General methodology for durability assessment

The methodology adopted by Task 27 includes three steps: a) initial risk analysis of poten­tial failure modes, b) screening testing/analysis for service life prediction and microclimate characterisation, and c) service life prediction involving mathematical modelling and life testing.

Initial risk analysis

The initial risk analysis is performed with the aim of obtaining (a) a checklist of potential failure modes of the component and associated with those risks and critical component and material properties, degradation processes and stress factors, (b) a framework for the selection of test methods to verify performance and service life requirements, (c) a frame­work for describing previous test results for a specific component and its materials or a similar component and materials used in the component and classifying their relevance to

the actual application, and (d) a framework for compiling and integrating all data on avail­able component and material properties.

The programme of work in the initial step of service life assessment is structured into the following activities: a) Specify from an end-user point of view the expected function of the component and its materials, its performance and its service life requirement, and the in­tended in-use environments; b) Identify important functional properties defining the per­formance of the component and its materials, relevant test methods and requirements for qualification of the component with respect to performance; c) Identify potential failure modes and degradation mechanisms, relevant durability or life tests and requirements for qualification of the component and its materials as regards durability.

Table 2 Specification of critical functional properties of booster reflectors and requirements set up by the IEA SHCP Task 27 group

Critical functional

Test method for determining functional

Requirement for functional



capability and long-term per-


Reflectance (specu­lar, A pspec, and dif­fuse, Pdif)

ASTM E903-96 „Standard test method for solar Absorptance, Reflectance, and Trans­mittance of Materials Using Integrating Spheres“

PC = 0.35-A pspec +(0.1/C)-Apdif < 0.05

with concentration ratio C=1.5

Adhesion between coating and sub­strate

Visual assessment

ISO 4624:2002 „Pull-off test for adhesion“ ISO 2409:1992 „Paints and varnishes — Cross cut test“

No blistering Adhesion > 1 MPa Degree 0 or 1

The first activity specifies in general terms the function of the component and service life requirement from an end-user and product point of view, and from that identifies the most important functional properties of the component and its materials. In Table 1 and Table 2 results are shown from the analysis made by the Task 27 group on booster reflectors. How important the function of the component is from an end-user and product point of view needs to be taken into consideration when formulating the performance requirements in terms of those functional properties. If the performance requirements are not fulfilled, the

particular component is regarded as having failed. Performance requirements can be for­mulated on the basis of optical properties, mechanical strength, aesthetic values or other criteria related to the performance of the component and its materials.

Potential failure modes and important degradation processes should be identified after failures have been defined in terms of minimum performance levels. In general, there exist many kind of failure modes for a particular component and even the different parts of the component and the different damage mechanisms, which may lead to the same kind of failure, may sometimes be quite numerous. In Table 3 an example from the Task 27 work on booster reflectors is presented.

Fault tree analysis is a tool, which provides a logical structure relating failure to various damage modes and underlying chemical or physical changes. It has been used for the static solar materials studied in Task 27 to better understand observed loss in performance and associated degradations mechanisms of the different materials studied. In Figure 1 and Figure 2 are shown examples on how the different failure modes and associated deg­
radation mechanisms can be represented for booster reflectors and antireflective glazing materials.



Degradation of protective coating on reflector

Insufficient coating of reflective

layer at production










Ageing with

Loss in

Loss in

Loss in





adhesion of

of substrate





and loss in

due to


layer to






al damage





of surface

Corrosion of reflective layer



tion and




Loss of reflector performance

Figure 1 Representation of failure modes and associated degradation mechanisms for booster reflectors from the IEA SHCP Task 27 study

The risk associated with each potential failure/damage is taken as the point of departure to judge whether a particular failure mode needs to be further evaluated or not. Risks may be estimated jointly by an expert group adopting the methodology of FMEA (Failure Modes and Efffect Analysis) [2,3]. In Table 4 the result of a risk analysis made by the Task 27 group on booster reflectors is presented.

Failure/Damage mode / Degradation process

Estmated risk number associated with damage mode (based on FMEA)

A1 Degradation of the protective layer — Ageing with material decomposition


A2 Degradation of the protective layer — Loss in protec­tive capability due to mechanical damage


A3 Degradation of the protective layer — Loss in adhe­sion to reflective layer


A4 Surface soiling


A5 Surface erosion


B1 Insufficient coating of reflective layer at production


C1 Corrosion of the reflecting layer (Result of mecha­nisms A1-A3, B1)


D1 Loss of adhesion of reflector from substrate


D2 Degradation of the substrate


Table 4 Risk assessment on different damage modes of booster reflectors made by the IEA SHCP group using the methodology of FMEA [2,3]______________________________

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