Both measure how well a bonded joint resists being pulled apart, but the force direction is completely different. Shear strength measures resistance to a force acting parallel to the bond line, trying to slide the two surfaces past each other, with stress spread across the whole bonded area. Peel strength measures resistance to a force lifting one edge of the bond away, with stress concentrated in a thin line right at the point of separation.
The Core Difference
| Property | Peel Strength | Shear Strength |
|---|---|---|
| Force direction | Lifts at an angle (typically 90° or 180°) away from the bond, usually starting at an edge | Parallel to the bond line, trying to slide surfaces apart |
| Stress distribution | Concentrated at the peeling edge | Spread across the entire bonded area |
| Substrate requirement | At least one substrate must be flexible | Substrates are typically rigid |
| Typical units | Force per unit width (N/25mm, lb/in) | Force or stress per bonded area (MPa, psi) |
| Common tests | 90° peel, 180° peel, T-peel | Lap shear, static shear (torsion test) |
| Important for | Tapes, labels, films, flexible laminates | Structural joints, metal-to-metal bonding |
A Simple Way to Picture It
Shear is like sliding a sticker sideways across a surface without lifting it — the whole contact area resists the motion together. Peel is like grabbing one corner of that same sticker and pulling it up — the bond “unzips” starting from wherever you first lifted it, and only a thin line of adhesive is resisting at any given moment.
The Broader Picture: Five Stress Types on a Bonded Joint
Shear and peel are two of five ways stress commonly loads an adhesive joint:
- Shear — pull parallel to the bond, distributed across the whole area
- Tensile — pull perpendicular to and away from the bond, distributed across the whole area
- Cleavage — stress concentrated at one edge between two rigid substrates
- Peel — stress concentrated at one edge where at least one substrate is flexible
- Compression — the opposite of tensile; force perpendicular to the bond, pushing inward
Cleavage and peel are easy to confuse: both concentrate stress at one edge, but cleavage assumes rigid substrates on both sides while peel assumes at least one flexible one. For most product design decisions, shear and peel are the two numbers that actually get specified.
Why You Can’t Judge an Adhesive on Shear Strength Alone
An adhesive can have high shear strength and low peel strength at the same time — meaning it holds firm under a load applied parallel to the bond, but fails easily the moment something starts lifting an edge. This is one of the most common selection mistakes: choosing an adhesive based on a strong shear number without checking whether the application will ever put an edge-peeling load on the joint. If the design includes any flexible substrate or any risk of an edge being lifted, peel strength needs to be checked independently — a good shear number doesn’t imply a good peel number.
Which One Matters for Your Application
- Structural bonding, metal panel-to-panel joints, load-bearing connections — shear strength is the number that matters most.
- Tapes, labels, films, flexible packaging, anything with a flexible substrate that could be lifted from an edge — peel strength is the number that matters most.
- Anything with both a rigid load path and a flexible/exposed edge — check both; a bond spec that only reports one of the two numbers is giving you an incomplete picture.
Related reading: How to Calculate Shear Strength
