Inhomogeneous Stress Distribution Research Articles

Tracking the evolution of the morphology and stress distribution of SIS thermoplastic elastomers under tension using atomic force microscopy

ABSTRACT Styrene-based ABA-type triblock copolymers and their blends are widely investigated thermoplastic elastomers (TPEs). The design of tough TPE materials with high strength and resilience requires further clarification of the relationship between microstructure and macroscopic properties of stretched samples. Here, we applied atomic force microscopy (AFM)-based quantitative nanomechanical mapping to study the deformation behavior of poly(styrene-b-isoprene-b-styrene) blends under tension. The results indicated that the glassy polystyrene (PS) domains deformed and inhomogeneous stress distributions developed in the initial stretching stage. At 200% strain, the glassy PS domains started to crack. The change in the peak value in the JKR Young’s modulus diagram during stretching was consistent with the stress – strain curve. Analysis of the particles before and after stretching suggested that the glassy domains separated and reorganized during stretching.

Bio-Informed Porous Mineral-Based Composites.

Certain biominerals, such as sea sponges and echinoderm skeletons, display a fascinating combination of mechanical properties and adaptability due to the well-defined structures spanning various length scales. These materials often possess high density normalized mechanical properties because they contain well-defined pores. The density-normalized mechanical properties of synthetic minerals are often inferior because the pores are stochastically distributed, resulting in an inhomogeneous stress distribution. The mechanical properties of synthetic materials are limited by the degree of structural and compositional control currently available fabrication methods offer. In the first part of this review, examples of structural elements nature uses to impart exceptional density normalized Young's moduli to its porous biominerals are showcased. The second part highlights recent advancements in the fabrication of bio-informed mineral-based composites possessing pores with diameters that span a wide range of length scales. The influence of the processing of mineral-based composites on their structures and mechanical properties is summarized. Thereby, it is aimed at encouraging further research directed to the sustainable, energy-efficient fabrication of synthetic lightweight yet stiff mineral-based composites.

Spatially correlated stress-photoluminescence evolution in GaN/AlN multi-quantum wells

In the manufacture of semiconductor devices, cracking of heterostructures has been recognized as a major obstacle for their post-growth processing. In this work, we explore cracked GaN/AlN multi-quantum wells (MQWs) to study the influence of pressure on the recombination energy of the photoluminescence (PL) from the polar GaN QWs. We grow GaN/AlN MQWs on a GaN(0001)/sapphire template, which provides 2.4% tensile strain for epitaxial AlN. This strain relaxes through the generation and propagation of cracks, resulting in a final inhomogeneous distribution of stress throughout the film. The crack-induced strain variation investigated by micro-Raman spectroscopy and X-ray diffraction mapping revealed a correlation between the spacing of the cracks and the amount of strain between them. We have developed a 2D model that allows us to calculate the spatial variation of the in-plane strain in the GaN and AlN layers. The measured values of compressive in-plane strain in the GaN QWs vary from -0.4 % away from cracks, to -0.7 % near cracks. PL from the GaN QWs exhibits a clear correlation to the varying strain resulting in an energy shift of ∼ 140 meV. As a result, we can experimentally calculate a pressure coefficient of PL energy of ∼ -60.4 meV/GPa for the ∼ 7 nm thick polar GaN QWs. This agrees well with the previously predicted theoretical results by Kaminska et al. in 2016 [DOI: 10.1063/1.4962282], which were demonstrated to break down for such wide QWs. We will discuss this difference with respect to the reduction in both the expected point defects and extended defects resulting from not doping and growth on a GaN template, respectively. As a result, our work indicates that cracks can be utilized for investigating some fundamental material properties related to strain effects.

Hot deformation behavior and microstructure evolution of a novel Mn-containing HEA

The deformation behavior and microstructure evolution of a novel Mn-containing HEA were explored during the deformation temperature ranges of 800∼1100 °C, and strain rate ranges of 0.001∼1s−1. The study results indicate that the true stress-strain curves after work hardening exhibit continuous softening type at low strain rate ranges of 0.001–0.01 s−1, while softening then steady-state type at high strain rate ranges of 0.1∼1s−1, respectively. The activation energy (Q) and stress exponent (n) are calculated to as 404.6 kJ/mol and 4.9016, respectively. As the strain increases, the flow instability is prone to expand towards the regions of lower strain rate and higher deformation temperature, and the optical processing window is determined at 1050–1100 °C and 0.001–0.002 s−1. The strain and stress field distribution of the FEM simulations reveal the inhomogeneous deformation and the damage field distribution predict the crack region of deformation specimen. The critical (εc)/peak strain (εp) for DRX do not display a proportional relationship with strain rate and an inversely proportional relationship with deformation temperature, and the relationship between εc and εp can be expressed as: εc = (0.3–0.5) εp. Only the discontinuous dynamic recrystallization (DDRX) mechanism is activated in the whole range of deformation conditions, but it is not fully carried out. Because of the inhomogeneous stress distribution, the dynamic recrystallized grains show bimodal grain size. Furthermore, the recrystallized grains do not grow significantly due to the synergistic contribution of sluggish diffusion effect, high solution hardening effect and the trimodal B2 precipitate phase distribution.

Analysis of the effect of the direction of reinforcement of a fibrous composite material on the inhomogeneity of localization of deformations and stresses using the method of thermoelastic response

The purpose of the article is to study the effect of the direction of reinforcement of a composite material relative to the cyclic load on the inhomogeneity of stress distribution by the thermal method of non-destructive testing using the thermoelastic effect. This method is widely used in all industries for the control of structural elements and in research work. The degree of stress inhomogeneity is used when choosing the safety factors necessary to ensure the safety of technical objects. In this study, the thermoelastic effect is used to show the possibility of estimating the local distribution of stresses using the temperature field of the test object, proceeding from the linear relation between the temperature of the elastic body and mechanical stresses. The results of studying composite laminates (fibrous filler (fiberglass) with an epoxy matrix) with different laying of fibers with respect to the direction of loading are presented. Four laminate samples were considered: single-layer [0] and [90], three-layer [0]3 and [90]3. Statistical data and estimates of the coefficients of variation, methods of clustering and averaging over the length of the laminates the values of the temperature distribution over the surface of the laminates. The numerical characteristics of the distribution of local temperatures are obtained. It is shown that a more uniform distribution occurs when reinforcing is directed along the direction of the material loading. The integral and local characteristics of the temperature distribution in the samples are compared. The differences in the local temperature distribution depending on the laminate thickness were noted visually for single-layer and three-layer samples. A more uniform distribution of the load is also observed in the direction of reinforcement and with an increase in the number of reinforcement layers. The results obtained lead to the conclusion that the choice of the value of the safety factor, which evaluates the level of stress inhomogeneity, depends on the design features of the laminates. During operation of products made of composite materials in the presence of significant accumulated damage, when the uneven distribution of stresses increases, the levels of safety factors should be adjusted.

Mechanical response of monolayer graphene via a multi-probe approach

Push-to-pull (PTP) testing is employed to probe the uniaxial tensile response of freestanding monolayer graphene. Various analytical approaches are employed to estimate the elastic modulus of end-clamped graphene samples, combining in-situ Raman spectroscopy and scanning electronic microscope (SEM) measurements. The utilization of spatially resolved Raman-derived strains for assessing the elastic properties of monolayer graphene leads to results consistent with previous experimental and theoretical values of the elastic modulus (approximately 1 TPa). Molecular dynamics (MD) simulations of (pristine and defective) freestanding graphene sheets uniaxially loaded under varying clamping conditions are performed to support the experimental observations. The computational results indicate that the mechanical responses of the sheets are affected by the type, the spatial profile, and the heterogeneity of the clamping. When uniaxial pulling of end-clamped graphene is applied by a substrate adhering to the graphene sheet through van der Waals forces (as in PTP testing), the elastic modulus may be highly underestimated due to often inhomogeneous stress distribution and slippage processes. The MD simulations predict that the elastic modulus of pristine monolayer graphene is approximately 1 TPa, whereas its fracture strength can reach values of up to 110 GPa. Overall, this study underscores the limitations of traditional analyses of PTP experiments (utilizing indentation readouts and SEM imaging) and proposes new potential avenues (involving Raman measurements) for future research on the elastic properties of 2D materials.

Excellent strength-ductility combination of interstitial non-equiatomic middle-entropy alloy subjected to cold rotary swaging and post-deformation annealing

The effect of cold rotary swaging and subsequent annealing on the microstructure, texture and mechanical properties of a Fe49.5Mn30Co10Cr10C0.5 middle-entropy alloy was studied. Finite element modeling predicted inhomogeneous stress distribution and temperature gradient during deformation. Microstructure analysis indicated the development of deformation-induced γ→ε martensitic transformation during earlier steps of cold rotary swaging (20–40 % reduction). However, a single-phase face-centered cubic structure was attained after 60–90 % reduction due to reverse ε→γ transformation. Meanwhile, a twin-matrix microstructure was observed in the bar center, whereas an ultrafine microstructure was formed at the bar edge. Post-deformation annealing at 600 °C caused the onset of static recrystallization and thereby the formation of ultrafine dislocation-free grains. Apparently, after 60–90 % reduction, a ⟨100⟩- and ⟨111⟩-fiber texture gradient along the bar cross-section was developed. Besides, at the bar edge, a pronounced B/B‾ shear texture was obtained that transformed into a Cube texture after annealing at 700 °C. Compared to the as-swaged material (yield strength (YS) = 1250 MPa; ultimate tensile strength (UTS) = 1520 MPa; elongation to failure (EF) = 6.5 %), annealing at 500 °C resulted in additional strengthening (YS = 1655 MPa; UTS = 1660 MPa), but EF did not change noticeably. Yet, annealing at 600°С was accompanied by an essential increase in both EF (to 11 %) and YS (to 1500 MPa). The applied approach can be used for obtaining a material with an attractive strength-ductility combination due to overcoming the strength-ductility trade-off.

A Theoretical Analysis for Arbitrary Residual Stress of Thin Film/Substrate System With Nonnegligible Film Thickness

Abstract Stoney formula is widely used in advanced devices to estimate the residual stress of thin film/substrate system by measuring surface curvature. Many hypotheses including that thin film thickness is ignored are required, thus bringing significant error in characterizing the inhomogeneous residual stress distribution. In this article, arbitrary residual stresses on thin film/substrate structures with nonnegligible film thickness are modeled and characterized. We introduce nonuniform misfit strain and establish the governing equations including mismatched strain, displacements, and interfacial stresses based on the basic elastic theory. The parameterization method and the method of constant variation are used in the process of equation decoupling. The expressions between displacements, surface curvatures, and misfit strain are determined through decoupling calculations. By substituting misfit strain, residual stresses are expressed by parametric equation related to surface curvature. It further indicates that there is a “non-local” part between the film stress and curvature at the same point. Compared to neglecting the film thickness, the proposed method eliminate relative errors up to 58.3%, which is of great significance for stress measurement of thin films and substrates.

Multicrystalline Informatics Applied to Multicrystalline Silicon for Unraveling the Microscopic Root Cause of Dislocation Generation.

A comprehensive analysis of optical and photoluminescence images obtained from practical multicrystalline silicon wafers is conducted, utilizing various machine learning models for dislocation cluster region extraction, grain segmentation, and crystal orientation prediction. As a result, a realistic 3D model that includes the generation point of dislocation clusters is built. Finite element stress analysis on the 3D model coupled with crystal growth simulation reveals inhomogeneous and complex stress distribution and that dislocation clusters are frequently formed along the slip plane with the highest shear stress among twelve equivalents, concentrated along bending grain boundaries (GBs). Multiscale analysis of the extracted GBs near the generation point of dislocation clusters combined with ab initio calculations has shown that the dislocation generation due to the concentration of shear stress is caused by the nanofacet formation associated with GB bending. This mechanism cannot be captured by the Haasen-Alexander-Sumino model. Thus, this research method reveals the existence of a dislocation generation mechanism unique to the multicrystalline structure. Multicrystalline informatics linking experimental, theoretical, computational, and data science on multicrystalline materials at multiple scales is expected to contribute to the advancement of materials science by unraveling complex phenomena in various multicrystalline materials.

On intergranular interaction and the crystallographic mechanism of plastically deformed polycrystalline metals

Taylor principle prevailing currently has been proved to be effective only when grains or grain clusters are deformed in a rigid environment, which disagrees with the reality. Grains realize their plastic deformation in elastoplastic environment, and the equilibrium of intergranular stress and strain need to be reached simultaneously and naturally. The intergranular reaction stress (RS) during deformation can be calculated according to Hooke’s law and the yield stress should be the up-limit of the RS. The combinations of constant external stress and changing RS in RS theory induce alternate activation of deformation systems including slips and twinning with different work hardening rates, while grain orientations and inhomogeneous distribution of external stress have important influence. Some additional deformation systems near the boundaries need to be locally activated when the reduced stress up-limits are reached frequently, to balance the intergranular stress and strain incompatibilities in elastoplastic way. Based on the simpler RS theory, the simulation results of rolling texture on aluminium, low carbon steel, austenite stainless steel, titanium are very close to the reality, indicating that the intergranular elastoplastic interactions and the corresponding crystallographic behaviours of plastic deformation have been reasonably and quantitatively described, therefore, Taylor principle becomes no more necessary.

Diffusion-Reaction-Deformation Coupled Modeling of Large-Deformed Germanium Thin Film Anodes

Germanium is known as a high-capacity material that reversibly stores large amounts of lithium, whereas the inevitable volume changes lead to mechanical failures and unstable reaction interfaces. According to the finite deformation theory, we establish a theoretical framework to capture the viscoplastic flow and the interfacial transfer kinetics during lithiation and delithiation under coupled diffusion-reaction-deformation environments. Many microcracks on the surface of germanium electrodes are observed by previous experiments, and we take this effect into consideration by associating the parameters of Li-Ge alloy with the degree of lithiation, such as the concentration-dependent elasticity modulus and yield stress. Subsequently, the framework is used to calculate the mechanical and electrochemical response of thin film electrodes during charge and discharge under the rigid substrate constraint. The results suggest that charge rate and electrode thickness determine the performance of thin film battery, which is in accordance with the experimentally observed phenomenon. The Cauchy stress in the thin film electrode is also subject to the effect of the inhomogeneous spatial distribution of stress, and the stress drop at the ends of the electrodes is the main source of material fracture failure.

From network to channel: Crack-based strain sensors with high sensitivity, stretchability, and linearity via strain engineering

High-performance stretchable strain sensors are highly desirable for various scenarios, such as health monitoring and human-robot interfaces. Here, we propose a universal strain engineering strategy that introduces an inhomogeneous spatial distribution of stress and promotes crack propagation behavior leading to a critical state between network and channel morphologies, achieving stretchable strain sensors with high sensitivity, a wide working range and good linearity. Approaches for introducing soft-rigid interfaces, enlarging elastic modulus mismatches and matching dimensions have been employed to execute the strategy for network-crack strain sensors with collapsed nanocone cluster structures as representatives. The strain sensors can be tuned to realize a gauge factor of 690.95 in a linear working range of 0–40% (R2 = 0.993) or a gauge factor of 113.70 in a larger linear working range of 0–120% (R2 = 0.999). Intraocular pressure monitoring and dynamic facial asymmetry assessment have been demonstrated based on these sensors to show their great application potential.

Performance of steel cable-stayed bridges in ship fires, part I: Numerical method and baseline fire scenario study

Over the past decade, the previous hysteretic recognition of the fire threat to bridges has provoked significant progress in understanding the fire performance of bridges. However, despite the growing maritime shipment of combustibles, scarce studies have contributed to evaluating the safety of steel cable-stayed bridges subjected to under-girder ship fires. Limited existing knowledge is still based upon the less precise temperature curve method and flame geometry modeling. This study developed an advanced numerical method to more accurately evaluate the response of steel cable-stayed bridges subjected to ship fires. First, the computational fluid dynamics (CFD) approach was used to reproduce realistic fire conditions. An efficient thermal boundary, adiabatic surface temperature, was then transferred to the thermal finite element model of affected girder segments for calculating the temperature propagation. Lastly, this paper used the multiscale structural finite element method to incorporate the local performance variation of exposed girders into the response of the entire bridge. The proposed method was validated by predicting a thermomechanical response of the steel beam exposed to realistic underneath fires, agreeing well with the data from two large-scale structural fire experiments by NIST. Based on the method, the response of a typical steel cable-stayed bridge subjected to ship fires was simulated. Results show that fires can introduce local damage to the girder segments above the fire and alter the internal force distribution of the entire bridge. The bending moment of the girder sections over the fire significantly increases, and the flexural characteristic of pylons is also altered. Both thermal convection and radiation contribute to the temperature increment, and the latter dominates at the locations above fires. Adopting temperature curves can overestimate or underestimate the fire impact, introducing uncertainties to the safety of steel cable-stayed bridges in ship fires and can cause unrealistic stress concentration on the border of heated and unheated domains of exposed floors. The developed advanced method is more competent in capturing the inhomogeneous temperature and stress distribution.

Phase-field simulation for voltage profile of [formula omitted] nanoparticle during lithiation/delithiation

Tin has become one of the most promising anode materials due to its large theoretical capacity. The multi-phase transformations occurring during the (de-)lithiation process have posed a severe challenge to study their thermodynamic, kinetic and elastic properties via computational methods. In this work, a phase-field framework consisting of phase-concentration equations and Allen-Cahn equations is established. Two concepts to deliver thermodynamic and elasticity data as input quantities for the phase-field model are proposed, namely from density functional theory calculations and the thermodynamic energy of stoichiometric compounds. Using the formulated and configured phase-field model, voltage profiles, composition profiles, phases, and stress distributions of the electrode during the charging/discharging cycles are simulated. The effects of varying applied current densities and the homogeneity/inhomogeneity of the eigenstrain within the electrode on the voltage profile are discussed. According to simulation results, the establishment of stable voltage output remains a challenge under a higher C-rate due to inhomogeneous phase or stress distribution within the nanoparticle. A simulation-based derivation of linkages between electrode morphology, charging rate, stress, and capacity retention could enable an advanced design of electrode microstructures.

Influence of nano-indentation depth on the elastic–plastic transformation of 6H-SiC simulated

To investigate the effect of nano-indentation depth on the elastic–plastic transition mechanism of 6H-SiC, the loading process of nano-indentation on a 6H-SiC⟨0001⟩ crystal surface is analyzed by using the molecular dynamics method. Diamond indenters and 6H-SiC amorphous nanolayers are constructed; Tersoff and Vashishta potential functions are established to describe the interactions between atoms; and simulated environmental factors, such as relaxation conditions, ensemble, and loading speed, are optimized. The stress distribution and internal deformation characteristics are analyzed by combining load–displacement curves, radial distribution curves, dislocation nucleation, and their evolutive processes. The load–displacement curve deviates from the theoretical value at a depth of 3.8 nm and enters the elastic to plastic transition phase, where the shear stresses generated by friction on the contact surfaces cause the atoms on the stacked layers in the elastic phase to move in a directional manner and form dislocations: edge-type dislocations connecting the two layer dislocations and horizontal spiral dislocations. The plastic deformation phase starts at a depth of 5.9 nm, the atoms keep migrating and reorganizing, the original dislocation rings break to form incomplete dislocations, and the new dislocations formed by the combination of partial edge-type dislocation rings expand internally until fracture. The formation and expansion of dislocations contribute to the elasto-plastic transformation of 6H-SiC, and amorphous states are found to be involved in the deformation process at both the indentation contact surface and the deformation layer. Moreover, there are differences in the direction of dislocation expansion due to the inhomogeneous stress distribution at the contact surface.

Bulk oxygen release inducing cyclic strain domains in Ni-rich ternary cathode materials

Nickel-rich layered transition metal oxide is limited by the poor structural stability during cycling as cathode materials for next-generation lithium-based automotive batteries. In the past, the poor electrochemical performance was mainly attributed to cracks and formation of rock-salt phase on the particle surface at high potentials. Rarely is the effect of bulk phase structure evolution on properties discussed. Here, we report a bulk oxygen release induced dynamic accumulative electrochemical–mechanical coupling failure mechanism. Domain-like rock salt phases are generated due to the oxygen release and transition metals migration in the bulk region of LiNi0.6Co0.2Mn0.2O2 (NCM622) particles at the first cycle high cutoff voltage. Then, reversible compressive/tensile lattice strain alternately dominate around the domain boundary and accumulate with cycling, leading to capacity fading and becoming the origin of intracrystalline cracks. The results suggest that, in addition to the side effects from the surface, the structural transformation of the bulk plays an important role in the capacity fading. The stabilization of lattice oxygen in bulk region is a feasible solution to suppress the structural transition and the inhomogeneous stress distribution.

Inhomogeneous intrinsic stress in sputtering-deposited platinum films and its effect on the formation of helical structures

Recently, we reported an approach to form helical nanostructures, namely nanocoils, through the spontaneous bending deformation of fiber-shaped Pt films. In that study, the inhomogeneous intrinsic stress developed during Pt sputtering was proposed to be the origin of the driving force for deformation. Unfortunately, the existence of inhomogeneous intrinsic stress in the Pt films was not verified experimentally. Moreover, the intrinsic stress, which affects the bending deformation and determines the shape of the nanocoils, was not quantitatively measured. Thus, the current study investigates the intrinsic stress in Pt films deposited by two magnetron sputtering systems by measuring the film forces; moreover, it discusses the effect of stress inhomogeneity on the formation of Pt nanocoils. An inhomogeneous intrinsic stress distribution along the thickness direction was confirmed, and the tensile and compressive stresses were found to reach hundreds of megapascals. The Pt films deposited by the two sputtering systems exhibited completely different stress distributions. Regardless of this, the Pt film with inhomogeneous intrinsic stress eventually led to the formation of Pt nanocoils, whereas that with a uniform stress did not. It was therefore verified that inhomogeneous intrinsic stress was the origin of the driving force for the formation of a helical structure. Finally, it was found that the bending radius could be estimated by measuring the intrinsic stress. The results of this study will provide a perspective on the application of intrinsic stress to helical-structure manufacturing.

Research on welding-induced inhomogeneous stress in thick plate EH40 butt joint considering phase transformation

In this work, the inhomogeneous residual stress distribution and evolution of thick plate in single-pass full penetration laser welding are systematically investigated using phase transformation model. The material constitutive model considering solid-state phase transformation according to the actual weld microstructure is proposed to predict the inhomogeneous stress distribution of the welded joint. The inhomogeneous stress distributions along the longitudinal, transverse and thickness direction surfaces of the welded joint are systematically analyzed and deeply discussed, respectively. The results show that there is an obvious inhomogeneous distribution of the temperature field between the start-welding, middle section and end-welding position of the weld. It is obviously observed that the top and bottom surfaces of welded joint produce remarkably high stress areas and high stress gradients. The maximum longitudinal and transverse stress gradient values in the top surface are up to 1240.1 MPa and 1175.7 MPa, respectively. It is found that martensitic transformation is the main reason for the inhomogeneous stress distribution including high stress areas and high stress gradients in thick plate welded joint. The current research is beneficial to reduce the detrimental effects of welding-induced inhomogeneous residual stress on thick plate welded joints.

Effect of Hollow Body Material on Mechanical Properties of Bubble Concrete

The bubble concrete (BC) produced by mixing high-strength hollow bodies into concrete has been proven to be effective in reducing density while maintaining strength. It has been verified that the position distribution of hollow bodies inside hardened BC significantly affects the compressive strength of the BC. However, the effect of hollow body material on the strength of BC has not yet been established. In the present paper, through concrete compression experiments and the nonlinear finite element analysis (F.E.A.), a comparative study of BC with three different hollow bodies—steel spheres, sand bonded stainless steel spheres, and ceramic spheres—has been carried out. The results show that the difference in elastic modulus of hollow bodies and concrete is the primary reason for the inhomogeneous stress distribution inside the concrete. The interface between the hollow body and hardened concrete could be increased using epoxy resin, thus enhancing the compressive strength of BC specimens by 26.6% compared to control concrete. The ceramic material is tightly integrated with the concrete, but as a brittle material, the ceramic cannot sustain colossal stress and subsequently breaks down.

Analysis of the extension behavior of reflective cracks in asphalt pavements based on dry shrinkage property

In order to study the extension behavior of dry shrinkage type reflective cracks in asphalt pavements, this paper uses the differential equation theory to derive the calculation formula for the inhomogeneous gradient distribution of dry shrinkage stress in the subgrade. Meanwhile, based on the traction separation rule and damage behavior equation, the extended finite element method is applied to simulate the extension path and tip stress response of dry shrinkage type reflection crack, and then analyze the influence of Semi-rigid subgrade structure and material parameters on the extension behavior of dry shrinkage type cracks. The results show that the dry shrinkage stress of Semi-rigid type base is related to the elastic modulus of base course material, dry shrinkage coefficient, uniform variation of moisture content, horizontal deformation resistance coefficient, and structural dimensions of Semi-rigid type base. Micro cracks in the Semi-rigid type base under the action of dry shrinkage stress will occur two-way extension behavior, one is to continue cracking below the base course, and the other one is the reflection extension into into the surface layer, in addition, the extension speed of the reflected cracks in the surface layer is much larger than the crack cracking speed in the base course. Reducing the modulus of the base course, increasing the thickness of the base course and controlling the variation of the moisture content of the base course within the limit value can effectively retard the emergence and extension rate of reflection cracks.