Experimental techniques

As discussed in Section 1.2.3, numerical methods are often required for sufficient insight during blast-structure interaction. Before using such computational methods their performance should be validated in terms of reliability, robustness and effectiveness in predicting both the loading and the response. Experimental validation is ideal since it represents the actual physics of the problem. Full-scale

1.2. Previous work 15 testing of realistic blast scenarios is normally too expensive and time consuming.

This requires explosive charges in the range of 100-30,000 kg TNT and large outdoor areas [99, 100]. Even medium-scale detonations (1-100 kg TNT at stand-off distances of several meters [101,102]) are challenging to perform in non-military laboratory settings. At these scales instrumentation becomes difficult which often results in a qualitative rather than a quantitative assessment of the experimental observations. Controlled small-scale experiments in laboratory environments should therefore be used to evaluate current computational methods and improve the understanding of the underlying physics during blast events.

Research on blast-loaded plates using small-scale explosive detonations typically involves plate dimensions up to 0.5 m, explosive charges are less than 100 g and detonated at distances up to 0.5 m from the test specimen [44,103,104]. This is often carried out in controlled laboratory environments at scaled distances corresponding to close-in or near-field detonations (Z <2.0 m/kg^1/3) using either the ballistic pendulum approach (see e.g. [44, 104–109]), or free-field airblast experiments using an explosive charge at a given stand-off distance from the plate (see e.g. [54, 68, 103, 110]). In the latter setup, the plate is typically installed in a mounting frame which is fixed to the ground. All setups are in general interested in an accurate quantification of the loading and on the resulting structural response. The ballistic pendulum approach uses the maximum angle reached by the pendulum to determine the impulse imparted to the plate, while pressure sensors positioned in the vicinity of the plate may be used to indicate the loading in free airblast tests. The spatial and temporal distribution of the blast loading are controlled by varying the explosive material, charge geometry, explosive mass and stand-off distance.

The structural response are often reported in terms of the permanent deflection and deformed shape of the plate.

Experimentation involving small-scale detonations have many benefits and are necessary to investigate the inherent complexity in such blast environments (e.g. highly non-uniform spatial and temporal pressure distributions and the interaction between the fireball and the blast overpressure in the vicinity of the target). However, such experiments also introduce some challenges (e.g.

ground reflections, light flashes and fireballs) and special care must be taken to ensure accurate geometries and alignments of the charge relative to the structure. Small geometric imperfections and deviations in the alignment may lead to non-symmetric spatial and temporal distributions of the pressure and variations in blast parameters between each test at the same configuration [111].

Moreover, high-explosives may be hazardous and involves legal restrictions which often make such experiments less available for research purposes.

Due to these challenges, alternative techniques have been developed to generate

a blast loading similar to those from actual free-field detonations. Examples of such techniques are the pressure blow down apparatus [65,66,112] and shock tube facilities [113–121]. These alternative techniques and scaled explosive detonations cover distinct loading regimes. Scaled explosive detonations are ideal for close-in and near-field testing, while the pressure blow down apparatus and shock tubes produce a blast environment resembling that of far-field detonations. Briefly stated, scaled explosive detonations typically result in peak reflected overpressures above 1 MPa and durations shorter than 1 ms, while the pressure blow down apparatus and shock tubes are characterized by peak reflected overpressures below 1 MPa and durations greater than 1 ms. In the pressure blow down apparatus, the transient loading is generated by using a pressure vessel where the test component is clamped between the centre flanges.

The test component then divides the pressure vessel in two pressure chambers and a rapid evacuation of the pressure in one of the chambers will produce a uniform pulse pressure loading on the test specimen. The shock tube technique is well-known within the field of gas dynamics using well-defined and easily controllable initial conditions [122–125]. It typically consists of a gas-filled tube in which a high-pressure chamber is separated from a low-pressure chamber using multiple diaphragms. A sudden opening of the diaphragms generates a shock wave propagating downstream the diaphragms and into the low-pressure chamber, while rarefaction waves expand into the high-pressure chamber. Using a relatively small ratio between the lengths of the two pressure chambers, this experimental setup differs from traditional shock tubes in the way that the reflected rarefaction waves catch up with the shock wave resulting in pressure profiles similar to that from an explosive detonation. It should be noted that the blast wave may also be generated using explosive-driven shock tubes where the pressurized air is replaced by an explosive detonation in the high-pressure chamber [118]. The interaction between a planar blast wave and a structure may then be studied by placing a test object inside or at the end of the tube.

Pressure blow down apparatuses and shock tube facilities therefore allow for the evaluation of blast-structure interaction without the need to considering the inherent complexity in close-in and near-field detonations.

Finally, it is emphasized that measurement techniques are equally important as the experimental setup since they determine the usefulness, reliability and validity of the experimental data. Until recently it was difficult to measure the deflection-time history of plates exposed to blast loading. However, the recent development of three-dimensional digital image correlation (3D-DIC) techniques has enabled such measurements of the complete deformation history during blast experiments [103,126,127]. The two most common techniques are the subset-based local DIC [128] and the finite element-based global DIC [129].

Tiwari et al. [126] and Zhao et al. [127] used subset-based local 3D-DIC to obtain transient deformations of thin aluminium plates during buried blast

1.3. Objectives 17