Case Studies 6: Steel Bridge Plate Welded Lap Joints and Cruciform, G40.21 350W (ASTM A572 Gr.50) Steel



Fatigue Design and Evaluation Committee
Web Page: https://fde.uwaterloo.ca/Fde/CaseStudies/GhahremaniPhD/ghahremaniCase6.html
Data References:
1. K.Ghahremani, "Fatigue Assessment of Repaired Highway Bridge Welds Using
Local Approaches," PhD Thesis, Dept. Civil Engr., U.of Waterloo, 2015
2. K.Ghahremani "Predicting the Effectiveness of Post-Weld Treatments Applied
under Load" MSc. Thesis, Civil Eng., U.of Waterloo 2010.
3. El Haddad, M.H., "A Study of the Growth of Short Fatigue Cracks Based on
Fracture Mechanics," PhD Thesis, Civil Eng., U.of Waterloo August 1978.

Material

The material is a bridge steel CSA standard G40.21 350W. Plate rolling direction
was in same direction as specimen loading. The transverse stiffners seem to be
welded with full penetration and were made 
      "... transverse stiffeners and laps, were welded to the plates using the
     flux-cored arc welding (FCAW) process with a nominal weld size of 6.4 mm (1/4 in.)"
CSA G40.21 350W is a higher manganese structural steel. The "350" denotes
minimum yield(mpa) and the "W" implies weldable. ASTM equivalent
is A572 grade 50. From other literature the nominal chemistry is 
 

Nominal Chem. for CSA G40.21 350W:  
       (All elements are maximums allowed)

    0.23C wt%  0.4Si  0.50-1.50Mn  0.04P  0.05_S
	
Crack Initiation and Propagation data for a simulated HAZ are available

                          here 

Baseline crack initiation and propagation specimens were machined from 
plates that were subjected to a simulated Heat Affected Zone (HAZ) operation:
      1. Heat in furnace at 990 C  for 30min
      2. Cool between two contacting plates
      3. Machine specimens (see PhD thesis for details)
The process increased the hardness from HRB=89 to HRB=94.

The digital "Fitted file" is available here:
https://fde.uwaterloo.ca/Fde/Materials/Steel/Other/Astm-A572/ghahremaniSimHAZ-350W_fitted.html


Constant amplitude Crack Propagation data was obtained by Ghahremani using
R=0.1 ΔS = 270mpa in the MSc thesis with welded specimens using a electric
potential drop technique. CT specimens were tested in the PhD thesis at R=0.8 and
R=0.9 with use of a travelling microscope. The crack propagation rate data is
plotted in figure 2 below along with data from the measurements of
El Haddad[3] (R=-1) on two similar materials. (The data in figure 2 were digitized
by others from graphs and may contain digitization errors.)



Fig. 2: Crack Propagation Rate vs. Stress Intensity for Ghahremani PhD and MSc
Tests of CSA G40.21 350W and El Haddad tests of CSA G40.11 and G40.21 Steels


The data points of figure 2 were fitted visually to obtain computation curves for
use in crack propagation calculations. The points are shown in figure 2 in black. Text files of these fitted curves are available:
da/dN vs ΔK CSA G40.21-350W Steel R=0.1, 0.8, 0.9
da/dN vs ΔK CSA G40.21 50A Steel R=-1


Geometry of Welded Specimens


Welded fatigue specimen cross-sections

Load Histories Applied to Welded Specimens :

Two constant amplitude (CA) specimens were tested for the transverse
stiffener (X) specimen, and two CA tests were performed on the double
lap-joint (L) version. The specifics of the constant amplitude tests
in terms of Smax and Smin nominal stresses are listed in Table 1: 

  Table 1      Condition:  As-Welded

          Name        Nf     Smax mpa       Smin mpa
          ----   ---------    -----          ----- 

          XCA-1    1995000     120             12
          XCA-2     920000     160             16

          LCA-1      27000     279             28
          LCA-2      17000     342             34

Two variable amplitude VA1 and VA2 histories were tested on several samples. Note that these histories have the same name as used in Ghahremani's MSc thesis, but are different histories in the PhD. The PhD histories were supplied by Ghahremani to the FD+E Comm. and are shown graphically in figures 3 and 4.



Fig. 3: Variable amplitude history VA1; max=200, min=10.07



Fig. 4: Variable amplitude history VA2; max=200, min=3.94


Digital Versions of Test Histories:



Variable Amplitude As-Welded Fatigue Test Results



  Table 2  Condition:  As-Welded, X section (transv. stiffener)
                #Kasra Ghahremani  PhD
       
          Name        Nf     Smax mpa    Smin mpa  History
          ----   ---------    -----       -----      ----

         XAV1-1    3843000     180          10       VA1
         XAV1-3    1131000     229          12       VA1
         XAV1-3     713000     252          13       VA1
         XAV1-4     397000     387          21       VA1

         XAV2-1    8203000     167           7       VA2
         XAV2-3    1921000     226           9       VA2
         XAV2-3    1041000     319          13       VA2
         XAV2-4     827000     387          16       VA2

  Table 2  Condition:  As-Welded, L section (double lap)
                #Kasra Ghahremani  PhD

          Name        Nf     Smax mpa    Smin mpa  History
          ----   ---------    -----       -----     ----

         LAV1-1     652000     148           8       VA1
         LAV1-2    1080000     159           8       VA1
         LAV1-3    1651000     161           9       VA1
         LAV1-4     468000     180          10       VA1
         LAV1-5      56000     387          21       VA1

         LAV2-1    9840000     136           6       VA2
         LAV2-2     964000     179           7       VA2
         LAV2-3     116000     387          16       VA2
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