Birla Institute of Technology & Science, Pilani.
II Semester 2024-25
Work Integrated Programme Division
DE ZG 631 Materials Technology and Testing.
Situated L earning assignment
(5x2 =10 Marks)
Duration: 02 May 2025 to 12 may 2025
Time permitted 30 minutes
Q1) A splined shaft manufactured by a company has failed as shown in the Figure given below:
Identify the following and give your observations.
1)
2)
3)
4)
Whether the crack has initiated at one point or multiple points.
What is the basic reason for the failure?
What type of fracture has occurred based on the surface of the fracture?
What is the remedy to avoid such failures?
(4 marks)
Answer:
This image shows a fractured splined shaft, and a close visual examination reveals several critical aspects
about the failure. Let's address each point:
1) Crack Initiation: Single Point
•
Observation: The fracture appears to have a relatively symmetric and uniform central initiation
point. The presence of a small protruding area (often referred to as a "chevron mark" or nipple)
at the center indicates crack initiation at a single point—likely at the center or near the key stress
concentration zone.
•
Conclusion: Crack has initiated at a single point, probably due to a stress concentration or defect.
2) Basic Reason for the Failure
•
Observation: The fractured faces exhibit radial marks and a dimpled or fibrous appearance,
suggesting a failure caused by torsional overload or fatigue under repeated loading.
•
Conclusion: The basic reason is likely torsional fatigue, possibly due to misalignment, overloading,
or inadequate material strength for the operational stresses.
3) Type of Fracture Based on Surface Appearance
•
Observation: The surface shows fibrous textures with some shear lips and no signs of a smooth
brittle fracture.
•
Conclusion: This indicates a ductile fracture, typically associated with plastic deformation before
final rupture.
4) Remedies to Avoid Such Failures
•
Improve material quality: Ensure the shaft material meets the required toughness and fatigue
strength.
•
Reduce stress concentration: Optimize spline geometry, ensure proper key fitting, and eliminate
sharp corners.
•
Ensure proper alignment and installation to reduce bending and torsional stresses.
•
Perform non-destructive testing (NDT) periodically to detect early cracks.
•
Apply surface treatments (e.g., shot peening) to improve fatigue resistance.
Q2) Identify the type of failure of the two components given below and suggest the remedial measure to
avoid the recurrence of such failures.
(3 Marks)
Answer:
Based on the SEM (Scanning Electron Microscope) images provided, we can analyze the fracture surfaces
of the two components to determine the failure modes and suggest appropriate remedial measures.
Image 1: Good Component Fracture
•
•
Failure Mode: Ductile Fracture
o
This image shows a classic ductile fracture surface characterized by numerous dimples or
microvoid coalescence.
o
These dimples indicate that the material underwent significant plastic deformation
before failure, which is typical of a tough and ductile material.
o
The label also confirms this as a “Good Component Fracture.”
Remedial Measures:
o
No specific action is needed here as this represents a desirable failure mode in many
ductile metals (e.g., steels, aluminum alloys).
o
Continue using the same material and processing conditions if ductility is desired for
energy absorption or toughness.
Image 2: Brittle Fracture
•
Failure Mode: Brittle Fracture
o
The second image shows a cleavage-like, faceted fracture surface with minimal plastic
deformation.
o
There are sharp, angular features, and no evidence of dimple formation—this is indicative
of brittle fracture.
o
•
Brittle fracture typically occurs suddenly and without significant plastic deformation,
often due to:
▪
Low temperatures,
▪
High strain rates,
▪
Material defects,
▪
Improper heat treatment,
▪
Presence of stress concentrators (e.g., inclusions or cracks).
Remedial Measures:
1.
Material Selection: Use a more ductile material or one with higher fracture toughness.
2.
Heat Treatment: Ensure proper tempering or annealing to relieve internal stresses and
improve toughness.
3.
Design Changes: Avoid sharp corners and use fillets to reduce stress concentrations.
4.
Operational Controls: Avoid operating at low temperatures if the material is susceptible to
ductile-to-brittle transition.
5.
Quality Control: Implement better inspection techniques (e.g., ultrasonic or X-ray) to detect
internal flaws or inclusions early.
Summary:
Image Fracture
Type
1
Ductile
Fracture
2
Brittle
Fracture
Cause
Remedial Measures
Normal/expected behavior in ductile None needed (desirable fracture mode)
material
Possible material defects, low Use tougher material, improve design,
temperature, or high stress
heat treatment, better inspection
Q3) Identify the type of failure from the microstructure given below and suggest suitable remedial
measure to avoid recurrence of such problem.
(3 Marks)
Answer:
The microstructures in the two images (a and b) show a material composed of primary and secondary
phases, which strongly indicates a two-phase alloy, such as a eutectic or cast alloy. Let's analyze each:
Analysis of the Microstructure Images
Image (a): Optical Micrograph
•
Shows a network-like distribution of phases.
•
The primary phase appears in larger, well-defined grains.
•
The secondary phase appears as a continuous or semi-continuous network at grain boundaries.
Image (b): SEM Micrograph
•
Offers clearer visualization of the grain boundaries and intergranular features.
•
The secondary phase is concentrated along grain boundaries.
•
This morphology is typical of materials that are prone to intergranular corrosion or intergranular
fracture.
Identified Failure Mode:
Intergranular Fracture (or Intergranular Corrosion, depending on application)
•
The accumulation of the secondary phase at grain boundaries weakens cohesion between grains.
•
Under mechanical or corrosive stress, cracks propagate along grain boundaries, causing
premature failure.
•
Common in cast alloys, stainless steels, and some aluminum and titanium alloys, especially when
sensitized or improperly heat-treated.
Root Causes:
•
Improper heat treatment or slow cooling rates leading to precipitation of brittle or corrosive
phases at grain boundaries.
•
Alloy composition leading to segregation of elements.
•
Thermal cycling or prolonged exposure to high temperatures.
•
Possible contamination or inclusions promoting intergranular attack.
Recommended Remedial Measures:
1. Heat Treatment Optimization:
o
Implement solution heat treatment followed by rapid quenching to dissolve and retain
alloying elements in solid solution.
o
Apply stabilization or aging treatments to control phase precipitation away from grain
boundaries.
2. Alloy Modification:
o
Use alloy grades with controlled impurity levels to reduce formation of undesirable grain
boundary phases (e.g., low carbon stainless steels to avoid chromium carbide
precipitation).
o
Introduce grain refiners or modifiers to alter morphology of secondary phases.
3. Control Cooling Rate:
o
Avoid slow cooling, which promotes segregation.
o
Employ controlled solidification techniques during casting or welding.
4. Grain Boundary Engineering:
o
Use thermomechanical processing to produce low-angle or special grain boundaries less
prone to intergranular attack.
5. Surface Protection or Coating:
o
In environments prone to corrosion, apply protective coatings or perform passivation to
shield grain boundaries.
Summary Table
Image Observation
(a)
Secondary phase at
boundaries
(b)
Clear
intergranular
phase presence
Failure Type
Intergranular fracture or
corrosion
Intergranular fracture
Remedy
Solution treatment, faster cooling, alloy
modification
Same as above; optimize heat
treatment and microstructure