Degradation Mechanisms - Classification and Definition

Degradation Management is about managing Degradation Mechanisms (DMs). So what really defines a DM? Let's have a look:

Definition

According to the British Standards terminology, degradation is defined as the “detrimental change in physical condition, with time, use, or external cause. Degradation may lead to a failure”. Failure is the “termination of the ability of an item to perform a required function” and a Failure Mode (FM) is then the “manner in which the inability of an item to perform a required function occurs”. See References - 2.

A Degradation Mechanism (DM) is the physical, chemical, or other process that will cause a specific degradation morphology to occur, which may then lead to a failure. There are many different DMs and morphologies, and it is important to identify the correct DM to manage the degradation properly.

In IMS all DMs are modelled according to their location, and type. They can also be further described in terms of the morphology that they cause (designation of degradation).

Location Types

DMs are divided into two location types: 

• Internal: Process-side degradation
• External: Degradation outside pressure envelope

Note: Most DMs are Internal.

Degradation Types

DMs are divided into three degradation types:

• Age-Related (AR): Modelled with Remnant Life (RL)
• Non-Age-Related (NAR): Modelled without RL
• Strategy Based (SB): Modelled with or without RL and pre-defined Inspection Strategy (IS)

See Age-Related and Non-Age-Related Failure Characteristics and also Corrosion and Remnant Life.

Morphologies

The DMs can be further described in terms of the morphology that they cause (designation of degradation):

• General Thinning
• Localized Thinning
• Pitting Isolated
• Internal Thinning and Pitting
• Environmental Cracking – surface breaking
• Environmental Cracking – internal
• Environmental Cracking – blistering
• Fatigue Cracking
• Creep Damage
• Brittle Fracture
• Delamination
• Material Change
• Dent Buckling
• Plastic deformation

Defining DMs in IMS

Degradation Mechanisms can be defined in the CL.