1 

Metallurgy of the U.S. Capitol Dome

T.A. Siewert, C.N. McCowan, and J.D. McColskey 

Materials Reliability Division 

National Institute of Standards and Technology 

This report describes the composition and some properties of the cast iron and 

wrought iron that form the dome of the U.S. Capitol. It is a compilation of two 

recent investigations, one by NIST and one by Lucius Pitkin, Inc. These 

investigations were implemented by the Office of the Architect of the Capitol to 

determine the dome’s condition and also to predict how the outer shell might 

respond to weld repair of cracks. However, the studies also provide an interesting 

glimpse into the technology that was in existence at the time of the dome’s 

construction, the mid-1800's, and document (perhaps for the first time) the 

microstructure and properties of materials that form the dome. Knowledge of the 

properties and other characteristics of the dome materials is fundamental to 

maintaining the dome and to predicting its future performance. 

Keywords: cast iron; corrosion; fatigue tests; tensile tests; U.S. Capitol dome. 

1. Historical Background on the Dome 

An understanding of a structure begins with information on its construction and maintenance. A 

good summary of the construction of the Capitol and its dome is available on the web site of the 

Office of the Architect of the Capitol [1]. 

The present dome of the Capitol was built between 1856 and 1866. It replaced an earlier 

wooden dome that was no longer considered to be in scale with the expansions to the House and 

Senate wings (expansions needed to accommodate legislators from the states that had just been 

added to the Union). Cast iron was chosen because it was fire resistant, could be formed in 

complex shapes, and could be erected with pieces of convenient sizes. The designers also 

recognized that the dome would be subject to movement due to heating and cooling cycles, and 

the design included features to accept this movement. The U.S. Capitol Dome was the second 

cast-iron dome in the world and it remains the largest iron dome to this day. 

Although the majority of the dome, particularly its inner and outer shells and lower skirt, is 

composed of cast iron, wrought iron is used in a few places. The main framing of the dome 

consists of 36 arched ribs that bear on 36 paired pillars, which in turn, bear on 36 pairs of cast 
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iron brackets that are embedded in the masonry walls of the Great Rotunda. The ribs are tied 

together at multiple levels by bands or hoops consisting of either cast-iron sections or wroughtiron riveted plates. From the main rib framing, an elaborate arrangement of cast-iron brackets 

support the outer shell of the dome and give it its distinctive shape. The inner shell is suspended 

from the main ribs with either wrought iron hangers or cast iron brackets. Also suspended from 

the main ribs near the top of the dome is a shell of cast iron grating to which the plaster base of 

the fresco entitled "The Apotheosis of Washington" is applied. At the top of the dome, the 36 

ribs converge into 12 which continue upward to support the Tholos and the Lantern levels, and 

the Statue of Freedom. These structural parts of the dome were all fabricated in the 1850's and 

1860's using the casting technology of the time. More information (including Thomas Walter's 

elevation and cross-section drawings from 1859) is available at the Office of the Architect of the 

Capitol web site [1]. 

Modern structures usually have a steel framework, and many of the components are joined by 

welding. However, the blast furnace and refining technologies that permitted large-scale 

production of rolled steel sheets and beams were not introduced until the second half of the 19th

century, while welding processes for structural fabrication were not commercialized until the 

1930's and 1940's [2]. Therefore, the dome construction in the 1850's was based on the structural 

metals that dominated before then, cast iron (the most economical) and wrought iron, and was 

joined by wrought bolts. Cast iron (high-carbon iron that is cast in molds of the desired shapes) 

is much less common in structures today than it was 140 year ago, because it is inherently more 

brittle than structural steel, and cannot be shaped by hammering or rolling. Thus, most structures 

today are constructed of steel. Yet, cast iron is very resistant to thermal shock and damps 

vibrations. Cast iron is still used in some demanding automotive applications where resistance 

to heat checking (in brake drums and clutch plates) and vibration (engine cylinder blocks and 

heavy gearboxes) make it the material of choice. 

 

2. Evaluations of Material in the Dome

In the early 1990's, a study investigating water penetration of the dome during heavy rainstorms 

revealed problems in several areas, including leakage due to numerous cracks and breaks in the 

cast iron plates that form the skin of the dome. In turn, the staff from the Office of the Architect 

of the Capitol sought information on the materials that had been used to construct the dome, as 

well as advice on how to repair cracks that were found in the skin of the dome. One response 

came in 1998 from Lucius Pitkin, Inc., an organization with expertise in areas such as 

mechanical property testing, weld engineering, nondestructive testing, and failure investigations. 

 Their report included an engineering assessment of various structural members comprising the 

dome [3]. Specifically, this assessment included 

· nondestructive characterization of the microstructure and mechanical properties of the 

ribs and skin of the dome and of the wrought iron tension ring, 

· metallurgical testing of samples selected from various structural members, and 

· measurement of the thermally induced strains. 

Data from their study are included in the next section on results. 
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During the tour in 1998, we saw that the cast iron rib framework of the dome is in excellent 

condition. The castings that form the skin have visibly degraded only where corrosion has 

penetrated the paint (especially where paint could not reach all the surfaces) and has led to 

rusting, or where the castings that form the skin of the dome have been stressed beyond their 

ductile limits and have cracked. This led to two studies of possible procedures for weld repair, 

one in 1998 and one in 2002. A separate report covers the weld procedures and their properties 

[4]. This report covers the baseline characterization data on the dome materials themselves. 

3. Results and Discussion

3.1. Microstructure

Cast irons are iron alloys in which the carbon content exceeds the solubility limit of the 

austenitic (high-temperature) phase of iron. By definition, cast irons have carbon contents 

above 2 mass percent, which distinguishes them from the various grades of steel. Besides 

carbon, alloying elements such as silicon are often added, while impurities can be carried along 

from the ore or come from the materials added during processing. These other elements have 

effects as well, however this discussion will concentrate on the carbon. During cooling and 

solidification, the excess carbon precipitates as either iron carbide or graphite. For the cast irons 

found in the dome, the excess carbon is found as flakes. This form is known as gray cast iron 

because the large graphite flakes in the structure give a gray appearance to a fracture surface. 

Gray iron was very common at the time of the construction of the dome. Since then, many other 

types of cast iron (ductile, malleable, etc.) have been developed, and are distinguished by the 

shape of the graphite flakes or nodules [5]. 

In turn, there are two different classes of gray cast iron in the dome, ferritic cast iron in the ribs 

and pearlitic cast iron in the skin. Here, the two names refer to the microstructure that surrounds 

the graphite flakes in the gray cast iron, either a ferritic phase (predominantly iron) or a pearlitic 

phase (a fine mixture of layers of iron and iron carbide). The ferritic versus pearlitic structure is 

determined primarily by the cooling rate, with ferritic structure promoted by a slower cooling 

rate and pearlitic promoted by a faster cooling rate. A secondary reason is that the rib castings 

have a slightly different composition, the implications of which are discussed in more detail in 

the following sections. Together, cooling rate and composition explain why a ferritic structure is 

observed in the massive rib beams, and the pearlitic structure is observed in the thinner skin 

castings (less than 1 cm thick plates) that form the skin. More information on the distinctions 

between ferritic and pearlitic iron is included in handbooks on the processing of metals [2, 5-6]. 

Other structural parts of the dome were constructed of wrought iron, a material produced by 

mixing molten, high-carbon iron (like cast iron) with oxides. The mixture was “puddled” in a 

furnace to remove most of the carbon and reduce the concentration of other impurities [2]. The 

resulting ball of iron was squeezed into the shape of a bar (to remove excess slag), which could 

then be rolled into plates or rods. In turn, some rods could be processed into bolts and some 
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plates processed into nuts and washers. The end result was a wide variety of structural 

components (from plates to nuts) composed of a lower-carbon iron, with layers of silicates, 

sulphides, and oxides. The tensile strength of the wrought iron is greater than that of gray cast 

iron, and it is much more ductile. In effect, the additional refining and processing steps added 

cost, but improved the mechanical properties for use in locations with more demanding 

requirements. The tension ring at the base of the dome and the bolts and nuts were made from 

wrought iron. 

The report by Lucius Pitkin includes a number of micrographs, some taken from metallurgical 

specimens and some from replicas taken by polishing the surface of the ribs and plates. None of 

these micrographs is included here because we only had a photocopy of their report, so the 

quality of the micrographs was low. 

The group from NIST also obtained a few pieces of the dome in 1998, and a few more in 2002, 

to assist with the two studies of weld repair procedures [4]. These pieces included a few circular 

coupons removed during the installation of new rain-water drains, and parts of a railing and a 

gutter that had been replaced in the past. Most of this material was used for the studies of weld 

repair procedures, but the remaining material further away from the weld (unaffected by the weld 

thermal cycles) was studied later to obtain data on the original structure. Thanks to the 

characterization work already done by Lucius Pitkin, NIST was able to concentrate on 

developing complementary data on the dome materials. Since the dome includes about 4,500 

tons of cast iron produced by a number of different foundries during a 10-year period, we tried to 

evaluate as many pieces as possible to assess the range of compositions and properties likely to 

occur through the dome. 

In the NIST laboratory, we examined several sections of the original skin and confirmed that the 

microstructure was primarily a pearlitic gray cast iron. The skin castings contained graphite 

flakes that fit two standard classifications. The castings had a combination of randomly 

dispersed flake graphite (type A) and graphite rosette (type B) structures, although the 

microstructure varies from casting to casting and is quite complicated. The type B graphite 

structure is typical of castings that are cooled more rapidly, such as those with section 

thicknesses below about 10 mm [5]. Figure 1 shows a region from a skin casting that has both 

rosettes and randomly oriented flakes between the rosettes. The marker in the lower right of the 

figure is 200 μm long, so the magnification is about 150 X. This figure is at a relatively low 

magnification and so is useful in showing the variation in the flake shapes over this section. This 

micrograph was prepared by grinding a piece of casting on a series of successively finer grits, at 

each stage removing surface damage introduced by the previous grit. To reveal the internal 

structure, the final surface was finished by polishing with a fine alumina powder. The black 

regions are the carbon (graphite) flakes, while the white areas are a mixture of pearlitic and 

ferritic iron. The type A flakes are the large angular chunks, while the type B flakes are the 

small clusters of fine flakes (rosettes). Other pieces of castings showed different ratios of the 

two graphite structures, varying from solely type A to solely Type B. Such variation in 

microstructure is not unusual for castings produced over a 10-year period by a number of 

different foundries, especially when the section thickness is near the transition of type A to type 
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B. 

Figures 2 and 3 show regions at magnifications about double that of Figure 1. Figure 2 shows a 

region with a typical type A structure, while Figure 3 shows one with a typical type B structure. 

At these higher magnifications, more details of the structure become visible, both variations in 

the shape of the graphite flakes and slightly different shadings in the areas with the highly 

reflective surface. The phases in the reflective areas can be better distinguished with the use of 

an etchant, a chemical solution that reacts at different rates with the various phases. Figure 4 

shows the effect of using Klemm’s reagent after a nital pre-etch, at a still higher magnification 

[7-8]. Here, the graphite flakes are still easily distinguished by their acicular shape, while many 

new phases have become visible. The large, rounded dark areas are pearlite, with fine lines in 

their interiors marking the alternating layers of iron and iron carbide. The large, rounded light 

areas are ferrite, so this region would be characterized as a mixed ferrite-pearlite structure, in 

which the ferrite dominates. The light-colored regions (with small etched islands) at the 

boundaries of the ferrite and pearlite grains are a low-melting-point eutectic of iron carbide and 

iron phosphide. 

The microstructure serves as a record of how the casting structure developed. As the casting 

cooled from the liquid, the graphite flakes formed along with austenite (a high temperature phase 

of solid iron), using up much of the iron, carbon, and silicon in the melt. Finally, the only liquid 

left had the composition of the low-melting-point eutectic, and filled in the gaps between the 

austenite grains. Just below the melting temperature, the casting was a mixture of graphite 

flakes (only a few percent of the volume), austenite (the majority of the volume), and the lowmelting-point eutectic (only a few percent filling in the gaps between the austenite grains). As 

the casting continued to cool, some of the remaining carbon diffused to the carbon flakes and the 

austenite transformed to a mixture of ferrite and pearlite, determined by the cooling rate and the 

amount of carbon in the austenite. 

With a color camera, even more details are visible. Figure 5 shows a region near that of Figure 

4. Here, there are differences in color within the ferrite grains, which indicate a graded change 

in composition from the center of the grain to the outside. This gradient is known as coring and 

is quite common in slowly cooled metals. Certain components in the molten metal solidify at 

slightly higher temperatures. As the liquid is depleted in these components, the layer that is 

solidifying slowly changes in composition. 

Metallography can also help to characterize the structure of the wrought iron plates. Figure 6 

shows a cross section through one of the plates, parallel to the rolling direction. Here, the ferrite 

structure is separated by a multitude of dark horizontal lines, which are slag islands left from the 

refining process. The slag islands are fairly soft at the rolling temperature and elongated during 

rolling into these long shapes known as stringers. Figure 7 shows the structure at a higher 

magnification. The large stringer in the center fractured into many chunks during the rolling 

operation, while the smaller stringers stretched elastically. This behavior is due to the variation 

in composition of individual stringers, including metal sulphides, silicates, and oxides. Of these 

constituents, some are quite ductile at the rolling temperatures, while other are quite brittle. 
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Figure 8 shows the wrought iron structure with a color etch, similar to that of Figure 5. Here, the 

ferrite that appeared only as reflective in figures 6 and 7 is revealed to be composed of a large 

number of smaller grains. This structure explains the good ductility of the wrought iron 

compared to the cast iron. There are no large graphite flakes to serve as crack starters, and the 

low level of impurities (such as iron phosphides) means that the ferrite is quite ductile. 

3.2. Composition

On one cast iron rib, the team from Lucius Pitkin found the composition (in mass percent) to be 

3.39 C, 1.07 Mn, 0.92 Si, 0.61 P, and 0.10 S. On one piece of wrought iron (boiler plate), they 

found the composition (in mass percent) to be 0.025 C, 0.13 Mn, 0.10 Si, 0.13 P, and 0.01 S. 

This wrought iron composition matches well with good practice for the production of wrought 

iron, which was carbon of less than 0.035 mass percent, silicon of 0.075 to 0.15 percent, sulfur of 

less than 0.02 percent, and phosphorus of 0.1 to 0.25 percent [4]. When composition standards 

came into use in later years, the U.S. specifications carried a maximum of 0.09 percent Mn, but 

even then there were exemptions such as “there was no logical ground for condemning an 

otherwise well-made product because of a relatively high manganese content” [4]. It should also 

be pointed out that a certain fraction of these elements are incorporated in the slag islands, not in 

the iron itself. Thus, they have less effect on the mechanical properties of the wrought iron. In 

other words, a crack in a brittle slag island is less likely to propagate through the wrought iron 

than a similar crack in a brittle phase in an iron casting. 

The group at NIST also measured the composition. Table 1 list the results of spectroscopic 

analysis of three specimens taken from different castings on the skin of the dome. In addition to 

the key elements reported by Lucius Pitkin, additional trace elements are listed for one specimen 

to document the levels of these impurities in the castings in the dome. These compositions can 

be considered characteristic of the castings produced in this time period. The contents of some 

key elements differ from those reported by Lucius Pitkin for the rib specimen, but this is not 

surprising because these castings are from the skin of the dome (and so are predominately 

pearlitic cast iron rather than the ferritic iron used in the ribs). It seems reasonable to assume 

that the different foundries produced castings using different formulations, and some adjustments 

may have been made to the melts to increase the liquidity for the thin skin castings over those for 

the ribs. For example, the liquidus temperature of a casting drops from 1295 °C to 1175 °C as 

the carbon content increases from 2.5 to 3.6 mass percent [5]. If the maximum temperatures in 

the furnaces were limited by the technology of the time, dropping the liquidus temperature by 

adding carbon would have been the easiest way to increase the superheat of the casting (the 

temperature difference between the melt and that of solidification) and so increase the ability of 

the liquid metal to fill the furthest corners of the mold. In addition, lower temperatures greatly 

reduce the damage to the mold during pouring of the iron, so increased carbon content is the best 

way to combine a complete fill of a thin mold with a good surface appearance. This reasoning 

further explains the higher carbon content in the skin castings. 

Incidently, the skin castings do have periodic rough areas on the inner surfaces that may mark 
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where risers were removed during cleaning of the castings. Riser is the casting term for a 

reservoir of liquid metal used to feed metal into the casting to offset solidification shrinkage. 

Only in a few places, such as changes in section size, did we find solidification shrinkage in the 

castings. One of these locations, beneath a tab on a railing section, is shown in figure 9. 

 

Table 1. Composition of pearlitic gray cast iron from the skin of the dome. 

Element Specimen 1 

(mass %) 

Specimen 2 

(mass %) 

Specimen 3 

(mass %) 

C 3.36 3.62 3.86 

Mn 0.67 0.82 0.48 

Si 3.20 2.18 2.31 

P 0.78 0.82 0.60 

S 0.11 0.08 0.06 

Cr 0.01 

Ni 0.04 

Mo 0.01 

Cu 0.02 

Al 0.001 

Ti 0.11 

Carbon equivalent 4.7 4.62 4.83 

Table 1 also includes a value for carbon equivalence (CE), a term used here to characterize the 

behavior of cast iron as being above or below the eutectic composition (4.3 mass percent C) on 

the iron-carbon phase diagram. While many different formulas have been developed to compute 

the carbon equivalence, one common and simple version is 

CE = C + ( Si + P)/3, (1) 

where the elements are in mass percent [9]. This equation for carbon equivalence indicates that 

both silicon and phosphorus function like carbon in determining the microstructure, but at onethird its effectiveness. The cast iron composition (from a rib, as reported by Lucius Pitkin) had a 

CE of 3.9, well below that of the eutectic composition (and so known as hypoeutectic), while the 

cast iron from the skin had CEs from 4.62 to 4.83, well above that of the eutectic composition 

(and so known as hypereutectic). This difference in carbon equivalent also suggests differences 

in the structures between the rib and skin castings. 
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Note that this CE combines the relative effect of the various components in the cast iron on the 

solidification mode. There are other CEs that calculate the relative effect of the various 

components in the cast iron on the crack resistance during reheating, such as when the casting 

might be heated prior to repair by welding. 

Modern gray cast irons usually have carbon contents between 2.5 and 4 mass percent and silicon 

contents between 1 and 3 mass percent [9]. Being at the high end of the range for both elements 

(also having a high P content) means that the carbon equivalents for the dome castings are 

unusually high. However, the high carbon equivalent is not the goal by itself, but it is rather a 

by-product of aiming to make the cast iron more fluid and to promote a gray cast iron structure 

in such a thin-wall casting. Solidification of a composition with a lower carbon content in such a 

thin section could favor the formation of a white iron structure, one even more brittle than gray 

iron [5]. 

While the carbon is the most important variable in cast irons, other elements have roles as well. 

Silicon promotes the formation of the carbon flakes and improves the corrosion resistance, and 

so is kept in the range of 1 to 3 mass percent. Phosphorus does not have a desirable effect for 

this application and is present only because it was carried along from the ore and refining 

operations. 

3.3. Mechanical Properties

The group from Lucius Pitkin used a few 12 mm (1/2 in) square or round tensile specimens to 

determine that the ferritic iron castings (dome ribs) had tensile strengths of between 120 and 130 

MPa (17.4 and 18.8 ksi), while the wrought iron had a tensile strength near 325 MPa (46 ksi). 

These values are typical of the materials produced at the time. To minimize the amount of 

material that they needed to remove from the dome, they evaluated several more ferritic and 

pearlitic castings using a Brinnell hardness tester, and then converted these values to equivalent 

strengths. For seven different cast ribs (ferritic structure), they measured hardnesses of 130 to 

160 BHN, which correspond to strengths of 110 to 165 MPa (16 to 24 ksi), in close agreement 

with the strength measurements. For three exterior cast plates (pearlitic) in the dome, they 

measured hardnesses of 158 to 179 BHN, which correspond to tensile strengths of 158 to 186 

MPa (23 to 27 ksi), slightly higher than those obtained from tensile tests on the ferritic cast irons. 

 All the casting strengths are lower than those of most current-technology gray iron castings, 

which usually have strength minimums of 210 to 280 MPa (30 or 40 ksi), although there are 

grades as low as 140 MPa (20 ksi) and as high as 420 MPa (60 ksi). The strength is determined 

by factors such as the composition and cooling rate. 

The group at NIST made several measurements of the tensile strength of the pearlitic cast iron 

from a piece of the gutter. The width of the gutter, about 115 mm, determined the length of the 

tensile specimens, and resulted in reduced-section specimens such as those shown in Figure 10. 

Other than in length and the use of clamp rather than pin grips, the specimens were machined to 
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the sheet-type specimen dimensions shown in Figure 1 “Rectangular Tension Test Specimens” 

of ASTM Standard E 8 “Standard Test Methods for Tension Testing of Metallic Materials” [10]. 

 The faces of the specimens were machined enough to remove the surface damage, and so had 

final thicknesses near 7 mm, which were just under the original thickness of the casting. The 

specimen width was machined to 12.5 mm, producing a cross-sectional areas near 90 mm2

 . 

Because of the low ductility expected for cast irons, the elongation was measured by strain gages 

bonded to the machined faces, as shown in Figure 11. The data from the tensile tests were 

developed following the procedures in E 8, and are listed in Table 2. 

Table 2. Tensile test data. 

Specimen Ultimate 

Strength (MPa) 

Ultimate 

Strength (ksi) 

Elastic Strain at 

Fracture (%) 

Plastic Strain at 

Fracture (%) 

BM-1 186 27 0.18 0.25 

BM-2 front 182 26.4 0.25 0.19 

BM-2 back 183 26.5 0.25 0.19 

BM-3 front 168 24.5 0.22 0.17 

BM-3 back 170 24.6 0.19 0.18 

The dual-displacement-strain measurements per specimen (front and back) were designed to 

detect any bending, after the relatively low elongation value was observed in BM-1. Any 

bending would have a relatively large effect on the measurement of small elongations such as 

those for the cast iron, but the almost exact correspondence between the data for the fronts and 

backs of the other specimens confirms that little bending occurred. 

These tensile strength data compare well with the values calculated by Lucius Pitkin from the 

hardness measurements. The strength measurements also compare well with predictions based 

on compositions and microstructure. The high carbon equivalent that helped to increase the 

fluidity during the casting operation negatively affected the strength. Increasing the carbon 

equivalent from 3.5 to 4.5 in gray iron castings lowers the tensile strength from about 350 MPa 

to 150 MPa [5]. Thus the low strength (compared to typical gray iron castings today) can be 

attributed partially to the high carbon equivalent, which can be attributed in turn to the practical 

aspects of foundry technology of the time. It reflects the balance between the desire to gain 

fluidity while keeping the melting temperature low. 

No yield strengths are reported for the skin specimens because all specimens failed before or just 

after meeting the 0.2 percent offset plastic criterion of E 8. However, the strain gages provided 

an accurate measure of the actual plastic deformation to failure, which ranged from 0.17 to 0.25 

percent. The plastic strain did not appear suddenly after a period of elastic loading, but occurred 

gradually and progressively as the load was applied. Figure 12 shows the record of a typical 

stress-strain test. Here, the stress is shown along the vertical scale and the strain along the 

horizontal scale. The record begins at no load or strain, in the lower left of the figure, and 
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continues to failure of the specimen in the upper right. A tangent is drawn parallel to the first 

part of the record to estimate the slope of the curve during the elastic loading. The curve begins 

to deviate from this straight line at a very low load, perhaps as low as 25 MPa (4 ksi). Thus, 

tensile damage begins to accumulate at very low stresses (about 20 percent of the ultimate 

strength), and confirms that the dome is very sensitive to slight bending or tensile loads, such as 

occur when the corrosion products build up in the joints. Several unloading cycles during the 

tensile tests provided a rough estimate of the modulus, around 83 GPa, somewhat lower than the 

handbook value of near 100 GPa [11]. Tensile loading is an unusual condition for most of the 

dome. Most of the structure is designed to be in compression. We did one uniaxial test in 

compression and measured an ultimate strength of about 540 MPa (77 ksi) and a strain of about 

1.6 percent. These values are only approximate, because the specimen started to buckle at this 

point, making further analysis very complicated. The real message is that the cast iron is about 

twice as strong in compression as in tension, and it has greater elongation. 

The group at Lucius Pitkin did not calculate yield or elastic strains for the rib specimens, but the 

chart records included in their report allow rough estimates of these properties to be made [2]. 

The strain curves for two rib specimens begin to deviate from pure elastic behavior at about 40 

MPa (6 ksi), a value similar to that for the skin castings. The curves reach stresses of about 100 

MPa and 120 MPa (15 and 17 ksi) at a plastic strain of 0.2 percent (the traditional definition of 

yielding), then continue to stretch to final elongations of 0.3 and almost 0.5 percent. The rib 

castings are distinctly more ductile than the skin castings. 

Gray cast irons are typically stronger by a factor of two in compression than in tension, 

supported both by the single test of skin material and literature [9]. Thus, the tensile values 

quoted in the table above can be nearly doubled for modeling of compression applications. The 

application of cast iron for compression members and wrought iron for tension members 

indicates a recognition of these characteristics by the designers of the dome, and helps to explain 

its good performance over the years. 

The curvature of the tensile test data (figure 12) suggested that plastic deformation begins at low 

loads and strains. To get a better understanding of the development of this damage, we prepared 

several more specimens like those used for the tensile tests, and exposed them to cyclic loads 

(fatigue tests). We used a servohydraulic test machine to apply a sinusoidal load spectrum. We 

clamped the ends of the specimens, then cycled between a minimum and maximum tensile load. 

Based on the low cyclic loads (mostly thermal expansion) on the dome, we started with a load 

just above the first deviation from elastic behavior in figure 12 (about 25 MPa or 4 ksi), a load 

thought to represent the intensity of variable loads on the dome due to snow, wind, and thermal 

cycles. We then cycled between this load and half this load until fracture (an R ratio of 0.5). 

Table 3 shows the data from these tests. 
11 

Table 3. Fatigue tests on skin material. 

Maximum Load (MPa) Minimum Load (MPa) Cycles to Failure 

35 17.5 > 180,000 

70 35 > 180,000 

105 52.5 60,000 

105 52.5 > 180,000 

140 70 > 180,000 

Table 3 shows some quite encouraging results from the fatigue tests. Below maximum tensile 

loads of 105 MPa (15 ksi), the fatigue specimens were still intact and crack-free up to 180,000 

cycles. The tests were terminated after this value because it corresponds to almost 500 years of 

daily thermal cycles (caused by the usual day to night temperature swings and the heating of the 

sun as it passes over the surface of the dome). Since the dome has experienced only about one 

third of this life, there should be ample remaining life for loads at this level. In addition, most of 

the loads on the dome are expected to be compressive, so these tensile test results would be 

conservative estimates of compression fatigue behavior. Only once at 105 MPa, did we note a 

fracture, and then only after 60,000 cycles. This failure initiated at a 2 mm deep corrosion pit on 

the surface of the specimen. Additional specimens at 105 MPa and at 140 MPa were still intact 

and free of cracks up to 180,000 cycles. Other than the one failure at a maximum load of 105 

MPa after 60,000 cycles at a prexisting corrosion pit, all specimens lasted 180,000 cycles at 

maximum loads up to 140 MPa. 

The group at Lucius Pitkin also calculated the equivalent stresses (based on modulus) of 

thermally induced strains to be +4.3 to -9.4 MPa (+630 to -1360 psi) over a temperature range of 

17 °C (30 °F). Even when expanded to cover the maximum temperature fluctuation from 

summer to winter, the thermally induced stresses are only a small fraction of the tensile strength 

of the cast iron or wrought iron. However, their report does not mention whether the strain gage 

data were corrected for thermal effects. 

4. Summary (based on evaluation of specimens taken from the dome) 

The rib castings 

· are gray iron with a type A (random) distribution of graphite flakes, 

· have a ferritic structure around the graphite flakes because their massive sections limited 

the cooling rate. The slow cooling rate is further supported by the large size of the 

graphite flakes, some near 0.8 mm long, and 
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· have yield strengths near 110 MPa, tensile strengths near 130 MPa, and elongations 

between 0.3 and 0.5 percent. 

 

The skin castings 

· are gray iron with a combination of type A (random) and type B (rosette cluster) 

distributions of graphite flakes, 

· have a mixture of pearlitic and ferritic grains around the graphite flakes because their 

thinner section promoted faster cooling. The faster cooling rate is further supported by 

the smaller graphite flakes, usually below 0.2 mm long, and 

· fracture at strengths near 180 MPa, but at elongations of only about 0.18 percent, not 

quite reaching the traditional criteria for a yield strength. However, this characteristic is 

not unusual, since yield strength is not required and seldom even measured for cast irons 

[5]. Only where the corrosion products have built up in the joints between castings, have 

there been any failures of the castings. 

· can handle fatigue loads of 70 MPa (10 ksi) at an R value of 0.5 for a lifetime beyond 

180,000 cycles. 

The tension ring and plates 

· are wrought iron, typical of that produced in the 19th century, 

· have a ferritic structure with long stringers of non-metallic inclusions (slag), and 

· have ultimate strengths near 325 MPa (46 ksi). 

5. References

[1] http://aoc.gov/projects/dome_ren/dome_ren_overview.htm [November 2002] 

[2] The Making, Shaping, and Treating of Steel, 9th Edition, U.S. Steel, Herbick and Held 

Publishers, Pittsburgh, 1971, pp. 8 to 28. 

[3] United States Capitol Dome Structural Member Assessment - Washington, D.C., Lucius 

Pitkin, Inc. Report M98028, July 14, 1998. 

[4] T.A. Siewert, C.N. McCowan, R.A. Bushey, J. Doherty, T. Christ, D.J. Kotecki, D.L. 

Olsen, L.W. Myers, Jr., E. Hinshaw, and S. Kiser, Weld Repair of the U.S. Capitol Dome, 

NIST TechNote 1500-10, 2002. 

. 

[5] Properties and Selection: Irons, Steels, and High-Performance Alloys, Metals Handbook, 

Tenth Edition, Volume 1, ASM International, Materials Park, Ohio, 1990, pp. 3 to 32. 

[6] R. Bushey, “Welding of Cast Irons,” chapter in Welding, Brazing, and Soldering, Metals 

Handbook, Tenth Edition, Volume 6, ASM International, Materials Park, Ohio, 1993, pp. 

708 to 721. 
13 

[7] “Metallographic Techniques for Cast Irons,” chapter in Metallography, Structures and 

Phase Diagrams, Metals Handbook, Eighth Edition, Volume 8, ASM International, 

Materials Park, Ohio, 1973, p. 92. 

[8] G. Vander Voort, Metallography: Principles and Practice, McGraw Hill, 1984, p. 642.

[9] Properties and Selection: Irons, Steels, and High-Performance Alloys, Metals Handbook, 

Ninth Edition, Volume 1, ASM International, Materials Park, Ohio, 1990, p. 4. 

[10] “Standard Test Methods for Tension Testing of Metallic Materials,” ASTM Standard E 8, 

Annual Book of ASTM Standards 2002, Vol 3.01, ASTM International, Philadelphia, 

Penn., 2002. 

[11] Smithells Metals Reference Book, Sixth Edition, Butterworths, 1983, . 15-2. 
14