Predicting dynamic compressive strength of frozen-thawed rocks by characteristic impedance and data-driven methods

In cold regions,the dynamic compressive strength(DCS)of rock damaged by freeze-thaw weathering significantly influences the stability of rock engineering.Nevertheless,testing the dynamic strength under freeze-thaw weathering conditions is often both time-consuming and expensive.Therefore,this study considers the effect of characteristic impedance on DCS and aims to quickly determine the DCS of frozen-thawed rocks through the application of machine-learning techniques.Initially,a database of DCS for frozen-thawed rocks,comprising 216 rock specimens,was compiled.Three external load parameters(freeze-thaw cycle number,confining pressure,and impact pressure)and two rock parameters(characteristic impedance and porosity)were selected as input variables,with DCS as the predicted target.This research optimized the kernel scale,penalty factor,and insensitive loss coefficient of the support vector regression(SVR)model using five swarm intelligent optimization algorithms,leading to the development of five hybrid models.In addition,a statistical DCS prediction equation using multiple linear regression techniques was developed.The performance of the prediction models was comprehensively evaluated using two error indexes and two trend indexes.A sensitivity analysis based on the cosine amplitude method has also been conducted.The results demonstrate that the proposed hybrid SVR-based models consistently provided accurate DCS predictions.Among these models,the SVR model optimized with the chameleon swarm algorithm exhibited the best performance,with metrics indicating its effectiveness,including root mean square error(RMSE)﹦3.9675,mean absolute error(MAE)﹦2.9673,coefficient of determination(R^(2))﹦0.98631,and variance accounted for(VAF)﹦98.634.This suggests that the chameleon swarm algorithm yielded the most optimal results for enhancing SVR models.Notably,impact pressure and characteristic impedance emerged as the two most influential parameters in DCS prediction.This research is anticipated to serve as a reliable reference for estimating the DCS of rocks subjected to freeze-thaw weathering.

Dynamic compressive strength and failure mechanisms of microwave damaged sandstone subjected to intermediate loading rate

To investigate the influence of microwave heating on the dynamic behavior and failure mechanisms of rock,dynamic compression tests were conducted on microwave-irradiated sandstone specimens using a modified split Hopkinson pressure bar(SHPB)system.Experimental results show that microwave radiation can effectively weaken the compressive strength of sandstone.Rock specimens show three different failure modes under impact load:tensile failure,tensile−shear composite failure and compressive−shear failure.The dynamic Poisson’s ratio,calculated using the measured P-and S-wave velocities,is introduced to describe the deformation characteristics of sandstone.With the increase in microwave power and heating time,the Poisson’s ratio declines first and then increases slightly,and the turning point occurs at 244.6℃.Moreover,the microstructural characteristics reveal that microwave radiation produces dehydration,pore expansion,and cracking of the rock.The damage mechanisms caused by microwave radiation are discussed based on thermal stress and steam pressure inside the rock,which provides a reasonable explanation for the experimental results.

Dynamic Uniaxial Compressive Tests on Limestone

The dynamic properties of limestone play a pivotal role while selecting the suitable explosives for any limestone mine.Since the application of explosives creates dynamic loading and is a dynamic event,the determination of dynamic modulus values is technically more appropriate than the static measurement.The rock fragmentation would significantly improve by investigating the dynamic uniaxial compressive strength as specific fracture energy,stress intensity factor,fracture toughness of any detonating blast hole depend heavily on dynamic rock property and not on static rock property.Most of the limestone projects globally are still accustomed with using static compressive strength to understand the rock fragmentation.The present papers deal with determination of dynamic uniaxial compressive property using split Hopkinson pressure bar(SHPB)system.The nano second high speed camera with laser captures the crack surface opening velocity during dynamic loading.It was observed during data analysis that dynamic compressive strength of limestone increases by 1.7-4.9 times of the static strength.It may be concluded by the study that determination of dynamic compressive strength is paramount for understanding the rock fragmentation.

Dynamic fragmentation of microwave irradiated rock

The microwave-assisted rock fragmentation has been proven to be a promising approach in reducing cutting tools wear and improving efficiency in rock crushing and excavation.Thus,understanding the influence of damage induced by microwave irradiation on rock fragmentation is necessary.In this context,cylindrical Fangshan granite(FG)specimens were exposed to microwave irradiation at a power of 6 kW for different durations up to 4.5 min.The damages of the specimens induced by irradiation were quantified by using both X-ray micro-CT scanning and ultrasonic wave measurement.The CT value and Pwave velocity decreased with increase of irradiation duration.The irradiated specimens were then tested using a split Hopkinson pressure bar(SHPB)system to simulate rock fragmentation.A momentum-trap technique was utilized to ensure single-pulse loading on the specimen in SHPB tests,enabling valid fragment size distribution(FSD)analysis.The dependence of dynamic uniaxial compressive strength(UCS)on the irradiation duration and loading rate was revealed.The dynamic UCS increased with increase of loading rate while decreased with increase of irradiation duration.Using the sieve analysis,three fragmentation types were proposed based on FSD,which were dictated by both loading rate and irradiation duration.In addition,an average fragment size was proposed to quantify FSD.The results showed that the average fragment size decreased with increase of loading rate.A loading rate range was identified,where a dramatic reduction of the average fragment size occurred.The dependence of fragmentation on the irradiation duration and loading rate was also discussed.

Mechanical properties of cement mortar in sodium sulfate and sodium chloride solutions

To investigate the mechanical properties of cement mortar in sodium sulfate and sodium chloride solutions, uniaxial compression test and ultrasonic test were performed. Test results show that the relative dynamic elastic modulus, the mass variation,and the compressive strength of cement mortar increase first, and then decrease with increasing erosion time in sodium sulfate and sodium chloride solutions. The relative dynamic elastic moduli and the compressive strengths of cement mortars with water/cement ratios of 0.55 and 0.65 in sodium sulfate solution are lower than those in sodium chloride solution with the same concentration at the420 th day of immersion. The compressive strength of cement mortar with water/cement ratio of 0.65 is more sensitive to strain rate than that with water/cement ratio of 0.55. In addition, the strain-rate sensitivity of compressive strength of cement mortar will increase under attacks of sodium sulfate or sodium chloride solution.

Experimental investigation on dynamic mechanical properties of sandy clay treated with alkali-activated metakaolin cement and discrete polypropylene fibers

The dynamic mechanical properties and dynamic energy absorption capacity of marine sandy clay,which was stabilized by cement with partial substitution of alkali-activated metakaolin(AAMK)and discrete polypropylene fibers,were experimentally investigated at strain rates of 80-280 s^(-1).The AAMK,as partial replacement of cement,is eco-friendly and economical,and polypropylene fibers with corrosion resistance can withstand severe environmental conditions.Dynamic mechanical properties of 16 different mix ratios were experimentally examined via split Hopkinson compression pressure bar(SHPB)tests.Typical macroscopic post-impact fragment patterns Ⅰ and Ⅱ were observed in dynamic stress-strain curves and macroscopic fragmentations.The results confirmed an obvious enhancement in the dynamic compressive strength and energy absorption density due to the use of cement with partial substitution of AAMK and the addition of polypropylene fibers and sand.Based on scanning electron microscopy(SEM)tests and nuclear magnetic resonance(NMR)tests,cemented sandy clay specimens treated with 0.2%fiber contents or higher exhibited a denser network of soil particles with hydration products.The connection mechanism and typical interface between fiber-sand-hydrate-sandy clay particles were observed via SEM tests.Furthermore,an optimal mix ratio was proposed to satisfy the demands of high dynamic mechanical properties,energy absorption capacity,and economic and environmental constraints.The optimal mix ratio corresponded to 0.2% fiber content or higher and sand content of up to 16%.Additionally,it was observed that the dynamic compressive strength of samples with 0.1% fibers or less deteriorated.Based on absorption energy density and failure modes analysis,the fiber content should be higher than or equal to 0.2% to effectively reduce the degree of fragmentation and increase the size of fragments.