Effect Of Specimen Geometry Research Articles

Influence of instrument compliance and specimen thickness on the polymerization shrinkage stress measurement of light-cured composites

Objectives The aim of our study was to investigate the effects of instrument compliance and specimen geometry on the polymerization shrinkage stress measurements of composites. Methods A custom designed stress–strain analyzer was made using a linear variable differential transformer probe and a cantilever load cell. A sandblasted glass rod was fixed at the free end of the load cell, and another glass rod was located on a base plate. A composite was placed between the two rods and light cured. The end displacement of the load cell during polymerization was recorded for 10 min. A flowable (Filtek Flow), a universal hybrid (Z100), and a packable (P60) composite were studied. To investigate the effect of specimen geometry and instrument compliance, specimen thickness was varied between 0.5, 1.0, and 2.0 mm, and three load cells with maximum capacities of 20, 100, and 500 kgf were used. Ten maxillary premolars were prepared with two sizes of MOD cavities; the bucco-lingual widths and depths of the cavities were 1.5 mm × 2 mm and 3 mm × 2 mm, and the cusp compliance and deflection were measured before and during composite polymerization. Results The measured polymerization stress decreased in the order of Filtek Flow, Z100, and P60 for all measurement conditions. As the specimen thickness was increased, the shrinkage stress per unit thickness (mm) decreased. The measured stress decreased with increasing instrument compliance. The cusp compliance (3.32 μm/N) of the 3 mm × 2 mm cavities was similar to that of instrument with a 20 kgf load cell (3.34 μm/N). Significance For determining the most clinically relevant values for shrinkage stress of dental composites, the instrument compliance should closely match that of the tooth walls in the cavity geometry to be tested.

The friction-free compressibility curve of bentonite block

Compressibility curves are widely used to describe the compaction behavior of powders. The friction between the powder and the wall of the mold during compaction results in over-estimation of the compaction force. This paper presents a method to correct this effect and obtain a ‘friction-free’ compressibility curve. The method is based on the friction force distribution theory. By eliminating the effects of wall friction force and specimen geometry, it is possible to determine the real compaction behavior of powders. A series of uniaxial compaction tests were conducted to establish the validity of the friction-free compaction concept. The data obtained from tests for various aspect ratios and lubrication conditions are shown to tend towards a unique friction-free compressibility curve, once results have been corrected for frictional effects.

An Investigation of Specimen Geometry Effects on the Fracture Behavior of a Polytetrafluoroethylene Polymer

Abstract Recent interest by the U.S. Navy in metal filled polymers for weapons applications has resulted in a desire to characterize interactions between polymer properties associated with strain rate sensitivity, pressure sensitivity, failure mechanisms, damage mechanisms, spall strength, crystalline response to shock loading, and dynamic fracture mechanics as well as the effects of loading the polymer with various metal fillers [1]. Polytetrafluoroethylene (PTFE) is an important structural polymer that in the “7C” derivative is used for gaskets, bearing pads, piston rings, and diaphragms and when 5μm aluminum spheres are added, has important ballistic properties. Prior work [2–4] evaluated the fracture toughness of PTFE and aluminum filled PTFE over a range of temperatures and loading rates using compact tension (C(T)) specimens and the normalization method [5–7] described in Annex A15 of ASTM E 1820. Moreover, prior work [3] has shown that filling the PTFE with 10 to 25 % aluminum 5μm spheres can increase stiffness and strength while the effect on toughness can be either positive or negative depending on the loading rate and test temperature. The overall effect on fracture toughness properties derived from adding the aluminum spheres to the PTFE is small and for that reason the unfilled PTFE material was used here to study the effects of geometry on fracture toughness in this material. The normalization method was initially chosen for this project because of the scarcity and cost of specimen material, especially the filled, reactive material, the many different test conditions to be investigated, and the inadequacy of alternative methods. It was quite fortunate that the normalization method was used initially to characterize this polymer since the fracture toughness resistance curves of this material often contains ductile instabilities or “pop-ins” which would not have been well characterized by multispecimen methods. The objective of this work is to compare the fracture toughness measurements obtained from C(T) and precracked Charpy (SE(B)) specimens tested under similar test conditions. A comparison is also made between J-R curves obtained using the single specimen normalization method and the “Basic” or multispecimen procedure. A limited study of the effects of side grooving on the crack extension and on the resulting J-R curves is included here.

Characterising performance of automotive materials at high strain rate for improved crash design

This paper investigates the effect of specimen geometry on the measured dynamic mechanical properties of a high strength sheet steel material derived from tensile testing at 15 m/s on a high speed servo-hydraulic test machine. In this study, stochastic modelling is used extensively to support experimental investigations. The current objective is to develop a tensile specimen design and test procedure, which more closely matches the capabilities of the new IARC high speed test machine, to enable accurate and precise measurement of tensile mechanical properties over a range of strain rates up to 600 s - 1.

Specimen-geometry effects on mechanical behavior of metallic glasses

The mechanical behavior of metallic glasses under bending, compression, impact, and fatigue experiments varies, depending on the specimen geometry. The dependence of the mechanical behavior of metallic glasses on specimen geometries is reviewed and studied in this work. Different specimen geometries are found to influence the shear-band formation and deformation processes, resulting in the variations of mechanical properties. The understanding of specimen-geometry effects on the mechanical behavior of metallic glasses is critical to the improvement of mechanical properties of metallic glasses.

Specimen geometry effect on creep crack growth in 316L(N)

ABSTRACTThe ASTM E1457‐98 standard describes the procedure to determine the master curve da/dt versus C* parameter, for creeping solids. However, the methodology is only to be applied to compact tension (CT) specimens. The European collaborative program CRETE aims at extending the application of the ASTM E1457‐98 standard to other types of laboratory specimens. In this paper, an existing database of creep crack growth on 316L(N) stainless steel is utilized, concerning three types of specimens: circumferentially cracked round bars (CCRBs) and double edge notched tensile (DENT) specimens for tensile mechanical loading whereas the classical CT specimen combines tensile and bending loading modes.A modified procedure based on the ASTM E1457‐98 standard has been applied to the database, resulting in a unique master curve of da/dt versus C*. The geometry effect is then investigated by introducing the Q* parameter by analogy to the Q parameter in the elastic–plastic J‐Q approach.

Yielding Phenomena Appeared in Impact Tensile Test Using a Split-Hopkinson Pressure Bar Method

To identify the yielding phenomena often observed in tensile test at high strain rates even for the constitutive equation of material without any yielding phenomena, numerical simulations using the dynamic FEM analysis have been performed to study the effect of specimen geometry and loading configurations in the Split-Hopkinson Pressure Bar apparatus. Stress distributions in the specimens and the loading bars are clarified, and it is found that the three-dimensional stress wave propagation from the specimen to the loading bar dominates the yielding phenomena due to radial oscillations associated with, even though a slight effect of the specimen geometry is involved to some extent. It is also found that the relevant shape of loading bar end is served to suppress the yielding phenomena.

The effect of consistency, specimen geometry and adhesion on the axial polymerization shrinkage measurement of light cured composites

Objectives This study investigated the effect of the consistency, specimen geometry and adhesion on the measurements of axial polymerization shrinkage of light cured composite resins using an axial shrinkage-measuring device. Methods Four commercially available composites were examined: an anterior posterior hybrid composite Z100, a posterior packable composite P60 and two flowable composites, Filtek flow and Tetric flow. The axial polymerization shrinkage of the composites was determined using a ‘bonded disc method’ and ‘non-bonded’ free shrinkage method at varying C-factors by altering the specimen geometry. These measured axial shrinkages were compared with the free volumetric shrinkages. The consistency of the composites was also compared using a squeeze test. Results Using the non-bonded method, the axial shrinkage was approximately one third of the true volumetric shrinkage as a result of isotropic contraction. However, in the bonded disc method, the axial shrinkage increased up to the volumetric shrinkage by anisotropic contraction as the bonded surface increased. The axial shrinkage increased with the increasing C-factor. It approached the true volumetric shrinkage and reached a plateau at near a C-factor of 5–6. However, in flowable composites, a lower level of axial shrinkage was measured by the compensational radial flow. Significance When estimating the volumetric shrinkage from the axial shrinkage measured using the bonded disc method, the C-factor of the specimen should be higher than 5 and the consistency of the composite is also an important factor that needs to be considered.

Effect of specimen geometries on the C∗ versus d a/d t master curve for type 316L stainless steel

This work deals with engineering components made of stainless steels working at high temperature and subjected to creep-fatigue loading history. The defect assessment procedures generally use the crack growth properties curve d a/d t versus C ∗ parameter for estimating the creep-crack growth. The ASTM E 1457-98 [ASTM E 1457-98. Standard test method for measurement of creep crack growth rates in metals, 1998] procedure proposes the rule to establish such a master curve. In particular, it is stipulated that this rule only applies for CT specimens. Previously [Laiarinandrasana L, Kabiri R, Drubay B. In: Gupta A, editor. Proceedings of the 16th international conference on structural mechanics in reactor technology, Washington, USA, 2001], some practical methodology to produce this crack growth curve on CT specimens has been described by introducing the way to determine the upper and lower limits of relevant experimental points and by adopting the ASTM E 1457-98 method to estimate the creep component of the load line displacement rate (d δ/d t behavior). This latter is the interesting part of the total displacement rate recorded during the test. This paper focuses on the application of the procedure proposed in [Laiarinandrasana L, Kabiri R, Drubay B. In: Gupta A, editor. Proceedings of the 16th international conference on structural mechanics in reactor technology, Washington, USA, 2001] on specimen geometries other than CT, such as circumferentially cracked round bar (CCRB) and double edged notched in tension (DENT) specimens. The d a/d t versus C ∗ curves issued from all of these specimens are compared. Discussion about the effect of geometry on these curves is carried out. Additionally, some finite element analyses have been performed in order to simulate the creep crack growth using the node release technique. These simulations allow to verify the validity of the proposed expressions of C ∗ and consequently the master curve of the 316L(N) stainless steel.

An Experimental Measurement of Toughness Locus for a Ductile Material

A toughness locus Jc-Q for a ductile steel, SA106 Grade C used in the main steam piping of nuclear power plants, has been experimentally evaluated. Along with the standard fracture test procedure for J-R curve, Q as the second parameter governing stress triaxiality nearby the crack tip is measured from the displacements nearby the side necking which occurs near the crack tip on the lateral surface of a fracture specimen. The displacements nearby the side necking are measured from the digital images taken during the fracture experiment based on Stereoscopic Digital Photography (SDP) and high resolution Digital Image Correlation (DIC) software. The crack length is monitored by Direct Current Potential Drop (DCPD) method and the J-R curve is determined according to ASTM standard E1737-96. The effects of crack length, specimen geometry and thickness of specimen are studied, which are included in the toughness locus Jc-Q.

Effect of geometry and electrical boundary conditions on R-curves for lead zirconate titanate ceramics

R-curve measurements on PZT poled in thickness direction were carried out on CT specimens under different electric boundary conditions. The effect of specimen geometry was evaluated by measuring R-curves in CT specimens of different thickness and comparing these with R-curves in bend bars. A low coercive stress is responsible for the development of a large switching zone. This switching zone is of high relevance for the computation of the actual stress intensity factor at the crack tip and for the R-curve calculation.

An investigation of moisture and leakage currents in GRP composite hollow cylinders

The applicability of using flat composite plates and hollow core composite cylinders for moisture absorption testing of unidirectional glass/polymer composites used in high voltage composite (non-ceramic) insulators was examined. Two main issues were addressed in this work. First, the effect of specimen geometry (cylinders vs. plates) on moisture absorption by the composites was investigated both numerically and experimentally. Both classical Fickian and non-Fickian diffusions were considered. Subsequently, hollow core cylinders made up of ECR (low seed)-glass fibers and epoxy resin were tested for their high voltage properties under controlled moisture diffusion conditions. The specimens were exposed to warm, moist air and their high voltage properties were ascertained using a modified version of the ANSI test (standard C29.11 Section 7.4.2) for water diffusion electrical testing. It was found that the behavior of the hollow core cylinder and flat plate composite specimens subjected to moisture compared reasonably well experimentally and very well numerically. From the high voltage tests, a direct correlation was found between the amount of moisture that had been absorbed by the specimens and the amount of leakage current that was detected. It was shown that using the thin walled composite cylinders leakage currents could be predicted based on the amount of absorbed moisture in the insulator composites. The predictions can be made based on relatively short term moisture data even if the diffusion process in the composites is anomalous in nature with long times required for full saturation. After additional verifications, considering other composite systems, the hollow core cylinder testing under controlled moisture and high voltage conditions could become a screening test for selecting suitable glass/polymer composites for insulator applications.

High strain rate tensile testing of automotive aluminum alloy sheet

The desire for more fuel efficient vehicles has resulted in a renewed interest in aluminum alloy sheet as a replacement for mild steel in automobile bodies due to its higher strength-to-weight ratio. When considering the crashworthiness of vehicles, the dynamic behavior of the materials used must be considered when simulating crash events, although traditionally, the strain rate sensitivity of aluminum alloys has been considered to be low. Room temperature and elevated temperature experiments were conducted on a tensile split Hopkinson bar apparatus to identify the constitutive response and damage evolution in AA5754 and AA5182 aluminum alloy sheet at high strain rates. Modifications were made to the apparatus in order to minimize wave distortions at the interface between the specimen and the bar, and to the specimen design to minimize specimen geometry effects. The rate dependency of flow stress was shown to be low; however, a marked increase in elongation was measured with increases in strain rate. The data was shown to be amenable to fits using the Johnson–Cook constitutive model.

Peak force as function of the embedded length in pull-out and microbond tests: effect of specimen geometry

We have derived the equations which explicitly express the peak force, F max, and the apparent interfacial shear strength, τ app, measured in the pull-out and microbond tests, as functions of the embedded length. Three types of test geometries were considered: (1) a fiber embedded in a cylindrical block of the matrix material; (2) microbond test with spherical matrix droplets; and (3) pull-out test in which the matrix droplet had the shape of a hemisphere. Our equations include the local interfacial shear strength (IFSS), τ d, and the frictional interfacial stress, τ f, as parameters; the effect of specimen geometry appeared in the form of dependency of the effective fiber volume fraction on the embedded length. The values of τ d and τ f were determined by fitting our theoretical curves to experimental F max (l e) plots by using the least squares method. Our analysis showed how the local IFSS and the frictional interfacial stress affected the measured F max and τ app values. In particular, it was revealed that intervals of embedded lengths could exist in which frictional interfacial stress had no effect on F max and τ app, even if the τ f value was high. We also derived an equation relating the scatter in the interfacial strength parameters (τ d and τ f) to the scatter in τ app, which is experimentally measurable, and proposed a procedure to determine the standard deviations of τ d and τ f from experimental pull-out and/or microbond test data.

Effects of Specimen Geometry and Loading Condition on Bifurcation of Fast Propagating Cracks

Effects of Specimen Geometry and Loading Condition on Bifurcation of Fast Propagating Cracks

コンクリートの直接引張試験における4つの誤解

The best way to investigate the tension softening process is by applying a uniaxial tension force directly on a concrete specimen, because it can measure both tensile strength and the tension softening curve from an identical specimen. However, no standard tests have been adopted to provide a direct measurement of the tension softening curves of concrete, though many inadequate test procedures were proposed until recently. There are four misunderstandings in investigating the tension softening process, concerning the effects of notches, secondary flexure, boundary conditions and specimen geometry. In this paper, the misunderstandings are discussed and clarified in detail with theoretical and experimental considerations. A test procedure free from these misunderstandings is proposed for the uniaxial tension test of concrete and some successful test results are presented.

Overload failure curve and fatigue behavior of spot-welded specimens

The mechanical behavior of a spot-welded specimen is generally approached in angles of overload and fatigue failures. The primary issue in an overload failure is to establish an overload failure criterion. Fatigue failure of spot-welded specimens can be dealt with a fracture parameter, since a spot-weld forms a singular geometry of external crack type. In this work, we express the limit loads in terms of base metal yield strength and specimen geometries. We then present a master overload failure curve for a single spot-welded specimen in a mixed-mode load domain. The coordinates of the domain are normalized by the limit loads of single spot-welded specimens. Recasting the load vs. fatigue life relations experimentally obtained, we attempt to predict the fatigue life of various spot-weld specimens with a single parameter denoting the equivalent stress intensity factor. This crack driving parameter is demonstrated to successfully describe the effects of specimen geometry and load type in an inclusive manner. The suggested fatigue life formula for a single spot-weld can be used in the assessment of spot-welded panel structures as the fatigue strength of multi-spots is eventually determined by that of each single spot-weld.

하이브리드 복합재료의 층간파괴인성치에 관한 연구

하이브리드 복합재료(Hybrid composites) 의 층간 파괴 인성치에 영향을 주는 인자 중 적층순서, 하중점변위율, 초기크랙길이를 변화 시켰을 때의 실험 결과는 다음과 같다. (1) CF/CF, CF/GF. GF/GF로 적층하였을 경우 층간파괴인성치값은 서로 같은 계면을 성형한 것보다 서로 다른 계면을 적층한 CF/GF의 경우가 강도면에서 가장 높게 나타나는 것을 알 수 있었다. (2) 하중점변위율은 미세한 변동은 있었으나, 하중점변위율의 영향은 거의 받기 않는 것을 알 수 있었다. (3) 초기크랙을 변화시켰을 때, 초기크랙길이의 영향은 일정하지 않았다. CF/CF인 경우는 초기크랙이 짧은 경우, CF/GF. GF/GF인 경우는 초기크랙이 긴 경우에 높은 값을 나타났다. 이것은 GF 섬유가 직조형태의 프리프레그로 되어 있어 크랙의 진전에 따라 섬유부스러기 등의 생성에 따른 영향이라고 생각된다. (4) 적층순서에 따라 파면의 섬유 분포 형태가 달랐으며 CF/GF인 경우가 섬유의 파손형태가 가장 복잡하게 나타났으며, 이것이 높은 층간파괴인성치를 나타내는 원인이라고 판단된다. This paper describes the effect of loading rate, specimen geometries and material properties for ModeⅠ and Mode Ⅱ interlaminar fracture toughness of hybrid composite by using double cantilever beam (DCB) and end notched flexure (ENF) specimen. In the range of loading rate 0.2~20mm/min, there is found to be no significant effect of loading rate with the value of critical energy release rate (Gc).The value of Gc for variation of initial crack length are nearly similar values when material properties are CF/CF and GF/GF, however, the value of Gc are highest with the increasing intial crack length at CF/GF. The SEM photographs show good fiber distribution and interfacial bonding of hybrid composites when the moulding is the CF/GF.

Residual stresses in alumina scales grown on different types of Fe–Cr–Al alloys: effect of specimen geometry and cooling rate

Residual stress measurements in alumina scales grown on Fe–Cr–Al alloys were conducted using the ruby fluorescence technique and the results compared with common theoretical considerations. The oxidation experiments were carried out at 1200 °C using different Fe–Cr–Al-based materials, different substrate thickness values and varying cooling rates. In case of a conventional wrought alloy the cooling rate significantly influences the residual stress state, whereas in case of a stronger, dispersion strengthened (ODS) alloy hardly any effect is visible. The results indicate at the very beginning of the oxidation process a high oxide growth stress level (up to −1.6 GPa), which does not substantially depend on the alloy type or specimen geometry. The growth stresses relax with time and this relaxation at oxidation temperature is faster in case of the wrought alloy than for the ODS alloy. Additionally, for all studied systems except the 2 mm thick PM 2000 specimens a transition from compressive to apparent tensile growth stresses after longer exposure times occurs. For very thin Aluchrom YHf specimens (hM = 50 μm) this transition might be explained by nearly total Al-depletion of the substrate material and the resulting decrease of the thermal expansion coefficient. However, the validity of the theoretical calculations based on an ideal flat surface cannot be assured at least for such thin substrates. In fact, geometrical irregularities, a non-even relaxation of the stress or micro-cracking in the scale are likely to affect the calculated residual and/or growth stresses even for thicker specimens.

Strength and Erosion Characteristics of Earth Blocks and Earth Block Masonry

Initially, the paper describes methods currently used for strength and erosion resistance testing of earth blocks. Following this, an experimental study undertaken to assess the influence of test procedure and specimen geometry on strength and erosion characteristics is presented. Cement stabilized pressed earth blocks were fabricated using different blended soils and compacted using a constant volume manual press. The effects of specimen geometry on experimental compressive strength are described and aspect ratio correction factors for unconfined unit strength outlined. Proposals for a unified approach to compression strength testing are also suggested. Bending strength testing is commonly used as an indirect method of strength assessment, as it is more readily suited to in-situ quality control testing than compression testing. The experimental correlation between compressive and bending strengths is presented. Results of compression tests on masonry walls are also presented together recommendations for design. Finally, the influence of test method and specimen geometry on erosion resistance and the correlation with block strength is discussed.