Quiz 2 Flashcards
Why do longer wires fail at lower loads than shorter wires with the same diameter?
Longer wires have a greater volume and surface area which leads to a greater chance of defects forming
What is true stress?
Load applied to actual cross-sectional area
What is engineering stress?
Load applied to cross-sectional area before load is applied
Is true stress or engineering stress greater?
True stress
What is the primary difference between elastic and plastic deformation?
Elastic deformation is reversible while plastic deformation is permanent
What is toughness?
The measure of energy absorbed until fracture; the total area under a stress-strain curve
How do dislocations affect elasticity and plasticity?
Dislocations have little effect on elasticity, but enhance plastic deformation by allowing easier atomic movement
Is the energy absorption capacity of a material entirely dictated by the strain to failure of the material? Explain.
No, energy absorption also depends on toughness which includes strength and ductility
What effect does temperature have on yield strength?
Higher temperatures reduce yield strength
What effect does strain rate have on yield strength?
Higher strain rates increase yield strength
Why do materials exhibit necking during tensile testing?
Necking occurs when the material’s cross-sectional area reduces under tension, concentrating stress to a localized region
How does grain size affect yield strength of a material?
Smaller grains increase yield strength by impeding dislocation (Hall-Petch effect)
What is the relationship between hardness and tensile strength?
Hardness is proportional to tensile strength, but the exact relationship varies depending on the type of material
Why is factor of safety important to design?
Ensures a structure operates within safe limits to prevent catastrophic failure
What role do grain boundaries play in plastic deformation?
Impede dislocation motion, enhancing the materials strength and resistance to plastic deformation
Why does a stress-strain curve change shape at different strain rates?
Higher strain rates result in increased stress, as dislocation motion is hindered, the material is strengthened
How does the Hall-Petch relation explain the strengthening of materials with finer grains?
Smaller grains increase yield strength by limiting dislocation movement across grain boundaries
What is the significance the modulus of elasticity in mechanical testing?
The modulus of elasticity measures a material’s stiffness and ability to resist elastic deformation
How does strain rate sensitivity vary between ductile and brittle materials?
Ductile materials strengthen more with increased strain rate compared to brittle materials
What role does dislocation motion play in work hardening?
Dislocation motion during plastic deformation leads to increased resistance, causing work hardening
How does Poisson’s ratio affect material deformation under loading?
Poisson’s ratio describes the ratio of lateral contraction to axial extension in a material under load
What is the difference between ductile and brittle fracture in terms of stress-strain behavior?
Ductile fracture occurs after significant plastic deformation, while brittle fracture happens with minimal deformation before failure
How does creep behavior change with increasing temperature?
At higher temperatures, creep accelerates, leading to faster material deformation under constant stress
What is the typical orientation of the fracture plane in tensile, ductile fracture? Explain.
Fracture plane occurs at 45 degrees to the tensile axis due to maximum shear stress; ductile materials by shear, so the fracture forms at an angle
What is the typical orientation of the fracture plane in tensile, brittle fracture? Explain.
Fracture plane is perpendicular to the tensile axis because it fails under maximum normal stress; brittle materials fail under tension, so fractures normal to the load
What is the fracture plane orientation for torsional loading of a brittle shaft? Explain.
Fracture plane is at 45 degrees to the shaft axis where tensile stress is the maximum; brittle materials fracture under tensile stress
What is the fracture plane orientation for torsional loading of a ductile shaft? Explain.
Fracture plane is perpendicular to the shaft axis due to maximum shear stress; ductile materials fail due to shear forces in torsional loading
What is the fracture plane orientation in bending?
The crack starts on the tensile side, perpendicular to the tensile axis and propagates toward the compression side
Does the shear lip develop on the tensile or compression side of a material bending?
Develops on the tensile side as this is where ductile fracture typically initiates
What information are you looking to obtain from the fracture surface examination?
Loading conditions (monotonic or cyclic), crack initiation site and propagation direction, environmental factors (temperature, corrosion), defects or material imperfections
Does macroscale inspection tell the whole story of material failure?
No, it shows features (radial marks, beach marks, or chevrons), further examination at the microscale (e.g. dimples, striations) provide more detailed insight into the fracture mechanism
What is the role of principle stresses on ductile materials, and how do they relate to fracture initiation?
Ductile materials yield by maximum shear stress, causing failure along shear planes
What is the role of principle stresses on brittle materials, and how do they relate to fracture initiation?
Brittle materials fail by maximum normal stress, leading to fracture along planes normal to the principle stress
What is the failure mechanism associated with beach marks?
Cyclic loading
What is the failure mechanism associated with radial marks and chevrons?
Point toward crack initiation and show the direction of crack propagation
How does fracture behavior differ under tensile loading for brittle and ductile materials?
Brittle fractures are perpendicular to the load while ductile fractures show shear lips at 45 degrees
How does fracture behavior differ under torsional loading for brittle and ductile materials?
Brittle fractures show helical pattern, and ductile fractures occur along the plane of maximum shear stress
How does shear stress affect the fracture surface in ductile materials under different loading conditions (tension, compression, torsion)?
Shear lips form at 45 degrees to the tensile axis (tension), barreling occurs due to maximum shear at the center of the sample (compression), fracture occurs on planes of maximum shear perpendicular to the shaft axis (torsion)
What are features of fracture surface roughness in brittle materials?
Smooth and may have faceted surfaces, indicating cleavage failure
What are features of fracture surface roughness in ductile materials?
Rough and matte with dimples
How does monotonic loading condition influence fracture surface features?
Lacks beach marks or striations
How does cyclic loading condition influence surface features?
Leaves beach marks and striations, indicating fatigue crack growth
Why is it important to examine both macroscale and microscale features of a fracture?
Macroscale features show crack propagation direction while microscale features like striations or dimples provide information about fracture mechanism
How do geometric constraints influence the fracture mechanism in materials?
Geometric constraints can create stress concentrations which can influence the location and direction of crack propagation
How do environmental factors such as temperature and corrosion affect the appearance and propagation of fracture surface?
Temperature and accelerate creep and change fracture mechanism; corrosion may cause discoloration or initiate cracks due to chemical degradation
What are the microscale features on the fracture surface that typically indicate ductile failure?
Dimples from microvoid coalescence (MVC)
Is Tresca or von Mises failure theory the most conservative?
Tresca is more conservative than von Mises
True or False: A ductile fracture surface will typically always have a cup and cone characteristic.
True; commonly seen in ductile, tensile fractures
What causes voids to nucleate in ductile material?
Stress concentrations from microscopic contaminants or inclusions
What are the elastic and plastic regions of the ductile failure envelopes?
Initial part of stress-strain curve (elastic region); after yield where permanent deformation occurs (plastic region)
What are the typical macroscopic ductile fracture features for tensile loading and torsional loading?
Necking, cup, and cone fractures (tensile); elongated shear dimples (torsional)
Where does torsion failure initiate on a round bar?
Initiates at the surface where shear stress is the highest
What is microvoid coalescence (MVC)?
A ductile fracture mechanism where voids form, grow, and coalesce under stress
What do dimples on a fracture surface indicate?
Dimples indicate ductile failure through microvoid coalescence
Why is von Mises theory preferred in modern design?
It is more accurate for isotropic materials and accounts for distortional energy
How does temperature affect ductile-brittle transition?
Lower temperatures can cause transition from ductile to brittle behavior, especially in BCC metals
What role do stress concentrators play in ductile fracture?
They nucleate voids and grow coalescence, leading to failure
How can mixed fracture modes complicate fracture surface analysis?
It becomes harder to determine whether the dominant fracture mechanism is ductile or brittle
True or False: Operating temperatures below ductile-brittle transition temperature (DBTT) increase the fracture energy in all metals.
False; operating temperatures below the DBTT decrease fracture energy, making metals more brittle
What is the correlation between brittle fracture propagation direction and the applied loading direction?
Brittle fracture generally propagates perpendicular to the direction of the applied stress; cracks follow the path of least resistance, often along crystallographic planes of grain boundaries
What are the macroscopic fracture features due to brittle tensile loading?
Flat fracture surface, no significant necking or plastic deformation, crystallized appearance
What are the macroscopic fracture features due to brittle torsional loading?
Helicoidal or spiral fracture surface at 45 degrees to the axis, reflecting the maximum shear stress
What are the macroscopic fracture features due to brittle bending loading?
Compression curls on the compression side, where the crack deflects from the perpendicular plane due to stress changes
What is cleavage fracture?
Occurs when atomic bonds break along specific crystallographic planes (cleavage planes) in a brittle manner, often in BCC and HCP metals at low temperatures
What are the two main categories of brittle fracture?
Transgranular: Crack propagates through the grains along cleavage planes; Intergranular: Crack propagates along the grain boundaries, often due to embrittlement or impurities at grain boundaries
What are microscale features of a brittle fracture surface?
River Patterns: indicate the direction of crack propagation; Cleavage Facets: Flat and smooth corresponding to crystallographic planes; No Plastic Deformation: fracture is clean and sudden
What is Charpy impact testing used for?
Measures the energy absorbed during fracture, which helps determine the toughness of a material and assess the ductile-brittle transition temperature (DBTT)
Which brittle failure theory is more conservative, Mohr-Coulomb or Maximum Normal Stress?
Maximum Normal Stress; it assumes failure occurs when the maximum principal stress reaches the material’s ultimate tensile strength, ignoring the contributions of shear stresses
How does temperature affect ductility and fracture mode?
Lower temperatures make reduce ductility, making materials more prone to brittle fracture
What is the significance of the ductile-brittle transition temperature?
DBTT is the temperature below which a material transitions from ductile fracture to brittle fracture, requiring less energy for fracture
What are river patterns in brittle fracture?
Meandering marks showing the direction of crack propagation in brittle fracture surfaces
Why is brittle fracture more dangerous than ductile fracture?
Brittle fracture occurs suddenly without significant plastic deformation, making it harder to detect
How does grain orientation affect fracture?
Fracture can propagate through grains (transgranular) or along grain boundaries (intergranular)
What role does crystallographic orientation play in brittle fracture?
Determines the plane along where cleavage occurs
What is the difference between cleavage and quasi-cleavage?
Cleavage is a clean break along crystallographic planes, while quasi-cleavage shows a mix of cleavage and dimpled (ductile) features, typically at higher temperatures
What type of fracture surface morphology is observed at temperatures below DBTT?
Fracture surfaces show brittle cleavage features with minimal deformation and distinct river patterns
