Stress Field Intensity Research Articles

Synergistic influence mechanism of microstructure type and loading mode on the long crack propagation in Ti-55531 alloy

This work aims to reveal the synergistic effect mechanism of microstructure type and loading mode on the long crack propagation in the Ti-55531 alloy. The fracture toughness (KIC) and fatigue crack propagation rate (da/dN) of the Ti-55531 alloy with lamellar microstructure (LM) and bimodal microstructure (BM) were investigated and compared at room temperature. The results show that both microstructural factors and the loading mode significantly affect the long crack propagation behavior of the alloy. Under static loading, the KIC of the LM is higher than that of the BM. Under cyclic loading, the da/dN of the BM is lower than that of the LM in the lower stress field intensity factor range (ΔK < 26.47 MPa·m1/2). However, when ΔK > 26.47 MPa·m1/2, the da/dN of the BM is higher than that of the LM. The results of the fracture surface morphology and the crack path analysis show that the difference in microstructure characteristics leads to various behaviors of the long crack propagation under both static and cyclic loadings. The key microstructural factors controlling the long crack propagation in the two microstructures under static and cyclic loadings are discussed. The influence of microstructure characteristics on the long crack propagation in different stages of crack growth is also explained. Moreover, a new calculation model of KIC is proposed. These results could provide support for the reliability design of engineering structures and promote the wider application of the alloy in engineering.

Intensity of singular stress field for bonded body with dissimilar adherends

In this study, the intensities of the singular stress field (ISSFs) at the interface corner and interface edge in 3D BJ models with similar/dissimilar adherends are calculated by changing the adhesive layer thicknesses by the proportional method. The differences between the ISSFs for the 3D BJ models with similar/dissimilar adherends are discussed. The ISSF for the BJ with dissimilar adherends is larger than that for the BJ with similar adherends. The differences between the ISSFs for the 3D BJ models with similar/dissimilar adherends increase as the adhesive layer thickness decreases. When the adhesive layer thickness is 0.05 mm, the ISSF at the interface corner in the BJ with dissimilar adherends is about 1.7 as large as that for the BJ with similar adherends. Then, the remarkable differences between the singular stresses at the interface corner and along the interface edge in the BJ with dissimilar adherends are found.

A convenient evaluation of intensity of singular stress field at the interface corner in single lap joint with fillet

A convenient evaluation of intensity of singular stress field at the interface corner in single lap joint with fillet

A stress gradient-based fatigue life prediction method for multiaxial notched specimen considering additional hardening effect

In this paper, the damage parameter considering stress field distribution near the notch root is employed to overcome the deficiency of the equivalent stress method (ESM) ignoring the influence of stress gradient, and a new multiaxial fatigue life prediction method is then developed taking into account stress gradient effects and additional hardening effects. Firstly, the equivalent stress distribution on the critical plane is analyzed using the finite element method (FEM), and the equivalent stress field intensity of fatigue damage affected area is defined as the multiaxial fatigue damage parameter to estimate the fatigue life of notched specimens. Secondly, a new non-proportional additional hardening coefficient is defined to consider the influence of phase difference and material characteristics on multiaxial fatigue life. Finally, in order to verify the reliability of the proposed model, the estimated lives using the proposed method and the equivalent stress method are compared with the test life, and the comparison results show that the accuracy of the proposed method considering stress gradient effect and additional hardening effect is higher than that of the equivalent stress method under proportional and non-proportional loading.

Surface roughness effect on fatigue strength of aluminum alloy using revised stress field intensity approach

The fatigue strength of a component is known to highly depend on its surface quality, and it is thus necessary to develop a reliable and appropriate mathematical model for fatigue strength assessment that consider the effect of surface roughness. In this paper, different underlying physical mechanisms of the roughness effect at different regions of specimens were studied by fatigue testing of 7N01 aluminum alloy. For a quantitative analysis of the surface roughness effect, a revised stress field intensity approach for a fatigue strength assessment of microsized notches was proposed as a theoretical support. In the new model, a new form of weight function was built to adapt the characteristics of microsized notches. In addition, the effect of the field radius was fundamentally weakened on solution of the stress field intensity and the difficulty of fatigue failure region definition in the traditional method was overcome correspondingly in the proposed model, which made the calculated field strength accurate and objective. Finally, to demonstrate the validity of the revised approach quantitatively, specimens with conventionally sized notches were subjected to stress field intensity calculations. The results showed that the revised approach has satisfactory accuracy compared with the other two traditional approaches from the perspective of quantitative analysis.

Analysis of Intensity of Singular Stress Field at the Stepped-Lap Joint

Analysis of Intensity of Singular Stress Field at the Stepped-Lap Joint

DYNAMIC FRACTURE PROPAGATION MECHANISM AND APPLICATIONIN TIGHT OIL RESERVOIR

Tight oil reservoirs have achieved certain oil increase effect by supplementing formation energy with water-injection huff and puff. However, formation pressure and production decrease rapidly after multiple rounds of water injection. In order to improve the oil enhancement effect of tight oil reservoirs, changing the development method quickly became hotspot research. This paper analysis the stress field distribution near the tip of type I fracture considered the complex fracture morphology of tight oil reservoirs based on Irwin theory and elastic mechanics. A multi-fracture cross-fracture propagation model is established based on seepage mechanics, fractured tight reservoir characteristics and dynamic fracture seepage characteristics. The fracture propagation length is obtained based on the fracture propagation mechanism and the energy conservation principle. It is proposed to turn water-injection huff and puff into unstable pulse water injection according to the principle of reverse imbibition in tight oil reservoirs. Comparative analysis of two energy supplementary generation methods, water-injection huff and puff and pulse water injection, predicting cumulative oil production, pressure and remaining oil distribution in 10 years. The results show that the net internal pressure of the fracture increases with the increase of water injection, and the stress field intensity factor also increases. When the stress field intensity factor reaches the fracture toughness, it will expand at the fracture tip. The expanded and extended natural fractures communicate with each other, presenting irregular and complex fracture networks. Reverse imbibition mainly occurs in the complex fracture networks. Pulse water injection has a high cumulative oil production, a wide area of water injection and strong reverse imbibition. The findings of this study can help for better understanding of the transformation of water-injection huff and puff into pulsed water injection from horizontal wells in fractured tight oil reservoirs. It can give full play to the effects of reverse imbibition and linear displacement. This research provides guidance for it can achieve the purpose of effective oil displacement of the dynamic fracture network.

Proportional method for evaluating intensity of singular stress field for butt joint with adhesive spew

In this paper, a proportional method for evaluating an intensity of singular stress field (ISSF) at an interface corner in a butt joint (BJ) with an adhesive spew is proposed. Square-shaped inclusion models under bi-axial tensile stress and under shear stress are used as reference models. The ISSFs for the BJ are calculated by superposing those for the reference models. The reference and BJ models are analyzed by FEM under the same mesh division and the same material combination, and the stresses applied to the reference models are determined by the FEM stresses at the interface corner. The validity of the present method is examined by applying to the BJ composed of the steel and the epoxy resin. The asymptotic solution obtained by the method is in agreement with the FEM stress distributions. Then, the ISSFs for the BJ are calculated by changing adhesive spew similarly. It is shown that the ISSF normalized by the adhesive thickness is constant independent of the thickness.

Transparent and toughening epoxy thermosets modified by linear telechelic polymer containing rigid spiroacetal moieties: Uncovering the relationship between the heterogeneous crosslinked network and thermoset performances

AbstractSpiroacetal moieties containing linear telechelic polymers (LTPs) with two terminal epoxy groups were facilely synthesized via the successive thiol‐ene and thiol‐epoxy click additions. The LTPs were incorporated into epoxy matrix to fabricate the transparent and toughening thermosets. In term of the miscibility of epoxy with the adduct of ethanedithiol and 3,9‐divinyl‐2,4,8,10‐tetraoxaspiro[5.5]undecane (BTU), it is proposed that the size of the formed nanostructures was small enough to allow light penetrate. Therefore, the modified thermosets were transparent, which was further confirmed by small‐angle X‐ray scattering (SAXS) measurements. The LTPs with an alternating structure of rigid spiroacetal moiety and soft thioether could formed heterogeneous crosslinked networks (HCNs) and obviously toughen epoxy thermosets. Flexural strength increased from 1.06 GPa for neat epoxy to 2.09 GPa for toughening thermosets modified with 15 wt% LTPs. The stress field intensity (KIC) value reached up to 3.93 by more than 125% compared with that of the control sample (1.75). This work uncovered the role of HCNs on performance and paved a way for technological advances toward transparent and toughening materials in epoxy thermosets.

Assessment of notch fatigue and size effect using stress field intensity approach

Notch features have shown significant effects on fatigue performance of engineering components, which deserve more attentions during structural integrity design and assessment. This work firstly revisits the stress field intensity (SFI) approach as well as its recent advances. Then, an improved weight function is developed to account for the inhibition effect, and yields a modified SFI approach for notch fatigue analysis considering size effect. In addition, the relationship between fatigue failure region (FFR) radius and fatigue life was explored and quantified. Fatigue tests on Ni-based superalloy GH4169 notch specimens with different sizes were conducted at 650 °C, and experimental data of En3B steels and Al 2024-T351 alloys were used for model validation and comparison. Results indicate that the modified SFI approach provides better predictions than the other two approaches.

Multiaxial fatigue life prediction method of notched specimens considering stress gradient effect

AbstractThis paper investigates the effect of stress gradient on the fatigue life of U‐notched specimens under multiaxial proportional loading by studying the shear stress gradient and normal stress gradient on the crack initiation plane separately. First, based on the energy method and critical plane method, an energy‐critical plane method for determining the direction of crack initiation is proposed. Second, with the help of the concept of stress field intensity, the shear stress gradient and normal stress gradient on the critical plane are studied. Finally, a multiaxial fatigue life prediction model is established by combining the equivalent stress field intensity, composed of the shear stress field intensity and normal stress field intensity, with the Manson–Coffin equation. Theoretical analysis is verified by the experiments of Q345 steel notched specimens, and the results show that the predicted accuracy of the proposed method is not conservative and is superior to the local stress–strain method.

Study on prediction method of crack propagation in absorber weld by experiment and simulation

As an important equipment for purifying crude oil and natural gas in energy industry, absorber is subjected to harsh working environment of high stress and severe corrosion. Especially, weld is the weak region of the absorber, which faces significant risk of crack propagation due to residual stress caused by the welding process, working pressure and temperature. Meanwhile, crack propagation in absorbers with high pressure, high temperature and toxic media can often cause disastrous consequences. Therefore, to solve this problem, a systematic method considering welding, heat treatment and working process is presented in this paper to predict crack propagation completely in the weld. In the case, based on actual conditions, a finite element model (FEM) of welding process considering transient heat transfer and heat radiation for residual stress field analysis is established to obtain the far-field stress for cracks. Then, the detailed analysis of stress field and stress intensity factor (KI) under the obtained far-field stress at the crack tip is carried out through the method of integrating the strain energy density (SED) in the crack tip region. Finally, experiments of fracture toughness (KIC) of undamaged weld specimens and post-corrosion weld samples are carried out to compare with KI obtained by FEM and evaluate the safety of existing cracks. The method presented in this paper is a complete system that can provide a universal technological basis for crack propagation prediction and safety assessment of pressure vessel.

Reference solution and proportional method to calculate intensity of singular stress field (ISSF) at the interface corner where reinforced fiber enters resin matrix

To evaluate fiber/matrix bonding properties, the average shear stress is generally used as the interface strength without considering the intensity of singular stress field (ISSF) at the interface ends. In this paper, therefore, how to calculate the ISSF is described in detail at the fiber entry point of the single-fiber under pull-out test. Recent studies showed the ISSF method is quite useful for controlling the debonding strength. As a reference solution, the ISSFs at the fiber entry point are tabulated and illustrated under arbitrary material combination for the specific geometry. By applying the proportional method with this reference solution, the ISSF can be calculated for other geometries conveniently. As an example, the ISSFs of Glass fiber and Carbon fibers are shown to be obtained from the reference solution within 1 percent error. Utilizing those results, the difference between the pull-out test and the micro-bond test is discussed by varying the reinforced fiber properties.

Fractographic identification of fracture origin mainly controlled by the intensity of singular stress field (ISSF) in prismatic butt joint with corner fillet

In this study, the fracture origin is identified for the prismatic butt joints whose debonding condition can be expressed as a constant value of the ISSF. The ISSF variation is newly analyzed along the interface side by considering the real specimen geometry with chamfer at the corner. The detail fractographic observation shows that most of the fracture starts from the maximum ISSF region at the interface side instead of the interface corner. When the bondline thickness h is larger, the fracture origin can be seen at sub-surface because the stress triaxiality decreases inside of the specimen. The fracture origin under thermal loading can be estimated similarly because of the coincidence the ISSF variations.

Study of the Double Wedge Cut Technique in Medium-Depth Hole Blasting of Rock Roadways

A double wedge cut was used to improve the cut effects in the medium-depth hole blasting of rock roadways. First, the cavity formation mechanisms of single wedge cut and double wedge cut were analysed theoretically. The results show that the cavity formation resistance of the second-order wedge cut is reduced by the first-order wedge cut. Second, simulations of the two cut modes were conducted, which visualize the stress wave evolution and indicate that the stress wave of the first-order cut holes can cause pre-damage to the rock mass in the second-order cut cavity. A comparison of the stress field distribution shows that the first-order cut holes significantly increase the stress field intensity within 0 ~ 1.6 m to enhance the failure of the rock mass in the first-order cut cavity. Hence, the rock mass in the first-order cut cavity can easily be thrown out to create a new free face, which reduces the clamping effect of the second-order wedge cut and exacerbates the tensile failure of the rock mass in the second-order cut cavity. Thus, the difficulty of second-order wedge cut is reduced. Finally, blasting experiments were performed in a rock roadway. Using the double wedge cut, the average footage increased by 0.31 m, the average utilization rate of blastholes increased by 12.4%, and the average specific charge decreased by 0.20 kg·m−3, which proves the superiority of this cut mode.

スカーフ継手における特異応力の強さ(ISSF)を用いた接着強度の評価法

スカーフ継手における特異応力の強さ(ISSF)を用いた接着強度の評価法

Adhesive strength evaluation of stepped-lap joint based on the intensity of singular stress field (ISSF)

Adhesive strength evaluation of stepped-lap joint based on the intensity of singular stress field (ISSF)

Adhesive strength evaluation of scarf joint based on the intensity of singular stress field (ISSF)

Adhesive strength evaluation of scarf joint based on the intensity of singular stress field (ISSF)

Analysis of Intensity of Singular Stress Field for Single Lap Joint with fillet

Analysis of Intensity of Singular Stress Field for Single Lap Joint with fillet

Fracture Origin of Prismatic Butt Joints in Relation to the Intensity of Singular Stress Field (ISSF)

接着接合は重量・価格・生産性の面で優れており，その接着強度は，界面端部に生じるISSF＝一定によって表現される。しかし，実際の接着構造は3次元形状を有するため，はく離が生じる起点を正確に把握し，その強度を適切に予測することが製品の信頼性を確保する上で重要である。本研究では，角柱状突き合わせ継手試験片において接着界面縁のどの部分から破壊が生じるのか実験的に明らかにし，その破壊起点が滑らかな界面縁のISSFの分布のどこで生じるのか，正確な解析をもとに考察した。接着界面コーナー部の周辺ではISSFの極大値が局所的に生じるため，破壊起点となりにくく，ほとんどの破壊起点はコーナー部以外の界面縁で生じる。