Metal Failure Analysis

Overview of Metal Failure

• Metal failure can cause equipment shutdown, production loss, and potential harm
• Understanding failure modes can help determine causes and prevent recurrences
• Keywords associated with metal failure: Chevron marks, transition temperature, notch, S-N curves, beach markings, stress concentration, creep curve.

Stages and Causes of Failure

• Failures typically progress through initiation, growth, and propagation stages
• Brittle materials can fail rapidly under unfavorable conditions, leading to catastrophic results
• Failure investigations require a systematic approach before corrective actions

Sources of Failure

• Poor designs lacking consideration for stress raisers can lead to early metal failure
• Correct consideration of stress type and environment is need for adequate material selection

Imperfections and Processing Effects

• Surface defects and internal flaws reduce the overall strength, initiating cracks and propagation
• Defects from processing, such as gas porosity in castings, serve as failure origins
• Cold forming produces high residual stresses, causing localized stress areas, cracks, and reduced ductility
• Surface defects from processing impact fatigue strength, brittle fracture resistance, and corrosion resistance
• Severe grinding can overheat and soften hardened steels, leading to cracking
• Misalignment of shafts, gears, bearings, seals, and couplings can cause service failures

Environmental Factors and Maintenance

• Equipment operation under severe conditions or without maintenance leads to failures
• Regular inspection and monitoring are essential for detecting defects and deterioration
• Inadequate maintenance is a frequent factor, requiring re-evaluation of maintenance procedures

Types of Metal Failure

• Metal failures are grouped into ductile fracture, brittle fracture, fatigue, and creep

Ducile Fracture

• Ductile fractures occur due to excessive tensile force on metals capable of permanent deformation
• Tensile specimens neck down before fracture in ductile failure
• Ductile failure are "overload failures"
• Metals show permanent deformation, with fractured surfaces displaying a 'cup and cone' appearance at a 45° angle to the tensile axis
• Transgranular cracking is seen microscopically
• Ductile fractures occur at stresses above the yield strength due to material weakness, incorrect heat treatment, unanticipated service loads, or abnormal loading conditions

Brittle Fractures

• Brittle fractures show minimal work absorption, very little deformation and a crystalline appearance
• Chevron patterns may indicate the origin
• Brittle fractures can occur suddenly at low stress
• Markings indicate failure originated from severe stress concentration like a crack
• Three conditions induce brittle failure: temperature below transition, presence of a notch, and tensile stress
• Transition temperature involves rapid changes from ductile to brittle behavior under notching

Transition Temperatures in BCC Metals

• BCC metals have a ductile/brittle transition temperature
• Charpy V notch tests determine the transition temperature of BCC metals like mild steels
• Temperature-energy plots and observation reveal fracture mode transition
• Stress needed for crack growth is large above the brittle-ductile transition temperature, but small below it
• Transition temperature varies based on material, test, and thickness
• Brittle fractures originate from pre-existing cracks or sharp defects

The Notch

• Localized stress depends on geometry and orientation of the crack or notch.
• Concentrated stress around a notch amplifies applied stress

Tensile Stress

• Tensile stress is required for fracture, from external loading, welding residual stress, or both
• High stress and necessary conditions lead to brittle failure
• Notches and cracks are more difficult to initiate than to propagate a brittle crack
• Weld defects and heat treatment cracks can initiate fractures due to stress concentration

Correcting Brittle Fractures

• Eliminating any of the three conditions can mitigate risk
• Above transition temperature, even large defects are manageable, focus shifts to static strength
• Absence of defects prevents fracture, even if the material is brittle at service temperature

Lowering Transition Temperature

• Decreasing C content below 0.15%
• Decreasing loading rate
• Decreasing the depth of the notch or increasing the radius of the notch
• Increasing the nickel content to about 2 to 5%
• Reducing the grain size by adding grain refining elements like A1, Nb

Identification of Brittle Fractures

• Brittle fractures have almost no plastic deformation and are flat and shiny, occuring below the yield stress
• Chevron markings can trace the fracture path to its origin, often a notch or small crack
• Microscopic examination reveals transgranular or intergranular fractures

Fatigue Failure

• Fatigue is the failure under repeatedly applied stress, at levels lower than needed for a single pull failure
• Steel structures under normal conditions are designed to a permissible static stress roughly equivalent to 2/3 of the YS, giving an adequate margin against the onset of yield and a bigger margin against ultimate failure
• Few structures experience purely static loading
• Fluctuations in working stresses arise from live load variations, temperature changes, vibrations, or dynamic loads
• Cyclic stress changes require fatigue failure considerations
• Designs must relate working stresses to fatigue strength data

Studying Fatigue

• The "Wohler" machine is a tool for understanding fatigue
• The specimen rotates which causes a sinusoidal variation of stress greatest at the surface and zero at the center
• Series of specimens endure stress reversals until failure or 10 million cycles
• Stress-cycle (S/N or S/log N) curves show stress-cycle relationship

Fatigue Curves

• After about 10 million cycles, the S/N curve becomes horizontal at the fatigue limit for ferritic steels below 200°C and Al-Mg alloys at room temperature
• Endurance limit is the stress below which a metal withstands indefinite cycles without fatigue failure
• Steels exhibit true endurance limits, while most nonferrous metals do not
• Endurance strength is used for materials without true endurance limits, defined as repeated stress without failure before a certain number of stress cycles

Visible Signs of Fatigue Fractures

• Fatigue fractures don't have any deformations, that are hard to see especially in the initial stages of the crack
• The fracture surface exhibits of two areas, one portion is smooth and shows ripple markings of striations spreading out from some discrete points indicating where the fractures were initiated
• Striations are a feature of service failures
• A crystalline and fibrous appearance indicates final tearing when the area cannot sustain the load, which may result in brittle or ductile fractures

Factors Affecting Fatigue

• Fatigue in metals can be reduced by stress concentration like processing methods and surface conditions
• Stress concentration effects, design, processing methods, surface conditions, and tensile strength also plays important roles

Effect of Stress Concentration on Fatigue

• S-N curves for mild steel plates show axial stress variation from zero to maximum tension
• Holes and notches in specimens reduce fatigue strength via stress concentration

Mitigating Stress Concentration

• Local stress concentration is caused by notches, keyways, oil holes, screw threads, machining marks, and weld defects
• Scratches, tool marks, rough surfaces, quenching cracks, sharp section changes, poor fillets, inclusions, and corrosion pits promote fatigue failures
• Design improvements reduce section change severity with fillets and rounded contours
• Rounded bottom keyways reduce fatigue failure, and liberal fillets replace sharp right-angle cuts

Processing

• Fatigue initiates at surfaces with higher stresses, especially under bending loads
• Surface irregularities from processing also are a factor

Improving Fatigue Resistance

• Processing methods can be modified to improve resistance to fatigue failure, such as changing manufacturing methods or improving a given method like mould design
• Better specifications for machinery operations and surface finish are also helpful

Influence of Tensile Strength

• Fatigue and tensile strength are directly relational
• Strength increase is achieved by alloying the metal in alloys for solid solution
• Strength increase can be limited in unstable alloys (example is age-hardened material) Surface Conditions

Improving Surface Conditions

• Fatigue resistance is boosted by surface hardening via nitriding, carburizing, or other types of improvements.
• This results in compressive stress through shot-peening, cold rolling, or case hardening

Creep

• Creep describes the phenomenon of deformation under constant stress at elevated temperatures.
• High-temperature service performance is related to creep
• Time is a factor because at high temperatures the metal section under stress will continue to deform although the load is maintained constant
• Metals at room temperature below the yield points leads to complete elastic deformation or strain.
• At elevated temperatures, below the yield point, it result in progressive stain
• Creep increases with increasing temperature.

Creep Resistance

• The use of metal or article under load is because of how long it will deform given it elevated temperatures
• Metals and alloys differ considerably in creep rate.
• At room temperature, only the low melting point undergo load.
• The material will creep sufficiently high temperatures. In that case this temperature and stress are high enough, the metal will creep until rupture occurs

Creep Curve

• Creep behavior is evaluated by subjecting a metal specimen to constant load at a certain temperature and plotting strain vs time
• A typical creep rupture curve in Figure 6 includes instantaneous elastic/plastic deformation and three stages:

Primary Creep

• Primary creep presents rapid extension at a decreasing rate
• Considerations should be made by the designer

Secondary Creep

• Secondary creep presents a nearly constant rate of strain
• Material properties are constant as a result of thermal softening processes.
• Creep strain involves sliding of grains boundaries so that fine grained materials creeps more rapidly that course grains.

Tertiary Creep

• Tertiary creep leads to rupture
• Creep use should be avoided
• Low temperatures and stresses have a significant effect on the rate that the material will rupture
• It may be challenging to measure what is going to happen when the specimen creeps
• Tertiary creep results in growth of cracks and ultimate catastrophic failure

Resistant Creep

• Creep deformation is permanent and the elongation remains if the load is removed
• Primary creep can be the factor where there is low temperatures and low stresses. In this case Extension May Eventually Cease
• High Temperature and High Stress May accelerate the process that leads to increase in cracks and stress

Creep Resistant Alloys

• These methods can further reduce creep:
  • High melting point metal
  • Solid solution strengthening
  • Precipitation or dispersion hardening using a matrix (the second phase cannot be easily dissolved