Rongxin Guo

Experimental Research on High Temperature Resistance of Modified Lightweight Concrete after Exposure to Elevated Temperatures

In order to improve the spalling resistance of lightweight aggregate concrete at high temperature, two types of modified materials were used to modify clay ceramsite lightweight aggregates by adopting the surface coating modification method. Spalling of the concrete specimens manufactured by using the modified aggregates was observed during a temperature elevation. Mass loss and residual axial compressive strength of the modified concrete specimens after exposure to elevated temperatures were also tested. Concrete specimens consisting of ordinary clay ceramsites and crushed limestone were manufactured as references for comparison. The results showed that the ordinary lightweight concrete specimens and the crushed limestone concrete specimens were completely spalled after exposure to target temperatures above 400°C and 1000°C, respectively, whereas the modified concrete specimens remained intact at 1200°C, at which approximately 25% to 38% of the residual compressive strength was retained. The results indicated that the modified lightweight concrete specimens have exhibited superior mechanical properties and resistance to thermal spalling after exposure to elevated temperatures.

Application of phase unwrapping algorithm based on least-squares and iteration in digital holography

Phase unwrapping is an important content of digital holography, which gets continual and real phase from wrapped phase. The least-squares phase unwrapping is a fast and effective method. But for wrapped phases with complicated variation and much noise, the unwrapped phase got by least-squares method will produce errors. In this paper iterative unwrapping of phase difference is combined with least-squares unwrapping to eliminate the errors. This method is used in digital holography to unwrap phases with complicated variation. The phase variation of object wave through a polymethyl methacrylate (PMMA) specimen with a hole under uniform tensile force is measured by holography. The phase is unwrapped by phase unwrapping based on least-squares and iteration. The cosine patterns of unwrapped phase and wrapped phase are consistent. Which means this method is correct and can be used to unwrap phases with complicated variation and much noise.

Research on collective cracks' strain field with digital real-time holography

Cracks widely exist in material and the interaction among cracks plays an important role in the damage process of material. It is necessary, therefore, to pay much attention to the strain field of collective crack region. The elastic field for the region which contains collective cracks is different from that of the region without inclusions due to the existence of collective cracks. The variation of elastic field demonstrates the interaction among cracks. Different kinds of cracks are prefabricated on PMMA plates by mechanical fatigue or laser cutting in this paper. In order to measure the strain field of transparent plate under applied load, a digital real-time holographic measurement system developed by us is used and the holograms are reconstructed for different kinds of cracks with the help of this system. The shielding effect and amplification effect can be easily observed with the help of the magnitude and distribution of fringe in holograms. It can be seen from the hologram that distribution of elastic field is much different for different kinds of crack configures. It can be concluded from the distribution and the magnitude of interference fringes that the different interaction effect exists for elliptical holes and cracks at the same equivalent distance. The interference fringes always originate from the crack tip and spread to the up and down sides of crack with the increase of applied load. That means the originating position is the highest strain position. It is very helpful for us to determine the interference fringes level and the dangerous position for a complicate specimen. When the distance among cracks are not very close, the amplification effect among cracks will mainly lead to high strain field, vary the propagation path of crack and propel the connection among cracks. There exists gradient strain stripe among the elliptical holes. This will vary the emergence and the propagation of crack. The measurement results are also compared well with those obtained by numerical computation.

Real-Time and Quantitative Measurement of Crack-Tip Stress Intensity Factors Using Digital Holographic Interferometry

Detection of the crack in an object is a critical problem for the health monitoring of a transparent object. The real-time and quantitative measurement of the crack-tip stress intensity factor (SIF) remains an open issue. In this paper, an approach for real-time and quantitative measurement for the SIFs of a Mode I crack is presented based on digital holographic interferometry (DHI). A transmission digital holographic system is established to measure the phase difference of an object wave during loading. The expression to achieve the SIF from the phase difference is formulated. To enhance the accuracy of measurement, calibrated phase unwrapping based on least-squares and iteration and median filtering is applied to retrieve the actual phase from the noisy wrapped one. The SIFs of the Mode I crack in a transparent polymethyl methacrylate (PMMA) specimen are measured by this approach. The results are compared with the theoretical ones to demonstrate the feasibility of the proposed approach.

Research of the interaction among holes with digital real-time holography

Hole is one of the most common damages in the material. It is necessary to take the interaction among holes into consideration in the research of damage process of materials. The digital real-time holographic measurement system developed by us is used to measure the out-of-plane displacement of PMMA plates containing holes in this paper. The elastic field for the region which contains multiple holes is different from that of the region with one hole. The variation of elastic field due to the existence of multiple holes demonstrates the interaction among holes. It can be seen from the hologram recorded by the digital real-time holography that the distribution of elastic field is related to the configuration, the orientation and the distance of holes. Both the shielding effect and the amplification effect can be easily observed with the help of the distribution of fringes in hologram. These experimental results testify the credibility of the conclusions proposed by theory and by numerical computation.