Digital Image Correlation System Research Articles

Determination of static and dynamic fracture initiation toughness and numerical simulation of dynamic 3-point bend experiments of Al6063-T6

The objective of this research is to find out the static and dynamic fracture initiation toughness of aluminium alloy Al6063-T6. An in-house modified Hopkinson Pressure Bar (MHPB) along with the digital image correlation (DIC) system has been used for 3-point bend experiments under dynamic loadings. However, conventional UTM has been used for quasi-static experiments. Dynamic experiments have been performed at different projectile velocities of 13.37, 14.23, 15.95, and 20.09 m/s, while quasi-static experiments have been performed at a 1 mm/min displacement rate. Dynamic force is calculated by using strain gauge measurement at two-point on the bar while the crack initiation time is calculated using combined results from strain gauge and digital image correlation. Experimental results found that the value of dynamic fracture initiation toughness (DFIT) is higher as compared to static fracture initiation toughness (SFIT). However, DFIT has continuously increased with increasing loading rates. A numerical simulation of pre notched specimen under 3-point bend dynamic loading has been performed in ABAQUS software using the J-C constitutive and fracture model. Simulation results were compared with the experimental results and found good agreement in both these results.

Failure mechanisms of TWCFS members considering various eccentricities

AbstractThin‐walled cold‐formed steel elements represent an attractive structural solution due to the fast manufacturing and erection time on‐site. Their small thickness leads to imperfection and sensitivities to eccentricities. An experimental programme was performed on short members with lipped channel cross section, subjected to eccentric compression about the minor and major axes, in a large range of eccentricities. The specimens were manufactured on a folding machine. Before testing, the dimensions of all specimens were measured. Shortening of all specimens was measured in two ways, i.e., 1) using the displacement gauge integrated with the machine crosshead beam and 2) from deformation fields obtained using digital image correlation (DIC) system. The quantitative results, presented as the ultimate loads versus the eccentricity curve, emphasise the influence of the eccentric load on the capacity of the element. Finite element (FE) analyses were performed to simulate the behaviour of short members in eccentric compression using the commercial FE software ABAQUS/CAE and ANSYS. Static non‐linear analyses were conducted in displacement control. Both geometrical and material nonlinearities were included. An isotropic linearly elastic‐perfectly plastic constitutive model was considered, with von Mises yielding the criterion and associated flow rule. Failure modes were identified due to numerical and experimental tests. Plastic mechanism models were derived, resulting in the derivation of post‐ultimate, rigid‐plastic curves, characterising a post‐ultimate structural behaviour.

Steel rebar effect on tensile and cracking behavior of UHPFRC based on direct tensile tests and digital image correlation

Ultra-high performance fiber-reinforced concrete (UHPFRC) is usually used as the reinforcement and protection layer for structural members, and adding steel rebars to UHPFRC to form reinforced UHPFRC (R-UHPFRC) is widely employed in engineering practice. This paper focuses on the differences in tensile and cracking behavior triggered by the addition of steel rebars in UHPFRC, which significantly impacts its strengthening effect and durability performance. Six UHPFRC and six R-UHPFRC specimens are tested by direct tensile tests; the mechanical responses and the full-filed and full-process cracking behaviors are characterized by the digital image correlation (DIC) system. The fiber number, dispersion, and orientation are quantitatively obtained and evaluated based on image recognition, while their variations initiated by the addition of rebars are determined. Moreover, the correlations between fiber distribution, tensile mechanical response, and cracking behavior are analyzed, and the interaction mechanism in the full tensile process between the steel rebars and UHPFRC component in R-UHPFRC is studied. Test results show that the UHPFRC component in R-UHPFRC has deteriorated fiber distribution quality and lower load-bearing capacity than UHPFRC. With the addition of steel rebars in UHPFRC, the cracking mode changed from chain-like to full-field development, leading to better multi-cracking behavior. Meanwhile, the employment of steel rebars significantly improved the capacity of UHPFRC to control the crack width and weakened its susceptibility to the non-uniformity of fiber distribution. During the cracking process of R-UHPFRC, a peculiar load redistribution exists between the UHPFRC component and steel rebars. The experimental and analytical results illustrate that the axial tensile yield and peak load of R-UHPFRC can be evaluated based on the superposition principle, while this method may overestimate the stiffness in the service limit state.

Seismic behavior of partially grouted masonry shear walls containing openings: Experimental testing

This article presents an experimental study aimed at understanding the effect of openings on the seismic response of partially grouted reinforced masonry (PG-RM) walls. For this purpose, four full-scale PG-RM walls with different opening sizes were built and tested under the combined action of constant axial compression and cyclic lateral loading. All the specimens had the same external dimensions. The effect of openings was assessed in terms of wall hysteresis and force–displacement curves and the quantification of different seismic parameters such as shear capacity, degradation of lateral stiffness, energy dissipated, equivalent viscous damping ratio, and displacement ductility. A 2D Digital Image Correlation (DIC) system was implemented to monitor principal strain fields and cracking patterns during the tests. The paper also includes the assessment of the shear capacity of the tested walls according to three existent approaches developed for in-plane shear panels, which rely on the definition of an effective height of piers.The results indicate that the shear capacity of the walls decreased when the aspect ratio of the piers increased. From DIC analysis results, it was observed that damage extended beyond piers also covering spandrels and that directionality of shear loads played a fundamental role in the damage development. Therefore, the identification of the pier’s effective height and direction of seismic loads should be incorporated into the estimation of the shear capacity. Finally, the performance of the selected shear expressions and different approaches to estimate the effective height of the piers was evaluated. It was concluded that the most accurate approach was to define an effective piers height according to Augenti (2006)’s proposal using the Calderón et al. (2022) shear expression to assess piers’ lateral resistance.

EVALUATING THE ACCURACY OF A DIGITAL CORRELATION SYSTEM TO PREDICT THE FATIGUE FAILURE OF ADDITIVELY MANUFACTURED PARTS

Internal defects that occur during the additive manufacturing process of direct metal laser sintering can have a significant influence on the fatigue performance of Ti-6Al-4V additively manufactured parts compared with parts that are made using traditional manufacturing methods. For the adoption of additively manufactured parts in the medical, automotive, and aerospace industries, the expected fatigue life of parts must be predictable and repeatable for successful application. Micro-CT scanning is commonly used to provide insight into the expected fatigue life of parts by examining internal defects that have occurred during melting. However, using a micro-CT for every part in the supply chain is not always feasible, so alternatives must be considered. One approach would be to analyse parts by applying a short-run fatigue test coupled with a digital image correlation (DIC) system to detect areas on parts where excessive distortion is taking place. This study investigated the level of accuracy that a DIC system could obtain as an alternative non-destructive test to predict where and when a test specimen would fail. To determine whether this alternative was viable, tests were carried out until the specimens failed. The DIC images were analysed at 50% of the fatigue life and at the point just before failure to determine whether the DIC system accurately indicated the strain concentration at the same point where the specimens failed. Experimental data from the study shows that the DIC system could accurately predict the point of failure at the fatigue half-life in only 10% of the test specimens that were investigated. The DIC system was able accurately to predict the point of failure right before failure occurred in only 25% of the test specimens that were investigated.

Characterization of shock-induced panel flutter with simultaneous use of DIC and PIV

In this experimental study, panel flutter induced by an impinging oblique shockwave is investigated at a freestream Mach number of 2, using the combination of planar particle image velocimetry (PIV) and stereographic digital image correlation (DIC) to obtain simultaneous full-field structural displacement and flow velocity measurements. High-speed cameras are employed to obtain a time-resolved description of the panel motion and the shockwave-boundary layer interaction (SWBLI). In order to prevent interference between the PIV and DIC systems, an optical isolation is implemented using fluorescent paint, dedicated light sources, and camera lens filters. The effect of the panel motion on the SWBLI behavior is assessed, by comparing it with the SWBLI on a rigid wall. The results show that panel oscillations occur with a maximum amplitude of ten times the panel thickness. The dominant frequencies observed in the panel oscillation (424 Hz and 1354 Hz) match the main spectral content of the reflected shockwave position. A further POD analysis of the panel displacement spatial distribution shows that these two frequency contributions are well captured by the first two POD modes, which correspond, respectively, to a first and a third bending mode shape and account for 92% of the total oscillation energy. The fluid-structure coupling is studied by identifying, in the flow, the regions of maximum correlation between the panel displacement and the flow velocity fluctuations. The results obtained prove that the inviscid flow region upstream of the SWBLI is perfectly in phase with the panel oscillation, while the downstream region has a delay of one quarter of the flutter cycle.

Application of Digital Image Correlation method for verification of topology optimization of 3D printed load-bearing element

This paper presents DIC-based experimental verification of topology optimization of the 3D printed cantilever plate load-bearing element. Test samples were 3D printed from ABS and PET-G materials using FDM technology. A 3D geometrical parametric model was created and FEA and topology optimization were performed using computer program Autodesk Inventor. In order to successfully define the optimization problem and obtain reliable results, it is extremely important to properly define the analysis input parameters, such as material parameters and boundary conditions. Young's moduli of ABS and PET-G were determined experimentally on samples which were made using 3D printing technology with identical settings as cantilever plate samples. Stresses and strains in optimized samples were determined using FEA. To verify the FEA and topology optimization results, an experimental setup for holding and loading of 3D printed samples was prepared and displacements were measured using digital image correlation system ARAMIS. Results obtained experimentally were compared with the results obtained by finite element analysis of optimized cantilever plate sample and show very good agreement for both ABS and PET-G samples, with deviation of around 4 % and 11 % respectively.

DIGITAL IMAGE CORRELATION – METHOD DEVELOPMENT, SCOPE, PRINCIPLE OF FUNCTIONING, AND FUTURE GOALS

This paper presents the basics of the Digital Image Correlation System, its algorithm of operation, methods of data recording and implementation. In addition, the paper characterises in detail the standard bench instrumentation necessary for the implementation of this type of measurement. The paper also describes the procedure of sample preparation and classifies the main methods of applying a marker to the surface of the sample. The article highlights the main advantages of the system and the main difficulties associated with its operation, and indicates the important parameters affecting the quality of the measurement. The paper shows a wide range of applications of Digital Image Correlation (DIC) and the possibilities of cooperation with other measurement systems as well as extended versions of the system, such as Digital Volumetric Correlation. The article also outlines further directions for the development of the DIC research methodology including, among others, extending the temperature range in which the method can be applied, as well as increasing the speed of camera image recording. Such modifications will allow the image correlation method to be used for research where it has not yet been possible.

Effect of ply thickness on tensile and bending performances of carbon fiber reinforced thermoplastic unidirectional laminate

AbstractIt has been explored that thin ply would improve the mechanical performances of carbon fiber reinforced epoxy composites laminate. It has not been known whether there are similar effects on thermoplastic composites. In this article, carbon fiber reinforced polyamide 6 (CF/PA6) unidirectional fabrics with two thicknesses of 0.045 and 0.105 mm were used to fabricate the unidirectional laminate (UDL). Tensile and bending behaviors of the UDL were recorded with a digital image correlation (DIC) system, their damage macro‐morphologies were observed with high‐speed camera during loading. The fracture microstructures of the samples were observed with SEM and optic microscope. The results showed that the tensile and bending strengths of the samples with 0.045 mm thick are 79.8% and 24.8% higher than that with 0.105 mm thick. Thinning ply can inhibit the crack expanding, and the time from crack originating to fracture of the sample is larger than that of the thick ply one. Morphologies of the fractures revealed that the crack mainly expanded in the matrix along the loading direction for thin‐ply laminate, while the crack developed in the fiber cluster for thick‐ply laminate. The results also revealed that the mechanical properties of the laminate can be improved obviously by using thin‐ply.

The application of digital image correlation (DIC) in fatigue experimentation: A review

AbstractIn recent years, the use of digital image correlation (DIC) in fatigue experiments has become widespread. It is estimated that ~1000 published works exist that outline fatigue experiments in which DIC is employed for displacement and strain measurement. Of these, ~900 were published in the last 10 years. DIC is a noncontact method that uses a series of digital images to calculate full‐field strains on the surface of an object, planer or curved. Typical commercial DIC systems compute strains at resolutions high enough to trace hysteresis loops in metals. Properly operated open‐source systems can do the same. The DIC method is applied not only on optically based digital images but also on digital images from ultra‐high resolution (ultra‐HR) microscopes like a scanning electron microscope (SEM) or on volumetric images from computed tomography (CT) scans. In fatigue analysis, DIC provides much more information than that of an extensometer. Full‐field strains from DIC can be acquired at different scales (i.e., microscale, macroscale, and nanoscale) and can be related to items such as microstructural features, interacting surfaces (e.g., fretting), fatigue crack growth phenomenon, and distinct forms of energy. Because fatigue is a highly complex, strain‐induced process, the DIC method is and will be an important tool for current and future research in fatigue. This review begins with an overview of the history and fundamentals of DIC including an evaluation of the overall performance and accuracy of the method. Publications selected for review are then presented and discussed. Remarks about the present state‐of‐the‐art and an outlook for future work to be done are then provided.

Experimental investigation on the buckling and post-buckling behavior of variable stiffness laminates

On the basis of adequate considering the process constraints and defect control methods of tow curve placement, two types of variable stiffness laminates were designed and manufactured. A comprehensive experiment has been carried out through the 3D digital image correlation system, strain gauges, and displacement meters. The measured results show that the buckling and failure loads of variable stiffness specimens are improved by at least 40% and 70%, respectively. The FEA results of out-of-plane displacement and strain agree well with the experimental results. Furthermore, the reason for the higher buckling and failure loads of the variable stiffness specimens was elucidated by numerical results.

The effect of thermal aging degradation of CFRP composite on its mechanical properties using destructive and non-destructive methods and the DIC system

This paper presents the results of laboratory tests for CFRP (Carbon Fiber Reinforced Polymer) composite subjected to aging in different conditions: temperature (250 °C and 300 °C) and time (60, 65, 75 and 90 min) to assess its thermal degradation and mechanical response. The tests were conducted for six batches of five samples each. The last batch was treated as reference and was not subjected to aging. To evaluate the effect of thermal degradation both nondestructive and destructive measurement techniques were used to study changes in mechanical properties including damage growth. The nondestructive ODS (Operational Deflection Shapes) technique concerned estimation of natural frequency measurements for cantilever beams, while the destructive technique involved uniaxial tensile tests. In both cases, the whole deformation and degradation processes were monitored by the Aramis system using the Digital Image Correlation (DIC) technique. The results show that despite changes in Young's modulus of aging specimens and their thickness, the natural frequency remains constant for the first and second modes. However, for specimens with higher levels of thermal aging degradation two additional modes of natural vibration were observed, i.e. the third and fourth. The aging process caused a 21% decrease in Young's modulus, an increase in Poisson's ratio by 340%, and finally a reduction in the strength of the CFRP composite by 34%.

Ballistic resistance of high-strength armor steel against ogive-nosed projectile impact

The advanced high-strength armor steel with the superior strength-to-weight ratio can extremely improve the ballistic resistance and mobility of military equipment and structures. Aiming to assess the ballistic resistance including the ballistic limit and failure modes of two types of advanced high-strength armor steels, the projectile perforation test and numerical simulations were conducted. Firstly, ten shots of 15 mm-caliber ogive-nosed projectile perforation test were performed under the initial impact velocities of 164–428 m/s. The failure mode and out-of-plane displacement (OPD) response of target plate were recorded by utilizing the high-speed 3D digital image correlation system. Then, the classical finite element (FE) and mesh-free Smoothed Particle Galerkin (SPG) methods were adopted to conduct the numerical simulations, respectively. The predicted residual velocities of projectile were compared with the test data to verify the accuracy of the calibrated constitutive model parameters and numerical analysis approaches. By further comparing the failure modes and OPD–time histories of target, it was drawn that the FE method is preferable to reproduce the failure mode of target, while the SPG method is recommended to quickly predict the residual velocity of projectile. Finally, by adopting the validated numerical analysis approach, the residual velocities for two types of high-strength armor steels under the initial impact velocity range of 150–600 m/s were obtained. The reliabilities of the existing seven analytical models in predicting the ballistic limit and residual velocity for high-strength armor steels were further evaluated.

Effects of steel fibers on the dynamic properties and failure process of ultra-high performance concrete

In the building area, an increasing number of applications have been made of ultra-high performance concrete (UHPC) because of its excellent properties. The steel fiber, as one of the popular admixtures of UHPC, can have a significant impact on the properties and failure process of UHPC under dynamic loading. A series of dynamic compression and Brazilian split tests were performed on UHPC specimens with six steel fiber contents based on the split Hopkinson pressure bar (SHPB) to investigate the effects of varying fiber content on the dynamic performance of UHPC. The digital image correlation (DIC) system equipped with a high-speed camera was incorporated to assist an in-depth knowledge of the UHPC failure process. Diverse experimental results were obtained and demonstrated that the presence of steel fibers can enhance the strength and toughness of UHPC, but the crack resistance of UHPC was unremarkable when the steel fiber content was 0.5%. Over the studied range, an increase in tensile strain rate of 10 s −1 resulted in a 36% growth of tensile dynamic increase factor (DIF t ) and the variation of fiber content had little effect on the growth ratio of DIF t . While with a 100 s −1 increase in compressive strain rate, the compressive dynamic increase factor (DIF c ) increased within 27% and higher fiber content decreased this value obviously. Based on the test data, logarithmic models for compressive and tensile dynamic increase factors were proposed in which the effects of strain rate and fiber content were both incorporated. DIC analysis revealed that the inclusion of steel fibers allowed for a larger peak strain of the specimen and slower macro-crack evolution after cracking. This research facilitates further understanding of the dynamic behaviors and damage mechanisms of UHPC. • The effects of steel fiber on dynamic behaviors of UHPC and corresponding mechanisms were analyzed. • DIF models incorporating the effects of strain rate and fiber content were proposed. • The crack evolution was explored in terms of strain field and the cracking speed was evaluated. • Inclusion of steel fibers allowed for greater deformation before failure and restrained the expansion of macro-cracks after failure.

Calibration of Ring Multicamera System With Transparent Target for Panoramic Measurement

The calibration of a multicamera system is a prerequisite for high-accuracy measurement. In this study, a method for calibrating a ring multicamera system with a transparent target combined with digital image correlation (DIC) technology is proposed to achieve panoramic dynamic and static measurement of the object. The transparent target is placed in the middle of the ring camera array, and the target can be driven by the turntable to calibrate multiple cameras simultaneously. A complete refraction correction model is built on the transparent side of the target, and the corrected calibration data are integrated into the multicamera DIC system. The global optimization further improves the overall calibration accuracy of the system. Special markers are attached to the target surface to automatically identify the refractive and nonrefractive surfaces of the target. This type of camera can establish a coordinate relationship with the cameras on the other side of the target although no overlapping field of view exists between adjacent cameras on the same side of the target. The calibration data are analyzed, and dynamic and static experiments are implemented to verify the measurement accuracy of the system. All experimental results show that the calibration and measurement accuracies of the proposed method are high, and the system can be fully used for high-precision panoramic dynamic and static measurements.

Elastic Wave Application for Damage Detection in Concrete Slab with GFRP Reinforcement

The aim of the presented examination is condition-monitoring of GFRP-reinforced concrete structural members using elastic wave propagation. As an example, a deck slab is selected. The deck slab is made of concrete of the targeted C30/37 class under three-point bending. During loading cycles, the specimen is observed with a digital image correlation (DIC) system, which enables calculation of the strain field. The measuring setup consists of two Baumer 12.3 Mpx cameras with VS-1220HV lenses, combined in a Q400 system by Dantec Dynamics GmbH. Elastic waves are also measured based on signals recorded with PZT (lead-zirconate-titanate) sensors. Additionally, the typical crack-opening measurements are made. The appearance of a crack and its growth causes changes in both the shape and amplitude of the registered signals. However, the changes are not obvious and depend on the location of the sensors. Due to the impossibility of determining simple parameters with respect to disturbingly wide cracks, for damage detection, an artificial neural network (ANN) is applied. Perfect determination of the specimen's condition (100% properly classified patterns) is possible based on whether the element is under loading or not.

Prediction of Residual Strains Due to In-Plane Fibre Waviness in Defective Carbon-Fibre Reinforced Polymers Using Ultrasound Data

Residual strains affect the properties and performance of composite components, therefore measuring and predicting them are important. The prediction of residual strains from a model can be achieved by two steps: the generation of a geometric ply map and the modelling based on that to predict 3D residual strains. A novel method for identifying the most effective algorithm for characterising fibre orientation for the geometric ply map using ultrasound C-scan data has been developed. Finite element models were generated based on the fibre-orientation data from three different algorithms: the Radon transform, 2D fast Fourier transform, and Sobel filter. The models were used to predict residual strains due to three different severities of in-plane fibre waviness induced in a set of 18 specimens. Stratified leave-one-out cross validation was applied to obtain optimum parameters for the three characterisation algorithms and to update the values of the coefficient of thermal expansion for the material. Residual strains on the surface of the specimens were obtained from calculations based on the out-of-plane displacements measured using a digital image correlation system. The predicted and measured residual strain maps were decomposed into feature vectors using orthogonal polynomials to reduce data dimensionality and make quantitative comparisons. The measured residual strains and the predictions based on models using optimised parameters showed good agreement. The differences in performance were quantified based on the accuracy of the predicted residual strains, which showed that the Radon transform performed best.

Study on Mechanical Properties and Fracture Behavior of Granite after Thermal Treatment under Brazilian Splitting Test

In this paper, the effects of thermal treatment on the mechanical properties, fracture characteristics, and roughness of the fracture surface of granite under the Brazilian splitting test are studied using an electro-hydraulic servo testing machine with an acoustic emission (AE) and digital image correlation (DIC) system. The test results show that: 1) Thermal treatment degrades the physical and mechanical properties. Density, P-wave velocity, and peak load of granite decrease with the increase in thermal treatment temperature (T). 2) With the increasing T, the fracture pattern changes from “I” type tensile fractures to “Y” type tensile-shear mixed failure. The DIC results show that the fracture process changes from transient to progressive. 3) AE counts show that granite gradually changes from brittleness to ductility with the increase of T. Furthermore, the variations in the RA and AF values of the AE events indicate that the increasing T leads to an increase in shear cracks in granite. After the test, 4) the roughness of the fracture surface of the specimen was analyzed using a spatial digitizer. The results show that thermal treatment significantly improved the fracture surface's asperity height and roughness. 5) Scanning electron microscopy (SEM) results revealed that thermal treatment leads to the generation of micro-cracks in granite and the desquamation of mineral grains, which influences the macro properties of granite.

The Effect of Preheating Temperature on the Forming Limit Diagram of AA1050/AA7050 Al Multilayered Sheets Produced by Accumulative Roll Bonding (ARB)

Multilayered sheets of AA1050 and AA7050 Al alloys produced by hot accumulative roll bonding (ARB) are tested to estimate their forming limit diagrams (FLDs). Sheets processed with preheating at 450 and 500 °C for up to six ARB cycles are submitted to Nakazima tests with an in situ digital image correlation system and tensile tests to analyze the mechanical behavior. X‐ray measurements and electron backscatter diffraction are performed to obtain the crystallographic texture and the mesotexture, respectively, and thus characterize the heterogeneous microstructure. A bimodal grain size distribution is observed in these sheets since AA7050 layers present elongated and fine grain size with nanometric precipitates and AA1050 layers present coarser and equiaxed grains. In addition, texture analysis indicates both rolling and shear components in the sheets. The FLDs show that the forming properties did not follow the expected rule of mixtures when compared with monolithic alloy sheets. Overall, the multilayered AA1050/AA7050 processed by means of ARB at 500 °C presents better formability than that at 450 °C, due to the combination of lower anisotropy measured by the Lankford coefficients values close to 1 and low normal anisotropy Δr values.

Crack Propagation and AE/EMR Response Characteristics of Pre-Holed Coal Specimens under Uniaxial Compression

Drainage boreholes in soft coal seams are prone to deformation and failure under the action of in situ stress and mining stress, which has a significant impact on gas drainage in coal mines. To simulate the development and propagation of cracks around the shaft wall caused by in situ stress, the crack propagation of coals with different diameters and strengths during the failure process, and the acoustic emission (AE) and electromagnetic radiation (EMR) law and response characteristics are explored. The results show that: The failure process of coal with pores is divided into four stages: initial compaction stage (OA), elastic deformation stage (AB), yield deformation stage (BC), and macroscopic crack development stage (CD). The crack propagation develops significantly in the post-load peak stage, the coal body damage is aggravated, and the coal body is unstable and fractured. For the pre-holed coal specimens with the same diameter, as the coal becomes softer, the peak stress decreases significantly (from 15.73 to 10.05 MPa). The cumulative value of AE counts of hard coal samples increased from 2.3 × 105 to 3.6 × 105 with increasing diameters. The Digital Image Correlation system (DIC) strain cloud diagram found that there are ‘I’-type cracks around the axial direction of the prefabricated holes. Coal samples with smaller hole have shorter cracks, indicating that the diameter of the holes significantly changes the axial loading limit. The research results have a certain reference significance for understanding the crack propagation of coal under static loads and evaluating the deformation characteristic and spatiotemporal stability of gas drainage in soft coal seams.