WHY DID IT BREAK? The Use of Microscopy in Failure Analysis

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1 WHY DID IT BREAK? The Use of Microscopy in Failure Analysis David Taylor Professor of Materials Engineering Mechanical Engineering Dept. Trinity College Dublin

2 How Things Break Mechanism Appearance Mechanical Property 1) Ductile Fracture Permanent deflection and failure Yield Strength (σ y ) 2) Brittle Fracture Cracking and sudden failure Toughness (K IC )

3 Ductile Failure

4 Bolts failing by ductile fracture (left) and brittle fracture (right)

5 SEM Appearance of Ductile Failure Typical of good-quality metal alloys (high energy failure) Dimples (craters) on fracture surface. Steel fracture surface Craters on the moon

6 Brittle Fracture aluminium ladder

7 ORIGIN OF BRITTLE FRACTURE Brittle fractures start from defects or stressconcentrations in the material: - Pre-existing cracks - Manufacturing defects - Notches and other stress concentrations

8 Brittle Fracture Sometime you see marks (Chevron Marks) leading to the origin, where there is a defect ( but beware: this defect was not the problem!)

9 Another example of brittle fracture starting from a defect (internal pore) A good rule of thumb is that, in most cases, if the defect is dangerous, then it will be big enough to see by eye (i.e. >1mm). There are some exceptions to this rule.

10 Surface Roughness is a good indicator of the amount of energy needed for fracture Flat surface (low energy) Rough surface (high energy)

11 SEM Appearance of Brittle Fracture Confusing, because sometimes cracks can grow by the same mechanism of ductile fracture, giving the same dimpled surface.. (I prefer to call this tearing )

12 but cracks can also grow by a brittle mechanism: cleavage Fracture surface (brittle fracture, steel) Transgranular cleavage (through the grains).

13 It also happens in brittle polymers (e.g. epoxy)

14 SEM cleavage fractures, intergranular Brittle fracture (inter- or trans-granular) is often an indication that something is wrong

15 Mechanisms of Failure - SLOW Mechanism Appearance Mechanical Property 3) Fatigue Crack initiation Fatigue limit ( σ o ) and and propagation Fatigue crack propagation threshold ( K th ) 4) Stress-Corrosion Crack initiation Limiting stress and and propagation Stress intensity threshold K SCC

16 Mechanisms of Failure - SLOW Mechanism Appearance Mechanical Property 5) Creep Gradual plastic Strain rate, deformation, internal damage limiting stress, crack propagation rates 6) Wear Loss of material Wear rate, from surface Surface hardness

17 FATIGUE Fatigue failure occurs over a period of time, due to cycles of stress. stress Fatigue is the most common reason for mechanical failure time

18 FATIGUE CRACKS Example: 737 aircraft landing gear. Fast Fracture (brittle) Beach Marks (clam shell marks) Fatigue (corroded)

19 Fatigue Cracks more complicated Artificial hip joint (Co-Cr alloy)

20 SEM - fatigue Typical appearance of fatigue fracture surface. Striations

21 STRESS-CORROSION FAILURE Also called Stress Corrosion Cracking (SCC), this is another type of failure that occurs by gradual crack growth, like fatigue. In this case you don t need a cyclic stress; you need a stress plus a corrosive atmosphere. Examples: Aluminium alloys + NaCl solution Copper alloys + ammonia SCC shows much the same appearance as fatigue, except at very high magnification (SEM see below).

22 STRESS-CORROSION FAILURE SCC is difficult to diagnose. Sometimes you see corrosion on the fracture surface and not elsewhere Sometimes you see multiple cracking and crackbranching.

23 SCC in Brass Sometimes you see intergranular or transgranular cleavage

24 CREEP Creep is a gradual increase in plastic strain over time which occurs at relatively high temperatures, usually more than 0.3T m (T m is the melting point on the absolute temperature scale (degrees Kelvin)). Creep failures look like ductile failures (which is what they are). During creep, damage occurs in the material in the form of small cracks and holes which form inside the material, usually on the grain boundaries. These can usually only be seen at high magnification (see SEM below).

25 SEM - Creep Voids on grain boundary Cracks on grain boundary Used to monitor creep in aging power plant, etc

26 WEAR Wear occurs when two surfaces rub together (e.g. in bearings and gears). Material is lost from one or both surfaces. This is usually obvious when looking at the surface. Wear can also occur on fracture surfaces caused by fatigue etc if they rub together. Fretting Fatigue is a combination of wear and fatigue; wear creates cracks near the surface which may then grow into the rest of the component due to cyclic stress.

27 Example: wear and fretting fatigue in an artificial knee joint Wear patch Fatigue started from here

28 Wear Debris Failed O-ring EDS in the SEM showed particles rich in Fe,Al, Si and O with smaller amounts of other elements including Ti and Zn. Typical wear debris. Damage on O-ring due to wear particle?

29 BASIC RESEARCH: Locust Wing In-situ testing under the microscope

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33 Basic Research Damage and Repair in Human Bone Microscopic cracks form and grow, and are repaired

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35 Bone cells near a crack (LSCM, antibody stain)

36 In-situ SEM: fatigue tests on cell processes Crack opening