PSI - Issue 19

Anne Coulon et al. / Procedia Structural Integrity 19 (2019) 665–673 Coulon / Structural Integrity Procedia 00 (2019) 000–000

667

3

Determination of load margin coefficient C 1 : In the requirement analysis phase, the first step in the process, the requirements of the system are collected by analyzing the needs of the user(s). In a remaining life context, the product is operational and actual operational loads act F are available by measurement in situ. In the V-Model, standard loads nom F used in the initial development process of the product are updated by the actual loads of the product. The initial design is evaluated regarding the comparison of standard and actual loads. To evaluate the remaining life, the load margin coefficient corresponding to the ratio between nominal and actual loads, for the same number of cycles, is defined:

F C F  1

act nom

(1)

Determination of design margin coefficient C 2 : Systems design is the phase where system engineers analyze and understand the business of the proposed system by studying the user requirements document. They figure out possibilities and techniques by which the user requirements can be implemented ensuring the structural mechanical behavior of the product. Many actual products and installations have been designed with old structural analyses, using less efficient FEA tools than current ones. An FE computation with the nominal load is performed. The initial design is criticized comparing the maximal computed stress to the nominal strength limit. The design margin coefficient is then determined thanks to the ratio between the nominal strength limit and the maximal computed stress on the whole structure under the nominal load:



lim,

C



(2)

nom

2



max,

nom

Determination of production margin coefficient C 3 : Then, the structural mechanical integrity of the product is studied by nondestructive examination (NDT, magnetoscopy, visual observation, etc). Strength and quality of material and assembly (welded joints etc) are observed and quantified. The strength limit considered in the initial design is updated and criticized regarding the manufacturing quality. A third correction factor corresponding to the ratio between actual and nominal strength limits is defined to evaluate the remaining life:



lim,

C



(3)

act

3



lim,

nom

At this step and considering Basquin law, the overall life evaluation can be done using the formula :

b nom act T T C C C 3 2 1 * * *  b b

(4)

Then the remaining life is given by: e T mainingLif act   Re The last step concerns the validation of the remaining life evaluation according to the potential appearance of cracks or failures on the actual fleet. The maintenance plan and the dated failure inventory must be analyzed since entry into service. This analysis must be completed by the nondestructive examination realized previously. Once the reliability of the remaining life evaluation is validated, operation road maps and maintenance plans for the future life are defined. Age