Material Failure Analysis

The Failure Analysis Process

A systematic failure investigation follows a structured approach. The first step is preserving evidence: the fractured component, operating records, maintenance history, and environmental conditions must all be documented before any destructive examination begins. Photography of the failure scene and the fracture surfaces from multiple angles creates a permanent record. The fracture surfaces themselves are the most critical evidence and must be protected from further damage, corrosion, or contamination, as they contain detailed information about the failure mechanism.

Background information gathering includes collecting the component design specifications, material certifications, manufacturing records, service history, and the events immediately preceding failure. Operating temperature, pressure, loading conditions, chemical environment, and any recent changes in service conditions are all relevant. Interviews with operators, maintenance personnel, and witnesses often reveal information not captured in written records.

The physical examination proceeds from macroscopic to microscopic. Visual and low-magnification examination identifies the fracture origin (where the crack started), the direction of crack propagation, the presence of corrosion or wear damage, and any manufacturing defects. The fracture origin is identified by tracing crack propagation features (beach marks, river patterns, chevron marks) back to their convergence point. Material testing verifies that the component material meets specification through chemical analysis, hardness testing, and metallographic examination.

Fracture Mechanisms

Ductile fracture occurs after significant plastic deformation and is characterized by a dull, fibrous fracture surface with a cup-and-cone appearance in round tensile specimens. At the microscopic level, ductile fracture proceeds by the nucleation, growth, and coalescence of microvoids at inclusions and second-phase particles. The resulting fracture surface shows a dimpled pattern when viewed in the scanning electron microscope, with each dimple marking the site of a former microvoid. Ductile fracture is generally the safest failure mode because the plastic deformation provides warning before final separation and absorbs significant energy.

Brittle fracture occurs with little or no plastic deformation and is characterized by a bright, flat, faceted fracture surface. In crystalline materials, brittle fracture follows specific crystallographic planes (cleavage), producing flat facets that reflect light and show characteristic river patterns that point toward the fracture origin. Brittle fracture is dangerous because it propagates at near the speed of sound in the material, giving no warning and releasing all stored elastic energy at once. The Liberty ship failures of World War II, where welded cargo ships broke in half in cold water, are the most infamous examples of catastrophic brittle fracture.

Fatigue fracture is caused by cyclic loading and is the single most common cause of mechanical failure, responsible for an estimated 80 to 90 percent of all structural failures. Fatigue fracture surfaces show characteristic beach marks (also called clamshell marks or arrest lines), concentric ridges spreading outward from the crack origin that record successive positions of the advancing crack front. Under the SEM, the crack growth region shows striations, microscopic ridges where each striation represents one loading cycle. The fatigue crack grows slowly and predictably until the remaining cross-section is too small to carry the applied load, at which point final fast fracture occurs.

Fatigue cracks almost always initiate at stress concentrations: sharp corners, notches, threads, keyways, corrosion pits, machining marks, or weld defects. The stress at a stress concentration can be several times the nominal stress, making these locations far more likely to nucleate fatigue cracks. Reducing stress concentrations through smooth fillet radii, shot peening surface residual compression, and careful surface finish is the most effective approach to preventing fatigue failure.

Environmental and Time-Dependent Failures

Stress corrosion cracking (SCC) results from the synergistic action of tensile stress and a corrosive environment. Neither the stress alone (below the yield strength) nor the environment alone (too mild for general corrosion) would cause failure. Together, they produce intergranular or transgranular cracking that can lead to catastrophic rupture with minimal corrosion product or warning. Specific material-environment combinations are susceptible: austenitic stainless steel in chloride solutions above 60 degrees Celsius, brass in ammonia, and high-strength steel in hydrogen sulfide.

Hydrogen embrittlement occurs when atomic hydrogen diffuses into a metal and reduces its ductility and fracture toughness. Sources of hydrogen include electroplating, welding, corrosion reactions, and high-pressure hydrogen gas exposure. High-strength steels (above approximately 1,000 megapascals tensile strength) are particularly susceptible because the hydrogen concentrates at regions of high triaxial stress, such as crack tips and notch roots, promoting brittle fracture. Hydrogen embrittlement is a critical concern for high-strength fasteners, spring steel, and components for hydrogen fuel cell and storage systems.

Creep failure occurs in components operating at elevated temperatures under sustained stress. Creep damage manifests as grain boundary void formation and growth, eventually linking into grain boundary cracks that cause rupture. Creep failures in power plant steam lines, gas turbine components, and petrochemical reactor tubes are detected by replica metallography (pressing a thin film against the polished surface to capture a microstructural impression) or by automated ultrasonic inspection that detects creep voids.

Common Causes of Failure

Design errors account for a significant fraction of failures, including inadequate stress analysis, ignoring fatigue loading, insufficient corrosion allowance, and poor material selection for the service environment. Manufacturing defects such as casting porosity, forging laps, welding defects (lack of fusion, porosity, slag inclusions, hydrogen cracking), and improper heat treatment create the flaws that initiate premature failure. Service conditions beyond the design envelope, including overloading, thermal excursions, unexpected vibration, and changes in chemical environment, can cause failure even in well-designed and well-manufactured components.

Improper maintenance is a common contributing factor. Missed inspections, use of wrong replacement materials, incorrect torque on fasteners, and inadequate corrosion protection all increase the probability of failure. The investigation must distinguish between the root cause (the fundamental reason the failure occurred) and contributing factors (conditions that made the failure more likely or more severe), because preventing recurrence requires addressing the root cause.

Case Studies in Failure Analysis

The Aloha Airlines Flight 243 fuselage rupture in 1988 demonstrated the consequences of multi-site fatigue damage. An 18-foot section of the Boeing 737 upper fuselage tore away in flight at 24,000 feet. Investigation revealed that fatigue cracks had initiated at multiple rivet holes along a lap joint and grew until they linked together, causing sudden explosive decompression. The root causes were disbonding of the adhesive in the lap joint (allowing water ingress and crevice corrosion at rivet holes), inadequate inspection for multi-site damage, and the aircraft high number of pressurization cycles (nearly 90,000 flights). The investigation led to mandatory inspection programs and fuselage aging research that improved safety for the entire commercial fleet.

The Silver Bridge collapse in Point Pleasant, West Virginia in 1967 killed 46 people when a single eyebar chain link failed by stress corrosion cracking. The 2-by-12-inch eyebar developed a small SCC crack at the pin hole over years of service in an aggressive atmospheric environment. When the crack reached critical size, the eyebar fractured, the chain failed, and the entire suspension bridge collapsed within one minute. This disaster led to the establishment of the National Bridge Inspection Program requiring regular inspection of all highway bridges in the United States.

Preventing Failure: Lessons Learned

The most valuable output of failure analysis is the corrective action that prevents recurrence. Effective corrective actions address the root cause rather than symptoms. If a component failed by fatigue from a stress concentration at a sharp corner, the corrective action is redesigning the corner with an adequate fillet radius, not simply replacing the failed component with an identical one. If a weld failed because of hydrogen cracking from inadequate preheat, the corrective action is revising the welding procedure and welder training, not just repairing the weld.

Damage-tolerant design philosophy, developed originally for aircraft structures, assumes that flaws exist in all structures and designs for safe operation with those flaws present. This requires knowing the fracture toughness of the material, the maximum flaw size that could escape inspection (determined by the capability of the NDE method used), and the rate at which the flaw will grow under service loading. Inspection intervals are then set so that flaws are detected and repaired before they grow to critical size. This approach requires far more materials data and analysis than traditional safe-life design but provides a rational framework for managing structural integrity throughout service life.

Failure databases and industry standards encode lessons learned from past failures. The ASME Boiler and Pressure Vessel Code, developed after devastating boiler explosions in the 19th and early 20th centuries, prescribes design rules, material requirements, fabrication procedures, and inspection requirements that prevent the failure modes identified through decades of experience. The Failure Analysis Society, a division of ASM International, publishes case histories and best practices that help engineers avoid repeating past mistakes. Sharing failure analysis results, though sometimes commercially sensitive, is essential for improving the safety and reliability of engineered systems across all industries.