Tempered glass impact test

In explosion-proof products, a large number of transparent parts of various specifications are used, and the most commonly used material is tempered glass based on various factors such as material stability, strength, and cost. The strength of tempered glass plays an important role in the explosion-proof performance of the product.

In the process of product use, except for some special cases, tempered glass may be subjected to static loads, most of which are shock loads, external shocks are in the form of concentrated impact loads, and the internal explosion shock wave of explosion-proof products can be regarded as uniformly distributed impact loads. When inspecting the explosion-proof performance of explosion-proof products, the impact resistance of transparent parts is tested by dropping weight impact test. Although the specific impact height and the weight of the falling weight are specified in the standard, the specific impact location is not specified, which has a significant impact on the test results.

Compared with ordinary glass, tempered glass is widely used by float glass quenching. Once the tempered glass is broken, the tensile stress at the end of the crack on the surface will exceed its tensile strength, and the crack will spread rapidly to the surrounding area. explosively, so when the tempered glass is broken by impact, the whole glass is honeycomb-like obtuse angled particles, and the particles have no sharp corners.

Because glass is a brittle material, it is impossible to repeat the test on the same sample, and this project is a destructive test, and the glass breakage process is extremely fast, the crack propagation speed can reach 1466m/s, and the shock wave detonation speed in the glass is as high as 5000m/s. And it is difficult to observe by ordinary means, that is, repeated tests with a large number of samples, which is extremely costly.

Impact process analysis

When a certain speed of falling weight comes into contact with the glass surface, due to the inertia of the material, the disturbance of the sudden load on each part of the specimen does not occur at the same time, but goes through a propagation process, gradually propagating from the local disturbance area to the undisturbed area, which is the process of stress wave propagation. Under the action of impact load, the spherical wavefront of the longitudinal wave and the shear wave propagates at different speeds from the point of action of the load. The longitudinal wave with a fast propagation speed is reflected as a tensile wave on the lower surface of the glass plate, causing a fairly high tensile stress somewhere near the free surface, and when the tensile stress meets the dynamic fracture criterion of the material, it will cause the fracture failure of the material there. This is what causes radial cracks.

In addition, during the impact process, the shear wave interacts with the upper surface of the glass to cause a ray wave. Reilei wave is not a simple transverse wave or longitudinal wave, it is similar to a water surface wave, like a particle in water, the object floating on the water does up-and-down movement and forward and backward movement at the same time, forming an elliptical trajectory with the flow of water. Rayleigh waves propagate slightly less than shear waves and only affect points located on or near the surface of the specimen. The ray wave is reflected into a tensile wave through the boundary, generating tensile stress, which is the cause of annular cracks. When the impact energy is low, if the glass is not broken, a shallow pit is often left on the surface, which is both the principle.

In related studies, it is found that shear stress reduces the compressive strength of glass materials, and shear is one of the main causes of material failure when glass materials are subjected to compression shear. The morphology of the glass failure process was observed by scanning electron microscopy, and it was found that the expansion of the shear area led to the fragmentation of the glass material. When the glass is tensile, tensile stress is used as the threshold for damage. When the glass is compressed, shear stress is used as the threshold for damage, and both tensile and compression are linear damage.

Build a model

In order to accurately grasp the specific situation of impact on different parts of tempered glass, the process is analyzed in detail by simulation methods.

With reference to the actual use situation, two typical flat glass types, round and rectangular, were selected as research objects. The dimensions are φ160×10 and 200×100×100, respectively, with the same area, and the metal frame is used for support, in order to reduce the amount of calculation, the rubber liner is omitted; The round glass impacts the center and near the border, and the rectangular transparent parts impact the center, the middle of the long side, the middle of the short side, and the corners. The impact height is 0.7m, the impact speed is 3.7m/s, and the head of the drop hammer is the same diameter 25mm hemispherical as in the standard, but the length is reduced to save the calculation time, which has no substantial impact on the test results

Due to the parameter setting, this impact energy will not break the glass, but it will reflect the difference in the results when impacting different parts.

Analysis of results

Calculations are performed on various cases and the results are summarized.

First, the case of round glass with simple conditions is analyzed. The shear stress, which has the most significant effect on the failure, is taken as the observation object, and the stress distribution map at different times after the impact occurs is intercepted. As can be seen from the figure, when the drop hammer touches the glass, the stress rises at the impact point, and then the stress wave spreads along the surface to the surroundings, and inside the glass, the stress wave also spreads outward in a spherical shape, and when it encounters the glass wall, it reflects back to the starting direction, and when it meets the shear wave, a new small stress peak appears, and then continues to spread outward and disappears quickly.

By analyzing the propagation process of the stress wave generated by the impact, the shear stresses were 111.51 MPa and 46.115MPa, respectively, when the edge and center of the transparent part were impacted The shear stress at the edge impact is significantly higher than that at the center impact, and the results are more stringent under this test configuration.

Similarly, the impact cases of different parts of the rectangular transparent parts are calculated and their stress propagation process is examined. It can be seen that the overall process is basically the same as that of circular glass, and it also spreads in a ring shape to the surrounding area at the impact position, but because the width direction is closer to the edge, it meets the reflected wave returning from the edge earlier, so that the stress wave shape becomes oblong.

In the case of statistical impact corner, long side, short side and center, the shear stresses are 101.17 MPa and 57.842, respectively MPa、55.723 MPa、55.076 MPaIt can be clearly seen that the test results are the most stringent when the impact is in the corner of the rectangular transparent part. This is also echoed in the existing research, when the support is at the four vertices of the transparent part, the failure probability distribution is the same, under the same impact height, the failure probability at the four corners is the highest, and the failure probability of the center part is the lowest. In the case of this study, the shear stress increases significantly when all sides are fixed.

brief summary

For the impact resistance test of flat glass of different shapes, the impact simulation model of tempered glass drop weight is established, and the numerical simulation of the impact process of tempered glass under different impact positions is calculated by using the finite element method, and the shear stress value under each impact position is compared to judge the severity of each impact position on the test results. The following conclusions are obtained:

1) When the round flat plate is tempered glass, the impact edge is tighter than the impact center;

2) When the rectangular flat plate tempered glass is impacted, the corner is the strictest, the center is the loosest, and the middle of the long side and the middle of the short plate are the middle, and appropriate attention should be paid to the test.

Due to the time relationship, this time only a simple case analysis and comparison was carried out, and the mathematical model of stress in the impact process was not elaborated in detail, nor did the detailed interpretation of each parameter in the impact process.