FCS 1 POST WORK

Written communication

The journal is full of grammatical errors in the language which makes it difficult to understand the information which the authors wanted to convey. The words of authors contradict their own previous statements at some places. The SEM images are not labelled. The crack formation is said to be started at the temperature of grains interlocking in the introduction. An approximated value of the temperature would help analysing the simulations there. The EDS analysis image is shown but the EDS defect which is the presence of oxides should have been labelled in the image.

Validation of the model

The confirmation of hot tearing effect by the simulations is mentioned while the casting simulations do not provide any information about the location where hot tearing is prevalent. The microscopy analysis talks about the SEM images of hot tearing failure zone with dendritic morphology. The images are ambiguous and not labelled which makes it difficult to locate the cracks and failure between the dendrites. The hot spot is said to be located on the stem change direction and the porosity prediction at this location in the simulations is mentioned as the cause for its failure whereas the femoral component shown in figure 2 states porosity at the head, not at the failure zone.

Justification of the modelling parameters

The images from ProCAST simulations do not provide much information. The modelling parameters are not stated which include time, the most important one. There is no justification for the mentioned parameters such as temperature and heat transfer conditions. The insulation of the upper part of the mould is suggested by figures 4 (e) and 4 (f) for directional solidification of the casting for homogeneity. There is no scale in these figures obtained from the simulations. Also, the recommendations could include more information such as numerical values about the process conditions. The simulation is validated for the section where no defects were seen. It should have been done for the worst case scenarios so as to prevent the failure.

FCS 10 PRE WORK

1. The non-uniform hardening layer is mentioned to be one of the causes of failure. There is no information about the effect of the hardening layer if it is desirable or not. The undulation in hardness profile curve is not explained. The desirable hardness is not mentioned anywhere.

2. The numbers 0.5 and 2.5 for the severity of inclusions do not have units. It is not clear if it is a comparison or a percentage. A value of maximum allowable inclusions to prevent failure could be mentioned. The microstructures are not labelled.

3. There is no support statement for it being a failure due to improper heat treatment. The hardness and brittle failure is discussed for site A but there is no discussion about the cause and effect of ductile failure at the middle. No suggestions to prevent failure are mentioned.

FCS 10 POST WORK

A little description and analysis of the fracture surface

The case of the shaft failed in a brittle manner while the core failure was ductile. If this was the situation, the authors should have provided some description about the site of crack initiation and source of the failure. There is no analysis of results from fractography. Furthermore, they did not mention if there were any surface defects in the material. It can be seen from figure 1(b) that there are some surface marks along the direction of shear failure. There is no information if these are threads or machining marks. If they are machining marks, they might have caused the fatigue failure. The microstructural analysis considers MnS inclusions might be the reason for failure whereas figure 3(b) clearly shows that MnS particles are along the longitudinal direction and cracks are perpendicular to them which inhibits crack propagation. So, MnS is not the reason of failure.

No mechanical analysis of failure

The shaft diameter is 7 cm. There is no detail if it is the standard component or one of its type. It is mentioned that failure occurred after 296 hours of service which is too early for a standard component. A thorough mechanical analysis should have been done as the authors have stated about a sudden jerk which might have caused overloading. Some calculations would be helpful to obtain maximum allowable torque on the shaft hence providing the design specifications. There is no explanation about the undulation in the hardness profile curve. The hardness values at the site of failure could have been used to calculate maximum load. The crack propagation is perpendicular to the applied stress for brittle failure while it is 45o for ductile failure. They should have justified if it is the reason for failure as figure 1(a) shows failure to be at 45o to the applied stress.

No suggestions provided

The conclusion was limited to the brittle failure mode of the case. The article itself lacks so much information which if incorporated could help in better analysis of failure and further suggestions. The authors should have provided some preventive measures to avoid the failure. Mechanical analysis could describe the relation between the design specifications and service conditions. Material can be changed from 42CrMo4 grade to heat-treated, tempered and induction hardened Chromoly alloy which is used by Alloy USA for rear axle shafts. It is mentioned that the case is martensitic and is very hard leading to brittle failure due to improper heat treatment. Using appropriate heat treatment would ensure safety, increasing the cost at the same time. So, if it is not a standard component, change of design might help.

PRE WORK CASE STUDY 7

1. The measurements from visual and stereography examination could be explained more to present their significance. Figure 5 from fractography should have been labelled as it is difficult to visualise the generation of crack at keyway as stated by the author.

2. It is recommended to use Magnesium and Calcium additions for better shape control during casting of steel. An estimate of allowable or minimum additions would add to credibility of the statement.

3. It is mentioned that yield strength and hardness decrease to the half of their initial values which further leads to decrease in the fatigue life of the component. Some calculation to show the value of yield strength and an estimation of fatigue life would be better.

PRE WORK CS 8

1. The dimensions and configurations of the pressure vessel are well-defined but there is no information if they are according to design specifications or not. Thinning of sections A and B is mentioned but reason for that is not clear.

2. The testing is done by ultrasonic method which is insufficient for identification of linear defects parallel to sound beam. It is tough to inspect rough surface (welds) from this NDE technique. There is no mathematical analysis of the failure and no suggestions for elimination have been provided.

3. The SEM images are not well explained. General observations of microstructure have been made but there is not enough analysis of how it led to failure. Again, the mechanical properties of the failed specimens are calculated but their significance is not explained. Very poor discussion and ambiguous conclusion.