Correlation of the maximum shear stress with micro-mechanisms of ductile fracture for metals with high strength-to-weight ratio

Lou, Yanshan,Yoon, Jeong Whan,Huh, Hoon,Chao, Qi,Song, Jung-Han Elsevier 2018 International journal of mechanical sciences Vol.146 No.-

<P><B>Abstract</B></P> <P>Mechanisms of ductile fracture are investigated experimentally in a wide range of loading conditions from compressive upsetting to the balanced biaxial tension for two metals with high strength-to-density ratio of DP980 (t1.2) steel sheets and a bulk aluminum alloy of AA7075. Specimens are carefully designed to achieve various loading conditions from shear at low stress triaxiality to the balanced biaxial tension at high stress triaxiality for DP980, while both tensile and compressive tests are conducted for AA7075. Fractured specimen surfaces are analyzed macroscopically focusing on their relations with the maximum shear stress. It is observed that all the specimens tend to fail along the direction of the maximum shear stress in various loading states of plane strain compression, uniaxial compression, shear, uniaxial tension, plane strain tension and the balanced biaxial tension. Scanning electron microscope analyses of fracture surfaces are also conducted to explore the underlying mechanism of void coalescence since coalescence of voids is viewed as the last step of ductile fracture after nucleation and growth of voids. It is noted that fractured voids elongate along the direction of the maximum shear stress for all specimens with the stress triaxiality ranging from about −0.57 in compression to 0.67 in the balanced biaxial tension. The experiments of DP980 and AA7075 reveal that ductile fracture takes place along the direction of the maximum shear stress in the wide loading conditions of compressive upsetting, shear, uniaxial tension, plane strain tension and the balanced biaxial tension with stress triaxiality below 0.67. Thus, ductile fracture is expected to be governed by the maximum shear stress in these wide loading conditions of compression, shear and tension. It is suggested that effect of the maximum shear stress must be correctly coupled in modeling of ductile fracture in these loading conditions with uncoupled and coupled ductile fracture criteria.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Mechanisms of ductile fracture is investigated experimentally in the wide range of loading conditions from compressive upsetting to the tension of notched specimens for two lightweight metals of DP980 (t1.2) steel sheets and a bulk aluminum alloy of AA7075. </LI> <LI> All the specimens tend to fail along the direction of the maximum shear stress in various loading states of plane strain compression, uniaxial compression, shear, uniaxial tension and plane strain tension. </LI> <LI> Fractured voids elongate along the direction of the maximum shear stress for all specimens with the stress triaxiality ranging from negative in compression to 0.57 in the plane strain tension. </LI> <LI> The experiments of DP980 and AA7075 reveal that ductile fracture takes place along the direction of the maximum shear stress in the wide loading conditions of compressive upsetting, shear, uniaxial tension, and plane strain tension with stress triaxiality below 0.6. </LI> <LI> Effect of the maximum shear stress must be correctly coupled in modeling of ductile fracture in these loading conditions. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>Shear fracture takes place in wide loading conditions of tension, shear and compression. </P> <P>[DISPLAY OMISSION]</P>

Involvements of Stress Triaxiality in the Brittle Fracture during Earthquakes in Steel Bridge Bents

Hiroshi Tamura,Eiichi Sasaki,Hitoshi Yamada,Hiroshi Katsuchi,Theeraphong Chanpheng 한국강구조학회 2009 International Journal of Steel Structures Vol.9 No.3

Stress triaxiality is proposed as one of the key parameters to discuss the cause of brittle fracture during earthquakes in steel structures. This study analytically investigated the features of stress triaxiality in steel bridge bent subjected to earthquakes. The target structure is a steel bridge bent actually fractured during the South Hyogo prefecture Earthquake. From the investigations, it was confirmed that high stress triaxiality was generated at a point supposed as fracture origin. There is a possibility that the triaxiality was involved in the fracture strongly through the increase of maximum principal stress. Moreover, from the analyses accounting for several kinds of large earthquake waveforms, it was indicated that threre is possibility that the distribution of triaxiality around the fracture origin was not affected by significantly by each cycle and each waveform. Stress triaxiality is proposed as one of the key parameters to discuss the cause of brittle fracture during earthquakes in steel structures. This study analytically investigated the features of stress triaxiality in steel bridge bent subjected to earthquakes. The target structure is a steel bridge bent actually fractured during the South Hyogo prefecture Earthquake. From the investigations, it was confirmed that high stress triaxiality was generated at a point supposed as fracture origin. There is a possibility that the triaxiality was involved in the fracture strongly through the increase of maximum principal stress. Moreover, from the analyses accounting for several kinds of large earthquake waveforms, it was indicated that threre is possibility that the distribution of triaxiality around the fracture origin was not affected by significantly by each cycle and each waveform.

Stress-strain Behaviour and Shear Strength of Municipal Solid Waste (MSW)

Xiulei Li,Jianyong Shi 대한토목학회 2016 KSCE JOURNAL OF CIVIL ENGINEERING Vol.20 No.5

An understanding of the stress-strain behaviour of Municipal Solid Waste (MSW) is important in landfill design and stability analysis of landfill slopes. A series of triaxial compression tests were carried out on reconstituted MSW specimens. The effect of stress path on the drained stress-strain response and shear strength of MSW were investigated. For the compression stress paths of Δσ3 ≥0, a straight-lined increase section and an upward curvature are observed in the stress-strain curves of MSW; the upward curvature results from the fibrous constituents (primarily plastic and paper) reinforcing the waste matrix; the hardening points defined in stress-strain curves are used as failure criterion to calculate the strength parameters of MSW. For the compression stress paths of Δσ3 < 0, the stress-strain responses of MSW exhibit a rapidly increasing section towards a slowly increasing section without upward curvature; the development of fibrous reinforcement is not enough due to the reduction of confining stress σ3; the failure points defined in stress-strain curves are also used as failure criterion to calculate the strength parameters of MSW. A new method to estimate MSW strength parameters is proposed in this study. The differences of MSW friction angle are very small for different stress path tests. However, the cohesions obtained in compression stress path tests of Δσ3 < 0 are much smaller than that in conventional triaxial compression tests.

저온용 고장력강(EH36)의 평균 응력 삼축비에 따른 파단 변형률 정식화

정준모(Joonmo Choung),남웅식(Woongshik Nam) 한국해양공학회 2013 韓國海洋工學會誌 Vol.27 No.2

Stress triaxiality is recognized as one of the most important factors for predicting the failure strain of ductile metals. This study dealt with the effect of the average stress triaxiality on the failure strain of a typical low-temperature high-strength marine structural steel, EH36. Tensile tests were carried out on flat specimens with different notches, from relatively smooth to very sharp levels. Numerical simulations of each specimen were performed by using ABAQUS. The failure initiation points in numerical simulations were identified from a comparison of the engineering stress vs. strain curves obtained from experiments with simulated ones. The failure strain curves for various dimensionless critical energy levels were established in the average stress triaxiality domain and compared with the identified failure strain points. It was observed that most of the failure initiation points were approximated with a 100% dimensionless critical energy curve. It was concluded that the failure strains were well expressed as a function of the average stress triaxiality.

취성파괴수준과 파괴개시시점에 관한 진삼축 모형실험연구

천대성(Dae-Sung Cheon),박찬(Chan Park),박철환(Chulwhan Park),전석원(Seokwon Jeon) 한국암반공학회 2007 터널과지하공간 Vol.17 No.2

상대적으로 저심도에 건설되는 암반구조물의 경우 단층이나 절리 등 암반 내 존재하는 불연속면이 굴착 후 생성된 경계면과의 교차에 의해 구조적인 형태의 파괴가 지배적으로 발생하나, 고심도에 건설되는 경우 높은 현지응력과 굴착에 따른 유도응력으로 인해 공동 경계면에서 스폴링이나 슬래빙과 같은 취성파괴가 발생할 수 있다. 취성파괴는 암반구조물의 안정성을 약화시키는 주된 원인으로, 고심도 영역에서 암반구조물의 안정성을 확보하기 위하여 응력조건에 따라 발생하는 취성파괴의 개시시점, 파괴수준 및 파괴범위 등과 같은 파괴특성이 규명되어야 한다. 본 연구에서는 고심도의 암반구조물에서 발생할 수 있는 취성파괴의 파괴수준 및 개시시점과 재하응력사이의 관계를 정량적으로 평가하고자 진삼축 응력조건을 구현할 수 있는 모형실험장치를 설계, 제작하여 여러 응력조건에서 모형실험을 수행하였다. 공동주변에서 발생한 파괴수준을 육안관찰과 미소파괴음 발생양상에 의해 3단계로 구분하고, 진삼축 응력조건에 따라 제시하였다. 그 결과 파괴수준은 공동단면에 작용하는 재하응력(Sv, SH2) 뿐 아니라 공동 축에 평행한 재하응력 SH1에 영향을 받으며, SH1와 SH2의 크기가 증가할수록 동일한 Sv에서 파괴수준은 감소하였다. 파괴개시점 역시 SH1와 SH2의 증가에 따라 파괴개시를 위한 응력수준은 증가하였으며, 다중회귀분석을 통해 파괴개시시점과 진삼축 응력조건의 관계식을 도출하였다. At low in-situ stress, the continuity and distribution of natural fractures in rock mass predominantly control the failure processes. However at high in-situ stress, the failure process are affected and eventually dominated by stress-induced fractures preferentially growing parallel to the excavation boundary. This fracturing is often observed in brittle type of failure such as slabbing or spalling. Recent studies on the stress- or excavation-induced damage of rock revealed its importance especially in a highly stressed regime. In order to evaluate the brittle failure around a deep underground opening, physical model experiments were carried out. For the experiments a new true triaxial testing system was made. According to visual observation and acoustic emission detection, brittle failure grades were classified under three categories. The test results indicate that where higher horizontal stress, acting perpendicular (SH2) and parallel (SH1) to the axis of the tunnel respectively, were applied, the failure grade at a constant vertical stress level (SV) was lowered. The failure initiation stress was also increased with the increasing SH1 and SH2. From the multi-variable regression on failure initiation stress and true triaxial stress conditions, f (SV, SH1, SH2) was proposed.

Experimental Study on the Mechanical Behaviour of Natural Loess Based on Suction-Controlled True Triaxial Tests

Fang Zheng,Shengjun Shao,Jiao Wang,Shuai Shao 대한토목학회 2020 KSCE JOURNAL OF CIVIL ENGINEERING Vol.24 No.8

Loess is mostly distributed in an unsaturated state in nature, and the complexity of the engineering properties of unsaturated soil is mainly due to the existence of matric suction. Therefore, matric suction must be considered in investigating the mechanical properties of unsaturated loess. However, soils are most often subjected to three principal stresses with different magnitudes in practical engineering. For the sake of examining and discussing the mechanical behaviour of unsaturated natural loess under a complex stress path, a suction-controlled true triaxial apparatus with a rigid-flexible boundary is used to test unsaturated natural loess under a complicated stress path. Four trials of isotropic consolidation tests are conducted on natural loess under suction-controlled conditions via the true triaxial apparatus. The consolidation yield characteristics of the natural loess under different matric suctions are investigated. Forty-eight trials of consolidated drained true triaxial tests under suction-controlled conditions are conducted on unsaturated natural loess to examine and discuss the influence of the matric suction and intermediate principal stress parameter (b-value). The consolidated drained trials are performed under a constant net mean stress and a constant matric suction with different intermediate principal stress parameters (b-values). Stress-strain curves and failure envelopes of the natural loess are also presented. The results indicate that the stress-strain-strength response of unsaturated natural loess depends on the matric suction and intermediate principal stress parameter under true triaxial conditions.

조선 해양 구조물용 강재 파단의 이론적 배경

정준모(Joonmo Choung),심천식(Chun-Sik Shim),김경수(Kyung Su Kim) 대한조선학회 2011 대한조선학회 학술대회자료집 Vol.2011 No.6

Main pillars of this paper are to provide theoretical backgrounds of fracture phenomena in marine structural steels. In this paper, various fracture criteria are theoretically investigated: shear failure criteria with constant failure strain and stress triaxiality-dependent failure strain (piecewise failure and Johnson-Cook criteria), forming limit curve failure criterion, micromechanical porosity failure criterion, and continuum damage mechanics failure criterion. It is obvious that stress triaxiality is a very important index to determine failure phenomenon for ductile materials. Assuming an piecewise failure strain curve as a function of stress triaxiality, the numerical results are well coincided with test results for smooth and notched specimens where low and high stress triaxialities are observed. Therefore it is proved that a failure criterion with reliable material constants presents plastic deformation process as well as fracture initiation and evolution.

조선 해양 구조물용 강재의 소성 및 파단 특성 Ⅱ

정준모(Joonmo Choung),심천식(Chun-Sik Shim),김경수(Kyung Su Kim) 한국해양공학회 2011 韓國海洋工學會誌 Vol.25 No.2

The main goal of this paper is to provide the theoretical background for the fracture phenomena in marine structural steels. In this paper, various fracture criteria are theoretically investigated: shear failure criteria with constant failure strain and stress triaxiality-dependent failure strain (piecewise failure and Johnson-Cook criteria), forming limit curve failure criterion, micromechanical porosity failure criterion, and continuum damage mechanics failure criterion. It is obvious that stress triaxiality is a very important index to determine the failure phenomenon for ductile materials. Assuming a piecewise failure strain curve as a function of stress triaxiality, the numerical results coincide well with the test results for smooth and notched specimens, where low and high stress triaxialities are observed. Therefore, it is proved that a failure criterion with reliable material constants presents a plastic deformation process, as well as fracture initiation and evolution.

Analysis on Slope Stability Considering Seepage Effect on Effective Stress

Yuanzhan Wang,Xufei Liu,Zhikai Zhang,Panbo Yang 대한토목학회 2016 KSCE JOURNAL OF CIVIL ENGINEERING Vol.20 No.6

Interaction between seepage field and soil stress field influences soil effective stress in general consideration. In addition, water erosion in soil skeleton can also affect soil effective stress. The current studies conducted triaxial compression experiments with seepage, Duncan-Chang model analysis, ABAQUS numerical simulation and followed by a case study analysis. Experiments show that the water erosion in soil skeleton reduces effective stress strength of soil, which is quantitatively described by the reduction of equivalent confining pressure. Through analyzing the characteristics of soil stress-strain curves, the equivalent confining pressure is applied to improve Duncan-Chang model. Using the modified Duncan-Chang model as well as considering the interaction between seepage field and soil stress field, ABAQUS software package is employed to simulate triaxial compression experiments with seepage. This method is validated by comparing simulation and experimental results. Furthermore, this method is applied in the case study of the overall stability of bank slope under seepage effect and finds that the modified model is more conservative on safety assessment of slope stability. The studies provided evidences to prove the importance of considering soil skeleton eroded by water flow in slope stability analysis quantitatively and proposed a modified Duncan-Chang model for engineering application with consideration of seepage effect.

A Stress-Dilatancy Relationship for Rockfill Incorporating Particle Breakage and Intermediate Principal-Stress Ratio

Wanli Guo,Li Chen 대한토목학회 2019 KSCE JOURNAL OF CIVIL ENGINEERING Vol.23 No.7

Rockfill material is widely used for construction of high rockfill dam, which undergoes significant particle breakage and threedimensional stress path induced by the water pressure, earthquake, etc. The dilatancy equation, defined as the function of the dilatancy dg and the stress ratio η, is regarded as the foundation of elastoplastic constitutive model. Therefore, a dilatancy equation considering the influence of particle breakage and intermediate principal stress ratio b was proposed in this paper. A series of test data from rockfill materials including conventional triaxial test, constant-b test and plane strain test were used to verified the applicability of the equation. It was found that the proposed dilatancy equation considering two important factors (i.e., particle breakage and bvalue) agree well with the rearranged test data of the rockfill material in terms of dg-η. While the other dilatancy equations showed significant poorer predictions when ignoring one or two of the factors. That is, the proposed dilatancy equation can well describe the dilatancy behavior of rockfill material when considering the combined influences of the particle breakage and b-value.