What is loading rate

Within our track and field program we had several distance runners that had experienced unilateral and bilateral lower leg stress fractures. As part of our research we compared the plantar surface contact of the foot to the individuals that experienced stress fractures and found that they all had a reduced amount of plantar surface contact compared to their peers that did not experience unilateral or bilateral stress fractures. This in turn created a higher loading rate onto the lower extremity.

As a result, the following year after completing the research every long distance runner was preventatively analyzed and custom orthotics were created to increase the plantar surface contact area of those that did not have a great plantar surface contact area, hoping to reduce the loading force on the distance runner, thus lower the potential for stress related fractures. By knowing force loading rates it allowed us to conduct research and create a potential solution to an overuse injury problem.

Oatis, C. Kinesiology the mechanics and pathomechanics of human movement 3 rd Ed. Your email address will not be published. Skip to content. References Oatis, C. Philadelphia, PA: Wolters Kluwer. Tam, N. The present paper provides an overview on test techniques for determining tensile and fracture toughness properties under dynamic conditions and examples of effects of dynamic rates on tensile properties and fracture toughness, as well as structural assessment considerations.

The emphasis is on ferritic steels with some consideration given to stainless steels. Dynamic test methods and standardisation of such methods are developing rapidly. This overview paper cannot cover every aspect in detail, and it is inevitable that there will be some omissions. Dynamic tests have been used and standardised to characterise qualitatively material properties for many years.

Such tests include Charpy and Pellini drop-weight tests. Although valuable for comparative purposes, they offer little detailed insight into failure mechanisms and quantifiable properties. This has led to instrumented versions of these tests, and to new test methods being developed over the past 20 years or so.

A number of test techniques, initially developed for research purposes, are now being refined and compared to establish if they can meet the strict requirements of standardisation. Repeatability, accuracy and ease of use are vital if such methods are to become accepted. There is co-operation within Europe and with the USA to ensure that any methods are truly international.

Dynamic testing is usually expensive. Special testing machines may have to be used. High speed recording equipment will be needed, and the analysis of the test date may be complex. Skilled and experienced personnel will be needed to perform the tests and evaluate the results correctly. Extra safety precautions need to be taken.

For all these reasons, it is increasingly realised in Europe and the USA that such tests should be performed and analysed using, as far as possible, agreed international standard methods. This paper will consider dynamic tensile testing and fracture testing in both linear-elastic and elastic-plastic regions. Medium rates, when inertial effects are negligible or can be controlled and quasi-static analyses remain applicable.

High rates where inertia dominates, and special measurement and analyses techniques must be applied. The British standard BS was the first standard for fracture testing at dynamic loading rates. It is based on static procedures with allowances made for dynamic condition. Round-robin testing carried out during the development of the standard is reported in Ref. It is to be replaced in due course by BS Part 3. For Symbol.

The main issue with such high loading rates is that conventional instrumentation is no longer appropriate to record the actual conditions experienced by the specimen. For example, Fig. It can be seen Fig. Load-time plots under static loading [30]. Load-time plots under dynamic loading [30]. Instrumentation of mm thick high rate fracture mechanics specimens. Load-time record for fracture toughness test. To characterise fully ductile behaviour, fracture toughness values at the initiation of tearing or tearing resistance curves are required.

There are no accepted standards yet to carry out such tests under dynamic loading conditions but a number of test methods have been suggested [] and reviewed in by MacGillivray and Turner [14]. One method, due to Chipperfield, is illustrated in Fig.

Shoulders are machined into the sides of an impact specimen which cause the specimen to 'slip' between the impact anvils at a given amount of load line displacement, associated with a given amount of tearing. Various shoulders can be machined to give various amounts of crack growth to establish a dynamic R-curve.

A ESIS TC5 round-robin programme has been performed by seven laboratories to compare the proposed standard with reasonable success reported [17]. It has been long known e. A further data collection for plain carbon steel reproduced from Ref. This plot reveals three regions. Up to a strain rate of about 1s -1 only a small to moderate increase in yield strength of about 70MPa for this steel type is obtained.

The strain rate sensitivity increases beyond 1s -1 , up to a value of about 10 3 s Data in Fig. Beyond 10 3 s -1 the sensitivity to strain rate becomes extreme. Increase in yield strength due to strain rate [16]. The effect of temperature on strain rate dependence of yield and tensile strength [18] is shown in Fig. The room temperature behaviour Fig. Hence, for the purpose of establishing constitutive equations, care must be taken when extrapolating or even interpolating data obtained at temperatures or strain rates not representative of the situation under consideration.

True stress versus strain rate at RT [18]. An example of the effect of increasing strain rate on stress-strain curves is shown in Fig. Effect of temperature and strain rate on crack toughness of ABS-C steel [19].

Effect of temperature and strain rate on crack toughness of 18Ni maraging steel [19]. At a given temperature, the effect of loading rate expressed as the rate of increase of the applied stress intensity factor, , in the elastic loading region of the specimen is generally to reduce the measured K Ic values.

Examples are shown in Fig. Figure 10c also shows that the loading rate sensitivity of AB steel fracture toughness increases with increasing temperature. However, not all steels exhibit negative fracture toughness loading rate sensitivity. Figure 11 shows data for alloy steels showing positive loading rate sensitivity over a range of temperatures Fig.

Whilst the reasons have not been investigated in detail, it is likely that strain rate dependent microstructural process such a dynamic strain ageing are affecting the test results. Effect of loading rate on fracture toughness of Rail steel [6].

Dynamic K Ic data for AB [21]. Loading rate and temperature dependence of K Ic for AB [22]. K Id data for 0. However, for the majority of steels, the K Ic dependence can be summarised as shown in Fig.

At very high loading rates an increase in K Ic may be observed indicated by the dotted line in Fig. Schematic K Ic vs Symbol. Ductile tearing resistance of steels is strongly affected by the tensile strength and the strain hardening behaviour of the material. As tensile properties tend to increase with loading rate, it is generally observed that increased loading rates lead to increased ductile fracture toughness. This behaviour is shown schematically in Fig. Effect of loading rates on R-curves [14].

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