Fiberglass reinforced plastics (FRP) equipment is fabricated by applying layers to build up the required thickness. Due to time and exothermic constraints, thickness may need to be built up in thinner increments with the first increment being allowed to cure before the next increment is applied. The interface between these two increments of thickness is called a secondary bond. This secondary bond is relatively strong when a design load is applied parallel to the bond area (lap shear), but is relatively weak when applied perpendicular to the edge of the bond area (peel).
Peel is a phenomenon peculiar to FRP equipment since the bonded layers could be peeled back at relatively low loads. Peel loading commonly occurs in areas such as lift lugs, holddown lugs, support clips, and nozzle attachments. Peel has been at the root of many equipment failures, but is still not well understood and very little research has been performed in this area.
This paper will provide a basis for others to begin with. It will cover peel definitions, how to design for peel, test methods, and peel strengths.
The only industry accepted design parameter for peel is referenced in ASME RTP-I’s Non-Mandatory Appendix, NM4-400 for Hold-down Lug Design1. This figure of 50 lb/in of bond width does not take the length, thickness or strength of material into account which one would logically assume to affect peel bond strength. I have developed a peel equation which takes these parameters into account and have performed limited testing to determine the peel resistance constants used.
Since peel is a relatively new frontier and there are no industry accepted definitions, I need to define various terms to be able to describe different aspects of peel
Figure I shows and defines the variables for a fiberglass welds and peel equations.
Figure 2 shows and defines the types of peel loads that can occur.
Figure 3 shows and defines variations to the types of peel loads in Figure 2.
Bond Factor = How many times the type of bond is stronger than the standard conservative face tensile strength of a polyester or vinyl ester secondary bond with no special surface preparation.
Resistance Factor = How much peel load a square inch of bond can resist per inch of weld thickness.
Peel cannot be designed using Finite Elemental Analysis or conventional "Metals" thinking. A fiberglass designer must be aware of peel situations and try to prevent those types of bonds from experiencing any significant loads. This is not always possible, hence the need for better peel design methods.
In order to have a design which takes all of the logical variables into account, I decided to attempt to define how much of the weld bond area can be used to resist peel The bond width is easily defined but the bond length is very subjective. As a weld debonds, the detached laminate becomes a cantilever beam with the maximum stress at the point of attachment.