INFLUENCE OF PARENT METAL STRENGTH ON THE FATIGUE …

1 INTRODUCTION

Trends towards more effective and lighter structures have increased the interest in using high strength steel for higher performance and reduced weight. In order to benefi t fully from those steels, design and manufactu-ring techniques have to be adjusted to the properties of

the steel and to the higher working stresses associated with effective use of these steels.

Design measures for avoiding welds in highly stressed regions and the introduction of post-weld improvement of critical welds mean that unwelded sections also have to be assessed in the design of welded structures.

There is need for more of such data and in particular how the strength of the parent material infl uences the fatigue resistance. It is also important to know how this resistance is affected by the roughness of rolled surfaces and cut edges.

INFLUENCE OF PARENT METAL STRENGTH ON THE FATIGUE STRENGTH OF PARENT MATERIAL WITH MACHINED AND THERMALLY CUT EDGES 79

INFLUENCE OF PARENT METAL STRENGTHON THE FATIGUE STRENGTH

OF PARENT MATERIAL WITH MACHINEDAND THERMALLY CUT EDGES

J-O. Sperle

Sperle Consulting AB(Sweden)

ABSTRACT

Trends towards more effective and lighter structures have increased the interest in using high strength steels for higher performance and reduced weight. Design measures for avoiding welds in highly stressed regions and the introduction of post-weld improvement of critical welds mean that unwelded sections also have to be assessed in the design of welded structures. In this investigation, hot rolled strip steels in thicknesses of 6-12 mm with mini-mum yield strengths from 240 to 900 MPa are fatigue tested under constant amplitude pulsating tensile loading. Specimens with as rolled surface and with machined, laser cut, plasma cut and oxygen cut edges are tested. The results are evaluated according to the IIW recommended practice. The surface roughness Rz has been measured for different surface conditions, including ground and as rolled surfaces, and compared to literature data. Thermal cutting methods have been developed appreciably over the years, especially for laser and plasma cutting. This means that the FAT classes evaluated here are higher than those recommended by IIW. The results show that if the quality of cutting can be kept high, the fatigue strength of strip steel, especially strip with laser and plasma cut edges, can be of the same order as that of steel strip with machined edges. Both empirical and physically based models are presented for the infl uence of surface roughness on the fatigue strength at different steel strength. A tentative recommendation is given for a steel strength enhancement factor for the infl uence of yield strength on the fatigue resistance at different values of surface roughness.

IIW-Thesaurus keywords: Fatigue strength; High strength steels; Mechanical properties; Parent material; Reference lists; Steels; Thermal cuts.

Doc. IIW-1861-07 (ex-doc. XIII-2174r1-07) recom-mended for publication by Commission XIII “Fatigue of welded components and structures”.

Welding in the World – Research Supplement


In this investigation, hot rolled strip steels in thicknes-ses of 6-12 mm with minimum yield strengths from 240 to 900 MPa are fatigue tested under constant ampli-tude pulsating tensile loading. Specimens with as rolled surface and with machined, laser cut, plasma cut and oxygen cut edges are tested. The results are evalu-ated according to the IIW recommended practice with m = 5, and a tentative recommendation is given to a steel strength enhancement factor for the infl uence of yield strength on the fatigue resistance.

2 EXPERIMENTAL DETAILS

2.1 Materials and test specimens

The materials tested were microalloyed HSLA steels characterised by low content of alloying elements and low carbon equivalent in combination with very good

cold forming properties. The steel strengths tested cover the range from mild steel with minimum yield strength of 240 MPa up to ultra-high strength steel with 900 MPa minimum yield strength. The thicknesses tested varied from 6 to 12 mm.

In order to minimize the stress concentration of the test specimens, a streamlined fi llet was used. The contour of the waist was approximated by fi ve radii, starting from r = 960 mm near the middle of the specimen to r = 30 mm at the end of the waist (Figure 1).

The chemical composition of the different steel grades tested is shown in Table 1 and tensile properties and edge conditions tested are shown in Table 2.

The results of tests on the 12 mm specimens have been published earlier in [1], although not the indi-vidual data points, nor have the results been exami-ned according to the IIW recommended practice.

Table 1 – Chemical composition of steels tested

SteelNo

GradeChemical analysis

C Si Mn P S N Cr Cu Mo Al Nb V Ti B

1 Domex 240 0.071 0.03 0.60 0.013 0.015 0.005 0.040

2 Domex 310 0.060 0.03 0.60 0.013 0.012 0.005 0.035 0.014

3 Domex 350 0.068 0.02 0.60 0.007 0.009 0.005 0.053 0.038

4 Domex 420 0.069 0.03 0.60 0.010 0.012 0.005 0.028 0.050

5 Domex 500 0.065 0.20 1.50 0.010 0.004 0.010 0.10 0.025 0.040 0.085

6 Domex 590 0.093 0.19 1.72 0.010 0.001 0.015 0.13 0.029 0.026 0.14

7 Domex 650 0.070 0.10 1.75 0.010 0.001 0.012 0.13 0.040 0.030 0.14

8 Domex 700 0.060 0.05 1.85 0.012 0.002 0.005 0.13 0.025 0.060 0.11

9 Domex 900 0.081 0.05 0.77 0.010 0.002 0.005 1.00 0.243 0.031 0.02 0.0017

10 Domex 350 0.057 0.02 0.59 0.007 0.005 0.005 0.037 0.028

11 Domex 590 0.091 0.20 1.61 0.008 0.003 0.015 0.18 0.038 0.023 0.15

12 Domex 700 0.062 0.05 1.87 0.008 0.002 0.005 0.023 0.062 0.12

Figure 1 – “Streamlined” specimen for fatigue test

INFLUENCE OF PARENT METAL STRENGTH ON THE FATIGUE STRENGTH OF PARENT MATERIAL WITH MACHINED AND THERMALLY CUT EDGES80


2.2 Edge preparation

Thermal cutting of the edges of the 12 mm specimens was carried at different sites at Volvo Construction Equipment (VCE) and the 6 mm specimens were laser cut by HIAB AB. All machining of edges was done by SSAB Tunnplåt AB. The corners between the rolled surface and the machined edge were lightly cham-fered with emery cloth. No treatment of the corners was done on the thermally cut specimens. In order to

minimize the scatter, only results of tests on as rolled and not blasted surfaces or edges are included in this report. The data that were available for the thermal cut-ting processes are shown in Table 3.

Measurements of the surface roughness were made on some specimens and the average values are given in Table 4.

Additional surface roughness measurements have been carried out on as rolled surfaces and thermally cut edges from other batches of similar steels. These results are discussed later in the report.

2.3 Fatigue tests

All fatigue tests were performed in either a 700 kN MTS hydraulic testing machine or in an Amsler 1 000 kN pul-sator at SSAB Tunnplåt. The test frequency was 7-10 Hz and the tests were performed in tension at constant amplitude loading with R = 0. The fatigue strength was determined in the life range of 5x104 – 2x106 cycles. However, it was diffi cult to reach the low cycle region, especially for the low strength steels, since the stress was limited by the yield strength. There were few test results from failed specimens that had fatigue lives lon-ger than 106 cycles, the constant amplitude fatigue limit being rather high and close to the yield strength.

3 RESULTS AND DISCUSSION

3.1 Primary results of fatigue tests

All individual test results are plotted in graphs 1-4 in Appendix I. Statistical evaluation of the S-N curves has been carried out in accordance with the IIW recom-mended practice [2] with a fi xed slope of m = 5, repre-


Table 2 – Tensile properties of steelsand tested edge conditions

SteelNo

GradeEdge

conditiont

mmReH

MPaRm

MPa

1 Domex 240 Machined 6 292 396








9 Domex 900 Machined 6 945 1 043

10 Domex 350 Laser cut 12 380 481

11 Domex 590 Laser cut 12 642 765

12 Domex 700 Laser cut 6 724 805

9 Domex 900 Laser cut 6 945 1 043

3 Domex 350 Plasma cut 12 428 514

6 Domex 590 Plasma cut 12 608 720

10 Domex 350 Oxygen gas cut 12 380 481

Table 3 – Data for thermal cutting

Cutting methodThickness

mmPower

kWSpeedm/min

CurrentkA

Pressurebar

Cutting gas

Laser 6 2.6 2.5 He, CO2, N2

Laser 12 1 1.1 0.6

Plasma 12 3.5 0.2 0.6

Oxygen 12 0.4

Table 4 – Results of surface roughness measurements on fatigue test specimens

GradeEdge

condition

t Surface Edge

mm Rz Ra Rz Ra

Domex 350 Laser cut 12 25

Domex 590 Laser cut 12 25

Domex 700 Laser cut 6 8.8 1.46 13.3 2.41

Domex 900 Laser cut 6 14.1 2.04 10.9 1.90

Domex 350 Plasma cut 12 8.1 1.23 30.6 6.70

Domex 350 Oxygen gas cut 12 36.0 6.60


senting unwelded parent material. Test results which do not belong to the statistical population when m = 5 are omitted in the analysis. These test results are both those that are affected by the yield strength and those that are affected by the fatigue limit. In accordance with normal practice, no run outs are included in the analysis. All results that are excluded in the statistical analysis are shown as open symbols in the graphs. S-N curves of the form log N = log C –m.log Sr are fi tted to the test results. The fatigue strength, Sr, at N = 106 and N = 2x106 cycles, according to graphs 1-5 in Appendix I, are given in Table 5. The table also shows the standard deviation in log C (Stdv), FAT5 %

and the point of crack initiation.

The point of crack initiation is governed by the balance in quality between the rolled surface and

the cut edges. The presence of secondary bending stresses, which are diffi cult to avoid, is one probable explanation for the high frequency of failures that start at the corner.

3.2 Infl uence of parent metal strength

For unwelded parent material, the fatigue strength nor-mally increases with increasing parent metal strength. This is also confi rmed by the results from this investi-gation (Figure 2).

Such behaviour was expected on specimens with machined edges, but is somewhat surprising on speci-mens with thermally cut edges. Fatigue tests on beams and specimens with oxygen cut edges in 20 mm plate

Table 5 – Results of fatigue tests, individual test series

SteelNo

Grade Edge condition Sr, 106 Sr, 2x10

6 Stdv FAT5%

Point of crack initiation %

Surface Corner Edge

1 Domex 240 Machined 281 245 0.14 206 - - -

2 Domex 310 Machined 298 259 0.06 236 100

3 Domex 350 Machined 264 230 0.17 188 100

4 Domex 420 Machined 366 319 0.09 287 100

5 Domex 500 Machined 359 312 0.12 269

6 Domex 590 Machined 358 311 0.12 271 50 50

7 Domex 650 Machined 392 341 0.15 290 - - -

8 Domex 700 Machined 418 364 0.12 316 - - -

9 Domex 900 Machined 450 392 0.10 350 - - -

10 Domex 350 Laser cut 294 256 0.12 217 25 75

11 Domex 590 Laser cut 351 306 0.16 245 89 11

12 Domex 700 Laser cut 411 358 0.17 304 83 17

9 Domex 900 Laser cut 438 381 0.16 321 13 87

3 Domex 350 Plasma cut 313 272 0.18 218 5 95

6 Domex 590 Plasma cut 353 308 0.08 273 100

10 Domex 350 Oxygen gas cut 261 227 0.14 190 100

Sr = 28,564R e0,3973

Sr = 39,269R e0,3426

Sr = 26,664Re0,4135

200

250

300

350

400

450

500

200 300 400 500 600 700 800 900 1000

Yield Strength, Re (MPa)

Fatig

ue s

tren

gth

Sr,E

6 (M

Pa)

Machined edges

Laser cut edges

Plasma cut edges

Oxygen cut edges

Figure 2 – Infl uence of yield strength on the fatigue strength at N = 106 cycles



steel did not show such an infl uence of parent metal strength [3]. This indicates that the cutting quality was worse in this case and that crack-like defects were present, which governs the fatigue process so that it is dominated by crack propagation rather than crack initiation.

Another aspect of this is that the as rolled surface itself plays a role in the fact that the surface roughness of plate steel is normally worse than that of strip steel. Sonsino et al. [4] have shown that removing the rolled surface by grinding after oxygen cutting of a 15 mm plate steel resulted in an increased fatigue strength with increasing yield strength (Re = 350, 690 and 890 MPa), while no such effect was noted for specimens when the rolled surface was left untreated.

A factor that would be of importance to the fatigue strength of thermally cut edges is the state of resi-dual stresses. No residual stress measurements were performed on the fatigue test specimens tested here. However, such measurements were reported in [4], where longitudinal residual tensile stresses of about 300 MPa were measured on the oxygen cut surface of specimens with the rolled surface ground off. Residual compressive stresses of -50 to -300 MPa were measured between 0.6 to 1.0 mm under the cut surface and again residual tensile stresses of 250-600 MPa appeared 1.6 to 2.8 mm below the surface. General plastic straining of the specimens with 0.2 % strain relaxed the residual compressive stresses but did not reduce the fatigue strength, while further plastic straining to 1 % resulted in a considerable reduction in fatigue life, probably due to the introduc-tion of micro cracks in the martensitic surface layer. However, this statement could not be fully clarifi ed by the test results.

3.3 Fatigue class FAT referred to mild steel

Fatigue classes, FAT, according to the IIW classifi ca-tion system have been evaluated by recalculating the fatigue data in Graphs 1 to 4 in Appendix I to the yield strength of mild steel chosen here to be Re = 280 MPa. The result of this recalculation, which is based on the infl uence of yield strength in Figure 2, is shown in Appendix II, Graphs 5 to 7. Since results on oxygen cut surfaces are only available for one yield strength level, this edge condition is not included in the ana-lysis. Evaluated fatigue classes FAT, which refer to 2x106 cycles, are shown in Table 6. The parameters

xm and k refer to the calculation of the characteristic value FAT5%.

The values in Table 5 refer to tests with R = 0. When FAT is evaluated from test results at R = 0, the IIW recommendation suggests that the FAT value should be reduced by 20 % to allow for high residual stresses (R = 0.5). If this is applied here, FAT for design becomes close to or just above the value IIW recommends for as rolled parent material with machined edges or seamless tube sections, i.e. FAT = 160, for all edge conditions analysed above. This also means that the FAT values for thermally cut edges is higher in this investigation than the values given in the IIW recommendation for thermally machine cut edges that have FAT values of 125 – 140, depending on subsequent treatment of cor-ners and cutting imperfections.

3.4 Measurement of surface roughness

In order to obtain a broader database for the surface and edge roughness, additional measurements have been carried out on different as rolled and thermally cut surfaces in steels similar to those tested under fatigue loading [5]. Results from those measurements, expressed as peak to valley height Rz and average height of the surface roughness Ra, are shown in Figures 3 and 4. When Rz is quoted in this report, it always refers to an average of fi ve measurements, i.e. Rz5.

The requirements on surface roughness of thermally cut edges according to EN ISO 9013:2002 [6] are also given in Figure 4.

We note that the laser cut surfaces in particular show a broad range of surface roughness and that the laser cut edges of Domex 700 and Domex 900 steels which are tested in fatigue in this investigation are placed low in the scatter band. These specimens, having a surface roughness of Rz =11-13 μm, conform to Range 1 in the ISO standard. One possible reason for these low values is that the laser cutting performed by HIAB was car-ried out with a cutting gas containing helium. This gas, which is mainly used for cutting stainless steel, gives good cutting quality and is used by HIAB in regular production.

In order to broaden the database even further, the above surface roughness ranges are compared in Figure 5 to data from the literature.

Table 6 – Fatigue class FAT at N = 2x106

Edge condition “FAT” 50% Stdv xm k FAT5% 0.8xFAT5%

Machined 243 0.14 18.23 1.90 216 173

Laser cut 242 0.18 18.22 1.95 206 165

Plasma cut 239 0.14 18.19 2.39 204 163



3.5 Relation between fatigue strength, surface roughness and parent metal strength

It is well known that steel surfaces con-tain minute flaws that can reduce the fatigue strength, and that the fl aw size at which this reduction becomes apparent decreases with increasing tensile strength of the parent metal. For machined surfaces, relations between a fatigue reduction factor, surface roughness and parent metal tensile strength have long been established [7].

Such relations, somewhat modifi ed, have proved to hold also for the infl uence on the fatigue strength of the type of surface roughness that appears on as rolled sur-faces [11, 12]. A baseline fatigue strength Sr* was then evaluated representing a fi ctitious value of the fatigue strength for

Figure 3 – Results of surface roughness measurements onthe strip surface

Figure 4 – Results of surface roughness measurements on the thermally cut edges

Figure 5 – Surface roughness

measurements compared

to literature data



smooth specimens. This calculation was carried out by applying a surface factor Kr to experimental results according to the equations below.

Sr* = Sr (Ra) .Kf.Kr (1)

where

Kf = f (Kt, r, Rm) according to conventional Neuber theory

Kr = f (Ra, Rm) is a surface factor defi ned by

1/Kr = 1 – 0.000254.Rm.ln(Ra + 1) Note 1 (2)

NOTE 1 The relation for Kr is valid as long as the fatigue process is governed by crack initiation. When the steel enhancement factor f(Re ) according to Figure 7 falls off, f(Re ) is set to its maximum value for the actual value of Rz and the curve continues horizontally, which represents a fatigue process governed by crack growth.

One hypothesis in this investigation is to see whether the relation between fatigue strength, surface rough-ness and steel strength found for as rolled surfaces could be extended to thermally cut edges. A recalcu-lation of the results in this investigation to a baseline fatigue strength Sr* is shown in Figure 6 for specimens with machined, laser cut and plasma cut edges.

Based on this curve, a steel strength enhancement factor, f(Re), for the infl uence of yield strength on the

fatigue resistance could be recalculated for different values of Rz, based on equations (1) and (2), Figure 7. Rz is then assumed to be 6 Ra. The fatigue strength of mild steel with R e= 280 MPa is chosen as the basis for the calculation of the factor f(Re). Data from this investi-gation as well as data for cold reduced steel from [11, 12] are plotted in Figure 7 for comparison.

It will be noted from this comparison that it looks as though the method used for as rolled steel surfaces can also be used for thermally cut surfaces. Howe-ver, there are, of course, factors such as different residual stresses as well as different types of sur-face morphology and cutting defects that will have an effect on the fatigue strength, even though the same surface roughness Rz is measured. However, the Rz value and the preparation of corners and cut-ting defects currently seem to be the factors mostly referred to in standards for cutting quality. There is a clear indication that the strip steels with laser cut edges included in this investigation, which meet to demands for Range 1 according to ISO 9013:2002 (Figure 4) give increasing fatigue strength with incre-asing steel strength, which is of the same order as for steel with machined edges.

Figure 6 – Results of fatigue tests from this investigation and the calculated baseline fatigue strength Sr*, N = 106 cycles

Sr* = 9,8989R e0,6071

200

300

400

500

600

700

800

200 300 400 500 600 700 800 900 1000

Yield Strength, R e (MPa)

Fatig

ue s

tren

gth

S r, E

6 (

MPa

)

Machined edges

Laser cut edges

Plasma cut edges

Sr*

Figure 7 – Steel strength enhancement factor, with results from this investigation plotted in,reference yield strength Re = 280 MPa

1,00

1,20

1,40

1,60

1,80

2,00

2,20

2,40

200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400

Yield strength Re (MPa)

Ste

el s

tren

gth

enha

ncem

ent f

acto

r f(

Re) Rz=2,5

Rz=5

Rz=10

Rz=15

Rz=40

Rz=30

Rz=20

Rz=120

Rz=60

Rz=25

Rz=13

Rz=11

HR surface, machined edges Rz= 10 - 20

Rz=31

HR surface, Laser cut edges

HR surface, Plasma cut edges

CR surface, machined edges Rz= 3 - 8



3.6 Metallographic investigationof test specimens

Test samples of specimens from this investigation have been examined, with special emphasis on the geometry of thermally cut edges. In summarizing the metallo-graphic observations, it can be said that the surfaces at low magnifi cation look very similar, although the plasma cut edge show a higher degree of waviness. The defect sizes at micro level are very small and a rough estimate is shown in Table 7. The fi gures given are not typical values, but are values found on ran-domly chosen specimens.

The hardness of the laser cut surfaces has been measured on steels with the highest strengths. The Vickers hardness Hv0.5 increased from 300 to 400 within 0.5 mm from the laser cut edge on Domex 700 and from 350-375 to 500-550 on Domex 900. A nar-row soft zone situated 0.3 to 0.4 mm below the cut surface, with a hardness drop down to 300, was recorded on Domex 900.

3.7 A fracture mechanics approachto the infl uence of surface roughnessand steel strength on the fatigue strength

The micro defect levels measured above do not indi-cate the sizes that are critical from the fracture mecha-nics point of view at normal fatigue strength levels. The

results show a clear effect of parent metal strength and the fatigue process must be dominated by crack initia-tion rather by crack growth. In investigations in which no effect of steel strength is found, larger crack-like defects are probably present. The relation between defect size and steel strength can be described with a so-called Kittagawa diagram, where the decreasing infl uence of steel strength with increasing defect size is well illustrated. Figure 8 describes schematically how this can look, with some ranges of defect sizes exem-plifi ed.

The problem of initiation and propagation of short cracks can be treated with methods proposed by Smith [13], Topper and El Haddad [14]. They present methods that have been developed further by Olsson [15, 16], models which catch both the infl uence of sur-face roughness and steel strength. Their work, which ends up in the following surface factor Kr, is physically based and its background is described in more detail in Appendix III.

A surface factor is defi ned by

Kr = (1+a.(2.Sro/ΔKth)2)0.5 (3)

where

Sro = fatigue strength of smooth specimen, here assu-med to be Sr*

ΔKth = threshold value, here assumed to be 2 for small defects that are assumed to be open and not experi-ence any crack closure

Table 7 – Rough estimate of micro defect geometry on specimens includedin the metallographic investigation

Surface/EdgeRadius in micro defect

(microns)Depth of micro defect

(microns)Surface roughness Rz

(microns)

As rolled (HR) 25 10-20 12

Laser cut 25-30 15 12

Plasma cut 6-10 20 31

Oxygen gas cut 50-80 14 36

Figure 8 – Kittagawa diagram showing the shift from crack initiation to crack growth governed fatigue process, shown schematically

0,001 0,01 0,1 1 10

Crack initiation

Crack growth

EHS

HS

MS

Weld toe

Root side of weld

Cold reduced surface Laser/Plasma cut edge

Hot rolled strip, rolled surface

Defect size a mm

Fat

igu

e S

tren

gth

(lo

g s

cale

)

Crack initiation

Crack growth

EHS

HS

MS

Weld toe

Root side of weld

Cold reduced surface Laser/Plasma cut edge

Hot rolled strip, rolled surface



a = crack depth, conservatively assumed to be equal to Rz

In the “long crack regime”, the fatigue strength and the infl uence of steel strength are limited by

Srp = (ΔKth/√a)/2 (4)

where

ΔKth = threshold value, here taken to be ΔKth = 6 - 4.56.R according to the IIW recommendation

Srp = fatigue stress needed to propagate a crack of size a

a = crack depth, conservatively assumed to be equal to Rz

The fatigue strength Sr = Sro/Kr according to the model above has been plotted with dotted lines in Figure 9, where the fatigue strength evaluated with the empirical formula, equation (2), which is also related to the steel strength enhancement factor f(Re) in Figure 7, is shown as unbroken lines. The limiting threshold stress accor-ding to equation (4) above and with data according to the IIW recommendation is also shown in the fi gure for R = 0 and R = 0.5.

From the physical point of view, there are two regimes, the fi rst of which is up to defect sizes of 0.1 - 0.2 mm, up to which the fatigue process is controlled by “crack initiation” with decreasing infl uence of steel parent metal strength as the defect size increases. The second regime is controlled by crack growth, with no infl uence of steel strength. This transition point would be related to the degree of crack opening and consequently to the state of residual stress. The local R value will therefore also infl uence the defect level (surface roughness) at which the infl uence of steel strength will “fade out”.

The fatigue behaviour of thermally cut edges is a rather complicated process, where the combination of sur-face geometry and state of residual stress will have an infl uence. This is one reason why we have omitted in this investigation all test series with blasted surfaces or blasted cut edges, so that we would have only the

infl uence of the inherent surface roughness and resi-dual stresses from the machining or cutting.

3.8 Results reported by otherson the infl uence of steel strengthon the fatigue strength of thermally cut edges

Not very many results have been reported in which the infl uence of steel strength could be evaluated for fatigue tests with the same type of specimen tested under the same loading conditions.

However, some results are included here and, wherever possible, the result have been re-analysed with a slope of m = 5. Bergqvist and Sperle [3] found no infl uence of yield strength on the fatigue strength when testing large, 20 mm thick H beams or on corresponding small specimens with oxygen gas cut edges tested with steels ranging in yield strength from 240 to 870 MPa. Data on the surface roughness of the cut surfaces is somewhat unclear but could have been up to Rz = 100, which can explain the absence of the infl uence of steel strength.

Sonsino, Kauffmann et al. [4] found the same beha-viour on 15 mm thick steel specimens with oxygen cut edges and with steel strength ranging from 355 to 890 MPa. However, tests with the same type of speci-mens with the millscale removed show a clear infl uence of steel strength (Figure 10). The explanation of this given by the authors is that the surface condition of the as rolled surface was more critical than the cut edge, considering both surface roughness and residual stres-ses. The tests were performed in tension/compression loading, R = -1. Based on a few reported comparative tests at R = 0, giving a factor of Sr, R=0/Sr, R= -1 of 0.79, which is close to the rule of thumb of 0.8, the results have been recalculated to R = 0. These results, which show a strong infl uence of steel strength, are included in Figure 10, where results from this investigation are compared to literature data.

Figure 9 – Fatigue strength as a function of surface roughness with steel strength as parameter

Crack growth, DKth=6, R=0

Crack growth, DKth=3.72 R=0.5

100

1000

0001001011

Surface roughness Rz (μm)

Fatig

ue s

tren

gth

S r E

6 (

MPa

)

Yield strength Re (MPa)

350

220

500

700

900 Dotted lines refer to the model based on fracture mechanics



Byggnevi and Mrden [9] present some recent results on 10 mm specimens with laser cut edges with R = -1. Unfortunately, only one steel strength level was tested. These results recalculated to R = 0 are shown in Figure 10 and are close to what would be expected, considering the results from laser cut edges presented in this investigation.

Eylmann and Paetzold [18] report results on 10 mm steel with 690 MPa yield strength with plasma cut as well as on fl ame cut edges and on underwater plasma cut edges. For the latter two, data is only given in the low cycle region and those results are not included here. Paetzold [19] also reports fatigue test results on the same type of specimens of low and medium strength steel with edges cut with underwater plasma. However, these results are surprisingly low considering the low surface roughness values given in the report. A test series with the plasma cut edges ground is just above the scatter band for machined edges from this investigation.

4 CONCLUSIONS

Fatigue tests at constant amplitude pulsating tensile loading show results for hot rolled strip specimens with as rolled surface and with laser and plasma cut edges which are in line with or close to those for specimens with machined edges. The fatigue strength of specimens with oxygen gas cut edges was somewhat lower.

A clear infl uence of steel strength on fatigue strength was found for specimens with machined, laser cut and plasma cut edges, even though no bevelling of the cor-ners was done on the thermally cut specimens. For gas cut edges, only one yield strength level was tested and no information on steel strength dependence could be evaluated for this edge condition.

Surface roughness measurements indicate values of Rz = 3-9 and Rz = 8-18 for cold reduced and hot rolled strip surfaces respectively. The surface rough-ness of laser cut edges varied from Rz = 8 to 47. The laser cut edges for specimens tested in this investiga-tion had Rz = 10-20, which is in line with Range 1 in ISO 9013:2002. Plasma cut edges varied from Rz = 15 to 42 and oxygen gas cut values are Rz = 32-47.

By recalculating the results from this investigation to the mild steel strength, FAT classes according to IIW were evaluated to FAT 163 for plasma and FAT 165 and laser cut edges after a 20 % reduction of FAT to allow for full residual stresses (R = 0.5), since the tests were perfor-med at R = 0. This is in good agreement with FAT 160, which is what IIW recommends for parent metal with machined edges.

The FAT values for thermally cut edges are higher in this investigation than the values given in the IIW recom-mendation for thermally machine cut edges which have FAT values of 125–140, depending on subsequent treat-ment of corners and cutting imperfections. The thermal cutting methods have been developed quite substanti-ally over the years, especially laser and plasma cutting and an adjustment of the IIW recommendation in this respect would be justifi ed.

Both empirical and fracture mechanics based models for the infl uence of steel strength on the fatigue strength at different surface roughness levels describe fairly well the behaviour obtained from test results.

A tentative recommendation is given to a steel strength enhancement factor for the infl uence yield strength on the fatigue resistance at different values of surface roughness. In brief, it results in a steel strength depen-dence at N = 106 cycles of Sr ~ Re

0.4 for hot rolled strip steel with machined and laser cut edges, and Sr ~ Re

0.35

for plasma cut edges. This is based on an evaluation of

Figure 10 – Literature data on the fatigue strength of specimens with thermally cut surfaces comparedto the results from this investigation, N = 106 cycles

200

250

300

350

400

450

500

550

600

200 300 400 500 600 700 800 900 1000 1100

Yield Strength, R e (MPa)

Fatig

ue s

tren

gth

Sr,E

6 (

MPa

)

Machined edges [This inv]

Laser cut edges [This inv]

Laser cut, 10 mm, R=-1, recalculated to R=0, Rz=36 [8]

Plasma cut edges [This inv]

Oxygen cut edges [This inv]

Oxygen gas cut, mill scale removed, R=-1, recalculated toR=0, Rz=40-60 [3]

Flame cut, 10 mm, as cut, Rz=114 [17]

Plasma cut in air, 10 mm, Rz=28 [17]

Underwater Plasma, 10 mm, Rz=7 [18]

Underwater Plasma, 10 mm, Rz=14-30 [18]

Underwater Plasma, 10 mm, edges ground, Rz=2 [18]



S-N curves according to IIW with a slope of m = 5 and with no consideration of a fatigue limit.

REFERENCES

[1] Trogen H., Olsson K-E., Spennare H.: Infl uence of cut-ting method and material static strength on the fatigue strength of test specimens, Proceedings of the Fist North European Engineering and Science Conference of Welded High Strength Steel structures, Stockholm, 1997.

[2] Hobbacher A.: Recommendations for fatigue design of welded joints and components, IIW doc. XIII-2151r1-07/XV-1254r1-07.

[3] Bergqvist L., Sperle J-O.: Fatigue strength of high strength steel beams with thermally cut surfaces, Doc. IIW-644-80 (ex doc. XIII-919-79), Welding in the World, 1980, Vol. 18, No 9/10, pp. 208-220.

[4] Sonsino C.M., Kaufmann H., Müller F., Berghöfer U.: Schwingfestigkeit von hochfesten Feinkornbaustählen im brenngeschnittenen Zustand, Studiengesellshaft Stahl-anwendung e.V., Projekt 185 – Forschungsbericht Juni 1992.

[5] Nilsson T., Jansson S.: A comparison of Domex steel and other cold forming steel with laser cut edges, SSAB Tunnplåt, Internal technical report no 970524 (in Swe-dish).

[6] SS-EN ISO 9013: Thermal cutting - Classifi cation of thermal cuts – Geometrical product specifi cation and qua-lity tolerances (ISO 9013:2002), 2003-03-21, revision 2.

[7] The effect of surface roughness on the fatigue limit of steels at zero mean stress, Engineering Sciences Data Unit (ESDU), Publication no 74027, 1974.

[8] Howse D.S., Hilton P.A.: A cutting process comparison, TWI report 605/1997.

[9] Byggnevi M., Mrden N.: Fatigue performance of gas cut edges at R= -1, Proceedings of Int. Symposium on Integrated Design and Manufacturing of Welded Structu-res, March 13-14 2007, Eskilstuna, Sweden.

[10] Fricke W., Weissenborn, Eggert: Erschliessung des hochfesten Stahls für den Schiffbau, Technische Univer-sität Hamburg-Harburg, Bericht Nr. 1/2004.

[11] Sperle J-O., Nilsson T.: The application of high strength steels for fatigue loaded structures, Proceedings of HSLA Steels Conference, Beijing, 1992.

[12] Gustafsson M. et al.: Internal database on fatigue test results, SSAB Tunnplåt AB.

[13] Smith R.A.: Fatigue thresholds – A design engineer’s guide through the jungle, Proceedings of the Conference on Fatigue Thresholds, Stockholm, 1981.

[14] Topper T.H., El Haddad M.H.: Fatigue strength pre-diction of notches based on fracture mechanics, Procee-dings of the Conference on Fatigue Thresholds, Stock-holm, 1981.

[15] Olsson K-E.: Infl uence of surface roughness and steel strength - Proposal for a surface roughness factor (in Swe-dish), Internal document, Kea Quality, November 2005.

[16] Olsson K-E.: Private communication.

[17] Takahashi K., Murakami Y.: Quantitative evaluation of the effect of surface roughness on the fatigue strength, Report from Kyushu University, Japan.

[18] Eylmann S., Paetzold H.: High tensile steel in ship structures - Fatigue tests on simple specimens, Tech-nische Universität Hamburg-Harburg, TUHH Report TD 03.52.05.01, 2004.

[19] Paetzold H.: Beitrag zur Gütesicherung Unterwas-ser – Plasmageschittener Blechkanten in Der Schiffskon-struktion, Institute für Schiffbau der Universität Hamburg, Bericht Nr. 211/1989.

APPENDIX I:1 – BASIC FATIGUE DATA

Graph 1 – Specimens with machined edges

100

1000

70+E,160+E,150+E,140+E,1 Cycles

Dx 240 Dx 240 ro Dx 240

Dx 310 Dx 310 ro Dx 310

Dx 350 Dx 350 ro Dx 350

Dx 420 Dx 420 ro Dx 420

Dx 500 Dx 500 ro Dx 500

Dx 590 Dx 590 ro Dx 590

Dx 650 Dx 650 ro Dx 650

Dx 700 Dx 700 ro Dx 700

Dx 900 Dx 900 ro Dx 900

Sr (MPa)



APPENDIX I:2 – BASIC FATIGUE DATA

Graph 2 – Specimens with laser cut edges

100

1000

70+E,160+E,150+E,140+E,1 Cycles

Dx 350 Dx 350 ro Dx 350

Dx 590 Dx 590 ro Dx 590

Dx 700 Dx 700 ro Dx 700

Dx 900 Dx 900 ro Dx 900

Sr (MPa)

Graph 3 – Specimens with plasma cut edges

100

1000

70+E,160+E,150+E,140+E,1 Cycles

Dx 350 Dx 350 ro Dx 350

Dx 590 Dx 590 ro Dx 590

Sr (MPa)

Graph 4 – Specimens with oxygen cut edges

100

1000

70+E,160+E,150+E,140+E,1 Cycles

Dx 350 Dx 350 ro Dx 350

Sr (MPa)



APPENDIX II:1

Baseline for Re= 280 MPa - correction performed for the infl uence of yield strength

100

1000

70+E,160+E,150+E,140+E,1 N cycles

Dx 240

Dx 310

Dx 350

Dx 420

Dx 500

Dx 590

Dx 650

Dx 700

Dx 900

Mean m=5

FAT 5%

Sr (MPa)

100

1000

70+E,160+E,150+E,140+E,1 Cycles

Dx 350

Dx 590

Dx 700

Dx 900

Mean m=5

FAT 5%

Sr (MPa)

100

1000

70+E,160+E,150+E,140+E,1 Cycles

Dx 350

Dx 590

Mean m=5

FAT 5%

Sr (MPa)

Graph 5 – Specimens with machined edges

Graph 6 – Specimens with laser cut edges

APPENDIX II:2

Baseline for Re= 280 MPa - correction performed for the infl uence of yield strength

Graph 7 – Specimens with plasma cut edges



APPENDIX III

Background to equation (3)

Normal fracture mechanics theory for “long” cracks is not applicable to small defects such as surface rough-ness. By adding a constant ao to the crack length,

Topper and Haddad assert that both short and long cracks could be handled with the same model, which has proved to be in agreement with experimental data [14]. Takahashi [17] has shown that surface defects act as cracks, although they sometimes rest as non propa-gating cracks, Figure III b below. Based on the above reasoning, the following equation is formulated for a surface crack.

ΔKth = 1.12.Sr. ( π. (a + ao))

0.5 (5)

When a approaches 0, the fatigue strength Sr approa-ches the fatigue strength for a smooth specimen Sro and with 1.12. √π ≈ 2 we arrive at

ao = (ΔKth / (2.Sro)

2 (6)

The surface factor Kr is Sro / Sr = (1 + a / ao)0.5 which gives

Kr = (1 + a. ( 2.Sro / ΔKth))0.5 (7)

Sr = Sro / Kr (8)

A fi rst approximation of Sro is 0.5.Rm . However, in this analysis, we use Sr* according to Figure 5. The term ao in equation (5) is a “material parameter” and how this as well as ΔKth relate to material strength is discussed further in [15].

a) Elastic stresses at a notch root b) Fatigue regimes in notched specimens

Figure III [13]


INFLUENCE OF PARENT METAL STRENGTH ON THE FATIGUE …

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