Simulation of nonstationary hydrogen diffusion processes near the crack tip in a field of inhomogeneous mechanical stresses

An elastic-plastic isotropic body is investigated, weakened by a rectilinear crack directed along the abscissa axis, under the action of stresses symmetric with respect to its plane. The hydrogen concentration near the crack tip is calculated. An approximate solution of this problem is constructed under the condition that the distribution of hydrostatic stresses along the crack extension is approximated by a parabola. For a numerical solution, a method of the third order of accuracy with a two-sided estimate of the main term of the local error is proposed.

Use of the AE Effect to Determine the Stresses State in AAC Masonry Walls under Compression

Safety and reliability of constructions operated are predicted using the known mechanical properties of materials and geometry of cross-sections, and also the known internal forces. The extensometry technique (electro-resistant tensometers, wire gauges, sensor systems) is a common method applied under laboratory conditions to determine the deformation state of a material. The construction sector rarely uses ultrasonic extensometry with the acoustoelastic (AE) method which is based on the relation between the direction of ultrasonic waves and the direction of normal stresses. It is generally used to identify stress states of machine or vehicles parts, mainly made of steel, characterized by high homogeneity and a lack of inherent internal defects. The AE effect was detected in autoclaved aerated concrete (AAC), which is usually used in masonry units. The acoustoelastic effect was used in the tests described to identify the complex stress state in masonry walls (masonry units) made of AAC. At first, the relationships were determined for mean hydrostatic stresses P and mean compressive stresses σ3 with relation to velocities of the longitudinal ultrasonic wave cp. These stresses were used to determine stresses σ3. The discrete approach was used which consists in analyzing single masonry units. Changes in velocity of longitudinal waves were identified at a test stand to control the stress states of an element tested by the digital image correlation (DIC) technique. The analyses involved density and the impact of moisture content of AAC. Then, the method was verified on nine walls subjected to axial compression and the model was validated with the FEM micromodel. It was demonstrated that mean compressive stresses σ3 and hydrostatic stresses, which were determined for the masonry using the method considered, could be determined even up to ca. 75% of failure stresses at the acceptable error level of 15%. Stresses σ1 parallel to bed joints were calculated using the known mean hydrostatic stresses and mean compressive stresses σ3.

Analisa Tegangan Sisa Paduan ZrNbMoGe Menggunakan teknik Difraksi Neutron

Pengukuran tegangan sisa paduan ZrNbMoGe dengan variasi komposisi unsur Ge telah dilakukan di PTBIN-BATAN. Bahan paduan disintesa via proses peleburan dalam tungku arc-melting yang di flaping hingga 4 kali. Bahan baku yang berupa Zr sponge, serpihano Mo, kawat Nb dan cuplikan Ge dilelehkan pada temperatur lebih dari 1850 C dengan atmosfir gas argon. Tiga buah spesimen berhasil dibuat; satu buah ingot zirkonium murni (ZG0) dan dua buah ingot paduan ZrNbMoGe dengan kandungan unsur Ge 1%wt (ZG1) dan 3%wt (ZG2). Spesimen ini kemudian diambil (running) pola difraksinya dengan instrumen difraktometer serbuk resolusi tinggi (HRPD) yang memiliki panjang gelombang 1,8195Å di Balai Spektrometri Neutron. Berdasarkan masukan parameter awal tertentu, difraktogram kemudian diolah dengan perangkat lunak Rietveld untuk mendapatkan parameter kisi dan parameter profil spesimen uji. Hasil analisis memperlihatkan bahwa paduan ZrNbMoGe memiliki empat buah fasa yakni: Zr berstruktur heksagonal (hcp), Zr3Ge berstruktur tetragonal (bct), ZrMo2 berstruktur kubik (bcc) dan ZrGe berstruktur ortorombik. Setiap fasa memperlihatkan pola regangan berbeda, tetapi kelakuan tegangan sisa pada setiap fasa berkecenderungan ke arah positif (Zr dan ZrGe) dan arah negatif (Zr3Ge dan ZrMo2). Tegangan sisa () hidrostatis dalam bahan paduan ZrNbMoGe dengan komposisi 1%wt. Ge dan 3%wt. Ge berturut-turut adalah 1401,14 GPa dan -771,77 GPa.Kata Kunci : Paduan ZrNbMoGe, Tegangan sisa, Difraksi neutron.AbstractThe residual stress measurements in ZrNbMoGe alloys with varied composition of Ge has been carried out in PTBIN-BATAN. The raw materials were melted at higher than 1850C in arc-furnace by 4 times flapping and argonised to have ingots free of oxidation. Three specimens were made; pure zirconium ingot (ZG0) and ZrNbMoGe ingots with composition of Ge 1%wt (ZG1) and 3%wt (ZG2). To obtain diffraction profiles, the specimens were measured by using high resolution powder diffractometer (HRPD) with 1,8195Å wavelength in the Neutron Spectrometry Laboratory. The data parameters were calculated by Rietveld refinements methode and analyzed. The results show that ZrNbMoGe alloys comprise of hexagonal (Zr), tetragonal (Zr3Ge), cubic (ZrMo2) and ortorombic (ZrGe) structures. Each phase tends to have different lattice strains, but the residuals tend to positive (Zr and ZrGe) or negative (Zr3Ge and ZrMo2) stresses.The hydrostatic stresses in ZrNbMoGe with 1%wt and 3%wt Ge are 1401.14 GPa and -771.77 GPa respectively.Keywords : ZrNbMoGe alloy, Residual stresses, Neutron diffraction.