Tracing hail stone impact on external thermal insulation composite systems (ETICS) – An evaluation of standard admission impact tests by means of high-speed-camera recordings

Steinbauer, Veit; Kaufmann, Josef; Zurbriggen, Roger; Bühler, Theodor; Herwegh, Marco (2017). Tracing hail stone impact on external thermal insulation composite systems (ETICS) – An evaluation of standard admission impact tests by means of high-speed-camera recordings. International journal of impact engineering, 109, pp. 354-365. Elsevier 10.1016/j.ijimpeng.2017.07.016

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Hail impact damage on External Thermal Insulation Systems (ETICS) is increasingly recognised by insurance companies owing to increased storm occurrence frequency and storm intensity as well as more widespread installations of ETICS. In order to develop hail resistant ETICS for houses and to evaluate existing admission tests, high-speed-camera recordings of ice ball impacts at an angle of 45° and steel ball impacts at angles of 90° and 45° were used to characterize the impact process and to derivate the damage mode of hail impacts on facades.
The major differences in the impact process of the European steel ball impact test (90°, ETAG 004) and the Swiss ice ball impact test (45°, VKF P. No. 8) are identified to be (1) a 20-40 % higher maximum indentation depth in the case of the 90° steel ball tests leading to more damage, (2) a shorter impact duration caused by the higher impact speed of the ice balls resulting in a higher strain rate and (3) the shattering of the ice ball at impact energies exceeding 6 Joule. Considerable surface parallel shear movements of the ball are observed for 45° impacts. Resulting shifts in the impact stress field cause the formation of an elongated damage pattern. The rebound of the impactor, an indicator for the elasticity of the system, is found to be 10 % higher in the 45° setups compared to the 90° setup.
High strain caused by deep indentation depths is identified as the main reason for damage . First sub-surface fractures already occur shortly (tenth of milliseconds) after impact. In contrast, visible surface fractures form later during the impact processes at average indentation depths of 3-4 mm, i.e. at a time when strain localizes at the depression shoulders.
Hence to avoid the observed brittle failure behaviour, the development of flexible materials with the ability to elastically accommodate impact strains is favourable to reduce hail stone impact damage. The European steel ball test is suitable to evaluate the hail resistance of materials in laboratory studies; however, ice ball tests provide more realistic conditions (impactor material, impact speed) and are therefore advisable for final admission of ETICS regarding hail resistance on-site.

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