Section 2 Fracture control

2.1 Grades of steel

2.1.1 The resistance to fracture is controlled, in part, by the notch toughness of the steel used in the structure. Steels with different levels of notch toughness are specified in the Rules for the Manufacture, Testing and Certification of Materials, July 2022. The grade of steel to be used is, in general, related to the thickness of the material and the stress pattern associated with its location.

2.1.2 In order to distinguish between the material grade requirements for different hull members at varying locations along the ship, material classes are assigned as shown in Table 2.2.1 Material classes and grades. For each class, depending on thickness, the material grade requirements are not to be lower than those given in Table 2.2.2 Steel grades.

2.1.3 Where tee or cruciform connections employ full penetration welds, and the plate material is subject to significant strains in a direction perpendicular to the rolled surfaces, it is recommended that consideration be given to the use of special plate material with specified through thickness properties, as detailed in Ch 3, 8 Plates with specified through thickness properties of the Rules for Materials.

2.1.4 Design for normal worldwide service assumes the navigation to areas of minus 10°C, where the design air temperature is to be taken as the lowest mean daily average air temperature in the area of operation:

where

Mean	=	statistical mean over a minimum of 20 years
Average	=	average during one day and one night
Lowest	=	lowest during the year
MDHT	=	Mean Daily High Temperature
MDAT	=	Mean Daily Average Temperature
MDLT	=	Mean Daily Low Temperature

Figure 2.2.1 Design air temperature shows the definition graphically.

The material grade of exposed structure of ships intended to operate in external air temperatures below minus 10°C is to be in accordance with Pt 3, Ch 2, 2.3 Structures exposed to low temperatures.

Figure 2.2.1 Design air temperature

2.1.5 For container ships only, measures for the prevention of brittle fracture of thick plates are to be taken in accordance with the requirements of Pt 4, Ch 8, 2.3 Requirements for use of thick steel plates.

2.2 Refrigerated spaces

2.2.1 Where the minimum design temperature of the steel falls below 0°C in refrigerated spaces, in addition to the requirements of Pt 3, Ch 2, 2.1 Grades of steel 2.1.2, the grade of steel for the following items is to comply, in general, with the requirements of Table 2.2.3 Grades of steel for refrigerated spaces with a minimum design temperatures below 0°C:

Deck plating.
Webs of deck girders.
Longitudinal bulkhead strakes attached to deck.
Shelf plates and their face bars supporting hatch covers.

2.2.2 Unless a temperature gradient calculation is carried out to assess the design temperature in the items defined in Pt 3, Ch 2, 2.2 Refrigerated spaces 2.2.1, the temperature to which the steel deck may be subjected is to be assessed as shown in Table 2.2.4 Assessment of deck temperature.

Table 2.2.1 Material classes and grades

Structural member category		Material class/Minimum grade
SECONDARY
A1.	Longitudinal bulkhead strakes, other than belonging to the Primary category
A2.	Deck plating exposed to weather, other than that belonging to the Primary or Special category	Class I within 0,4L amidships
A3.	Side plating	Grade A/AH outside 0,4L amidships
PRIMARY
B1.	Bottom plating, including keel plate
B2.	Strength deck plating, excluding that belonging to the Special category
B3.	Continuous longitudinal plating of strength members above strength deck, excluding hatch coamings	Class II within 0,4L amidships
B4.	Uppermost strake in longitudinal bulkhead	Grade A/AH outside 0,4L amidships
B5.	Vertical strake (hatch side girder) and uppermost sloped strake in top wing tank
SPECIAL
C1.	Sheerstrake (or rounded gunwale) and stringer plate at strength deck, see Note 1	Class III within 0,4L amidships
C2.	Deck strake at longitudinal bulkhead excluding deck plating in way of inner skin bulkhead of double hull ships, see Note 1	Class II outside 0,4L amidships
C2.		Class I outside 0,6L amidships
C3.	Strength deck plating at outboard corners of cargo hatch openings (and plating intersections of the longitudinal underdeck girders and the cross-deck strips) in container carriers and other ships with similar hatch opening configurations	Class III within 0,4L amidships
		Class II outside 0,4L amidships
		Class I outside 0,6L amidships
		Minimum Class III within cargo region
C4.	Strength deck plating at corners of cargo hatch openings in bulk carriers (see Pt 3, Ch 2, 1.1 General 1.1.3), ore carriers, combination carriers and other ships with similar hatch opening configurations	Class III within 0,6L amidships
C5.	Trunk deck and inner deck plating at corners of openings for liquid and gas domes in membrane type liquefied gas carriers	Class II within rest of cargo region
C6.	Bilge strake in ships with double bottom over the full breadth and length less than 150 m	Class II within 0,6L amidships
		Class I outside 0,6L amidships
C7.	Bilge strake in other ships , see Note 1	Class III within 0,4L amidships
		Class II outside 0,4L amidships
		Class I outside 0,6L amidships
C8	Longitudinal hatch coamings of length greater than 0,15L including coaming top plate and flange	Class III within 0,4L amidships
		Class II outside 0,4L amidships
C9.	End brackets and deck house transition of longitudinal cargo hatch coamings	Class I outside 0,6L amidships Not to be less than Grade D/DH
ADDITIONAL MINIMUM REQUIREMENTS FOR SINGLE STRENGTH DECK SHIPS OF LENGTH GREATER THAN 150 m
D1.	Longitudinal plating of strength deck where contributing to the longitudinal strength	Grade B/AH within 0,4L amidships
D2.	Continuous longitudinal plating of strength members above strength deck
D3	Continuous longitudinal trunk deck plating of membrane type liquefied gas carriers	Class II within 0,4L amidships
D4	Single side strakes for ships without inner continuous longitudinal bulkhead(s) between bottom and strength deck	Grade B/AH within cargo region
ADDITIONAL MINIMUM REQUIREMENTS FOR SHIPS OF LENGTH GREATER THAN 250 m
E1.	Sheerstrake (or rounded gunwale) and stringer plate at strength deck, see Note 2	Grade E/EH within 0,4L amidships
E2.	Bilge strake, see Note 2	Grade D/DH within 0,4L amidships
ADDITIONAL MINIMUM REQUIREMENTS FOR SINGLE SKIN BULK CARRIERS SUBJECTED TO SOLAS REGULATION XII/6.4
F1.	Lower bracket of ordinary side frame, see Notes 6 and 7	Grade D/DH
F2.	Side shell strakes included totally or partially between the two points located to 0,125l above and below the intersection of side shell and bilge hopper sloping plate or inner bottom plate, see Note 7	Grade D/DH
Note 1. Single strakes required to be of Class III and within 0,4L amidships are to have breadths not less than 800 + 5L mm, but need not be greater than 1800 mm, unless limited by the geometry of the ship’s design. Note 2. Single strakes required to be of Grade D/DH or Grade E/EH and within 0,4L amidships are to have breadths not less than 800 + 5L mm, but need not be greater than 1800 mm, unless limited by the geometry of the ship’s design. Note 3. For strength members not mentioned, Grade A/AH may be generally used. Note 4. Steel grade is to correspond to the as-fitted thickness. Note 5. Plating materials for sternframes supporting the rudder and propeller boss, rudders, rudder horns and shaft brackets are, in general, not to be of lower grades than corresponding to Class II. For rudder and rudder body plates subjected to stress concentrations (e.g. in way of lower support of semi-spade rudders or at upper part of spade rudders) Class III is to be applied. Note 6. The term ‘lower bracket’ means webs of lower brackets and webs of the lower part of side frames up to the point of 0,125l above the intersection of side shell and bilge hopper sloping plate or inner bottom plate. Note 7. The span of the side frame, l, is defined as the distance between the supporting structures. Note 8. Corner inserts in way of complex openings such as for lifts and side doors which may impinge on the deck plating or stringer plate are to be of Grade D/DH for t ≤ 20 mm and Grade E/EH for t > 20 mm. Note 9. The material class used for reinforcement and the quality of material (i.e. whether mild or higher tensile steel) used for welded attachments, such as waterway bars and bilge keels, is to be similar to that of the hull envelope plating in way. Where attachments are made to rounded gunwale plates, special consideration will be given to the required grade of steel, taking account of the intended structural arrangements and attachment details. Note 10. The material class for deck plating, sheer strake and upper strake of longitudinal bulkhead within 0,4L amidships is also to be applied at structural breaks of the superstructure, irrespective of position. Note 11. Engine seat top plates outside 0,6L amidships may be Grade A/AH. Steel grade requirement for top plates within 0,6L amidships will be specially considered.

Table 2.2.2 Steel grades

Thickness, t, in mm	Material class
	I		II		III
	Mild steel	H.T. steel	Mild steel	H.T. steel	Mild steel	H.T. steel
t ≤ 15	A	AH	A	AH	A	AH
15 < t ≤ 20	A	AH	A	AH	B	AH
20 < t ≤ 25	A	AH	B	AH	D	DH
25 < t ≤ 30	A	AH	D	DH	D	DH
30 < t ≤ 35	B	AH	D	DH	E	EH
35 < t ≤ 40	B	AH	D	DH	E	EH
t > 40	D	DH	E	EH	E	EH
Note See Notes under Table 2.2.1 Material classes and grades

Table 2.2.3 Grades of steel for refrigerated spaces with a minimum design temperatures below 0°C

Minimum design temperature, in °C	Thickness, in mm		Grades of steel
<0 to –10		t ≤ 12,5	B/AH
	12,5	t ≤ 25,5	D/DH
		t > 25,5	E/EH
<–10 to –25		t ≤ 12,5	D/DH
		t > 12,5	E/EH
<– 25 to –40		t ≤ 12,5	E/EH
		t > 12,5	FH/LT–FH, see also Ch 3, 6 Carbon–manganese and nickel alloy steels for low temperature service of the Rules for Materials

Table 2.2.4 Assessment of deck temperature

Arrangement	Deck temperature
(1) Deck not covered with insulation in the refrigerated space	Temperature of the refrigerated space
(2) Deck covered with insulation in the refrigerated space and not insulated on the other side	Temperature of the space on the uninsulated side
(3) Deck covered with insulation on both sides
(a) Temperature difference not greater than 11°C	Mean of the temperatures of the spaces above and below the deck
(b) Temperature difference greater than 11°C but not greater than 33°C	Mean of the temperatures of the spaces above and below the deck less 3°C
(c) Temperature difference greater than 33°C	Deck temperature will be specially assessed
Note Where one of the internal spaces concerned is not refrigerated, the temperature of the space is to be taken as 5°C.

2.3 Structures exposed to low temperatures

2.3.1 For ships intended to operate in areas with low air temperatures (below −10°C), e.g. regular service during winter seasons to Arctic or Antarctic waters, the material grade requirements of structures above the lowest ballast water line exposed to air are not to be of lower grades than those given in Table 2.2.5 Material classes and grades for structures exposed to low air temperatures , Table 2.2.6 Materials for Class I for low air temperatures, Table 2.2.7 Materials for Class II for low air temperatures and Table 2.2.8 Materials for Class III for low air temperatures

2.3.2 Where the material class in Table 2.2.1 Material classes and grades is higher than in Table 2.2.5 Material classes and grades for structures exposed to low air temperatures , the higher material class is to be applied.

2.3.3 For ships where the optional Winterisation H notation is applied, see Rules for the Winterisation of Ships, July 2022, note that the external design air temperature defined in the Winterisation Rules is taken as 13°C lower than the design air temperature, i.e. if the design air temperature is −11°C, then the external design air temperature for the application of the Winterisation Rules is −24°C.

2.4 Carriage of cold cargoes

2.4.1 For ships other than liquefied gas carriers, which are intended to be loaded with liquid cargo having a temperature below −10°C, e.g. loading from cold onshore storage tanks during winter conditions, the grade of steel for the cargo tank boundary plating is to comply with the requirements of Table 2.2.6 Materials for Class I for low air temperatures where the design air temperature is to be taken as the design minimum cargo temperature in °C. The design minimum cargo temperature is to be specified in the Loading Manual, see Pt 3, Ch 4, 8.2 Loading Manual 8.2.4.

Table 2.2.5 Material classes and grades for structures exposed to low air temperatures

Structural member category	Material class
Structural member category	Within 0,4 L amidships	Outside 0,4 L amidships
SECONDARY	I	I
Deck plating exposed to weather, in general Side plating above CWL, see Note 5 Transverse bulkheads above CWL, see Notes 5 and 6
PRIMARY	II	I
Strength deck plating Continuous longitudinal members above strength deck, excluding longitudinal hatch coamings Longitudinal bulkhead above CWL, see Notes 5 and 6 Top wing tank bulkhead above CWL, see Notes 5 and 6
SPECIAL	III	II
Sheerstrake at strength deck, see Note 1 Stringer plate in strength deck, see Note 1 Deck strake at longitudinal bulkhead, see Note 2 Continuous longitudinal hatch coamings, see Note 3
Note 1. Not to be less than Grade E/EH within 0,4L amidships in ships with length exceeding 250 m. Note 2. In ships with breadth exceeding 70 m at least three deck strakes are to be Class III. Note 3. Not to be less than Grade D/DH. Note 4. Within 0,4L amidships, single strakes which are required to be of Class III or of Grade E/EH or FH are to have breadths not less than 800 + 5L, but need not be taken greater than 1800 mm. Note 5. The Cold Waterline (CWL) is to be taken as 0,3 m below the minimum design Ballast Waterline (BWL). Note 6. Applicable to plating attached to hull envelope plating exposed to cold air. At least one strake is to be considered in the same way as exposed plating and the strake width is to be at least 600 mm. If thermal stress calculations are performed, then the extent of plate requiring consideration is to be adjusted accordingly.

Table 2.2.6 Materials for Class I for low air temperatures

Thickness, mm	Design air temperature
	−11°C to −15°C		−16°C to −25°C		−26°C to −35°C		−36°C to −45°C		−46°C to −55°C
	MS	HT	MS	HT	MS	HT	MS	HT	MS	HT
t ≤ 10	A	AH	A	AH	B	AH	D	DH	D	DH
10 < t ≤ 15	A	AH	B	AH	D	DH	D	DH	D	DH
15 < t ≤ 20	A	AH	B	AH	D	DH	D	DH	E	EH
20 < t ≤ 25	B	AH	D	DH	D	DH	D	DH	E	EH
25 < t ≤ 30	B	AH	D	DH	D	DH	E	EH	E	EH
30 < t ≤ 35	D	DH	D	DH	D	DH	E	EH	E	EH
35 < t ≤ 45	D	DH	D	DH	E	EH	E	EH	n/a	FH
45 < t ≤ 50	D	DH	E	EH	E	EH	n/a	FH	n/a	FH
Note MS and HT are defined as Mild Steel and High Tensile Steel respectively.

Table 2.2.7 Materials for Class II for low air temperatures

Thickness, mm	Design air temperature
	−11°C to −15°C		−16°C to −25°C		−26°C to −35°C		−36°C to −45°C		−46°C to −55°C
	MS	HT	MS	HT	MS	HT	MS	HT	MS	HT
t ≤ 10	A	AH	B	AH	D	DH	D	DH	E	EH
10 < t ≤ 20	B	AH	D	DH	D	DH	E	EH	E	EH
20 < t ≤ 30	D	DH	D	DH	E	EH	E	EH	n/a	FH
30 < t ≤ 40	D	DH	E	EH	E	EH	n/a	FH	n/a	FH
40 < t ≤ 45	E	EH	E	EH	n/a	FH	n/a	FH	n/a	n/a
45 < t ≤ 50	E	EH	E	EH	n/a	FH	n/a	FH	n/a	n/a
Note. MS and HT are defined as Mild Steel and High Tensile Steel respectively.

Table 2.2.8 Materials for Class III for low air temperatures

Thickness, mm	Design air temperature
	−11°C to −15°C		−16°C to −25°C		−26°C to −35°C		−36°C to −45°C		−46°C to −55°C
	MS	HT	MS	HT	MS	HT	MS	HT	MS	HT
t ≤ 10	B	AH	D	DH	D	DH	E	EH	E	EH
10 < t ≤ 20	D	DH	D	DH	E	EH	E	EH	n/a	FH
20 < t ≤ 25	D	DH	E	EH	E	EH	E	FH	n/a	FH
25 < t ≤ 30	D	DH	E	EH	E	EH	n/a	FH	n/a	FH
30 < t ≤ 35	E	EH	E	EH	n/a	FH	n/a	FH	n/a	n/a
35 < t ≤ 40	E	EH	E	EH	n/a	FH	n/a	FH	n/a	n/a
40 < t ≤ 50	E	EH	n/a	FH	n/a	FH	n/a	n/a	n/a	n/a
Note MS and HT are defined as Mild Steel and High Tensile Steel respectively.