FRACTOGRAPHIC STUDY OF FATIGUE CRACK SURFACES IN OPENHOLE SPECIMENS

1

• FRACTOGRAPHIC STUDY OF FATIGUE CRACK SURFACES IN
  OPEN-HOLE SPECIMENS

• Paulo F P de Matos, Pedro M G P Moreira
• I Nedbal, Paulo M S T de Castro
• IDMEC, University of Porto, Portugal
• Faculty of Nuclear Sciences Physical
  Engineering
• Czech Technical University in Prague, Czech
  Republic

2

• Objectives

1. Fatigue life fractographic reconstitution 2.
Comparison between micro and macro crack growth
measurements.
3
Specimens tested
Normal open hole specimen Cold worked open hole
specimen
Fatigue tested at MPa
Video surface CGR measurements
4
Surface crack growth in the normal open hole
specimen (left side crack)
5
Surface crack growth in the cold worked open hole
specimen (left side crack)
6
Fractographic measurements
7
Mean striation spacing along the crack length a
for both specimens
8
Fractographic reconstitution of fatigue crack
history
Nedbal et al.
9
Fractographic reconstitution of fatigue crack
history
Simple numerical integration e.g. rectangle rule
10
Fatigue crack history reconstitution using D1
and DD(s) - normal hole
11
Fatigue crack history reconstitution using D1
and DD(s) - CW hole
12
Fractographic reconstitution and macroscopic
surface measurements
13
Crack growth rate v as function of number of
cycles N Normal and CW
14
CGR macroscopic vs striation spacing normal hole
specimen
CGR measurements (macroscopic) and CGR
reconstructed inside fatigue crack from striation
spacing. Normal open hole specimen.
15
CGR macroscopic vs striation spacing cold worked
hole specimen
CGR measurements (macroscopic) and CGR
reconstructed inside fatigue crack from striation
spacing. CW open hole specimen.
16
Difference between surface crack and striation
spacing measurements
During fatigue crack growth
Final stage of cracking
17
Conclusions
1) Cold expansion of the hole increases the
fatigue life of open-hole flat specimen of
Al-alloy 2024-T3. 2) Although the specimens used
in the study were not designed for crack growth
measurements, an attempt to evaluate the crack
growth rate was performed using a recording
camera for the surface macroscopic
measurements. The method allowed observation
during a reduced interval of the specimens
fatigue life approximately 5 for the
untreated, and 1.5 for the cold worked
specimen.
18
Conclusions (cont.)
3) As another information source, the
quantitative microfractography of fracture
surfaces was used the SEM measurements of
striation spacing have afforded the input data
for fractographic reconstitution of fatigue
crack growth. Two different approaches were
confronted a simplified presumption v da/dN
s (i.e., D1), and more realistic supposition
that the ratio v/s is not constant, i.e.,
DD(s). 4) Results of fractographic
reconstitution of the fatigue crack growth
offers a plausible evaluation of the movement of
a short segment of crack front propagating
along the longitudinal axis of bearing section
of the specimen...
19
Conclusions (cont.)

• 4) This information, attainable only by help
  of fractography,
• yields a new piece of knowledge about the
  effect of the hole
• cold expansion it seems that the main source
  of specimen
• life prolongation is not a longer crack
  initiation time, but
• an intensive braking of early crack growth
  stage, by effect of
• the field of residual stresses.