Virtual infinite stiffness load frame 무한강성 load farme

영신컨설턴트 (02) 529 8803 ystcha@naver.com 19 July 2021

 

 

VIS는 큰 하중 작용 시 frame이 변형되어 하중과 변형이 제어가 부정확하여 frame의 강성을 고려한 load frame입니다.

 

The GDSVIS is extremely stiff and designed principally for rock testing to allow minimum backlash at the point of sample shearing (other, less stiff load frames do not give analogous results at this critical point in the test due to the recovery of the load frame under fast unload conditions). 큰 하중 최대 전단하중에서 톱니바퀴의 작은 틈으로 엉킴이 발생하지 않게 한다.

25톤 Loqad Frame

 

Accurate determination of soil and rock stiffness is difficult to achieve in routine laboratory testing. Conventionally, the determination of axial stiffness of a triaxial test specimen is based on external measurements of displacement which include a number of extraneous movements. For example, the true strains developed in triaxial tests can be masked by deflections which originate in the compliances of the loading system and load measuring system. Such equipment compliance errors add to a variety of sample bedding effects to give a poor definition of the stress-strain behaviour of the material under test, particularly over the small strain range. Most triaxial tests therefore tend to give apparent material stiffnesses far lower than those inferred from field behaviour

 

Jardine, R.J., Symes M.J. & Burland, J.B. (1984). The measurement of soil stiffness in the triaxial apparatus. Geotechnique 34, No.3, 323-340.

 

load frame stiffness를 측정하여 보정하여 측정 제어

For the entire loading range, both the measurement and control of platen displacement is automatically corrected so that it corresponds to the deformation that occurs between the platen and the load button of the load cell. In this way, the platen displacement is corrected for strain in the load cell and side columns, bending flexure of the cross beams, and distortion within the motorised mechanical transmission. 

 

The GDS VIS Axial Loading System is works calibrated by computer to provide precise data on the load-deformation relationship of the entire load application and load measuring system. These measurements are made with the adjustable upper cross beam in the maximum and minimum positions. For each position, measurements are made with the platen at each end of its travel.

 

The calibration data is loaded into the read only memory (ROM) of the system which constantly monitors the axial load and uses the calibration to apply a correction to the platen displacement. It therefore appears to the observer (or controlling computer) that the measurement of platen displacement (resolved to 0.1 micrometre) is derived from a machine with infinite stiffness. In this way the system has the characteristic of Virtual Infinite Stiffness.

 

 

Applications in the geotechnical laboratory 적용

• classic compression and extension, 압축 인장시험

• creep (constant axial total stress),

• stress paths defined in terms of the stress invariants p and q,

• cyclic stress paths,

• low frequency cyclic loading.

 

Axial load

Ranges: 100 kN (10 ton), 250 kN (25 ton), 400kN or 500 kN (50 ton).

Resolution: +/- 1 in 10,000.

Platen displacement

Range: 100mm. Resolution: 0.1 micrometre. Accuracy: 0.05% over 20mm.

 

Weight 850 kgf

Size 2.3m x 0.96m x 1.0m.

 

Frame stiffness Greater than 100kN/mm.

 

Typically for a 100 kN machine at 100 kN this is about 1mm. This means the machine has a stiffness of about 0.1 kN/μm

 

Consolidated Drained 시험 50kN load frame 4000초 최대하중 큰 변형 시작하여

load frame 자체 변형 감소

 

Target Cell pressure : 4000kPa Target Back pressure : 10kPa, 온도 영하 5

Loading Velocity : 0.1596mm/min

 

시험 결과 하중 변위

 

 

Time-Load 그래프

 

 

Time-Displacement 그래프

 

Frame Displacement, LVDT (반드시 LVDT부착), 4000초에서 시료 최대하중 이후 하중 감소로 Load Frame 자체 변위 감소하여 LVDT 변위감소.