Determining the Fracture Mode: Brittle Fracture vs. Fatigue in a Pump Shaft 2

[Vahid.A]

Hello,
A broken pump shaft made of 1.4021 stainless steel failed at room temperature. The broken surface looks smooth and shiny, with some sparkles when tilted. Under the microscope, the cracks mostly followed the grain boundaries (intergranular fracture), which usually points to brittle fracture. We initially thought the material might have been weakened (e.g., due to bad heat treatment). But mechanical tests don’t back this up: the impact toughness test gave 32 Joules, which is higher than the required value for this steel.

Why it’s likely not fatigue:
No classic fatigue clues like beach marks, ratchet marks, or fine striations on the surface.
Fatigue cracks usually grow through the grains (transgranular), but here the cracks followed the grain boundaries. While rare, intergranular fatigue can happen in special cases (like high heat), but this failed at normal temperatures.

The confusion:
The grain-boundary cracking suggests brittleness, but the material’s impact toughness is good. What’s going on? Please Let me know your thoughts!

 

[mfgenggear]

OP
Please provide the AISI. Steel type
The heat treat procedure
Was there NDT prior to and after heat treat

I not experience with SEM
But am an with all mention above.
From the broken shafts the appears defects on the surface of the shaft , it appears to have discolored areas. Which could be visible cracks.

May or may not have been caused by heat treat.

 

[EdStainless]

So why aren't you calling those curved lines beach marks?
It sure looks like fatigue to me.

 

[mfgenggear]

Here is a great link

A Mechanic's Story: Basic Component Fatigue | Reliability

We are trying to create an intellectual curiosity within the front lines about 'making the call'...is it Fatigue, or some other fracture pattern?

reliability.com

 

[SJones]

There is an interesting striation at the two o’clock surface that should be investigated further. Depending on the fracture toughness (note: fracture toughness, not Charpy energy) it could be that a short fatigue crack created the conditions for a brittle fracture across the bulk of the cross section.

 

[Vahid.A]

OP
Please provide the AISI. Steel type
The heat treat procedure
Was there NDT prior to and after heat treat

I not experience with SEM
But am an with all mention above.
From the broken shafts the appears defects on the surface of the shaft , it appears to have discolored areas. Which could be visible cracks.

May or may not have been caused by heat treat.

Thank you for participating in this thread. Similar grade in ASTM would be ASTM A276-420. The heat treatment was QT800 (950 0C -1050 0C quenched by forced air and tempered at 650-750 0C) and was tested according to ASTM A388-11.

 

[Vahid.A]

So why aren't you calling those curved lines beach marks?
It sure looks like fatigue to me.

Thank you for the comment. When examined under light and tilted, the fracture surface exhibits a sparkling effect. This characteristic appears uncommon for fatigue-induced failures, as such fractures occasionally display such reflective properties. Additionally, the intergranular morphology of the fracture surface further contrasts with fatigue cracks, which are more typically transgranular. That said, I value your expertise and would appreciate your thoughts on this observation. If possible, could you suggest a reference or resource that addresses this distinction? I would greatly appreciate your guidance.

 

[Vahid.A]

There is an interesting striation at the two o’clock surface that should be investigated further. Depending on the fracture toughness (note: fracture toughness, not Charpy energy) it could be that a short fatigue crack created the conditions for a brittle fracture across the bulk of the cross section.

Thank you for participating in this thread. I assume this is the most probable scenario. I was unable to observe any striations in the SEM images. Could you please elaborate on your observation? Are you referring to the lines with the darker background in the photo (Broken Shaft.jpg) that resemble beach marks? Just want to confirm if I’m interpreting this correctly.

 

[mfgenggear]

OP
Make a fish bone diagram of possibility causes.
Here are possibilities.
Damaged surface from heat treat in unprotected plate or atmosphere.
Quench in air insufficient, or improper
Hardness improper temper.
Chemical etch with out proper pre and post hydrogen embrittlement bake.
Improper alignment and support of shafts
Shafts had defects on the surface causing or adding to the failure.

Combination of fatigue and Brittle fracture
Because of some of the above improper procedures.

Shafts if hot or cold rolled must be machined to remove surface imperfections.
Cold straightening can cause cracking if not done correctly. Must be heated and NDT inspected.
Must NDT prior to heat treat and after heat treat and final machining

 

[Vahid.A]

As I was initially impressed by the intergranular microstructure and shiny appearance of the fracture, I overlooked the presence of beach marks, and my interpretation was biased toward a brittle fracture mechanism. Fractography requires a trained eye and an unbiased review of the entire story. While this result is not yet final, metallographic analysis revealed localized regions of poor microstructure. Additionally, abnormal vibrations in the pump were reported. Based on these observations, the most probable scenario is as follows: Fatigue cracking likely initiated due to the imposed vibrations and propagated initially under cyclic loading. Subsequently, the crack progression transitioned to a brittle fracture mode, driven by the presence of the localized poor microstructure.

 

[dtimberlake]

May I have some pictures of the remainder of the shaft and the machine or device that "broke" it?

Also some details about the service history.

Would this happen to be a belt driven device ?

 

[mrfailure]

Hello,
A broken pump shaft made of 1.4021 stainless steel failed at room temperature. The broken surface looks smooth and shiny, with some sparkles when tilted. Under the microscope, the cracks mostly followed the grain boundaries (intergranular fracture), which usually points to brittle fracture. We initially thought the material might have been weakened (e.g., due to bad heat treatment). But mechanical tests don’t back this up: the impact toughness test gave 32 Joules, which is higher than the required value for this steel.

Why it’s likely not fatigue:
No classic fatigue clues like beach marks, ratchet marks, or fine striations on the surface.
Fatigue cracks usually grow through the grains (transgranular), but here the cracks followed the grain boundaries. While rare, intergranular fatigue can happen in special cases (like high heat), but this failed at normal temperatures.

The confusion:
The grain-boundary cracking suggests brittleness, but the material’s impact toughness is good. What’s going on? Please Let me know your thoughts!

Your should pay attention to Ed's comment. There are definite beachmarks at the 2:30 position that indicate a fatigue origin. This is at the outer edge of a feature that may have once been a keyway though it may have distorted over time or from the failure.

Most of the fracture looks like fatigue. Under fatigue, ferritic steels including stainless often do not show any evidence of striations because mating surfaces contact each other while the crack grows. Macroscopically, I think I see multiple origins around the circumference of this battered surface, but they do not show up as such in an SEM. I also see a small final fracture area around the 9:00 position. This morphology indicates the shaft had a low nominal (i.e. steady state) stress and that applied cyclic stress during service likely accounted for crack initiation and growth.

A broken pump shaft made of 1.4021 stainless steel failed at room temperature. The broken surface looks smooth and shiny, with some sparkles when tilted. Under the microscope, the cracks mostly followed the grain boundaries (intergranular fracture), which usually points to brittle fracture. We initially thought the material might have been weakened (e.g., due to bad heat treatment). But mechanical tests don’t back this up: the impact toughness test gave 32 Joules, which is higher than the required value for this steel.

Why it’s likely not fatigue:
No classic fatigue clues like beach marks, ratchet marks, or fine striations on the surface.
Fatigue cracks usually grow through the grains (transgranular), but here the cracks followed the grain boundaries. While rare, intergranular fatigue can happen in special cases (like high heat), but this failed at normal temperatures.

The confusion:
The grain-boundary cracking suggests brittleness, but the material’s impact toughness is good. What’s going on? Please Let me know your thoughts!

 

[mfgenggear]

To determine whether a load is fatigue-related or static in engineering, consider the nature of the loading, its duration, and the material's response. Static loads are constant and sustained, while fatigue loads are cyclic or repeated, potentially leading to material failure over time.

Here's a more detailed breakdown:


1. Understanding the Types of Loads:

• Static Loads:
  These are constant and sustained loads, meaning the magnitude and direction of the load remain the same over time. Examples include the weight of a structure or the force exerted by a stationary object.
  
  
• Fatigue Loads:
  These are cyclic or repeated loads, where the load changes in magnitude and/or direction over time. Examples include the vibrations of a machine, the stresses on a bridge due to traffic, or the force on a rotating shaft.
  

2. Identifying Fatigue vs. Static Failure:

• Static Failure:
  Occurs when a material or structure is subjected to a single load that exceeds its static strength, leading to immediate failure.
  
  
• Fatigue Failure:
  Occurs when a material is subjected to repeated or cyclic loading, leading to crack initiation and propagation over time until failure.
  
  
• Key Differences:
  • Time Dependency: Static failure is instantaneous, while fatigue failure is time-dependent, occurring over many cycles.
    
    
  • Load Type: Static failure is caused by static loads, while fatigue failure is caused by fatigue loads.
    
    
  • Failure Mechanism: Static failure is often characterized by ductile or brittle fracture, while fatigue failure is characterized by crack propagation.
    
    
  • Predictability: Static failure is generally more predictable than fatigue failure, as the material shows signs of yielding or cracking before breaking.
    

3. Factors Influencing Fatigue:

• Material Properties:
  Different materials have different fatigue strengths and resistance to crack propagation.
• Stress Levels:
  Higher stress levels under cyclic loading lead to faster fatigue failure.
• Loading Type:
  The type of cyclic loading (e.g., tension-compression, bending) affects fatigue life.
• Surface Conditions:
  Surface defects or imperfections can act as stress concentrators and initiate fatigue cracks.
• Environment:
  Temperature, humidity, and corrosive environments can accelerate fatigue damage.
  

4. Tools and Techniques for Analysis:

• Static Structural Analysis: Used to determine stresses and displacements under static loads.
  
  
• Fatigue Analysis: Used to predict the life of a component under cyclic loading.
  
  
• Fatigue Testing: Involves subjecting a material or structure to cyclic loading and measuring the resulting fatigue damage.
  
  
• Finite Element Analysis (FEA): A numerical method used to simulate the behavior of structures under various loading conditions, including fatigue.
  
  
• S-N Curves: Graphical representations of the relationship between stress amplitude and the number of cycles to failure, used in fatigue analysis.
  
  
• Rainflow Counting: A technique used to analyze complex stress histories and identify the relevant stress cycles for fatigue analysis.

 

[mrfailure]

Your should pay attention to Ed's comment. There are definite beachmarks at the 2:30 position that indicate a fatigue origin. This is at the outer edge of a feature that may have once been a keyway though it may have distorted over time or from the failure.

Most of the fracture looks like fatigue. Under fatigue, ferritic steels including stainless often do not show any evidence of striations because mating surfaces contact each other while the crack grows. Macroscopically, I think I see multiple origins around the circumference of this battered surface, but they do not show up as such in an SEM. I also see a small final fracture area around the 9:00 position. This morphology indicates the shaft had a low nominal (i.e. steady state) stress and that applied cyclic stress during service likely accounted for crack initiation and growth under unidirectional fatigue