Material Selection and Reliability

Physical products are made up of materials.

The materials including metals, polymers, adhesives, and many others experience loads and stresses during assembly, transport, storage, and use.

Selecting the right materials such that they both meet the needs of the customer and are sufficiently reliable relies on understanding how the material will respond to the applied stresses over time.

As with parts selection, one way to determine if a material is suitable for your application and end use is to monitor the material’s performance over time in your products. The trouble is it also transfers the risk of failure (the unknown risk of failure) to the customer.

In many cases this is unacceptable.

Supplier data sheets and reliability claims

The design team may select a material for use bases on their experience and the supplier’s assurances that the material will work as expected in the application.

In some cases, this is true, and others not so much. The trouble is the supplier rarely fully understand, as does the design early in the process, the full set of loads and stresses the material will experience.

You should identify any new material and ask the supplier for additional information. Specifically, given the expected application set of stresses how will the material respond? How will it shift, drift, degrade, deteriorate, or corrode over time? In short, how will the material lead to failure?

Once you know the likely failure mechanisms then compare the available data to the expected use conditions to estimate the material’s reliability performance. It may be suitable and have sufficient margin (both for safety and process variability). The point is to check.

Given the number and variety of new materials available for use in today’s design establishing a material reliability, characterization process may be in order. Identifying new materials and the associated loads the material will experience permits the entire team to fully evaluate the reliability and functional performance of the material.

Engineering experience along with sufficient supplier characterization results may indicate there is little risk for the given application. Or, there may be sufficient uncertainty to warrant reliability testing to discover the salient failure mechanisms and expected reliability performance.

Materials and types of reliability concerns

The best way to minimize the risk posed by using new materials is to minimize their use.

Often a new material provides a compelling benefit for product performance or safety, thus understanding how the material will perform over time becomes essential.

To provide a start for your evaluation of a new material here is a listing of different types of materials and common reliability related concerns to consider.

Of course, this list is incomplete and you and your team will have to carefully evaluate the new materials as applied in your products and environments to fully understand the risks and reliability performance.

Metals/Alloys

Cavitation is a putting erosion due to liquid/air bubble collapse and resulting in a high force impact on the metal surface
Corrosion given the process and use environments including the presence of contaminants
Cracking and crack propagation given shock, vibration, stress concentration or cyclic loading
Deformation due to applied loads or stress relation after forming
Embrittlement due to chemical interactions or work hardening or cyclic loading
Fatigue most often due to cyclic loading
Fracture could be ductile fracture due to cyclic loading or shear fracture due to excessive loading or a combination
Friction an increase or decrease in friction depending on the application due to lubricating failure, contamination, tolerances, surface finish changes
Wear often due to abrasion in the normal course of use for some application causing a change in dimension over time

Also consider the physical properties of the metal/alloy such has strength, stiffness, hardness and how they may change over time.

Many metals/alloys may have surface treatments or coatings applied, while not specifically a metal/allow material they are part of that materials system for your use.

Another set of considerations for metals/alloys is the galvanic corrosion that may occur with specific pairs of dissimilar metals in contact. Another is the effect of electromagnetic forces on specific metals. If the metal is used to deliberately or potentially to conduct current or experience a voltage potential, this is another form of loading to consider how the material behaves over time.

Metals in fitted parts or connectors likewise require careful review as they have to perform well in a difficult set of stresses and achieve high reliability. Mating/un-mating cycles wear the surfaces. Thermal cycles may cause micromotion leading to fretting corrosion.

Electrical arcing in some application may pit or deteriorate mating surfaces increasing contact resistance.

Lubricates and greases may flow, creep, or simply fail increasing mating forces, or wear, for example.

Plastics/Rubber

Abrasive wear due to surface sliding forces
Bad resilience is the loss or change of the function to accommodate specific loading the loss of a spring-like function
Chain scissioning is the breakdown of polymer chains thus changing the material properties such as hardness. It is a chemical process.
Compression set due to sustained compression loading
Dieseling is a form of cyclic loading
Explosive decompression is the loss of seal function due excessive loading
Extrusion due to excessive or sustained loading
Friction due to changes in material hardness and surface ruffles can either increase or decrease friction
Hardening caused by thermal, cyclic, chemical or UV stress is a chemical change to the material
Installation damage
Nibbling is the separation of small portion of the material due to faulty processes, mixing, or uneven abrasion, or scratching loads
Shrinking due to off-gassing, chain scissioning, or relaxation
Spiraling often due to either installation errors or forming process residual strain
Swelling due to absorption of material int the body of the material

Ceramics

Dialectic changes often due to contamination, high-temperature exposure, or in some cases simply stress relaxation
Cracking due to improper or uneven loads or cyclic loading
Fracture including ductile, brittle or impact

Ceramics have a wide range of applications thus may experience a very broad range of failure mechanisms similar to metals/alloys or plastics/rubbers.

Composites/Adhesives

Erosion often due to flowing solvents
Peeling may defeat a bond’s overall strength by applying sufficient load to separate a small area of the bond then propagate the separation across the bond area
Delamination is the separation of assembled layers potentially due to wrapping, thermal cycling
Substrate/backing failure due to excessive or cyclic loading
Adhesion failure due to excessive or cyclic loading
Cohesion failure due to excessive or cyclic loading or the deterioration of the chemical bonding due to thermal or chemical exposure

Composites/adhesives have a wide range of constructions and applications thus a very wide range of potential failure mechanisms.

The listing of concerns is short and each mechanism may deserve a dedicated article to fully describe. A key element of any material selection process is the ability to assess the potential risks of failure for that material in your application. Being aware of the broad range of common and not-so-common failure mechanisms will serve your and your team well.

What’s your favorite failure mechanism?

Part Selection Process and Reliability (article)

Sources of Reliability Data (article)

What is Reliability Optimization? (article)