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, adhe-sives, 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. Anal-ysis 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 struc-tured systems through kirigami and architectural geometries.