Effect of strain rate, thickness, welding on the J–R curve for polyethylene pipe materials

Highlights

• Mechanical properties of HDPE are significantly affected by the welding technique.

• For all HDPE specimens the fracture toughness decreases as the thickness increases.

• The fracture toughness of HDPE decreases with decreasing the crosshead speed.

Abstract

The main purpose of the present paper is to investigate the effect of strain rate, specimen thickness and welding on the fracture toughness for the specimens made from high-density polyethylene (HDPE). The welds at the pipe junction are produced by butt-fusion (BF) welding. Tensile tests specimens are cut longitudinally from the pipe with thickness, T = 30 mm, and are tested at different crosshead speeds (10–500 mm/min), and gauge lengths, G = 20 mm, to determine the mechanical properties of welded and unwelded specimens. Curved three-point bend (CTPB) fracture specimens are used. The effect of specimen thickness variation for welded and unwelded specimens is studied at room temperature, (T_a = 23 °C), and at different crosshead speeds, V_C.H, (10–500 mm/min). The study reveals that the crosshead speed has a significant effect on the fracture toughness of both welded and unwelded specimens. The fracture toughness, J_IC, is greater for unwelded than welded specimen.

Introduction

Many failure records in metal pipes have indicated that corrosion has always taken place in cheaper pipe metals and these have required post-treatment such as galvanizing or subsequent coating. Unfortunately, a small defect or installation damage will result in premature failure. Plastic pipes avoid much of these problems and there is now a great trend to use them instead of metallic pipes [1], [2], [3], [4], [5]. Plastic pipes have in general, good thermal and electrical insulation properties, low density and high resistance to chemicals, but are mechanically weaker and exhibit lower modulus of elasticity than metallic pipes [6]. The most common plastic pipes are polyethylene, PE which are used in local gas and water transmission systems. One of the advantages of PE pressure pipes is their suitability to squeeze-off when flow is to be stopped in a section of pipe or connections. PE gas pipes have many superior features compared with steel gas distribution alternatives; for example, PE hardly corrodes and is very flexible, so it is safe against earthquakes and ground subsidence. It is also light and easily constructed. PE pipes can be used in range of temperatures from −40 °C to 60 °C considering the change of operating pressure. It is commercially available in different grades, such as low-density polyethylene (LDPE) linear low-density polyethylene (LLDPE), high-density polyethylene (HDPE), ultra-high molecular weight polyethylene (UHMWPE), and cross-linked polyethylene (PEX or XLPE). [7], [8]. HDPE is the most widely used polymeric material for plastic pipe applications. The fracture toughness of HDPE is lower than that of commercial metallic and ferrous alloys. In addition, its strength and rigidity are low compared to both metals and concrete, which is partially offset by its low weight (specific strength).

Thermoplastic pipes can be easily joined with simple welding techniques using relatively simple machines with no need to highly specialized operator skills. Nowadays, most of the plastic pipes are joined by; butt-fusion (BF), (hot-plate) welding or electro fusion (EF) welding. Both butt fusion and electro fusion welding can produce high quality welds. However, butt fusion welding is a slow and complicated process. On the other hand, electro-fusion welding has an expensive consumable cost of the coupling and potential flaws like incomplete insertion of the pipe into the socket and deviation of the wire. Butt- fusion welding should only be used for joining pipes of the same standard dimension ration (SDR), value. Electro fusion welding method is capable to weld pipes having different wall thicknesses. They are available in a choice of 10 bar or 16 bar (water) and 5.5 bar or 7 bar (gas) rating. Care should be undertaken to ensure that the pressure rating of the fittings is equal to or greater than that of the pipe.

The theory of connection of both parts to be joined together is based on the hypothesis that when two molten pipe ends are welded together under a given pressure, some molten materials will be forced to flow out of the pipe wall. Some of molten materials will flow towards the pipe’s exterior surface and some towards the pipe’s interior surface. This means that there is a point in the joining plane, where the flow is separated into two opposite directions, as shown in Fig. 1 [9]. During the welding process the material at this separation point, O, has theoretically zero displacement. At other points the material displaces outwards from the separation point, O, along the joining plane and forms a bead.

The butt fusion welds can have the same strength as the parent material if the correct joining instructions are applied. However, the inner and outer beads formed during welding can cause stress concentrations during service. This form of welding has been widely employed in diverse industries such as aerospace, shipbuilding, nuclear, bridge and machinery and pipelines. The welding process leads to a nonuniform temperature distribution, which is associated with thermal strains and localized plastic deformation. The resulting welding residual stresses pose significant problems such as inducing fracture and degrading the buckling strength of a structure. Therefore, estimating the magnitude and distribution of welding residual stresses are necessary for achieving the safest design [10].

The determination of crosshead speed effect on fracture toughness parameters (K_IC, J_IC, and COD) is difficult due to the changes from obviously ductile, invalid tests, to brittle fractures in some cases as the speed rate increases [11], [12]. This trend is observed for plastic pipe materials [13]. Many researchers have studied the correlation between, strain rate, fracture behavior, and mechanical properties of HDPE pipe material under different operating conditions [14], [15]. Flueler et al. [16] have examined the applicability of plane strain fracture toughness of plastic pipes (PVC, and PE) in two orientations (longitudinal, and transverse) at different crosshead speeds, (V_C.H = 5–500 mm/min). It has been proven that the fracture toughness change considerably with the rate of loading, the annealing conditions, and the specimen orientation. The results of K_Q, and peak load show a gradual increase of the K_Q with increasing strain rate. The initial slopes are identical in both directions. The mean deviation between the longitudinal, and transverse fracture toughness lies within the range of 5%. O’Connell et al. [17] have studied the mode of failure of a number of polyethylene materials under plane strain conditions. The crosshead speed has been varied from 0.005 to 500 mm/min under different operating temperatures; 23, 38, 50, 85, and 110 °C. The results have shown that the failure mode changes from brittle to ductile failure as a function of crosshead speed at a specific constant testing temperature.

Gensler et al. [18] have investigated the fracture behavior of isotactic polypropylene iPP and impact modified isotactic polypropylene at test speeds from 0.1 mm/s to 14 m/s, using compact tension (CT) specimens. The results revealed that the deformation behavior of iPP indicate a ductile–brittle transition as the test speed increased. Roberts et al. [19], [20] have presented a study to apply the plane strain fracture toughness testing technique on the pipe made from HDPE pipes material. In their experimental work three-point bend (TPB) specimen is used. A parametric variational analysis has been conducted in which several operating conditions have been varied; such as notch shape, notch root, specimen thickness, testing temperature, and strain rate. From the experimental variational analysis it has been shown that at higher temperatures and slower strain rates the fracture is ductile. On the other hand, at lower temperatures and higher strain rates the fracture is brittle. Chan and Williams [21] have used the multiple specimen resistance curve technique as a basic method to determine J_IC for HDPE material. They used a mathematical formulation based on the finite difference method to obtain J_IC and then compare its value with the corresponding J_IC obtained from the multiple specimen method. The tests are performed in the temperature range from +23 °C to −80 °C at different crack to width ratio (a/W = 0.3, 0.5). The multiple specimen resistance curve technique is reliable for establishing a suitable J_IC prediction point. The results show that the cooling temperature has a significant effect on the fracture toughness parameters K_IC, and J_IC. At lower temperatures K_IC is greater due to greater stiffness whereas that, J_IC has lower energy absorption as the fracture mode becomes less ductile and unstable. Mandell et al. [22], [23] have investigated the plane strain fracture toughness, K_IC, for pipe made from MDPE pipe material over a large range of test temperatures and strain rates. The fracture test specimens are fabricated both as directly cut from pipe walls as well as from flat plates. Curved three point bend (CTPB) specimens are prepared directly from the pipe wall. The flat specimens are prepared from pipes by heating curved sections up to 150 °C then flattening them under an adequate press. Various geometry test specimens have been produced from the plates namely; CTPB, double edge notch (DEN), three point bend (TPB), and single edge notch (SEN). The results revealed that the behavior of fracture toughness is affected considerably by temperature and strain rate. The fracture toughness decreases with increasing strain rate and decreasing temperature at all specimen’s configurations. It has been proven that CTPB specimen (cut directly from pipe wall) has a higher fracture toughness compared with other configurations.

Kapp et al. [24] have studied the variations in wall thickness on the stress intensity factor for C-shaped specimen which, may be caused by the non-concentricity of the inner and outer surfaces of the cylinder. This non-concentricity allows shear stresses to develop in the plane of the crack producing fracture from the type mode II sliding in addition to mode I opening. The ratio K_II/K_I does not exceed 0.04. This means that for practical purposes, specimen non-symmetry induces only a negligible amount of mode II sliding. Jones [25] has used the FEM to compute the stress intensity factor, crack mouth opening displacement coefficient for a cylindrical segment (CTPB). Three variables are associated with geometry namely: curvature, thickness, and span. The stress intensity factor and crack mouth opening displacement coefficients are independent of the span length for a large range of crack lengths, and angular span of ring segment. Tracy [26] and Darwish et al. [27], [28] have investigated a similar specimen as Jones [25] CTPB specimen with the crack at the inner surface and used a Mapping Collocation technique to obtain the stress intensity factor equation. Chan and Williams [29] have studied the effect of specimen size (thickness B = 5–30 mm, and width, W = 3–40 mm, crack to width ratio, a/W = 0.1–0.5, and the mode of loading (SENT, and SENB) on the plane strain fracture toughness, K_IC, of HDPE pipe material. The tests are carried out at crosshead speed range, V_C.H = 5, 10, and 20 mm/min. The linear elastic fracture mechanics, LEFM, theory has been applied on three different grades of HDPE in an attempt to determine the fracture behavior in terms of plane strain fracture toughness, K_IC. The results show that, decreasing the thickness, B, leads to a transition state from plane strain to plane stress. However, a reduction in the width, W, leads to an increase in the yielding phenomenon at the crack tip, which decreases K_I. Gross and Srawley [30] have studied the curved three-point bend (CTPB) specimen by using the boundary collocation method to compute mode I stress intensity factor and crack mouth opening displacement coefficients. In their study the stress intensity factor and crack mouth opening displacement coefficient have been derived as a function of angular span of ring segment θ₁. For pipe made from MDPE, Mandell et al. [22], [23] have studied the effect of specimen thickness, B on the plane strain fracture toughness, K_IC. The specimen thickness is varied within the range 3–23 mm. TPB specimens have been prepared from the pipe after pre-flatting curved sections at 150 °C. The tests are carried out at test temperature equal = −51 °C, and crosshead speed, V_C.H = 3060 mm/min. The study demonstrated that the effect of specimen thickness is similar to those found in metals; namely higher fracture toughness in case of thinner specimens. This means that with increasing specimen thickness the fracture toughness decreases. The plane strain fracture toughness, K_IC is valid at specimen thickness equal to 18 mm.

Many researchers have studied the correlation between, welding, fracture behavior, and mechanical properties of HDPE pipe material under different operating conditions [31], [32]. Pfeil et al. [33] have presented a scheme based on the principles of fracture mechanics for failure assessment of butt fusion joints made from similar and dissimilar polyethylene PE pipes due to slow crack growth, SCG. The tests are performed at ambient temperature (23 °C) and elevated temperatures (60 and 70 °C) on TPB specimens containing a centrally notched joint. The fracture specimens are cut from the pipe wall in longitudinal direction. During butt fusion welding, the razor blade is precisely placed at the interface of the pipe just before joining to produce the initial start notch impression. The razor blade is preheated to 220 °C to prevent the formation of a cold joint. The orientation of the weld joint is similar to that of the pipe in a plane normal to the longitudinal axis. The results revealed that the fracture mechanics method can be used to quantify the influence of a crack-like defect on the life expectancy of heat fusion joints. The life expectancy of a joint produced by cross fusion welding of two different PE pipes is proven to be greater than that of a similar joint in the lower fracture resistant PE.

Deblieck et al. [34], [35] and Frank et al. [36], [37], [38] recently developed new test methods for the characterization of crack resistance of polyethylene pipes and pipe materials like the strain hardening test and the cyclic cracked round bar test. Both methods are suitable to characterize differences between pipe behavior and welding zone behavior in terms of crack resistance.

Most studies on polyethylene pipe materials concentrate on studying bulk material behavior. A higher focus on specific component behavior as in the current example with the butt weld could be of high practical importance. The main objective of the present study is determining the fracture toughness, J_IC, of welded and unwelded specimens made from HDPE pipes. The first step toward understanding the fracture toughness of HDPE pipes is to quantify the mechanical properties of the material. The second objective is to determine the effect of strain rate (10–500 mm/min), specimen thickness (10–45 mm), and welding on the fracture toughness, J_IC, of HDPE (PE100) pipes.