crack, and a load is applied, the

stresses and strains at the crack tip are intensified. When these are

increased to a critical value called fracture toughness, K.sub.IC), the

crack propagates and the part fractures. Low values of K.sub.IC

indicate ease of crack propagation.

The value of K.sub.IC in certain carbon and alloy steels shows an

abrupt change in a narrow temperature range which is called the

ductile-brittle transition temperature. Fracture above this transition

temperature is predominantly by movement of dislocations in slip

planes and by microvoid coalescence. The Pearls-Nabarrow lattice

friction stress increases with decreasing temperature, and, at

temperatures below the ductile-brittle transition, K.sub.IC becomes

very low and fracture occurs by cleavage.

Consider the example of the brittle fracture of a steel storage tank

used to store I million bushels of corn (see Figure 4). The tank

collapsed without warning on a cold winter day when its wall split and

was pushed outward by the flow of corn. The tank was 49 m in

diameter, 15 m tall, and was constructed by welding 6 rings of steel

plates. It had a 1.4-cm thick bottom and a 2.1 by 2.1 m access door

that was reinforced by a 1.9-cm thick steel plate. Fracture occurred

when the ambient temperature suddenly dropped to about - 23' C.

Chevron markings on fracture surfaces all indicated that the crack

began near a sharp rectangular corner of the reinforcing plate weld

joint at the top of the access door.

Laboratory investigations found that the steel plates used were in

conformance with the ASTM A283 Grade C carbon steel requirement.

They had 471-MPa tensile strength, 249-MPa yield strength, 34

percent elongation, and 75- to 78-RB hardness. The weldjoint was

made by the

multipass fusion weld method and the weld metal exhibited little

porosity, a columnar grain structure, and 24-RC hardness. Scanning

electron fractography near the crack origin area showed a cleavage-

type brittle fracture with flat grain facets and a river pattern typical of

a steel fracture below the ductile-brittle transition temperatures (see

Figure 5).

Attempts were made to determine the ductile-brittle transition

temperature of the steel using the Charpy V-notch impact test

method, and to determine the fracture toughness K.sub.IC with the

centercrack test method using specimens machined from the tank

wall. At the fracture temperature of – 23ºC, the impact energy for

fracture measured 12 J with 0.1 percent lateral expansion. The

fracture toughness K.sub.IC measured 59 MPa.√m. The critical crack

size was about 3.5 cm, which is close to the value that can be

predicted by fracture mechanics analysis.

Stress calculations indicated that the hydrostatic pressure stress at

the fracture origin area for the full tank load of corn, based on

Rankine's formula, was 128 MPa, and the vertical compressive stress

due to steel weight was 0.97 MPa. Thus, the allowable design stress

was about 129 MPa, or 3.6 times smaller than the fracture strength of

steel. If we assume a temperature gradient of about 22o C due to the

sudden drop in ambient temperature, the thermal stress that may

develop on the tank wall would be 45 MPa. The square-cut reinforcing

plate can cause an additional stress concentration, which was

estimated as 2.0 by Roark's formulas. The total stress including these

factors is still below the load-carrying capacity of steel. Fusion

welding creates residual weld stress and when it is superimposed on

the local applied stress, the total can reach the fracture stress of steel

and initiate a crack. Once the crack was formed below the ductile-

brittle transition temperature, it propagated rapidly and caused the

catastrophic fracture of the storage tank.

It should be pointed out that because of the interrelation of the

different effects, the exact combination of stress, temperature, weld

structure, and flaws that will cause brittle fracture in a given steel

structure cannot be accurately calculated. Therefore, the general

design practice is to select a steel with an appropriate ductile-brittle

transition temperature for the application. This is common practice for

the design of pressure vessels and should have been followed in the

design of this steel storage tank for cold climate use. The court found

the engineering company responsible for the collapse of the storage

tank and for damages.

Hydrogen-induced Cracking

Chemical attack and thermal effects can also cause brittle fractures in

metals and metal alloys. These effects include hydrogen-induced

cracking, stress-corrosion cracking, and cracking by structural

change.

Hydrogen embrittlement occurs when high strength steels and certain

other alloys absorb excessive amounts of hydrogen in a variety of

environments, such as in the presence of hydrocarbons or hydrogen

sulfide, or during pickling in acids, plating, welding, and heat treating.

If steels have a flaw, hydrogen tends to diffuse to the stress

concentration region and can initiate cracking. The crack propagates

in a brittle manner through grain boundaries, giving a "rock-candy"-

type fracture appearance.

A steel tank was fractured catastrophically in an oil refinery. The tank

was 2.4 m in diameter, 18-m tall, and was constructed from 2.54cm

thick steel plate, in accordance with the ASME Boiler and Pressure

Vessel Code, by welding six cylindrical rings together between two

ellipsoidal ends. it was used to remove H.sub.2S from a

propane/butane liquid mixture by the counterflow of an aqueous

solution of monoethanol amine. The hydrocarbon liquid entered the

tank near its bottom at 38' C and 1.4 MPa pressure, and the amine

solution entered at the top and exited at the bottom. The H.sub.2S

gas tended to collect in the lower part of the tank and produced

hydrogen blisters in the tank wall. After five years in service, the

second ring from the bottom was replaced by a new steel ring welded

in place by the shielded metal arc process. Two years later, the tank

developed a leak near the inlet valve and fractured catastrophically,

causing an explosion and fire, and extensive damage.

Field investigation indicated the fracture path followed the weld line

of the replacement ring. When the fracture surfaces were cleaned

with high pressure steam, it was noted that a black deposit extended

from the inner surface of the wall for nearly 30 percent of the

circumference. Chemical analysis showed the black deposit was rich

in sulfur. Since it was formed prior to fracture, it delineated the crack

that existed at the time of final rupture. This pre-existing crack was as

deep as 2.36 em when the leakage occurred.

Laboratory investigations indicated that the steel used was ASTM

A5]6 Grade 70 carbon steel with 540-MPa tensile strength, 304-MPa

yield strength, and 29 percent elongation. Metallographic

examination showed the pre-existing crack was in the heat-affected

zone of the repair weld area and consisted of hard spots with 410 to

490 KHN.sub.500 (41 to 47 RC) hardness with martensitic structure. A

typical scanning electron micrograph of the pre-existing crack surface

exhibited rock-candy"-type brittle intergranular fracture generally

associated with hydrogen-induced cracking.

Operating pressure of 1.4 MPa in a steel tank 2.54-cm thick with a

2.4-m diameter produced an axial stress of about 34 MPa. When the

tank wall thickness was reduced by a crack, the value of axial stress

increased to close to the value of the breaking stress of steel. The

crack propagated until it reached a critical size and the remaining

thin, solid metal shell fractured, causing leakage. At the same time,

the through-crack continued to grow. When it reached 80 cm,

catastrophic crack propagation occurred.

Fracture mechanics analysis based on Irwin's empirical relation

indicated that the crack tip opening displacement (CTOD) was about

0.064 mm. Tests conducted by the National Bureau of Standards

using notched ASTM A516 steel plate specimens yielded a CTOD

measurement of about 0. 17 mm. Therefore, hydrogen embrittlement

reduced the fracture resistance of the steel by more than half.

It should be pointed out that special precautions are required in

welding ASTM A516 carbon steel plates to avoid hydrogen

embrittlement. AWS Di.1 Standards stipulate that low hydrogen-

coated electrodes preheated to 22' C or higher must be used, and

that these temperatures must be maintained when welding 1.9 cm or

thicker steel plates. Neglecting these standard welding procedures

appears to have been the primary cause for the hard spots in the

weld joint which resulted in hydrogen-induced cracking of the steel

tank.

Stress-Corrosion Cracking

Stress-corrosion cracking (SCC) is the acceleration of cracking from

the combined effects of stress and a corrosive environment. These

stresses can include residual stress due to fabrication, or a

combination of residual and operating stresses. Stress corrosion

normally is initiated at several sites by localized pitting or

intergranular attack at the metal surface. It continues slowly, and

eventually a crack develops. The crack propagates either between

granules (intergranularly) or across them (transgranularly), and when

a critical size is reached the remaining solid metal ligament ruptures

by mechanical overloading. Crack propagation often is difficult to see

and detailed microscopic examination is necessary to confirm that the

fracture was indeed the result of SCC.

Many chemical, electrochemical, and mechanical mechanisms have

been suggested to interpret the features of stress-corrosion behavior

of metals. They include chemisorption of surface active species, as in

liquid metal embrittlement, fracturing of surface oxide films,

anodically active paths through dissolution, or formation of co-planar

dislocation arrays.

The fracture of a steel cylinder by SCC is shown in Figure 1. The

cylinder was manufactured in accordance with Department of

Transportation Specification 178.36 (OT-3A480). It was made of

seamless steel to handle a 480-psi (33.6 kg/cm.sup.2) service

pressure, and had 85-kg tare weight, 126-liter volume, 68-kg

ammonia capacity, 38-cm outside diameter, and 5.58-mm wall

thickness. It was used to supply anhydrous ammonia to a blueprint

machine. One day it exploded, discharging toxic fumes and hurling

pieces of steel some 10 meters across the room, causing property

damage.

Laboratory investigations indicated that the fracture occurred

longitudinally with a fishmouth thick-lip fracture almost through the

entire length. Tensile tests indicated 415-MPa tensile strength, 276-

MPa yield strength, and 32 percent elongation. The cylinder was

manufactured in 1938, was in good appearance, and was

hydrostatically tested at five-year intervals. The latest test was two

years prior to the accident at 63 kg/cm 2 , and showed no permanent

deformation. The ammonia vapor pressure at room temperature is

about 9 kg/cm 2, which can develop hoop stresses in the cylinder wall

(30 MPa) considerably below the stress developed by the test

pressure of 63 kg/cm.sup.2. It is clear that the cylinder wall thickness

was reduced by internal cracks to cause catastrophic failure.

Visual examination of the chevron pattern indicated that the fracture

began at the site of the maximum bulge in the cylinder wall (see

Figure 2). The wall at the fracture site had a reduced thickness (of

about 4.4 mm as compared to 5.58-mm normal thickness) resulting

from a bulge and exhibited a flat surface. The inner diameter edge at

the fracture site had a 1.0-mm deep layer with interference colors

ranging from light yellow brown to blue, followed by a rust color zone

almost to the outer diameter of about 10 cm in length. The inner

diameter surface had longitudinal microcracks with a depth of 2.5

mm; these microcracks extended transgranularly (see Figure 3).

Scanning electron micrographs of the fracture initiation site showed

transgranular cleavage and

radial microcracks.

Fracture mechanics analysis showed that at hydrotest pressures of

209 MPa, the cylinder could resist cracks 2.3-mm deep without

fracture, and could resist a critical through-crack size of 3.0 cm

without crack extension. Assuming that the rust region of fracture of

about 10 cm in length represents the critical crack size, the cylinder

wall must have been reduced in thickness to a very thin value (about

0.4 mm) after hydrotest before catastrophic failure occurred.

The ammonia used was pure-grade with 33 ppm water content. (The

DOT requires that commercial grade ammonia cylinders contain 50

ppm or more of water.) Addition of 0.08 wt percent of water was

found to inhibit SCC. It has been reported that carbon steels do not

exhibit SCC in pure grade ammonia unless it is contaminated with

oxygen. The formation of an oxygen-iron-ammonia film, along with

fracturing by slip in the base metal, was necessary for crack

propagation.

The use of pure-grade, water-uninhibited liquid ammonia, failed to

prevent air contamination when filling the cylinders, and infrequent

and improper testing and inspection to detect microcracks were the

contributing factors in the catastrophic fracture of the steel cylinder.

In general, austenitic stainless steels exhibit excellent corrosion

resistance to many environments and are widely used in the process

industries. The chromium and nickel in steel provide an effective

passive layer and reduce the corrosion rate. Certain environments

that reduce the breakdown of passivity can initiate pitting, where

cracks start in a pit and propagate transgranularly. The low stacking

fault energy favors the formation of co-planar dislocation arrays and

tends to increase solute atom concentration at the crack tip, thus

increasing the susceptibility of austenitic stainless steels to stress

corrosion.

Structural Change-induced Cracking

There is a strong relationship between the microstructure of metals

and alloys and the effect of environmental factors on crack

propagation. Cast-iron underground pipes, for example, generally

have a long service life. However, when such a pipe is located in

mildly corrosive soils that are constantly moist or contain high

calcium sulfate and chlorides, it is subject to graphitic corrosion.

For example, a gray-iron underground water pipe 20 cm in diameter

was fractured after 20 years in service, causing flooding (see Figure

6). Metallographic examination of the fractured section indicated the

presence of type B graphite flakes in pearlite and ferrite matrix.

Graphitic corrosion occurred, where the graphite in the pipe acted as

a cathode and iron was selectively leached out onto the outer

diameter surface of the pipe. The leaching process left a lacy porous

residue that had the shape of the original pipe, but practically no

strength. The effective wall thickness of the pipe was severely

reduced, decreasing the pipe's ability to handle loads. As a result, a

slight external bending force (added to the pressure of the water

flowing through the pipe) was sufficient to cause a sudden brittle

fracture to occur.

This article is adapted from a paper presented at the ASME Winter

Annual Meeting, San Francisco, Calif., Dec. 10-15, 1989.

A283 does not have adequate toughness at the temperature you mention.  A temperature of -30F or -30C is not cryogenic, its just a low temperature.