Material failure analysis Failure and Root Cause Analysis

Types of Analysis Two types of analysis: Failure analysis how the material failed Root cause analysis how it could have been prevented

Failure Analysis Determining: How did the material fail? Why did it fail? Who or which party is responsible?

Root Cause Analysis Emphasizes managerial role in failures How managerial techniques can be improved Focus on prevention, not failure determination Large plants Construction sites Manufacturing facilities

Steps in Failure Analysis Assess the situation Photography and sample handling Determine product and material standards Macroscopic examination Microscopic inspection

Steps in Failure Analysis Material characterization Mechanical testing Hardness testing Crack testing Bend testing Strain testing Determine how failure occurred from initial to final state Vickers Hardness tester Hardness tester Public domain image from: https://upload.wikimedia.org/wikipedia/commons/thumb/2/2f/Vickers-tester.png/373px-Vickers-tester.png

Baseball Madness Suspect 1 Suspect 2 Suspect 3

Baseball Madness Principle: Fact: Conclusion: Glass is brittle and will fracture if struck by a medium to high velocity projectile. Fact: A glass window was broken. A baseball was found in the vicinity of the broken window. Suspect 3 has a baseball bat. Conclusion: Suspect 3 hit a baseball that struck the window and caused the glass to fracture. Using the Engineering Method and Investigation Pyramid

Failure Analysis Just like pathologists determine cause of death, mechanical or materials engineers must determine cause of failure

Failure Analysis Components can fail because of: Material selection Design Produce usage Method of production Defect within product

Failure modes of a book shelf Unloaded Books in middle causing sag in wood Shelf made of thermoplastic material Support place close together Manufactured crack in middle of shelf Shelf infested with wood worms To illustrate what is meant by failure, look at the failure modes of a bookshelf placed on two supports in the following conditions… Here you can see that in each example, the product failed due to one of several reasons: Material selection Design Produce usage Method of production Defect within product

Causes of Demise Congenital Disease Consequence of previous operation Manufacturing defect that leads to failure of material Disease Corrosion, degradation, wear, etc. that leads to failure Consequence of previous operation Failure stemming from a previous repair or adjustment Trauma Sudden failure caused by gross mechanical overstress (e.g., collision)

Example Car (black) pulls out of driveway and is side swiped by on coming car (white). A manufacturing fault in the white car’s brakes leads to progressive weakening of the clutch plate over a period of service.

Example Disease: Degradation of clutch brake due to surface defect in the in forging initiates a fatigue crack that runs to completion after many thousands or millions of stress cycles. Clutch plate of a truck

Example Consequence of previous operation: Failure stemming from an earlier repair to fix the clutch plate resulted in welding that altered the microstructure. This introduced residual stress and cracks in heat affected zones that lead to eventual failure by fatigue. Clutch plate of a truck

Answering the three main questions How did it fail? Clutch plate failure Why did it fail? Old repair lead to crack formation leading to fatigue crack failure. Who or which party was responsible? Owner of the white car.

Failure Mechanisms What caused the component to fail?

Mechanisms of Failure Manufacturing defects Material overload Flaw introduced during manufacturing process Material overload Inappropriate consumer use of the material Loading material beyond specifications Normal wear through use Fatigue, corrosion, creep, etc.

Manufacturing Defects Congenital faults in a product Result in (1) failure the first time it comes under load or (2) progressive weakening and eventual failure by fatigue

Material Overload Materials have limits Products should be used according to material and engineering specifications Materials cannot be used universally

Material Overload The strength of a material is measured by laboratory tests ASTM (American Society for Testing and Materials) and BSI (British Standards Institution) provide standards covering safety, performance, and reliability of products Most common measurement is tensile strength; other strength data includes compression tests and shear tests

Sample of ASTM Standards Example: Hardness testing

Example: ASTM E9 Old clutch plate New clutch plate Hardness tester Start with old clutch plate and brand new clutch plate Test hardness using ASTM standard protocol and hardness tester Determine if hardness is same as regular clutch plate Theorize why the hardness varies between failed plate and new plate

Material Overload Types of materials exhibit different behaviors depending on the way their atoms and molecules are bonded and how they stack together to form crystal structures copper diamond

Material Overload Tensile test curves for 3 different types of material: Brittle (ceramics) Ductile (most metals) Thermoplastic polymers

Material Overload Illustration showing the characteristic fracture paths in ductile (left) and brittle (right) materials that are being subjected to different modes of material overstress (overloading). Looking at how these materials fracture with axial, shear, and bending forces.

Normal Wear Ultimately, products will fail with time Wear, fatigue, corrosion, creep

Normal Wear Wear Fatigue Corrosion Creep Damaged rendered through normal period of use Fatigue Repeated cycles of loading and unloading Corrosion Wasting away of metal surfaces due to chemical reactions among positively charged ions Reduces material’s capacity to support loads Creep Gradual deformation under steady tensile load Eventually causes creep rupture failure Creep is when a material component gradually deforms under a steady tensile load. The atoms or molecules in the microstructure of the material have to continually adjust to accommodate the forces being applied. This causes damage to the crystal structure of the material, creating voids and cracks in the material that will eventually spread and lead to a creep rupture failure. This usually takes a good amount of time. Ex: roof gutters will eventually split after a long period of steady loading, and after being exposed to extreme temperatures. Grocery bag handles stretching and breaking under constant stress of heavy groceries.

Summary Engineers determine to source of material failure Deductive reasoning as well extensive laboratory techniques are utilized Manufacturers often introduce source of failure into product Materials will ultimately fail over period of use of the product from manufacturing flaws, material overload, or wear