![Table 1. Chemical composition (weight %) of wire feedstock and deposited walls (reference and porosity). [NA: data not available].](/figures/table-1-chemical-composition-weight-of-wire-feedstock-and-ghnnqd4c.webp)
Q2. What have the authors stated for future works in "Criticality of porosity defects on the fatigue performance of wire + arc additive manufactured titanium alloy" ?
Two groups of specimens were made by the wire + arc additive manufactured titanium alloy ( WAAM Ti-6Al-4V ) and tested to study the material performance of the reference group ( manufactured with clean wire ) and porosity group ( manufactured with contaminated wire at the specimen gauge section ). Following conclusions can be drawn:
Q3. How much stress is required to cause the crack to grow unhindered?
Given the stress concentration factor of 2 in the porosity specimens, an applied stress amplitude of 270 MPa at stress ratio 0.1 is high enough to cause the crack to grow unhindered, leading to reduced scatter in the fatigue life.
Q4. What is the reason for the poor performance and data scatter?
The process-induced defects, such as gas pores and lack of fusion cracks, are reported to be the cause for the poor performance and data scatter [5–11].
Q5. Why were the rest of literature studies focused on powder based AM processes?
The rest of literature studies on notch fatigue and fracture mechanics methods were focused on powder based AM processes due to the concerns of higher porosity density.
Q6. What is the definition of a characteristic length?
Characteristic length is defined as the width of α colony [46], whichis 20-30 µm for WAAM Ti-6Al-4V and according to [47], when the pore diameter is less than 8 times of the characteristic length, the micro-crack growth will be influenced by the surrounding microstructure.
Q7. What are the main challenges to the widespread use of AM technologies?
One of the main challenges to the widespread use of AM technologies to produce safety criticalstructural components has been widely recognised as the issues of material properties and repeatability, in particular the fatigue and fracture properties.
Q8. Why is the crack initiation rate increased exponentially?
The reason is that when the crack growth driving force, ΔK, is below its threshold value ∆Kth, crack growth rate is dropped exponentially, resulting in much increased fatigue life, hence the change in the slope of the best fit curves [51].
Q9. What is the cause of the test data scatter?
Another cause for the test data scatter has been identified as the AM microstructure characteristics that are controlled by the cooling rate and peak temperature during deposition [12].
Q10. What is the reason for the higher fatigue life?
the higher fatigue life for the specimen with porosity size comparable to the microstructure characteristic length (i.e. width of lamellar α colony [46]) was due to the interaction of the pore with the microstructure.
Q11. What is the way to predict fatigue life?
A study based on the notch fatigue method, which is implemented in the FEMFAT fatigue post-processor [27], reported better fatigue life prediction accuracy, using the average stress acting on a finite volume of material in the notch root that is subjected to stress level greater than 90% of the maximum stress.
Q12. What is the stress intensity of a spherical gas pore?
According to Murakami’s approach [28], a spherical gas pore can be treated as a planar crack of size equal to the square root of the projected area of the pore.
Q13. What is the criterion for non-propagating defects?
Region II sets the criterion for non-propagating defects based on the LEFM condition of SIF rangeequal to the threshold SIF range (∆K=∆Kth).
Q14. How wide is the average lath width at the intersection between two layers?
at localised regions such as the intersection between two layers (Fig. 4e), the average α lath width was reduced to 1±0.2 µm.
Q15. What is the effect of the El Haddad model on the fatigue life of AM materials?
the El Haddad model [29] and the root area parameter were adapted by Beretta and Romano to determine the non-propagatingcrack condition for the lack of fusion defects encountered in AM materials [30].
Q16. What was the first proposed parameter for the notch fatigue model?
Murakami and Endo [39] proposed a parameter based on the projected area of defect to represent an effective crack size for embedded defects.