Peel tests for quantifying adhesion and toughness: A review

The peeling apart of layered materials is common in nature and has been used by humans in myriad applications since prehistoric times. Over the past century, a wide range of peel tests has been proposed, standardized, and used to characterize important properties of materials, adhesives, and interfaces. Understanding the relative merits and limitations of these tests, and meaningful ways to analyze the results, is essential for current and emerging applications – from tough, high strength aerospace structures and 3D printing to soft hydrogels and wearable devices. Here, we review the historical development of peel tests and then convey a categorization scheme that is applicable to various peel configurations. Four categories are presented, depending on whether peeling is elastic or inelastic, and if debonding is self-similar or not, to capture nearly all types of peel experiments from reversible interfaces to extremely tough permanent joints. Analysis methods and peel metrics are reviewed and discussed, along with recommendations for interpretation. We further consider the effects of geometric and material properties, viscoelastic effects, and structure-property relationships. We show that these topics are highly relevant for emerging areas like biological and bio-inspired mechanisms to control peeling as well as structured systems through kirigami and architectural geometries.