Sources of Metal Failure in Engineering Plants

• Failure of metals can arise from different causes and lead to equipment damage or loss of life in engineering plants.
• Understanding failure modes aids in determining causes and preventing recurrence.
• Metal failures can be categorised as ductile fracture, brittle fracture, fatigue failure, and creep.
• Keywords include chevron marks, transition temperature, notch, S-N curves, beach markings, stress concentration, and creep curve.

Sources of Metal Failure

• Poor design can cause stress raisers or uneven stress distribution.
• Using sharp fillet radii or including keyways and drilled holes can introduce stress concentrations.
• Material selection should consider the stress type and environment, including static or cyclic stress, and corrosion.

Material Imperfections And Processing Affecting Failure

• Surface defects and internal flaws reduce material strength and initiate cracks.
• Defects from processing, such as gas porosity in castings, can lead to metal failures.

Deficiencies in Processing

• Cold forming can cause high residual stresses, localized stress areas, and loss of ductility.
• Surface defects from processing influence fatigue strength, brittle fracture resistance, and corrosion resistance.
• Machining and grinding can cause residual stresses and softening, potentially cracking hardened steels.

Misalignment and Service Conditions

• Shaft, gear, bearing, seal, and coupling misalignment is a common factor in service failures.
• Abnormally severe operating conditions, chemical environments, or irregular maintenance contribute to service failures.
• Regular inspection and monitoring are important for checking defects and deterioration rates.

Maintenance and Ductile Fracture

• Inadequate maintenance is a frequent factor in service failures. Effective maintenance procedures should be re-evaluated when failures recur.
• Ductile fracture stems from excessive tensile force applied to a metal with the ability to deform permanently prior to fracture, often called overload failure

Ductile Fractures: Characteristics and Causes

• Metals that fail through ductile fracture show significant permanent deformation, with fractured surfaces exhibiting a 'cup and cone' appearance.
• Ductile fractures typically show transgranular cracking under microscopic examination which can be identified by a cup and cone fracture surface with irregular surfaces at 45° to the tensile axis
• Ductile fractures take place in areas with stresses exceeding the yield strength due to using weak material, unexpected service loads, or abnormal overload or stress.

Brittle Fracture

• Brittle fracture is characterized by minimal energy absorption, minimal deformation, and a crystalline fracture surface, usually containing river-like chevron patterns that point to the origin of the fracture.
• It can occur with little applied stress with great suddenness
• Markings on brittle fracture surfaces identify that it began at a point of severe stress concentration or crack-like defects

Conditions for Brittle Fracture

• Three conditions are required: ambient temperature beneath the transition temperature, the presence of a notch that causes severe stress concentration, and tensile stress.
• Transition temperature depends on when notched material quickly changes from ductile to brittle.
• BCC (Body-Centered Cubic) metals exhibit a ductile/brittle transition temperature, which is usually measured using the Charpy V-notch test.

Factors Affecting Transition Temperature and Notch Impact

• The effects of temperature are critical for notched ductility, but the size and thickness of material, rate of loading, and microstructure also contribute.
• Any brittle fracture starts either from a pre-existing crack or sharp defect, showing that it is necessary to understand the effects of a notch
• Localized stress amount relies on the geometry and orientation of the notch.

Tensile Stress and Remedies for Brittle Fracture

• Tensile stress is needed for fracture, even when using residual stress from welding.
• The risk of brittle fracture may be eliminated by removing any of the three conditions necessary, which is a temperature above the transition temperature or removal of any defects
• In steels, transition temperature can be reduced by decreasing the carbon content to below 0.15%, decreasing rate of loading, decreasing notch depth or increasing radius, raising nickel content to between 2 to 5%, and reducing grain size.

Identifying Brittle Fractures

• Brittle fractures show very little to no obvious plastic deformation which happen at stresses lower than the yield strength, commonly appear flat and shiny.
• Fracture path can be determined by the chevron markings
• Fractures commonly start at a notch or small crack and may be either transgranular or intergranular when observed microscopically.

Fatigue Failure: Introduction

• Fatigue "describes the point when a material fails under a repeatedly applied stress"
• These stresses are less than the normal amount needed to break than when using a single pull

Cyclic Stresses and Fatigue Testing

• Fluctuations in operational live load, temperature, pressure changes, vibrations from wind or machinery result in changes to working stress
• The working stresses must be related to fatigue strength data for the particular service condition.
• The "Wohler" machine helps compare different material fatigue properties, it does this by applying dead loading to the specimen through a ball bearing
• To establish relationship between stress and number of cycles for failure, specimens are subjected to multiple stress reversals at different values until failure or after 10 million cycles.

S-N Curves and Endurance Limit in Fatigue

• Stress-cycle (S/N) curve is plotted from machine to determine the relationship between stress and number of cycles with any particular type of loading.
• The fatigue limit is the highest stress that causes fracture, ferritic steels reach stress after about 10 million cycles
• Fatigue failure can happen at an indefinitely large number of cycles below the endurance limit. In nonferrous metals that do not show endurance limits, endurance strength describes repeated stress at which failure will not occur with stated number of stress cycles.

Identification of Fatigue Fractures

• Fatigue fractures don't show any visible signs of material deformation, therefore they may not easy to see, in the initial stages
• Any type of fatigue crack starts as a very narrow opening
• Fracture surfaces have smooth areas with ripple markings or striations from where the fractures started, and the remainder can appear crystalline or fibrous, where final tearing takes place.

Factors Affecting Fatigue: Stress Concentration

• Factors that influence the behaviour include stress concentration effects, design, processing methods, surface conditions, and tensile strength.
• A stress concentration can cause the fatigue strength to change based on a small hole/ V notches
• Local stress concentration, is the stress at the surface that makes a fatigue to promote failure ,it's important to note any inclusions to avoid stress concentration if this occurs.

Design and Processing Considerations for Resisting Fatigue

• Design changes like using fillets and rounded contours helps reduce fatigue failure.
• Surface irregularities and stress raisers cause fatigue to start on the surface, usually a result from processing.
• To reduce fatigue failure, processing methods must be improved and modified (change of manufacture).

Strengthening and Conditioning for Resisting Fatigue

• The fatigue and tensile strength of metal is related, even if not directly proportional. However, this can be effected by solid solution strengthened alloying metals
• Fatigue can be weakened by any bad conditions, but strengthened when hardening the surface with nitriding, carburising, etc.
• Uniformly hardening the surface is useful method for improving the fatigue or surface can be hardened by shot-peening/ cold rolling

Creep of Metals: Introduction

• Creep: slow plastic deformation of metals under constant stress. Leads to fracture at static stresses less than conventional fracture points.
• Creep becomes important when metals and alloys are stressed at temperatures > 1/3 of their melting point in Kelvin.

Environmental Factors and Stages of Creep

• Creep happens in steam and chemical plants operating at 450-550°C and gas turbines working at high temps of 800-900°C, in environments that parts need to operate in such as furnace parts.
• High service temperatures are controlled more by strength and ductility which affects room temps. The section that is under stress deforms although the load is kept constant.
• Metals continue to deform at such high temperatures but the amount that they creep/load is the limiting factor for the metals application.

Understanding the Creep Curve

• High Temperature evaluation of the behavior, puts specimen under load with specified temp, a curve of strain vs time is produced, in which shows an elastic deformation.
• The curve of elongation vs time shows three stages: primary, secondary, and tertiary.

Creep Stages in Metals

• Primary Creep is relatively rapid with a decreasing rate ( designer interest since extension affects clearances)/ strain and work hardening since the dislocation movement is becoming more difficult
• Secondary Creep/steady rate, is the period for constant creep , material remains same where work hardening is equal to the thermal softening processes
• Tertiary Creep involves when rate of extension increases and cause rupture, alloys should typically be avoided.

Variables of Creep

• Creep deformation stays even when load is removed. Only low temps and stresses has primary conditions, unlike elastic.
• High temperatures and stresses makes tertiary creep lead the growth of material cracks.

Methods for Achieving Creep Resistance

• To reduce creep, use:
  • High melting point metals
  • Solid solution strengthening
  • Precipitation or dispersion hardening (second phase doesn't dissolve)
• Finely dispersed precipitates are needed for high creep resistance;
• Nickel-based superalloys contain Al and/or Ti, makes precipitates form for intermetallic compounds.
• Oxide particles helps in in creep resistance in the material.

Explore causes of metal failure in engineering, including poor design, material selection, and material imperfections, and their impact on equipment damage.