Understanding Metal Failure in Engineering

Metal Failure

• Metal failure can cause a complete shutdown or serious loss of production in an engineering plant
• It can result in equipment damage or loss of life
• Understanding failure modes helps determine causes and prevent recurrences
• Failures typically occur gradually in three stages initiation, growth, and propagation
• The propagation stage is reached when growth becomes unstable and rapid
• Brittle materials can fail rapidly under unfavorable conditions, leading to catastrophic failure
• Investigating failures requires a systematic and comprehensive approach before corrective action

Sources of Metal Failure

• Poor designing can arise from a lack of understanding stress raisers or stress distribution
• Stress concentrations leading to early failure can be caused by sharp fillet radii, keyways, and drilled holes
• Material selection considerations should include the type of stress and the environment a material is subjected to, including static, cyclic, and corrosion factors
• The rate of wear or erosion and temperature effects should be considered when time is a factor

Imperfections and Deficiencies Leading to Failure

• Surface defects and internal flaws can reduce overall material strength and initiate or propagate cracks
• Material defects, like gas porosity, may originate from casting or welding processes
• Cold forming operations (deep drawing, bending) can produce high residual stresses, localized stress areas, cracks, microcracks, and loss of ductility
• Processing-induced surface defects and metallurgical changes affect fatigue strength, brittle fracture resistance, and corrosion resistance
• Machining and grinding often leave residual stress
• Severe grinding can overheat and soften hardened steels, leading to cracking
• Misalignment of shafts, gears, bearings, seals, and couplings can lead to service failures

Service Conditions and Maintenance

• Operation of equipment under abnormal conditions (speed, loading, temperature, chemical environment) or without scheduled maintenance contributes to failures
• Regular inspections using non-destructive testing are important to check for defects and deterioration
• Inadequate maintenance is a frequent contributor to failures and maintenance procedures should be re-evaluated when failures reoccur

Types of Metal Failure

• Metal failures are categorized as ductile fracture, brittle fracture, fatigue, and creep

Ductile Fracture

• Results from excessive tensile force on a metal capable of permanent deformation
• A classic example is a tensile specimen necking down before fracture, also known as overload failure
• Metal shows considerable permanent deformation, with fractured surfaces exhibiting a 'cup and cone' appearance and irregular surfaces at a 45° angle to the tensile axis
• It is characterized by transgranular cracking when examined microscopically
• Ductile fractures occur at stresses above the yield strength, which may be due to weak material, unexpected service conditions, or abnormal loading

Brittle Fracture

• Characterized by minimal work absorbed, little deformation, and a crystalline appearance
• Surfaces often feature chevron patterns pointing towards the origin of the fracture
• Brittle fracture can occur suddenly at low stress
• Markings on fracture surfaces indicate the failure originated from a severe stress concentration
• Ambient temp below transition temp, presence of a notch, and tensile stress must be present to produce brittle fractures
• Only BCC metals can demonstrate ductile/brittle transition temperatures
• Transition temperature of BCC metals can be found by the Charpy V notch test

Notch Sensitivity & Tensile Stress

• Realizing a brittle fracture starts from a pre-existing crack or sharp defect is important to recognize
• Applied stress may be amplified or concentrated at the tip of a notch, the magnitude of localized stress depends on crack or notch geometry/orientation
• Tensile stress is required to produce any fracture
• Tensile stress produced by external loading, residual stress from welding, combination of the two
• Notches and cracks should be avoided in materials or structures that may fail by brittle fracture
• Weld defects and heat treatment cracks can form severe stress concentrations as fracture initiation points

Remedy for Brittle Fractures

• Risk may be eliminated by removing any of the conditions necessary to cause it
• Even large crack-like defects can be tolerated above transition temperature
• If the material is brittle at the service temperature there is no risk of fracture even if there are no defects present
• Lowering the transition temperature in steels can be achieved by decreasing C content (to below 0.15%), decreasing loading rate, decreasing notch depth, increasing notch radius, increasing Ni content (2-5%), and reducing grain size

Identifying Brittle Fractures

• Brittle fractures are distinguished by a near plastic deformation absence
• Fractures happen at below yield stress, flat and shiny
• Fracture path can be traced to its origin by interpreting chevron markings
• Fracture origins are at a notch or small crack
• Microscopically, brittle fractures may be transgranular or intergranular

Fatigue Failure

• Describes material failure under repeated stress, which can be much less than needed to break the material with a single pull
• Steel structures under normal conditions may be designed to a permissible static stress of about 2/3 the yield stress in cases when stress remains constant with time
• Fatigue failure possibility must be considered when variations in live load during normal operation, temperature changes or pressure vibrations, or dynamic loads in machinery cause fluctuation
• The design of a structure subjected to fatigue loading should relate working stresses to fatigue strength data

Fatigue Testing with the Wohler Machine

• The Wohler machine is used to compare the fatigue properties of different materials
• Fatigue test uses a specimen in the form of a cantilever, which forms the extension of a shaft that is driven by an electric motor
• Dead loading is applied to the specimen through a ball bearing
• As the specimen rotates there is a sinusoidal variation of stress which is greatest at the surface and zero at the centre
• Specimens are subjected to reversals of stress until failure occurs or the specimen has endured 10 million cycles
• S/N curve is plotted from the results to determine the relationship between the stress and the number of cycles to failure for a type of loading

S-N Curves and Fatigue Limits

• For ferritic steels below 200°C and Al-Mg alloys at room temperature, S/N curves become horizontal after about 10 million cycles
• Endurance limit is the highest stress which regardless of the number of times it is repeated, will not cause fracture
• Endurance limits - the limiting stress below which a metal will withstand an indefinitely large number of cycles of stress without fatigue fracture
• Steels have true endurance limits but most nonferrous metals do not show true endurance limits
• Endurance strength applies to cases when the repeated stress at which failure will not occur before a stated number of stress cycles

Identifying Fatigue Fractures

• No deformation sign exhibited, initial stage difficult
• Fatigue crack begins as a very narrow opening, grows as repeated stressing is maintained
• Consists of portion of the facture surface is smooth and shows ripple markings of striations
• Remainder surface has either crystalline or fibrous appearance, indicates tearing that happens when the area can no longer sustain the load

Factors in Fatigue

• Behavior of metals under fatigue loading is affected by factors related to several conditions
• Factors - stress concentration effects, design, processing methods, surface conditions, and tensile strength
• S-N curves for mild steel plates specimens showed axial stress variation

Stress Concentrations

• Local stress concentrations - notches, keyways, oil holes, screw threads, machining marks, etc.
• Stress raisers include scratches, tool marks, rough surfaces, quenching cracks, and poor fillets
• Avoid creating areas of stress concentration

Processing, Design and Tensile Strength

• Achieved by fillets and rounded contours
• Fatigue often initiates at surfaces because stresses are normally higher there, especially with bending loads
• Improve and modify processing methods to increase resistance to fatigue failure, or change to a different method of manufacture
• Fatigue and tensile strength has a direct relationship
• Can affect alloys by alloying the mental for which can be solid solution straightened

Surface Conditions

• Strengthening or weakening fatigue resistance by hardening the surface (nitriding, carburizing)
• Uniformly hardening the surface of the material improves fatigue behavior
• Introducing compressive stress - work hardening or case hardening: Shot-peening (stream of steel shots), cold rolling, case hardening (for steel components)

Creep in Metals

• Producing alloys supporting stress at high temps is major task
• Alloys for aircraft engines are used at bright red heat (800 °C)
• Constant demand for steels for power industry boils and turbine equipment
• Understanding natures of creep processes ensures creep resistance
• Creep - slow plastic deformation of metals under constant stress
• Occurs and leads to fracture at static stresses much smaller than those to break specimen when loaded quickly
• Creep occurs when metals and alloys are stressed at temps > 1/3 Tm (Tm = melting point in Kelvin)
• Example: Lead and in creep at room temperature while molybdenum Tungsten/nickel-based alloys creep at about 1000°C
• Steam and chemical plants operating at 450° to 550°C and gas turbines working at high temps (800-900°C) also experience creep

High Temperatures and Creep Curves

• Performance of metals at high temperatures governed by time in addition to strength and ductility
• Rate is rapidly increased with increased temperature Behavior of metal can be evaluated under temperature by strain vs time curve.
• Three distinct stages:
  • Primary Stage: Rapid extension
  • Secondary Stage: Creep at constant rate
  • Tertiary stage: Extension accelerates to rupture

Strain under Creep

• Creep deformation (elongation) remains if load is removed
• Only temperature related, acceleration in growing cracks can lead to stress concentration or failures

Creep Resistance

• Following strengthening methods used to reduce creep:
• High melting point metal
• Solid solution strengthening
• Precipitation or hardening dispersion, must have the following characteristics Fine dispersed precipitates, must have elements such as aluminum or titanium