FROM THE EDITOR’S DESK
LESSONS TO BE LEARNED FROM
EN 13121-3:2016
EN 13121 is the European standard for GRP chemical
equipment which replaces BS 4994.In the May issue of this
journal, I wrote about the features of this new standard.One
important feature is the introduction of Limit state design
concept in conjunction with partial factor method. For the benefit
of readers, a full feature on this design method is presented in
this issue.Part of this articlewill be continued in the next issue.
The limit state design concept has been in use since 1974 in
European codes for the design of structures made of steel,
aluminium, reinforced concrete, timber and masonry structures.
This method is now used in the 2016 edition of EN 13121-3.
Though many more improvements are needed in its use,
adoption of this limit state concept for chemical equipment is
certainly a welcome forward step. The standard brings GRP on
par with other major engineering construction materials. The
benefits of design based on this methodology will be better
understood only when the users start using it for practical
designs. Do not equate GRP with ordinary plastics.
I used to emphasize that Composites technology is both
“knowledge based and knowledge driven”.Every step of design
and manufacture shall be executedwith proper scientific
understanding and decision making.Recognition of this fact is
the reason for the success of composites in aerospace and
defense sectors bringingmuch needed weight savings. Such a
rigorous knowledge based analysis is costly to be adopted in the
industrial and social sectors. Yet, EN 13121 has brought in
scientific decision making for selection of fibres, resins, product
wall structure and laminate property evaluation methods.
Product is designed for both serviceability requirements during
normal operation and structural resistance for overload
conditions. The arbitrariness of selection of safety factor is
removed by adopting partial factors for each type of uncertainty.
I believe that extensive research might have gone into the
formulation of this standard. As I wrote in the last editorial, the
standard for the first time brought a proper grading of resins.
The standard clearly brings out that it is the resin and not the
fibrethat controls the serviceability and durability. Many believe
that resin is only a bonding medium and any resin will do the
job. The standard shows it is not true. Good resin shall be used
for creating good products.
The value of partial safety factor is an indication of the level of
uncertainty in that parameter. The Factor A1 relating to the test
verification of material properties is the largest among all
factors. It indicates the batch to batch property variation that
exists for fibres and resins. Standard does not recognize test
reports prepared before 18 months. When the test report is 18
months' old, the safety factor is 50% more than that of samples
prepared just before the design. This means that 50 % more
materials must be used. It is cheaper to test materials and use
the proper value for design rather than add more materials and
increase cost. Since it is the purchasers who suffer the financial
loss due to premature failure of the product or due to cost of
excess materials used, they mustinsist to get the designs made
with proper material properties. Very often design is being done
after making the product for clearing the payment. Purchaser
must ask for design before the product is made. They must also
insist guarantee of service life that they expect from the product.
Another tendency seen is to sideline the many stringent
provisions given in the standard. We shall not sideline the
various provisions of the standard, because the reliability and
durability must be lost.Do not get quality certificate for one set of
materials and use other set of materials.
Dr. N.G. NAIR
Honorary Editor
06 COMPOSITES TODAY VOLUME 02 ISSUE 06 JUNE 2017